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The Morne Trois Pitons-Micotrin volcanic complex on the island of Dominica (Lesser Antilles) emitted a series of plinian eruptions between 18 and 9 ka BP. We studied it to constrain magma storage conditions and volatile degassing balances, by comparison with the three previous ignimbrites (~60-24 ka BP). Volatile concentrations in glass inclusions and mineral-melt thermobarometry indicate storage at ≤200 MPa (~6-8 km) and 860-880°C. The magmas feeding these plinian eruptions were stored at a shallower depth than those that older ignimbrites from the same volcanic complex and stored at ~16 km. Close magma composition and similar halogen ratios, however, suggest a common source for the magmas feeding both the plinian eruptions and the ignimbrites. The large eruptive fluxes of F, Cl, and Br to the atmosphere (up to 1.4-2.8 × 10-1 Mt/km³, 1.5-4.0 Mt/km³, and 2-4 × 10-2 Mt/km³, respectively), estimated by the petrological method, support the potentially important role of volcanic halogens in modifying the chemistry of the atmosphere, though Cl is underestimated here because of buffering in a fluid phase. The behavior of S, potentially partitioned in the same fluid phase, prevents here the calculation of an eruptive outgassing budget. [ABSTRACT FROM AUTHOR]

Trace amounts of water dissolved in minerals affect density, viscosity and melting behaviour of the Earth's mantle and play an important role in global tectonics, magmatism and volatile cycle. Water concentrations and the ratios of hydrogen isotopes in the mantle give insight into these processes, as well as into the origin of terrestrial water. Here we show the presence of molecular H2 in minerals (omphacites) from eclogites from the Kaapvaal and Siberian cratons. These omphacites contain both high amounts of H2 (70 to 460 wt. ppm) and OH. Furthermore, their ∂D values increase with dehydration, suggesting a positive H isotope fractionation factor between minerals and H2–bearing fluid, contrary to what is expected in case of isotopic exchange between minerals and H2O-fluids. The possibility of incorporation of large quantities of H as H2 in nominally anhydrous minerals implies that the storage capacity of H in the mantle may have been underestimated, and sheds new light on H isotope variations in mantle magmas and minerals. Trace amounts of water dissolved in minerals play an important role in global tectonics through changing the density, viscosity and melting behaviour of the Earth's mantle. Here, the authors identify the presence of molecular hydrogen in nominally anhydrous ecolgite minerals from the Kaapvaal and Siberian cratons, indicating that the storage capacity of H in the mantle may have been underestimated. [ABSTRACT FROM AUTHOR]

In the French Armorican Variscan belt, most of the economically significant hydrothermal U deposits are spatially associated with peraluminous leucogranites emplaced along the south Armorican shear zone (SASZ), a dextral lithospheric scale wrench fault that recorded ductile deformation from ca. 315 to 300 Ma. In the Pontivy-Rostrenen complex, a composite intrusion, the U mineralization is spatially associated with brittle structures related to deformation along the SASZ. In contrast to monzogranite and quartz monzodiorite (3 < U < 9 ppm; Th/U > 3), the leucogranite samples are characterized by highly variable U contents (~ 3 to 27 ppm) and Th/U ratios (~ 0.1 to 5) suggesting that the crystallization of magmatic uranium oxide in the more evolved facies was followed by uranium oxide leaching during hydrothermal alteration and/or surface weathering. U-Pb dating of uranium oxides from the deposits reveals that they mostly formed between ca. 300 and 270 Ma. In monzogranite and quartz monzodiorite, apatite grains display magmatic textures and provide U-Pb ages of ca. 315 Ma reflecting the time of emplacement of the intrusions. In contrast, apatite grains from the leucogranite display textural, geochemical, and geochronological evidences for interaction with U-rich oxidized hydrothermal fluids contemporaneously with U mineralizing events. From 300 to 270 Ma, infiltration of surface-derived oxidized fluids leached magmatic uranium oxide from fertile leucogranite and formed U deposits. This phenomenon was sustained by brittle deformation and by the persistence of thermal anomalies associated with U-rich granitic bodies. [ABSTRACT FROM AUTHOR]

