Chondrite-normalized Platinum-group Elements Patterns Research Articles

The chromitites of the Herbeira massif (Cabo Ortegal Complex, Spain) revisited

The ultramafic rocks of the Herbeira Massif in the Cabo Ortegal Complex (NW Iberia) host chromitite bodies. The textural and compositional study of the host rocks and the chromitites classified them into: (1) Type-I chromitites, forming massive pods of intermediate-Cr chromite (Cr# = 0.60–0.66) within dunites; and (2) Type-II chromitites forming semi-massive horizons of high-Cr chromite (Cr# = 0.75–0.82) interlayered with dunites and pyroxenites. Minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V and Sc) contents in the unaltered chromite cores from both types show patterns very similar to fore-arc chromitites, mimicked by the host dunites and pyroxenites. Calculated parental melt compositions suggest that Type-I chromitites crystallized from a melt akin to fore-arc basalt (FAB), while Type-II chromitites originated from a boninite-like parental melt. Both melts are characteristic of a fore-arc setting affected by extension during rollback subduction and have been related to the development of a Cambrian-Ordovician arc. These chromitites are extremely enriched in platinum-group elements (PGE), with bulk-rock PGE contents between 2,460 and 3,600 ppb. Also, the host dunites and pyroxenites exhibit high PGE contents (167 and 324 ppb, respectively), which are higher than those from the primitive mantle and global ophiolitic mantle peridotites. The PGE enrichment is expressed in positively-sloped chondrite-normalized PGE patterns, characterized by an enrichment in Pd-group PGE (PPGE: Rh, Pt and Pd) over the Ir-group PGE (IPGE: Os, Ir and Ru) and abundant platinum-group minerals (PGM) dominated by Rh-Pt-Pd phases (i.e. Rh-Ir-Pt-bearing arsenides and sulfarsenides, Pt-Ir-Pd-base-metal-bearing alloys, and Pt-Pd-bearing sulfides). The PGM assemblage is associated with base-metal sulfides (mostly pentlandite and chalcopyrite) and occurs at the edges of chromite or embedded within the interstitial (serpentinized) silicate groundmass. Their origin has been linked to direct crystallization from a S-As-rich melt(s), segregated by immiscibility from evolved volatile-rich small volume melts during subduction. At c. 380 Ma, retrograde amphibolite-facies metamorphism occurred during the exhumation of the HP-HT rocks of the Capelada Unit, which affected chromitites and their host rocks but preserved the primary composition of chromite cores of the chromitites. This event contributed to local remobilization of PGE as suggested by the negative slope between Pt and Pd and high Pt/Pd ratios in the studied chromitites, and host dunites and pyroxenites. In addition, it promoted the alteration of primary PGM assemblage and the formation of secondary PGM. Nanoscale observations made by focused ion beam high-resolution transmission electron microscopy (FIB/HRTEM) analysis of a composite grain of Rh-bearing arsenide with PGE-base-metal bearing alloys suggest the mobilization and accumulation of small nanoparticles of PGE and base-metals that precipitated from metamorphic fluids forming PGE-alloys. Finally, we offer a comparison of the Cabo Ortegal chromitites with other ophiolitic chromitites involved in the Variscan orogeny, from the Iberian Peninsula to the Polish Sudetes. The studied Cabo Ortegal chromitites are similar to the Variscan chromitites documented in the Bragança (northern Portugal) and Kraubath (Styria, Austria) ophiolitic massifs.

Mantle-hosted ophiolitic chromitites from Colombia: implications for petrogenesis and geodynamic evolution

ABSTRACT Chromitite bodies can be found within ophiolitic mantle sections in three locations in Colombia: Medellín, Planeta Rica, and La Guajira. These chromitites are typically massive pods that are often surrounded by lateritic soil in Medellín and Planeta Rica, or small discoidal bodies in La Guajira. Inclusions in chromite are preserved to varying degrees in the three localities: olivine and pargasite are partially preserved in Medellín and Planeta Rica, whereas inclusions in La Guajira chromitites are completely altered to chlorite. Compositionally, chromite from the chromitites is high-Al in Medellín (Cr#[Cr/(Cr+Al) atomic ratio] = 0.41–0.51) and Planeta Rica (Cr# = 0.49–0.57), and high-Cr in La Guajira (Cr# = 0.75–0.76), whereas TiO2 contents are low in the three localities (0.12–0.43 wt%). Regarding trace elements composition, the chromitites are similar to others formed in supra-subduction zone settings; the chromite from the high-Al chromitites is similar to that in MORB/BABB, whereas chromite from the high-Cr chromitites has a strong boninitic affinity. Chromitites from Planeta Rica are richer in Ti, possibly due to the presence of nearby gabbroic bodies and impregnated peridotites. The alteration in chromite rims and silicate matrix is related to metamorphic processes. Bulk-rock platinum-group element (PGE) contents range from 30 to 487 ppb, and chondrite-normalized PGE patterns show the typical enrichment in IPGE relative to PPGE observed in most ophiolitic chromitites. The estimated melts in equilibrium with the different chromitites correspond to MORB-like melts for the Medellín and Planeta Rica chromitites and to boninites for La Guajira chromitites. However, the estimated TiO2 content in the melt is too low for a typical MORB, indicating back-arc basin basalts and a back-arc setting for the formation of the Medellín and Planeta Rica chromitites during the Late Triassic and the Late Jurassic, respectively. La Guajira chromitites are interpreted to have formed in a fore-arc basin related to subduction-initiation during Cretaceous times.

