Podiform chromitites in the mantle sections of ophiolites belong to either high-Cr (metallurgical) or high-Al (refractory) varieties. Their highly variable compositions are reflected by different ...Cr#s 100Cr/(Cr+Al) and Cr2O3 and Al2O3 contents of the chromite, falling in the boninitic and MORB fields, respectively. Parental magmas of high-Cr chromitites have higher Sc, Mn, Co and Ni, and lower Ti, V, Zn and Ga concentrations than MORB melts; their trace-element patterns are similar to those of boninites, except for Ni and Zn. In contrast, high-Al chromitites have parental magmas characterized by generally flat MORB-normalized patterns, showing slight enrichments in V, Mn and Co, and depletion in Ni and Zn. Regardless of their compositions, both types of chromitites have chondrite-normalized platinum group element (PGE) patterns showing enrichment in IPGE and depletion in PPGE. A variety of platinum group minerals are typically present in both types, occurring either as euhedral inclusions or along fractures in chromite grains. These minerals have a wide span of Re–Os isotopic compositions, reflecting a variety of origins.
There is a diversity of unusual minerals and mineral inclusions associated with podiform chromitites. The presence of these minerals suggest that grains of amphibolite (plagioclase, amphibole and zircon) and eclogite (coesite, kyanite and garnet) were present in the magmas from which chromite crystallized. Multiphase mineral inclusions demonstrate that podiform chromitites form from hydrous mafic magmas in suprasubduction zone environments (SSZ). We propose a new model in which chromitite formation was involved in intra-oceanic subduction zones initiated in closing oceanic basins. Continued subduction carries oceanic and possibly continental crustal materials to deep levels where they are metamorphosed under greenschist, amphibolite and eclogite facies conditions. The tearing and breakoff of the subducted slab, possibly along the transitional contact between amphibolites and eclogites, create a slab window through which the underlying asthenosphere rises and melts to generate Cr-rich mafic magmas. These upward-migrating magmas pass through the subduction zone and assimilate the subducted slab. As a result of slab contamination, these magmas become more siliceous, more oxidized and more hydrous, rapidly triggering chromite crystallization. Minute grains of chromite are suspended in the upward-moving magmas as they migrate through the overlying metasomatized mantle wedge. Such chromite-bearing magmas eventually deposit chromite in magma conduits in the uppermost mantle close to the Moho where the upward flow changes from vertical to subhorizontal and velocity is greatly reduced.
Highly reduced and ultrahigh pressure minerals including diamonds are reported in literature both in podiform chromitites and host peridotites of ophiolites. Some of these minerals in association with host peridotites may have been brought by the uprising asthenosphere at mid-oceanic ridges due to the mantle convection. It is also possible that some diamonds may have formed in the subducted slab below about 150km. Some minerals of subducted slabs are preserved because they are encapsulated in chromite grains where they are protected from the SSZ melts. Some of these SSZ mantle wedges are emplaced on land to become podiform chromitite-bearing ophiolites.
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•Podiform chromitites crystallize from hydrous and high-Mg SSZ melts.•Crustal minerals suggest crystallization of chromite after slab assimination.•Asthenospheric melts pass through slab window.•Chromite deposits in mini-magma conduits near Moho.
High-precision Mg isotopic measurements on diverse mantle pyroxenite xenoliths collected from Hannuoba, North China Craton, revealed multi-stage interactions between the lithospheric mantle and melts ...of different origins. The garnet-bearing pyroxenites yield variable δ26Mg values from −0.48‰ to −0.10‰, consistent with their origin as reaction products between mantle peridotites and melts from subducted oceanic slab with highly heterogeneous δ26Mg. Most of their constituent olivine, clinopyroxene, and orthopyroxene have indistinguishable δ26Mg ratios around the normal mantle range (−0.25±0.07‰, Teng et al., 2010). The lack of fractionation among these three mineral phases agrees with their similar bonding environments for Mg (6-fold), and hence indicates a general isotopic equilibrium among them. By contrast, garnet has variably lighter δ26Mg values (−0.75‰ to −0.37‰, n=15), consistent with its higher coordination number for Mg (8-fold), and thus weaker Mg–O bonds. The magnitude of fractionation between garnet and olivine/pyroxene, however, is not correlated with equilibrium temperature, and therefore reflects disequilibrium Mg isotope partitioning. Considering the metasomatic origin of these garnets, the disequilibrium isotopic fractionation is most likely the result of rapid and incomplete metasomatic reaction during which garnets were formed at the expense of isotopically heavier co-existing minerals, particularly spinels. The two garnet-free clinopyroxenites, which display highly enriched light rare earth element (LREE) patterns and very low Ti/Eu ratios, are characterized by extremely light δ26Mg (as low as −1.51‰). Their formation possibly indicates an episode of carbonatite infiltration. In comparison, the three Cr websterites and one Al websterite, as well as an orthopyroxenite, all have mantle-like whole-rock and mineral δ26Mg ratios, with equilibrated clinopyroxene–orthopyroxene pairs. Their presence thus implies different episodes of asthenosphere-derived silicate melt injection into the mantle. Collectively, these observations suggest that mantle metasomatism plays an important role in producing inter-mineral Mg isotopic disequilibrium and local Mg isotopic variation in the lithospheric mantle. Therefore, Mg isotopes can be used to trace metasomatic activities in the mantle.
