Mantle Section Of Ophiolite Research Articles

Decoupled Trace Element and Isotope Compositions Recorded in Orthopyroxene and Clinopyroxene in Composite Pyroxenite Veins from the Xiugugabu Ophiolite (SW Tibet)

Abstract Pyroxenite veins and dikes are commonly observed in the mantle section of ophiolites. Because of their mantle occurrence, these pyroxenites are free from crustal contamination and offer a unique opportunity for studying mantle compositions and melt–rock interaction processes. We conducted an integrated petrological and geochemical study of a suite of composite orthopyroxenite, websterite, and pyroxene-bearing dunite veins from the Xiugugabu ophiolite located on the western segment of Yarlung–Zangbo Suture Zone. The dunite is separated from the host peridotite by a layer of pyroxenite, forming a composite vein system. Systematic variations in major, minor, and trace element compositions in minerals across the composite veins are observed. Two generations of orthopyroxenes in the pyroxenites are characterized by high Mg#, low TiO2 concentrations, and depleted patterns of incompatible trace elements. Clinopyroxenes in the pyroxenites are characterized by high Mg#, low contents of TiO2 and Na2O, spooned shaped REE patterns, and a negative Zr anomaly. Through major and trace element modeling, we showed that both orthopyroxene and clinopyroxene were in equilibrium with melts with different compositions. This hypothesis is further confirmed by distinct initial Nd and Hf isotope ratios in the two pyroxenes. A model for the formation of composite pyroxenite veins is developed, whereby hydrous and silica-rich melts percolate along the margins of a dunite channel. The orthopyroxenite was formed by the reaction between a hydrous, silica-rich melt and the surrounding peridotite. The websterite is formed by reactive crystallization of a hybrid melt produced by mixing silica-rich melt and the melt formed by remelting of previously depleted peridotite in the deeper part of the mantle column. The extremely enriched Nd–Hf isotope compositions of the pyroxenite veins (εNd = −20.3 to +11.5 and εHf = −13.2 to +25.3, 125 million years ago) can be explained by the addition of ancient, recycled sediments to the mantle source in a supra-subduction setting. Based on the low-Cr# spinel in the Xiugugabu dunites (Cr# = 19–50) and the depleted nature of the parental melt of the Xiugugabu pyroxenites, we deduced that the formation of pyroxenites postdate the formation of the Xiugugabu ophiolite at ~125–130 Ma. Collectively, results from this study have provided support to the hypothesis that the Xiugugabu ophiolite experience a two-stage evolution, i.e., firstly formed in a mid-ocean ridge setting and subsequently modified in a supra subduction zone.

Multistage petrogenetic evolution of Neoproterozoic serpentinized ultramafic rocks and podiform chromitites at Hagar Dungash, Eastern Desert of Egypt

Podiform chromitites are found in the mantle sections of ophiolites in many places and times in the geologic record. Leading theories for their origin invoke melt-rock reaction processes involving hydrous melts in supra-subduction zone settings. Here we develop a case study of a Neoproterozoic example and a quantitative thermodynamic model of the multistage process leading to nearly monomineralic chromitite pods. We demonstrate that the Hagar Dungash ophiolite of the Eastern Desert of Egypt — despite being dismembered, incomplete, and affected by variable degrees of alteration and metamorphism — is nevertheless plainly a supra-subduction zone ophiolite containing podiform chromitite deposits. Textural, mineralogical, and chemical evidence from the serpentinized ultramafic rocks point to mantle protoliths residual to high degrees of partial melt extraction, characteristic of fore-arc ophiolites. The chromitite pods are distinguished into massive and nodular types; Cr-spinel in the massive type is almost fresh, with intermediate to high Cr# (0.60–0.85) and extremely low Fe3+# (nil-0.04). Cr-chlorite with up to ∼ 15 wt% Cr2O3 is a common interstitial mineral in the chromitites. The observations are consistent with the conceptual multistage model wherein boninitic melt develops as the end-product of melt-harzburgite reaction in the shallow upper mantle. Mixing between this boninitic melt and ascending peridotite-derived basaltic melt drives the liquid deep into the Cr-spinel stability field and precipitates nearly monomineralic chromitite. Our thermodynamic model of this mixing and precipitation process reproduces the mineral chemistry trends of the fresh chromite samples and yields an estimate of the efficiency of chromite production relative to melt flux through the system, which can be applied to development of podiform chromitites in supra-subduction systems worldwide and across geologic eras.