Santorini caldera has had a long history of plinian eruptions and caldera collapses, separated by 20-40 kyr interplinian periods. We have carried out a study to constrain magma storage/extraction depths beneath the caldera. We analysed H2O in 138 olivine-, pyroxene- and plagioclase-hosted melt inclusions from plinian and interplinian products from the last 200 kyr, and CO2, S, Cl, F and dD in various subsets of these. The dataset includes 64 inclusions in products of the Minoan plinian eruption of the late 17th century BCE. All the melt inclusions were ellipsoidal and isolated, with no textural evidence for volatile leakage. Mafic melt inclusions contain 1-4 wt % H2O and up to 1200ppm CO2, 1200ppm S, 2000ppm Cl and 400ppm F; silicic inclusions contain 2-7 wt % H2O, up to 150ppm CO2, up to 400ppm S, 2000-6000ppm Cl and 600-1000ppm F. The δD values of 27 representative inclusions (-37 to -104‰) are intermediate between mantle and slab values and rule out significant H2O loss by hydrogen diffusion from olivine-hosted inclusions. H2O, S and Cl behave compatibly in melt inclusion suites varying from mafic to silicic in composition, showing that entrapment of many melt inclusions took place under volatile-saturated conditions. Most Santorini melts are saturated in a free COHSCl vapour phase at depths of less than ~10 km; the only exceptions are basaltic melts from a single interplinian eruption, which were volatile-undersaturated up to K2O contents of ~1 wt %. The rhyolitic melt of the Minoan eruption probably contained a free hypersaline liquid phase. H2O+CO2 saturation pressures were calculated using suitably calibrated solubility models to estimate pre-eruptive magma storage depths. Magmas feeding plinian eruptions were stored at >4km (>100 MPa) and extracted over depth intervals of several kilometres. Plagioclase phenocrysts in rhyodacitic pumice from the Minoan eruption have cores containing melt inclusions trapped at depths up to 10-12km (320 MPa), and rims (also orthopyroxene and clinopyroxene) containing inclusions trapped at 4-6km (100-160 MPa). This records late-stage silicic replenishment of a <2km thick shallow magma chamber, rather than extraction of melts syneruptively over the entire depth range. The plagioclase cores were carried from depth in the ascending melt, then overgrown by the rims in the shallow chamber. Exsolution of volatiles during ascent may have caused the replenishment melt to inject as a bubbly plume, causing mixing prior to eruption. This would explain (1) the homogeneity of the Minoan rhyodacitic magma, and (2) extraction of melt inclusions from the entire pressure spectrum during the first eruptive phase. Most silicic magmas feeding eruptions of the interplinian periods were stored in reservoirs at shallow depths (2-3 km) compared with those feeding the plinian eruptions (>4 km). Melt inclusions from the AD 726 eruption of Kameni Volcano yield a pre-eruptive storage depth of ~4 km, which is similar to that estimated from geodetic data for the inflation source during the 2011-2012 period of caldera unrest; this supports a magmatic origin of the unrest. The level of pre-AD 726 magma storage beneath Kameni was deeper than that of earlier silicic interplinian eruptions, perhaps owing to changes in crustal stress caused by the Minoan eruption. Combined with previously published results, the melt inclusion data provide a time-integrated image of the crustal plumbing system. Mantle-derived basalts are injected into the lower crust, where they fractionate to produce evolved melts in bodies of hot crystal mush. Evolved residual melts separate from their parent mushes in the 8 to >15km depth interval, then ascend rapidly into the upper crust, where they either crystallize or accumulate as bodies of eruptible, crystal-poor magma. [ABSTRACT FROM AUTHOR]