Genesis and high-pressure evolution of the Köyceğiz ophiolite (SW Turkey): Mineralogical and geochemical characteristics of podiform chromitites

Textural and compositional characteristics of mineral inclusions in podiform chromitites flourish thoughtful information about the genesis of the Köyceğiz ophiolite (SW Turkey). In the chromitite bodies, olivine, orthopyroxene, clinopyroxene, and sulfide inclusions are mostly sub-rounded or globular. Less abundant needle-shaped silicate inclusions (10–50 μm-long) are aligned along crystallographic orientations in the host magnesiochromite grains. Raman spectroscopy and micro analytical data indicate that these lamellar inclusions have consistently a diopside composition. Uncommon, discrete micro-diamonds and moissanite (SiC) particles, observed in some magnesiochromite grains, may suggest formation under ultra-high pressure (UHP) super-reduced (SuR) conditions.The Köyceğiz chromitites are characterized by variable platinum-group element (PGE) abundances (80–441 ppb), with unfailing IPGE > PPGE contents. Fractionated chondrite-normalized PGE patterns of chromitites and associated peridotites and data scatter in thePd/Ir versusPt/Pt* space indicate chromitite formation by partial melting of the associated mantle rocks. The variable 187Os/188Os(i) (0.1278–0.1380) and γOs (−0.38 to +7.54) values demonstrate extensive contamination of crustal components, likely in a supra-subduction zone (SSZ) setting. Combined with the occurrence of unusual ‘exotic’ mineral inclusions, geochemical and isotopic characteristics of the Köyceğiz chromitites replicate superimposed deep mantle processes, recycling and subduction-related phase transformations.

Mineralogy and geochemistry of peridotites and chromitites in the Jijal Complex ophiolite along the Main Mantle Thrust (MMT or Indus Suture Zone) North Pakistan

The Jijal Complex, one of the largest Neo-Tethyan ophiolites (ca. 150 km2) in Pakistan, occupies a deep-level section of the Cretaceous Kohistan Arc that was obducted along the Main Mantle Thrust or Indus Suture Zone. Peridotites of the Jijal Complex consist mainly of harzburgite and dunite with numerous podiform chromitites. In this paper, we present new mineral compositions, whole-rock major elements, trace elements, rare earth elements (REEs) and platinum group elements (PGEs) to evaluate the melt evolution and petrogenesis of these rocks. Chromian spinel in the chromitites is fairly uniform in composition with Cr#s ranging from 75 to 81, Mg#s from 59 to 63 and TiO2 < 0.2 wt%, which suggest crystallization from boninitic magmas. Chromian spinel in the peridotites has relatively lower Cr#s of 71–74, Mg#s of 38–48 and very low TiO2 contents (averaging 0.09 wt%). The Fe3+#s of chromian spinels both in the chromitites and peridotites are very low (maximum 0.02), reflecting crystallization under low oxygen fugacity (fO2). The harzburgites and dunites have low average CaO, Al2O3 and REE values and contain Al-poor clinopyroxene and high-Cr spinels. Mantle-normalized trace element patterns of the harzburgite, dunites and chromitites reflect high-degrees of partial melting (25%–30%). The high-Cr chromitites all show similar chondrite-normalized platinum group element patterns characterized by enrichment in Os, Ir, Ru and Rh relative to Pt and Pd. The mineral chemistry, whole-rock compositions and PGE geochemistry indicate that the chromitites in the Jijal Complex were generated from a boninitic parental magma generated under low to high fO2 in a supra-subduction zone environment. The calculated compositions of the parental melts of the Jijal chromitites are consistent with differentiation of arc-related magmas.

