Peridotite Minerals Research Articles

Heavy magnesium isotopic signatures in arc lavas may be attributed to dehydration of subducting hydrated mantle

Fluids released from subducting slabs profoundly affect mantle composition, rock melting points, and arc magma generation. However, identifying fluid sources (sediments, crust, or mantle) and their ascent paths remains challenging. Magnesium isotopes are potential tracers for subduction-related fluids, though their behavior during hydrous peridotite dehydration remains unclear. Here we determined the equilibrium magnesium isotope fractionation factors between aqueous fluids and hydrous peridotitic minerals using first-principles calculations. Aqueous fluids prefer heavy magnesium isotopes relative to mantle silicate minerals, indicating that fluids released during hydrous peridotite dehydration are enriched in heavy magnesium isotopes relative to the residual minerals. Our simulations proposed that magnesium isotope variations in arc lavas from different subduction zones could be attributed to different dehydration reactions influenced by subduction zone thermal structures. This study highlights the potential of magnesium isotopes for tracing fluids originating from subducting hydrated mantle, providing insights into the thermal structure of various subduction zones.

Detrital peridotite minerals reveal recycled arc mantle beneath Marion Rise, Southwest Indian Ridge

AbstractThe recycling of subcontinental lithospheric mantle back into the asthenosphere during continental breakup is potentially a major but underappreciated process in plate tectonics. Here, we report major and trace elements for detrital peridotite minerals in carbonate breccias dredged from the Marion Rise, Southwest Indian Ridge. With an emphasis on the trace elements of the relatively abundant orthopyroxene detrital grains, the results show that a portion of pyroxene grains was likely derived from a mantle source that experienced hydrous melting and slab‐derived melt metasomatism above a subduction zone. This finding provides a novel view into the mantle geochemistry at midocean ridges, as well as evidence that Marion Rise is at least partially supported by chemically buoyant recycled refractory mantle wedge material.

The water-poor and reduced mantle beneath the Neo-Tethys Ocean: Tectonic setting of the western Yarlung-Tsangpo ophiolites

Water contents and deformation behaviors of minerals in mantle peridotites are diagnostic features to distinguish the physical conditions between mid-ocean ridges and subduction zones, which could serve as effective indicators for discriminating the tectonic setting of ophiolites. In this study, we report a systematic dataset of petrology, geochemistry, water contents and deformation behaviors for the mantle peridotites from the Cuobuzha ophiolite in the western Yarlung-Tsangpo suture zone (YTSZ), southern Tibet, to provide new insights into the long-standing debate for the tectonic setting of ophiolites therein. Geochemical data revealed that the Cuobuzha mantle peridotites experienced low to moderate degrees of partial melting (∼10–15%), although some harzburgites have been intensively modified by late-stage melt-rock interactions. The Cuobuzha peridotites, on the other hand, are characterized by low orthopyroxene water contents (46–95 wt ppm in average), clear olivine crystallographic preferred orientations of A- and E-types, and low oxygen fugacity (logfO2, QFM −2.55 to −0.81, where QFM means quartz-magnetite-fayalite buffer), thus indicative of the occurrence of high-temperature, water-depleted, and reduced mantle in this ophiolite. The features above of the Cuobuzha mantle peridotites are quite similar to those of abyssal peridotites at modern mid-ocean ridges and systematically different from the relatively refractory, water-rich, and oxidized characteristics typical of forearc peridotites in suprasubduction zones. Based on the data from this study and a compiled dataset for other ophiolites from the western YTSZ, we revealed that mantle peridotites with a water-poor and reduced nature are common in these ophiolites, thus providing strong evidence supporting for their origination from a mid-ocean ridge of a mature ocean basin. From the perspective of mantle peridotites, the ophiolites in the western YTSZ now geographically distributed in two branches (i.e., the northern and southern sub-belts) might have formed in a single ocean basin of the Neo-Tethys and were dismembered by late-stage tectonic events to their current locations.

Lattice-preferred orientation (LPO) of olivine and amphibole in amphibole peridotites and neighboring hornblendites from Gapyeong, South Korea and implications for seismic anisotropy