Introduction: High pressure minerals from meteoritic shock melt veins are key to understand the collisional history of the Solar System. The L chondrites, the most shocked meteorites [1], present abundant shock melt veins from which we can retrace their history. To this day, their family of origin is still debated. The shock timescale of 7 chondrites was measured to deduce their parent body diameters. Moreover, we use tuite, a high pressure phosphate mineral to perform U-Pb datation by SIMS. We find a bimodal distribution of ages, which match closely the ages previously obtained for Creston and Novato, two other shocked L6 chondrites [2]. Samples and Methods: Seven samples were first studied by optical microscopy and Raman spectroscopy to identify high pressure (HP) polymorphs. Scanning Transmission Electron Microscopy (STEM) was then used to study microstructures and transformation/growth mechanisms. The EDX was used to locate eligible minerals for datation (phosphates). Combined STEM-EDX (UMET, Lille, France) and NanoSIMS (MNHN, Paris, France) chemical maps were finally collected on the same FIB sections in order to compare these two analytical approaches and produce chemical maps. The U and Pb isotope concentrations where then measured on the identified tuite grains with the help of a Cameca IMS 1280 LG SIMS (CRPG, Nancy France) for datation. Results: Multiple high pressure textures were observed in all seven samples. Polycristalline assemblages of ringwoodite are typically the dominant texture, but more exotic textures were also found. Some meteorites present ringwoodite as lamellae inside olivine crystals, whereas others seem to present an assemblage of MgSiO3 glass with akimotoite crystallites. Both these textures allowed us to investigate elemental diffusion induced by structural changes. We therefore calculated shock timescales following the methods described in [3,4]. For all our samples, assuming a temperature of 2400K [5], shock timescales ranging between 0.5 and 20 seconds were derived. The meteorites that do not contain ringwoodite lamellae have significantly higher shock timescales, between 11 and 16 seconds. These larger shock timescales were caused by an impact between larger bolides, including a parent body of at least 150km wide. This is significantly higher than parent-bodies with diameters around 70km required by the other group. In almost all meteorites, we find tuites inside the shock melt veins. The meteorites that do not contain tuite have at least shocked apatites and whitlockites, with shifted Raman spectra indicating a change in structure. We date both the host rock apatites and the shock vein phosphates. This allows us to obtain conclusive collision ages from a normal concordia diagram for two of our samples, one of each group. The tuites record collisions ages of 461+-57Ma for the group with the largest parent body, and 650+-160Ma for the group with the smaller parent body. The host rock minerals record upper intercepts of 4481Ma in both meteorites. Conclusion: Shocked L chondrites seem to define two groups, which are texturally different and appear to have a completely different history. A smaller, 70km-wide parent body exploded first (650 Ma ago) and yielded most of the shocked meteorites, and a larger one exploded in the cataclysmic collision known today as the "L chondrite parent body breakup" 470Ma ago. The upper intercept of 4481Ma could date the early separation of the original L chondrite parent body into different families. [ABSTRACT FROM AUTHOR]

The 2007 caldera-forming eruption of Piton de la Fournaise (PdF) erupted the largest volume of magma (210 Mm3) recorded at this volcano in at least three centuries. Major and trace element and Sr–Nd isotope data for bulk-rocks, groundmasses and olivine phenocrysts have been combined with melt inclusion data (major, trace and volatile elements) to track magma evolution over the whole eruptive sequence. We show that each eruptive phase had a distinctive geochemical and petrological signature and that caldera collapse on 5 April was preceded by a marked shift in bulk magma composition and crystal content and size. Aphyric basalt erupted at the beginning of the sequence (February 2007) had relatively high Sr isotope ratio (87Sr/86Sr = 0·70420–0·704180) and low Nd isotopic ratio (143Nd/144Nd = 0·51285–0·51286). Olivine-basalts extruded on 2–5 April just before caldera collapse are less enriched in radiogenic Sr (87Sr/86Sr = 0·70412–0·70416), but characterized by the same Nd isotopic composition. This magma is interpreted as a new deep input, which pressurized the shallow PdF plumbing system and triggered the 2007 activity. Post-collapse oceanite lavas represent the main volume of magma extruded in 2007. Their bulk-rocks and groundmasses have 87Sr/86Sr (∼0·70418) intermediate between those of February and 5 April, and similar to those of the March 2007 and 2001–2006 lavas. We show that the Steady State Basalts (SSB) commonly erupted at PdF are hybrid melts, which result from multistep mixing between ‘alkaline’ and ‘transitional’ end-members. Our results lead us to propose a new model of the PdF plumbing system to reconcile the petrological, geochemical and geophysical observations: (1) the shallow portion (above sea level) of the PdF plumbing system hosts several small sills, in which magma experiences variable degrees of degassing, cooling and crystallization; (2) oceanite lavas result from the withdrawal of shallow harrisitic mushes stored at low pressures (<48 MPa; <1800–2400 m depth) below both the volcano summit and its eastern flank; (3) water degassing plays a major role in fast magma crystallization at shallow depths. Multistep ascent and periodic extrusion of the shallow magmas is promoted by injections of deeper and hotter basaltic magma, containing up to 1·3 wt % H2O and 1630 ppm S. In 2007, the new deep input was the ultimate source of the large excess in sulfur degassing detected by satellites. Lateral draining and intrusion of magma below the eastern flank of the volcano are the cause of major volcano deformation, flank sliding and summit caldera collapse. [ABSTRACT FROM AUTHOR]

