Search

We present a geochemical study on olivine and clinopyroxene-hosted melt inclusions (MIs) from 2001 to 2006 Etna basaltic lavas and pyroclastites. Three MI suites are distinguished on the basis of trace element fingerprinting. Type-1 MIs (from 2001 Upper South and 2002 Northeast vents) share their trace element signature with low-K lavas erupted before 1971. Critical trace element ratios (e.g., K/La, Ba/Nb), along with Pb isotope data of Type-1 MIs provide evidence for a heterogeneous mantle source resulting from mixing of three end-members with geochemical and isotopic characteristics of EM2, DMM, and HIMU components. Type-1 MIs composition does not support involvement of subduction-related components. Type-2 (from 2001 Lower and 2002 South vents) and Type-3 (2004 eruption) MIs reveal “ghost plagioclase signatures,” namely lower concentrations in strongly incompatible elements, and positive Sr, Ba, and Eu anomalies. Both Type-1 and Type-2 MIs occur in 2006 olivines, which highlight the occurrence of mixing between Type-1 and Type-2 end-members. Type-2/Type-3 MIs testify to en route processes (plagioclase assimilation and volatile fluxing) peculiar for “deep dike fed” eruptions. The latter are strongly controlled by tectonics or flank instability that occasionally promote upraise of undegassed, more radiogenic primitive magma, which may interact with plagioclase-rich crystal mush/cumulates before erupting. Type-2/Type-3 MIs approach the less radiogenic Pb isotopic composition of plagioclase from prehistoric lavas, thus suggesting geochemical overprinting of present-day melts by older products released from distinct mantle sources. Our study emphasizes that MIs microanalysis offers new insights on both source characteristics and en route processes, allowing to a link between melt composition and magma dynamics.

International audience; A large late Cretaceous (~ 90 Ma) volcanic field (the Natash volcanic province) crops out in southeast Egypt at the northwestern boundary of the Arabian-Nubian shield. The lavas are mainly of alkaline affinity and exhibit a continuous compositional range from alkali olivine basalt (AOB) to trachyte and rhyolite. All basaltic lavas in the province record various extents of fractional crystallization of olivine, clinopyroxene, plagioclase and spinel. The basaltic lavas show variations in Sr-Nd-Pb-Hf isotopic ratios [(87Sr/86Sr)i = 0.7030–0.70286; (143Nd/144Nd)i = 0.512653–0.512761; (206Pb/204Pb)i = 19.28–19.94; (177Hf-176Hf)i = 0.28274–0.28285], that correlate markedly with the major and trace element ratios and abundances. Assimilation of crustal material cannot explain these correlations, and we invoke instead melting of a multicomponent mantle source. We infer the existence of High-μ (HIMU), Enriched mantle type-I (EM-I) and Depleted mantle (DM) domains in the melting source, with a predominant contribution from the HIMU-type. We suggests further that the basaltic lavas originate from low degrees of partial melting (F < 5%) at moderate potential temperatures (TP) 1391–1425 °C and pressures of 2.0–2.6 GPa. The melting pressure estimations imply that melting entirely occurred within lithospheric mantle, most likely in the presence of residual amphibole as presence negative K-anomalies in the primitive mantle-normalized patterns of the fractionation-corrected melts. The presence of amphibole within the lithosphere is a strong evidence that the lithospheric mantle underwent metasomatic enrichment prior to melting in Late Cretaceous. This metasomatic event affected on the Pb isotopic compositions of the Natash volcanics by adding Th and U to the melting source. Time-integrated calculations to remove the decoupling between 206Pb and 207Pb isotopes that most probably resulted from the metasomatic event indicate a tentative link between the metasomatism occurring in the Pan-African lithospheric mantle and the formation of juvenile crust during the Pan-African Orogeny. A two stage evolution model is therefore proposed for volcanism in the Natash area: fluxing of the lithosphere by hydrous fluids during Pan-African Orogeny forming a hybrid lithospheric mantle that in Late Cretaceous underwent thermal erosion and melting in response to upwelling asthenosphere, possibly at the onset of the extensional fracturing preceded the doming of the Afro-Arabian Shield.

Mantle xenoliths collected from Fuerteventura, one of the easternmost Canary Islands, exhibit a complex evolutionary history comprising events of depletion, serpentinization, dehydration and melt metasomatism. Each of these events left imprints on both the texture and chemistry of the xenoliths. Extensive partial melting is shown by complete lack of primary clinopyroxene, the ultra-refractory trace element composition of orthopyroxene porphyroclasts, and low heavy rare earth element contents as compared with abyssal peridotites sampled along mid-ocean ridges and oceanic fracture zones, in the xenoliths least affected by later metasomatism. In many xenoliths the original orthopyroxene porphyroclasts and some olivines are replaced by fibrous aggregates of orthopyroxene and/or large, deformed olivine porphyroclasts with mottled rims with stringy glass and fluid inclusions. Such features are very rare in ocean island xenoliths. Unusually high H2O and Cl concentrations, together with very high H2O/Ce and Cl/K ratios in interstitial glasses, suggest that the fibrous orthopyroxene formed by local serpentinization by hot seawater. The volume increase accompanying the serpentinization caused extensive fracturing of adjacent olivine porphyroclasts. The most likely scenario for local mantle invasion by hydrous fluids is along deep faults and fractures caused by tectonic movements along the continent‐ocean transition during the early phases of the opening of the Atlantic Ocean. The peridotites were later (probably during the Canary Islands magmatism) dehydrated, causing the serpentine minerals to be replaced by porous domains of fibrous orthopyroxene. Hydrous fluids released by the deserpentinization escaped into neighbouring and overlying rocks leaving trails of fluid inclusions along fractures and grain boundaries causing mottled rims and zones in olivine porphyroclasts. During the Canary Islands magmatism the upper mantle beneath Fuerteventura was also infiltrated by enriched silicate magmas that caused different degrees of Fe‐Ti-metasomatism. A higher degree of melt metasomatism in rocks with fibrous orthopyroxene and mottled olivine than in the massive harzburgites strongly suggests that the sublithospheric Canarian magmas reused serpentinized extensional faults during their rise to the surface. The strongest degree of melt metasomatism appears to have resulted in the formation of lherzolites, wehrlites, and dunites.

Partition coefficients (Min/L D ) for a series of geochemically relevant elements have been calculated from combined EMP/LA-ICP-HRMS analyses of plagioclase, alkali feldspar, Ti-magnetite, biotite, apatite, and trachytic/trachyphonolitic melt pairs in selected Campi Flegrei rocks. Pl/L D and Kfs/L D values are generally very low for most of the trace-elements but Sr, Ba, and Eu. Kfs/L D for the latter elements record a systematic increase as the melt composition changes from trachyte to trachyphonolite, likely due to increasing structural compliance of the sanidine in the trachyphonolites related to larger Na/K values. Conversely, Kfs/L D values for transitional, highly charged incompatible elements (e.g., LREE) decrease from trachyte to trachyphonolite, possibly in response to the decrease of melt polymerization. Min/L D values for titanomagnetite generally decrease with the increasing melt evolution, the highest values being those measured for Ti, V, and Sc. Ti, Ba, Sc, Rb, Nb, Ta, and V are compatible in biotite in equilibrium with trachytic melt, whereas Cs, Sr, and Pb are incompatible and REE are strongly incompatible, as supported by the extremely low Bt/L D Y (0.003–0.008). Partition coefficients for apatite and trachyphonolitic glass pairs are high for Sr, REE (particularly MREE), and Y, large for Th, U, and V, generally low for HFSE, and variable for other LILE. The comparison of measured Min/L D values for Campi Flegrei trachytes/trachyphonolites with other sets of partition coefficients reported in literature for evolved systems suggests that a reliable data set for magma evolution modeling requires: (1) a thorough preliminary selection of natural samples; (2) the adoption of accurate microanalytical techniques; (3) the direct measurement of Min/L D values for each specific melt composition.

The continental crust (CC) and the depleted mantle (DM) are generally assumed to be complementary reservoirs within the Earth. However, the mixture between CC and upper mantle does not generate the Nb/Ta and Nb/La ratios of chondrites. A reservoir with superchondritic ratios for Nb/Ta and Nb/La is thus required in the Earth's system. The occurrence of a hidden amphibole reservoir in the lower arc crust has been recently proposed. This, coupled with the capability of calcic amphibole to give rise to a superchondritic Nb/Ta and Nb/La reservoir, led us to determine to what extent amphibole-rich ultramafic rocks can account for the Nb (and Nb/Ta, Nb/La as well) imbalance on Earth. We have considered lower crust mafic and ultramafic amphibole-rich intrusive rocks from collisional settings worldwide. Because CC is considered to have primarily formed in collisional setting these rocks are important for its genetic model. We modeled Nb, Ta and La contents of the hidden Nb reservoir by mass balance calculations between continental crust, depleted mantle and primitive mantle. Modeling shows that amphibole-rich mafic lower crust can solve the so-called Nb paradox if large volumes of materials are supposed to be returned into the mantle during the Earth's history. A possible mechanism is recycling, particularly in Precambrian times, of eclogites that underwent pre-eclogitic melting in the amphibolite facies field and then recrystallized under eclogite-facies conditions.

Sapphirine is generally interpreted to be of metamorphic origin in high-MgO-Al2O3 rocks. Igneous sapphirine, i.e. sapphirine crystallised from melt, is very rare. We examined sapphirine-bearing magmatic veins in the Finero Phlogopite-Peridotite Massif, Western Italian Alps, to investigate a possible igneous origin for sapphirine by crystallisation from a melt, acquiring particularly high Al2O3 content via melt-rock reaction and fractional crystallisation. Sapphirine locally occurs in melanocratic zones placed between leucogabbroic veins and the host peridotite. The leucogabbroic veins cut at high angle the mantle foliation and the lithological layering of the peridotite massif, which is defined by alternating phlogopite-rich harzburgites and pyroxenites. This observation, along with their peculiar major-element mineral chemistry, indicates that leucogabbroic veins were unrelated to the pervasive metasomatic recrystallisation of the host mantle sequence, recording a later, distinct event of melt injection. Melanocratic seams are observed on both sides of the leucogabbroic veins. They show a marked zoning of the mode: an orthopyroxene-rich zone overgrows upon the host peridotite (OPX zone), whereas an amphibole-rich zone occurs towards the leucogabbroic vein (AMPH zone). Sapphirine commonly mantles spinel or occurs as discrete grains, located either (1) within large light-brown pargasite crystals in the AMPH zone or (2) interstitially, between light-brown pargasite in both AMPH zone and OPX zone. Light-brown pargasite can also enclose spinel that does not have sapphirine envelope. To explain the petrochemical features of the sapphirine-bearing veins and the host peridotite a four-stage process involving melt-rock reactions and fractional crystallisation is here proposed. During the first stage (Stage A), the interaction between the uprising SiO2-saturated melt and the host peridotite caused the replacement of peridotite olivine, amphibole and phlogopite by newly formed orthopyroxene close to the contact and Al2O3, TiO2, FeO enrichments in the host peridotite beyond the recrystallisation front. This mineralogical reaction resulted in high Al2O3/SiO2 and MgO/FeO ratios in the migrating melt. The modified melt crystallised Al2O3-rich dark-brown pargasite (16.5 wt% Al2O3) and apatite in the open conduit (Stage B). Sapphirine/spinel saturation was actually achieved later, in the Stage C, in presence of a more differentiated melt, which reacted with the early dark-brown pargasite locally producing pseudo-symplectite textures made by light-brown pargasite and spinel/sapphirine. A peculiar Al2O3-enriched composition for the parent melt segregating sapphirine is indicated by the composition of the associated light-brown pargasite (17.5 wt% Al2O3), phlogopite and spinel. Locally, this melt percolated also the OPX zone, segregating sapphirine-bearing mineral assemblages. The sapphirine-free leucogabbroic vein was finally segregated during the Stage D, after splitting of the AMPH zone likely due to hydraulic fracturing.

