Ophiolitic Mantle Sequence Research Articles

Origin of high-Cr chromite deposits in nascent mantle wedges: Petrological and geochemical constraints from the Neo-Tethyan Luobusa ophiolite, Tibet

The petrography and compositions of chromitites and peridotites from the Tibetan Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere, are used to study the origin of high-Cr chromite deposits and relevant magmatic evolution in nascent mantle wedges during subduction initiation. The mantle sequence of the ophiolite has a two-layered structure with cpx-poor and cpx-rich harzburgites dominating the upper and lower layers, respectively, and locally contains dunite and chromitite bodies in the upper layer. The cpx-rich and cpx-poor harzburgites contain clinopyroxene having finger-like protrusions and chromite showing both irregular and euhedral shapes, and display spoon-shaped and U-shaped REE patterns, respectively. The dunites and chromitites followed a crystallization sequence Ol-Chr-Cpx and present spoon-shaped to U-shaped REE patterns. Clinopyroxene grains in the cpx-rich harzburgites have major and REE compositions slightly more depleted than those in abyssal peridotites, whereas clinopyroxene grains in the cpx-poor harzburgites and dunites have boninitic affinity. Chromite grains in the chromitites and cpx-rich harzburgites have the highest and lowest Cr#s [100 × Cr/(Cr + Al)] (75–80 vs. 20–40), respectively, whereas those in the cpx-poor harzburgites and dunites have intermediate Cr#s. The dataset above indicates that the Luobusa harzburgitic mantle wedge was once modified by slab-derived fluids and later refertilized by melts with compositions intermediate between MORB and boninitic lavas. Under high geothermal conditions induced by slab rollback and asthenospheric upwelling, subsequent partial melting in the refertilized H2O-bearing mantle formed high-Ca boninitic to arc picritic melts, which reacted with the overlying cpx-rich harzburgites and generated high-Cr chromitites, dunites and cpx-poor harzburgites under conditions of diminishing melt/rock ratios. Chromitites and peridotites in mantle sequences of forearc ophiolites keep important information for reconstructing the geodynamic evolution of subduction initiation.

Genetic classification, distribution and ore genesis of major PGE, Co and Cr deposits in China: A critical review