Pyroxenites from mantle section of Voykar Ophiolite – Melt/peridotite reaction and crystallization in SSZ mantle

Pyroxenites from Voykar Ophiolite mantle section provide a great record of processes happening during final stages of evolution of an ophiolite. They are represented by the complete range from clino- to ortho-pyroxenites with websterites and complex dunite-pyroxenite veins also present. Their formation is attributed to various degrees of fractional crystallization and reaction of melts and possibly fluids with host harzburgites during changing P and T conditions. Most of pyroxenite parental melts were migrating through that part of the mantle section from a more depleted and higher temperature harzburgites. Presence of subduction signature is evident from mineral and whole rock compositions. Parental melt with boninite-like trace element patterns is proposed with most likely positioning in lithospheric part of forearc mantle. Orthopyroxenites formed on earliest stages from parental melt/fluid most depleted in REE and highest in silica. As the melt reacted with residual peridotites Si content was decreasing and websterites and clinopyroxenites started to form with increase of Cpx/Opx ratio with decreasing temperature. Olivine websterites and clinopyroxenites formed from the melt saturated in olivine and therefore not reacting with residual peridotites. Formation of pyroxenites triggered minor melting of surrounding harzburgites and some of thin cm sized pyroxenites represent accumulations of such melts.

Compositional Variations and Tectonic Settings of Podiform Chromitites Associated with Ultramafic Rocks of the Pan‐African Proterozoic Ophiolites from South Eastern Desert, Egypt

AbstractThe Precambrian podiform chromitites associated with ophiolites are abundant in Pan‐African belt in central Eastern Desert (CED) and south Eastern Desert (SED), Egypt and range from 690 to 890 Ma in age. The studied chromitites associated with Neoproterozoic ophiolites are distributed in southern Eastern Desert, Egypt in Baranis ‐ Shalaten sheet and occur as lenticular bodies with variable dimensions in ultramafic component (serpentinites). We present geochemical and mineralogical data from three areas of ophiolites and associated chromitites namely Gebel Abu Dahr (D), Gebel Arais (A) and Gebel Anbat in the Wadi Hodein area (H) (Fig. 1). The paper studies the compositional variations and tectonic settings of podiform chromitites associated with ultramafic rocks, in addition to the alteration process of chromite during metamorphism.The ophiolite in the present areas is composed of the ultramafic rocks (mainly serpentinites) with minor relics of fresh dunite and harzburgite. All these rocks are affected by metamorphism and subsequent retrograde during subduction and exhumation.Six samples selected from the serpentinites geochemically analyzed for major, trace and some REE elements and the geochemical results reflect that harzburgite and dunite compositions are typical of depleted mantle peridotite. Microprobe analyses and SIMS investigations were carried out for three massive podiform chromitite ore bodies and disseminated chromites in serpentinites (1215 spot probe analyses), and silicate minerals in serpentinite rocks such as serpentine and olivine (102 spots). Serpentine minerals are mainly antigorite with some chrysotile in serpentinite rocks and in chromitites, mainly filling cross‐cutting veins.In this study, we consider that the alteration occurred in two stages: during the first one chromite reacted with olivine and water to form Cr‐ and Fe‐rich, porous chromite and chlorite; during the second event magnetite filled the pores, created in the porous chromite and defused into this chromite to form homogeneous magnetite.According to this, the composition of chromite is a key factor controlling the metamorphic reaction between olivine and chromite because if the primary chromite is very poor in Al, the chlorite‐forming reaction hardly takes place. In this case, during the second event, the addition of magnetite only contributes to create a magnetite corona around the former chromite grains without any diffusion at the chromite‐magnetite boundary as suggested by Gerbilla et al. (2012).Barnes (2000) studied the chromite in komatiites and modification during green schist to mid amphibolite facies metamorphism. He suggested that the chromite cores continually equilibrated with magnetite rims document metamorphic grade conditions. Barnes (2000) suggested that the relative proportions of Cr3+, Al3+ and Fe3+ of chromite are not affected by metamorphism up to lower temperature amphibolite facies implying restricted mobility of these elements occurred under lower amphibolite facies. So, the chromite in lower temperature amphibolite facies preserves its primary igneous chemistry and can be used to estimate the metamorphic grade. Sack and Ghiorso (1991) and Barnes (2000) suggested that all chromite cores are equilibrated at temperature below ∼ 500–550°C corresponding to lowest amphibolite facies metamorphism and reflect magmatic composition not influenced by metamorphism. In this study, there is no alteration but only nearly pure magnetite deposition and development with restricted Cr‐solubility at < 500°C in the chromite rims on crystal boundaries and within fractures as shown in Fig 2a, b. Also magnetite alters later to hematite.The podiform chromitites are common in the Moho transition zone (MTZ) to the mantle section of ophiolites or harzburgite dominant peridotite massifs (e.g., Arai, 1997; Miura et al., 2012). They have been interpreted as a product of peridotite / melt reaction and subsequent melt mixing within the MTZ to the upper mantle; they are basically magmatic cumulates that formed at the upper mantle level (e.g., Arai and Yurimoto, 1994; Zhou et al., 1994). They are thus a good marker of peridotite / melt reaction (e.g., Arai, 1997). The Pan‐African podiform chromitites may have formed in the same way as the Phanerozoic, namely by melt‐ harzburgite reaction and subsequent melt mixing. The podiform chromatites and disseminated chromites are high‐Cr chromites and have range in Cr#(Cr/Cr+Al) atomic ratio from 0.75 to 0.95 and low Ti with boninitic affinity (Fig. 3a), indicating an island arc setting in supra ‐ subduction zone setting. The present massive chromitites and disseminated chromites in serpentinites fall in the field of chromites de Bou Azer, chromites de Cordoba, Argentinia in the Cr# versus Mg# diagram (Fig. 3b, c) (Gervilla et al., 2012)The studied chromatites contain some grains of platinum‐group minerals (PGM) such as sulfides (Os‐rich laurite) and Os–Ir alloy as shown in Fig. 4 and as reported in South Eastern Desert by Ahmed (2007).

