Search

Kızıldağ ophiolite (G-Turkey) is known as oceanic lithosphere remnant rocks formed in the southern arm of the Neotethys ocean. The ophiolite sequence, from bottom to top, mantle peridotites, ultramafic-mafic cumulates, isotropic gabbros, sheet dyke complex and pillow-shaped basaltic cover rocks. Mantle peridotites are in harzburgite composition and varying degrees of serpentinization are observed. Whole-rock major oxide-trace element Lanthanide Group Element (LGE) contents of mantle peridotites indicate that harzburgites are residues after partially molten layers of upper mantle. Harzburgites have low whole-rock Al and Ca compositions and reflect the depleted character of SSZ peridotites. Depleted mantle harzburgites have been found to be enriched in some large ionic radius elements (LILE) and light rare earth elements (LREE), indicating the melt-mantle interaction occurring during subduction. When whole rock MgO contents and CaO and Al2O3 contents of harzburgites are compared, it is seen that these rocks are partial melt residue rocks of 20-30% additionally Ti and Yb contents, which are known to behave incompatibly during partial melting, indicate meltrock interaction. [ABSTRACT FROM AUTHOR]

The Late Cretaceous ophiolite mélange in the Salmas area of NW Iran is a part of the Neotethys ophiolites. The mélange includes serpentinized harzburgite, serpentinites, mafic rocks, radiolarite, layered red pelagic limestones and grey and white marbles. Harzburgite main primary mineral phases are olivine, orthopyroxene, clinopyroxene and Cr-spinel. Cr-spinel has Cr2O3 contents of 21.13 to 30.18 wt% and high Al2O3 (38.67–48.52 wt%), FeO (15.18–18.13 wt%) and MgO (15.18–17.51 wt%) contents. The 100 × Cr/(Cr + Al) ratios of 23 to 34 indicate 9 to 13 % partial melting in the Mid Ocean Ridge (MOR) environment for the origin of the peridotites. An alteration zone is developed around the altered Cr-spinel. Fine-grained minerals assemblage at the spinel crystals’ margin includes Cr-rich chlorite, Cr-rich garnet and spinel-silicate mixture. A 2–5 μm wide transitional zone is developed between the chromite-silicate assemblage and the Cr-rich garnet zone. The chemical variations of major oxides across the alteration zone are mainly diffusion controlled. Al, Cr and Mg have diffused out from the primary spinel and Fe and Mn have diffused into the spinel. Cr-spinel is altered in two stages due to serpentinization. During the first stage and following hydration, spinel reacted with olivine and orthopyroxene to form Cr-rich chlorite and ferrian chromite. Silica formed at this stage. At the second stage, the reaction between the chromite-silicate assemblage and Cr-rich chlorite plus silica form the first stage and Ca2+ in the fluid (released from clinopyroxene alteration) produced Cr-rich garnet and H2O-rich fluid, at temperature between 400 and 600 °C.

Ultra-depleted peridotites have been reported from the mantle sections of a number of ophiolitic complexes (e.g., Papua New Guinea and New Caledonia). Such rocks are residues derived from boninitic melts in supra-subduction zones, particularly in forearcs. In the mantle wedge, slab-derived fluid-melt containing H2O increases the degree of melting and metasomatizes the peridotites. However, the relationship between high-degree melting under hydrous conditions and metasomatism is not well understood. In this paper, we report petrological features of amphibole-bearing ultra-depleted peridotites from the Horokanai ophiolite, Hokkaido, Japan. The peridotite complex in the Horokanai ophiolite consists of clinopyroxene-free harzburgite, orthopyroxene-poor harzburgite, and dunite. The harzburgites contain high-Cr# spinel (0.70-0.85) and orthopyroxene with very low abundances of incompatible elements (e.g., Y < 0.01 μg/g; Ti ~ 2 μg/g). Pargasitic-edenitic amphibole occurs as spinel-hosted inclusions and as an interstitial phase to olivine and orthopyroxene. The harzburgite is a residue formed by the high-degree melting caused by fluid flux. The hydrous conditions cause incongruent melting of orthopyroxene, producing a depleted Si-rich melt (i.e., a boninitic melt). Because such Si-rich melt can coexist with Cr-rich spinel under these conditions, amphibole is ultimately crystallized in the harzburgite. Such amphibole formation is important in producing secondary sodic amphibole during alteration of ultra-depleted peridotites. [ABSTRACT FROM AUTHOR]

The ca. 1.95-Ga Jormua Ophiolite Complex (JOC), Finland, is a rare Paleoproterozoic ophiolite that preserves a record of diverse upper mantle materials and melting processes. Meter-scale grid sampling of four JOC outcrops, as well as non-grid samples, permits evaluation of meter- to kilometer-scale mantle heterogeneity within the JOC. Significant heterogeneity is observed between the four grids, and also among a number of the non-grid samples examined. Variations in the concentrations of fluid-mobile elements are particularly large among different samples and locations. New whole-rock major, lithophile trace, and highly siderophile element data (HSE: Os, Ir, Ru, Pt, Pd, Re), including 187Re–187Os isotopic data, for serpentinized harzburgites indicate the presence of two distinct compositional types and probable modes of origin within the JOC. This is consistent with prior findings. Type 1 is similar to modern refractory abyssal-type mantle. Type 2 is more highly refractory than Type 1, and most likely represents samples from sub-continental lithospheric mantle (SCLM). Type 1 mantle is moderately heterogeneous with respect to major and trace element and Os isotopic compositions at both the meter and kilometer scales. By contrast, Type 2 mantle is considerably more homogeneous than Type 1 grids at the meter scale, but is more heterogeneous at the kilometer scale. The median initial γOs value for Type 1 mantle, calculated for 1.95 Ga, is ~ −2.0 (where γOs is the % deviation in 187Os/188Os relative to a chondritic reference calculated for a specified time). This isotopic composition is consistent with a moderate, long-term decrease in Re/Os relative to the estimate for primitive mantle, prior to JOC formation. The similarity in this γOs value to the value for the modern abyssal mantle, as well as the initial values for several Phanerozoic ophiolites, suggests that the upper mantle achieved a Re/Os ratio similar to the chondritic reference by ~2 Ga, then evolved along a subparallel trajectory to the chondritic reference since then. For this to occur, only limited Re could have been permanently removed from the upper mantle since at least the time the JOC formed. A localized secondary metasomatic event at ~2 Ga, concurrent with the estimated obduction age for the JOC and subsequent Svecofennian Orogeny, affected the HSE systematics of some Type 1 samples. By contrast, late Archean Os TRD model ages for Type 2 rocks indicate a depletion event superimposed upon the long-term Re depletion of the abyssal mantle. This event was established no later than ~2.6 Ga and may have occurred during a period of significant, well-documented crustal production in the Karelia craton at ~2.7 Ga. [ABSTRACT FROM AUTHOR]

New petrological and geochemical data for lherzolite, harzburgite, and gabbros in the Darbut ophiolitic mélange of west Junggar are combined to constrain the geological evolution of the Darbut ophiolite. Lherzolite, consisting of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with low Cr# values (34–39), is analogous to fertile abyssal peridotite. Harzburgite, composed of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with relatively high Cr# values (48–55), is similar to the supra–subduction zone (SSZ) peridotite. Isotropic gabbro, characterized by a flat rare earth element (REE) pattern as well as low Nb/Yb and high Ti/V ratios, is comparable to mid–ocean ridge basalt (MORB). Hornblende gabbro, displaying relative enrichments of fluid-soluble elements and elevated Th/Yb ratios, is similar to that of fore–arc basalt. Geochemical modelling of partial melting suggests that lherzolite samples are compatible with their formation after relatively low-degree (11–16%), anhydrous dynamic melting of the primitive mantle, while harzburgite samples have undergone 5–10% secondary-stage partial melting based on the already 16% depleted primitive mantle. These data suggest that the Darbut ophiolite was generated in a forearc setting. The upwelling asthenosphere triggered by the subduction initiation of the Junggar oceanic lithosphere led to low-degree, anhydrous decompression melting, producing lherzolite as well as the MORB–like melts at Late Silurian period. Increasing slab–derived fluids influx, accompanied by the progressively sinking slab, largely enhanced the partial melting degrees of the depleted mantle, and formed refractory harzburgite. [ABSTRACT FROM AUTHOR]

Myanmar has a drier sub-tropical climate than countries that typically contain Ni laterite deposits, but hosts a Ni laterite deposit at Tagaung Taung. Given that Ni enrichment processes in the Tagaung Taung deposit are poorly understood, we investigated the geochemical and mineralogical evolution of two weathering profiles developed on different bedrocks in the central part of Myanmar: a partly serpentinized harzburgite at Tagaung and an almost completely serpentinized peridotite at Budaung. The whole-rock geochemical data indicate that Si was retained relative to Fe and Al in the weathering profiles. Nickel has been enriched to contents as high as 4.89 wt.% NiO in the saprolite layers at Tagaung, whereas the saprolite layers at Budaung contain ≤ 1.55 wt.% NiO. Smectite is the main mineral that formed in the saprolite layers at Tagaung, whereas secondary serpentine dominates the saprolite layers at Budaung. Microscopic observations indicate that Ni-smectite (> 10 wt.% NiO), which is only observed at Tagaung, formed as a replacement product of orthopyroxene. In addition to the high Ni fixation capacity of smectite relative to secondary serpentine, Ni-rich pore water derived from the dissolution of olivine likely contributed to the high Ni contents of smectite. Our results imply that high-grade Ni laterite deposits may develop on unaltered or partly serpentinized harzburgite under the climatic conditions typical of Myanmar. [ABSTRACT FROM AUTHOR]