Major platinum-group element (PGE), cobalt (Co) and chromium (Cr) deposits in China are commonly associated with mafic-ultramafic rocks regardless magmatic or hydrothermal origins. Nearly all PGE and half of Co resources in China are from magmatic Ni-Cu-(PGE) sulfide deposits hosted in mafic-ultramafic intrusions. Major resources of Cr are from podiform chromite deposits associated with the mantle sequences of ophiolites, including the Luobusha chromite deposit in Tibet and the Sartohay chromite deposit in Xinjiang. However, there is a lack of large PGE and chromite deposits related to layered mafic-ultramafic intrusions. Sedimentary rock-hosted stratiform Cu deposits and Ni-Co laterites are two dominant types of Co-bearing deposits among terrestrial Co resources and are sporadically distributed in China, although these are insignificant. Platinum-group elements from three Ni-Cu-(PGE) sulfide-bearing mafic-ultramafic intrusions that formed within-plate settings account for 74% of the total PGE reserves of China. Among these, the Jinbaoshan and Yangliuping intrusions are parts of the Emeishan large igneous province (LIP) in SW China, and the Jinchuan intrusion emplaced in a rifting margin of the southern North China Craton. The Jinbaoshan intrusion hosts the only PGE-dominated deposit in China, but has not yet been mined due to the difficulty for access. In the Jinchuan and Yangliuping Ni-Cu deposits, PGEs are by-products. The Jinchuan intrusion is by far the largest PGE producer in China. Cobalt resources are from deposits associated with mafic-ultramafic rocks in China, including Ni-Cu sulfide deposits, Ni-Co laterites, volcanogenic massive sulfide (VMS) deposits and magmatic Fe-Ti oxide deposits, which make up 41% of total Co reserves of China. Major Ni-Cu sulfide deposits in both within-plate and convergent margin settings contain economic Co resources, such as the Jinchuan intrusion and those in the Emeishan LIP, and the Xiarihamu intrusion in the East Kunlun orogenic belt, the Jianchaling intrusion in a Neoproterozoic subduction zone in the northern margin of the Yangtze block and those in the central Asian orogenic belt. Ni-Co laterites that contain economic Co resource have been only discovered locally in Hannan, Yunnan and Taiwan. The Derni Cu-Co deposit in the A’nyemaqen suture zone of the East Kunlun orogenic belt is a special type of VMS deposits closely associated with ophiolite suites, which is also very rare worldwide. Magmatic Fe-Ti oxide deposits in the Emeishan LIP and hydrothermal iron oxide deposits in the East Tianshan orogenic belt have likely potential Co resources. Podiform chromite deposits in China occur in ophiolites, such as the Sartohay, Hegenshan and Suolunshan ophiolites in the central Asian orogenic belt, the Dadao Erji and Yushi ophiolites in the Qilian-Qinling orogenic belt, the Dongqiao ophiolite in the Bangong-Nujiang suture zone and Luobusha ophiolite in the Yarlung-Zangbo suture zone. Understanding the mechanisms of PGE enrichment in major Ni-Cu-(PGE) sulfide deposits such as the Jinchuan and Jinbaoshan intrusions has been a focus of studies in the past. The dissolution and re-precipitation processes that may lead to the enrichment of PGE and Co during hydrothermal overprints on magmatic Ni-Cu sulfide deposits have gained popularity. The origin of podiform chromite deposits is generally thought to be triggered by melt-mantle interaction and melt mixing. But it appears that such a traditional thought is challenged by the discoveries of various high-pressure and hydrous minerals that are trapped in chromite. Given PGE, Co and Cr are all strategic and critical metals to China, it is very important to conduct new exploration projects near active mines, such as those in the Emeishan LIP and the Jinchuan intrusion. However, additional experimental and mineralogical studies on the enrichment processes of PGE, Co and Cr are desired to make breakthroughs in order to better understand the behaviors of PGE, Co and Cr in magmatic-hydrothermal systems. Main issues that are worthy to be closely examined in the future include (1) micro- to nano-scale occurrence of PGE, (2) enrichment mechanism and occurrence of Co in mafic-ultramafic rocks and related ore deposits, and (3) mobilization and enrichment processes of Cr in the formation of podiform chromite deposits.

Evolution of nascent mantle wedges during subduction initiation: Li-O isotopic evidence from the Luobusa ophiolite, Tibet

The Li-O isotopes of olivine and pyroxene from the mantle sequence of the Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere in Tibet, reveal a series of magmatic processes and geochemical evolution in a nascent mantle wedge during subduction initiation. Olivine grains from the ophiolitic mantle sequence display δ18O values similar to those of normal mantle (+5.18 ± 0.2‰), but have large δ7Li variations, ∼+4‰ to +13‰ in harzburgites and ∼+1‰ to +11‰ in high-Cr chromitites. Clinopyroxene grains in cpx-bearing harzburgites have finger-like protrusions and LREE-depleted patterns comparable to those of clinopyroxene in abyssal peridotites, and also show high Li concentrations (∼2–8 μg/g) and negative δ7Li values (∼−15‰ to −8‰), which are beyond the ranges of normal upper mantle (<1.3 μg/g and +4 ± 2‰, respectively). Our dataset suggests that the nascent Luobusa harzburgitic mantle wedge was penetrated by limited amounts of slab-derived fluids at an early stage, leading to decoupling of high δ7Li but normal mantle-like δ18O values of the mantle rocks. The hydrated harzburgites were later refertilized by depleted mantle-derived melts. This process generated the cpx-bearing harzburgites with light-δ7Li clinopyroxene, and also imposed normal mantle-like Li isotopic signatures on some previously hydrated high-δ7Li mantle areas at deeper levels. Subsequent melting in both the high-δ7Li and normal mantle-like Li isotopic mantle regimes generated the parental magmas of the high-Cr chromitites with high-Ca boninitic affinities, accounting for the large δ7Li variation of olivine in the chromitites. Slab rollback below the nascent mantle wedge induced asthenospheric upwelling and a high geothermal gradient, which played a more direct role than slab dehydration in triggering the magmatic events of refertilization and formation of the high-Cr chromitites. Numerical modeling of inter-mineral Li diffusion in cpx-bearing harzburgites suggests an interval of <1000 years between the two magmatic events and rapid slab rollback before the generation of the high-Cr chromitites. Intra-oceanic emplacement of the Luobusa proto-forearc lithosphere was possibly finished within 106 years after the chromitite formation.