Amphibole peridotites and neighboring hornblendites are often found in subduction zones. To understand the effect of amphibole-rich rocks on seismic anisotropy in subduction zones, we studied the lattice-preferred orientation (LPO) of olivine and amphibole in amphibole peridotites and neighboring hornblendites in Gapyeong, South Korea. The major minerals of amphibole peridotites were olivine (31–51% in volume), amphibole (28–47%), and orthopyroxene (7–16%). Amphibole in amphibole peridotites showed relatively high SiO2 and low TiO2 and Na2O contents (S-type amphibole), indicating that it was formed under supra-subduction conditions. Amphibole in amphibole peridotites showed the type-I, type-II, and type-IV LPO, whereas amphibole in neighboring hornblendites showed the type-III and type-IV LPO. In the case of olivine, most samples showed a mixture of A- and B-type LPO, and one sample showed a mixture of B- and C-type LPO. Many serpentine inclusions were found in olivines. Fourier-transform infrared (FTIR) analysis of the samples showed that the olivines contained a large amount of water (∼29000–45000 ppm H/Si). We also found many dislocations in olivines. These observations indicate that samples showing a mixture of A- and B-type LPO and a mixture of B- and C-type LPO of olivine were deformed under water-rich conditions by dislocation creep. In amphibole peridotites, the P-wave anisotropy of olivine was relatively low (0.9–4.8%), whereas the P-wave anisotropy of amphibole was high (6.5–17.7%). The maximum S-wave anisotropy of olivine was also relatively low (0.87–2.89%), whereas the maximum S-wave anisotropy of amphibole was high (3.81–15.19%). In hornblendites, the P-wave anisotropy and maximum S-wave anisotropy of amphibole were high (6.9–13.6% and 4.27–10.61%, respectively). The P-wave anisotropy and maximum S-wave anisotropy of the amphibole peridotites were in the range of 2.6–8.4% and 1.73–7.30%, respectively. The seismic velocity and anisotropy pattern of amphibole peridotites were more similar to those of amphibole than those of olivine, indicating that the seismic properties of amphibole peridotites were more strongly affected by amphibole than by olivine. Furthermore, the seismic anisotropy of the mixture of amphibole peridotite and hornblendite in subduction zones was also found to be significantly affected by the amphibole LPO in hornblendite.

Experimental Modeling of the Interaction between Garnets of Mantle Parageneses and CO2 Fluid at 6.3 GPa and 950–1550 °C

Abstract —Experimental modeling of the interaction of eclogitic and lherzolitic garnets with CO2 fluid was carried out on a multianvil high-pressure apparatus of the “split-sphere” type (BARS) in platinum ampoules with inner graphite capsules, using a buffered high-pressure cell with a hematite container, at a pressure of 6.3 GPa and in the temperature range 950–1550 °C. It has been established that the main interaction processes at 6.3 GPa and 950–1250 °C are partial dissolution, recrystallization, and carbonation of garnet which lead to the formation of magnesian carbonate, kyanite, and coesite, a decrease in Mg contents in the recrystallized garnet, and the formation of carbonate, silicate, and oxide inclusions in it. Under these conditions, crystallization of metastable graphite and growth of diamond on the seed at ≥1250 °C were observed. In the temperature range 1350–1550 °C, the garnet underwent partial dissolution and recrystallization in CO2 fluid; no carbonation took place. These processes were accompanied by a decrease in the portion of the grossular component in the garnet and by the enrichment of the fluid phase with calcium. We have established the indicative characteristics of garnet that interacted with CO2 fluid: zoning, with low contents of CaO and MgO in the rims of crystals relative to the cores, and the presence of carbonate, kyanite, coesite, and CO2 inclusions. The compositions of the produced garnet and carbonates are consistent with the data on these minerals in mantle peridotite and eclogite parageneses and in inclusions in diamonds, which suggests a significant role of metasomatism with the participation of CO2 fluid in the evolution of deep-seated rocks and in the diamond formation. In this experimental research, we have first studied the processes of diamond crystallization and determined the boundary conditions for diamond growth in the system silicate–carbonate–CO2, which simulates natural diamond formation media. In general, the established regularities can be regarded as potential indicators of mantle metasomatism and mineral formation with the participation of CO2 fluid.

Botuobinskite and mirnyite, two new minerals of the crichtonite group included in Cr-pyrope xenocrysts from the Internatsionalnaya kimberlite