Komatiites are ancient volcanic rocks, mostly over 2.7 billion years old, which formed through >30% partial melting of the mantle. This study addresses the crucial relationship between volcanology and physical manifestation of primary magmatic water content in komatiites of the Agnew-Wiluna greenstone belt, Western Australia, and documents the degassing processes that occurred during the emplacement and crystallization of these magmas. The Agnew-Wiluna greenstone belt of Western Australia contains three co-genetic komatiite units that (1) display laterally variable volcanological features, including thick cumulates and spinifex-textured units, and (2) were emplaced as both lava flows and intrusions at various locations. Komatiite sills up to 500 m thick contain widespread occurrence of hydromagmatic amphibole in orthocumulate- and mesocumulate-textured rocks, which contain ca. 40-50 wt% MgO and <3 wt% TiO. Conversely, komatiite flows do not contain any volatile-bearing mineral phases: ~150-m-thick flows only contain vesicles, amygdales and segregation structures, whereas <5-10-m-thick flows lack any textural and petrographic evidence of primary volatile contents. The main results of this study demonstrate that komatiites from the Agnew-Wiluna greenstone belt, irrespective of their initial water content, have degassed upon emplacement, flow and crystallization. More importantly, data show that komatiite flows most likely degassed more water than komatiite intrusions. Komatiite degassing may have indirectly influenced numerous physical and chemical parameters of the water from the primordial oceans and hence indirectly contributed to the creation of a complex zonation at the interface between water and seafloor. [ABSTRACT FROM AUTHOR]

Caldera-forming volcanic eruptions are low-frequency, high-impact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days, with devastating effects on local and global scales. Because no such eruption has been monitored during its long build-up phase, the precursor phenomena are not well understood. Geophysical signals obtained during recent episodes of unrest at calderas such as Yellowstone, USA, and Campi Flegrei, Italy, are difficult to interpret, and the conditions necessary for large eruptions are poorly constrained. Here we present a study of pre-eruptive magmatic processes and their timescales using chemically zoned crystals from the 'Minoan' caldera-forming eruption of Santorini volcano, Greece, which occurred in the late 1600s bc. The results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption. Despite the large volume of erupted magma (40-60 cubic kilometres), and the 18,000-year gestation period between the Minoan eruption and the previous major eruption, most crystals in the Minoan magma record processes that occurred less than about 100 years before the eruption. Recharge of the magma reservoir by large volumes of silicic magma (and some mafic magma) occurred during the century before eruption, and mixing between different silicic magma batches was still taking place during the final months. Final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period, with major growth phases immediately before eruption. These observations have implications for the monitoring of long-dormant, but potentially active, caldera systems. [ABSTRACT FROM AUTHOR]

The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re-equilibration of these garnet crystals and to reconstruct the pressure-temperature-fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100-200 μm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell-shaped profile of spessartine component (7-3 mol.%), an increase in grossular from 14 to 22 mol.% and δO values between 9.5 ± 0.3‰ and 10.4 ± 0.2‰. Garnet inner rims (90-130 μm) are fluid inclusion-rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The δO values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2‰. The outer rims (<10 μm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four-stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re-equilibration by coupled dissolution-reprecipitation at the temperature peak (630 < T < 650 °C). This causes the creation of porosity as the coupled dissolution-reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rim's domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolution-reprecipitation is a possible re-equilibration process for garnet in metamorphic rocks and that intra-mineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements. [ABSTRACT FROM AUTHOR]