In the first article, we have reported petrological data for a new, glass-bearing orthopyroxenite vein cutting a sub-arc mantle xenolith from Kamchatka. As similar veins from the West Bismarck arc, this orthopyroxenite is sulfide-rich and formed by cooling of parental melts derived by partial melting of spinel harzburgite sources. Here, I report new data for the abundances of major base metals and chalcophile and highly siderophile trace elements in vein sulfides from the two localities. Kamchatka vein sulfides are all Cu-poor monosulfide solid solution (MSS). West Bismarck veins contain MSS and a ternary (Fe, Cu, Ni)S solid solution (“xSS”), which ranges between MSS and intermediate solid solution (ISS) in composition. Sulfides follow Ni and Cu enrichment trends and have chondrite-normalized platinum-group element (PGE) patterns with elevated Pt relative to Os, Ir, Ru, and Rh. Pt alloys are frequently associated with sulfides and vugs formed from hydrothermal fluids, which also contain metallic Fe and wüstite. Vein sulfides, ranging from Fe-rich MSS (ca. 1,050–1,100°C) to xSS (≤850°C) through Ni-rich MSS, were formed in a sulfide liquid line of descent under oxygen and sulfur fugacity conditions (fO2 and fS2) down to one log unit below the fayalite–magnetite–quartz and close to the Pt-PtS buffers, respectively. The Ni and Cu enrichment trends in MSS are consistent with cooling and fractionation of Ni-rich and Cu-poor sulfide liquids (original atomic ∑metal/S∼0.9), which will finally solidify as xSS or ISS. Chondrite-normalized Pt/Pd>1 in some of the sulfides is a signature of spinel harzburgite sources. Because it occurs at relatively low fS2, the crystallization sequence of these sulfide liquids is accompanied by the formation of abundant PGE alloys and other metallic phases. Melts derived from spinel harzburgite sources can be originally oxidized to carry up to ∼2,600 ppm S (predominantly as S6+) and follow a sulfide-undersaturated evolution trend, until they are rapidly cooled to crystallize as orthopyroxenite dykes or sills. There, S6+-Fe2+ redox reactions with host rocks, together with the production of high-Mg# andesite derivatives with low S solubility and high-temperature, hydrothermal fluids at decreasing fO2 and fS2, will lead to the local precipitation of abundant sulfides and alloys.

The reliability of eight Fe–Mg exchange geothermobarometers for garnet-bearing peridotites, pyroxenites and eclogites has been examined using a database comprised of more than 300 published peridotite, pyroxenite and eclogite experiments conducted from 10 to 70 kbar and 850 to > 1650 ∘ C . We have tested Fe–Mg exchange geothermometers suitable for a range of mantle lithologies, including websterite, harzburgite, wehrlite and eclogite. All geothermometers maintained an average difference in experimental and calculated temperature (T) Δ T = T exp - T calc of less than ± 50 °C with a standard deviation of Δ T between ± 50 to 150 °C. Most geothermometers performed well across a narrow range in ln Kd Fe - Mg A - B (where Kd Fe - Mg A - B = Fe A × Mg B (Fe B × Mg A) ), however, systematic overestimation and underestimation of T were observed outside of the optimal range of lnKd Fe - Mg A - B . Increases in experimental pressure (P) adversely affected several geothermometers, particularly those calibrated empirically using natural samples. All previously published calibrations of the garnet-clinopyroxene geothermometer were unable to reliably reproduce the experimental T for both peridotite and eclogite experimental compositions, which hinders their confident application to natural datasets. To improve the state of mantle geothermobarometry we have used our experimental database to recalibrate the (1) garnet-clinopyroxene Fe–Mg exchange geothermometer, and (2) garnet-orthopyroxene Fe–Mg exchange geothermometer. Each geothermometer has been recalibrated across an extended P, T, and compositional range. The inclusion of eclogitic experiments in the calibration for the garnet-clinopyroxene geothermometer permits application to both eclogitic and peridotitic/pyroxenitic assemblages equilibrated under a wide range of PT conditions in the upper mantle. Using multiple linear regression to solve for lnKd, we found the following expressions best reproduced the experimental T (℃) of our dataset: T Fe - Mg grt - cpx (∘ C) = 3356.34 - 0.008 × P kbar + 0.259 × X Ca grt + 0.914 × X Mg grt + - 0.159 × Jd cpx + ln Kd Fe - Mg grt - cpx + 1.265 - 273 T Fe - Mg grt - opx (∘ C) = 1851.85 - 0.007 × P kbar + - 1.83 × X Ca grt + ln Kd Fe - Mg grt - cpx + 1.08 - 273 . where, X Ca grt = Ca Ca + Fe + Mg , Kd Fe - Mg grt - opx = Fe grt × Mg opx (Fe opx × Mg grt) , X Mg grt = Mg Ca + Fe + Mg , Jd cpx = Na - Cr - 2 × Ti , Kd Fe - Mg grt - cpx = Fe grt × Mg cpx (Fe cpx × Mg grt) , with all elements calculated on the basis of 12 oxygen anions in garnet and 6 oxygen anions in clino- and orthopyroxene. Fe2+ = total Fe. Our updated calibrations resolve several issues with earlier calibrations, including a poor performance at elevated P and compositional limitations. An improvement in precision and accuracy has been demonstrated through application to the experimental calibration dataset, a second independent set of published experimental data, and to natural peridotites, pyroxenites and eclogites from on and off craton settings. Iterative PT estimates on natural datasets calculated using our updated calibrations compare well with estimates from widely used calibrations such as the Taylor (1998) two-pyroxene solvus geothermometer. We anticipate that this contribution will provide an important reference for the reliability of mantle geothermometers and that our updated calibrations will be used in future studies on peridotite, pyroxenite and eclogite inclusions in diamond and mantle-derived xenoliths. [ABSTRACT FROM AUTHOR]

Interaction of groundwater with the serpentinized harzburgites of the upper mantle plays an important role in the geologic carbon cycle and serpentinization processes. In this study, an intact serpentinized harzburgite with more than 60% lizardite from the Semail ophiolite in Oman was reacted with water at near neutral pH and low P CO2 to observe its dissolution behavior and reaction path within the CaO-MgO-SiO 2 -CO 2 -H 2 O system at 20 ° C and 30 bars. In the experiment, mass (i.e. H 2 O, ions) transfer is by diffusion and the selected temperature and pressure conditions mimic water-rock interactions at the shallow portions of most ultramafic rock hosted aquifers. Equilibrium activity-activity diagrams and thermodynamic data were used to compare the experimentally determined reaction path with the theoretical stability boundaries for minerals involved to investigate the evolution of bulk chemical composition, mineralogy, and water composition. The experiments demonstrated that (i) the water-rock interactions increased the pH of the aqueous phase from 5.9 to 7.5 over a period of 18 weeks; (ii) the dissolution of lizardite, orthopyroxene (enstatite), and clinopyroxene (diopside and augite) increased the concentration of Mg, Ca, and Si in the aqueous phase; (iii) the dissolution was incongruent with respect to Mg and Si, favoring Si release at pH > 6 due to (a) breaking of more reactive cation‑oxygen bonds that is consistent with the stoichiometry of magnesium for proton exchange reaction which favors a surface that is enriched in Mg at basic conditions, and (b) preferential dissolution of clinopyroxene Mg 1.30 Ca 0.52 Fe 0.06 Si 2 O 6 ; (iv) the aqueous phase was undersaturated with respect to carbonate (e.g. calcite, magnesite, and hydromagnesite) and hydrous (e.g. lizardite, chrysotile, brucite, and talc) minerals; (v) the bulk chemical composition and mineralogy of the intact serpentinized harzburgite matrix did not change; (vi) the thermodynamic data can successfully predict water-intact serpentinized harzburgite behavior if the water chemical composition can be constrained; and (vii) no detectable macroscopic form of damage (e.g. microcracks forming as a result of differential stress fields due to changes in volume during chemical reaction) was observed. • Water-intact serpentinized harzburgite interactions increased the pH from 5.9 to 7.5. • The concentrations of Ca and Si increased in the aqueous phase. • The intact serpentinized harzburgite dissolved incongruently with respect to Mg and Si. • Thermodynamic data can successfully predict system behavior. [ABSTRACT FROM AUTHOR]