AbstractTwo new mineral species of the crichtonite group: botuobinskite, ideally SrFe2+(Ti4+12Cr3+6)Mg2[O36(OH)2] and mirnyite, ideally SrZr(Ti4+12Cr3+6)Mg2O38, occur as inclusions in mantle-derived Cr-pyrope xenocrysts from the Internatsionalnaya kimberlite pipe, Mirny field, Siberian craton. Botuobinskite forms needle- and blade-like acicular crystals up to 1 mm in length and up to 30 μm in diameter, a large platy inclusion (700 × 700 × 80 μm) and roughly isometric grains (up to 80 μm). Mirnyite occurs as needle-and blade-like elongated inclusions (up to 1 mm). Both minerals are jet-black, opaque and exhibit a metallic lustre. In plane-polarised reflected light, botuobinskite and mirnyite are greyish-white with a weak brownish tint. Between crossed polars, the new species show distinct anisotropy in shades of bluish grey to greenish-brown. Neither bireflectance nor pleochroism is observed. Calculated densities for botuobinskite and mirnyite are 4.3582(5) and 4.3867(3) gm/cm3, respectively. The crystal structures of botuobinskite and mirnyite have been refined (R = 0.0316 and 0.0285, respectively) from single crystal X-ray diffraction data. The minerals are trigonal, crystallise in the space group R$\bar{3}$ (No. 148) and are isostructural with other members of the crichtonite group. The unit cell parameters are a = 10.3644(8) Å, c = 20.6588(11) Å and V = 1921.9(2) Å3 for botuobinskite and a = 10.3734(8) Å, c = 20.6910 (12) Å and V = 1928.2(2) Å3 for mirnyite, with Z = 3 for both. The Raman spectra of the minerals show strong peaks at 133, 313 and 711 cm–1. Infrared spectroscopy data for botuobinskite indicates H–O stretching of the hydroxyl groups. Botuobinskite and mirnyite have been approved by the IMA–CNMNC under the numbers 2018-143a and 2018-144a, respectively. Botuobinskite and mirnyite are named after the Botuobinskaya exploration expedition and Mirny town, respectively. The minerals may be considered as crystal-chemical analogues of other crichtonite-group species occurring in the lithospheric mantle (i.e. loveringite, lindsleyite and mathiasite). Both species commonly occur in intimate association with Cr-pyrope as well as other peridotitic minerals and exert an important control on the partitioning of incompatible elements during mantle metasomatism.

Zinc isotope fractionation in mantle rocks and minerals, and a revised δ66Zn value for the Bulk Silicate Earth

The isotope composition of zinc (Zn) can be an important tracer of planetary formation and differentiation, but estimates of the Bulk Silicate Earth (BSE) and the extent and origin of Zn isotope variations in the mantle continue to be debated. We report 153 high-precision Zn isotope analyses of whole-rock samples and minerals of well-characterized, mantle-derived spinel and garnet peridotites from five sites in Siberia, Mongolia and China: 38 xenoliths in basaltic rocks (fertile to strongly melt-depleted as well as variably metasomatized or veined peridotites) and 3 orogenic peridotites. Data quality was rigorously tested by twenty-two duplicated analyses using several dissolution techniques. Spinel (Sp) contains 760–1460 μg/g Zn and hosts up to 58 % of whole-rock Zn budget in the peridotites followed by olivine (Ol, 30–66 % of whole-rock Zn budget), orthopyroxene (Opx), clinopyroxene (Cpx) and garnet, with abundance ratios of [Zn]Sp/[Zn]Ol = 15–47, [Zn]Ol/[Zn]Opx = ∼1.5, [Zn]Ol/[Zn]Cpx = ∼6. Spinel has higher δ66Zn values than coexisting silicates (∼0.12 ‰ higher than in Ol). Minerals in orogenic peridotites show disequilibrium Zn element and isotope partitioning due to slow inter-grain Zn diffusion at retrograde metamorphism.Fertile peridotites (≥3 wt% Al2O3) contain 52–60 μg/g Zn, usually higher than melt-depleted (<3 wt% Al2O3) rocks (40–53 μg/g). The δ66Zn values in the majority of spinel and garnet peridotites range from 0.11 to 0.26 ‰ and show no systematic co-variations with melt extraction indices (e.g., Al2O3, Mg#), suggesting, unlike some previous studies, that partial melting does not appreciably affect Zn isotope composition of residual mantle. Garnet lherzolites yield an average δ66Zn of 0.22 ± 0.09 ‰ (2SD), similar to that (0.18 ± 0.07 ‰, 2SD) for shallower spinel peridotites, indicating uniform Zn isotope composition with depth. Four fertile lherzolites from earlier work, duplicated in this study to assess inter-laboratory bias, fall in the same δ66Zn range, suggesting that previously reported higher δ66Zn for these samples could be due to analytical problems. We thus estimate the δ66Zn of the BSE at 0.19 ± 0.08 ‰ (2SD) based on suitable fertile peridotites from several localities in this study and the literature.The δ66Zn values (−0.10 to 0.04 ‰) are much lower in Fe-rich (9.3–13.1 wt% FeOT) and veined spinel peridotites as well as in spinel pyroxenite veins from Mongolia (−0.20 to 0.11 ‰). The Fe-rich peridotites and pyroxenites are rich in Zn (up to 113 μg/g) and were previously found to have anomalously low δ56Fe (−0.33 to − 0.27 ‰) and δ53Cr (−1.36 to − 0.05 ‰). We attribute these features to kinetic isotope fractionation during reaction with mafic melts migrating in conduits in the lithospheric mantle or intrusion of Fe-Zn-rich melts with light Fe-Zn isotope compositions. Kinetic Zn isotope fractionation, rather than equilibrium partial melting, may be a major cause of Zn isotope heterogeneity in the mantle.