We present in situ microanalyses of Fe3+/ΣFe in mantle-derived kaersutites as measured by electron probe microanalysis (EMP) based on the "self absorption induced FeLα peak shift" method. The EMP results are not in agreement with bulk (wet chemistry) data. The heterogeneities revealed for some kaersutite megacrysts, when comparing bulk and EMP Fe3+/ΣFe results, cannot explain the differences with the EMP measurements. It is thus proposed that any EMP overestimation of Fe3+/ΣFe results from a beam-induced dehydrogenation and a subsequent oxidation of Fe2+ to Fe3+ according to the known relation: Fe2+ + OH- = Fe3+ + O2- + 1/2 H2. To demonstrate this phenomenon, H losses were measured by secondary ion mass spectrometry (SIMS) after EMP irradiation at different beam currents on two amphiboles with 1.1 and 1.7 wt% H2O, respectively. In both amphiboles, H losses were observed under high beam currents (240 and 100 nA). No dehydrogenation is observed under lower beam currents for the 1.1 wt% H2O amphibole, but still occurs, down to at least 50 nA, for the amphibole with the greatest H2O contents. Amphiboles with low H2O contents (below ~0.5 H+) are less affected by beam damage. For amphiboles with higher H2O contents, the electron beam current density should be reduced with consideration given to the resulting high statistical errors. [ABSTRACT FROM AUTHOR]

Detailed electron- and ion-microprobe analysis of diopsides extracted from a block of marble collected in the Mt. Marcy anorthosite massif (Adirondack Highlands, New York) shows that single crystals have preserved Al-Si zoning features from early contact metamorphism later modified by regional metamorphism. Diopsides show Al and Si complementary zoning from core to rim; average Al concentration in rims is constant (0.11 pfu), but values in the cores vary with crystals from 0.06 to 0.14 pfu. A δ18O profile measured with the ion microprobe shows no zoning. Modeling of the Al-diffusion profiles affected by regional metamorphism of the Grenville orogeny shows that original Al zoning was sharp and corresponds to a rapid change of crystal-growth conditions during early contact metamorphism: (1) diopside cores crystallized with variable Al contents due to restricted fluid circulation or differing sedimentary composition, (2) then pervasive fluid infiltration crystallized the homogeneous rim of diopside, and (3) core to rim Al zoning was later smoothed by granulite-facies metamorphism. Homogeneity of δ18O in core and mantle (near rim) of diopsides suggests that the crystals were isotopically homogeneous prior to and during regional metamorphism. Values of Al-Si diffusion coefficient deduced from modeling are in agreement with low-temperature extrapolation of experimental data. [ABSTRACT FROM AUTHOR]

Detailed CL image, U-Pb age and oxygen isotope composition in situ analyses for two metamorphic zircons from ultra-high pressure Xindian eclogite, Southern Dabie Terrain were carried out. There are two kinds of zircon in eclogite. The major subpopulation is podgy and isometric, and interpreted as protolith magmatic zircon with partial recrystallization. Its formation age is 811±22 Ma. The less abundant one is euhedral prismatic with core-rim structure and the rim being metamorphic overgrowth and formed at 221-217 Ma. Zircon CL image strength is mainly controlled by U and Th contents. The magmatic zircon has its Th/U ratio around 1.3 and decreasing with recrystallization, whereas metamorphic zircon has Th/U ratio of lower than 0.1. Magmatic and metamorphic zircon domains all show very low oxygen isotopic compositions with average δ18O values of 1.8‰ and 2.8‰, respectively. The low δ18O values in protolith zircon indicated meteoric water involvement in their magma source region. It may correlate with snowball earth event during Neoproterozoic. It also indicates that protolith zircon could survive its oxygen isotope signature during Dabie eclogite facies UHP metamorphism. [ABSTRACT FROM AUTHOR]

Diffusion modeling of zoning profiles in eclogite garnets from three different tectonic units of Mt. Dabie, UHPM unit, HPM unit and northern Dabie, was used to estimate the relative time span and cooling rates of these rocks. Modeling result for the Huangzhen eclogite garnet shows that the maximal time span for the diffusion-adjustment process is about 22 Ma since the peak-temperature metamorphism, which is the maximum time span from amphibolite facies metamorphism to greenschist facies metamorphism. The Bixiling eclogites had subjected to a cooling process at a rate of ∼ 10°C/Ma from 750°C to 560°C during 20 Ma. The second cooling stage of the Raobazhai eclogite following granulite-facies metamorphism is an initial fast cooling process at a rate of about 25°C/Ma and then slowed down gradually. All these belong to a cohereni Dabie collision orogen with differences in subduction depth and exhumation/uplifting path. [ABSTRACT FROM AUTHOR]