Mineralogy and geochemistry of peridotites and chromitites in the Aladag Ophiolite (southern Turkey): melt evolution of the Cretaceous Neotethyan mantle

The Aladag Ophiolite (also known as the Pozanti-Karsanti Ophiolite) is one of the largest Neotethyan ophiolites in Turkey and represents Cretaceous oceanic lithosphere derived from the Inner Tauride seaway of Neotethys. Its peridotites consist of spinel harzburgites, dunites and chromitites. The harzburgites display a depleted mineral and whole-rock geochemistry, indicating high degrees of partial melting. They contain Pd- and Pt-rich interstitial sulphides, which crystallized from S-saturated melts during melt–rock interactions. Their spoon-shaped rare earth element patterns indicate metasomatism by fluids, derived from a subducting slab, which were enriched in light rare earth elements. The chromitites in both the mantle and mantle–crust transition zone rock units show similar geochemical compositions, typical of high-Cr chromitites. They also show similar chondrite-normalized platinum group element patterns characterized by Os, Ir, Ru and Rh enrichments relative to Pt and Pd. The parental melts of the Aladag Ophiolite chromitites were S-undersaturated and boninitic in composition. The presence of microdiamond, moissanite, zircon and needle-shaped clinopyroxene exsolutions in chromite (chr) grains suggests that their formation might have originally involved high-temperature and high-pressure conditions within the mantle transition zone before their participation in further melting and the formation of oceanic crust at shallow depths in a forearc tectonic setting during the initiation of intra-oceanic subduction within the Inner Tauride seaway. Supplementary material: Sampling and analytical methods, representative microprobe analyses and whole-rock major and trace element compositions are available at https://doi.org/10.6084/m9.figshare.c.4153631 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Mineralogy, geochemistry, and melt evolution of the Kalaymyo peridotite massif in the Indo-Myanmar Ranges (western Myanmar), and tectonic implications

We present new whole-rock major, trace, and platinum group element (PGE) and mineral chemistry data from the Kalaymyo peridotite massif in the central part of the Indo-Myanmar Ranges (western Myanmar) and discuss its mantle melt evolution. The Kalaymyo peridotites consist mainly of harzburgites, which show typical porphyroclastic or coarse-grained equigranular textures. They are composed of olivine (forsterite, Fo = 89.8–90.5), orthopyroxene (enstatite, En 86–91 , wollastonite, Wo 1–4 , ferrosilite, Fs 8–10 ; Mg# = 89.6–91.9), clinopyroxene (En 46–49 Wo 47–50 Fs 3–5 ; Mg# = 90.9–93.6), and spinel (Mg# = 67.1–78.9; Cr# = 13.5–31.5), and have relatively homogeneous whole-rock compositions with Mg#s of 90.1–90.8 and SiO 2 (41.5–43.65 wt%), Al 2 O 3 (1.66–2.66 wt%), and CaO (1.45–2.67 wt%) contents. They display light rare earth element (LREE)–depleted chondrite-normalized (CN) REE patterns with (La/Yb) CN = 0.04–0.21 and (Gd/Yb) CN = 0.40–0.84, and show a slight enrichment from Pr to La with (La/Pr) CN in the range of 0.98–2.36. The Kalaymyo peridotites are characterized by Pd-enriched chondrite-normalized PGE patterns with superchondritic (Pd/Ir) CN ratios (1.15–2.36). Their calculated oxygen fugacities range between the quartz-fayalite-magnetite (QFM) oxygen buffers, QFM–0.57 and QFM+0.90. These mineralogical and geochemical features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees (5%–15%) of partial melting at a mid-oceanic ridge environment. The observed enrichment in LREE and Pd was a result of their reactions with enriched mid-oceanic ridge basalt–like melts percolating through these already depleted residual peridotites. The Kalaymyo and other ophiolites in the Indo-Myanmar Ranges therefore represent mid-oceanic ridge–type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward-migrating subduction-accretion system along the eastern margin of India.

Petrogenesis of Cenozoic basalts in central-eastern China: Constraints from Re–Os and PGE geochemistry

We present Re–Os and PGE (platinum group elements), together with major–, trace–element and Sr–Nd isotope data, to further constrain the petrogenesis for the Cenozoic basalts in central-eastern China. The basalts include low-Ca alkali, high-Ca alkali and tholeiitic varieties. Most low-Ca alkali basalts have unradiogenic 187Os/188Os (mostly <0.14), flat chondrite-normalized PGE patterns and depleted Sr–Nd isotopic compositions, indicating that they probably originated from a depleted mantle source with insignificant shallow-level crustal contamination. Minor high-Ca alkali/tholeiitic basalts have low PGE concentrations, particularly Os (down to 0.0056ppb), and highly suprachondritic initial 187Os/188Os ratios (up to >0.4), indicating involvements of sulfide/PGE-alloy segregation–AFC (assimilation–fractionation–contamination) processes. Most tholeiites and high-Ca alkali basalts have relatively high Os concentrations, highly variable and suprachondritic Os isotopic compositions, high Pd/Ir ratios and enriched Sr–Nd isotopic compositions. Modeling results reveal that the Os isotopic variations in these basalts cannot be ascribed to crustal contamination during ascent but mainly reflect source characteristics. Combined with their low CaO, low Mg#, high FC3MS values (FeOT/CaO–3×MgO/SiO2, all in wt.%), and high Fe/Mn signatures, a large portion of pyroxenite is inferred as an enriched component in the mantle sources to account for their highly suprachondritic Os isotopic compositions. The formation of the pyroxenites was probably due to the subducted sediment-bearing Pacific oceanic crust or the recycled lower continental crust resulted from the collision between the Yangtze and North China blocks.