We studied the elemental and isotopic (Pb, B and Li isotopes) composition of melt inclusions hosted in highly forsteritic (Fo(83-91)) olivines that were collected from San Bartolo lava and pumice (ST79p, ST82p and ST531p) samples erupted by Stromboli in historical times. The studied melt inclusions have primitive calcalkaline to shoshonitic basaltic compositions. They cover a compositional range far wider than that exhibited by the whole-rocks and differ in key trace element ratios. San Bartolo melt inclusions are characterized by lower incompatible trace element abundances, higher ratios between fluid-mobile (B, Pb, U and LILE) and less fluid-mobile (REE, Th, HFSE) elements and lower La/Yb ratios relative to the pumice-hosted melt inclusions and pumiceous melts erupted during paroxysmal events. Trace elements, along with different Pb, B and Li isotopic signatures, attest to source heterogeneity on the small scale and provide new insights into subducted components beneath Stromboli. Results of a mixing model suggest that metasomatism of the mantle source of pumice-hosted melt inclusions was driven by solute-rich high-pressure fluids (< 20%) expelled from the deep portion of the slab. Heterogeneous Pb isotopic composition together with light delta B-11 (-8.6 to -13.7aEuro degrees) and delta Li-7 (+2.3 to -1.7aEuro degrees) indicates that high-pressure liquids were released in variable proportions from highly dehydrated metabasalts and metasediments. On the other hand, the elemental and isotopic (delta B-11 similar to -1.9 to -5.9aEuro degrees) composition of San Bartolo melt inclusions is better explained by the addition of a prevalent aqueous component (similar to 2 to 4%) escaped at shallower depths from sediments and altered basaltic crust in almost equivalent proportions, with a smaller contribution by high-pressure fluids. Owing to the high-angle dip of the subducted cold Ionian slab, aqueous fluids and high-pressure fluids would rise through the mantle wedge and locally superimpose on each other, thus giving origin to variously metasomatized mantle domains.

In situ Sr-isotope data by microdrilling, cou- pled with major and trace element analyses, have been performed on plagioclase and clinopyroxene from seven samples collected during the 2002–2003 eruptive crisis at Stromboli volcano (Aeolian Islands, Italy). On 28 December 2002, the persistent moderate explosive activity was broken by an effusive event lasting about 7 months. A more violent explosion (paroxysm) occurred on 5 April 2003. Two magma types were erupted, namely a volatile- poor and highly porphyritic magma (HP-magma) poured out as scoria or lava and a volatile-rich, phenocryst-poor magma (LP-magma) found as pumice. LP-magma differs from the HP-magma also for its slightly less-evolved chemistry, the groundmass composition and the lower Sr- isotope ratios. Micro-Sr-isotope data show the presence of zoned minerals in strong isotope disequilibrium, as previ- ously found in products erupted in 1984, 1985 and 1996 AD, with 87Sr/86Sr values generally decreasing from cores to rims of minerals. Only some outer rims testify for equilibrium with the host groundmass. The internal mineral zones with high Sr-isotope ratios (0.70665–0.70618) are interpreted as ‘antecrysts’, crystallised during the previous activity and recycled in the present-day system since the opening shoshonitic activity of the Recent Period, which occurred at about 2.5 ka ago. This result has implications for the dynamics of the present-day plumbing system of Stromboli at intermediate pressure (about 2–3 km depth) and allows us to propose a model whereby an HP-magma reservoir is directly interconnected at the bottom with a cumulate crystal much reservoir. Efficient mixing between residing HP- and input LP-magmas can occur in this res- ervoir, due to more similar rheological characteristics of the two magmas than in the conduit, where crystallisation is enhanced by degassing. Antecrysts (and possibly melts) re-enter in the HP-magma reservoir both from the bottom, recycled by ascending LP-magmas crossing the crystal mush, and from the top, recycled by descending degassed and dense HP-magma, residual of the periodic Strombolian explosions at the surface. The isotope variation measured in the groundmasses allows calculating the proportion of the LP-magma entering the shallow HP-magma reservoir at *20%. From this proportion, we estimate that the total volume of LP-magma input during 2002–2003 closely matches the magma volume erupted in the effusive event, suggesting a steady-state system at broadly constant vol- ume. The comparison with estimates of the LP-magma volume ejected by the paroxysm indicates that the LP-magma amount directly reaching the surface during the 5 April paroxysm is minimal with respect to that entering the system.

The composition and geochemical signatures of the mantle wedge beneath the Lesser Antilles arc are documented by the ultramafic xenoliths included in alkali basalts (M-series) on Grenada. Xenoliths consist of harzburgites, lherzolites, dunites and subordinate wehrlites and pyroxenites. Primary minerals phases are olivine, low-Al and high-Al orthopyroxene, clinopyroxene and Cr-Spinel. In addition to the primary assemblage, Grenada xenoliths contain metasomatic phases such as Al-rich clinopyroxene, plagioclase, Al-rich spinel, pargasitic amphibole and Si- and Al-rich glasses. The trace-element signatures of pyroxenes and glasses have been determined on selected samples by LA-ICP-MS. Pyroxenes from both lherzolite and harzburgite xenoliths have U-shaped rare earth element (REE) profiles, unusually high Th, U and Sr concentrations and large negative Nb, Ta and Zr, and Hf anomalies. The geochemical signatures of metasomatic clinopyroxene are different from those reported for clinopyroxene from fluid-metasomatised mantle wedge, and are clearly distinct from those of clinopyroxene in equilibrium with host lavas. Si-rich glasses show a narrow compositional range, with trace-element characteristics closely similar to those of reacted pyroxenes. This, along with the general lack of chemical gradients of LILE and LREE over more compatible elements suggests dacitic glasses represent the products of in-situ melting caused by temperature increase before and during the uptake of xenoliths by host lavas. Dacitic melts are believed to represent local re-melts of regions metasomatically enriched by earlier arc magmas that had stalled, fractionated, and solidified in the upper mantle. These local re-melts thus reflect the metasomatic component formed by earlier arc-related metasomatic agents and liable to be re-mobilised. This also appears to be the easiest way to explain the compositional similarities between erupted arc lavas and the metasomatised peridotites. The results of this study suggest that the mantle wedge beneath the Lesser Antilles underwent complex peridotite–melt reaction processes operated by sub-arc melts and, later on, by magmas similar in compositions to the host alkali basalts. The majority of the compositional range of erupted Grenada magmas, but adakites found at surface, can be obtained by the interaction of basalts, possibly formed by hydrous melting of MORB-source mantle, with the overlying mantle wedge.

International audience; The evolutionary stages of lithospheric mantle in the Alboran domain are recorded by peridotites and associated pyroxenites outcropping in the Betic–Rifean orogenic belt. We show here that the scale of lithospheric mantle interpretation can be extended up to at least 300 km farther south of Beni Bousera thanks to the occurrence in Middle Atlas Cenozoic volcanics of rare pyroxenite mantle xenoliths, which unlike most peridotite xenoliths still preserve their pre-metasomatic geochemical signatures. These pyroxenites closely match mafic layers from Ronda and Beni Bousera occurrences and include, in addition to more common Spl-facies pyroxenites and former Grt-bearing pyroxenites accreted to the lithosphere and equilibrated under Spl-facies condition, also former Pl-bearing cumulates that represent old sectors of delaminated lower crust. The close similarity of Middle Atlas pyroxenite xenoliths to mafic layers in the Alboran domain makes the region comprising the Betic-Rifean belt and Middle Atlas one of the few occurrences that allow petrological and chemical information from exhumed mantle sectors to be combined with that from non-exhumed sectors, which only a few million years later were brought to the surface by volcanic activity.

A method for the in situ single spot δ11B characterisation of geological materials with laser ablation multicollector ICP mass spectrometry (LA-MC-ICPMS) has been developed. The mass spectrometer was equipped with both Faradays and multiple ion counters. Four samples with different B contents (12–31,400 ppm) and isotopic compositions (δ11B are between −8.71 and +13.6‰) were analysed. Samples include the B4 tourmaline and 3 MPI-DING glasses (StHs6/80-G, GOR132-G and GOR128-G). All sources of B isotopic fractionation during the analysis (mass bias, laser-induced isotopic fractionation and detector efficiency drift) have been evaluated and quantified. Instrumental mass bias is the major source of fractionation, altering the original isotopic ratio up to 13%. Fractionation related to laser sampling and transport to the ICP was found to be very low (less than 0.0015% s−1). Fractionation effects due to drift in ion counter efficiencies were found to be significant. Nevertheless, the “standard-sample-standard” bracketing approach could be used to correct for the above fractionation effects using NIST SRM 610 as external standard. With spot sizes of 60–80 μm in diameter, geologically meaningful results can be achieved on samples containing at least 10 ppm B, i.e., results with precisions that can discriminate between the different reservoirs on Earth. Data obtained with Faraday detectors on NIST SRM 610 and B4 tourmaline show high precision (down to 0.04‰, 1σ) and accuracy. Boron isotope ratios measured in the glass samples using multiple ion counting show significantly higher standard deviations (up to 2.5‰, 1σ), but they are very close to the values that can be expected from counting statistics. No significant variations with spot size or B contents were observed. Most of the values are within 1σ level of the reference values. The developed method was applied to a series of ashes from Mt. Etna erupted in 1995 having B contents between 14 and 20 ppm. The B isotope compositions of the ashes are between −4.8 and −10.7‰, with a weighted average value of −8.0 ± 1.9‰ (1σ).

Ultramafic (lherzolites, metasomatized peridotites, harzburgites, websterites and clinopyroxenites) and mafic igneous (basalts, dolerites, diorites and gabbros) rocks exposed at the sea-floor along the West Iberia continental margin represent a rare opportunity to study the transition zone between continental and oceanic lithosphere. The igneous rocks are enriched in LREE, unlike North Atlantic MORB. A correlation between their Nd/Nd isotopic composition and Ce/Yb ratio suggests that they originate from mixing between partial melts of a depleted mantle source similar to DMM and of an enriched mantle source which may reside within the continental lithosphere. Clinopyroxenes and amphiboles in the ultramafic rocks are LREE depleted and have flat HREE patterns with concentrations higher than those of abyssal peridotites. Clinopyroxenes in the harzburgites are less LREE depleted but have lower HREE concentrations. The clinopyroxenes in the Galicia Bank (GB) lherzolites have radiogenic Nd (Nd/Nd ranging from 0 512937 to 0 513402) and unradiogenic Sr ( Sr/Sr ranging from 0 702100 to 0 702311) isotopic ratios similar to, or higher than, DMM (Depleted MORB Mantle) whereas the clinopyroxenes in the Iberia Abyssal Plain websterites have low-Nd isotopic compositions (Nd/Nd ranging from 0 512283 to 0 512553) with high-Sr isotopic ratios ( Sr/Sr ranging from 0 704170 to 0 705919). Amphiboles in Galicia Bank lherzolites and diorites have Nd–Sr isotopic compositions (Nd/Nd from 0 512804 to 0 512938 and Sr/Sr from 0 703243 to 0 703887) intermediate between those of the clinopyroxenes from the Galicia Bank and the Iberia Abyssal Plain, but similar to the clinopyroxenes in the 5100 Hill harzburgite (Nd/Nd 1⁄4 0 512865 and Sr/Sr 1⁄4 0 703591) and to the igneous rocks (Nd/Nd ranging from 0 512729 to 0 513121 and Sr/Sr ranging from 0 702255 to 0 705109). The major and trace element compositions of cpx in the Galicia Bank spinel lherzolites provide evidence for large-scale refertilization of the lithospheric upper mantle by MORB-like tholeiitic melts. The associated harzburgites did not undergo partial melting during the rifting stage, but, in earlier times, probably during, or even before, the Hercynian orogeny. Iberia Abyssal Plain websterites are interpreted as high-pressure cumulates formed in the mantle. Their high Sm/Nd ratios ( from 0 43 to 0 67) coupled with very low-Nd isotopic compositions are best explained by a twostage history: formation of the cumulates from the percolation of enriched melts long before the rifting, followed by low-degree partial melting of the pyroxenites, accounting for their LREE depletion. This last event probably occurs during the rifting episode, 122 Myr ago. The isotopic heterogeneities observed in the ultramafic rocks of the Iberia margin were already present at the time of the rifting event. They reflect a long and complex history of depletion and enrichment events in an old part of the mantle, and provide strong arguments for a sub-continental origin of this part of the upper mantle.