Zircon U-Pb thermal ionization mass-spectrometer (TIMS) and secondary ion mass-spectrometer (SIMS) dating, mica and amphibole 40Ar-39Ar dating and mineral Sm-Nd isotopic compositions of Huangzhen low temperature eclogite and country granitic gneiss are carried out. The zircon U-Pb weighted average SIMS age is (231.6∓9.7) Ma for one eclogite. The mica 40Ar-39Ar isochron age is (232.6∓2.1) Ma and the lowest plateau age is (221.7∓2.4) Ma from same sample. U-Pb TIMS concordant ages from other eclogite zircons are from (221.3∓1.4) Ma to (222.5∓2.3) Ma. U-Pb SIMS low intercept age from country granitic gneiss is (221∓35) Ma. The retrograde amphibole 40Ar-39Ar isochron age is (205.9∓1.0) Ma. Except for mica, which may contain excess 40Ar, all the ages represent peak and retrograde metamorphism of low temperature eclogites. It is indicated that the Huangzhen low temperature eclogites differ from Xiongdian low temperature eclogites of north of the Northern Dabie Terrain in metamorphic ages. Huangzhen low temperature eclogites share one coherent HP-UHP terrain with high temperature eclogites from Southern Dabie Terrain and they may have differences in subduction depth and cooling rates during exhumation. [ABSTRACT FROM AUTHOR]

Summary¶Detailed petrographic, electron microprobe and ion probe studies of Archaean hydromagmatic amphiboles from the Abitibi greenstone belt, Canada, yield new insights into the origin of Al-undepleted komatiitic and Al-depleted tholeiitic and ferropicritic melts. The amphiboles are present in peridotite layers and basal chill zones of thick differentiated basic and ultrabasic sills and flows, and are titanian pargasite–hastingsite in composition. They can be grouped into two petrographic types: (1) amphibole in the groundmass; and (2) amphibole-bearing melt inclusions. The groundmass amphiboles are oikocrysts, rims and interstitial grains, present in minor to major amounts. The oikocrysts host cumulus olivines (Fo83–84) that are rounded in shape, embayed, and smaller in size. The amphibole-bearing melt inclusions are hosted in cumulus olivines (Fo83–84 in komatiitic rocks and Fo79 in tholeiitic rocks), spherical to ovoid in shape, 50–500 µm in size, and dominated modally by amphibole. The melt inclusions also contain euhedral chromite and aluminous spinel and micrometric clinopyroxene and glass, and sub-micrometric iron–nickel sulphide, chloro-apatite and ilmenite. In-situ ion probe analyses indicate the amphibole is: (1) enriched in Nb, LREE and Zr and depleted in Sr and HREE relative to primitive mantle; (2) contains up to 1–3 wt% H2O; and (3) overall displays δD values from 50‰ to -140‰, including many values in the accepted magmatic range of -60‰ to -90‰. The petrographic relationships and geochemical compositions, and comparisons to experimental systems, indicate amphibole formation by subsolidus reaction of residual hydrous silicate melt with olivine and clinopyroxene. Some of the hydrous melt intruded and was entrapped as secondary melt inclusions within relict olivine. Rapid crystallization of the hydrous melt inclusions formed amphibole+clinopyroxene±glass±spinel or solely glass. Bulk compositions of the melt inclusions, comparisons to experimental phase equilibria, and presence of magmatic water suggest amphibole crystallisation from olivine → pyroxene residual melts with at least 2–3 wt% H2O during rapid solidification of the host units. Adjustment for anhydrous phase crystallization (mainly olivine) suggests the initial melts contained 1–2 wt% H2O. Such high H2O contents and the magmatic δD compositions are consistent with the participation of H2O in melt petrogenesis. However, most Abitibi komatiites and tholeiites lack hydromagmatic minerals, making it difficult to attribute all basic and ultrabasic melts to melting in hydrous Archaean mantle. The favoured model is that some Abitibi basic and ultrabasic melts were wet and some were dry, as well as Al-depleted or Al-undepleted. [ABSTRACT FROM AUTHOR]

Focuses on measuring oxygen isotopic compositions of zircons from pyroxenite of Daoshichong, Dabieshan by an ion microprobe. Finding that oxygen isotopic ratios are homogeneous within the single grain and among various grains; Indication that the mantle-derived parent magma pyroxenite have interacted with crustal materials; Suggestion that crust-mantle interaction is from source mixing other than crustal contamination; The time interval between crust-mantle interaction and formation of the parent magma of Daoshichong pyroxenite; Indication the crustal component in crust-mantle interaction is mafic lower crust; Suggestion the parent magma of pyroxenite could contain a large proportion of this.