We report abundances of major trace and volatile elements in an orthopyroxenite vein cutting a sub-arc, mantle-derived, spinel harzburgite xenolith from Kamchatka. The orthopyroxenite contains abundant sulfides and is characterized by the presence of glass (formerly melt) both interstitially and as inclusions in minerals, comparable with similar veins from the West Bismarck arc. The glass formed by quenching of residual melts following crystallization of abundant orthopyroxene, amphibole, and minor olivine and spinel. The interstitial glass has a low-Ti, high-Mg# andesite composition, with a wide range of H2O and S contents but more limited F and Cl variations. We calculate trace element partition coefficients using mineral and glass data, including those for halogens in amphibole, which agree with experimental results from the literature. Despite having a similar, high-Mg# andesite composition, the orthopyroxene-hosted glass inclusions usually contain much more H2O and S than the interstitial glass (4–7 wt% and ∼2,600 ppm, respectively). The initial vein-forming melts were oxidized, recording oxygen fugacity conditions up to ∼1.5 log units above the fayalite–magnetite–quartz oxygen buffer. They intruded the sub-arc mantle lithosphere at ≥1,300°C, where they partially crystallized to form high-Mg# andesitic derivative melts at ca. 1,050–1,100°C. Comparison with literature data on glass-free orthopyroxenite veins from Kamchatka and the glass-bearing ones from West Bismarck reveals fundamental similarities indicating common parental melts, which were originally produced by low-degree melting (≤5%) of spinel harzburgite at ≥1,360°C and ≤1.5 GPa. This harzburgite source likely contained ≤0.05 wt% H2O and a few ppm of halogens. Volatile evolution inferred from glass compositions shows that (i) redox exchange between S6+ in the original melt and Fe2+ in the host mantle minerals, together with (ii) the formation of an S-bearing, (H2O, Cl)-rich hydrothermal fluid from the original melt, provides the conditions for the formation of abundant sulfides in the orthopyroxenites during cooling. During this process, up to 85% of the original melt S content (∼2,600 ppm) is locally precipitated as magmatic and hydrothermal sulfides. As such, melts derived from spinel harzburgite sources can concentrate chalcophile and highly siderophile metals in orthopyroxenite dykes and sills in the lithosphere.

Accessory zircon extracted from serpentinized dunite and harzburgite of the Tekturmas Zone of Central Kazakhstan, which is one of the largest ophiolite zones in the western part of the Central Asian Belt, has been studied for the first time. A U–Th–Pb (SIMS) geochronological study of this zircon was carried out and the age of its crystallization was estimated at 467 ± 3 Ma, which corresponds to the boundary between the Dapingian and Darriwilian stages of the Middle Ordovician. The presence of accessory Middle Ordovician zircon in dunite and harzburgite is most likely a reflection of the last stage of ultramafic depletion associated with melting of mafic rocks that form the upper parts of the ophiolite section in the northern part of the Tekturmas Zone. [ABSTRACT FROM AUTHOR]

Thermal diffusivity (D) and thermal conductivity (κ) of harzburgite and dunite from Luobusha ophiolite were simultaneously measured up to 3 ​GPa and 823 ​K using the transient plane-source method in a multi anvil apparatus. The results show that the values of D and κ of both samples systematically decrease with increasing temperature and increase with increasing pressure. By combination of the thermal physical data of rocks and minerals and geophysical constraints, we performed numerical simulation on the thermal evolution of Tibet vary over depth, distance and geologic ages. The present results provide new constraints on occurrence of partial melting and its geophysical significance beneath Tibetan crust. Image 1 • Thermal diffusivity and conductivity of harzburgite and dunite were measured at 300–823 ​K and 0.8–3 ​GPa. • D and κ systematically decrease with increasing T and increase with increasing P. • Numerical simulations provide new constraints on occurrence of partial melting beneath Tibetan crust. [ABSTRACT FROM AUTHOR]

An abundance of the trace elements has been determined in olivine of the Achaean spinel-bearing phlogopite harzburgite enclave (sample UR17/2) from the Bug granulite complex of the Ukrainian Shield by SIMS analysis. Major elements were analyzed in the olivine by SEM-EDS and EPMA analysis. The olivine shows a homogeneous chemical composition: MgO — 45.20- 45.64 wt.%, FeO — 13.66-14.23 wt.%, with Fo85-86 and Fe/Mn ratio of 68.3-68.6 which corresponds to the ratio of 60-70 in olivine of peridotites. Ni content in the UR17/2 olivine ranges from 4730 to 5612 ppm, which is higher than in olivine from mantle peridotites, high magnesium OIB and Hawaiian picrites. The olivine has average content of Ti — 20.6 ppm, Nb — 0.03 ppm, Zr — 0.32-0.60 ppm, the low total REE (0.1-0.5 ppm). The olivine crystallization temperature, calculated using an Al-in-olivine thermometer, corresponds to ~900 °C. Enrichment in Fe and Ni is the main feature that distinguishes the UR17/2 olivine from other mantle-derived olivine. Composition of the UR17/2 olivine and host harzburgite indicates an influence of T, P, the oxygen fugacity and composition of parent magma on the partition coefficients of Fe and Ni. Composition and structure (degree of polymerization) of magma are the main factors responsible for the olivine enrichment by iron and nickel. [ABSTRACT FROM AUTHOR]

Reactions between a boninitic or basaltic magma and harzburgite at shallow mantle depths are thought to be closely related to the formation of podiform chromitites, but little experimental data is available on these reactions. In this study, a series of experiments were conducted at 1.5 GPa and 1 000–1 400 °C to investigate the interactions between boninitic magma and harzburgite in homogenous mixed systems with varied bulk concentrations of water (∼0.7 wt.%–10 wt.%) and Cr2O3 (∼0.2 wt.%–4 wt.%). In the experimental charges, chromite grains can be observed coexisting with orthopyroxene, clinopyroxene±olivine, and quenched melt in the Cr-bearing systems. The bulk concentration of Cr2O3 in the starting material has a slight effect on compositional changes in the chromites generated. However, the Cr# (Cr#=100×Cr/(Cr+Al)) and Mg# (Mg#=100×Mg/(Mg+Fe)) values for the chromites exhibit positive and negative correlations, respectively, with the bulk H2O concentrations. At 1 100 °C, chromite Cr# values range from ∼33–35 to ∼58–65, and chromite Mg# values range from ∼70–73 to ∼55–58 when bulk H2O contents in the starting material are increased from ∼0.7 wt.% to ∼10 wt.%. The experimentally produced chromites have compositions (as expressed by Cr#, Mg#, and NiO and MnO contents) similar to natural chromites from low-Cr# chromitite bodies. We suggest that the interactions between boninitic magmas with varied H2O contents and harzburgite in a shallow mantle wedge could be a possible mechanism that forms the low-Cr# chromitites found in ophiolites. We emphasize here that H2O may play an important role in the compositional evolutions of natural chromitites. [ABSTRACT FROM AUTHOR]

Cratonic lithospheric mantle is believed to have been formed in the Archean, but kimberlite-hosted coarse peridotites from Udachnaya in the central Siberian craton typically yield Paleoproterozoic Re-depletion Os isotope ages (T RD). By comparison, olivine megacrysts from Udachnaya, sometimes called "megacrystalline peridotites", often yield Archean T RD ages, but the nature of these rare materials remains enigmatic. We provide whole-rock (WR) Re-Os isotope and PGE analyses for 24 olivine-rich xenoliths from Udachnaya as well as modal and petrographic data, WR and mineral major and trace element compositions. The samples were selected based on (a) high olivine abundances in hand specimens and (b) sufficient freshness and size to yield representative WR powders. They comprise medium- to coarse-grained (olivine < 1 cm) dunites, a megacrystalline (olivine > 1 cm) dunite, olivine megacrysts and low-orthopyroxene (11–21% opx) harzburgites equilibrated at 783–1154 °C and 3.9–6.5 GPa; coarse dunites have not been previously reported from Udachnaya; two xenoliths contain ilmenite. The harzburgites and dunites have similar WR variation ranges of Ca, Al, Fe, Cr and Mg# (0.917–0.934) typical of refractory cratonic peridotites, but the dunites tend to have higher MgO, NiO and Mg/Si. Mineral abundances and those of Ca and Al are not correlated with Mg# WR ; they are not due to differences in melting degrees but are linked to metasomatism. Several samples with high 187Re/188Os show a positive linear correlation with 187Os/188Os with an apparent age of 0.37 Ga, same as eruption age of host kimberlite. Robust T RD ages were obtained for 16 xenoliths with low 187Re/188Os (0.02–0.13). T RD ages for low-opx harzburgites (1.9–2.1 Ga; average 2.0 ± 0.1 Ga, 1 σ) are manifestly lower than for dunites and megacrysts (2.4–3.1 Ga); the latter define two subsets with average T RD of 2.6 ± 0.1 Ga and 3.0 ± 0.1 Ga, and T MA of 3.0 ± 0.2 Ga and 3.3 ± 0.1 Ga, respectively. Differences in olivine grain size (coarse vs. megacrystalline) are not related to age. The age relations suggest that the dunites and megacrysts could not be produced by re-melting of harzburgites, e.g. in arc settings, nor be melt channel materials in harzburgites. Instead, they are relict fragments of lithospheric mantle formed in the Archean (likely in two events at or after 2.6 Ga and 3.0 Ga) that were incorporated into cratonic lithosphere during the final assembly of the Siberian craton in the Paleoproterozoic. A multi-stage formation of the Siberian lithospheric mantle is consistent with crustal basement ages from U-Pb dating of zircons from crustal xenoliths at Udachnaya and detrital zircons from the northern Siberian craton (1.8–2.0, 2.4–2.8 and 3.0–3.4 Ga). The new data from the Siberian and other cratons suggest that the formation of strongly melt-depleted cratonic lithosphere (e.g. Mg# ≥0.92) did not stop at the Archean-Proterozoic boundary as is commonly thought, but continued in the Paleoproterozoic. The same may be valid for the transition from the 'Archean' (4–2.5 Ga) to modern tectonic regimes. [ABSTRACT FROM AUTHOR]