Re-Os Isotope Systematics of Sulfides in Chromitites and Host Lherzolites of the Andaman Ophiolite, India

Laser ablation MC-ICP-MS was used to measure the Os-isotope compositions of single sulfide grains, including laurite (RuS2) and pentlandite [(Fe,Ni)9S8], from two chromitite bodies and host lherzolites from ophiolites of North Andaman (Indo-Burma-Sumatra subduction zone). The results show isotopic heterogeneity in both laurite (n = 24) and pentlandite (n = 37), similar to that observed in other chromitites and peridotites from the mantle sections of ophiolites. Rhenium-depletion model ages (TRD) of laurite and pentlandite reveal episodes of mantle magmatism and/or metasomatism in the Andaman mantle predating the formation of the ophiolite (and the host chromitites), mainly at ≈0.5, 1.2, 1.8, 2.1 and 2.5 Ga. These ages match well with the main tectonothermal events that are documented in the continental crustal rocks of South India, suggesting that the Andaman mantle (or its protolith) had a volume of lithospheric mantle once underlaying this southern Indian continental crust. As observed in other oceanic lithospheres, blocks of ancient subcontinental lithospheric mantle (SCLM) could have contributed to the development of the subduction-related Andaman–Java volcanic arc. Major- and trace-element compositions of chromite indicate crystallization from melts akin to high-Mg IAT and boninites during the initial stages of development of this intra-oceanic subduction system.

Petrology of a Neoproterozoic mantle peridotite–chromitite association from Abu Dahr area, Eastern Desert, Egypt: Infiltration of a boninitic melt in highly depleted harzburgite

Peridotites of Abu Dahr represent the main litho–unit of a Neoproterozoic dismembered ophiolite sequence and are among the best–preserved and well–exposed mantle rocks in South Eastern Desert of Egypt. Here, we present new geochemical and mineral chemical data for peridotites and associated pyroxenites and for chromitites and their platinum–group minerals to constrain their petrogenesis and geotectonic setting. The Abu Dahr ophiolite mantle section consists mainly of harzburgites, cut by pyroxenite dykes and containing dunite–chromitite lenses. The harzburgites are composed of olivine, orthopyroxene, spinel and minor clinopyroxene (˂1.0 vol %) and amphibole. Olivine from harzburgites is highly magnesian (Fo 91–93) and Cr–spinel shows a wide–range of Cr2O3 and Al2O3 contents. The enstatite component of orthopyroxene decreases from harzburgite (En = 90–91) to orthopyroxenite (En = 84–87). Amphiboles are represented by magnesiohornblende and tschermakite. The chromitites are massive to disseminated and composed of magnesiochromite with high Cr# (83–93) and Mg# (66–79), and low TiO2 (<0.1 wt%) content. Solid inclusions in chromite include olivine, orthopyroxene and hornblende. Laurite (RuS2) is the most common PGM detected in the investigated chromitite samples and forms micrometer–size inclusions in fresh chromite. Various Ni–sulfides are found both in fresh chromite and along serpentine veinlets. Harzburgites have a refractory composition with a very low Al2O3 (0.4–0.8 wt%) and CaO (0.2–1.6 wt%) contents and high bulk–rock Mg# (89–92). Geochemical data suggest that the Abu Dahr peridotites are highly depleted SSZ peridotites formed in a forearc mantle wedge setting by high degrees of hydrous partial melting and emplaced as a result of the collision of the intra–oceanic arc with the Beitan gneisses. The podiform chromitites and orthopyroxenites were formed due to impregnation of mantle wedge harzburgites by boninitic melt. The highly depleted nature of the harzburgite is responsible for the small reserves of chromite ore at Abu Dahr and in the South Eastern Desert in general.