Episodic Nature of Magmatic Ascent in a Dynamic Conduit System: Evidence From a Late Gabbroic Intrusion Associated with the Eagle Ni-Cu Sulfide Deposit in Northern Michigan, USA

Abstract New exploration studies have revealed a near-vertical, plug-like gabbroic intrusive body emplaced along the feeder conduit of the funnel-shaped Eagle East ultramafic intrusion in the Upper Peninsula of Michigan. U-Pb baddeleyite analysis on this gabbro yielded an age of 1103.4 ± 1.2 Ma. Although a sulfide-deficient gabbro is the principal rock unit, peripheral zones of the intrusion contain feldspathic pyroxenite and melagabbro with minor blebs of chalcopyrite and pyrrhotite. A small pocket-like body of peridotite with significant quantities of sulfide minerals occurs along the edge of the main gabbroic unit. The modes of occurrence of sulfide minerals in the pocket peridotite are similar to the larger Eagle East and Eagle intrusions nearby. The composition of olivine in the main gabbroic rock is Fo24–30, but olivine in pocket peridotite shows a composition of ~Fo74. The Ni-concentration in the olivine of the main gabbro unit is below 100 ppm, but in the Mg-rich olivine of the pocket peridotite, it is between 2,100 and 2,800 ppm. Although δ34S values of sulfide minerals in the pyroxenite rim and in the pocket peridotite range from –0.6 to 7.0‰, the average value is higher than those reported from sulfides in the Eagle East and Eagle intrusions. The origin of the gabbro unit could be explained by a closed-system crystallization of a fractionated late derivative of an original high-MgO magma, while the peridotite pocket is a xenolithic representative of the old peridotitic intrusion dislodged and entrained in a late magmatic pulse.

Multiscale physical–chemical analysis of the impact of fracture networks on weathering: Application to nickel redistribution in the formation of Ni-laterite ores, New Caledonia

Supergene processes related to downward water migration and dissolution of primary minerals in the New Caledonian peridotites result in Ni enrichment at the bottom of the lateritic profile near bedrock. However, the combined effects of relief, lateral fluid flow, and complex inherited discontinuity networks filled by serpentine generate a more complex Ni distribution than a simple flat horizon. In surficial environments, fluid flow and mass transfer are not limited to the vertical scale; they can also be related to lateral fluid circulation and local drainage along damage zones in the vicinity of faults. In this study, we present new structural, petrophysical and geochemical data for core samples to shed light on the impact of a fracture network on the alteration and progression of the Ni-enrichment front during weathering. The analysis of element redistribution by XRF mapping coupled with fracture extraction from computed tomography (CT) establishes that the serpentine fracture network comprises sets of sub-orthogonal fractures whose orientations are potentially determined by pre-existing magmatic foliation. The progression of the olivine dissolution front leads to the redistribution and concentration of Ni into the serpentine fracture network as secondary Mg-Ni talc-like phases. The increase in porosity from < 1 % in the bedrock to > 10 % in the saprock is mainly driven by the formation of nanopores. Nanoporosity may control the development of the weathering front and the redistribution of Ni by small-scale diffusion. Structural and geochemical characteristics identified at the sample scale are analogous to those observed on the larger scale of the outcrop and deposit. Scale invariance in the structural organization and geochemical evolution indicates the potential upscaling of the processes involved in the redistribution of Ni.

Formation of mixed paragenesis diamonds during multistage growth – Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds

Inclusion-bearing diamonds from the Koidu kimberlite complex, Sierra Leone (West African Craton) were analyzed in situ for carbon and nitrogen isotope compositions, nitrogen concentrations and nitrogen aggregation states. In a suite of 105 diamonds, 78% contain eclogitic mineral inclusions, 17% contain peridotitic mineral inclusions, and 5% − an unusually high proportion − contain co-occurring eclogitic and peridotitic mineral inclusions indicating a mixed paragenesis. Major and trace element compositions of mineral inclusions from two mixed paragenesis diamonds (one with omphacite + Mg-chromite, the other with eclogitic garnet + forsteritic olivine) were determined. The presence of positive Eu anomalies in centrally located omphacite and eclogitic garnet inclusions indicates derivation from subducted protoliths, formed as igneous cumulates in lower oceanic crust. Mg-chromite (Cr# 85.5; Mg# 65.2) and olivine (Mg# 94.5) inclusions, located in outer portions of the mixed paragenesis diamonds, have compositions indicative of derivation from strongly depleted cratonic peridotites. Given that the olivine Mg# of 94.5 is the highest reported to date for the West African Craton, the eclogitic and peridotitic inclusions in these mixed paragenesis diamonds cannot have precipitated during infiltration of peridotitic substrates by eclogite-derived fluids, as the consequent fluid-rock interaction should lead to Mg# lower than that for the original peridotitic diamond substrate. The different origins of eclogitic and peridotitic inclusions could be explained by physical transport of their host diamonds from eclogitic into peridotitic substrates, possibly along high-strain shear zones, before renewed diamond growth.Based on the δ13C–δ15N systematics of the entire inclusion-bearing diamond suite from Koidu, three major compositional clusters are identified. Cluster 1 (eclogitic diamond cores; δ13C = –33.2 to −14.4‰ and δ15N = −5.3 to +10.1‰) bears the isotopic signature of recycled crustal material (± a mantle component). Cluster 2 (peridotitic diamonds and including the core of a diamond containing omphacite + Mg-chromite; δ13C = −6.0 to −1.1‰ and δ15N = −4.2 to +9.7‰) likely involves mixing of carbon and nitrogen from subducted and mantle sources. Cluster 3 (rims of eclogitic diamonds and including the eclogitic garnet + olivine included diamond and the rim of the omphacite + Mg-chromite included diamond; δ13C = −7.8 to −3.6‰ and δ15N = −7.9 to −2.1‰) matches convecting mantle-derived fluids/melts. The distinct isotopic signatures of the three diamond clusters, together with differences in nitrogen aggregation and cathodoluminescence response between diamond cores and rims, suggest episodic diamond growth during multiple fluid/melt pulses.