Interaction between eclogite-derived melt and peridotite has played an important role in modifying the cratonic lithospheric mantle and generation of high-Mg igneous rocks. To better understand the effect of peridotite physical state on melt-peridotite interaction, we conducted reaction experiments of lherzolite with two basaltic andesites and a ferro-basalt at temperatures of 1300 °C, 1375 °C, and 1425 °C and a pressure of 2 GPa using the reaction couple method. At 1300 °C, when lherzolite is subsolidus, its dissolution rate is slow, and it is mineralogically and texturally unchanged. Garnet and clinopyroxene precipitate at the melt-rock interface. The low-temperature reaction enriches the melt with SiO 2 and Na 2 O, and depletes the melt with Al 2 O 3 , FeO, and CaO. At 1375 °C and 1425 °C, when the lherzolite is partially molten, the dissolution and re-equilibration rates are fast due to grain-scale process that involves dissolution, precipitation, and reprecipitation. Dissolution of olivine and precipitation of orthopyroxene produce a melt-bearing orthopyroxene-rich lithology at the melt-rock interface followed by a melt-bearing harzburgite and a melt-bearing lherzolite. The lithology near the interface can be an orthopyroxene-rich harzburgite or an orthopyroxenite depending on the reacting melt composition. The high-temperature reaction produces melt with increased MgO, FeO, CaO, and Mg#, and decreased SiO 2 , Al 2 O 3 , and Na 2 O. The dissolution rate obtained from the experiments is used to assess the survival of mantle xenoliths in the early Cretaceous high-Mg diorites. Combined with field observations from mantle samples, the experimental results provide insight into the style of eclogite-derived melt and peridotite interaction in the lithospheric mantle beneath the North China Craton. At the lithosphere-asthenosphere boundary, the reaction is dominated by the high-temperature regime, which produces orthopyroxene-rich lithologies such as orthopyroxene-rich harzburgite and orthopyroxenite-veined peridotite. Above the lithosphere-asthenosphere boundary where peridotite is subsolidus, the reaction is characterized by the low-temperature regime, which produces garnet-bearing or garnet-rich lithologies such as garnet pyroxenite, garnetite, and high-Mg granulite. Interactions between eclogite-derived melts and peridotite in the two regimes are responsible for the geochemical features of the early Cretaceous high-Mg igneous rocks from the North China Craton. [ABSTRACT FROM AUTHOR]

Introduction Khoy ophiolite at the global scale is in the middle part of the Alp-Himalaya orogenic belt and it is extended over 3900 Km2 which indicates remnant Neotethys oceanic lithosphere in the Mesozoic era (Kananian et al., 2010). In this paper, in addition to a review of previous investigations about Khoy ophiolite, we will try to determine the nature and kind of minerals, origin and partial melting rate as well as the equilibrium pressure and temperature of harzburgites from the Eastern Metamorphic Complex of Khoy ophiolite. Materials and methods Thin sections microscopy studies were carried out following field investigations. EPMA analysis was carried out with using a Superprobe JEOL, JXA 8200 Microprobe unit at the state of WDS and under condition of 15kv accelerating voltage, 10nA current beam, 1µm beam diameter and collection of natural and synthetic standards for calibration. Results The study area is located at the NW of Iran and north of the Khoy city in the west Azarbaijan province. This area is part of the ophiolitic complex of NW Iran and belongs to its Eastern Metamorphic Complex. This metamorphic zone has large tectonically segments of the metamorphic ophiolites which mainly include serpentinized peridotites with associated metagabbros. There are three types of peridotitic rocks in this area which are: Lherzolites, harzburgites and dunites. Lherzolites are composed of olivine (60-70%), orthopyroxene (10-30%) and clinopyroxene (~10-20%) with minor amounts (~2%) of Cr-spinel mineral. Harzburgites are composed of olivine (70-80%), orthopyroxene (10-20%) and clinopyroxene (~5%) with minor amounts (~2%) of Cr-spinel mineral. Dunites are composed of olivine (90-95%), orthopyroxene (5-10%) with minor amounts (~1-2%) of Cr-spinel mineral. Composition range of olivines is between Fo89.46 Fa10.37 to Fo89.86 Fa10.0 as well as NiO content range is 018-046 (wt %). The calculated Mg# of olivines is 0.90 and the composition of olivines in Fo-Fa diagram is plotted in forsterite field. The end-members composition of clinopyroxenes is between En44.159 Wo46.910 Fs4.323 Ac1.459 to En46.803 Wo49.589 Fs4.786 Ac2.081. The calculated Mg# of clinopyroxenes is 0.91. The composition of clinopyroxenes in En-Wo-Fs (Morimoto et al., 1988) diagram is plotted on a diopside field. The plot of clinopyroxenes on Na versus Cr diagram (Kornprobst et al., 1981) indicates that the studied harzburgits are formed in the oceanic setting. Moreover, the Mg# versus Al2O3 in the clinopyroxenes is an indication of their relation to the Abyssal peridotites (Johnson et al., 1990). The end-members composition of orthopyroxenes is between En86.022 Wo2.491 Fs9.368 to En87.314 Wo6.719 Fs10.474. The calculated Mg# of orthopyroxenes is 0.90. The composition of orthopyroxenes in En-Wo-Fs diagram is plotted on an enstatite field. The plot of orthopyroxenes on Mg# versus Al2O3 diagram indicates their relation to Abyssal peridotites (Johnson et al., 1990). Compositions of spinels in the studied harzburgites indicates that they are high-Al type with Mg# and Cr# 0.67-0.72 and 0.19-0.26, respectively. Their TiO2 content are 0.01-0.11(Wt %) and Al2O3 content are 44.009-49.894 (Wt %). In Cr# versus Mg# diagram, spinels are plotted on the Abyssal peridotites field and indicate that the host peridotite has experienced 10-15% partial melting. In addition, using the equation F%=10Ln (Cr/Cr+Al) spinel+24) (Hellebrand et al., 2001) 7.6-10.4 (average 9.7%), the partial melting degree was obtained that means a slow spreading rate for the study area. Using the Cr# versus Mg# diagram, alpine I-type, abyssal and back arc setting for the studied harzburgites are determined. PTMAFIC Software (Soto and Soto, 1995) is used for orthopyroxene (single pyroxene), clinopyroxene-olivine, and orthopyroxene-clinoproxene thermometry. Based on these thermometers, overall temperatures of 1100 ±100 °C are estimated for the equilibrium stage of the minerals. For pressure estimation, the Cr content in clinopyroxene (Nimis and Taylor, 2000) barometer is used. Using this calibration and temperatures from single pyroxene thermometer a pressure of ~22±2.4 Kbar for equilibrium clinopyroxene and associated minerals in the studied harzburgites was estimated. Discussion Mineral chemistry studies indicate that these harzburgites may be related to oceanic settings. Moreover, the high Mg# in orthopyroxenes and clinopyroxenes and the high Fo% in olivines are indications of their tectonite origin. Calculation of partial melting degree using spinels compositions indicate that they are experienced 7.6-10.4 partial melting. In this regard, they are consistent with the partial melting degree in the Atlantic and Indian oceans. The spreading rate studies indicate that harzburgites are produced in the region with slow spreading rate. Their tectonic setting is more consistent with MOR peridotites. Based on geothemobarometry studies an overall ~1100±100°C temperature and ~22±2.4Kbar pressure are estimated which are consistent with mantle spinel lherzolite facies. Acknowledgements The author would like to thank the reviewers for the constructive comments which greatly contributed to the improvement of the manuscript. References Hellebrand, E., Snow, J.E., Dick, H.J.B., and Hofmann, A.W., 2001. Coupled major and trace elements as indicators of the extent of melting in mid-ocean ridge peridotites. Nature, 410(6829): 677–681. Johnson, K.T.M., Dick, H.J.B., and Shimizu, N., 1990. Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites. Journal of Geophysical Research, 95(B3): 2661-2678. Kananian, A., Ataei, M., Mirmohammadi, M., and Emamalipour, A., 2010. Petrography, mineral chemistry and genesis of Aland and Gheshlagh Chromite deposits, Khoy ophiolite (NW of Iran). Iranian Journal of Crystallography and Mineralogy, 18(3): 369-380. (in Persian with English abstract) Kornprobst, J., Ohnenstetter, D., Ohnenstetter, M., Ohnenstetter, M., 1981. Na and Cr contents in Cpx from peridotites: a possible discriminant between subcontinental and sub-oceanic mantle. Earth and Planetary Science Letters, 53(2): 241-254. Morimoto, N., Fabries, J., Ferguson, A.K., Ginzburg, I.V., Ross, M., Seifert, F.A., Zussman, J., Aoki, K., and Gottardi, D., 1988. Nomenclature of pyroxenes. American Mineralogist, 73(1-2): 1123-1133. Nimis, P., and Taylor, W.R., 2000. Single clinopyroxene thermobarometry for garnet peridotites. Part I. Calibration and testing of Cr-in-Cpx barometer and an enstatite-Cpx thermometer. Contributions to Mineralogy and Petrology, 139(5): 541–554. Soto, J.I., and Soto, V.M., 1995. PTMAFIC: software package for thermometry, barometry and activity calculations in mafic rocks using IBM-compatible computer. Computers and Geosciences, 21(5): 619-652.