Secondary ion mass spectrometry (SIMS) measurements of δ18O in nine chromite (Mg, Fe)(Al, Cr, Fe)2O4 samples (Cr# = 0.58–0.79, Mg# = 0.39–0.72) from chromitites and harzburgite have confirmed their ...intra‐ and inter‐grain isotopic homogeneity, making them suitable reference materials for in situ analysis. The SIMS measurements of the oxygen isotopic composition of these chromites exhibit a range of instrumental mass fractionation (IMF) as a function of the end‐member compositions of the chromite, particularly the magnetite (FeOFe2O3) and spinel (MgAl2O4) components. Functional relationships have been developed to correct for IMF over the compositional range XMag = 0.01 to 0.05, and XSpl = 0.14 to 0.22. We conclude that, to be accurate, SIMS measurements of δ18O must be combined with electron probe microanalyses of the individual analysed spots.
Key Points
Nine potential chromite reference materials for SIMS oxygen isotope measurements.
All these materials exhibit good reproducibility in terms of oxygen isotope ratios.
These chromites display varying instrumental mass fractionation, influenced by their end‐member compositions, notably the magnetite and spinel components.
We present petrological investigations and mineral chemistry of several Tethyan ophiolites to reveal the occurrence, origin, and fate of water in podiform chromitites. The results show that ...clinopyroxene and olivine in chromitites have H2O contents of 801-366 and 53-17 ppm, respectively. The highest water contents of olivine occur in massive chromitite and the lowest always in the clinopyroxene-bearing ores because much of the available hydrous fluids was taken up by the clinopyroxene during crystallization. The major and trace elemental and Li isotopic compositions of clinopyroxene associated with chromite and olivine in podiform chromitites indicate formation from a mixture of surface hydrous fluids on chromite grains and evolved melts from which olivine crystallized. The hydrous fluids initially originated from dehydration of a subducting slab as revealed by Li isotopic compositions of clinopyroxene and olivine in the chromitites. High fluid/rock ratios facilitated concentration of chromite to form chromitite, suppressing crystallization of olivine. The hydrous fluids that were collected on the chromite grain surface during crystallization allowed chromite grains to rise via decreasing density in the form of bubbles, thus promoting their gathering and concentration. The fate of these hydrous fluids depends on ambient physical and chemical conditions. Mostly they hydrate adjacent olivine grains in the chromitite or penetrate the surrounding dunite envelope. In some cases, the fluids dissolve into silicate melts to produce water-bearing clinopyroxene and/or hydrous minerals, such as amphibole, or infiltrate silicate and chromite grains to form inclusions, which may exsolve later in the form of mineral lamellae. Our investigations provide direct natural evidence for the presence and importance of water in the formation and evolution of chromite deposits, as inferred by earlier experimental studies.