Gabbro xenoliths reported in this paper were collected in northern Fuerteventura, the Canary Island located closest to the coast of Africa. The xenoliths are very fresh and consist of Ti–Al-poor clinopyroxene + plagioclase (An87–67) + olivine (Fo72–86) ± orthopyroxene. Clinopyroxene and orthopyroxene are constantly and markedly depleted in light rare earth elements (LREE) relative to heavy REE (HREE), as expected for cumulus minerals formed from highly refractory N-MORB-type melts. In contrast, whole-rock Primordial Mantle-normalized trace element patterns range from mildly S-shaped (mildly depleted in Pr–Sm relative to both the strongly incompatible elements Rb–La and the HREE) to enriched. Estimates show that the trace element compositions of the rocks and their minerals are compatible with formation as N-MORB gabbro cumulates, which have been infiltrated at various extents (≤1% to >5%) by enriched alkali basaltic melts. The enriched material is mainly concentrated along grain boundaries and cracks through mineral grains, suggesting that the infiltration is relatively recent, and is thus associated with the Canary Islands magmatism. Our data contradict the hypothesis that a mantle plume was present in this area during the opening of the Atlantic Ocean. No evidence of continental material that might reflect attenuated continental crust in the area has been found. Gabbro xenoliths with REE and trace element compositions similar to those exhibited by the Fuerteventura gabbros are also found among gabbro xenoliths from the islands of La Palma (western Canary Islands) and Lanzarote. The compositions of the most depleted samples from these islands are closely similar, implying that there was no significant change in chemistry during the early stages of formation of the Atlantic oceanic crust in this area. Strongly depleted gabbros similar to those collected in Fuerteventura have also been retrieved in the MARK area along the central Mid-Atlantic Ridge. The presence of N-MORB oceanic crust beneath Fuerteventura implies that the continent–ocean transition in the Canary Islands area must be relatively sharp, in contrast to the situation both further north along the coast of Morocco, and along the Iberian peninsula.

The analytical capabilities of laser ablation (LA)-ICP-MS in determining Li, Be and B at trace levels in geological samples have been tested on a series of glass reference materials and natural samples. The LA-ICP-MS instrument used consisted of a sector-field ICP-MS coupled with a laser ablation microprobe operating at either 266 or 213 nm wavelength. Reference glasses from NIST (SRM 612, 614 and 616) and MPI-DING (KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, T1-G and ATHO-G) were selected to develop the analytical method and to assess the best instrumental configuration. A series of calcic amphiboles with different Li, Be and B concentrations were also analysed using both LA-ICP-MS and SIMS to test the applicability of the method to natural minerals. Results indicated that with a spot size of 40 μm the agreement between measured and reference values of Li, Be and B is generally better than 10% for NIST SRM 612 and 20% for NIST SRM 614. Average reproducibility at the 2s level was 10% for Li, 20% for Be and 15% for B. Limits of detection were approximately 100 ng g-1 for Be and B and 200 ng g-1 for Li. These results were confirmed by analyses carried out on natural amphiboles and compared well in terms of precision and accuracy with those commonly achieved by SIMS. Les performances analytiques de l'ablation laser (LA-ICP-MS) pour la determination de Li, Be, B en trace dans les echantillons geologiques ont ete testees sur une serie de materiaux de reference vitreux et d'echantillons. L'instrument LA-ICP-MS utilise consiste en un ICP-MS a secteur magnetique couplea une microsonde d'ablation laser pouvant operer a des longueurs d'onde de 266 ou de 213 nm. Les verres de reference SRM 612, 614 et 616 provenant du NIST ainsi que les verres KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, T1-G et ATHO-G provenant du MPI-DING ont ete choisis pour developper la methode analytique et determiner la meilleure configuration instrumentale. Une serie d'amphiboles calciques ayant des concentrations en Li, Be et B variables a ete aussi analysee a la fois par LA-ICP-MS et par SIMS, afin de tester l'applicabilite de la methode a des echantillons naturels. Les resultats montrent que, avec une taille de faisceau de 40 μm, l'accord entre les valeurs mesurees et les valeurs de reference pour Li, Be et B est en general meilleur que 10% pour le verre NIST SRM 612 et de 20% pour le verre NIST SRM 614. la reproductibilite moyenne (2s) est de 10% pour Li, 20% pour Be et 15% pour B. Les limites de detection sont approximativement de 100 ng g-1 pour Be et B et de 200 ng g-1 pour Li. Ces resultats ont ete confirmes par les analyses effectuees sur les amphiboles naturelles et supportent tres bien la comparaison, en termes de precision et de justesse, avec celles effectuees par SIMS.

The importance of Li and B geochemistry has long been recognised, owing specifically to their characteristic behaviours during processes involving fluid phases. However, the lack of a set of validated reference data for the “normal” Earth mantle has hampered the development of models for Li and B metasomatic effects on mantle rocks. In particular, the concentration of B in the mantle is still a matter of debate. An estimate of 0.1 ppm B seems to be consistent as a source for non-arc basalts, but to date such data have not been directly confirmed. Li and B literature contents for peridotites are derived from samples whose non-metasomatized character has not been established for both elements, due to the aforementioned lack of a comprehensive metasomatism model for Li and B. We have looked at two groups of mantle rocks, with and without clear compositional evidence that they are metasomatized. The latter rocks provide the best constraints on “normal” mantle B and Li contents. We propose a diagnostic diagram based on (Ce/B) vs. (Li/Yb) as measured by SIMS in peridotite clinopyroxenes, which is useful in identifying metasomatized samples. After discovering samples with no metasomatic alteration, which are considered representative of the “normal” mantle, we derived for each mantle mineral phase (ol, opx, cpx and sp) the Mg#, Li and B partial-melting evolution trends. Additionally, considering that the “normal” mantle rocks have evolved through partial melting only, we assessed Li and B contents in the parental mantle of our samples, and assumed that the calculated values (1.6–1.8 ppm Li and 0.07–0.10 ppm B) are representative of the contents in MORB mantle sources. These new data are consistent with current melting models of fertile peridotites.

Partition coefficients between rutile and silicate melts were determined experimentally for Nb and Ta with melt compositions varying from rhyolite to basalt. Experimental conditions were 1.7–2.5 GPa, 950–1300 °C, at oxygen fugacities between QFM−2 and QFM+3.5. Both rt/meltDNb and rt/meltDTa increase by almost one order of magnitude with SiO2 content and polymerization, but decrease with TiO2 content in the melt. The ratio rt/meltDNb/DTa is 0.45–0.55 for basaltic melt compositions, around 0.6 for andesitic melts and 0.8–1.0 for more silicic melts, remaining ≤1 for all examined silicate melts. The fact that rt/meltDNb/DTa is smaller than unity can be explained by a slightly smaller ionic radius of Ta5+ than Nb5+ and thus a preferred incorporation of Ta into rutile. The variation of rt/meltDNb, rt/meltDTa, and rt/meltDNb/DTa strongly depends on melt composition without any significant correlation with rutile composition. The strong positive correlation of rt/meltDNb and rt/meltDTa with rt/meltDTi and SiO2 contents is explained with the decreasing solubility of high charge cations in an increasingly polymerized melt where the concentration of non-bridging oxygens decreases. The positive correlation of rt/meltDNb/DTa with rt/meltDTi and SiO2 contents is more difficult to understand and might be related to the higher polarizibility of Nb5+ compared to Ta5+. Magmas resulting from slab melting with residual rutile are slightly Nb-enriched relative to Ta and do not explain the subchondritic Nb/Ta ratio of continental crust. Rutile in the residue during partial melting or dehydration of subducting crust is not capable of significantly enriching Nb over Ta in the residue. Excluding residual rutile as a reason for high Nb/Ta reservoirs outsources this problem to either partial melting of low-Mg amphibolite or metasomatic Nb-enrichment of rutile-bearing eclogite-lenses in the source region of kimberlites. However, both of these processes cannot produce Nb-enriched reservoirs sufficiently large to balance the silicate Earth's Nb/Ta ratio to chondritic, thus, this study supports previous suggestions that the “missing” Nb is stored in the core.

Amphibole ± phlogopite ± apatite-bearing mantle xenoliths at Gobernador Gregores display modal, bulk-rock and phase geochemical characteristics held as indicators of carbonatitic metasomatism. However, part of these xenoliths has high TiO₂/Al₂O3 and those displaying the most pronounced carbonatitic geochemical markers modally trend towards harzburgite. Bulk-rock, clinopyroxene and amphibole show Zr, Hf and Ti negative anomalies, which increase at decreasing Na₂O and high field strength elements (HFSE) concentrations. Steady variation trends between xenoliths which have and do not have carbonatitic characteristics suggest a control by reactive porous flow of only one agent, inferred to be initially a ne-normative hydrous basalt (because of the presence of wehrlites) evolving towards silica saturation. Variation trends exhibit cusps when amphibole appears in the mode. Appearance of amphibole may explain the Ti anomaly variations, but not those of Zr and Hf. Numerical modelling [Plate Model (Vernieres et al. in J Geophys Res 102:24771–24784, 1997)] gives results consistent with the observed geochemical features by assuming the presence of loveringite. Modest HFSE anomalies in the infiltrating melt may be acquired during percolation in the garnet-facies., Facultad de Ciencias Naturales y Museo, Centro de Investigaciones Geológicas

The Northern Apennine ophiolites are remnants of the Middle Jurassic---Early Cretaceous lithosphere from the Ligurian Tethys. New trace element and Nd---Sr isotope investigations were performed on: (1) the rare gabbros associated with the subcontinental mantle rocks from the External Liguride ophiolites; (2) the gabbro--peridotite association from the poorly known ophiolitic bodies from Cecina valley (Southern Tuscany). Clinopyroxenes from the External Liguride and Cecina valley gabbros have similar trace element compositions, which are consistent with formation from normal mid-ocean ridge basalt (N-MORB) magmas. Sm---Nd mineral isochron ages are 179 9Ma for an External Liguride gabbro and 170 13Ma and 173 5 4 8Ma for two different gabbroic bodies from the Cecina valley ophiolites. These ages are interpreted to date the igneous crystallization of the gabbros and are slightly older than the oldest pelagic sediments of the Ligurian Tethys. Initial eNd (þ8 5 toþ8 9) and Sr/Sr of clinopyroxene are consistent with the interpretation that the studied gabbros were derived from N-MORB magmas. The least serpentinized mantle rocks from the Cecina valley ophiolites are porphyroclastic spinel lherzolites displaying a residual geochemical signature. They are similar to the least depleted residual peridotites from modern oceans. Nd and Sr isotopic ratios for separated mantle clinopyroxene are respectively higher (e.g. eNd 1⁄4 þ11) and lower than those of clinopyroxene from associated gabbros at the time of the gabbro intrusion. The gabbro---peridotite associations from the Northern Apennine ophiolites record the progression of the rifting process that led to opening of the Ligurian Tethys.