This paper reports detailed studies on harzburgite and serpentinite in the Hegenshan ophiolitic mélange. Harzburgite consists mainly of olivine and orthopyroxene with trace amounts of clinopyroxene and chromian spinel. Clinopyroxene occurs as isolated crystals or in the intergrowth of chromian spinel–clinopyroxene–orthopyroxene. Harzburgite is moderately to highly depleted, displaying high Fo contents in olivine (90.8–92.2), moderate Al 2 O 3 contents in orthopyroxene (1.59–2.79 wt%), low heavy REE abundances in clinopyroxene, and moderate Cr# values of spinel (0.50–0.62). The modal proportions of olivine and orthopyroxene pseudomorph grains imply that the parent of the Hegenshan serpentinite should be harzburgite. Whole-rock compositions of the harzburgite and serpentinite samples are characterized by depletions in Al 2 O 3 and CaO and enrichments in light REE, Sr, and U. Geochemical modeling suggests that the Hegenshan harzburgite represents residues after 17–18% partial melting of the primitive mantle. The melt in equilibrium with clinopyroxene is more depleted than typical forearc basalt and boninite. Various pyroxene thermobarometers yield equilibrated temperatures of 945–1067 °C and pressures of 4.8–8.0 kbar for the Hegenshan harzburgite. The oxygen barometer yields results of +0.4 to +1.7 log units above the fayalite–magnetite–quartz buffer for the Hegenshan harzburgite. These petrological and geochemical characteristics, as well as the estimated P–T– f O 2 conditions support a back-arc setting for the Hegenshan ophiolitic mélange. Unlabelled Image • clinopyroxene and rock compositions indicate 17–18% partial melting of primitive mantle for the Hegenshan harzburgite in a back-arc basin. • Mineral assemblage in harzburgite was equilibrated at conditions with f O 2 values of ΔFMQ+0.4 to ΔFMQ+1.7. [ABSTRACT FROM AUTHOR]

Camarioca Sur is a sector in Moa-Baracoa ophiolitic complex composed of pyroxene ultramaphites, gneissic amphiboles, chromium-spinels and amphibolitic granofels, in which the presence of compact and hard rock dykes has been documented, white to dark gray color and poor weathering crust development. The aim of this research is to characterize the associated leucocratic dykes and the ultramafic lithological association of Moa-Baracoa ophiolitic complex in Camarioca Sur sector by using petrological and geochemical techniques. It was found harzburgite dunites are the predominant rocks. Peridotites are mainly composed of olivine, with the presence of chromium-spinels and to lesser extent pyroxenes. The rocks are partial or totally transformed into minerals of serpentine group (antigorite, lizardite and chrysotile). Some peridotites are metamorphosed, developing talcum + tremolite + chlorite. The leucocratic rocks, which cut through the peridotite dikes, consist mainly of plagioclase (oligoclase-andesine), quartz and actinolite, and apatite, Muscovite and rutile as accessories. These dikes show a slight metamorphism, developing a foliation and the formation of albite + clinozoisite + kaolinite, which denotes conditions of metamorphism of degree from low to medium. [ABSTRACT FROM AUTHOR]

Abstract The most outstanding features of Archaean cratons are their extraordinary thickness and enduring longevity. Seismically, Archaean cratonic fragments are sharply-bounded deep roots of mechanically strong, cold lithospheric mantle, clearly distinguishable from non-cratonic lithosphere. Rhenium-depletion of deep cratonic xenolith whole rocks and sulphide inclusions in diamond indicate that melting was broadly coeval with formation of the overlying proto-cratonic crust, which was of limited mechanical strength. A very important process of proto-cratonic development was vertical crustal reorganisation that eventually yielded a thermally stable, cratonised crust with a highly K-U-Th-rich uppermost crust and much more depleted deeper crust. Clastic sedimentary rocks available for geochemical study are predominantly found in the youngest parts of supracrustal stratigraphies and over-represent the highly evolved rocks that appeared during cratonisation. Vertical crustal reorganisation was driven by crustal radiogenic heat and emplacement of proto-craton-wide, incubating and dense supracrustal mafic and ultramafic volcanic rocks. Statistical analysis of these cover sequences shows a preponderance of basalt and a high abundance of ultramafic lavas with a dearth of picrite. The ultramafic lavas can be grouped into Ti-enriched and Ti-depleted types and high pressure and temperature experimental data indicate that the latter formed from previously depleted mantle at temperatures in excess of 1700 °C. Most mantle harzburgite xenoliths from cratonic roots are highly refractory, containing very magnesian olivine and many have a high modal abundance of orthopyroxene. High orthopyroxene mode is commonly attributed to metasomatic silica-enrichment or a non-pyrolitic mantle source but much of the excess silica requirement disappears if melting occurred at high pressures of 4–6 GPa. Analysis of experimental data demonstrates that melting of previously depleted harzburgite can yield liquids with highly variable Si/Mg ratios and low Al 2 O 3 and FeO contents, as found in komatiites, and complementary high Cr/Al residues. In many harzburgites, there is an intimate spatial association of garnet and spinel with orthopyroxene, which indicates formation of the Al-phase by exsolution upon cooling and decompression. New and published rare earth element (REE) data for garnet and orthopyroxene show that garnet has inherited its sinusoidal REE pattern from the orthopyroxene. The lack of middle-REE depletion in these refractory residues is consistent with the lack of middle- over heavy-REE fractionation in most komatiites. This suggests that such pyroxene or garnet (or precursor phases) were present during komatiite melting. In the Kaapvaal craton, garnet exsolution upon significant cooling occurred as early as 3.2 Ga and geobarometry of diamond inclusions from ancient kimberlites also supports cool Archaean cratonic geotherms. This requires that some mantle roots have extended to 300 to possibly 400 km and that early cratons must have been much larger than 500 km in diameter. We maintain that the Archaean-Proterozoic boundary continues to be of geological significance, despite the recognition that upper crustal chemistry, as sampled by sedimentary rocks, became more evolved from ca. 3 Ga onwards. The boundary coincides with the disappearance of widespread komatiite and marks the end of formation of typical refractory cratonic lithosphere. This may signify a fundamental change in the thermal structure of the mantle after which upwellings no longer resulted in very high temperature perturbations. One school of thought is that the thermal re-ordering occurred at the core-mantle boundary whereas others envisage Archaean plumes to have originated at the base of the upper mantle. Here we speculate that Archaean cratonic roots may contain remnants of older domains of non-convecting mantle. These domains are potential carriers of isotope anomalies and their base could have constituted a mechanical and thermal boundary layer. Above laterally extensive barriers, emerging proto-cratons were protected from the main mantle heat loss. The eventual collapse of these mechanical barriers terminated very high temperature upwellings and dismembered portions of the barrier were incorporated into the cratonic mantle during the final Neoarchaean 'superplume' event. The surviving cratons may therefore preserve biased evidence of geological processes that operated during the Archaean. [ABSTRACT FROM AUTHOR]

We determined phase relations of harzburgite and basalt (mid‐ocean ridge basalt, MORB) at 12–28 GPa and 1600–2200 °C with a large number of experiments using a multianvil high‐pressure apparatus. These phase relations were precisely compared with those of pyrolite simultaneously determined by multisample cell technique. The post‐spinel (pSp) transition of harzburgite occurs at 23 GPa and 1600 °C with the boundary slope of −3 ± 1 MPa/°C. The post‐garnet (pGt) transition boundary of MORB, defined as the beginning of the majorite garnet‐bridgmanite transition, is located at 25 GPa and 1600 °C, with a slope of 1.5 ± 1 MPa/°C. The pSp transition of harzburgite occurs at higher pressure by 0.5 GPa at 1600 °C and has the more negative slope than that of pyrolite (−1 ± 1 MPa/°C). The pGt transition of MORB occurs at higher pressure by 3 GPa than the pSp transition of harzburgite. At 1600 °C, the density of pyrolite is smaller than those of harzburgite and MORB before the pSp transition of pyrolite (pyrolite–harzburgite/MORB = −0.17 g/cm3). After the pSp transition of pyrolite, harzburgite has lower densities than that of pyrolite at 22–27 GPa (pyrolite–harzburgite = 0.31 and 0.08 g/cm3 before/after the pSp transition in harzburgite, respectively). On the other hand, the density of MORB becomes the highest among the three rocks above 25 GPa after the pGt transition of MORB (pyrolite–MORB = −0.18 g/cm3). The present density contrasts suggest that harzburgite and MORB may stagnate and accumulate in the transition zone by slab subduction, resulting in that the pyrolitic part of slabs is subducted into the lower mantle. Key Points: The post‐spinel transition boundary of harzburgite is located at higher pressure and has a more negative slope than that of pyroliteThe post‐garnet transition of mid‐ocean ridge basalt occurs at higher pressure than the post‐spinel transitions of harzburgite and pyroliteHarzburgite is less dense than pyrolite at 1000 to 1600 °C at depth >660 km, suggesting accumulation of harzburgite in the transition zone [ABSTRACT FROM AUTHOR]