Presents information on a study which investigated the ages and chemical compositions of zircons from pyroxenites in northern Dabie Terrain in China. Methodology; Results and discussion; Conclusions.

Bromine diffusion was measured in two natural phonolitic melts: (1) a K2O-rich (~10 wt%) one synthesized from the white pumice phase of the 79 AD eruption of Vesuvius (Italy), and (2) a Na2O-rich (~10 wt%) one corresponding to the most differentiated melt of the 12 000 BC eruption of the Laacher See (Germany). Experiments were performed at 0.5 and 1.0 GPa, 1250 to 1450 °C, at anhydrous and hydrous (2.65 ± 0.35 wt% of dissolved water) conditions. Experiments conducted with the diffusion-couple technique in the piston cylinder were performed with only bromine diffusing and with the simultaneous diffusion of a halogen mixture (F, Cl, Br) to evaluate the interactions between the halogens during diffusion. The diffusion profiles of Br were measured by X-ray fluorescence using synchrotron radiation microprobe (SYXRF), ID18F, at the European Synchrotron Radiation Facility (ESRF, France). Bromine diffusion displays Arrhenian behavior under anhydrous conditions that is similar when it diffuses alone and when it diffuses with F and Cl. The Br diffusion coefficients range between 2 × 10-12 m2/s at 1250 °C and 1.5 × 10-11 m²/s at 1450 °C for the Na-rich melt and between 3 × 10-12 m²/s at 1250 °C and 2.5 × 10-11 m²/s at 1450 °C for the K-rich melt, at 1.0 GPa. Although Br mobility is independent of F and Cl in anhydrous phonolitic melts, its behavior may be dependent on the dominant alkali in the melt, as previously observed for Cl, but not F. For hydrous experiments, although the data are scattered, the Br diffusivity increases slightly with water and the Na/K ratio seems to influence Br diffusivity. Similarly to noble gases, halogen diffusivity at a given temperature in the phonolitic melts appears related to the ionic porosity of the silicate structure. Compared to basaltic melt, Br diffusivities are approximately one order of magnitude lower in the Na-phonolite melt, because of the difference of the pre-exponential factor. Br mobility appears to be decoupled from melt viscosity, considering the results here. [ABSTRACT FROM AUTHOR]

Volatiles contribute to magma ascent through the sub-volcanic plumbing system. Here, we investigate melt inclusion compositions in terms of major and trace elements, as well as volatiles (H2O, CO2, SO2, F, Cl, Br, S) for Quaternary Plinian and dome-forming dacite and andesite eruptions in the central and the northern part of Dominica (Lesser Antilles arc). Melt inclusions, hosted in orthopyroxene, clinopyroxene and plagioclase are consistently rhyolitic. Post-entrapment crystallisation effects are limited, and negligible in orthopyroxene-hosted inclusions. Melt inclusions are among the most water-rich yet recorded (≤ 8 wt% H2O). CO2 contents are generally low (< 650 ppm), although in general the highest pressure melt inclusion contain the highest CO2. Some low-pressure (< 3 kbars) inclusions have elevated CO2 (up to 1100-1150 ppm), suggestive of fluxing of shallow magmas with CO2-rich fluids. CO2-trace element systematics indicate that melts were volatile-saturated at the time of entrapment and can be used for volatile-saturation barometry. The calculated pressure range (0.8-7.5 kbars) indicates that magmas originate from a vertically-extensive (3-27 km depth) storage zone within the crust that may extend to the sub-Dominica Moho (28 km). The vertically-extensive crustal system is consistent with mush models for sub-volcanic arc crust wherein mantle-derived mafic magmas undergo differentiation over a range of crustal depths. The other volatile range of composition for melt inclusions from the central part is F (75-557 ppm), Cl (1525-3137 ppm), Br (6.1-15.4 ppm) and SO2 (< 140 ppm), and for the northern part it's F (92-798 ppm), Cl (1506-4428 ppm), Br (not determined) and SO2 (< 569; one value at 1015 ppm). All MIs, regardless of provenance, describe the same Cl/F correlation (8.3 ± 2.7), indicating that the magma source at depth is similar. The high H2O content of Dominica magmas has implications for hazard assessment. [ABSTRACT FROM AUTHOR]