The Pozantı-Karsantı ophiolite is one of the well-preserved supra-subduction-zone oceanic lithosphere slices. It offers a unique opportunity to study oceanic crust formation, and to specify ...subduction initiation processes of the Inner Tauride ocean in the Neo-Tethyan subduction system. The Pozantı-Karsantı ophiolite is tectonically underlain by a subophiolitic metamorphic sole and mélanges. The metamorphic sole is mostly amphibolites that are enriched in light rare earth elements and large-ion lithophile elements, low
ε
Nd
(
t
) values (+ 5.4 to + 8.3) and high
207
Pb/
204
Pb ratios, suggesting an OIB-like geochemical affinity. Secondary ion mass spectrometry (SIMS) zircon and titanite U–Pb ages reveal a protolith age at ~ 122.2 ± 1.3 Ma and metamorphic age at 92–90 Ma for the amphibolites. Gabbro and diabase dikes intruded the metamorphic sole rocks and ophiolitic units at ~ 91 to 90 Ma. Their elemental and Sr–Nd–Pb isotopic characteristics indicate a MORB-like mantle source, different from subduction fluid-altered dikes formed at 86.9 ± 3.1 Ma. The temporal-geochemical sequence therefore suggests that the intra-oceanic subduction initiated at 91.9 ± 0.8 Ma within an older lithosphere (> 122 Ma) of the Inner Tauride ocean. Incipient subduction led to forearc (ultra-)slow-spreading seafloor with mafic dikes of 91–90 Ma. The fast switch from MORB- to SSZ-type oceanic spreading might have occurred in less than 3 Myr and marks the inception of a mature subduction zone.
Ophiolites commonly sample the uppermost parts (15–20 km) of fossil oceanic lithosphere. However, in recent years, the documentation of diamonds, super‐reducing (e.g., SiC), and other “unusual” ...minerals from several ophiolitic peridotites and chromitites (e.g., Tibet and the Polar Urals) has caused debate concerning their origin (i.e., deep vs. shallower upper mantle). Here we report on symplectite‐bearing lherzolites from the Purang ophiolite in Tibet, which preserve the first compelling evidence of garnet‐facies protoliths. These lherzolites were previously formed and stabilized at a depth of ~85–100 km, which is much deeper than generally suggested and approaches the depth (~120 km) required for stabilizing the diamonds. Combining with other key observations, we suggest the Purang garnet‐bearing peridotites may represent mixtures of oceanic lithosphere domains with diverse origins; they were rapidly exhumed at a variety of mantle depths within a subduction channel associated with oceanic slab retreat.
Plain Language Summary
Ophiolites are remnants of fossil ocean basins that have been emplaced on land and usually sample the upper 15–20 km of oceanic lithosphere. However, the discovery of diamonds, super‐reducing, and other “unusual” minerals from several ophiolitic peridotites suggests that these peridotites might originate in the deep upper mantle (>300 km), challenging conventional models of ophiolite formation. In this study, we show that symplectite‐bearing lherzolites from the Purang diamond‐bearing ophiolite (Tibet) were originally garnet‐bearing peridotites, with a previous equilibration depth of ~85–100 km. Our results suggest that the Purang peridotites may represent mixtures of oceanic lithosphere domains with different nature and origins, which were exhumed and mixed within an oceanic subduction channel associated with slab retreat.
Key Points
Symplectite‐bearing lherzolites from a Tibetan diamond‐bearing ophiolite formed from garnet peridotite protoliths
These lherzolites were previously formed and stabilized at a depth of ~85‐100 km, much deeper than previously suggested
An oceanic subduction channel model is suggested for exhumation of some of the Tibetan diamond‐bearing peridotites
Magnesium isotopic compositions are reported for twenty‐four international geological reference materials including igneous, metamorphic and sedimentary rocks, as well as phlogopite and serpentine ...minerals. The long‐term reproducibility of Mg isotopic determination, based on 4‐year analyses of olivine and seawater samples, was ≤ 0.07‰ (2s) for δ26Mg and ≤ 0.05‰ (2s) for δ25Mg. Accuracy was tested by analysis of synthetic reference materials down to the quoted long‐term reproducibility. This comprehensive dataset, plus seawater data produced in the same laboratory, serves as a reference for quality assurance and inter‐laboratory comparison of high‐precision Mg isotopic data.
Les compositions isotopiques du magnésium sont fournies pour vingt‐quatre matériaux géologiques de référence internationaux, comprenant des roches ignées, métamorphiques et sédimentaires, ainsi qu'une phlogopite et des serpentines. La reproductibilité à long terme de la détermination isotopique du Mg, basée des analyses sur quatre ans d’échantillons d'olivine et d'eau de mer, était ≤ 0.07% (2s) pour δ26Mg et ≤ 0.05% (2s) pour δ25Mg. La précision a été testée par l'analyse de matériaux de référence synthétiques jusqu’à la reproductibilité à long terme indiquée. Cette base de données complète, ainsi que des données d'eau de mer produites dans le même laboratoire, servent de référence pour l'assurance qualité et la comparaison inter‐laboratoires de haute précision des données isotopiques du Mg.