Geochemistry of platinum-group elements and mineral composition in chromitites and associated rocks from the Abdasht ultramafic complex, Kerman, Southeastern Iran

The Abdasht complex is a major ultramafic complex in south-east Iran (Esfandagheh area). It is composed mainly of dunite, harzburgite, podiform chromitites, and subordinate lherzolite and wehrlite. The podiform chromitites display massive, disseminated, banded and nodular textures. Chromian spinels in massive chromitites exhibit a uniform and restricted composition and are characterized by Cr# [=Cr/(Cr+Al)] ranging from 0.76 to 0.77, Mg# [=Mg/(Mg+Fe2+)] from 0.63 to 0.65 and TiO2<0.2wt.%. These values may reflect crystallization of the chromian spinels from boninitic magmas. Chromian spinels in peridotites exhibit a wide range of Cr# from 0.48 to 0.86, Mg# from 0.26 to 0.56 and very low TiO2 contents (averaging 0.07wt.%). The Fe3+# is very low, (<0.08wt.%) in the chromian spinel of chromitites and peridotites of the Abdasht complex which reflects crystallization under low oxygen fugacities.The distribution of platinum group elements (PGE) in Abdasht chromitites displays a high (Os+Ir+Ru)/(Rh+Pt+Pd) ratio with strongly fractionated chondrite-normalized PGE patterns typical of ophiolitic chromitites. Moreover, the Pd/Ir value, which is an indicator of PGE fractionation, is very low (<0.1) in the chromitites.The harzburgite, dunite and lherzolite samples are highly depleted in PGE contents relative to chondrites. The PdN/IrN ratios in dunites are unfractionated, averaging 0.72, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru anomaly, and their PdN/IrN ratio averages 2.4 and 2.3 respectively. Moreover, the PGE chondrite and primitive mantle normalized patterns of harzburgite, dunite and lherzolite are relatively flat which are comparable to the highly depleted mantle peridotites.The mineral chemistry data and PGE geochemistry indicate that the Abdasht chromitites and peridotites were generated from a melt with boninitic affinity under low oxygen fugacity in a supra-subduction zone setting. The composition of calculated parental melts of the Abdasht chromitites is consistent with the differentiation of arc-related magmas.

Platinum-group minerals in the LG and MG chromitites of the eastern Bushveld Complex, South Africa

The chromitites of the Bushveld Complex in South Africa contain vast resources of platinum-group elements (PGE); however, except for the economic upper group (UG)-2 chromitite seam, information on the distribution of the PGE in the ores and on the mineralogical nature, assemblages, and proportions of platinum-group minerals (PGM) is essentially missing. In the present geochemical and mineralogical study, PGE concentrates originating from the lower group (LG)-6 and middle group (MG)-1/2 chromitites were investigated with the intention to fill this gap of knowledge. Chondrite-normalized PGE patterns of bulk rock and concentrates are characterized by a positive slope from Os to Rh, a slight drop to Pt, and an increase to Pd again. The pronounced similarities of the PGE patterns indicate similar primary processes of PGE concentration in the chromitites, namely “sulfide control” of the PGE mineralization, i.e., co-precipitation of chromite and sulfide. Further, the primary control of PGE concentration in chromitites appears to be dual in character: (i) base-level concentrations of IPGE (up to ∼500 ppb) hosted within chromite and (ii) co-precipitation of chromite and sulfide, the latter containing virtually the entire remaining PGE budget. Sulfides (chalcopyrite, pentlandite, and pyrite; pyrrhotite is largely missing) are scarce within the chromitites and occur mainly interstitial to chromite grains. Pd and Rh contents in pentlandite are low and erratic. Essentially, the whole PGE inventory of the ores occurs in the form of discrete PGM. The PGM are almost always associated with sulfides. The dominant PGM are various Pt-Pd-Rh sulfides (cooperite/braggite [(Pt,Pd)S] and malanite/cuprorhodsite [CuPt2S4]/[CuRh2S4]), laurite [RuS2], the main carrier of the IPGE (Os, Ir, Ru), sulfarsenides [(Rh,Pt,Ir)AsS], sperrylite [PtAs2], Pt-Fe alloys, and a large variety of mainly Pd-rich PGM. The LG and MG chromitites have many characteristics in common and define a general, “typical” PGM spectrum of Bushveld chromitites. This PGM assemblage is characterized by the predominance of PGE-sulfides including elevated proportions of malanite, variable proportions of (sulf) arsenides, and Pt-Fe alloys in conjunction with a paucity of (bismutho)tellurides. The formation of this specific PGM spectrum is related to the distinct chromitite environment and its depositional and post-depositional history, whereby desulfurization reactions have probably played an important role. The LG-6 samples have higher contents of PGE-sulfides, including extraordinary high proportions of malanite but low PGE-arsenide and PGE-sulfarsenide contents compared to the MG-1/2 samples. This indicates a higher availability of arsenic either in the stratigraphically higher MG-1/2 samples (compared to the LG-6) or regionally in the chromitites south of the Steelpoort lineament.