We present an inventory of B, Cl and Li concentrations in (a) key minerals from a set of ultramafic samples featuring the main evolutionary stages encountered by the subducted oceanic mantle, and in (b) fluid inclusions produced during high-pressure breakdown of antigorite serpentinite. Samples correspond to (i) nonsubducted serpentinites (Northern Apennine and Alpine ophiolites), (ii) high-pressure olivine-bearing antigorite serpentinites (Western Alps and Betic Cordillera), (iii) high-pressure olivine–orthopyroxene rocks recording the subduction breakdown of antigorite serpentinites (Betic Cordillera). Two main dehydration episodes are recorded by the sample suite: partial serpentinite dewatering during formation of metamorphic olivine, followed by full breakdown of antigorite serpentine to olivine+orthopyroxene+fluid. Ion probe and laser ablation ICP-MS (LA ICP-MS) analyses of Cl, B and Li in the rock-forming minerals indicate that the hydrous mantle is an important carrier of light elements. The estimated bulk-rock B and Cl concentrations progressively decrease from oceanic serpentinites (46.7 ppm B and 729 ppm Cl) to antigorite serpentinites (20 ppm B and 221 ppm Cl) to olivine–orthopyroxene rocks (9.4 ppm B and 45 ppm Cl). This suggests release of oceanic Cl and B in subduction fluids, apparently without inputs from external sources. Lithium is less abundant in oceanic serpentinites (1.3 ppm) and the initial concentrations are still preserved in high-pressure antigorite serpentinites. Higher Li contents in olivine, Ti-clinohumite of the olivine–orthopyroxene rocks (4.9 ppm bulk rock Li), as well as in the coexisting fluid inclusions, suggest that their budget may not be uniquely related to recycling of oceanic Li, but may require input from external sources. Laser ablation ICP-MS analyses of fluid inclusions in the olivine–orthopyroxene rocks enabled an estimate of the Li and B concentrations in the antigorite breakdown fluid. The inclusion compositions were quantified using a range of salinity values (0.4–2 wt.% NaClequiv) as internal standards, yielding maximum average fluid/rockDB∼5 and fluid/rockDLi∼3.5. We also performed model calculations to estimate the B and Cl loss during the two dehydration episodes of serpentinite subduction. The first event is characterized by high fluid/rock partition coefficients for Cl (∼100) and B (∼60) and by formation of a fluid with salinity of 4–8 wt.% NaClequiv. The antigorite breakdown produces less saline fluids (0.4–2 wt.% NaClequiv) and is characterized by lower partition coefficients for Cl (25–60) and B (12–30). Our calculations indicate that the salinity of the subduction fluids decreases with increasing depths. fluid/rockDB/fluid/rockDCl

International audience; Pyroxenites and garnet pyroxenites are mantle heterogeneities characterized by a lower solidus temperature than the enclosing peridotites; it follows that they are preferentially involved during magma genesis. Constraining their origin, composition, and the interactions they underwent during their subsequent evolution is therefore essential to discuss the sources of magmatism in a given area. Pyroxenites could represent either recycling of crustal rocks in mantle domains or mantle originated rocks (formed either by olivine consuming melt-rock reactions or by crystal fractionation). Petrological and geochemical (major and trace elements, Sr–Nd and O isotopes) features of xenoliths from various occurrences (French Massif-Central, Jordan, Morocco and Cameroon) show that these samples represent cumulates crystallized during melt percolation at mantle conditions. They formed in mantle domains at pressures of 1–2 GPa during post-collisional magmatism (possibly Hercynian for the French Massif-Central, and Panafrican for Morocco, Jordan and Cameroon). The thermal re-equilibration of lithospheric domains, typical of the late orogenic exhumation stages, is also recorded by the samples. Most of the samples display a metasomatic overprint that may be either inherited or likely linked to the recent volcanic activity that occurred in the investigated regions.The crystallization of pyroxenites during late orogenic events has implications for the subsequent evolution of the mantle domains. The presence of large amounts of mantle pyroxenites in old orogenic regions indeed imparts peculiar physical and chemical characteristics to these domains. Among others, the global solidus temperature of the whole lithospheric domain will be lowered; in turn, this implies that old orogenic regions are refertilized zones where magmatic activity would be enhanced.

Spinel-facies mantle xenoliths occur in a diatreme cutting through the Neogene Southern Patagonia Plateau at Gobernador Gregores (Santa Cruz Province, Argentina). This plateau is in a back-arc position with respect to the Chile trench. Xenoliths differ in their whole-rock composition from other South America occurrences, having higher CaO/Al2O3 ratios and, in some samples, TiO2 enrichment, whereas the Na2O/Al2O3 variation range is similar. Three assemblages can be distinguished. Assemblage 1, in anhydrous protogranular lherzolites and harzburgites, contains clinopyroxene with a depleted major and trace element composition, indicating pre-metasomatic depletion processes. This assemblage fully recrystallized to Assemblage 2 (amphibole ± phlogopite ± Cl-apatite-bearing) during a metasomatic episode. This causes clinopyroxene to acquire geochemical characteristics often attributed to carbonate-melt metasomatism. Noticeably, amphibole is markedly enriched in Nb (up to 298 ppm), especially when depleted in Ti. A further event, related to decompression during xenolith uplift to the surface, induces closed-system (perhaps with the exception of CO2 addition) disequilibrium melting of Assemblage 2, dominantly of amphibole. It is found in pockets (where amphibole is a residual phase) consisting of Na–Si-rich glass and carbonate (Mg-rich calcite) drops, and in veins originating from the pockets (Assemblage 3). Euhedral olivine, clinopyroxene and spinel crystallize only in the silicate glass. So do new, euhedral apatite crystals when glass is in contact with previous Assemblage 2 apatite. Textural evidence and comparison with experimental work suggest that silicate glass and carbonates are the result of unmixing of a former homogeneous melt. Because of the different flow rates of carbonate and silicate melt, the xenoliths become enriched in carbonate, which is found in the veins during their migration. Thus, the high CaO/Al2O3 ratio of whole rocks provides inconclusive evidence of carbonatite metasomatism. This factor, and other minor deviations from the expected results of carbonatite metasomatism, lead us to hypothesize an aqueous, Cl-rich fluid, possibly slab derived, as an alternative agent. Amphibole, resulting from reactive porous flow of this agent in the mantle, could fully explain the observed geochemical features, as indicated by estimates of its partition coefficients. Fil: Laurora, Angela. Dipartamento Di Scienze Della Terra, Universitá Di Modena e Reggio Emilia, Modena ; Italia Fil: Mazzucchelli, Maurizio. Dipartamento Di Scienze Della Terra, Universitá Di Modena e Reggio Emilia, Modena ; Italia Fil: Rivalenti, Giorgio. Dipartamento Di Scienze Della Terra, Universitá Di Modena e Reggio Emilia, Modena ; Italia Fil: Vannucci, Riccardo. Dipartamento Di Scienze Della Terra, Universitá Di Pavia and Cnr Centro Di Studio per la Cristallochimica e Cristallografia, Via Ferrata, Pavia ; Italia Fil: Zanetti, Alberto. Dipartamento Di Scienze Della Terra, Universitá Di Pavia and Cnr Centro Di Studio per la Cristallochimica e Cristallografia, Via Ferrata, Pavia ; Italia Fil: Barbieri, María Adelaide. Dipartamento Di Scienze Della Terra, Universitá Di Modena e Reggio Emilia, Modena ; Italia Fil: Cingolani, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina

Two spinel dunite xenoliths (Fo89.8–91.2 in olivine) from La Palma contain minor amounts ( directions are common, implying commensurate or incommensurate modulations along directions. Raman spectra show peaks typical of both lazurite and hauyne. It is concluded that the mineral has a structure intermediate between those of pure lazurite and pure hauyne, and it is here referred to as hauyness. The hauyness occurs together with strongly nepheline-normative glass in thin veinlets (

Constraints on the calculation and use of mineral/melt and two-mineral partition coefficients for Ti (D Ti ) have been derived from current knowledge of the distinct crystal-chemical mechanisms for the incorporation of Ti 4+ in the amphibole structure as follows: (1) mineral/melt partition coefficients for Ti, and other tetravalent high field-strength elements (HFSE), can be compared only when considering the fraction of Ti 4+ that enters the same structural site; (2) accurate two-mineral partition coefficients can be obtained only when considering the fraction of Ti 4+ that is involved in the same crystal-chemical mechanism in the two relevant phases (i.e., M2 Ti 4+ and M1 Ti 4+ for amphibole and clinopyroxene, respectively). The complete crystal-chemical characterization of synthetic titanian pargasite and kaersutite and of synthetic richterite (all crystallized under P , T , X , f O 2 conditions of interest for upper-mantle studies) shows that the site preference of Zr and Hf differs between the two amphibole compositions; these elements are essentially ordered at M2 in pargasite and kaersutite, but preferentially enter M1 in richterite. In the latter case, Ti segregates into the split M1′ site with distorted coordination and shorter Ti-O3 distances, whereas Zr and Hf most likely prefer the larger and more regular M1 site. The observed site preference is strongly controlled by the relative dimensions of the available sites. The crystal-chemical mechanisms that govern the incorporation of octahedral high-charge cations are the local charge balance of [IV] Al (by R 3,4+ at M2) and of dehydrogenation (by R 3,4+ at M1); thus the incorporation of Zr and Hf depends on distinct intensive parameters in the two amphibole compositions. Calculation of partition coefficients and of elastic-site parameters under the assumption that all Ti and other HFSE 4+ order at the M2 site in amphibole, as is currently done in geochemical studies, is strongly biased. In the presence of significant dehydrogenation, amphibole/melt D 0 values obtained from modeling based on the elastic-strain theory starting from the more-accurate site populations for Ti may be only 1/4 of those obtained by using the total Ti content, and the derived site parameters E and r 0 are more consistent with octahedral coordination. This result has important consequences for the prediction of D values under P - T conditions different from those of the experimental work. Applying the above concepts to data from natural assemblages, we obtained a significantly narrower (0.3–2.4 vs. 1.5–8.9) and more reasonable range of variation for amphibole/clinopyroxene D Ti . A relationship between these values for D Ti and pressure is also now apparent.

Incompatible trace element abundances have been determined in mantle-derived clinopyroxenes by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in a comparative study with established microanalytical and bulk techniques. Individual clinopyroxene grains were sampled for the laser ablation study from sieved mineral separate fractions, from which similar mineral grains had been previously extracted for microprobe (SIMS) and bulk (INAA and ID-TIMS) analysis. Mineral grains were ablated with variable degrees of surface spatial resolution (50-200 μm) in order to maximise ICP-MS analyte count rates and to improve detection limits. A comparison of results from the different techniques reveals that for the most homogeneous samples LA-ICP-MS can achieve excellent levels of agreement with other techniques (10%) and good precision for most of the studied elements (1-5% RSD). Variations in calculated concentrations by LA-ICP-MS confirm inter- and intra-mineral heterogeneity determined by SIMS, reflecting changes in sample composition and texture. The long-term reproducibility of the technique is shown by the consistency of results for one sample analysed on thirteen occasions over a period of nine months. Les abondances d'elements en traces incompatibles ont ete determinees dans des clinopyroxenes derives du manteau par ablation laser couplee a une ICP-MS et comparees avec les resultats de techniques eprouvees d'analyse globale ou de microanalyse. Des grains individuels de clinopyroxene ont eteechantillonnes pour l' ablation laser, a partir de fractions minerales tamisees, fractions qui ont fourni aussi les grains analyses auparavant par microsonde (SIMS) et analyses globales (INAA et ID-TIMS). L' ablation de ces mineraux a ete realisee avec differentes resolutions spatiales (50-200 μm) afin de maximiser les taux de comptage ICP-MS et d'ameliorer les limites de detection. La comparaison des resultats obtenus par les differentes techniques montre que, pour les echantillons les plus homogenes, la methode LA-ICP-MS donne des resultats en excellent accord avec les autres techniques (10%) et une bonne precision pour la plupart des elements etudies (1-5% RSD). Les variations dans les concentrations calculees par LA-ICP-MS confirment les heterogeneites inter- et intra-minerales determinees par SIMS, refletant les variations de texture et de composition des mineraux. La reproductibilitea long terme de la technique est demontree par la consistance des resultats pour un echantillon analyse 13 fois sur une periode de 9 mois.