Petrological characteristics of the Neoproterozoic Ess ophiolite mantle section, Arabian Shield, Saudi Arabia: a mineral chemistry perspective

The Ess ophiolite, one of the most important ophiolitic massifs of the Arabian Shield, consists of a lower succession of serpentinized mantle rocks overlain by an ultramafic cumulate sequence, layered and isotropic gabbros, sheeted dykes, pillow lavas and pelagic sediments. The Ess mantle section is composed mainly of serpentinized peridotites with chromitite pods, dunite, wehrlite and pyroxenite. Extensive metasomatism and alteration has transformed the ultramafic rocks to talc-carbonates, magnesite deposits and listvenite, especially along shear zones and fault planes. Nevertheless, relics of primary chromian spinel, olivine and pyroxenes are observed. Both primary and metamorphic olivines can be recognized in dunite; the latter is marked by very high forsterite content (97–98), low NiO content ( 0.6) and low TiO2 content (< 0.14 wt%) of fresh Cr-spinel and the high forsterite (0.90–0.93) and NiO contents (0.4–0.5 wt%) of fresh olivine are all consistent with residual mantle rocks that experienced high degrees of partial melt extraction. Orthopyroxene and clinopyroxene in the Ess peridotites have low CaO, Al2O3 and TiO2 contents resembling those typically found in depleted harzburgites from fore-arcs. Consequently, we propose that the Ess mantle peridotites formed in a forearc setting during subduction initiation that developed as a result of northwest subduction due to the convergence between East and West Gondwana, leading eventually to closure of the Mozambique Ocean during the Pan-African orogeny.

Importance of permeability and deep channel network on the distribution of melt, fractionation of REE in abyssal peridotites, and U-series disequilibria in basalts beneath mid-ocean ridges: A numerical study using a 2D double-porosity model

A plethora of observations have been made at mid-ocean ridges and mantle sections of ophiolites: presence of tabular replacive dunites, highly depleted LREE in residual abyssal peridotites, U-series disequilibria in fresh basalts, crustal thickness, and highly attenuated seismic and magnetotelluric structures beneath spreading centers. These independent observations can become more powerful if they are jointly analyzed in a self-consistent model. The difficulty for such a model is to resolve and evaluate the effect of fine scale petrologic feature such as dunite channels in a tectonic scale geodynamic model. Here we present a two-dimensional double-porosity ridge model that consists of low-porosity residual lherzolite and harzburgite matrix and high-porosity interconnected channel network. The geodynamic simulation features a spatially distributed channel network and anisotropic permeability that gradually develops as the upwelling mantle is deformed. We compute the porosity, melt and solid flow fields for several choices of channel distribution and permeability model. We use the calculated porosity distribution and velocity fields to model the variations of REE in residual mantle and U-series disequilibria in melts.The present study underscores the importance of deep channel networks and permeability model to the interpretation of first order geophysical and geochemical observations at mid-ocean ridge spreading centers. The anisotropic permeability of channels can enhance melt focusing by 60%, resulting in thicker crust. The attenuated seismic and magnetotelluric structures require a channel network starting from 60 km depth beneath the ridge axis. The depleted REE patterns in clinopyroxenes in residual abyssal peridotites are also consistent with melt extraction into channels starting from 60 km depth. Although deep channels are conducive to producing U-series disequilibria in eruptible melts, the present model still cannot explain the full ranges of the observed U series disequilibria data in MORB samples. Additional factors, processes, and models are discussed. And finally, we found that, within the uncertainty of the permeability, the porosity varies by three folds and the excess of 230Th varies by up to two folds.

Solid-state redistribution of mineral particles in the upwelling mantle flow as a mechanism of chromite concentration in the ophiolite ultramafic rocks (on the example of Kraka ophiolite, the Southern Urals)