Spinel peridotite, olivine websterite and the textural evolution of the Purang ophiolite complex, western Tibet

Spinel peridotite and associated olivine websterite form an ultramafic massif at the base of the Purang ophiolite complex of western Tibet. The spinel peridotites consist chiefly of harzburgites with lesser amounts of lherzolite and dunite. All of the peridotites are rich in MgO, but depleted in Al2O3, CaO and TiO2 relative to primary mantle values, indicating depletion by several episodes of partial melting. The olivine websterite was probably generated from evolved melts that had reached olivine saturation and that reacted with the host harzburgite. The evolution of the Purang ultramafic rocks from lherzolite through harzburgite and dunite to olivine websterite, indicates multistage melting and melt/rock reaction processes. Decompression-related textures such as exsolution lamellae of clinopyroxene and spinel in orthopyroxene, melting and melt/rock reaction textures, such as embayed, flame-like grains of spinel and partial chain spinel textures are widespread in the ultramafic rocks. Impregnation textures are reflected by the presence of late intergranular and interstitial clinopyroxene and spinel in the harzburgite, dunite and olivine websterite. Mineral textures vary from lherzolite to harzburgite and dunite to olivine websterite, and equilibrium temperatures of corresponding mineralogical assemblage increase as the degree of melt metasomatism or melt–rock reaction increase. Chromer spinels in the lherzolite with Cr#s (100Cr/(Cr+Al) of 26–33 may have formed beneath a mid-ocean ridge system or in an evolved back-arc basin, whereas spinel in the harzburgite, which has Cr#s of 39–57, is thought to record both abyssal and supra-subduction zone processes. Dunite and olivine websterite containing Cr-spinel with Cr#s of 60–64 and 52–87, respectively, reflect melt–rock reaction in a supra-subduction zone environment. Plots of TiO2 vs Al2O3 for spinel and Al2O3, TiO2 and Cr# vs Mg# for orthopyroxenes of the Purang ophiolite also support these conclusions. The Purang spinel peridotites thus provide a good example of multistage melt extraction, as well as melt/rock and metasomatic reactions in an ophiolitic mantle sequence.

Re–Os isotopic and platinum group elemental constraints on the genesis of the Xiadong ophiolitic complex, Eastern Xinjiang, NW China