Metamorphic alteration of the massive sulfide horizon from the Salt River VMS deposit (South Africa)

Sulfides from the upper sulfide zone of the Salt River VMS deposit (South Africa) were analyzed for PGE (platinum group elements), Re, Au, Sb, As, Se, Te, Cr, Co, Ni, Mn, Mo, Ag, Hg and Pb using in-situ LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) techniques. The analysis of a plethora of trace elements during the ablation of one single spot was possible by using a multi-standard approach, involving the analysis of several external standards before, after and between the analysis of unknowns, including a new in-house standard for As, Sb, Se, and Te. Chondrite normalized PGE patterns best fit data from Besshi-type deposits, and show that metamorphism was isochemical with respect to the PGE and Au. Although most sulfide minerals experienced melting no migration into discrete pockets could be observed.

Reevaluation of siderophile element abundances and ratios across the Cretaceous–Paleogene (K–Pg) boundary: Implications for the nature of the projectile

The discovery over 30years ago at Gubbio (Italy) and Caravaca (Spain) of an enrichment in the concentrations of iridium (Ir) and the other platinum group elements (PGE) by up to four orders of magnitude (Irmax=0.10–87ng/g) compared to average continental crustal background levels remains one of the most important discoveries in the Earth sciences. Since then, similar anomalies have been detected in more than 120 Cretaceous–Paleogene (K–Pg) boundary sites worldwide. Highly elevated Ir and other siderophile element abundances in roughly chondritic ratios are considered strong indicators for the presence of a meteoritic contribution in impact-related lithologies (melt rocks, impact ejecta material, etc.), delivered when an extraterrestrial object strikes Earth. The presented work adds 113 unpublished PGE analyses of 38 K–Pg sections worldwide to the existing literature. The analytical protocol relied on for this purpose consisted of a combination of a nickel-sulfide fire assay pre-concentration technique and subsequent trace metal determination via inductively coupled plasma-mass spectrometry (ICP-MS). Through repeated determination of key siderophile elements (i.e., Cr, Co, Ni, and PGE), the importance of sampling, nugget effects, and analytical methodologies applied becomes more apparent. Even more critically, these analytical effects are superimposed by the local syn- and post-depositional conditions that have affected the pristine meteoritic signature of the K–Pg impactor, including potential fractionation during vaporization and condensation, dissimilar PGE carrier phases, terrestrial PGE input, sedimentation rate, reworking, diagenesis, bioturbation, and chemical diffusion. While chondrite-normalized PGE patterns of individual sites appear relatively flat (i.e., chondritic), strong variations in siderophile element content and inter-element ratios exist between K–Pg locations, inter-laboratory measurements, and replicate analyses, hampering a precise projectile identification using (highly) siderophile elements. Only when considering improved databases of siderophile element concentrations in meteorites, in combination with linear regression analysis to calculate inter-element ratios from a large suite of ejecta deposit sites, the nature of the K–Pg projectile can be resolved. Application of this methodology to an extensive data set of continental and marine sites, very proximal to distal to the Chicxulub impact structure, supports a carbonaceous chondritic impactor (type CM or CO).

Juvenile subcontinental lithospheric mantle beneath the eastern part of the Central Asian Orogenic Belt