Mantle xenoliths hosted by the historic Volcan de San Antonio, La Palma, Canary Islands include veined spinel harzburgites and spinel dunites. Glasses and associated minerals in the vein system of veined xenoliths show a gradual transition in composition from broad veins to narrow veinlets. Broad veins contain alkali basaltic glass with semi-linear trace element patterns enriched in strongly incompatible elements. As the veins become narrower, the SiO2-contents in glass increase (46 ->, 67 wt% SiO2 in harzburgite, 43 -> 58 wt% in dunite) and the trace element patterns change gradually to concave patterns depleted in moderately incompatible elements (e.g. HREE, Zr, Ti) relative to highly incompatible ones. The highest SiO2-contents (ca. 68% SiO2, low Ti-Fe-Mg-Ca-contents) and most extreme concave trace element patterns are exhibited by glass in unveined peridotite xenoliths. Clinopyroxenes shift from LREE-enriched augites in basaltic glass, to REE-depleted Cr-diopside in highly silicic glass. Estimates indicate that the most silicic glasses represent melts in, or near, equilibrium with their host peridotites. The observed trace element changes are compatible with formation of the silicic melts by processes involving infiltration of basaltic melts into mantle peridotite followed by reactions and crystallization. The Fe-Mg interdiffusion profiles in olivine porphyroclasts adjacent to the veins indicate a minimum period of diffusion of 600 years, implying that the reaction processes have taken place in situ in the upper mantle. The CaO-TiO2-La/ Nd relationships of mantle rocks may be used to discriminate between metasomatism caused by carbonatitic and silicic melts. Unveined mantle xenoliths from La Palma and Hierro (Canary Islands) show a wide range in La/Nd ratios with relatively constant, low-CaO contents which is compatible with metasomatism of "normal" abyssal peridotite by silicic melts. Peridotite xenoliths from Tenerife show somewhat higher CaO and TiO2 contents than those from the other islands and may have been affected by basaltic or carbonatitic melts. The observed trace element signatures of ultramafic xenoliths from La Palma and other Canary Islands may be accounted for by addition of small amounts (1-7%) of highly silicic melt to unmetasomatized peridotite. Also ultramafic xenoliths from other localities, e.g. eastern Australia, show CaO-TiO2-La/Nd relationships compatible with metasomatism by silicic melts. These results suggest that silicic melts may represent important metasomatic agents.

New experimental amphibole/melt partition coefficients from a variety of geologically relevant amphibole (pargasite, kaersutite, and K-richterite) and melt compositions obtained under conditions of interest to upper-mantle studies are combined with the results of X-ray single-crystal structure refinement. The ideal cation radii (r0), calculated using the lattice-site elastic-strain model of Blundy and Wood (1994) under the hypothesis of complete REE (rare earth elements) ordering at [8]M4, mostly differ significantly from those obtained from both the structure refinement and the ionic radius of [8]Ca2+. Heavier REE may also strongly deviate from the parabolic trends defined by the other REE. On the basis of the crystal-chemical knowledge of major-element site-preference in amphibole and the occurrence of two sites with different co-ordination within the M4 cavity (M4 for Ca and Na, M4′ for Fe2+ and Mg), we propose a new model for REE incorporation. LREE order at the [8]M4 site, whereas HREE prefer the M4′ site with lower co-ordination in amphiboles with a significant cummingtonite component, and may also enter the M2 octahedron, at least in richterite. This more complex model is consistent with the observed Amph/LD, and drops the usual assumption that REE behave as a homogeneous group and order at the M4 site. The availability of multiple crystal-chemical mechanisms for REE3+ incorporation explains why measured and estimated Amph/LDHREE may differ by up to one order of magnitude. When REE enter two different sites within the same cavity, a fit performed on the basis of a single curve may appear correct, but the values obtained for r0 are biased towards those of the dominant site, and the Young's modulus is underestimated. When REE are incorporated in multiple sites in different cavities, the observed pattern cannot be reduced to a single curve, and the partition coefficients of heavy REE would be strongly underestimated by a single-site fit. The simplistic assumption that REE occupy a single site within the amphibole structure can thus substantially bias predictive models based on the elastic-strain theory. Our combined approach allows linkage between fine-scale site preference and the macroscopic properties of minerals and provides more reliable predictive models for mineral/melt partitioning. After the possible site-assignments have been identified, the shape of the Onuma curves constructed from accurately determined Amph/LDREE now allows the active mechanisms for REE incorporation in amphiboles to be recognised even where site populations are not available. The REE preference for polyhedra with smaller size and lower co-ordination than those occupied by Ca invalidates the general idea that Ca acts as a “carrier” for REE.

Partition coefficients for Rb, Sr, Sm, Nd, U, Th, Pb, La, Ce, Ba, Nb, and Li between phlogopite, clinopyroxene and melt were determined in order to characterize the effect of metasomatically grown minerals on the isotopic evolution of the earth's lithospheric mantle. Experiments were performed in a piston–cylinder-apparatus at 15 kbar and 1040–1175°C with leucite lamproite as starting composition. Three phlogopite/glass and one clinopyroxene/glass pairs were analyzed by secondary ion mass spectrometry (SIMS). Partition coefficients ( D -values) for phlogopite/melt of Rb, Cs and Ba and Ti are ≥0.5. D Cs is significantly lower than D Rb (0.50/1.34); in contrast to these two elements, D Ba varies between 0.3 and 1.6 in phlogopites of different major element composition. A similar behavior is suggested for the elements Sr ( D =0.02–0.06) and Pb ( D ≈0.02). The partition coefficients measured for REE, Th and U are in the order of 10 −4 –10 −6 , those for Zr and Nb are 0.015 and 0.07 respectively, and D Li for phlogopite and melt is 0.2. Partition coefficients for REE and Sr between clinopyroxene and melt are relatively high compared to other studies, presumably because of the high wollastonite component in the clinopyroxenes of this work. The data for clinopyroxene as well as the data for phlogopite were used to calculate crystal lattice-site parameters for the M2-site in clinopyroxene and the X-site in phlogopite. Parameters for clinopyroxene are in good agreement with data from other works. For the phlogopite interlayer site, an optimum ionic radius of 1.71 A and polyhedral Young's moduli of 552 kbar (1 + -cations) and 1087 kbar (2 + -cations) were found. Recent data and the results of this study show that phlogopite is able to store significant amounts of Rb, Cs and Ba in the lithospheric mantle, thus causing a radiogenic evolution of Sr. In contrast, the influence of phlogopite on the Nd-isotope evolution is negligible because of the very low partition coefficients for Sm and Nd. The concentrations of Sr, Nd and Sm and the evolution of Nd isotopes in metasomatized lithospheric mantle are controlled by clinopyroxene, which significantly contributes to a less radiogenic evolution of Sr relative to the one caused by phlogopite. This implies that the evolution of Sr isotopes depends on the modal proportion of phlogopite and clinopyroxene.

The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite (websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (LaN/YbN = 8–17) and depleted in Nb. It has high radiogenic Sr (87Sr/86Sr(270) = 0.7055–0.7093), low radiogenic Nd (ɛNd(270) = −1 to −3) and EMII-like Pb isotopes. Two pyroxenite – peridotite sections examined in detail show the virtual absence of major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes exhibit LREE-enriched patterns (LaN/YbN ∼16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative Sr anomaly, but incompatible element concentration ∼1.9 (Sr) to ∼7.9 (Ti) times higher than that of coexisting clinopyroxene. Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg## and higher in Na2O, and spinels and phlogopite are richer in Cr2O3. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical signatures of “carbonatite” metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and Sr concentrations than amphibole (amph/cpxDREE,Sr = 0.7–0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling, the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics. In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process, which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence of some CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and with the occurrence of modal metasomatism before peridotite incorporation in the crust.

Ion microprobe analyses of clinopyroxenes in equilibrium with glasses (SiO2 from 42 to 69 wt%) formed as the result of AFC-type reactions between infiltrating basaltic melt and peridotite wall-rock in the upper mantle under La Palma (Canary Islands) reveal trace element signatures usually ascribed to carbonatite metasomatism (i.e. strong REE-enrichment and Zr,Ti-depletion). Cpx/glass partition coefficients for REE, Y and Sr progressively increase with increasing SiO2 in response to liquid composition/structure effects, approaching unity for La, Ce and Sr, and exceeding unity for the other REE in the most silicic glasses. Partition coefficients for Zr and Ti remain constant or decrease, probably as the combined effect of melt composition/structure and difficulty of charge-balancing 4+ cations in the crystal. Incorporation of the higher-charged HFSE in the cpx lattice requires complex coupled substitutions (e.g. M1Zr+1TAl+2M1R2+−1TSi−2 or M2Na+1M1Zr+1TAl+1M2Ca−1M1R2+−1TSi−1). Our results demonstrate that REE-enrichment and REE/HFSE fractionation of clinopyroxene do not necessarily reflect these characteristics in the metasomatic agent and that cpx/basaltD cannot confidently be used to infer the geochemical nature of equilibrium melts under upper mantle conditions. Moreover, we provide evidence that the strong REE-enrichment and Zr,Ti-depletion of some clinopyroxene from La Palma xenoliths is a feature commonly observed in clinopyroxenes from various mantle occurrences often interpreted as reflecting chemical interaction with metasomatic fluids and melts of different composition (either alkaline or carbonatitic) and provenance (from either the enriched asthenospheric mantle or the subducted slab). Therefore, claims of carbonatite metasomatism as opposed to silicate melt infiltration based on trace element signatures of investigated clinopyroxene should be regarded with caution unless the role of liquid composition/structure and of crystal chemical control is investigated.

This paper presents the results of new petrochemical studies carried out on mantle xenoliths hosted in Pleistocene basaltic rocks from the Agua Poca volcano in central-western Argentina. Mantle xenoliths studied are shown to be mainly anhydrous spinel lherzolites with minor amounts of harzburgite and banded pyroxenite, showing highly variable equilibrium temperatures ranging from 820°C to 1030°C at 1.0 to 2.0 GPa. This constitutes evidence that the mantle xenoliths are representative of a large portion of the lithospheric mantle column and that the geothermal gradient is not very elevated as reported in some other Patagonian provinces. Geochemical characteristics of clinopyroxene in the mantle xenoliths allow classification into two groups; Groups 1 and 2. Group 1 contains most of the lherzolites and has light-REE depletion, with slightly positive anomalies of Eu in some samples and extreme Nb and Ta depletion. Group 2 consists of two harzburgitic samples, has flat REE patterns with lower Sm to Lu concentrations, with enriched Sr and negative HFSE anomalies. Based on mineral and residua compositions estimated assuming equilibrium with clinopyroxenes, Group 1 can be considered to be refractory residua after up to 7%, non-modal, near-fractional melting of a spinel-facies Primitive Mantle. Group 2 can be considered to be after ca. 13% of partial melting. It is inferred that partial melting events in the lithospheric mantle beneath the Agua Poca occurred in different ages since the Proterozoic, but compared with Group 1, the metasomatic overprint is dominant in Group 2 mantle xenoliths. The calculated melt compositions from Group 2 are interpreted to be transient liquid compositions developed during melt-peridotite interaction, and are different from the host alkaline basalts. The HFSE-depleted composition estimated for the rising melt suggests the presence of a slab-derived component, although the possibility cannot be disregarded (on the basis of present data) that such a geochemical feature is due to segregation of HFSE-bearing minerals during the interaction with the peridotite. Thus, we attribute the metasomatic agent to a basaltic melt and to a minor amount of slab-derived fluids. Fil: Bertotto, Gustavo Walter. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Ciencias de la Tierra y Ambientales de La Pampa. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales. Instituto de Ciencias de la Tierra y Ambientales de La Pampa; Argentina Fil: Mazzucchelli, Maurizio. Università di Modena e Reggio Emilia. Dipartimento di Scienze Chimiche e Geologiche; Italia Fil: Zanetti, Alberto. Università di Pavia. Istituto di Geoscienze e Georisorse; Italia. Consiglio Nazionale delle Ricerche; Italia Fil: Vannucci, Riccardo. Università di Pavia. Dipartamento di Scienze della Terra; Italia

Ultramafic xenoliths from the basaltic tuff of Kapfenstein (Graz Basin) are mainly protogranular and protogranular- to- porphyroclastic spinel lherzolites. In addition to these common rocks, a coarse-grained spinel harzburgite and a heterogeneous spinel peridotite consisting of a clinopyroxene-poor harzburgite and a clinopyroxene-rich lherzolite have been found. Interstitial amphiboles (pargasite and kaersutite) are present in most xenoliths. The magmatic activity which brought these xenoliths to the surface is part of the Plio-Pleistocene extension-related alkali basalt volcanism which took place in the Carpatho-Pannonian Region, post-dating the subduction-related calc-alkaline volcanism which formed the Inner Carpathian arc.

