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Abstract: Spinel-bearing mantle xenoliths have been recovered in the pyroclastic breccia surrounding the Lake Nyos maar. These include spinel lherzolites, spinel harzburgites and olivine websterites. They exhibit coarse granular or protogranular to weakly porphyroclastic textures, and show variations in mineral chemistry, modal compositions and equilibrium temperature. The xenoliths consist of four mineral phases typical of upper mantle origin: olivine (Fo89–Fo91.5, NiO=0.29–0.38wt%, CaO=0.02–0.17wt%), enstatite (Mg#=90–92, Cr2O3 =0.35±0.04wt%), Cr-diopside (Mg#=92–98, Cr2O3 =0.7–1.65wt%, TiO2 =0.26–0.6wt%) and spinel (high Mg# of 70–80, low TiO2 ≤0.4wt%). Spinels are aluminous (Cr#=9.7–11) in most lherzolites, and become increasingly chromiferous from websterites (Cr#Sp =15.3–19.8) to harzburgites (Cr#Sp =19–33.6). The lherzolites are composed of olivine (48–58%), orthopyroxene (22–30%), and clinopyroxene (8–15%). The harzburgites modes are olivine (60–81%), orthopyroxene (11–29%), and clinopyroxene (<5%). The websterites are mainly composed of pyroxene (∼62%) with variable amounts of olivine (23–31%). Temperatures of mineral equilibration in the xenoliths have been estimated from the two-pyroxene thermometer of and range between 850 and 1050°C, corresponding to about 10–30kbar at a depth mantle of 30km at least. These P–T conditions show significant variations between different petrographical types, the maximum conditions being recorded in two spinel lherzolites (NY-05 and NY-23) that have atypical chemical compositions and textures suggesting that they were initially formed in an environment close to the garnet stability field, then re-equilibrated within the spinel stability field prior to their incorporation in the host magma. With the exception of minerals from these two lherzolite nodules, all the minerals exhibit depletion of light REE, a typical feature of abyssal peridotites implying that some xenoliths from the Cameroon volcanic line were probably sampled in a part of the sub-continental mantle that is chemically similar to sub-oceanic mantle. The variations observed in the mineral chemistry and modal compositions of xenoliths suggest that the spinel harzbugite nodules which represent residues of a significant degree of partial melting of lherzolitic mantle were affected by infiltration of alkali-enriched metasomatizing melts (or fluids) within the uppermost mantle to produce pargasitic amphiboles prior to their sampling by the host lava. The features of this metasomatism event occur in the rocks of all three petrographical facies xenoliths from Lake Nyos. [Copyright &y& Elsevier]

Abstract: Liquidus phase relations were experimentally studied in model kimberlite melts at 6–12GPa. Four starting materials were used with different CO2 and H2O contents but almost identical proportions of other components (~35wt.% SiO2, 3wt.% Al2O3, 10wt.% FeO, 33wt.% MgO, 15wt.% CaO, 0.3wt.% Na2O, and 1wt.% K2O on a 100% volatile-free basis). The liquidus phase of the CO2 richest mixture (33wt.% CO2 and no H2O) is coesite at P>6–8GPa and low-Ca pyroxene at lower pressures. In an H2O-free mixture with lower CO2 content (18wt.%) low-Ca pyroxene is the main liquidus phase and coesite was never observed in the experimental products. The addition of water (mixtures with 10wt.% H2O+9wt.% CO2 and 12wt.% H2O+5wt.% CO2) depresses the liquidus temperature and expands the crystallization field of olivine. At high pressures (>8GPa), garnet crystallization dominates near-liquidus phase relations. Based on the experimental data, possible conditions of the simultaneous saturation of kimberlite melt with olivine+low-Ca pyroxene±garnet were estimated. High-Ca pyroxene was never found in the near-liquidus assemblages despite the high-CaO compositions of the starting mixtures. It was shown that the supposed primary kimberlite melt can be in equilibrium with the garnet harzburgite assemblage at 6–8GPa and variable volatile contents, from ~15wt.% CO2 under anhydrous conditions to 20wt.% H2O and low CO2. These conditions correspond to possible parameters of kimberlite magma generation by partial melting of carbonated garnet harzburgite in the lithospheric mantle. The primary CO2 content could be higher if the kimberlite magma was poorer in SiO2 (more ‘carbonatitic’). The maximum CO2 content is limited by the appearance of magnesite on the liquidus at 20–22wt.%, which is much lower than CO2 solubility in kimberlite melt at 7–8GPa (>30wt.%). The content of H2O in primary kimberlite magmas depends on the bulk H2O content in the source, because no hydrous phases are stable near the kimberlite liquidus. [Copyright &y& Elsevier]

Abstract: The Zhongba ophiolite is located in the western part of the Yarlung Zangbo Suture Zone (YZSZ) separating Eurasia to the north from the Indian plate to the south. This ophiolite comprises a well-preserved mantle sequence dominated by harzburgites with minor dunites. Highly depleted modal, mineral and bulk rock compositions of the harzburgites indicate that they are residues after moderate to high degrees of partial melting (13–24%) mainly in the spinel-stability field. These rocks display typical U-shaped chondrite-normalized Rare Earth Element (REE) patterns and fractionated chondrite-normalized Platinum Group Element (PGE) patterns. These characteristics, in combination with their hybrid mineral and whole-rock compositions intermediate between those of abyssal and forearc peridotites, indicate melt-rock interaction resulting in the selective enrichment of LREE and Pd. We propose a two-stage model to explain the generation of the Zhongba harzburgites: 1) original generation from a MORB-source upper mantle, and 2) subsequent trapping as part of a mantle wedge above a subduction zone. Comparable observations from the ophiolitic massifs along the whole YZSZ allow us to propose that a ca. 2500-km long complex subduction system was active between India and the Lhasa terrane, Burma, and the Karakoram microcontinent within the Neo-Tethys during the Early Cretaceous, similar to the modern active intra-oceanic subduction systems in the Western Pacific. [Copyright &y& Elsevier]