Mineral compositions, clinopyroxene trace-element and Sr–Nd–Hf isotopic compositions, whole‐rock major‐element and Platinum Group Element (PGE) contents, and Re–Os isotopes of peridotite xenoliths in the Cenozoic alkali basalts from Abaga, Inner Mongolia (China) help to constrain the nature of the subcontinental lithospheric mantle beneath the eastern part of the Central Asian Orogenic Belt. The mantle xenoliths are mainly spinel lherzolites, with minor olivine websterite and spinel harzburgite. Clinopyroxene–orthopyroxene equilibration temperatures are higher than 840°C. An olivine websterite (sample A2-2) has the lowest MgO and highest Al2O3, CaO and TiO2 contents, whereas a harzburgite (sample A2-1) has the highest MgO and low Al2O3, CaO and TiO2 contents. The spinel lherzolites have major-element concentrations transitional between the harzburgites and websterites. According to the chemical characteristics and Sr–Nd–Hf isotopic compositions of clinopyroxenes, two groups of spinel lherzolites can be identified. Group 1 spinel lherzolites are moderately depleted in Light Rare Earth Elements (LREEs) and with flat Heavy Rare Earth Element (HREE) patterns, indicating a low degree of melt extraction and minimal metasomatism. They have clinopyroxenes with homogeneous 87Sr/86Sr (0.7030−0.7033), 143Nd/144Nd (0.5131–0.5132) and 176Hf/177Hf (0.2834–0.2835) ratios. They have flat chondrite-normalized PGE patterns and radiogenic 187Os/188Os ratios (0.1232–0.1256), within the range of modern convective mantle. Group 2 spinel lherzolites have low HREE contents with variable LREE concentrations, suggesting complex melt extraction and mantle metasomatism processes. They have relatively heterogeneous Sr–Nd–Hf isotopic compositions with 87Sr/86Sr of 0.7040–0.7054, εNd values of +6.1 to +12.1 and εHf values of +19.2 to +474. The Group 2 spinel lherzolites have flat chondrite-normalized PGE patterns and radiogenic 187Os/188Os ratios (0.1231–0.1252). The websterite has a magmatic origin with weakly decoupled Nd–Hf isotopes and the lowest 187Os/188Os ratio of 0.1219. The harzburgite has a 187Os/188Os ratio of 0.1280 similar to that of the modern convective mantle. All these characteristics indicate that the Group 2 spinel lherzolites represent ancient lithospheric mantle that experienced high‐degree melt extraction and subsequent extensive silicate metasomatism. Thus, it is proposed that the SCLM beneath the Abaga region of the eastern Central Asian Orogenic Belt (CAOB) is mostly juvenile with minor relicts of old depleted lithospheric mantle, which cooled from near-adiabatic to near-lithospheric temperatures to form new lithospheric mantle during the formation of the eastern CAOB.

Effects of melt percolation on platinum group elements and Re–Os systematics of peridotites from the Tan-Lu fault zone, eastern North China Craton

Abstract: Concentrations of platinum group elements (PGE) and Re–Os isotopic compositions were determined for a suite of peridotite xenoliths from Cenozoic Beiyan volcanoes within the Tan-Lu fault zone, eastern North China Craton (NCC). Previous petrographic and geochemical data indicate the existence of considerable modification by melt–peridotite interaction for these xenoliths. Thus, this suite of xenoliths provides an unusual opportunity to examine the effects of melt percolation on PGE abundances and Re–Os isotopes. Our data show that most of the Beiyan peridotites have flat chondrite-normalized PGE patterns whereas a considerable variation in total PGE abundances has been observed in lherzolites to highly fertile cpx-rich lherzolites and wehrlites. Their total PGE contents range between 0.97 and 61.4 ppb and their PGE relative abundances from 0.1 to 0.0001 × CI-chondrites, respectively. In these rocks, the total PGE budget was reduced by more than 90% from lherzolite to cpx-rich lherzolite and to wehrlite, probably because intergranular sulphides were completely removed by silicate melt. However, Beiyan peridotites have low and variable Re (0.0002–0.5118 ppb) and Os (0.194–10.4 ppb) abundances and high 187 Os/ 188 Os (0.12167–0.14978) ratios. The 187 Os/ 188 Os ratios in some wehrlites are much higher than the value for the primitive mantle. The high 187 Os/ 188 Os ratios have also been observed in peridotites found in other localities within the Tan-Lu fault zone such as Shanwang and Nüshan. This could be the result of the addition of radiogenic Os during the melt percolation. Extremely low Os abundances in Beiyan peridotites suggest that Os may behave as an incompatible element during melt percolation and could mobilize with the dissolution of sulphides. The suprachondritic 187 Os/ 188 Os ratios in peridotites within the Tan-Lu fault zone relative to those away from it indicate that the Tan-Lu fault zone played an important role as a melt infiltrating channel in the radiogenic Os enrichment induced by melt percolation. This could be the reason for the Os isotopic heterogeneity observed in the eastern NCC. This study also confirms that Os isotopes and PGE appear to be mobile during massive melt percolation.