Clinopyroxenes from peridotite, Cr-diopside and Al-augite websterites of the Balmuccia massif have been analyzed by ion-probe for REE, Sr, Zr, Ti, V, Y, Sc and Cr. In the peridotites, the REE pattern of clinopyroxene varies from markedly LREE depleted [(Ce/Yb)(n)=0.03] to moderately LREE enriched [(Ce/Yb)(n)=1.96]. Clinopyroxenes of the Cr-Diopside suite vary from moderately LREE depleted to enriched [(Ce/Yb)(n)=0.53-1.36], but HREE are always depleted with respect to MREE [(Sm/Yb)(n) = 1.19-2.06]. The clinopyroxenes of the Al-Augite dikes have higher trace element concentrations and are relatively depeleted in LREE. Two types of peridotite to websterite transitions have been documented. One type characterizes the transitions from peridotite to Cr-diopside websterite. The depletion of peridotite increases, and cpx ''fertility'' decreases, towards the contact; i.e. REE from Sm-Eu to Yb and Ti, V, Y, Sc and Cr in cpx define abundance trends consistent with melt extraction. LREE, Sr, and perhaps Zr, on the contrary, increase in the contact region over a distance of 0-40 cm. The variation of the latter elements and of CaO and Al2O3 from peridotite to websterite is smooth, whereas it is steep for the other elements. Alumina, HREE, Y and Sc are lower, Ca is higher, and the MREE/HREE pattern is steeper in the clinopyroxene of the websterite with respect to peridotite. In the second type of transition, as the contact is approached, there is an increase in modal cpx and decrease of the opx/cpx ratio. Cpx trace element patterns are similar in peridotite and websterite. This transition is generally at the Al-augite websterite contacts, but also was found at two Cr-diopside contacts. The wall rock depletion of the first transition type is interpreted as caused by melt extraction induced by intrusion of melts derived from deeper mantle (garnet-facies), whereas the LREE and Sr variation is interpreted in terms of diffusive redistribution of trace elements after emplacement of the dikes and after the modal clinopyroxene has been removed from the peridotite. Diffusive exchange occurred under stagnant conditions but in the presence of a partial melt or a ''metamorphic fluid'', rather than under dry subsolidus conditions. There is no conclusive evidence favouring either a partial melt or an aqueous fluid as a medium for diffusive transport. However, we favour transport via a fluid phase because the presence of such a fluid is indicated by ubiquitous traces of amphibole in the peridotite. The second transition type is interpreted in terms of melt infiltration and re-equilibration of the melt with the peridotite matrix.

Eleven clinopyroxenes from websteritic (Cr-diopside- and Al-diopside-bearing) and Al-augite±orthopyroxene±spinel-bearing megacrystic xenoliths occurring in alkaline volcanics of the Hyblean Plateau (SE Sicily) were analysed for rare-earth and other trace elements (Sc, Ti, V, Cr, Sr, Y, Zr) with an ion microprobe. Trace-element analysis shows that Hyblean pyroxenites are crystal/liquid segregates of basaltic magmas characterized by different composition (from tholeiitic to alkaline) and variable LILE enrichment. Megacrystic clinopyroxenes are, most probably, high-pressure liquidus phases of OIB-type alkaline basalts characterized by smooth chondrite-normalized trace-element abundance plots, except for a slight negative Sr anomaly. Clinopyroxenes from websterites crystallized from either alkaline or tholeiitic melts and later experienced metamorphic (orthopyroxene exsolution) and/or metasomatic (interaction with fluids or melts) processes which modified to variable extent their original major-and trace-element signatures. In Hyblean high-pressure alkaline basalts, compositional factors (i.e. major-element activities in the melt and clinopyroxene crystal chemical configuration) caused a significant D Ti increase, mild D REE and D Zr increases, and virtually no D Sr variations with increasing degree of fractionation. In particular, clinopyroxene crystal chemical constraints, namely electrostatic balance and M1-site geometry, appear to play a major role in the creation of HFSE/REE and Sr/REE decoupling in clinopyroxene-bearing mantle rocks. A sharp decrease of D Zr with increasing pressure is predicted from crystal chemical considerations.

Mantle peridotites of the External Liguride (EL) units (Northern Apennines) represent slices of subcontinental lithospheric mantle emplaced at the surface during early stages of rifting of the Furassic Ligurian Piemontese basin. Petrological, ion probe and isotopic investigations have been used to unravel the nature of their mantle protolith and to constrain the timing and mechanisms of their evolution. EL peridotites are dominantly fertile spinel lherzolites partly recrystallized in the plagioclase lherzolite stability field. Clinopyroxenes stable in the spinel-facies assemblage have nearly flat REE patterns (Ce-N/Sm-N = 0.6-0.8) at (10-16) x C1 and high Na, Sr, Ti and Zr contents. Kaersutitic-Ti-pargasitic amphiboles also occur in the spinel-facies assemblage. Their LREE-depleted REE spectra and very low Sr, Zr and Ba contents indicate that they crystallized front hydrous fluids with low concentrations of incompatible elements. Thermometric estimates on the spinel-facies parageneses yield lithospheric equilibrium temperatures in the range 1000-1100 degrees C, in agreement with the stability of amphibole, which implies T < 1100 degrees C. Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, plot within the depleted end of the MORB field (Sr-87/Sr-86 = 0.70222-0.70263; Nd-143/Nd-144 = 0.513047-0.513205) similar to many subcontinental orogenic spinel lher- zolites from the western Mediterranean area (e.g. Ivrea Sone and Lanzo N). The interpretation of the EL lherzolites as subcontinental lithospheric mantle is reinforced by tire occurrence of one extremely depleted isotopic composition (Sr-87/Sr-86 = 0.701736; Nd-143/Nd-144 = 0.513543). Sr and Nd model ages, calculated assuming both CHUR and DM mantle sources, range between 2.4 Ga and 780 Ma. In particular, the 1.2-Ga Sr age and the 780-Ma Nd age can be regarded as minimum ages of differentiation. The transition from spinel- to plagioclase-facies assemblage, accompanied by progressive deformation (front granular to tectonite-mylanite textures), indicate that the EL lherzolites experienced a later, subsolidus decompressional evolution, starting from subcontinental lithospheric levels. Sm/Nd isochrons an plagioclase-clinopyroxene pairs furnish ages of similar to 165 Ma. This early Furassic subsolidus decompressional history is consistent with uplift 63, means of denudation in response to passive and asymmetric lithospheric extension. This is considered to be the most suitable geodynamic mechanism to account for the exposure of huge bodies of sub-continental lithospheric mantle during early stages of opening of an oceanic basin.

Ultramafic xenoliths from alkali basalts in the Per§ani Mountains in the Eastern Transylvanian Basin (ETB) of Romania are mainly spinel Iherzolites, although spinel harzburgites, websterites, clinopyroxenites and amphibole pyroxenites are also present. Amphibole veins cut some spinel peridotite samples. All are derived from the shallow lithospheric upper mantle. In general, textural variations are restricted to protogranular and porphyroclastic types, compared with the more varied textures found in mantle xenoliths from the alkali basalts of the neighbouring Pannonian Basin. Also, ETB peridotites are richer in amphibole. Thus, the mantle beneath the edge of the ETB is less deformed but more strongly metasomatized than the mantle closer to the centre of the Pannonian Basin. Mineralogical and bulk-rock geochemical variations resemble those of spinel Iherzolites from other sub-continental shallow mantle xenolith suites. There is no apparent correlation between deformation and geochemistry, and much of the major and trace element variation is due to variable extraction of picritic melts. The REE patterns of separated clinopyroxenes from the peridotite xenoliths are mostly LREE depleted, although clinopyroxenes from regions adjacent to amphibole veins have experienced an enrichment in La and Ce and a change in their Sr and Nd isotopic values towards those of the vein, while still retaining an overall LREE depletion. Clinopyroxenes from the websterites and clinopyroxenites are more variable. Amphibole in the hydrous pyroxenites and amphibole veins is strongly LREE enriched and is considered to be metasomatic in origin. 87Sr/86Sr and

Field, petrographic-structural and geochemical data are reported for spinel and plagioclase peridotites from the northern domain of the Lanzo peridotite massif (Western Alps). The North Lanzo peridotites are extremely heterogeneous in terms of mineral mode, texture and chemistry. They can be referred to four major groups: (1) spinel harzburgites with coarse granular to porphyroclastic structures; (2) pyroxene-depleted spinel harzburgites recording olivine-forming or pyroxene-consuming microtextures; (3) spinel lherzolites with porphyroclastic to foliated and banded structures; (4) plagioclase-enriched spinel lherzolites. Major and trace element characterization of whole-rocks and their constituent minerals allows reconstruction of the complex series of pre- to syn-rift events this mantle sector underwent. Sr, Nd and Hf isotope data provide information on the nature of infiltrating melts and time constraints. More depleted harzburgites represent refractory protoliths that after melt extraction, possibly in the presence of residual garnet, underwent a first episode of refertilization by enriched mid-ocean ridge basalt (E-MORB)-like melts, whereas harzburgites and spinel lherzolites with ocean island basalt (OIB) signatures document the successive migration of alkaline melts. The most noticeable feature of this group of rocks is their Nd-Hf decoupling, specifically the very high Hf-176/Hf-177 coupled with very low Nd-143/Nd-144. Lu-Hf data for these peridotites define an similar to 260 Ma errorchron that is interpreted as evidence of mixing during relatively recent times between old (most probably Proterozoic) refractory subcontinental mantle and OIB-type melts. This event most probably occurred during extension (Triassic times) or during the onset of exhumation (Liassic times). Plagioclase peridotites document the last refertilization episode, involving the shallow-level impregnation of harzburgite mantle by evolved MORB melts before its sea-floor emplacement. This Middle Jurassic event caused the almost complete resetting of the original trace element and Sr-87/Sr-86 composition. The combination of structural, petrological and geochemical information for a north-south Lanzo traverse, from the North Massif to the South Massif, notwithstanding the effects of the alpine orogeny, allows the study of the complete evolution of a sector of old (Proterozoic?) mantle since the early stages of melt removal and allows reconstruction of the tectonic and magmatic events during continental extension leading to the opening of the Jurassic Ligurian-Piedmontese basin. North Lanzo fundamentally preserves the record of pre-syn-rift ancient episodes, whereas South Lanzo better highlights the processes that deeply modified and refertilized the older lithosphere during subsequent lithosphere extension. Slow to very slow extension led to sea-floor exposure of the subcontinental lithospheric mantle (North Lanzo) at a marginal position, close to the Adria continental margin, and of the deeply melt-modified lithospheric mantle (Lanzo South) in a more distal setting of the basin. In this respect, the Lanzo traverse is closely similar to the ocean-continent transition in slow- or ultraslow-spreading oceanic basins, such as the North Atlantic.