Abstract: It is widely recognized that H2O and other volatile components play a crucial role in mantle melting in subduction zones in both continental and intraoceanic arc settings. Here we report the results of a study aimed at determining the H2O-undersaturated, near-liquidus phase relations for two primitive arc magma compositions with the goal of determining the P–T–H2O conditions of mantle melt segregation beneath an arc of each type. These samples, JR-28, a basaltic andesite (SiO2 =53.2wt.%) from Volcán Jorullo, Mexico, and ID-16, a basalt (SiO2 =48.9wt.%) from Okmok Volcano, Aleutian Islands, have compositions consistent with near-primary, mantle-derived origins. H2O-undersaturated (2–7wt.% added H2O) piston-cylinder experiments have been carried out at upper mantle pressures and temperatures (1.0–2.0GPa and 1100–1350°C). The near-liquidus mineralogy of these compositions was mapped in P–T–H2O space to constrain the conditions under which these melts are multiply saturated with a mantle residue (lherzolite or harzburgite). Previous measurements of dissolved H2O in olivine-hosted melt inclusions suggest pre-eruptive H2O-contents for JR-28 of ≥5.5wt.% H2O. Inverse and forced saturation experiments show JR-28 melts to be saturated with a harzburgite assemblage at ~1175°C and ~1.2GPa with 5wt.% H2O and ~1150°C and ~1.4GPa with 7wt.% H2O. The experimentally determined phase relations of ID-16 suggest equilibrium with a lherzolite residue with ~2wt.% H2O at 1280°C and 1.4GPa. The presence of Ca-rich pyroxene in these latter experiments likely reflects the higher bulk CaO content of ID-16 (~10.7wt.% CaO) relative to JR-28 (~8.3wt.% CaO). The temperatures of melt–mantle equilibration for both samples are higher than those predicted by steady-state geodynamic models at the inferred equilibration depths. Our results thus support the hypothesis that ascent of melts from the hottest region of the mantle wedge perturbs the isotherms upward to create a hot region in the shallow mantle beneath the arc. We propose that final melt–mantle equilibration of primitive, H2O-undersaturated melts occurs in the shallow mantle prior to their ascent into the crust and that heterogeneities in uppermost mantle mineralogy therefore influence the compositions of erupted primitive melts. We suggest that basalt ID-16 last equilibrated with lherzolite, whereas the primitive basaltic andesite JR-28 formed by melting of a depleted source and last equilibrated with a harzburgite residue. [Copyright &y& Elsevier]

Abstract: This study examines the major element composition of mantle-derived garnets recovered from heavy mineral concentrates of several Proterozoic kimberlites of the diamondiferous Wajrakarur Kimberlite Field (WKF) and the almost barren Narayanpet Kimberlite Field (NKF) in the Eastern Dharwar Craton of southern India. Concentrate garnets are abundant in the WKF kimberlites, and notably rare in the NKF kimberlites. Chemical characteristics of the pyropes indicate that the lithology of the sub-continental lithospheric mantle (SCLM) beneath both the kimberlite fields was mainly lherzolitic at the time of kimberlite eruption. A subset of green pyropes from the WKF is marked by high CaO and Cr2O3 contents, which imply contribution from a wehrlitic source. The lithological information on SCLM, when studied alongside geobarometry of lherzolite and harzburgite xenoliths, indicates that there are thin layers of harzburgite within a dominantly lherzolitic mantle in the depth interval of 115–190km beneath the WKF. In addition, wehrlite and olivine clinopyroxenite occur locally in the depth range of 120–130km. Mantle geotherm derived from xenoliths constrains the depth of graphite–diamond transition to 155km beneath the kimberlite fields. Diamond in the WKF thus could have been derived from both lherzolitic and harzburgitic lithologies below this depth. The rarity of diamond and garnet xenocrysts in the NKF strongly suggest sampling of shallower (<155km depth) mantle, and possibly a shallower source of kimberlite magma than at the WKF. [ABSTRACT FROM AUTHOR]

We present petrographic, major element and trace element data for bulk-rocks and minerals for a suite of 34 remarkably fresh peridotite xenoliths in the Udachnaya kimberlite recovered from deep horizons of the open pit mine. The xenoliths are spinel peridotites, granular and deformed garnet peridotites (as well as a megacrystalline dunite) in proportions similar to those reported by previous work on altered xenoliths from less deep levels in the mine. Equilibration temperatures (T) range from 760–965°C for the spinel peridotites to 1200–1320°C for sheared garnet peridotites; the latter yield pressures (P) of 5·4–6·6 GPa. The majority of the granular garnet peridotites equilibrated at 860–1000°C and 2·6–5·5 GPa, but two samples yield much higher T and P values (1176°C, 6·1 GPa; 1340°C, 6·8 GPa) indicating the presence of undeformed rocks near the base of the lithosphere. Strong enrichments in silica and opx relative to dry melting residues are not common in our granular peridotites (e.g. 11 samples out of 18 analyzed plot on Boyd’s ‘oceanic trend’); they have distinctly lower modal opx than low-T peridotites from the Kaapvaal craton and could be formed by ∼30–45% of melt extraction at 1–5 GPa. Minor to moderate enrichments in silica relative to experimental melting trends seen in some xenoliths may indicate that subduction-related settings were involved in their origin. Garnets are usually unzoned but some are enriched in Ti at the rims. Their contents of CaO (4–7 wt %) and Cr2O3 (2–12 wt %) show inverse power-law correlations with whole-rock CaO, Al2O3 and Al/Cr ratios; the latter can be estimated from garnet analyses. Garnets from the dominant cpx-free and cpx-bearing harzburgites and less common low-cpx (5–6%) lherzolites fall into the G9 ‘lherzolite’ field; garnets from three cpx-rich (9–15%) lherzolites are in the G5 ‘pyroxenite’ field. Rare earth element (REE) patterns in cpx and garnet range from sinusoidal to bell-shaped and light REE (LREE)-depleted; the latter are common in sheared rocks and may be close to equilibrium with metasomatic liquids. Garnets from some granular rocks show low heavy REE (HREE) and continuous depletion in LREE, as in melting residues. Trace element patterns and ratios in many bulk peridotites are distinct from those in the host kimberlites and may not be much affected by contamination. Bulk-rock abundances of moderately incompatible elements are lowest in spinel harzburgites (20–50 times lower than in primitive mantle for HREE) and highest in sheared rocks. Some bulk-rocks show depletion from Lu to Er attributed to 30–40% of melt extraction first in the presence of garnet, then in the spinel field. Overall, the observations on fresh Udachnaya peridotites show less evidence for silica and opx enrichments and smaller compositional effects of syn- and post-eruption alteration than earlier work. The abundances of cpx and garnet and bulk-rock Al2O3 do not vary systematically with depth except that rare sheared xenoliths anomalously enriched in cpx and garnet occur near the base of the lithosphere. The remarkable preservation of olivine and opx shows in detail the transformation of granular microstructures into different types of porphyroclastic fabrics and their relations to chemical compositions. [ABSTRACT FROM PUBLISHER]