Dynamic Metasomatism Experiments Investigating the Interaction between Migrating Potassic Melt and Garnet Peridotite

Dynamic metasomatism experiments were performed by reacting a lamproite melt with garnet peridotite by drawing melt through the peridotite into a vitreous carbon melt trap, ensuring the flow of melt through the peridotite and facilitating analysis of the melt. Pressure (2–3 GPa) and temperature (1050–1125 °C) conditions were chosen where the lamproite was molten but the peridotite was not. Phlogopite was formed and garnet and orthopyroxene reacted out, resulting in phlogopite wehrlite (2 GPa) and phlogopite harzburgite (3 GPa). Phlogopites in the peridotite have higher Mg/(Mg + Fe) and Cr2O3 and lower TiO2 than in the lamproite due to buffering by peridotite minerals, with Cr2O3 from the elimination of garnet. Compositional trends in phlogopites in the peridotite are similar to those in natural garnet peridotite xenoliths in kimberlites. Changes in melt composition resulting from the reaction show decreased TiO2 and increased Cr2O3 and Mg/(Mg + Fe). The loss of phlogopite components during migration through the peridotite results in low K2O/Na2O and K/Al in melts, indicating that chemical characteristics of lamproites are lost through reaction with peridotite so that emerging melts would be less extreme in composition. This indicates that lamproites are unlikely to be derived from a source rich in peridotite, and more likely originate in a source dominated by phlogopite-rich hydrous pyroxenites. Phlogopites from an experiment in which lamproite and peridotite were intimately mixed before the experiment did not produce the same phlogopite compositions, showing that care must be taken in the design of reaction experiments.

Partitioning of Fe2O3 in peridotite partial melting experiments over a range of oxygen fugacities elucidates ferric iron systematics in mid-ocean ridge basalts and ferric iron content of the upper mantle

Basalts and peridotites from mid-ocean ridges record fO2 near the quartz-fayalite-magnetite buffer (QFM), but peridotite partial melting experiments have mostly been performed in graphite capsules (~ QFM-3), precluding evaluation of ferric iron’s behavior during basalt generation. We performed experiments at 1.5 GPa, 1350–1400 °C, and fO2 from about QFM-3 to QFM+3 to investigate the anhydrous partitioning behavior of Fe2O3 between silicate melts and coexisting peridotite mineral phases. We find spinel/melt partitioning of Fe2O3 ({D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{spl}/mathrm{melt}}) increases as spinel Fe2O3 concentrations increase, independent of increases in fO2, and decreases with temperature, which is consistent with new and previous experiments at 0.1 MPa. We find {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} = 0.63 ± 0.10 and {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{cpx}/mathrm{melt}} = 0.78 ± 0.30. MORB Fe2O3 and Na2O concentrations are consistent with a modeled MORB source with Fe2O3 = 0.48 ± 0.03 wt% (Fe3+/ΣFe = 0.053 ± 0.003) at potential temperatures (TP) from 1320 to 1440 °C. The temperature-dependence of the {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{spl}/mathrm{melt}} function alone allows ~ 40% of the variation in MORB compositions. If we allow {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} and {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} to also vary with temperature by tying them to spinel Fe2O3 through intermineral partitioning, then all the MORB data are within error of the model. Our model Fe2O3 concentration for the MORB source would require that the convecting mantle be more oxidized at a given depth than recorded by continental mantle xenoliths. Our result is supported by thermodynamic models of mantle with Fe3+/ΣFe = 0.03 that predict fO2 of ~ QFM-1 near the garnet-spinel transition, which is inconsistent with fO2 of MORB. Our results support previous suggestions that redox melting may occur between 200 and 250 km depth.