The Xiadong ultramafic complex, Eastern Xinjiang, NW China, was tectonically emplaced in Proterozoic rocks of the Middle Tianshan Terrane in the Central Asian Orogenic Belt. It consists of harzburgite, dunite and websterite and is associated with amphibolite. The amphibolite is interpreted as metamorphosed oceanic crust, and contains igneous zircon grains with an age of 368±3Ma.All of the ultramafic rocks have highly variable PGE contents. Both the harzburgite and dunite have high PGEs but low Cu and Ni, whereas the dunite and websterite show significant depletion of Ir and Pt. The dunite and harzburgite generally have low 187Re/188Os ratios (0.074–0.81) with relatively uniform 187Os/188Os ratios from 0.1320 to 0.1375, slightly higher than the primitive upper mantle (PUM) value (0.1296), except for one harzburgite sample with much higher 187Re/188Os (24.6) and 187Os/188Os ratios (0.4581). The websterite has significantly high 187Re/188Os (202–1661) and radiogenic 187Os/188Os (1.166–19.90) ratios relative to the dunite and harzburgite.PGE patterns and Re–Os isotopic compositions clearly indicate that the Xiadong complex is a part of an ophiolitic mantle sequence, and the PGE geochemistry suggests a relatively reduced mantle source. Rocks from the complex may have undergone two stages of formation; early partial melting at a mid-ocean ridge generated a depleted mantle sequence with low Re/Os ratios. The harzburgite represents the residuum after partial melting of the lherzolite, whereas the dunite was formed by later melt/rock interaction. The websterites are cumulates derived from SSZ magmas above an intra-oceanic subduction zone during the second stage of formation. This second stage of magmatism resulted in metasomatism and enrichment of Re/Os ratios and 187Os/188Os isotope ratios of the mantle rocks. Our new data suggest that the Xiadong complex records the former presence of a major ocean basin between the Tarim Block and the Middle Tianshan Terrane at ~368Ma. The ophiolite was emplaced shortly after the second stage of magmatism.

Meter-scale Nd isotopic heterogeneity in pyroxenite-bearing Ligurian peridotites encompasses global-scale upper mantle variability

Research Article| October 01, 2013 Meter-scale Nd isotopic heterogeneity in pyroxenite-bearing Ligurian peridotites encompasses global-scale upper mantle variability Giulio Borghini; Giulio Borghini 1DISTAV, Università di Genova, corso Europa 26, 16132 Genoa, Italy2Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA3Dipartimento di Scienze della Terra, Università di Milano, via Botticelli 23, 20133 Milan, Italy Search for other works by this author on: GSW Google Scholar Elisabetta Rampone; Elisabetta Rampone 1DISTAV, Università di Genova, corso Europa 26, 16132 Genoa, Italy Search for other works by this author on: GSW Google Scholar Alberto Zanetti; Alberto Zanetti 4Istituto di Geoscienze e Georisorse, CNR, via Ferrata 1, 27100 Pavia, Italy Search for other works by this author on: GSW Google Scholar Cornelia Class; Cornelia Class 2Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar Anna Cipriani; Anna Cipriani 2Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA5DSCG, Università degli Studi di Modena e Reggio Emilia, Piazza S. Eufemia 19, 41100 Modena, Italy Search for other works by this author on: GSW Google Scholar Albrecht W. Hofmann; Albrecht W. Hofmann 2Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA6Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany Search for other works by this author on: GSW Google Scholar Steven L. Goldstein Steven L. Goldstein 2Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA7Department of Earth and Environmental Sciences, Columbia University, 61 Route 9W, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (10): 1055–1058. https://doi.org/10.1130/G34438.1 Article history received: 31 Jan 2013 rev-recd: 30 May 2013 accepted: 31 May 2013 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Giulio Borghini, Elisabetta Rampone, Alberto Zanetti, Cornelia Class, Anna Cipriani, Albrecht W. Hofmann, Steven L. Goldstein; Meter-scale Nd isotopic heterogeneity in pyroxenite-bearing Ligurian peridotites encompasses global-scale upper mantle variability. Geology 2013;; 41 (10): 1055–1058. doi: https://doi.org/10.1130/G34438.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Pyroxenites embedded in peridotite are often invoked as a major cause of short-length scale isotopic heterogeneities in the upper mantle, but there has been little direct evidence. We report spatially controlled chemical and Sr-Nd isotopic compositions of pyroxenites and their host peridotites from an ophiolitic mantle sequence in the Northern Apennines, Italy, with depleted mantle compositions, representing a surface exposure of veined upper mantle, a potential source for mid-oceanic-ridge basalts (MORB). Interaction between pyroxenites and adjacent mantle rocks results in centimeter-scale chemical modifications in the host peridotites, systematically lowering their Sm/Nd ratios. Over time, this interaction causes the host peridotite at >0.1 m scale to acquire an isotopic heterogeneity larger than the range defined by the peridotite and pyroxenite end-members. Moreover, the 143Nd/144Nd variation of a single outcrop covers most of the global Nd isotopic variability documented in abyssal peridotites. Such pyroxenite-peridotite veined mantle domains may represent the enriched component rarely found in abyssal peridotites, but often invoked to account for the low end of 143Nd/144Nd variations in MORB. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area, Central Eastern Desert, Egypt