Abstract Most of Hyblean serpentinized peridotite xenoliths (south-eastern Sicily, Italy) are characterized by the abundance of some incompatible trace elements, particularly Sr, whose origin is still debated. A cryptic metasomatic process is traditionally invoked, although there is also mineralogical evidence for ancient hydrothermal alteration. In this work, two selected peridotite xenoliths were investigated by combining scanning electron microscopy (SEM), electron probe microanalyses and vibrational (Infrared and Raman) spectroscopy. The results, based on both new and previously published data, confirm that the Sr contents in Hyblean partially serpentinized peridotite xenoliths correlate linearly with the loss on ignition (LOI, wt%), indicating that alteration strongly influences whole-rock compositions of these rocks. The Sr abundance is related mainly to the presence of aragonite veins (SrO = 0.65–5.10 wt%), substituting serpentine veins. A tentative timing of alteration of Hyblean deep-seated xenoliths, based on published data on hydrothermal zircons and the Sm Nd isotope compositions of some peridotite xenoliths, suggests a Permo-Triassic time range, in agreement with palaeo-geographical reconstructions suggesting that Sicily and much of the present Mediterranean are presently rooted in a preserved in-situ remnant of the (Early Devonian) Palaeo-Tethys realm. Highlights • Hyblean serpentinized peridotite xenoliths are commonly crosscut by carbonate veins. • Vibrational (Infrared and Raman) spectroscopy reveals that the carbonate phase is aragonite. • Aragonite veins contain significant Sr amounts (1.5 ± 0.8 wt%, on average). • Whole-rock Sr contents correlate linearly with the degree of alteration. • Seawater-related carbonate metasomatism is dominant in Hyblean peridotites. [ABSTRACT FROM AUTHOR]

The research on petrochemistry and geochemistry of rare, rare earth and platinum group elements in the rocks Shaman massif - the largest ultramafic protrusion body in eastern branch of the Baikal-Muya ophiolite belt, performed using a different of analytical techniques (X-ray fluorescence analysis, mass spectrometry analysis with inductively coupled plasma and laser ablation; synchrotron radiation method). Harzburgites and subordinate dunites, composing massif, depleted TiO2, Na2O, K2O and CaO, and rare earth elements (REE). The total content of the latter in harzburgites somewhat higher (0.29-3.01 ppm) than in dunites (0.12-1.54 ppm). The patterns of chondrite-normalized REE contents in ultramafic rocks are arched downward configuration, due to their abnormal enrichment nonstructural admixture of light elements, centered as it is assumed in the intergranular and within granular microcracks. Massif is characterized by higher contents of Zr and Hf, the hub of which is supposed to be an accessory zircon. The total content of platinum group elements (PGE) in the harzburgite and dunite are from 12.50 to 24.35 ppb, while the predominant element is Ru. There was an inverse relationship between its content and the content of heavy rare earth elements (Er, Tm, Yb, Lu), and between the total content of PGE and REE, which are supposed to be due to divers directional fractionation of these groups of elements with contrasting properties in the process of partial melting of upper mantle protolithe. Prevalence of Ru in the rocks suggested that PGE concentrated mainly in the such minerals as rutenosmirid and laurite.

We describe an unusual pattern of spheroidally weathered surface developed in a mineralized harzburgite of Mount Kumuzhya, Monchepluton layered complex of Early Proterozoic age, Kola Peninsula, Russia. This rock is composed of a matrix and spheroids, which differ in their paragenesis and textures. The relief spheroids consist principally of large oikocrysts (up to 3-4 cm) of orthopyroxene (En86), enclosing aggregates of finegrained chromian spinel (chromite-magnesiochromite). The matrix is mostly composed of olivine (Fo90.5), surrounded by patches of chromite. We infer that this pattern represents a new variety of spheroidal weathering of rock surface, which is not related to “normal” spheroidal weathering. Presumably, it has resulted mainly from the primary magmatic characteristics, whereas other factors, such as differential rates of weathering of olivine and orthopyroxene, were of secondary importance.

Abstract: This paper reports the results of high-pressure experimental modeling of interaction between glaucophane schist and harzburgite or websterite for the evaluation of the influence of mantle material on the input-output of components and character of metasomatic transformations at the crust-mantle boundary in the subduction zone. In all experiments, glaucophane schist (proxy for oceanic crust) containing volatile components (H2O and CO2) incorporated in hydrous minerals (amphiboles, phengite, and epidote) and calcite was loaded into the bottom of each capsule and overlain by mantle material. During the experiments at a temperature of 800°C and a pressure of 2.9 GPa, which correspond to the conditions of a hot subduction zone, the schist underwent partial (up to 10%) eclogitization with the formation of the anhydrous assemblage omphacite + garnet + quartz ± magnesite ± potassic phase. Carbonate and a potassic phase were formed only in the experiments with websterite in the upper layer. A reaction zone was formed at the base of the websterite layer, where newly formed omphacite, quartz, and orthopyroxene replaced in part initial pyroxenes. Orthopyroxene and phlogopite (or an unidentified potassic phase) were formed in the reaction zone at the base of the harzburgite layer; among the initial minerals, only orthopyroxene relicts were preserved. Above the reaction zones produced by diffusion metasomatism, new phases developed locally, mainly at grain boundaries: newly formed orthopyroxene and magnesite were observed in harzburgite, and omphacite and quartz, in websterite. Alterations along grain boundaries extended much further than the reaction zones, which indicates that fluid infiltration dominated over diffusion in the experiments. The experiments demonstrated that the H2O-CO2 fluid with dissolved major components released from the glaucophane schist can produce mineral assemblages of different chemical compositions in mantle materials: Na-bearing in websterite and K-bearing in harzburgite. The complementary components, K2O and CO2 for the websterite layer and Na2O for the harzburgite layer, are fixed in the initial glaucophane schist layer. The distinguished separation of alkalis and CO2 at the crust-mantle boundary can affect the character of metasomatism in the mantle wedge, primary magma compositions, and the chemical evolution of the rocks of the subducting slab. [ABSTRACT FROM AUTHOR]

Mineral carbonation has been suggested as a safe carbon capture and storage (CCS) strategy for the mitigation of climate change. This study investigates the effect of ball milling on the CO 2 sequestration capacity of peridotites, which are among the most promising readily available lithologies for CCS on the Earth’s surface. A partially serpentinized harzburgite from the Troodos ophiolite (Cyprus) was subjected to different degrees of ball milling to produce nanoscale ultramafic powders with enhanced CO 2 uptake. The optimum ball milling conditions were found (16 h of wet ball milling with 50 wt% ethanol as process control agent) through CO 2 chemisorption followed by temperature-programmed desorption (CO 2 -TPD) experiments. The results clearly showed that the CO 2 uptake of the initial rock material was increased by a factor of 4.5 after ball milling. Detailed characterization of the unmilled and ball-milled samples indicated that this enhancement is mainly attributed to (i) the reduction of particle size down to the nanoscale range and (ii) the structural disordering of the constituent Mg-silicate minerals due to mechanical deformation. [ABSTRACT FROM AUTHOR]

Magnetite bodies of the Late Cretaceous Nain ophiolite mélange (Central Iran) are hosted by a small volume of a serpentinized peridotite nappe. These ore bodies and their host peridotites have been studied in detail with respect to their mode of occurrence, petrography and mineral chemistry. The investigated ore deposits consist of m-scale lenses and pods of massive magnetite rocks exposed along a semi-brittle shear zone between pervasively serpentinized harzburgites, upper one with higher-Cr# [=Cr/(Cr + Al), 0.6–0.7] spinel and lower one with lower Cr# (0.5). Silicate mesostasis of the ore bodies is composed of stringy serpentine and chlorite. Cr-spinel grains are occasionally found dispersed within the magnetite rocks. Serpentines of the host serpentinized harzburgites show mesh and bastite textures, and were in part replaced by fibrous serpentine showing an interpenetrating texture. The Cr-spinels show a wide range of Cr# (0.5–0.7) and Mg# [=Mg/(Mg + Fe 2+ ), 0.5–0.7], low TiO 2 contents (<0.1 wt%), and relatively high contents of ZnO (0.11–0.26 wt%) and MnO (0.33–0.43 wt%). The magnetites are close in chemistry to the end-member Fe 3 O 4 , but some grains show high-SiO 2 (up to 1.5 wt%) cores. They strongly resemble accessory Cr-spinels in the host serpentinized harzburgites in chemistry. The magnetite rocks have total precious-metal abundances (ΣPGE + Au = 66–262 ppb) analogous to those of mantle peridotites from other Mesozoic ophiolites. They are poor in PGE (21–67 ppb) relative to Au (45–195 ppb). Our analyses and observations are inconsistent with an igneous origin for the Nain magnetite deposits, but indicate a replacement origin from harzburgite, not from chromitite, via hydrothermal fluids. Overall data indicate that the Nain magnetite bodies were generated by multi-episodic serpentinization of harzburgites. The magnetite ore deformation style itself indicates syn-kinematic emplacement of the ore bodies within the semi-brittle shear zone, where the serpentinization-derived fluid/(altered) peridotite ratio became so high that eventually caused precipitation of magnetite at the ore level. [ABSTRACT FROM AUTHOR]