Abstract: The Antalya ophiolites (AO) in SW Turkey comprise remnants from the southern branch of the Neotethyan oceanic basins. The mantle peridotites of the AO include spinel lherzolite, harzburgite, and dunite. Harzburgites are dominant over spinel lherzolites in the AO, whereas dunites occur exclusively as ‘envelopes’ surrounding chromitite pods hosted by harzburgite. Petrographic and geochemical data from the peridotites show a progression from fertile to ultra-refractory character, reflected by a progressive decrease in bulk rock contents of Al2O3, CaO, SiO2, TiO2, Sc, V and Y, modal clinopyroxene, Al2O3 content of pyroxenes, and an increase in mafic phase Mg#s and spinel Cr#s (Cr#: 18.0–39.6% in spinel lherzolites, 33.2–77.8% in harzburgites, and 68.7–72.2% in dunites). The chondrite-normalized REE patterns of the AO spinel lherzolites have LREE depleted signature showing steep slopes from HREE to LREE, and are compatible with the residue of a low degree (8–18%) of partial melting of N type MOR mantle. Their mineral composition, whole-rock chemistry and oxygen fugacities between FMQ −1.01 and FMQ −0.57 are consistent with mid-ocean ridge-type mantle. On the other hand, the AO harzburgites have slight enrichment in LREE relative to the patterns expected for residues of partial malting thereby indicating reaction with a LREE-enriched melt. Thus, their chondrite-normalized REE patterns show ‘V-shaped’ REE distributions similar to those of forearc harzburgites. The AO harzburgites have oxygen fugacities between FMQ −1.08 and FMQ +0.72. The correlation of spinel Cr# and oxygen fugacity values indicates polygenetic association in the AO harzburgites as encountered in modern forearcs (e.g. Izu–Bonin–Mariana and South Sandwich forearc complexes). The spinel Cr# and oxygen fugacity correlation and the LREE enrichment, despite their ultra-refractory character, confirm the notion that the AO harzburgites could be residue from interaction of a previously melted mantle source (the AO spinel lherzolites) with an ascending LREE-enriched melt in a supra-subduction environment. The interaction must have caused modest melt fractions of the pre-existing mantle lithosphere. Therefore, the harzburgites experienced higher degree partial melting (17 to 25%) than the spinel lherzolites. The AO dunites, the most ‘refractory’ of the peridotites, appear to have formed at oxygen fugacities between FMQ +0.72 and FMQ +1.72, and show V-shaped REE distributions similar to those of supra-subduction zone (SSZ) dunites. Phase equilibria, field evidence and petrographic features show that the AO dunites are formed by dissolution of pyroxene from host harzburgites during the melt–mantle interaction. Focused melt flow caused the highest of melt fractions around the reaction zone. The AO podiform chromitites composed of a very refractory olivine–spinel assemblage (Fo: 94.8 and 96.1 – Cr#: 72.2–81.2%), corroborating a boninitic parentage. Therefore, the chromitites are considered to be the product of boninitic melt evolved by melt–rock interaction in a supra-subduction environment. Our results support a model involving both melt–rock interaction and differential partial melting to explain (a) precipitation of podiform chromitite from boninitic magma and (b) peridotites showing a sequence from MORB-like to subduction-related affinity. [Copyright &y& Elsevier]

To better understand the dynamical processes of partial melting, melt transport, and melt–rock reaction in the mantle, mass conservation equations for a two-porosity double lithology continuum have been developed using the method of volume averaging. Here the region of interest is treated as two overlapping continua occupied by a low-porosity peridotite matrix and a high-porosity dunite channel network. Conservation equations for the matrix and channel continua are coupled through mass transfer terms that include matrix dissolution and diffusive and advective mixing between the melt in the channel and that in the matrix. Essential features of the two-porosity double lithology model have been investigated through simple case studies. In general, the composition of the channel melt is a weighted average of the matrix melt extracted along the melting column (as a result of melt suction) and the matrix materials dissolved into the channel melt (as a result of matrix–channel transformation). Dissolution of pyroxene in the peridotite matrix into the channel melt and precipitation of olivine lowers the compatible trace element abundances in the channel melt. In the absence of matrix dissolution, the channel melt is equivalent to the aggregated or pooled matrix melt. The incompatible trace element abundance in the channel melt is dominated by the less depleted small-degree melts from the lower part of the melting column and not very sensitive to the details of how the melt suction rate and matrix–channel transformation rate vary spatially. The incompatible trace element abundances in the matrix melt and solid are very sensitive to the direction of melt flow across the matrix–channel interface, the magnitude and variation of the relative melt suction rate, and the depth of dunite channel initiation, and depend moderately on variations in channel volume fraction in the double lithology region. Percolation of the enriched channel melt into the more depleted residual matrix in the upper part of the melting column may provide a viable mechanism for late-stage melt refertilization or mantle metasomatism. Understanding the first-order characteristics of the channel and matrix melts and solids is essential in deciphering the melting and melt migration histories of residual mantle rocks and erupted basalts. [ABSTRACT FROM PUBLISHER]

The mantle above a subducting slab is the site of complex interactions between deformation, partial melting, fluid migration and magma transport. To constrain these interactions and their effects on olivine deformation, we analyze microstructures, crystal preferred orientations, and water contents of peridotite xenoliths entrained by andesites of the Avacha volcano, southern Kamchatka arc. These xenoliths are refractory spinel harzburgites that have coarse-grained microstructures with widely spaced subgrain boundaries and sinuous grain boundaries in olivine, consistent with deformation by dislocation creep under low deviatoric stress (≤13 MPa) and with a significant contribution from diffusional processes. Analysis of crystal preferred orientations (CPO) indicates dominant activation of high-temperature, low-stress {0kl}[100] slip systems in olivine and of(100)[001] in orthopyroxene. In most samples, coarse opx crystals, elongated parallel to the lineation, enclose small olivine grains in crystallographic continuity with neighbouring crystals, indicating crystallization of orthopyroxene at the expense of olivine as a result of reactive percolation of Si-rich fluids coeval with the high-temperature deformation. Secondary crystallization of interstitial orthopyroxene led locally to development of opx-rich lenses parallel to the foliation, characterized by a decrease in olivine grain size and dispersion of the olivine CPO without changing the dominant slip systems. Half of the samples also show acicular orthopyroxene aggregates with which is associated a fine-grained matrix composed of rounded strain-free olivine, orthopyroxene, spinel, and rare amphibole crystals. This matrix occurs pervasively along grain boundaries or forms millimeter-scale irregular lenses and anastomosing veinlets that crosscut the coarse crystals and their ductile deformation structures. Both acicular orthopyroxene and the fine-grained matrix are interpreted as resulting from reactive transport of H2O-rich fluids under static conditions, probably in the lithospheric mantle. Infrared analyses show that olivine contains 1–8·6 ppm by weight of water. These low water contents are similar to those observed in spinel peridotites from other subduction zones and probably record both the low solubility of water in olivine at low pressure and dehydration during exhumation of the xenoliths. Water contents in orthopyroxene are highly variable (25–506 ppm H2O), probably recording spatially heterogeneous interaction with fluids or melts and compositional disequilibrium in the studied samples. Change in the dominant percolation mechanism from porous flow to fracturing suggests cooling, consistent with the low temperatures estimated from pyroxene thermometry (≤ 800–900°C). The Avacha xenoliths therefore record pervasive deformation of a region of the mantle under asthenospheric conditions, followed by its accretion to the base of the lithosphere, probably as a result of cooling of the mantle wedge. Percolation of Si-rich fluids or hydrous melts is recorded at all stages; this probably enhanced diffusion and lowered deviatoric stresses during ductile deformation, but did not change the dominant slip direction in olivine from [100] to [001]. [ABSTRACT FROM PUBLISHER]