This research aims at integrating remote sensing data and field studies for lithological mapping of different rock units. Geological interpretation of Landsat ETM+ image and field studies revealed that the study area is mainly covered by ophiolitic ultramafic rocks, ophiolitic metgabbros, schistose metavolocanics and alkaline granites.The serpentinites masses occur in the form of dipping or steeply inclined sheets or lenses generally emplaced along thrust faults .They are elongated in a ENE-WSW direction. They are composed essentially of antigorite, clinochrysotile and lizardite with subordinate amount of magnesite, chromite, magnetite, talc and chlorite.Geochemically, the studied serpentinites are rich in MgO, Cr, Ni and high Ni/Co ratio. The examined serpentinites belong to the ultramafic cumulate ophiolite rocks and it was originally a harzburgite that represents a fragment of the oceanic lithosphere formed in a back arc basin, i.e., they belong to an ophiolitic mantle sequence which was formed above subduction zone (SSZ).Several mineral deposits are associated with the ophiolitic ultramafic rocks such as magnesite, talc and chromite. Chromite lenses are mainly encountered at wadi level in a podiform shape. Microprobe analyses of chromite show that the chromite is an aluminum chromite type. The podiform chromite is characterized by low TiO2 (<0.3%) and MnO (<0.27% contents) which are considered as distinct feature of Alpine type chromite deposits. The presence of podiform chromites in the serpentinites are related typically to supra subduction zone ophiolites (SSZ). The studied serpentinites show similarities to the Alpine type serpentinites.

Neoproterozoic ophiolitic peridotites along the Allaqi-Heiani suture, South Eastern Desert, Egypt

The Wadi Allaqi ophiolite along the Egyptian-Sudanese border defines the southernmost ophiolitic assemblage and suture zone in the Eastern Desert. Ophiolite assemblages comprise nappes composed mainly of mafic and ultramafic rocks that were tectonically emplaced and replaced by serpentine and carbonates along shear zones probably due to CO2-metasomatism. Serpentinites, altered slices of the upper mantle, represent a distinctive lithology of dismembered ophiolites of the western YOSHGAH suture. Microscopically, they are composed of more than 90 % serpentine minerals with minor opaque minerals, carbonate, brucite and talc. The mineral chemistry and whole-rock chemical data reported here indicate that the serpentinized peridotites formed as highly-depleted mantle residues. They show compositions consistent with formation in a suprasubduction zone environment. They are depleted in Al2O3 and CaO similar to those in fore-arc peridotites. Also, high Cr# (Cr/ (Cr+Al)) in the relict chrome spinels (average ~0.72) indicates that these are residual after extensive partial melting, similar to spinels in modern fore-arc peridotites. Therefore, the studied serpentinites represent fragments of an oceanic lithosphere that formed in a fore-arc environment, which belongs to an ophiolitic mantle sequence formed in a suprasubduction zone.

Petrological and mineralogical studies of Pan-African serpentinites at Bir Al-Edeid area, central Eastern Desert, Egypt