The main regularities of the structure of chromitite-bearing zones of ultramafic rock of the ophiolitic association are considered on the example of Kraka massifs. In all studied chromitite-bearing zones, olivine demonstrates a strong preferably crystallographic orientation, indicating that plastic flow was one of the main factors of petrogenesis and ore formation. A critical review of existing ideas about the origin of ophiolitic chromitites has been carried out. It is shown that for models involving the reaction and magmatic formation of dunites and chromitites, there are a number of difficulties. In particular, the application of the magma mixing model to the mantle ultramafiс rocks for the formation of chrome ores is faced with the problem of “free space”. Free space is necessary for the deposition of large volumes of ores, which is absent in a very low-porous crystalline upper mantle. In the “melt-mantle” interaction model, it is difficult to explain the often observed abrupt contacts of dunites and harzburgites, as well as an increase in the content of orthopyroxene in the near-contact parts of harzburgites, which is very often observed in ophiolite massifs. In addition, there is no mechanism for the formation of chromitites as geological bodies in this model. We have shown that the main trend in the composition and structure of the mantle section of ophiolites is stratification, accompanied by the separation of the rheologically most “weak” aggregates of polycrystalline olivine (dunites), which are host rocks for chrome ores. The stratification of the mantle material occurred during the solid-phase redistribution of minerals in the rocks, which are a dispersion system. In this work, a thermodynamic model is substantiated, which demonstrates the possibility of the emergence of solid-state flows in the conditions of the upper mantle and which makes it possible to eliminate some of the difficulties and contradictions characteristic of the magmatic and reaction-magmatic hypotheses.

Fluid rock interactions on residual mantle peridotites overlain by shallow oceanic limestones: Insights from Wadi Fins, Sultanate of Oman

In the southeastern Oman Mountains the mantle section of the Samail ophiolite is unconformably capped by large units of Maastrichtian to Eocene limestones deposited in a shallow marine environment after ophiolite emplacement. In the vicinity of the town of Fins, a deep canyon carved by a stream has exposed mantle sections of the ophiolite. This section is composed of altered peridotites with high concentrations of calcium and small enrichments of silica compared to the Samail mantle protolith suggesting that the peridotites reacted with a hydrous fluid derived from interaction of seawater with the overlying sediments composed of limestones with minor amounts of chert. This is further affirmed by average δ13C (−0.25‰VPDB) δ18O (−5.53‰VPDB) and 87Sr/86Sr (0.70788) in the carbonate veins, consistent with values in the sediments. Clumped isotope thermometry on calcite veins in peridotite establish that they formed at 25–60 °C. Reaction path modeling of carbonate-quartz derived fluids with peridotite reproduces the observed mineral assemblage composed of carbonate and serpentine with similar Mg# and MgO/SiO2 at high water to rock ratios, with carbon, H2O and silica added to the rock by the reacting fluid.

Geochemistry of volcanic and plutonic rocks from the Nahlin ophiolite with implications for a Permo–Triassic arc in the Cache Creek terrane, northwestern British Columbia

In northwestern British Columbia, the Permian Nahlin ophiolite in the northern Cache Creek terrane comprises spinel harzburgite tectonite with minor lherzolite, lower crustal mafic and ultramafic cumulates, gabbroic rocks including dikes intruding mantle harzburgite, and basaltic volcanic and volcaniclastic rocks. New lithogeochemical data from the Menatatuline Range area confirm that plutonic and volcanic rocks of the ophiolite are tholeiitic and arc related, while only a minor component of volcanic rocks are alkaline intraplate basalts. Tholeiitic basalts of the Nahlin ophiolite represent the products of 2%–20% fractional melting, and their complementary residue may be peridotite from the ophiolite mantle section. Correlative tholeiitic volcanic sections can be found elsewhere in the northern Cache Creek terrane, and they may be linked to a regionally extensive (∼200 km) intraoceanic arc. The arc tholeiite geochemistry of the plutonic and volcanic rocks, and the highly depleted nature of the mantle residues, imply that the Nahlin ophiolite formed in a supra-subduction zone environment. The Nahlin ophiolite therefore occupied the upper plate during intraoceanic collision prior to emplacement of the Cache Creek terrane. The volumetrically minor ocean island basalt type volcanic rocks in the northern Cache Creek terrane are associated with carbonate successions bearing Tethyan fauna. These sequences are likely fragments of oceanic plateaux and their carbonate atolls sliced off of the subducting plate and are unrelated to the Nahlin ophiolite-arc system.

Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes

Since ultra-high pressure (UHP) minerals have been discovered in ophiolites from Tibet and the Polar Urals, it is speculated that the mantle sections of ophiolites may originate deep within the mantle. The UHP minerals are frequently found together with ultra-reduced silicides, carbides, and nitrides. Consequently, it is argued that the deep mantle, or at least domains within it, must be highly reduced, so reduced that practically all transition elements at depth are present in the metallic state. We find it problematic to rewrite the history of ophiolite complexes based on these observations and suggest we should search for alternative and more realistic modes of origin. Electric discharge experiments at >6000 K reported here show that the UHP and highly reduced phase assemblages may precipitate from plasmas. We argue that the mineral assemblages may originate by lightning strikes. As such, they may not record the origin and emplacement history of the mantle lithologies within which they occur.