Abstract: The Slovenska Bistrica ultramafic complex (SBUC; Eastern Alps, Slovenia) occupies the south-easternmost part of the Pohorje Mountains, which represent an exhumed piece of continental crust subducted during the Cretaceous Eo-Alpine orogeny. The SBUC is composed of serpentinised harzburgites with local occurrences of garnet lherzolite, and is the only known occurrence of ultramafic rocks within the high- to ultrahigh-pressure nappe system apart from a few small dismembered pieces in the near vicinity. The harzburgites are highly depleted following melting within the spinel stability field, as exemplified by high whole-rock MgO contents (41.5–44.3 wt.%), low Al2O3 (0.7–1.2 wt.%), low LuN (0.1–0.7), and high Cr# of Cr-spinel (ca. 0.5). Fluid-immobile incompatible trace elements (Ti, Sc, V, Zr, HREE, Th) correlate well with MgO, consistent with a melt depletion trend. Other incompatible elements (Ba, Sr, LREE) show little correlation and are probably modified by the serpentinisation process or later metamorphic overprint. However, comparable LREE enrichment of all samples and absence of negative Nb and Th anomalies suggests that this piece of mantle was already metasomatised by melts or fluids before serpentinisation. Garnet lherzolite in the SBUC recorded an UHP stage (4 GPa, 900 °C) not visible in the harzburgites. Because of the evidence of an earlier lower pressure stage within the spinel stability field, the SBUC represents a piece of subducted mantle. The protolith of the harzburgites is probably oceanic mantle, considering the high degree of melt depletion yet the lack of a subduction-zone signature. It therefore most likely represents a part of previously subducted Meliata oceanic mantle, which was part of a deeper section of the hanging wall along which subduction of the continental crust that is now exposed in Pohorje took place. Alternatively, it may represent mantle depleted and metasomatised in a continental rift zone, which was later incorporated in the hanging wall of the subduction zone and subsequently dragged down to UHP conditions. [Copyright &y& Elsevier]

Ophiolitic rocks (chromitites and serpentinized peridotites) were petrologically examined in detail for the first time from Rayat, in the Iraqi part of the Zagros thrust zone, an ophiolitic belt. Almost all the primary silicates have been altered out, but chromian spinel has survived from alteration and gives information about the primary petrological characteristics. The protolith of the serpentinite was clinopyroxene-free harzburgite with chromian spinel of intermediate Cr# (= Cr/[Cr + Al] atomic ratio) of 0.5 to 0.6. The harzburgite with that signature is the most common in the mantle section of the Tethyan ophiolites such as the Oman ophiolite, and is the most suitable host for chromitite genesis. Except for one sample, which has Cr# = 0.6 for spinel, the Cr# of spinel is high, around 0.7, in chromitite. The variation in Cr# of spinel in chromitite observed here has been also reported in the Oman ophiolite. The peridotite with chromitite pods exposed at Rayat was derived from an ophiolite similar in petrological character to the Oman ophiolite, one of the typical Tethyan ophiolites (fragments of Tethyan oceanic lithosphere). This result is consistent with the previous interpretation based on geological analysis. [ABSTRACT FROM AUTHOR]

• Rutiles and zircons occur in forearc mantle wedge rocks in New Caledonia. • Age spectra fingerprint xenocrysts to possible sources in down-going slab. • First direct dating of podiform chromite using co-occurring magmatic zircons. • First ever U-Pb ages determined from rutiles in harzburgite. • Interplate transfer of xenocrysts occurred at around 50-60 km subduction depth. Voluminous flux of a hydrous component from subducting oceanic lithosphere into the forearc mantle and its contribution to arc magmas have long been recognised but the transfer of solid-phase materials has received less consideration. Although xenocrystic zircons are known from some arc magmas and ophiolitic chromitites their origins remain enigmatic. How and when such materials are transferred into the overlying wedge of mantle lithosphere and the length of their residency therein are unclear. Using zircons and rutiles recovered from ophiolitic rocks in New Caledonia, we demonstrate unambiguous association with subducting sediments and hence evidence for inter-plate transfer. The 'fingerprints' of age spectra for both minerals can be matched to lithospheric slab components subducted during Eocene time. Their occurrence in forearc harzburgite suggests that they were incorporated into their mantle wedge host at subduction depths beyond the onset of serpentinisation at ca. 300 °C but before the ca. 600 °C destabilisation temperature of the rutile U-Pb system. Relocation from slab to forearc mantle is therefore likely to have occurred at ca. 50-60 km depth within the subduction channel. Fluid-assisted conveyance of high field strength elements within solid-state accessory mineral phases into supra-subduction mantle wedges may represent a little-recognised but significant phenomenon that is both predictable and pervasive at convergent margins globally. [ABSTRACT FROM AUTHOR]

A study of potassium- and silica-rich glass (SiO2 = 65.3–67.4%, K2O = 7.1–9.8%, Na2O = 4.4–6.5%) in spongy clinopyroxene rims from anhydrous spinel harzburgite, collected from Damaping (Hannuoba, north China), is reported here. The corroded surface of clinopyroxene along with clear chemical zonation (homogeneous core and partially melted rim) suggests that incongruent melting of primary clinopyroxene is responsible for generating the Si-rich glass in clinopyroxene rims. The degree of clinopyroxene melting is estimated to be higher than 15%. In order to generate glass with K2O contents of 7.0 to 9.8% by clinopyroxene melting at a degree of 15% in a closed system, K2O contents in the primary clinopyroxene should be greater than 1.0 wt%, suggesting a very deep origin for the Damaping harzburgite. [ABSTRACT FROM AUTHOR]