The studied serpentinites occur as isolated masses, imbricate slices of variable thicknesses and as small blocks or lenses incorporated in the sedimentary matrix of the mélange. They are thrusted over the associated island arc calc-alkaline metavolcanics and replaced by talc-carbonates along shear zones. Lack of thermal effect of the serpentinites upon the enveloping country rocks, as well as their association with thrust faults indicates their tectonic emplacement as solid bodies. Petrographically, they are composed essentially of antigorite, chrysotile and lizardite with subordinate amounts of carbonates, chromite, magnetite, magnesite, talc, tremolite and chlorite. Chrysotile occurs as cross-fiber veinlets traversing the antigorite matrix, which indicate a late crystallization under static conditions. The predominance of antigorite over other serpentine minerals indicates that the serpentinites have undergone prograde metamorphism or the parent ultramafic rocks were serpentinized under higher pressure. The parent rocks of the studied serpentinites are mainly harzburgite and less commonly dunite and wehrlite due to the prevalence of mesh and bastite textures. The serpentinites have suffered regional metamorphism up to the greenschist facies, which occurred during the collisional stage or back-arc basin closure, followed by thrusting over a continental margin. The microprobe analyses of the serpentine minerals show wide variation in SiO 2, MgO, Al 2O 3, FeO and Cr 2O 3 due to different generations of serpentinization. The clinopyroxene relicts, from the partly serpentinized peridotite, are augite and similar to clinopyroxene in mantle-derived peridotites. The chromitite lenses associated with the serpentinites show common textures and structures typical of magmatic crystallization and podiform chromitites. The present data suggest that the serpentinites and associated chromitite lenses represent an ophiolitic mantle sequence from a supra-subduction zone, which were thrust over the continental margins during the collisional stage of back-arc basin.

Platinum group minerals in ophiolitic chromitites from Tehuitzingo (Acatlán complex, southern Mexico): implications for post-magmatic modification

Podiform chromitite bodies occur in serpentinites at Tehuitzingo (Acatlan complex, southern Mexico). Serpentinite and chromitite are believed to represent a fragment of Paleozoic ophiolitic mantle formed in a supra-subduction zone setting. The ophiolitic mantle sequence is associated with eclogitic rocks, enclosed in a metasedimentary sequence. This association suggests that serpentinites, chromitites and eclogitic rocks underwent a common metamorphic evolution, starting from high pressure (eclogite facies) followed by retrogression (epidote-amphibolite and greenschist facies). The chromitites are strongly altered so that chromite grains are transformed to ferrian chromite; no primary silicates (i.e. of magmatic origin) have been preserved. The chromitites are Al-rich, and contain up to 303 ppb platinum group elements (PGE), with a marked predominance of Os + Ir + Ru over Rh + Pd + Pt, resulting in a characteristic negative-slope of the chondrite-normalized PGE pattern. Consistent with the geochemical data the platinum group minerals (PGM) assemblage is dominated by Ru–Os–Ir minerals, occurring both as single-phase or as composite grains generally less than 10 µm in size. The PGM mineralogy includes laurite, osmium, irarsite and Ru–Fe oxide or hydroxide. Based on textural relations, paragenesis and composition, it was possible to establish that Os-rich laurite and irarsite were early liquidus phases, which now occur as inclusions in unaltered chromite. However, most of the PGM are found in the alteration assemblages of the chromitites in close association with ferrian chromite, chlorite, and heazlewoodite. Laurite from the secondary assemblage is Os-poor and commonly shows overgrowths of Os–Ir alloys. Internal zoning of some laurite grains indicates that Os-poor laurite formed from a Os-rich laurite by release of Os and some Ir, that are readily incorporated in the Os–Ir alloys. Such process requires a decrease of sulfur fugacity with decreasing temperature; this is not consistent with the fS2-T trend in magmatic systems. It is proposed, therefore, that the magmatic PGM assemblage underwent mineralogical reworking starting from relatively high temperature during metamorphism. Temperatures, estimated from chlorite geothermometry (399–210 °C), possibly reflect effects of low-grade metamorphism. After that the PGM and the associated sulfides started to be oxidized. Although it is difficult to determine the extent of PGE mobilization on the basis of mineralogical observations, our data suggest that the metamorphism affecting the Tehuitzingo chromitites caused only re-distribution of PGE on a small scale. Thus, we conclude that metamorphism modified the primary PGM assemblage without having changed the whole-rock PGE concentration.