Platinum-group elements (PGEs) are one of the key tracers to reveal early differentiation processes of the Earth due to their preferential distribution into the metallic core. Meanwhile, informative evidence for early differentiation has been greatly disturbed through metasomatic PGE disturbance, which has been demonstrated through a number of PGE data for natural mantle peridotites as well as base-metal sulfide and platinum group mineral grains therein. The mechanism and process of metasomatic PGE mobilization should be investigated in detail for an appropriate estimation of PGE abundance in the primitive upper mantle. However, this has not yet been achieved, because sub-micrometer-scale ( i.e. scale of less than a micrometer) descriptions for metasomatic effects imprinted in the mantle peridotites have not been sufficiently recorded. Here, we report a sub-micrometer-sized sulfide–glass inclusion array in a Tahitian harzburgite xenolith. The textural and chemical characteristics were disclosed with employing synchrotron X-ray and transmission electron microscope analyses. The results demonstrate that the sulfide and glass contain appreciable amounts of PGE (9.7 at.% Ir, 4.3 at.% Rh and 5.8 at.% Pt) and carbon (21.2 at.% C), respectively. The sulfide–glass inclusion array is hosted in sodium-enriched clinopyroxene (up to 1.8 wt% Na 2 O) that shows vein-like distribution and partly replaces orthopyroxene. Primitive mantle-normalized trace-element patterns of the clinopyroxene show a general increase from heavy rare-earth elements (REEs) to light REEs with negative anomalies in Pb and high-field-strength elements such as Zr, Hf and Ti, which indicate equilibration with Mg-rich carbonatitic melt. These results suggest that Na-bearing Mg-rich carbonatitic melts were involved in the harzburgite formation and that Ir, Rh and Pt were mobilized through carbonatitic metasomatism and eventually distributed in the sulfides. [ABSTRACT FROM AUTHOR]

Tectonically emplaced peridotites from North Hebei Province, North China Craton, have retained an original harzburgite mineral assemblage of olivine (54%-58%) + orthopyroxene (40%-46%) +minor clinopyroxene (<1%)+spinel. Samples with boninite-like chemical compositions also coexist with these peridotites. The spinels within the peridotites have high-Al end-members with Al2O3 content of 30 wt%-50 wt%, typical of mantle spinels. When compared with experimentally determined melt extraction trajectories, the harzburgites display a high degree of melting and enrichment of SiO2, which is typical of cratonic mantle peridotites. The peridotites display variably enriched light rare earth elements (REEs), relatively depleted middle REEs and weakly fractionated heavy REEs, which suggest a melt extraction of over 25% in the spinel stability field. The occurrence of arc- and SSZ-type chromian spinels in the peridotites suggests that melt extraction and metasomatism occurred mostly in a subduction-related setting. This is also supported by the geochemical data of the coexisting boninite-like samples. The peridotites have 187Os/188Os ratios ranging from 0.113-0.122, which is typical of cratonic lithospheric mantle. These 187Os/188Os ratios yield model melt extraction ages (TRD) ranging from 981 Ma to 2054 Ma, which may represent the minimum estimation of the melt extraction age. The Al2O3-187Os/188Os-proxy isochron ages of 2.4 Ga-2.7 Ga suggest a mantle melt depletion age between the Late Achaean and Early Paleoproterozoic. Both the peridotites and boninite-like rocks are therefore interpreted as tectonically exhumed continental lithospheric mantle of the North China Craton, which has experienced mantle melt depletion and subduction-related mantle metasomatism during the Neoarchean- Paleoproterozoic. [ABSTRACT FROM AUTHOR]

Ultramfits of the studied territory are perspective on detection of Cu-Ni with platinum-group elements deposits that causes the relevance of their research. The main aim of the study. Petrologic researches were directed to establishing the formational accessory of ultramafits of Kulibinskaya area, extraction of the main petrographic varieties and identification of potentially ore-bearing types of rocks. The methods used in the study. Detailed petrographic studying of ultramafits realized on a polarisation microscope «AxioScop 40» (Carl Zeiss) that allowed revealing their main varieties and determining their formational accessory by structural features. Composition of minerals and rocks was generally studied in the Center of collective use «Analytical center of a geochemistry of natural systems» at Tomsk State University. Chemical composition of rock-forming minerals was estimated on the electronic scanning microscope «Tescan Vega II LMU» equipped with the energy-dispersion spectrometer INCA Energy 350 and the wave-dispersion spectrometer INCA Wave 700. Contents of rare and rare-earths elements was defined by the ICP-MS method on a spectrometer of the Agilent 7500 series. Bulk chemical composition was defined by the quantitative chemical analysis in the laboratory of the «Minusinsk GRE» (Minusinsk). The results. It was ascertained that ultramafits in the explored territory are divided into two formational types and united in two complexes: kingashsk magmatic dunite-verlite-picritic and idarsk restite dunite-harzburgitic. The ultramafits of a kingashsk complex composing bodies of the subvolcanic facies are the subject of the greatest practical interest. They are presented by dunites, verlites, their serpentinised varieties and picrites with various structures, which are derivants of primary high-magnesian pikritic magma. Cumulative or porphyryc structures are tipical for them. They form a persistent trend of a variation of chemical composition of minerals, which reflects serial magmatic differentiation of an initial high-magnesian picritic melt. It is established that ore components stand apart, mainly, in the interstices, or in a groundmass. Cumulative dunites and their serpentinised varieties which form ore-bearing Kingashsk massif are optimum for Cu-Ni-PGE mineralization. The subsequent more differentiated varieties are depleted by valuable components. Restite ultramafits are presented by the rocks of dunite-gartsburgitic association with the reference metamorphic granoblastic structures. They differ in more homogeneous high-magnesian chemical composition of minerals caused by intensive depletion of initial mantle substratum.

Introduction Geological background Ophiolites have played a major role in our understanding of Earth’s processes ranging from seafloor spreading, melt evolution and magma transport in oceanic spreading centers, and hydrothermal alteration and mineralization of oceanic crust to collision tectonics, mountain building processes, and orogeny. They provide the essential structural, petrological, geochemical, and geochronological evidence to document the evolutionary history of ancient continental margins and ocean basin. Ophiolites include a peridotitic mantle sequence, generally characterized by high-temperature plastic deformation and residual chemistry, and a comagmatic crustal sequence (gabbros, diabase dikes, and submarine basalts), weakly or not deformed. According to this interpretation, ophiolites were allochthonous with respect to their country rocks. They were assembled during a primary accretion stage at an oceanic spreading center, and later tectonically emplaced on a continental margin or island arc (Dilek, 2003). The indigenous dikes of pyroxenites and gabbros that were injected into a melting peridotite, or intrusive dikes of pyroxenite and gabbro that injected when the peridotite was fresh and well below its solidus, are discussed in different ophiolite papers. Pyroxenite formation and contact of gabbro and mantle peridotite are discussed in different articles (Dilek, 2003). When a gabbro intrude a fresh mantle peridotite could not significantly react with it, but if intrusion occurs during the serpentinization, the gabbro will change to rodingite. Geological setting The Naein ophiolitic melanges comprise the following rock units: mantle peridotites (harzburgite, lherzolite, dunite, with associated chromitite), gabbro, pyroxenite, sheeted and swarm dikes, massive basalts, pillow lava, plagiogranite, radiolarian chert, glaubotruncana limestone, rodingite, listvenite, and metamorphic rocks (foliated amphibolitic dike, amphibolite, skarn, banded meta-chert, and succession of schist and marble) (Davoudzadeh, 1972; Jabbari, 1997; Pirnia Naeini, 2006; Torabi, 2012; Shirdashtzadeh, 2006). In this ophiolite, the leucogabbro intrusions crosscut all other rock units. Materials and Methods Mineralogical analyses were conducted by wavelength-dispersive EPMA (JEOL JXA-8800R) at the Cooperative Centre of Kanazawa University (Japan). The analyses were performed under an accelerating voltage of 15 kV and a beam current of 15 nA. JEOL software using ZAF corrections was employed for data reduction. Natural and synthetic minerals of known composition are used as standards. The Fe3+ content in minerals was estimated by assuming mineral stoichiometry. Results In the contact zone of leucogabbros and mantle peridotites of the Naein ophiolite, wehrlite and olivine clinopyroxenite are formed. Rock-forming minerals of these wehrlites are olivine (chrysolite), clinopyroxene (diopside), Cr-spinel, serpentine, amphibole (tremolite and tremolitic hornblende), epidote and magnetite. Comparison of mineral chemistry of olivine, clinopyroxene and chromian spinel in wehrlites and mantle peridotites indicate that chemical composition of clinopyroxene and olivine in these rocks are different, but chemistry of Cr-spinels in harzburgite and wehrlite are nearly same. Discussion According to the resistance of Cr-spinel against the metamorphism and alteration, it can be concluded that the wehrlites in contact zone of gabbros and mantle peridotites are formed at the expense of harzburgite. Olivine and clinopyroxene of wehrlites are formed by serpentine metamorphism and interaction of serpentine and calcium of gabbro, respectively. Field study of the research area shows that the leucogabbro intrudes the harzburgite. This research shows that after the serpentinization of mantle harzburgite, the gabbro intrusions crosscut the serpentinized peridotites, and wehrlite and olivine clinopyroxenite formed in the contact zone. Acknowledgements The authors thank the University of Isfahan for financial support. References Dilek, Y., 2003. Ophiolite concept and the evolution of geological thought. Geological Society of America Special Paper 373, 504. Davoudzadeh, M., 1972. Geology and Petrography of the area north of Nain, Central Iran. Geological Survey of Iran, Tehran, Report 14, 92 pp. Jabbari, A., 1997. Geology and petrology of the northen Naein ophiolites. M.Sc. Thesis, University of Isfahan, Isfahan, Iran, 162 pp. (in Persian) Pirnia Naeini, T., 2006. Petrology of mantle peridotites of the Naein ophiolite. M.Sc. Thesis, University of Isfahan, Isfahan, Iran, 191 pp. (in Persian with English abstract) Torabi, G., 2012. Central Iran ophiolites. Jahad daneshgahi, Isfahan, 443 pp. (in Persian) Shirdashtzadeh, N., 2006. Petrology of metamorphic rocks in the Naein ophiolite (Isfahan province). M.Sc. Thesis, University of Isfahan, Isfahan, Iran. 194 pp. (in Persian with English abstract)