We present major- and trace-element analyses of mineral phases present in a suite of 16 garnet-peridotite xenoliths from the western terrane of the Kaapvaal craton. The xenoliths were entrained by a Group II Finsch kimberlite at 118 Ma, shortly prior to a major metasomatic event that caused widespread enrichment of the Kaapvaal lithospheric mantle. Compositionally homogeneous grains of olivine, orthopyroxene, garnet and clinopyroxene and coarse–equant textures indicate equilibrium relationships between mineral phases in the majority of xenoliths. Pressure and temperature estimates suggest that clinopyroxene-bearing garnet peridotites last equilibrated at 1130–1270°C and 45–59 kbar whereas clinopyroxene-free xenoliths record temperatures of 1000–1070°C and pressures of 34–42 kbar. The Finsch xenoliths plot on a conductive palaeogeotherm with a surface heat flux of ∼46 mW m2. Combined Ca and Cr abundances of Finsch pyrope garnets suggest that both lherzolitic and harzburgitic parageneses are present. Samples bearing sub-calcic (harzburgitic) garnets are from the shallowest depths. The lherzolitic garnets are depleted in light rare earth elements (LREE) relative to the middle and heavy REE (MREE and HREE) and have ‘smooth’ chondrite-normalized patterns. In contrast, the sub-calcic garnets are characterized by sinusoidal chondrite-normalized REE patterns that peak at Nd and Lu and exhibit lows at La and Er. The sub-calcic garnets also have lower Zr, Hf, Ti and HREE, and higher LREE and Sr, than lherzolitic garnets. The variations in REE ratios correlate with temperature and pressure and also Cr/Ca ratio. The high Cr content of harzburgitic and some lherzolitic Finsch garnets may have a significant effect on the crystal framework. Substitution of the larger Cr3+ ion for the smaller Al3+ ion increases with decreasing temperature and pressure and distorts the crystal lattice; this permits a greater substitution of Ca by large cations, such as Sr and the LREE, but also limits the replacement of Al by Ti, Zr and Hf. Positive HREE slopes displayed by harzburgitic garnets on chondrite-normalized plots are believed to result from metasomatic enrichment by a melt that had already undergone significant garnet fractionation during ascent through the lithospheric mantle. The low-temperature Finsch peridotites are characterized by much lower orthopyroxene (< 17%) and higher olivine (up to 96%) modal abundances than have been reported from xenolith suites elsewhere in the Kaapvaal craton. Significantly, they resemble residues generated in partial melting experiments. The Finsch harzburgites have very low Al2O3 (0·18 wt %) and CaO (0·38 wt %) and high MgO contents (49·75 wt %) and appear to be highly refractory. They also have high bulk-rock Mg/(Mg + Fe) and high modal olivine contents, and in this respect resemble some of those recently described from NW Canada and Greenland. We suggest that some of the Finsch low-temperature peridotites represent Kaapvaal lithospheric mantle that formed as a residue of adiabatic decompression melting between 4·5 and 1·5 GPa. The inferred mantle potential temperature of 1550°C would have been similar to that of ambient Archaean mantle. Importantly, it appears that the sub-Finsch lithospheric mantle has remained unmodified by the silica enrichment that has been so prevalent elsewhere in the craton. This may reflect the remoteness from the subduction zone that is believed to have been in existence at 2·9 Ga on the eastern margin of the craton. [ABSTRACT FROM PUBLISHER]

Chromitites from a single section through the mantle in the Oman ophiolite are of two different types. Low-cr# chromitites, of MORB affinity are found in the upper part of the section, close to the Moho. High-cr# chromitites, with arc affinities are found deeper in the mantle. Experimental data are used to recover the compositions of the melts parental to the chromitites and show that the low-cr# chromitites were derived from melts with 14.5–15.4 wt% Al2O3, with 0.4 to 0.9 wt% TiO2 and with a maximum possible mg# of 0.76. In contrast the high-cr# chromitites were derived from melts with 11.8–12.9 wt% Al2O3, 0.2–0.35 wt% TiO2 and a maximum melt mg# of 0.785. Comparison with the published compositions of lavas from the Oman ophiolite shows that the low-cr# chromitites may be genetically related to the upper (Lasail, and Alley) pillow lava units and the high-cr# chromitites the boninites of the upper pillow lava Alley Unit. The calculated TiO2–Al2O3 compositions of the parental chromitite magmas indicate that the high-cr# chromitites were derived from high-Ca boninitic melts, produced by melting of depleted mantle peridotite. The low-cr# chromitites were derived from melts which were a mixture of two end-members—one represented by a depleted mantle melt and the other represented by MORB. This mixing probably took place as a result of melt–rock reaction. [ABSTRACT FROM AUTHOR]

Abstract: Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol1) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite–eclogite is derived from a troctolitic protolith, whereas the epidote–eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO2, and high Al2O3 and MgO. The kyanite–eclogite shows inherited cumulate layering. The phengite–eclogite has high TiO2, low Al2O3 and MgO with incompatible trace elements resembling enriched-type MORB. Sr–Nd isotope data indicate that the protoliths of both kyanite–eclogite and epidote–eclogite ([87Sr/86Sr] i ~0.703–0.704; ε Nd(T)~5.9–8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower ε Nd(T) value (1.4–4.1) of phengite–eclogite is more or less consistent with an enriched MORB protolith, their high [87Sr/86Sr] i ratio (0.705–0.716) points to an additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P ≥2.7 GPa). The harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ≤3.0 GPa. Zircon U–Pb SHRIMP dating shows that the eclogites have a protolith age of 516±8 Ma and a metamorphic age of 445±7 Ma. These data indicate the presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of subducted oceanic crust prior to the subduction of continental materials during Ordovician–Silurian times and ultimate continent collision. These rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision. [Copyright &y& Elsevier]

Modal composition and mineral composition of harzburgites from the southern Mariana fore-arc show that they are highly refractory. There are a few modals of clinopyroxene (0.7 vol %) in harzburgites. Two types of amphibole are found in these harzburgites: magnesiohornblende accompanied by clinopyroxene with higher Al2O3 content (>7%) and lower Mg#; tremolite around orthopyroxene with lower Al2O3 content (< 2%) and higher Mg#. Trace element of clinopyroxene and two types of amphibole are analyzed. Primitive mantle-normalised REE patterns for clinopyroxene and magnesio hornblende are very similar and both show HREE enrichment relative to LREE, while magnesiohornblende has higher content of trace element than clinopyroxene. The contents of trace element of tremolite are much lower than those of magnesiohornblende. Clinopyroxene shows enrichment of most of the trace element except HREE and Ti relative to clinopyroxene in abyssal peridotites. Petrology and trace element characteristic of clinopyroxene and two types of amphibole indicate that southern Mariana fore-arc harzburgites underwent two stages of metasomatism. The percolation of a hydrous melt led to mobility of Al, Ca, Fe, Mg, Na, and large amounts of trace element. LILE and LREE can be more active in hydrous melt than HREE and Ti, and the activities of most of the trace element except some of LILE are influenced by temperature and pressure. [ABSTRACT FROM AUTHOR]

Abstract: Spinel-anthophyllite rocks that may be classified as ultrabasic low-Ca spinel amphibolites have been first discovered in the Kokchetav collision zone (northern Kazakhstan). They outcrop 2 km west of Enbek-Berlyk Village among schists and quartzites and are closely associated with spinel harzburgites and garnet pyroxenites. The main hosted minerals are spinel (hercynite) and anthophyllite. The rocks bear magnetite-hornblende-spinel-anthophyllite pseudomorphs with rounded and polygonal sections, which might have been resulted from the replacement of garnet grains. The prismatic anthophyllite crystals and scarce olivine relics contain elongate parallel spinel inclusions resembling spinel-olivine syntactic intergrowths in the Enbek-Berlyk spinel harzburgites. The spinel-anthophyllite rocks are similar to the associated spinel harzburgites in CaO, MnO, SiO2, and Al2O3 contents but are richer in FeO and poorer in MgO (F = FeO/(FeO + MgO) = 57% against 35% in the harzburgites). Geological, mineralogical, and geochemical data suggest that the spinel-anthophyllite rocks formed during the isochemical contact metamorphism of garnet-bearing spinel harzburgites, which contained more FeO and less MgO than garnet-free harzburgites of the same area. Variations in FeO and MgO contents in both types of harzburgites seem to be due to different chemical compositions of the chlorite protoliths of these rocks. [Copyright &y& Elsevier]