Clinopyroxenes from mantle xenoliths from the Massif Central, France, have been chosen for a comparative study of rare-earth element (REE) abundances obtained by means of conventional and in situ analyses. Xenoliths from Hungary and the Canary Islands showing unusual trace-element or mineralogical features have also been investigated. Secondary ion mass spectrometry (SIMS) has been shown to be a powerful technique for both in situ and bulk investigations, competitive with instrumental neutron activation analysis (INAA) and isotope dilution mass spectrometry (IDMS) in the quantification of REE in clinopyroxenes. In addition, the application of SIMS for microspot analysis provides reliable information about REE concentrations with respect to texture. When SIMS is applied for bulk investigations, its major advantages are the simple and fast sample preparation, and the small quantity of material required. The results obtained in this paper indicate that grain-by-grain and internal homogeneity of clinopyroxenes, which is related to texture, determines the degree of agreement between SIMS, INAA and ID. The data for clinopyroxenes from equigranular xenoliths, which recrystallized to almost equilibrium conditions, show the best agreement. For clinopyroxenes from protogranular LREE-depleted xenoliths, the agreement is good when the whole clinopyroxene population has the same degree of depletion, and the metasomatic La and Ce enrichment is negligible. When complex geochemical processes are investigated, such as subsolidus reactions and interactions with fluids and melts, conventional bulk techniques are often inadequate and the SIMS technique is the only one capable of thoroughly describing the effects of element migration at a microscale. For a proper understanding of geochemical processes in the mantle it is here suggested that SIMS investigations in thin section are made prior to mineral selection for subsequent elemental and isotopic analysis by ID.

This paper presents new petrological and geochemical data on peridotites and associated pyroxenites from the Zabargad Island (Northern Red Sea), and, particularly, new data on REE bulk rock composition of pyroxenites, and trace element composition of amphiboles (determined by ion probe) from both peridotites and pyroxenites. Results are discussed in the context of field, structural-textural, petrological, geochemical and isotopic knowledge on the Zabargad peridotite. The Zabargad peridotite consists of variably depleted mantle ultramafics. The dominant rock type is a four-phase, granular, spinel(sp)-lherzolite characterized by: i) relative abundance of clinopyroxene(cpx) (15-20% by volume), ii) high contents of fusible components in the constituent minerals, iii) high bulk-rock REE concentrations with C1-normalized pattern almost flat from H- to MREE, at more than 2 times C1, and a slight negative LREE fractionation. Other lithologies consist of anhydrous Al-diopside(Di) pyroxenite layers and hydrous (Ti-pargasite-bearing) Cr-Di pyroxenite dykes. Field, petrological and geochemical evidence suggests the following sequence of events developed at mantle depth: 1) Near-solidus evolution of lherzolites and the formation of anhydrous Al-Di pyroxenite layers. The compositional features of the anhydrous Al-Di pyroxenites indicate that they crystallized from tholeiitic melts as garnet-free and garnet-bearing clinopyroxenites, at T higher than 1200°C. 2) Static recrystallization at lower T (and probably P) conditions, most probably around 1000°C. Lherzolites and pyroxenites recrystallized to granular, equilibrium sp-bearing (gnt- and pl-free) assemblages. 3) Widespread kaersutitic amphibole crystallization in equilibrium with the sp-bearing granular association, probably related to infiltration of H 2 O-rich mantle fluids. 4) Decompression: tectonite-mylonite fabrics were formed along localized shear zones and plagioclase(pl)-bearing assemblages developed in both massive and deformed rocks. During decompression, Zabargad peridotites were intruded by hydrous melts which generated amphibole-bearing, Cr-Di websterite dykes, characterized by dominant subhedral pyroxenes with interstitial Ti-pargasite and Ti-phlogopite. Both clinopyroxenes and Ti-pargasites are strongly LREE-enriched, suggesting that these rocks crystallized from alkaline melts

Ion microprobe data (REE, Na, Sc, Ti, V, Cr, Sr, Zr) of unaltered clinopyroxenes in the ophiolitic basalts from the Northern Apennines have been used in a cpx-based geochemical modelling of MORB magmatism from both External (EL) and Internal (IL) sectors of the Ligurian Tethys (i.e. Jurassic Ligure-Piemontese basin), alternative to the more common whole-rock approach. Clinopyroxenes from EL basalts display slightly fractionated LREE (Ce(N)/Sm(N) approximately 0.5) and HREE (Gd(N)/Yb(N) approximately 1.5) patterns and large variations in the REE composition (up to 6 times from microphenocryst cores to interstitial clinopyroxenes). Interstitial clinopyroxenes in IL basalts are similar to the microphenocrysts from the most primitive EL basalts. By contrast, IL microphenocrysts are characterized by greater LREE (Ce(N)/Sm(N) approximately 0.3) and lesser HREE (Gd(N)/Yb(N) < 1.2) fractionation. The comparison of trace element variations in whole-rocks and clinopyroxenes clearly shows that the olivine and plagioclase portion of the fractionation sequence is poorly represented by the EL and IL basalts. In fact, ophiolitic basalts mainly consist of a minor interstitial glass (now deeply altered) associated with a prevailing plagioclase-clinopyroxene assemblage crystallized from liquids significantly evolved along the olivine-plagioclase-clinopyroxene saturation boundary. Thus, bulk rock chemistry is largely governed by clinopyroxene composition. This, in addition to alteration, indicates that the bulk rock chemistry does not provide reliable chemical information to constrain the composition and the generation of the parental magmas. Unfortunately, most clinopyroxenes are characterized by complex zoning, probably caused by disequilibrium partitioning during crystal growth as a result of kinetic factors. On this ground, estimation of melt chemistry and inferences about the origins of these basalts are only allowed by the core compositions of microphenocrystic clinopyroxenes. Modelling. of (Nd/Yb)N and Ti/Zr in the parental magmas, as deduced from the clinopyroxene compositions, indicates thata EL and IL basalts do not represent products of different mantle source compositions. Rather, they were generated by varying degrees of fractional melting in the spinel stability field, lower for the EL (a few percent) relative to IL, totalling no more than 10% of an asthenospheric MORB source, and leaving in the residua clinopyroxene with REE patterns similar to those shown by IL suboceanic type peridotites. Accordingly, these latter are interpreted as refractory residua after MORB-generating fractional melting occurred during rifting and opening of the Ligure-Piemontese basin. By contrast, residual clinopyroxenes from the EL subcontinental type peridotites are not consistent with low degrees of fractional melting, in agreement with the current interpretation that EL peridotites are unrelated to the MORB magmatism in the Ligure-Piemontese basin and represent lithospheric mantle material already emplaced towards the surface by a tectonic denudation mechanism during the early stages of oceanic rifting.

Mantle peridotites of the External Liguride (EL) Units (Northern Apennines) mainly consist of fertile spinel-lherzolites partially recrystallized to plagioclase-facies assemblages. and are consequently appropriate to investigate the interphase element partitioning related to the transition from spinel- to plagioclase-facies stability field. Evidence for the development of the plagioclase-facies assemblage is mainly given by: (1) large exsolution lamellae of orthopyroxene and plagioclase within spinel-facies clinopyroxene, (2) plagioclase rims around spinel; (3) granoblastic domains made up of olivine + plagioclase +/- clino- and orthopyroxene. In situ major and trace [REE (rare-earth elements). Ti, Sc, V, Cr, Sr, Y, Zr and Ba] element mineral analyses have been performed, by electron and ion probe, on selected samples which show the progressive development of the plagioclase-bearing assemblage. The main compositional variations observed during the change from spinel- to plagioclase-facies minerals are as follows: (1) clinopyroxenes decrease in Al, Na, Sr, Eu/Eu* and increase in Y, V, Sc, Cr, Zr and Ti; (2) amphiboles decrease in Eu/Eu*, Sr, Ba and increase in Zr and V; (3) spinels decrease in Al and increase in Cr and Ti. The most striking feature is the decoupling in the behaviour of similarly incompatible elements (D about 0.1) in clinopyroxene, e.g. Sr decrease is mirrored by Zr increase. Mass-balance calculations indicate that the trace element interphase redistribution documented in the EL peridotites occurred in a closed system and in response to the metamorphic reaction governing the transition from the spinel- to the plagioclase-facies stability field. The observed element partitioning reveals, moreover, that subsolidus re-equilibration processes in the upper mantle produce HFSE (high-field-strength element)/REE fractionation in minerals, which must be evaluated for a reliable determination of mineral-melt distribution coefficients. The results of this study furnish evidence for subsolidus metamorphic evolution during decompression, without concomitant partial melting processes. This is consistent with the interpretation that the EL. peridotites represent subcontinental lithospheric mantle emplaced at the Surface in response to lithospheric thinning and tectonic denudation mechanisms related to the Triassic-Jurassic rifting of the Ligure-Piedmontese basin.

During its 1800-year-long persistent activity the Stromboli volcano has erupted a highly porphyritic (HP) volatile-poor scoriaceous magma and a low porphyritic (LP) volatile-rich pumiceous magma. The HP magma is erupted during normal Strombolian explosions and lava effusions, while the LP one is related to more energetic paroxysms. During the March–April 2003 explosive activity, Stromboli ejected two typologies of juvenile glassy ashes, namely highly vesicular LP shards and volatile-poor HP shards. Their textural and in situ chemical characteristics are used to unravel mutual relationships between HP and LP magmas, as well as magma dynamics within the shallow plumbing system. The mantle-normalized trace element patterns of both ash types show the typical arc-lava pattern; however, HP glasses possess incompatible element concentrations higher than LP glasses, along with Sr and Eu negative anomalies. HP shards are generally characterized by higher Li contents (to ~20 ppm) and lower δ7Li values (+1.2 to −3.8‰) with respect to LP shards (Li contents of 7–14 ppm and δ7Li ranging between +4.6 and +0.9‰). Fractional crystallization models based on major and trace element compositions, combined with a degassing model based on open-system Rayleigh distillation and on the assumption that melt/fluidDLi > 1, show that abundant (~30%) plagioclase precipitation and variable degrees of degassing can lead the more primitive LP magma to evolve toward a differentiated (isotopically lighter) HP magma ponding in the upper conduit and undergoing slow continuous degassing-induced crystallization. This study also evidences that in March 2003 Stromboli volcano poured out a small early volume of LP magma that traveled slower within the conduit with respect to later and larger volumes of fast ascending LP magma erupted during the April 5 paroxysm. The different ascent rates and cooling rates of the two LP magma batches (i.e., pre- and post-paroxysm) resulted in small, but detectable, differences in their chemical signatures. Finally, this study highlights the high potential of in situ investigations of juvenile glassy ashes in petrologic and geochemical monitoring the volcanic activity and of Li isotopes as tracers of degassing processes within the shallow plumbing system.