Carbonatite metasomatism plays an important role in modifying the composition of Earth's mantle, however, its effect on mantle Mg isotopic composition is poorly constrained. Here, we report ...high-precision mineral Mg isotope data for three suites of mantle peridotite xenoliths that experienced variable degrees of carbonatite metasomatism. The δ26Mg values of minerals in these xenoliths are variable and range from −0.32 to −0.11‰ in olivine, from −0.28 to −0.09‰ in orthopyroxene, from −0.27 to −0.05‰ in clinopyroxene, from 0.06 to 0.44‰ in spinel and from −0.61 to −0.37‰ in garnet. Calculated bulk-rock δ26Mg values of the peridotites vary from −0.27 to −0.10‰, falling within and slightly higher than the normal mantle range (−0.25 ± 0.07‰). The coexisting minerals are in isotopic equilibrium, with clinopyroxene δ26Mg values correlated with the carbonatite metasomatic indices such as MgO and Na2O in orthopyroxene. These results suggest that carbonatite metasomatism does not produce light Mg isotopic signature in mantle peridotites as previously suggested, instead it might slightly elevate their δ26Mg values. Therefore, carbonatite-metasomatized peridotites in the mantle cannot be the primary source rocks of low-δ26Mg mantle-derived magmas. Instead, fractional crystallization and accumulation of chromite during ascent of the basaltic magmas may explain the isotopically light basalts, as supported by the covariations of δ26Mg with chemical indices of chromite crystallization (e.g., Cr, V, Fe and Ti). Consequently, chromite crystallization may significantly influence the physiochemical processes on the genesis of basalts, which would require comprehensive evaluation in future studies.
•First report of Mg isotopic compositions of mantle carbonatite-metasomatized peridotites.•Overall normal-mantle Mg isotopic compositions of major constituent minerals.•No relation of carbonatite metasomatized peridotites with low-δ26Mg mantle-derived magmas.•Significant effects of chromite crystallization on Mg isotopic compositions and genesis of continental basalts.
Here we attempt to constrain the P–T evolution of ultrahigh‐temperature granulites using textures coupled with multiple thermobarometric approaches. Sapphirine‐bearing granulites were collected from ...a quarry in the central part of the Highland Complex of Sri Lanka. Three sapphirine‐bearing domains were selected and petrographically studied. Homogeneous sample domains were thermodynamically modelled using their bulk compositions (forward phase equilibria modelling). One heterogeneous sample from a single domain, composed of irregularly distributed residuum and melt, was also used. The bulk composition of its residual part was calculated using mineral compositions and their respective modes. Equilibrium T–X(Fe2O3) phase diagrams were constructed in the chemical system NCKFMASHTO to estimate the bulk ferric/ferrous iron ratio, and conventional geothermometers (garnet–orthopyroxene and Al in orthopyroxene) were applied. The Ti in zircon trace element thermometer was also applied to calculate peak metamorphic conditions. Modal abundance isopleths of each mineral in equilibrium phase diagrams and textural observations were combined to constrain the retrograde P–T path. Our hybrid approach of forward and inverse phase equilibria modelling and conventional thermobarometric calculations indicate that the sapphirine‐bearing granulites have reached their peak T of 920–940°C at P ~10 kbar under relatively highly oxidizing conditions. Subsequently, the rocks followed a near‐isobaric cooling path down to 890–860°C, prior to near‐isothermal decompression up to 6 kbar. The results highlight the importance of dealing with Fe3+. Multiple thermobarometric approaches on carefully observed mineral textures are required to retrieve the most reliable P–T conditions of HT/UHT mineral assemblages.
Contrary to what is currently known, archetypal zircon samples from gneisses and intrusive leucogranites in the Palaeoproterozoic Suhum Basin, SE Ghana, suggest the involvement of Neoarchean crustal ...material in the formation of the Palaeoproterozoic juvenile crust of the Birimian terranes in Ghana. The zircons dated using U–Pb dating methods and subjected to Lu–Hf isotopic analysis suggest that crustal‐forming events from different contemporaneous magmatic episodes within the Suhum Basin took place over a time interval of 139 Ma from 2224 to 2085 Ma. Whole‐rock Lu–Hf data obtained for the gneissic and leucogranitic rocks gave model ages (T
DM2
) ranging from 2789 to 2456 Ma with ɛHf(t) values ranging from −1.1 to +5.4. These model ages imply that the magmas that formed these rocks were sourced from the early Palaeoproterozoic juvenile mantle with substantial Neoarchean crustal reworking.
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