Abstract: Recent studies of the upper mantle above the flat slab in central Chile and Argentina indicate the seismic velocity structures in this area are very different from those found in subduction zones with “normal,” steeper slab geometries. The mantle above the horizontal section of the flat slab is characterized by low P-wave velocities, high S-wave velocities, and low V p / V s ratios. As the slab begins to transition to a more normal dip to the south, the mantle above it changes as well. Above this “transition zone”, the mantle is characterized by high P-wave velocities, high S-wave velocities, and generally high V p / V s ratios. Above this high velocity anomaly, in the corner between the south-dipping slab of the transition zone and the east-dipping slab of the normal subduction zone, lies a small shallow anomaly distinguished by its very low P- and S-wave velocities and its moderately high V p / V s ratio. We interpret these anomalies to represent very depleted cold lithosphere, cooled trapped asthenosphere, and frozen pooled melt, respectively. Recent research looking at the elastic properties of cratonic xenoliths indicate that a low V p, high V s, and low V p / V s signature is indicative of cratonic lithosphere. This suggests that the material above the flat slab may be ancient Laurentian lithosphere, or other melt-depleted, dry lithospheric material. The presence of frozen asthenosphere to the south of the flat slab may indicate that large volumes of asthenosphere were displaced in advance of the eastward progression of the flattening slab between 10 and 2Ma, forcing a change in flow patterns from trench normal to SSW around the south eastern corner of the flattening slab. As flattening progressed this asthenosphere became trapped above the transition zone. Eventually, temperatures dropped sufficiently to freeze the last remaining pocket of extracted melt which had collected in the corner of the descending slab. [Copyright &y& Elsevier]

Abstract: Geotherm and water content profiles in the upper mantle above the stagnant slab of the Pacific back-arc were estimated from the electrical conductivity and seismic P-wave velocity () structures. The geothermal profiles were determined by using the electrical conductivity and seismic structures, which, assuming a dry hartzburgite or a dry pyrolite composition, are designated as electrical and seismic geotherms, respectively. In a deeper part of the upper mantle, neither the dry pyrolite nor the dry harzburgite condition provides consistent electrical and seismic geotherms. This discrepancy can be explained by allowing for a small amount of water (500–1000 ppm H/Si) with the seismic geotherm. In a shallower part of the upper mantle, the electrical and seismic geotherms are consistent with each other within 1500–1700  C under the dry harzbutgite condition, whereas they are inconsistent by more than 100  C under the dry pyrolite condition. Alternatively, the wet pyrolite condition applied to the deeper part of the upper mantle also satisfies the electical conductivity and seismic structures in the shallower part. [Copyright &y& Elsevier]

Abstract: Harzburgite is the predominant type of peridotite in the mantle sections of ophiolite complexes. The residual character of harzburgite suggests that the magma extraction is efficient, and that harzburgite form a residual layer above a subsequent mantle source. This residual layer form an upper lid for subsequent magma accumulation. A theoretical estimate of the depletion rate of a veined mantle source region show that it is possible for a veined source region to become nearly completely depleted for melt within a year. This period of time is comparable to the durations of abyssal and subaerial volcanic eruption episodes, which varies from a few months to a few years. [Copyright &y& Elsevier]

Abstract: Numerical simulations of thermochemical mantle convection are used to investigate the influence of multicomponent phase changes on mass transport between the upper and lower mantle. The model includes both olivine and pyroxene–garnet components based on a pyrolite composition. The simulations reveal large and focused upwellings through the 660-km phase boundary in the vicinity of subducting slabs. The upwellings are composed of depleted harzbergitic material, which contributes to the depletion of the upper mantle and may generate distinct reservoirs of trace elements in the upper and lower mantle as well. The position and composition of the upwelling is attributed to the development of compositional stratification at the top of the lower mantle, where dense harzbergitic material underlies enriched basaltic material due to a density inversion in the combine phase system. Subducting slabs disrupt the stratification in the vicinity of subduction, permitting a counter flow of harzbergitic material into the upper mantle. This behavior is not observed in a pure olivine phase system, suggesting that realistic mantle compositions are needed to assess the generation of distinct reservoirs in mantle convection simulations. Phase transitions in a multicomponent system provide a plausible mechanism for explaining the formation of a depleted reservoir in the upper mantle without implementing dehydration-induced melting and differentiation in the transition zone. [Copyright &y& Elsevier]

Abstract: A new lithotectonic framework for the northwestern Reindeer Zone of the Trans-Hudson Orogen divides rocks into five northwest- to north-dipping volcano-sedimentary assemblages: (1) at the structural base, the 1.92–1.87Ga largely sedimentary Levesque Bay Assemblage (partly equivalent to former ‘MacLean Lake gneisses’), which lies within the confines of the Kisseynew Domain and is tectonically imbricated with metasedimentary rocks of the <1.85Ga McLennan and Burntwood groups; (2) the turbiditic Duck Lake Assemblage, also located along the northern edge of the Kisseynew Domain; it contains detrital zircons ranging in age between 1.92 and 1.87Ga; (3) the ?1.92Ga mafic–ultramafic volcano-plutonic Lawrence Point Assemblage of the La Ronge Domain; (4) the ≥1.88Ga felsic to intermediate volcano-plutonic Reed Lake Assemblage of the La Ronge Domain; and (5) the turbiditic Milton Island Assemblage of the Rottenstone Domain, which contains detrital zircons ranging in age between 2.83 and 1.86Ga. The assemblages are intruded by a variety of 1.91–1.78Ga mafic to felsic plutons. The Lawrence Point Assemblage is interpreted as a dismembered supra-subduction zone ophiolite. High-MgO refractory harzburgite (‘Group 1’ ultramafic rocks), at the structural base of the assemblage, is geochemically identical to the upper mantle section of selected supra-subduction zone ophiolites and mantle tectonites. Chromite and olivine compositions of the ‘Group 1’ ultramafic rocks are also comparable to those of ophiolitic harzburgite and mantle tectonite. Mafic metavolcanic rocks of the assemblage are classified as subalkaline tholeiitic basalts. Their trace element patterns and Hf, Ta, Th, Y, Nb, and La element ratios resemble those of modern back-arc basin basalts. The Reed Lake Assemblage represents a subduction-generated arc complex that was built on top of the Lawrence Point Assemblage; its mafic metavolcanic rocks are subalkaline basalts, with calc-alkaline trends, and elevated Th and Ce concentrations and negative Nb anomalies. Feldspar porphyry dykes intruding the Lawrence Point and Duck Lake assemblages constrain timing of Lawrence Point ophiolite emplacement onto the Duck Lake Assemblage to 1.86–1.84Ga. The trace element geochemistry of the dykes suggests continued arc volcanism after ophiolite emplacement. Mafic metavolcanic rocks of the Levesque Bay Assemblage are geochemically similar to those of the Lawrence Point Assemblage. Other ultramafic rocks (peridotite to pyroxenite) are abundant in the Lawrence Point Assemblage, but have similar geochemistry to small ultramafic bodies intruding the Reed Lake, Duck Lake and Levesque Bay Assemblages. They represent a separate, later phase (?1.86Ga) of ultramafic plutonism, which post-dates ophiolite emplacement. Timing of Lawrence Point ophiolite emplacement (between 1.86 and 1.84Ga) and geochemistry of later felsic and mafic/ultramafic volcanism suggest that the Lawrence Point ophiolite and overlying Reed Lake arc assemblage were not accreted to the Hearne Craton prior to 1.86Ga, but were first accreted to the Flin Flon–Glennie Complex after 1.86Ga. [Copyright &y& Elsevier]