High-Cr chromites from the Late Proterozoic Miaowan Ophiolite Complex, South China: Implications for its tectonic environment of formation

Chromian-spinel (chromite) is a common mineral in ophiolitic rocks and the study of chromite from the mantle sections of ophiolites can help shed light on their petrogenetic origin and tectonic setting. The Proterozoic Miaowan Ophiolite Complex (MOC) in the Yangtze Craton contains disseminated chromite grains in mantle harzburgites and podiform chromitites associated with serpentinised dunites. Most chromite grains display compositional zoning due to alteration but the fresh cores preserve primary igneous compositions. Podiform chromitites in the MOC dunites and harzburgites are compositionally similar to typical ophiolitic chromitites elsewhere. The chromite grains contain numerous inclusions of clinopyroxene and amphibole with minor amounts of olivine, chlorite, base-metal sulphides (BMS) and platinum-group minerals (PGM). The abundant hydrous mineral inclusions within the chromite grains suggest a hydrous mantle source. Core compositions of chromite grains indicate that the parental melts of the chromitites were similar to typical boninitic melts in a forearc setting. However, chromite grains in the harzburgites show mixed MORB and arc signatures. Thus, the mineralogy and geochemistry of the MOC peridotites suggest that the chromitites in the MOC formed in a forearc tectonic setting during reaction between boninitic melts and MORB-type harzburgite in a supra-subduction zone (SSZ) mantle wedge.

Surprises from the top of the mantle transition zone

Recent studies of chromite deposits from the mantle section of ophiolites have revealed a most unusual collection of minerals present as inclusions within the chromite. The initial discoveries were of diamonds from the Luobosa ophiolite in Tibet. Further work has shown that mantle chromitites from ophiolites in Tibet, the Russian Urals and Oman contain a range of crustal minerals including zircon, and a suite of highly reducing minerals including carbides, nitrides and metal alloys. Some of the minerals found represent very high pressure phases indicating that their likely minimum depth is close to the top of the mantle transition zone. These new results suggest that crustal materials may be subducted to mantle transition zone depths and subsequently exhumed during the initiation of new subduction zones—the most likely environment for the formation of their host ophiolites. The presence of highly reducing phases indicates that at mantle transition zone depths the Earth's mantle is ‘super’‐reducing.

Podiform chromitites do form beneath mid-ocean ridges

Podiform chromitites are commonly found within the Moho transition zone to mantle section of ophiolites, as well as in non-ophiolitic peridotite massifs. However, they have rarely been observed in the present-day ocean floor, even though some ophiolites are considered slices of oceanic lithosphere. One of the factors controlling podiform chromitite formation is the chemistry of the host mantle peridotite. A moderately refractory harzburgite that contains chromite with an intermediate Cr# (Cr/(Cr+Al)) of 0.4–0.6 is the optimum host for chromitites. Such a harzburgite represents the most typical lithology of oceanic lithosphere where peridotite–melt reactions, another requirement for chromitite formation, are possibly common. Thus, the oceanic upper mantle is potentially a suitable host for podiform chromitites. In particular, off-ridge magmatism may lead to the formation of podiform chromitite. The apparent rarity of chromitites in the present-day ocean floor is simply a reflection of the under-sampling of mantle material from fast-spreading ridges and the center segment of slow-spreading ridges. However, in addition to ophiolitic chromitites forming at ordinary mid-ocean ridges from genuine MORB, they also form in the typical ophiolite tectonic setting, at supra-subduction zone spreading centers, from wet MORB. Future mantle drilling as an active way of sampling, on the ocean floor will possibly reveal the occurrence of podiform chromitite in present-day oceanic lithosphere, analogous to ophiolitic chromitite.

Peridotitic minerals of the Nidar Ophiolite in the NW Himalaya: sourced from the depth of the mantle transition zone and above