Trace element and isotopic data obtained for mantle spinel lherzolites and diorite dykes from the Baldissero massif (Ivrea–Verbano Zone, Western Italy) provide new, valuable constraints on the petrologic and geodynamic evolution of the Southern Alps in Paleozoic to Mesozoic times. Whole rock and mineral chemistry indicates that Baldissero lherzolites can be regarded as refractory mantle residues following limited melt extraction. In particular, the Light Rare Earth Elements (LREE)-depleted and fractionated compositions of whole rock and clinopyroxene closely match modelling results for refractory residues after low degrees (~ 4–5%) of near-fractional melting of depleted mantle, possibly under garnet-facies conditions. Following this, the peridotite sequence experienced subsolidus re-equilibration at lithospheric spinel-facies conditions and intrusion of several generations of dykes. However, lherzolites far from dykes show very modest metasomatic changes, as evidenced by the crystallisation of accessory titanian pargasite and the occurrence of very slight enrichments in highly incompatible trace elements (e.g. Nb). The Re–Os data for lherzolites far from the dykes yield a 376 Ma (Upper Devonian) model age that is considered to record a partial melting event related to the Variscan orogenic cycle s.l. Dioritic dykes cutting the mantle sequence have whole rock, clinopyroxene and plagioclase characterised by high radiogenic Nd and low radiogenic Sr, which point to a depleted to slightly enriched mantle source. Whole rock and mafic phases of diorites have high Mg# values that positively correlate with the incompatible trace element concentrations. The peridotite at the dyke contact is enriched in orthopyroxene, iron and incompatible trace elements with respect to the lherzolites far from dykes. Numerical simulations indicate that the geochemical characteristics of the diorites can be explained by flow of a hydrous, silica-saturated melt accompanied by reaction with the ambient peridotite and fractional crystallisation. The composition of the more primitive melts calculated in equilibrium with the diorite minerals show tholeiitic to transitional affinity. Internal Sm–Nd, three-point isochrons obtained for two dykes suggest an Upper Triassic-Lower Jurassic emplacement age (from 204 ± 31 to 198 ± 29 Ma). Mesozoic igneous events are unknown in the southern Ivrea–Verbano Zone (IVZ), but the intrusion of hydrous melts, mostly silica-saturated, have been well documented in the Finero region, i.e. the northernmost part of IVZ and Triassic magmatism with calc-alkaline to shoshonitic affinity is abundant throughout the Central–Eastern Alps. The geochemical and chronological features of the Baldissero diorites shed new light on the geodynamic evolution of the Southern Alps before the opening of the Jurassic Tethys.

An updated REE measurement of Gabbro GOG-1 was carried out by means of secondary ion mass spectrometry (SIMS). The availability of high amount of rock powder allowed the sample to be well ground and homogenized before analysis. Fused glass pellets were employed for SIMS investigations. REE data were compared with new ICP-AES and NAA analyses. REE results from all employed analytical techniques show good agreement and are consistent with in-situ SIMS measurements of REE in major mineral phases forming GOG-1. Extensive homogeneity tests seem to confirm the possibility to using this rock as a reference material for earth sciences.

International audience; Mantle xenoliths enclosed in the Plio-Quaternary alkaline basalts from Bou Ibalrhatene, Middle Atlas, Morocco, are characterized by a wide range of lithological and chemical heterogeneity, consistent with metasomatism of their lithospheric mantle source. Subordinate porphyroclastic to protogranular spinel-lherzolites associated with websterites exhibit major and trace element signatures, along with a depleted mantle isotopic affinity, that testify to ancient melt extraction processes, possibly during Neo- to Paleoproterozoic times. These samples show large ion lithophile element (LILE) enrichments that have been imparted a long time after the accretion of their protoliths to the lithosphere, induced by alkaline melts rising from upwelling HIMU-like asthenosphere since the late Cretaceous or Eocene. Extensive sectors of the percolated lherzolite and harzburgite mantle progressively approached chemical equilibrium with the migrating melts, forming porphyroblastic amphibole-rich ultramafic rocks with strong LILE enrichment, highly variable (enriched to strongly depleted) high field strength element (HFSE) abundances and HIMU-like isotopic signatures. As a result, the older depleted lithosphere was progressively refertilized. The presence among the Bou Ibalrhatene ultramafic xenoliths of both Fe- and Mg-rich wehrlites is a rare occurrence in xenolith suites. The Fe-rich wehrlites formed by the migration through dunitic channels, which opened during earlier reactive porous flow, of larger and larger volumes of rising alkaline melts, becoming progressively less evolved in composition and more similar to the erupted lavas. In contrast, subordinate Mg-rich wehrlites characterized by the presence of apatite, extreme LILE enrichment and HFSE depletion testify to the localized chemical effects of either carbonatite melts related to the oldest (i.e. Eocene) volcanic activity in the area or, more probably, to highly evolved SiO2-saturated silicate melts that vanished through interaction with the lithosphere. As a whole, the textural, petrographic and chemical characteristics of the Bou Ibalrhatene xenoliths suggest prolonged thermo-chemical erosion of an originally heterogeneous lithosphere with a dominant depleted mantle isotopic signature by melts rising from a HIMU-like asthenospheric mantle source in response to intraplate tectonic reactivation and rifting of the Pan-African basement. The overall scenario envisaged for lithosphere-asthenosphere interaction beneath the Middle Atlas is similar to that inferred for other Cenozoic Central-Northern African volcanic centres, such as Hoggar (Algeria) and Gharyan (Libya). The majority of the mantle sectors beneath the NE-SW volcanic alignment associated with the Trans-Moroccan fault system consist of rejuvenated lithosphere formed by small-scale, shallow-mantle upwelling and accretion of asthenospheric material since late Cretaceous or Eocene times.

A suite of mantle-wedge amphibole + phlogopite-bearing spinel peridotite xenoliths in Plio-Pleistocene alkali basalts from Southern Patagonia (Gobernador Gregores, Santa Cruz Province, Argentina) contains carbonic fluid inclusions, glass and carbonate in several textural domains. Here we present a microstructural and fluid inclusion study showing that fluid (corresponding to pure CO 2 ) and glass post-date the hydrous mantle assemblage and formed soon before and/or during xenolith entrainement in the host alkali basalt. The high densities preserved by a number of CO 2 inclusions indicate that fluid infiltration took place at mantle depths. The low densities pertaining to the majority of analyzed fluid inclusions derive from inclusion re-equilibration during xenolith ascent. Glass occurs in reaction haloes around clinopyroxene, amphibole and phlogopite, where it hosts microlites of new pyroxene, olivine and locally carbonate. Glass veins cut the mantle minerals and locally contain primary carbonate. Glasses vary widely in composition depending on the textural domains and attain Si- and alkali-rich compositions (SiO 2 = 47.0–68.3 wt.%; Na 2 O + K 2 O = 5.8–12.2 wt.%). Incompatible trace element patterns of glasses in anhydrous xenoliths are closely similar to those of the host alkali basalts, whereas the compositions of interstitial and vein glasses in the hydrous xenoliths indicate that a compositional control has been exerted by the local mineral assemblage (mainly amphibole). The δ 18 O values of carbonate from the glass pockets and veins in the xenoliths, as well as of carbonate globules and amygdales in the host basalts are in the range 19.62 to 21.04‰. Corresponding δ 13 C values are − 9.25 to − 10.12‰ and − 7.59 to − 9.32‰, respectively. These values are very different from those of primary carbonatites and the δ 18 O values clearly exceed those expected for minerals and glasses from mantle assemblages. The similarity of isotopic ratios of carbonates from both xenoliths and host lavas and their shift towards low δ 13 C and high δ 18 O values may be the result of basalt-peridotite interaction during ascent of the mantle xenoliths. Our study points to a close relationship between the infiltration of carbonic fluid together with fractions of the host alkali basalt, and melting of hydrous peridotite-forming minerals. Assuming an initial average content of 1.5–2 wt.% CO 2 in the primary alkaline melt and considering that the dissolved amounts of CO 2 and H 2 O in such a melt at 400 MPa can be in the order of 0.3 wt.% and 3.5% respectively, approximately 75 to 50% of the total carbon dioxide load was released by the uprising host magma. This process led to infiltration and entrapment of high-density CO 2 inclusions in the GG mantle rocks, to hydrous phase breakdown and to carbonate precipitation in veins and at some reaction sites after the primary mantle minerals. We propose that formation of CO 2 inclusions, glass and carbonate in hydrated mantle xenoliths is not unique of carbonatite metasomatism: comparable effects can be produced by degassing alkaline magmas.

Rare-earth and selected trace elements (Ti, Cr, V, Sr and Zr) have been analyzed by secondary ion mass spectrometry (SIMS) in pyroxenes and amphiboles from both the lower and upper zones of the post-orogenic Proterozoic Ikasaulak intrusive complex (Nagssugtoqidian mobile belt of southeast Greenland). The orthopyroxenes from the harzburgites and olivine-orthopyroxenites (lower zone) are LREE-depleted. In contrast. orthopyroxenes from gabbros (the most mafic members of the upper zone) display U-shaped chondrite-normalized patterns and REE contents which increase from core to rim. In the lower zone, clinopyroxene coexists with pargasitic amphibole, both exhibiting unusually high LREE, Sr and Zr contents. In the upper zone, clinopyroxene is less LREE-enriched and is replaced by late- or post-magmatic hornblende. The variations observed in both the major- and trace-element composition of the ferromagnesian phases indicate that the Ikasaulak intrusive complex did not result from a single magma injection. An early injection of magma was responsible for the lower zone and strongly interacted with the country rocks. Fractional crystallization of this magma was probably accompanied by metasomatism effected by highly LREE-enriched hydrous fluids (Ce abundances up to 1300 × chondrite). The most mafic members from the upper zone were derived from a late injection of magma. This magma probably mixed with the residual liquid of the earlier injection, thus retaining the geochemical signature of the contamination processes which had occurred in the lower zone. Mixing and assimilation processes led the pyroxenes to concentrate REE contents higher than those theoretically predicted on the basis of the usual distribution coefficients for basaltic systems. The late growth of hornblende is unrelated to the LREE-enrichment processes. The ion microprobe analyses of the main mafic rock types of the Ikasaulak intrusive complex indicate that: (i) the parental magmas (tholeiites probably marked by a slight LREE-enrichment) were contaminated during the early cumulus processes; (ii) the degree of LREE-enrichment and HFSE (high field strength elements) depletion in parental magmas, as deduced from the whole-rock chemistry, may be overestimated.

Rare earth elements were determined by secondary ion mass spectrometry for 13 silicate rock samples, ranging from ultramafic-mafic to acidic compositions, 2 feldspar and 1 biotite separates. As a whole, the investigated samples are characterized by matrices and rare earth elements spectra covering most geological applications. The present data are compared with reference values. The advantage of using secondary ion mass spectrometry as a fundamental tool for trace element detection in bulk samples in the few ppm-ppb region is demonstrated.

[1] The Campi Flegrei (CF) caldera is one of the most dangerous quiescent volcanic systems in the world. Its activity mostly resulted in low-magnitude explosive eruptions, such as that of the Monte Nuovo tuff cone that represents the last eruptive event within the caldera (A.D. 1538). However, there have been more energetic Plinian events, e.g., the Agnano Monte Spina eruption (4.1 ka), and very highly explosive, caldera-forming eruptions, e.g., the Campanian Ignimbrite eruption (39 ka). Here, we integrate new and literature data on the groundmass texture and composition of pyroclastic products from the three above eruptions with the aim of unraveling how volatiles content, degassing mechanisms, and crystallization processes influence magma explosivity and eruption dynamics at CF. Previous studies indicate that the investigated rocks share similar major element bulk and phenocryst chemistry; also similar is the water content of their trapped melt inclusions. These observations suggest that the magmas feeding these eruptions had comparable physicochemical properties during storage in the shallow crust. However, our investigations indicate that the studied rocks differ in texture and composition of the groundmass and viscosity of the related magmas. We ascribe such differences to the variable style of volatile exsolution and outgassing from the melt, primarily in response to changes of the rate of magma ascent to the surface. We conclude that the magma ascent rate was the key parameter in driving explosive eruptions at CF, and we suggest that this parameter may be influenced by magma-water interaction and/or magma chamber geometry and replenishment.