Porphyroclastic enstatite in a garnet lherzolite xenolith from the Monastery Mine kimberlite, South Africa, has exsolved pyrope garnet, Cr-diopside and Al-chromite, and the specimen is interpreted as representing a transition from fertile harzburgite, (containing high Ca-Al-Cr enstatite) to granular garnet lherzolite. Although the exsolved phases occur in morphologically different forms (fine and coarse lamellae; equant, ripened grains), indicating textural disequilibrium, the exsolved grains are very constant in composition, indicating chemical equilibrium. Theoretically, the exsolution could have been due to a fall in temperature, but the close association of exsolution and deformation of the host enstatite suggests that exsolution was also aided by straining of the enstatite lattice. The phase compositions can be broadly matched with those in other mantle peridotites, except that all phases are characterised by a virtual absence of Ti. In the garnet and diopside Ti, Co, Zr and most of the REE are lower than in published analyses of garnet and diopside in both granular and sheared garnet lherzolites from Southern African kimberlites, and diopside/garnet partitioning for Sr and the REE is higher. Comparison with the trace element chemistry of an enstatite from a fertile harzburgite indicates that, except for Nb, the trace element content and distribution found in the Monastery phases could arise by isochemical exsolution from such an enstatite. On the assumption that (a) the Monastery specimen represents a transition from harzburgite to garnet lherzolite, and (b) many garnet lherzolites are of exsolution origin (as suggested by their modal compositions), the inference is that most garnet lherzolites, and not just the sheared variety, have been subject to varying degrees of Ti, Zr, Sr and REE metasomatism. [Copyright &y& Elsevier]

The composition, structure and thermal state of the lithosphere beneath the Slave craton have been studied by analysing over 300 peridotitic mantle xenoliths or multiphase xenocrysts entrained within kimberlites in the Lac de Gras area. These xenoliths are derived from seven kimberlites located on the Ekati Diamond Mine™ property and define a detailed stratigraphic profile through the central Slave lithosphere from less than 120 km down to ∼200 km. Two dominant peridotite types are present, namely garnet-bearing harzburgite and lherzolite with rare occurrences of chromite-facies peridotite, websterite and wehrlite. The pressures and temperatures (P–T''s) defined by the entire data-set range from 28 to 62 kbar and 650 to 1250 °C, respectively, and approximately intersect the diamond stability field at 900 °C and 42 kbar. There is no apparent change in the geotherm with depth that is discernable beyond the resolution of the various thermobarometers. The peridotites can be divided into two compositional zones—a shallow layer dominated by garnet harzburgite that straddles the diamond–graphite boundary and a deeper layer that is strongly dominated by garnet lherzolite. Compositionally, the harzburgites (and to a lesser extent, the shallow lherzolites) are ultra-depleted relative to the more fertile deeper layer, irrespective of whether they reside within the graphite or diamond stability field. This ultra-depleted layer beneath Ekati continues to ∼150 km. [Copyright &y& Elsevier]

The mechanisms and kinetics of harzburgite reactive dissolution in basaltic liquids were examined using a combined experimental and numerical approach. Dissolution experiments were conducted at 1250–1290°C and 0.6–0.75 GPa using dissolution couples consisting of pre-synthesized rods of alkali basalt and harzburgite in graphite and platinum-lined molybdenum capsules. Reactive dissolution of harzburgite produces a melt-bearing, orthopyroxene-free dunite with a sharp mineralogical front at the dunite–harzburgite interface. The thickness of the dunite layer is proportional to the square root of experimental run time, and its growth rate is limited by the rates of diffusion of major components in the melt. Around the sharp mineralogical front there exists a broad composition boundary layer where major and trace element abundances in the interstitial melt and olivine vary systematically as a function of distance and time. The sharp mineralogical front and the broad composition boundary layer result from a combined effect of orthopyroxene and olivine dissolution at the dunite–harzburgite interface, olivine re-precipitation within the dunite, as well as diffusive exchange between the crystals and the melts. Based on experimental observations a simple model for harzburgite reactive dissolution was developed and used to extrapolate the experimentally measured dissolution rates and concentration profiles to conditions relevant to melt transport under the mid-ocean ridge. Model calculations demonstrate that diffusive dissolution alone is incapable of producing dunite dikes wider than a few meters within the time scale of mantle upwelling under the mid-ocean ridge. Prevalent melt percolation, hence large melt–rock ratios, is required in the formation of large dunite channels in the mantle. The composition of the reacting melt is of particular importance in determining the composition gradients in olivine across the dunite–harzburgite interface. By adjusting the reacting melt compositions we were able to produce concentration profiles broadly similar to those observed in our experiments as well as those reported across the dunite–harzburgite sharp contacts in the mantle sections of ophiolites and peridotite massifs around the world. [Copyright &y& Elsevier]