Abstract The Nidar Ophiolite Complex (NOC) consists of a c. 10 km thick ophiolite suite in the NW Himalaya, India. The c. 7 km thick lower ultramafic part of the ophiolite body terminates against the Tso Morari Crystallines, which represent the leading edge of the Indian continental margin. Mineral inclusions from the peridotites in the lower ultramafic part of the NOC were studied, including C 2 /c clinoenstatite, disordered coesite and high-pressure Mg 2 SiO 4 (probably β-Mg 2 SiO 4 ). These minerals, found in two lherzolite bodies from the ophiolite's mantle section, were characterized by laser Raman spectroscopy and electron micro-probe analysis. Textural evidence supporting decompression from an ultra-high-pressure condition was also observed, such as Cr spinel exsolution needles in olivine crystals. The systematic mineral phase transitions of coesite→quartz, high-pressure clinoenstatite→orthoenstatite and β-Mg 2 SiO 4 →Cr spinel exsolution needles in olivine suggest that the mantle section of the Nidar Ophiolite evolved from the deep mantle beneath a palaeo-spreading centre. The phase stabilities of these high-pressure minerals require derivation from the depth of the mantle transition zone (410–660 km). A transport mechanism for these minerals is suggested via dunite channels along a mantle adiabat in the focused convective flow below the spreading centre. This mechanism brought these deep mantle phases into the ultramafic part of the NOC. These observations suggest that some part of the mantle section of the NOC in the NW Himalaya originated in a mid-ocean ridge setting. Supplementary material: Representative mineral chemical compositions of the lherzolites (1M1 and 1NU27) and the host channelized dunites are available at http://www.geolsoc.org.uk/SUP18836 .

Origin of podiform chromitite, a new model based on the Luobusa ophiolite, Tibet

Podiform chromitites have been interpreted as the result of melt–rock reaction and related melt mixing in upper mantle sections of ophiolites. However, the discovery of ultrahigh-pressure (UHP) minerals, especially diamond and coesite, in many podiform chromitites and host peridotites, raises fundamental questions about the validity of this model. Chromitites in the Luobusa ophiolite of Tibet range from massive, to nodular to disseminated. Chromite grains in both the chromitites and peridotites have variable but relatively high MgO and are classified as magnesiochromite. Many magnesiochromite grains in the massive chromitites contain inclusions of forsterite and pyroxene, as well as diamonds and other unusual minerals. Forsterite inclusions have Fo numbers of 97–99 and NiO contents of 1.11–1.29wt.%. Mg#s (=100∗Mg/(Mg+Fe)) of clinopyroxene inclusions are 96–98 and those of orthopyroxene are 96–97. X-ray studies show that the olivine inclusions have very small unit cells and short cation–oxygen bond distances, suggesting crystallization at high pressure. In contrast, magnesiochromite grains in nodular and disseminated chromitites lack pyroxene inclusions and their olivine inclusions have lower Fo numbers of 94–96 and lower NiO contents of 0.35–0.58wt.%. In addition, magnesiochromite in massive ores has higher Fe3+/Fetotal (0.42) than that in nodular and disseminated ores, which have ratios of 0.22. Disseminated chromitites also show systematic changes in olivine and magnesiochromite compositions from the dunite envelope to the massive ore, indicating melt–rock reaction. These observations suggest that the formation of podiform chromitites is a multi-stage process. Magnesiochromite grains and perhaps small bodies of chromitite crystallize deep in the mantle under low ambient ƒO2 from partial melts of peridotite. UHP minerals and highly magnesian olivine and pyroxene inclusions are trapped in these magnesiochromite grains. When oceanic crustal slabs are trapped in suprasubduction zones (SSZ), they are modified by island arc tholeiitic and boninitic magmas, which change the magnesiochromite compositions and deposit chromitite ores in melt channels.

A comparative study of melt-rock reactions in the mantle: laboratory dissolution experiments and geological field observations

Systematic variations in mineralogy and chemical composition across dunite-harzburgite (DH) and dunite-harzburgite-lherzolite (DHL) sequences in the mantle sections of ophiolites have been widely observed. The compositional variations are due to melt-rock reactions as basaltic melts travel through mantle peridotite, and may be key attributes to understanding melting and melt transport processes in the mantle. In order to better understand melt-rock reactions in the mantle, we conducted laboratory dissolution experiments by juxtaposing a spinel lherzolite against an alkali basalt or a mid-ocean ridge basalt. The charges were run at 1 GPa and either 1,300°C or 1,320°C for 8–28 h. Afterward, the charges were slowly cooled to 1,200°C and 1 GPa, which was maintained for at least 24 h to promote in situ crystallization of interstitial melts. Cooling allowed for better characterization of the mineralogy and mineral compositional trends produced and observed from melt-rock reactions. Dissolution of lherzolite in basaltic melts with cooling results in a clinopyroxene-bearing DHL sequence, in contrast to sequences observed in previously reported isothermal-isobaric dissolution experiments, but similar to those observed in the mantle sections of ophiolites. Compositional variations in minerals in the experimental charges follow similar melt-rock trends suggested by the field observations, including traverses across DH and DHL sequences from mantle sections of ophiolites as well as clinopyroxene and olivine from clinopyroxenite, dunite, and wehrlite dikes and xenoliths. These chemical variations are controlled by the composition of reacting melt, mineralogy and composition of host peridotite, and grain-scale processes that occur at various stages of melt-peridotite reaction. We suggest that laboratory dissolution experiments are a robust model to natural melt-rock reaction processes and that clinopyroxene in replacive dunites in the mantle sections of ophiolites is genetically linked to clinopyroxene in cumulate dunite and pyroxenites through melt transport and melt-rock reaction processes in the mantle.