К вопросу о вхождении редкоземельных элементов в структуру хромпироповых гранатов

Rare earth elements (REE) in garnet are of interest in various fields of modern geology. The geochemistry of REEs in magmatic minerals is widely used in determining the distribution coefficients of crystal/melt and crystal/fluid, modeling the processes of melting and crystallization of magmatic rocks, studying deep mantle processes, age estimates and other issues of petrogenesis. The aim of the present work was a synthesis of a peridotite mineral association including the garnet containing REE at high pressure and high temperature. The initial sample consisted mainly of natural serpentine collected from ophiolites of the Eastern Sayan (Russia). As is known, the extreme stage of the regressive metamorphism of peridotites is serpentinization. It is depleted in calcium, but can recrystallize at high PT conditions into a harzburgite paragenesis, and at the initial stage of the experiment the chemical composition of the sample was a model harzburgite depleted in calcium and chromium, as well as a fluid of predominantly aqueous composition. As a source of chromium, chromite grains of 1–2 mm in size from peridotite xenoliths of the Udachnaya kimberlite pipe (Yakutia) were used. REE were added to the initial charge in the form of water-soluble salts. The experiment at a pressure of 5 GPa and temperature 1300 was performed on a multi-anvil high-pressure apparatus of the “split sphere” type (BARS) designed and developed at the V.S. Sobolev Institute of Geology and Mineralogy SB RAS. A container based on refractory oxide ZrO2 was used as a highpressure medium. The pressure in the cell before sample heating was estimated using the reference substances Bi and PbSe. The temperature was measured by a platinum-platinum-rhodium thermocouple PtRh30-PtRh6. The quenched was performed by switching off the voltage in the heater circuit. The experiment products contain an association of olivine + garnet + orthopyroxene + newly formed spinel. The predominant phase was olivine of a forsterite composition. A low-Fe orthopyroxene (1.49 – 1.68 wt% FeO) was found in elongated grains uniformly distributed throughout the sample. The newly formed spinel shows the faceted grains. The chromium content in the spinel significantly exceeds that of the initially added to the initial charge, 61.63 and 54.04 wt% Cr2O3, respectively. The garnet is characterized by a purple color, and was identified in the sample volume between olivine grains in the form of individual faceted crystals or their clusters. The largest garnets reached 0.5 mm in size. The synthesized garnet was determined as a high-Cr low-Ca pyrope variety. The contents of Cr2O3 and CaO are 10.15-11.21 and 0.06-0.11 wt%, respectively. The total content of REE in the garnet identified by the microprobe analysis is relatively high reaching 5-7 wt%. As a result of the work a mineral association corresponding to the peridotite paragenesis was obtained, including the subcalcic Crrich pyrope containing rare earth elements in significant amounts. It was estimated that their content in garnet mainly depends on the size of the ionic radius and, accordingly, on the atomic weight. This is consistent with the known facts about the preferable position of heavy REEs into the garnet structure compared to the light REEs.

Geological Evidence does not Support a Shallow Origin for Diamonds in Ophiolite

Geological Evidence does not Support a Shallow Origin for Diamonds in Ophiolite

Enormous Lithium Isotopic Variations of Abyssal Peridotites Reveal Fast Cooling and Melt/Fluid‐Rock Interactions

AbstractFast diffusing Li isotopes provide important insights into the “recent” transient events or processes for both modern and ancient times, but questions remain concerning the large Li isotopic variations of mantle peridotites, which greatly hampers their usage as a geochemical tracer. This study investigates in situ Li content and isotopic profiles of the constituent minerals of abyssal peridotites from the Gakkel Ridge and Southwest Indian Ridge. The complicated and large variations of Li isotopic profiles in Clinopyroxene (Cpx) and Orthopyroxene (Opx) indicate Li isotopic disequilibrium at millimeter scale. The negative correlations of a wide range of Li contents (0.5 to 6.5 ppm) and δ7Li values (−10 to +20‰) of olivine, Opx and Cpx grains/relicts, trace element zoning of Cpx, the occurrence of plagioclase, olivine serpentinization along cracks, together with numerical modeling demonstrate the observed Li characteristics to be a manifestation of high‐temperature mineral‐melt Li diffusion during melt impregnation overprinted by low‐temperature mineral‐fluid Li diffusion during dissolution and serpentinization. The preservation of the Li isotopic diffusion profiles requires rapid cooling of 0.3–5°C/year after final‐stage melt impregnation at the Moho boundary, which is consistent with the low temperature at very slow spreadin g ridges caused by conductive cooling. Compared with the well‐studied melt‐rock interaction process, our study indicates that low‐temperature fluid‐rock interaction can induce Li diffusion even in the visibly unaltered mineral relicts of partially altered rocks.