The Sabzevar ophiolite is a highly dismembered ophiolite complex located along the northern boundary of the central Iranian microcontinent (CIM), and is one of the internal Iranian group of ophiolites and colored melanges. The igneous rocks of this complex consist of peridotites (harzburgite, dunite and lherzolite), serpentinite, minor pyroxenite, gabbros, and a volcanic sequence that exhibits a wide range of composition from basalts and basaltic andesites to rhyodacite–dacites, rhyolites and basanites. Sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-marine rocks. These include pelagic fossiliferous carbonates that are mixed with the pillow basalt and basaltic andesite as interlayers or exotic blocks ranging in size from 10 to 100 m. Also present are extensive units of radiolarian chert, which are interbedded within the basalts and basaltic andesites. A combination of petrographic observations and analyses of incompatible trace elements and rare earth elements indicates the presence of at least four different types of extrusive rocks in the Sabzevar ophiolite. The geochemical data clearly identifies some of the extrusive rocks to have formed from three distinct types of basaltic melts; (i) the group-1 basaltic rocks, which formed from an initial melt with N-MORB-like (LREE depleted) chemical signatures and are petrogenetically related to the gabbros, (ii) group-2 basaltic rocks which have E-MORB chemical signatures, and (iii) group-3 basaltic rocks with LREE-enriched signatures and incompatible trace element patterns that suggest an island arc affinity. The data also suggest that the gabbros and the group-1 basaltic rocks are petrogenetically related and produced by fractionation of plagioclase and to a lesser extent clinopyroxene and hornblende. Similar to other studied Iranian ophiolites (e.g. Khoy, Band-e-Zeyarat/Dar Anar, Neyriz, Kermanshah and Baft) the results from the Sabzevar ophiolite demonstrate the presence of relics of mid-ocean ridge basalt (MORB)-like signatures in the extrusive units of Neo-Tethyan ophiolites and suggests that the fossil oceanic lithosphere preserved in these ophiolites developed at an oceanic spreading center. In addition, the Sabzevar ophiolite contains other volcanic rocks that have non-MORB-like geochemical signatures (e.g. within-plate, island arc). The differences between these geochemical signatures could be a result of a differing upper mantle composition, or different degrees of partial melting of the same upper mantle. The presence of sizable Nb anomalies in the extended rare earth element (REE) patterns for andesites and the group-3 basalts is characteristic of volcanic arc magmas, or possibly are products of the ascending magma that was modified by crustal contamination. The presences of the basanites and the silica-undersaturated rocks could be attributed to extrusive activity following ophiolite emplacement. Plate reconstructions suggest that rocks of the Sabzevar ophiolite were part of the Tethyan oceanic crust that was formed during rifting of a narrow but deep seaway that separated the CIM from the Eurasian plate. The ophiolite was emplaced during northeast dipping subduction (i.e. closure) of this segment of the Tethyan seaway (Sabzevar Ocean). [Copyright &y& Elsevier]

Abstract Abundant peridotite xenoliths have been found in pyroclasitics of Avacha (Avachinsky) volcano, the south Kamchatka arc, Russia. They are mostly refractory harzburgite with or without clinopyroxene: the Fo of olivine and Cr/(Cr + Al) atomic ratio of spinel range from 91 to 92 and from 0.5 to 0.7, respectively. They are metasomatized to various extents, and the metasomatic orthopyroxene has been formed at the expense of olivine. The metasomatic orthopyroxene, free of deformation and exsolution, is characterized by low contents of CaO and Cr2 O3 . The complicated way of replacement possibly indicates low viscosity of the metasomatic agent, namely hydrous fluids released from the relatively cool slab beneath the south Kamchatka arc. This is a good contrast to the north Kamchatka arc, where the slab has been hot enough to provide slab-derived melts. High content of total orthopyroxene, 40 vol% on average, in metasomatized harzburgite from Avacha suggests silica enrichment of the mantle wedge, and is equivalent to some subcratonic harzburgite. Some subcratonic harzburgites therefore could have been formed by transportation of subarc metasomatized peridotites to a deeper part of the upper mantle. [ABSTRACT FROM AUTHOR]

The olivine melilitites from the southern part of the 6.8 Ma-old Takarindiona volcanic field (Eastern Madagascar) are olivine ± chromite -phyric lavas, with zoned titanaugite, perovskite, melilite, nepheline, monticellite, Ba–Ti-mica and Fe–Ti oxides as microphenocrysts and groundmass phases. The rocks are very primitive, rich in incompatible trace elements (e.g., Ba = 1049 ± 153 ppm, Sr = 1050 ± 167 ppm, Nb = 98 ± 13 ppm; La/Yb n = 41 ± 5; La/Nb = 0.88 ± 0.05), and have restricted ranges of initial 87Sr/86Sr (0.70391–0.70410) and 143Nd/144Nd (0.51272–0.51282). The rocks follow a differentiation trend controlled by ab. 20% removal/addition of phenocryst olivine ± chromite. The olivine melilititic magmas are the product of small degrees of partial melting (1–3%) of a peridotitic source, enriched in highly incompatible trace elements by CO 2 -, F-, and H 2 O-rich melts, located within the garnet stability field (3–3.5 GPa and ~100 km depth) of sub-continental lithospheric mantle, where carbonates (dolomite) and possibly phlogopite were stable phases. Mantle xenoliths within the volcanics are mostly spinel harzburgites having mineral modes and chemical compositions suggesting variable degrees of " basalt " melt extraction. Based on textural and chemical evidence, and quantitative thermobarometric estimates, the xenoliths were incorporated at a pressure of ~1.1 GPa (~35–40 km depth), far shallower than the source of the melilititic magmas, and along a predictably cool geotherm beneath Archean continental lithosphere. Highly resorbed orthopyroxene xenocrysts mantled by augite indicate that the melilitites may have also entrained lower crustal materials or underplated subalkaline rocks. The mantle sources of the lavas and mantle xenoliths of the Takarindiona district indicate stratification of the lithospheric mantle, and help constraining the lithospheric features and the magmatic history of the Eastern Madagascar craton. • The Takarindiona olivine melilitites are highly primitive. • The sources are lithospheric and rich in olivine and carbonates. • Xenoliths and xenocrysts are lherzolites, harzburgites and reacted orthopyroxene relicts. [ABSTRACT FROM AUTHOR]

The Edessa ophiolite complex of northern Greece consists of remnants of oceanic lithosphere emplaced during the Upper Jurassic-Lower Cretaceous onto the Palaeozoic-Mesozoic continental margin of Eurasia. This study presents new data on mineral compositions of mantle peridotites from this ophiolite, especially serpentinised harzburgite and minor lherzolite. Lherzolite formed by low to moderate degrees of partial melting and subsequent melt-rock reaction in an oceanic spreading setting. On the other hand, refractory harzburgite formed by high degrees of partial melting in a supra-subduction zone (SSZ) setting. These SSZ mantle peridotites contain Cr-rich spinel residual after partial melting of more fertile (abyssal) lherzolite with Al-rich spinel. Chromite with Cr# > 60 in harzburgite resulted from chemical modification of residual Cr-spinel and, along with the presence of euhedral chromite, is indicative of late melt-peridotite interaction in the mantle wedge. Mineral compositions suggest that the Edessa oceanic mantle evolved from a typical mid-ocean ridge (MOR) oceanic basin to the mantle wedge of a SSZ. This scenario explains the higher degrees of partial melting recorded in harzburgite, as well as the overprint of primary mineralogical characteristics in the Edessa peridotites. [ABSTRACT FROM AUTHOR]