Base metal (BM) and platinum-group elements (PGE) mineralogy and geochemistry of the elmaslar chromite deposit (Denizli, sw turkey): implications for a local BM and PGE enrichment

The Elmaslar Chromite Deposit (ECD) in Denizli (SW Turkey) consists of chromitite enveloped by highly refractory replacive dunite within a depleted harzburgite. The composition of chromian spinel is Cr-rich suggesting derivation from boninitic melt-peridotite interaction beneath a supra-subduction zone. The chondrite normalized PGE patterns show predominance of Ir-group (IPGE) over Pd-group PGE (PPGE) that is consistent with ophiolitic podiform chromitites in general. The primary PGM paragenesis of ECD mainly consists of laurite-erlichmanite series with minor Pd bearing phases. Dominance of Osrich laurite over the Os-poor ones suggests initially high fS2 during crystallization of chromitite. It also indicates a slow cooling rate of the chromitite parent melt. Coexistence of irarsite with erlichmanite suggests increase in As activity during late stages of chromitite crystallization. One peculiar contact zone sample with disseminated-massive chromitite enveloped by dunite shows unusually high PPGE content and positive-sloped chondrite normalized PGE pattern. This enrichment is attributed to a two stage process: (i) primary and local sulfur saturation, (ii) followed by secondary hydrothermal alteration causing smallscale remobilization and precipitation of PGE. No mode of occurrence is suggested for the primary and local sulfur saturation stage, because there is no direct field evidence on the geological setting of the sample. However, it is clear that the effects of this event were limited and not deposit wide. The secondary enrichment of PGE is mainly attributed to introduction of fluids during serpentinization. The amphibole-pyroxene-spinel-silica-carbonate rocks found in the stockpiles of ECD are remarkable. The role of these rocks, in the secondary PGE enrichment in ECD stays to be questioned. They mark further low-temperature CO2-metasomatism in ECD and provide promising indicators for investigating secondary PGE occurrences in chromitites.

PGE distribution in massive sulfide deposits of the Iberian Pyrite Belt

We present the first platinum group elements (PGE) data on seven massive sulfide deposits in the Iberian Pyrite Belt (IPB), one of the world largest massive sulfide provinces. Some of these deposits can contain significant PGE values. The highest PGE values were identified in the Cu-rich stockwork ores of the Aguas Tenidas Este (Σ PGE 350 ppb) and the Neves Corvo (Σ PGE 203 ppb) deposits. Chondrite normalized PGE patterns and Pd/Pt and Pd/Ir ratios in the IPB massive, and stockwork ores are consistent with the leaching of the PGE from the underlying rock sequence.

Platinum-Group-Element Systematics of Peridotites from Ophiolite Complexes of Northwest Anatolia, Turkey: Implications for Mantle Metasomatism by Melt Percolation in a Supra-subduction Zone Environment

Platinum-group-element (PGE) studies of peridotites from the supra-subduction zone (SSZ) ophiolites of northwest Anatolia provide evidence for the nature of melt extraction within the uppermost mantle, and interactions between subduction-related magma and oceanic lithosphere. The peridotite samples from the mantle section of the ophiolites are mainly spinel-harzburgites and dunites, accompanied by subordinate amounts of spinel-lherzolite. Whole-rock major-trace element and mineral chemical characteristics indicate that the peridotites originated as the solid residues of varying degrees of partial melting (~5 to ~20%), and were subsequently modified by interaction with metasomatizing melts. The samples have non-chondritic, fractionated chondrite-normalized PGE patterns. Melt-depleted (e.g., low Al2O3 and CaO contents) mantle harzburgites and dunites show moderate to strong enrichments in the palladium group relative to the iridium group PGEs (PdN/IrN = 1.81±0.23; N = CI-chondrite normalized), and in most samples, pronounced Rh and Pd enhancements relative to Pt (RhN/PtN = 2.31 ± 0.66; PdN/PtN = 1.93 ± 0.20). These signatures cannot be reconciled with a simple in situ melt extraction and removal of sulfide phases, but most likely reflect a multi-stage petrogenetic process that selectively enriched the local mantle environment in incompatible and less refractory siderophile elements that are mobilized during continuous melt percolation, while relatively depleting the mantle wedge in Pt, which was not as effectively mobilized by silicate melts (or fluids). The results of quantitative model calculations indicate that the addition of sulfides that originated from interaction between solid mantle and percolating hydrous basaltic melts may account for the strongly supra-chondritic ratios of both Pd/Ir and Ir/Os, as well as for the formation of abundant chromite deposits within the ophiolite complex. The peridotites show no systematic variation of Ir-group PGE (Ir, Ru, Os; I-PGE) abundances relative to melt depletion indices such as Mg#, Al2O3, CaO, or spinel Cr#, despite their remarkable inter-element PGE variations. These along with elevated values of strongly incompatible lithophile elements (e.g., Ba, U, and LREE) in the reactive harzburgites and dunites suggest a post-melting metasomatism and melt impregnation in a suprasubduction zone environment. Enrichment in various incompatible elements (Hf, U, Ta, Sr) relative to the expected values for melt-depleted mantle residues and pronounced negative anomalies in fluid-insoluble high-field-strength elements (Ta, Nb, Zr, Hf, Ti) further suggest that both siliceous melts and slab-derived hydrous fluids were involved in mantle metasomatism.