Metasomatic control of hydrogen contents in the layered cratonic mantle lithosphere sampled by Lac de Gras xenoliths in the central Slave craton, Canada

Whether hydrogen incorporated in nominally anhydrous mantle minerals plays a role in the strength and longevity of the thick cratonic lithosphere is a matter of debate. In particular, the percolation of hydrogen-bearing melts and fluids could potentially add hydrogen to the mantle lithosphere, weaken its olivines (the dominant mineral in mantle peridotite), and cause delamination of the lithosphere's base. The influence of metasomatism on hydrogen contents of cratonic mantle minerals can be tested in mantle xenoliths from the Slave Craton (Canada) because they show extensive evidence for metasomatism of a layered cratonic mantle. Minerals from mantle xenoliths from the Diavik mine in the Lac de Gras kimberlite area located at the center of the Archean Slave craton were analyzed by FTIR for hydrogen contents. The 18 peridotites, two pyroxenites, one websterite and one wehrlite span an equilibration pressure range from 3.1 to 6.6 GPa and include samples from the shallow (≤145 km), oxidized ultra-depleted layer; the deeper (∼145–180 km), reduced less depleted layer; and an ultra-deep (≥180 km) layer near the base of the lithosphere. Olivine, orthopyroxene, clinopyroxene and garnet from peridotites contain 30–145, 110–225, 105–285, 2–105 ppm H2O, respectively. Within each deep and ultra-deep layer, correlations of hydrogen contents in minerals and tracers of metasomatism (for example light over heavy rare-earth-element ratio (LREE/HREE), high-field-strength-element (HFSE) content with equilibration pressure) can be explained by a chromatographic process occurring during the percolation of kimberlite-like melts through garnet peridotite. The hydrogen content of peridotite minerals is controlled by the compositions of the evolving melt and of the minerals and by mineral/melt partition coefficients. At the beginning of the process, clinopyroxene scavenges most of the hydrogen and garnet most of the HFSE. As the melt evolves and becomes enriched in hydrogen and LREE, olivine and garnet start to incorporate hydrogen and pyroxenes become enriched in LREE. The hydrogen content of peridotite increases with decreasing depth, overall (e.g., from 75 to 138 ppm H2O in the deep peridotites). Effective viscosity calculated using olivine hydrogen content for the deepest xenoliths near the lithosphere-asthenosphere boundary overlaps with estimates of asthenospheric viscosities. These xenoliths cannot be representative of the overall cratonic root because the lack of viscosity contrast would have caused basal erosion of lithosphere. Instead, metasomatism must be confined in narrow zones channeling kimberlite melts through the lithosphere and from where xenoliths are preferentially sampled. Such localized metasomatism by hydrogen-bearing melts therefore does not necessarily result in delamination of the cratonic root.

From Decompression Melting to Mantle-Wedge Refertilization and Metamorphism: Insights from Peridotites of the Alag Khadny Accretionary Complex (SW Mongolia)

This study reports on mineral and bulk rock compositions of metaperidotites from the Alag Khadny accretionary complex in SW Mongolia, to reveal their nature and relationships with associated eclogites. The peridotites preserved original porphyroclastic textures and are composed of olivine, orthopyroxene relics, Cr-spinel, interstitial (not residual) clinopyroxene, and secondary chlorite, tremolite, olivine, Cr-magnetite, clinopyroxene, and antigorite. Cr-spinel has Cr# of 0.3–0.5, and primary olivine shows Mg# of 0.90–0.92. The pyroxenes are high-magnesian with low Al2O3 and Cr2O3. The bulk rocks have U-shaped normalized trace-element patterns with enrichment in LILE, L-MREE relative to HREE, and weak Pb–Sr peaks and Nb–Zr–Hf minima. Interstitial clinopyroxene exhibits V- and U-shaped normalized REE patterns with (La/Yb)N &gt; 1 (Yb = 1.2–3 of chondritic values) and enrichment in fluid-mobile elements and Zr. HREE abundances of clinopyroxene can be simulated by 23–26% partial melting of depleted mantle starting at garnet-facies (6–8%) depths, followed by hydrous or anhydrous melting at spinel-facies depths L-MREE characteristics of clinopyroxenes can be simulated by further interaction of harzburgites with an island-arc basaltic melt in a supra-subduction environment. The association of hydrous secondary minerals in the Alag Khadny peridotites suggests their retrograde metamorphism at 1.6–2.0 GPa and 640–720 °C, similar to P–T conditions reported earlier for the spatially associated eclogites. This supports metamorphism of the Alag Khadny peridotites in a mantle wedge, followed by joint exhumation of peridotites and eclogites. Given the findings above and implying the regional geological background, we advocate for a sequential Neoproterozoic evolution the Alag Khadny harzburgites from (1) their formation by decompression partial melting in an Early Neoproterozoic or older spreading center of a mid-ocean or back-arc setting, and (2) refertilization by supra-subduction melts, followed by (3) Late Neoproterozoic–Early Cambrian hydrous-fluid metamorphism and juxtaposition with eclogites.