Oman diopsidites: a new lithology diagnostic of very high temperature hydrothermal circulation in mantle peridotite below oceanic spreading centres

Mafic–ultramafic dykes scattered in the mantle section of ophiolites are generally crystallisation products from common silicate melts. In the frame of a global survey of these melt migration relics in the Oman mantle harzburgites, we discovered a peculiar lithology made essentially of pure diopside ( 0.95 < M g M g + F e total < 1 ) whose characteristics do not match a magmatic or mantle origin. Arguments against a magmatic or mantle origin for these diopsidites combine compositional and textural evidence. In spite of their refractory composition, they are strongly depleted in Cr (Cr 2O 3 0.2 wt.%). By the same way other minor elements (Al, Ti, Na…) and rare earth elements have peculiarly low abundances and plot away from magmatic differentiation trends. In a few samples, the paragenetic association includes pure anorthite (An% up to 0.99), minor amounts of forsterite (Fo > 0.95) or traces of andradite. When not deformed, the diopsides are automorphic and their texture points to metamorphic growth in a matrix composed of antigorite and/or carbonate. Diopsidites have textures reminiscent to that of skarns developing in contact metamorphic halos or that of rodingite frequently present in the serpentine bodies of the ophiolitic crust. They frequently appear as dykes (former cracks), with a few mm to few tens of cm wide transitional zones, which contain high amounts of hydrous minerals, between the diopsidite facies and its host rock. The diopsidites are not randomly distributed in the Oman ophiolite, being more abundant near former asthenospheric diapirs emplaced at shallow depth in the lithosphere. We interpret the diopsidites as the footprint of very high temperature circulation of seawater and carbonated fluids (> 800 °C), which may have leached plagioclase rich lithologies before penetrating the mantle (as shown by a well developed positive Eu anomaly). Our data confirm the prediction of McCollom and Shock [T.M. McCollom, E.L. Shock, Fluid-rock interactions in the lower oceanic crust: Thermodynamic models of hydrothermal alteration, J. Geophys. Res. 103 (B1) (1998) 547–575.] who proposed that common anhydrous minerals like pyroxene, plagioclase and olivine may crystallise from high temperature fluids intermediate between silicate melts and supercritical water. This confirms that there is no clear-cut thermal and chemical boundary between the fields of magmatic and hydrothermal crystallisations.

An experimental study of the kinetics of lherzolite reactive dissolution with applications to melt channel formation

The kinetics of lherzolite dissolution in an alkali basalt and a basaltic andesite was examined experimentally at 1,300°C and 1 GPa using the dissolution couple method. Dissolution of lherzolite in basaltic liquids produces either the melt-bearing dunite–harzburgite–lherzolite (DHL) sequence or the melt-bearing harzburgite–lherzolite sequence depending on whether the reacting melt is or close to olivine saturation (alkali basalt) or olivine + orthopyroxene saturation (basaltic andesite). The dunite in the DHL sequence is pyroxene-free and the harzburgites in both sequences are clinopyroxene-free. The melt fraction and olivine grain size in the dunite are larger than those in the harzburgite. The olivine grain size in the dunite and harzburgite in the DHL sequence also increases as a function experimental run time. Across the sharp dunite–harzburgite and harzburgite–lherzolite interfaces, systematic compositional variations are observed in the reacting melt, interstitial melt, olivine, and to a lesser extent, pyroxenes as functions of distance and time. The systematic variations in lithology, grain size, mineral chemistry, and melt compositions are broadly similar to those observed in the mantle sections of ophiolites. The processes of lherzolite dissolution in basaltic liquids involve dissolution, precipitation, reprecipitation, and diffusive transport in the interstitial melts and surrounding minerals. Preferential dissolution of olivine and clinopyroxene and precipitation of orthopyroxene in the basaltic andesite produces the melt-bearing harzburgite–lherzolite sequence. Preferential dissolution of clinopyroxene and orthopyroxene and precipitation of olivine results in the melt-bearing DHL sequence. Preferential mineral dissolution can also affect the composition of the through-going melt in a dunite channel or harzburgite matrix. Systematic variations in melt fraction and mineral grain size in the peridotite sequences are likely to play an important role in the development of channelized or diffuse porous melt flow in the mantle.