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.

Platinum-group element abundances in the upper mantle: new constraints from in situ and whole-rock analyses of Massif Central xenoliths (France)

Fourteen peridotite xenoliths collected in the Massif Central neogene volcanic province (France) have been analyzed for platinum-group elements (PGE), Au, Cu, S, and Se. Their total PGE contents range between 3 and 30 ppb and their PGE relative abundances from 0.01 to 0.001 × CI-chondrites, respectively. Positive correlations between total PGE contents and Se suggest that all of the PGE are hosted mainly in base metal sulfides (monosulfide solid solution [Mss], pentlandite, and Cu-rich sulfides [chalcopyrite/isocubanite]). Laser ablation microprobe-inductively coupled plasma mass spectrometry analyses support this conclusion while suggesting that, as observed in experiments on the Cu-Fe-Ni-S system, the Mss preferentially accommodate refractory PGEs (Os, Ir, Ru, and Rh) and Cu-rich sulfides concentrate Pd and Au. Poikiloblastic peridotites pervasively percolated by large silicate melt fractions at high temperature (1200°C) display the lowest Se (<2.3 ppb) and the lowest PGE contents (0.001 × CI-chondrites). In these rocks, the total PGE budget inherited from the primitive mantle was reduced by 80%, probably because intergranular sulfides were completely removed by the silicate melt. In contrast, protogranular peridotites metasomatized by small fractions of volatile-rich melts are enriched in Pt, Pd, and Au and display suprachondritic Pd/Ir ratios (1.9). The palladium-group PGE (PPGE) enrichment is consistent with precipitation of Cu-Ni-rich sulfides from the metasomatic melts. In spite of strong light rare earth element (LREE) enrichments (Ce/YbN < 10), the three harzburgites analyzed still display chondrite-normalized PGE patterns typical of partial melting residues, i.e., depleted in Pd and Pt relative to Ir and Ru. Likewise, coarse-granular lherzolites, a common rock type in Massif Central xenoliths, display Pd/Ir, Ru/Ir, Rh/Ir, and Pt/Ir within the 15% uncertainty range of chondritic meteorites. These rocks do not contradict the late-veneer hypothesis that ascribes the PGE budget of the Earth to a late-accreting chondritic component; however, speculations about this component from the Pd/Ir and Pt/Ir ratios of basalt-borne xenoliths may be premature.

PLATINUM-GROUP MINERALS FROM THE PALEOPROTEROZOIC CHROMITITE, OUTOKUMPU OPHIOLITE COMPLEX, EASTERN FINLAND

This paper describes the presence of platinum-group minerals in the Vasarakangas chromitite from the 1.97 Ga Outokumpu ophiolite complex in eastern Finland. The platinum-group mineral assemblages as well as the compositions of laurite, irarsite and orarsite, and also the chondrite-normalized platinumgroup element patterns are compatible with those described in younger Mesozoic ophiolitic chromitites. In contrast to these younger chromitites, where Irgroup minerals occur mainly as inclusions in chromite, the platinum-group minerals at Vasarakangas are principally found as enclosed minor grains in euhedral matrix gersdorffites. The PGE distribution between separated chromite and gersdorffite fractions is in accordance with the observed exceptional mode of PGM occurrence.

Precious metal abundances in selected iron meteorites: in-situ AMS measurements of the six platinum-group elements plus gold

We have measured the abundances of seven precious metals in NiFe phases (kamacite and plessite) in six iron meteorites. These in-situ analyses, obtained by accelerator mass spectrometry (AMS) on small polished samples previously characterized by electron microprobe techniques, constrain the distribution of the rare siderophile elements. Within our set of irons, a small but varied suite, concentrations measured by AMS vary by factors of 9–13 for Au, Pd, Pt, Rh and Ru, and by factors of 90 and 250 for Ir and Os respectively.Data are presented for all six platinum group elements (PGE) plus gold. The AMS data suggest a variation in overall precious-metal abundances of a factor of 16 between the most-enriched (Negrillos, ΣPGE + Au = 270 ppm) and the least-enriched (Welland, 16–19 ppm).A clear illustration of the use of AMS data for provenance studies of meteoritic iron is presented for the Welland IIIA iron, an 1888 find from Ontario. Few published data are available for Welland: comparison of a type sample with a smaller piece of unknown metal, with respect to chondrite-normalized PGE patterns, major-element chemistry and textures of the metals, strongly support a suggestion that the latter is a fragment of the same iron.