The Kapalagulu layered ultramafic and mafic intrusion is emplaced between the Paleoproterozoic Ubendian basement and overlying Neoproterozoic Itiaso Group metasedimentary rocks, located near the western shore of Lake Tanganyika. High-grade platinum group element (PGE) mineralization (1–6 g/t Pt + Pd + Au) is associated with chromitite and sulfide-bearing harzburgite within the southeastern extension of the intrusion, known as the Lubalisi Zone, which is covered by a layer of nickel-rich (0.2–2%Ni) laterite regolith that contains linear areas of PGE mineralization. In the Lubalisi Zone, the mineralization may be divided into several significant geometallurgical domains: (a) high-grade PGE mineralization (1–6 g/t Pt + Pd + Au) associated with stratiform PGE reefs and chromitite seams within a harzburgite unit; (b) high-grade PGE mineralization (up to 12 g/t Pt + Pd + Au) associated with small bodies and veins of nickel massive sulfide within harzburgite below PGE-bearing reefs and chromitite seams; (c) low-grade PGE mineralization (0.1–0.5 g/t Pt + Pd + Au) associated with a sulfide-mineralized harzburgite unit above the PGE-bearing reefs; (d) laterite style residual PGE mineralization (0.2–4 g/t Pt + Pd + Au) associated with chromite concentrations in the saprolite and overlying red clay horizons of the laterite regolith; and (e) supergene Ni associated with the saprock and overlying saprolite clay. Mineralogical study of three samples from the PGE reef consisting of high grade PGE chromitite and harzburgite indicate that this mineralization will give a good metallurgical response to conventional grinding and floatation due to the relatively coarse-grained nature of the PGM (P80 from ∼37 to 52 µm), association with base metal sulfides, and unaltered gangue minerals (Wilhelmij and Cabri, 2016). In contrast, mineralogical and metallurgical study of the Ni and PGE mineralized laterite indicate that it cannot be processed using conventional mineral processing techniques but that a hydrometallurgical route should be used to recover the base and precious metals. Because any process is very much deposit-controlled, significant metallurgical and geometallurgical testing of mineralized samples, as well as pilot plant testing, will be required to arrive at feasibility studies. [ABSTRACT FROM AUTHOR]

In the Dun Mountain Ophiolite Belt of east Nelson, New Zealand, chromite segregations (chromitites) occur in a lower zone of serpentinised harzburgite and dunite from Wooded Peak (Dun Mountain) to Little Ben, and in dunite-rich zones in the Red Hills massif and Baldy Ridge outlier. Electron probe microanalyses of chromite grains from the chromitites and also the Red Hills peridotite reveal a range in chromite compositions that are consistent with depletion of peridotite and reaction with island arc tholeiite and boninitic melts in a forearc setting of a supra-subduction zone. Podiform chromitite samples in the harzburgite zone show elevated Ir and Ru values, similar to the levels of enrichment of these platinum group elements (PGE) found in podiform chromite deposits in ophiolites worldwide. The PGE values in dunite-hosted chromitite from Baldy Ridge (Matakitaki) have elevated Pt, Pd and Rh, as is found in cumulate dunite zones in some other ophiolites. Disseminated Ni mineralisation as awaruite ± magnetite ± ferritchromit ± heazlewoodite ± pentlandite ± native Cu occurs in serpentinised peridotites, and was likely produced as a by-product of hydration as a result of detachment faulting allowing penetration of seawater into underlying mantle peridotite. [ABSTRACT FROM PUBLISHER]

Orthopyroxene is especially suited to decode and testify to the behavior of highly immobile elements during hydrous mantle melting. Laser ablation ICP-MS analyses from orthopyroxene hosted within peridotite from the Coast Range ophiolite (CRO) demonstrates that Group A peridotites (lherzolites) have similar compositions to mid-ocean-ridge abyssal peridotite, whereas other peridotites (Groups B and C; harzburgites) retain depleted signatures, but display ‘spoon-shaped’ enrichments for the light-REE. These patterns are consistent with variable degrees of partial melting of MORB-source asthenosphere initiated within the garnet stability field (< 10%) and continuing into the spinel stability field (< 15%). A few samples may have been subjected to subsequent melt/rock interaction. The supra-subduction zone (SSZ) environment represented by the CRO is illustrated by enriched fluid mobile elements (Li, Be, B, Pb) in all samples - up to 200 × depleted-MORB mantle (DMM). New applications of trace-element addition calculations [Shervais J. and Jean M.M. (2012) Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone. GCA 95, 270–285] modified for orthopyroxene reveals that tens to hundreds of ppm were added to the DMM-source region. Our purpose is to demonstrate that orthopyroxene, in the absence of clinopyroxene, can be a constructive ( and perhaps better ) indicator of tectonic environment and magmatic processes that occurred within the North American Cordillera mantle wedge. Through this investigation we have captured all three stages of Coast Range ophiolite petrogenesis: starting with initial SSZ-coupled forearc spreading dominated by decompression melting, to a mature subduction zone with fluid-assisted partial melting, and the transition between the two. [ABSTRACT FROM AUTHOR]

Highly siderophile element (HSE) concentration and 187 Os/ 188 Os isotopic heterogeneity has been observed on various scales in the Earth’s mantle. Interaction of residual mantle peridotite with infiltrating melts has been suggested to overprint primary bulk rock HSE signatures originating from partial melting, contributing to the heterogeneity seen in the global peridotite database. Here we present a detailed study of harzburgitic xenolith 474527 from the Kangerlussuaq suite, West Greenland, coupling the Re–Os isotope geochemistry with petrography of both base metal sulfides (BMS) and silicates to assess the impact of overprint induced by melt-rock reaction on the Re–Os isotope system. Garnet harzburgite sample 474527 shows considerable heterogeneity in the composition of its major phases, most notably olivine and Cr-rich garnet, suggesting formation through multiple stages of partial melting and subsequent metasomatic events. The major BMS phases show a fairly homogeneous pentlandite-rich composition typical for BMS formed via metasomatic reaction, whereas the 187 Os/ 188 Os compositions determined for 17 of these BMS are extremely heterogeneous ranging between 0.1037 and 0.1981. Analyses by LA-ICP-MS reveal at least two populations of BMS grains characterized by contrasting HSE patterns. One type of pattern is strongly enriched in the more compatible HSE Os, Ir, and Ru over the typically incompatible Pt, Pd, and Re, while the other type shows moderate enrichment of the more incompatible HSE and has overall lower compatible HSE/incompatible HSE composition. The small-scale heterogeneity observed in these BMS highlights the need for caution when utilizing the Re–Os system to date mantle events, as even depleted harzburgite samples such as 474527 are likely to have experienced a complex history of metasomatic overprinting, with uncertain effects on the HSE. [ABSTRACT FROM AUTHOR]

Cr-spinels from ultramafic rocks from Lokris (Megaplatanos and Tragana), and Beotia (Ypato and Alyki) ophiolitic occurrences were studied. These rocks comprise principally harzburgite with minor dunite. Small amounts of clinopyroxene-rich harzburgite and lherzolite have been observed along with the harzburgite in Alyki. The Cr# in the studied spinels displays a wide variability. The spinels hosted in harzburgite and cpx-rich harzburgite display low Cr# (<0.6), typical for oceanic (including back-arc basins) ophiolites, whereas the spinels hosted in dunite with Cr# (>0.6) characterize arc-related ophiolitic sequences. Cr-spinels from Alyki indicate a moderate fertile character and are analogous to those from abyssal peridotites. The dunitic and harzburgitic spinel-olivine pairs are consistent with a Supra-Subduction Zone origin. The relatively large range in spinel Cr# and Mg# may have been resulted from a wide range of degrees of mantle melting during the evolution of the host peridotites. [ABSTRACT FROM AUTHOR]