Paleozoic serpentinite-enclosed chromitites from Tehuitzingo (Acatlán Complex, southern Mexico): a petrological and mineralogical study

The serpentinites and associated chromitite bodies in Tehuitzingo (Acatlán Complex, southern Mexico) are in close relationship with eclogitic rocks enclosed within a metasedimentary sequence, suggesting that the serpentinites, chromitites and eclogitic rocks underwent a common metamorphic history. Primary chromites from the chromitite bodies at Tehuitzingo are of refractory-grade (Al-rich) and have a chemical composition similar to that expected to be found in an ophiolitic environment. The chromite grains in chromitites and serpentinites are systematically altered to ‘ferritchromite’. The alteration trend is usually characterized by a decrease in the Al, Mg and Cr contents coupled by an increase in Fe 3+ and Fe 2+. The Tehutizingo chromitites have low Platinum Group Elements (PGE) contents, ranging from 102 to 303 ppb. The chondrite-normalized PGE patterns are characterized by an enrichment in the Ir-subgroup elements (IPGE=Os, Ir, Ru) relative to the Pd-subgroup elements (PPGE=Rh, Pt, Pd). In addition, all chromitite samples display a negative slope from Ru to Pd [(Os+Ir+Ru)/(Pt+Pd)=4.78−14.13]. These patterns, coupled with absolute PGE abundances, are typical of ophiolitic chromitites elsewhere. Moreover, all the analyzed samples exhibit chondrite-normalized PGE patterns similar to those found for non-metamorphosed ophiolitic chromitites. Thus, the PGE distribution patterns found in the Tehuitzingo chromitites have not been significantly affected by any subsequent Paleozoic high-pressure (eclogite facies) metamorphic event. The chondrite-normalized PGE patterns of the enclosing serpentinites also indicate that the PGE distribution in the residual mantle peridotites exposed in Tehuitzingo was unaffected by high-pressure metamorphism, or subsequent hydrothermal alteration since the serpentinites show a similar pattern to that of partially serpentinized peridotites present in mantle sequences of non-metamorphosed ophiolites. Our main conclusion is that the chromitites and serpentinites from Tehuizingo experienced no significant redistribution (or concentration) of PGE during the serpentinization process or the high-pressure metamorphic path, or during subsequent alteration processes. If any PGE mobilization occurred, it was restricted to individual chromitite bodies without changing the bulk-rock PGE composition. Our data suggest that the Tehuitzingo serpentinites and associated chromitites are a fragment of oceanic lithosphere formed in an arc/back-arc environment, and represent an ophiolitic mantle sequence from a supra-subduction zone, the chemical composition of which remained essentially unchanged during the alteration and metamorphic events that affected the Acatlán Complex.

Crystallization of Podiform Chromitites from Silicate Magmas and the Formation of Nodular Textures

Abstract: Podiform chromite deposits consist of numerous individual accumulations of chromite in the mantle sequences of ophiolites, suggesting formation in separate, mini‐magma conduits in the upper mantle. They may show unique nodular and orbicular textures. Simple mixing of two distinct magmas, invoked for chromite deposits in layered intrusions, is inadequate to explain the formation of podiform chromite deposits. More likely, melt/rock interaction triggers the precipitation of chromite by addition of newly‐formed droplets of melt to the main body of magma passing through a conduit, a process similar to that of magma mingling but involving a turbulent, moving magma so that newly‐formed melt droplets behave like snowballs. These droplets concentrate chromite to form an outer shell and, while the magma is moving upwards, less dense silicate melts are squeezed out of the droplets as the shell collapses to form a nodule. Upon cooling, both orbicular and nodular textures are preserved in the chromitite.

Mafic segregations in ophiolite mantle sequences

Mafic segregations of picritic composition and high-temperature–high-pressure mineralogy are described from the mantle sequences of ophiolites and show similarities with parental magma to oceanic tholeiite and have escaped significant fractionation and re-equilibration due to ascent through an unperturbed geothermal gradient. Such conditions are uncharacteristic of mature speading centres and indicate that the host ophiolites were generated during incipient rifting.