Genesis of pyroxenite veins in supra-subduction zone peridotites: Evidence from petrography and mineral composition of Egiingol massif (Northern Mongolia)

Swarms of orthopyroxenite and websterite veins are found within Egiingol residual SSZ peridotite massif of Dzhida terrain (Central Asian Orogenic Belt, Northern Mongolia). The process of Egiingol pyroxenite veins formation is investigated using new major and trace element analyses of pyroxenite minerals, calculations of closure temperatures and composition of equilibrium melt. The pyroxenites show abundant petrographic and geochemical evidence for replacement of the residual peridotite minerals by ortho- and clinopyroxene due to melt-rock interaction. Relics of peridotite olivines are found in pyroxenites, Cr^# of spinel increases from peridotites to pyroxenites, and compositions of ortho- and clinopyroxene change from peridotite to pyroxenite. The authors show that calculated equilibrium melts for investigated pyroxenites are very similar to compositions of boninite lavas from the Dzhida terrain. Therefore, formation of pyroxenite veins most likely resulted from percolation of boninite melts through the Egiingol peridotites. Orthopyroxenite veins formed at first, followed by websterite veins. Thus, the authors assume that pyroxenite veins represent the channels for boninitic melts migration in supra-subduction environment.

Fracture-mediated deep seawater flow and mantle hydration on oceanic transform faults

Fluid-rock interaction on oceanic transform faults (OTFs) is important for both the deformation behavior of the lithosphere and volatile cycling in the Earth. Rocks deformed and exhumed at OTFs preserve information about the depth extent of fluid percolation and the nature of fluid-rock interactions within these fault zones. In this study, we focus on five dredges from the Shaka and Prince Edward OTFs on the ultraslow spreading Southwest Indian Ridge that recovered significant volumes of deformed mantle rocks. Samples are predominantly mylonites that have been deformed to high strains in the fault zone, but also contain several generations of fractures. Based on the mineral assemblages in fractures and shear bands combined with thermobarometry analysis, we identified three distinct temperature ranges of fluid-mantle interactions associated with deformation. At low temperature (LT), this leads to crystallization of serpentine (± talc ± amphibole ± chlorite) at <500–550°C. At medium temperature (MT), chlorite and amphibole crystallized at ∼500–750°C. At high temperature (HT), amphibole (± second generation peridotitic minerals) crystallized. The composition of minerals in HT fractures and shear bands indicates that fracturing and fluid flow occur up to temperatures of at least 850–875°C. Combining these results with modeled geotherms for both faults suggests that seawater percolation extended to depths of 20–25 km and that serpentinization extended to ∼11–13 km. The evolution of fault zone structure induced by deep fluid-rock interaction and progressive formation of LT, MT and HT mylonites on OTFs results in weakening and strain localization within the oceanic lithosphere, and suggests that the global transform system may represent a large reservoir of volatiles in the Earth's lithosphere.

Experimental Study of the Peridotite–Basalt–Fluid System: Phase Relations at Subcritical and Supercritical Р-Т Conditions

The paper reports experimental data on the partial melting of hydrous peridotite and basalt at pressures up to 4 GPa and temperatures up to 1400°С, as well as peridotite–basalt association in the presence of alkaline water–carbonate fluid as an experimental model of mantle reservoir with subducted oceanic protoliths. At partial melting of the hydrous peridotite at 3.7–4.0 GPa and 1000–1300°С, critical relations were observed over the entire studied pressure and temperature interval. At partial melting of hydrous basalt, critical relations between silicate melt and aqueous fluid were recorded at 1000°С and 3.7 GPa. The Na-alkaline silicate melt coexists with garnetite at 1100°С and with clinopyroxenite at 1150 and 1300°С. The peridotite–basalt–alkaline-aqueous–carbonate fluid at 4 GPa and 1400°C shows signs of critical relations between a carbonated silicate melt and a fluid. Reaction relations in the residual peridotite minerals with replacements of olivine ← orythopyroxene ← clinopyroxene ← potassic amphibole-type testify to a high chemical activity of the supercritical liquid. Experimental results suggest the existence of regions of partial melting (asthenospheric lenses) in the fluid-bearing upper mantle under supercritical conditions. These lenses contain near-solidus highly reactive supercritical liquids enriched in incompatible elements. Mantle reservoirs with supercritical liquids are geochemically similar to undepleted mantle and could serve as sources of magmas enriched in incompatible elements. The modal and cryptic upper mantle metasomatism under the effect of supercritical liquids leads to the refertilization of peridotite via enrichment of residual minerals in incompatible elements.