Ru Anomalies Research Articles

Platinum-group elements in Late Quaternary high-Mg basalts of eastern Kamchatka: Evidence for minor cryptic sulfide fractionation in primitive arc magmas

The geochemical variations of magmas across and along supra-subduction zones (SSZ) have been commonly attributed to profound changes in the phase and chemical compositions of the mantle source and subduction-derived melt and fluid fluxes, as well as the physical parameters (e.g. depth, temperature, oxygen fugacity etc) of slab dehydration, mineral breakdown and melting. Here we test the variability of the Late Quaternary primitive magmas in the southern and northern parts of the meridionally oriented Eastern Volcanic Belt (EVB) of Kamchatka, with a slab depth varying from 60 to 160 km. Eight high-Mg (Mg# > 60 mol%) basalts were characterized for major, trace and platinum-group element (PGE) abundances, as well as the compositions of olivine phenocrysts and olivine-hosted spinel inclusions. The basalts in our study are geochemically typical of SSZ magmas and contain similar liquidus assemblages of forsteritic olivine (Mg# 78–92 mol%), low-Ti Cr-spinel and clinopyroxene. Although the absolute abundances of major and trace elements, and their ratios, in the basalts fluctuate to some extent, the observed variability cannot be correlated with any of considered parameters in the geometry of the Kamchatka SSZ and conditions of melting. This unexpected result led to the evaluation of the platinum-group element (PGE) systematics against the lithophile and chalcophile trace element geochemistry and the compositions of phenocrysts. Total whole-rock PGE content varies from 2.3 to 11.7 ppb, whereas the normalized PGE concentration patterns are typical for supra-subduction zones magmas and broadly similar in all studied samples. They are enriched in Rh, Pd and Pt relative to mid-ocean ridge basalts (MORB) and have nearly identical concentrations of Ir-group PGE. The only parameter that correlates well with PGE contents is the average Mg# of olivine phenocrysts from 84 to 90.3 mol%. This is interpreted to result from minor cryptic fractionation of sulfide melt, together with primitive olivine, in low-to-mid crustal conditions. Negative Ru anomalies on chondrite-normalized diagrams correspond to the Fe2+/Fe3+ ratios in spinel (a proxy for magma redox conditions), which reflects a replacement of monosulfide solid solution by laurite in the mantle wedge during oxidation.

Multistage melt impregnation in Tethyan oceanic mantle: Petrochemical constraints from channelized melt flow in the Naga Hills Ophiolite

Ophiolitic sequences obducted onto continental margins allow field based observations coupled with petrochemical interrogations of upper mantle lithologies thereby aiding evaluation of compositional heterogeneity of oceanic mantle, depletion-enrichment events and geodynamic conditions governing oceanic lithosphere formation. The Naga Hills Ophiolite (NHO) suite preserves a segment of the Neotethyan oceanic lithosphere encompassing a package of mantle and crustal lithologies. This paper for the first time reports the occurrence of melt flow channels traversing the mantle section near Molen of the NHO and presents a comprehensive study involving chromite-spinel chemistry, bulk rock major, trace and PGE geochemistry to understand the petrogenesis and evolution in a geodynamic transition from mid oceanic ridge (MOR) to suprasubduction zone (SSZ). The spinel chemistry of peridotitic melt channels depicts both MOR-type and SSZ signatures underlining a transitional tectonic frame. Chromite chemistry and high Al2O3/TiO2 ranging from 15.98–35.70 in concurrence with low CaO/Al2O3 ranging from 0.03–0.53; and chondrite normalised LREE > MREE < HREE patterns confirm the influx of boninitic melts into the refractory mantle. The boninitic signature shared by melt channels and host rock invokes a geochemical and geodynamic transition from anhydrous melting of depleted mantle to hydrated fluid flux melting resulting in boninitic melts, that subsequently impregnate and refertilise the fore arc mantle wedge in a SSZ regime at the nascent stage of subduction. The high Ba/Nb, Ba/Th, and Ba/La for the studied peridotites highlight the influx of subduction derived fluids in the supra subduction mantle. Further higher Zr/Hf and Nd/Hf with respect to primitive mantle values in concurrence with lower Nb/Ta suggest progressive refertilisation due to fluid- and melt-driven metasomatism of the refractory fore arc mantle wedge. The chondrite normalised PGE patterns suggest positive Ir and Ru anomalies stipulating the source to be refractory while enriched Pt and Pd underpins the mobilisation of these elements by subduction derived fluids and melts. The elevated abundances of PPGEs than IPGEs as cited by PPGE/IPGE > 1; and Pd/Pt avg. 0.85 for melt channels and 0.84 for host peridotites indicate fluid-fluxed metasomatism of fore arc mantle wedge with a S-undersaturated trend coupled with boninitic affinity. The mineral, trace, REE and PGE chemistry collectively emphasizes that the mantle peridotites of the NHO formed in a transitional geodynamic tectonic setting caused by fore arc extension during subduction initiation followed by rejuvenation by subduction derived fluids and boninitic melts, which typically are of the SSZ tectonic regime. The harzburgitic melt channels and host rock are refractory in nature, reflecting multiple episodes of melt extraction of about 5–15% and ~10–20% respectively from a spinel peridotite mantle source. The occurrences of these melt channels indicate segregation and percolation of melt through porous and channelized network in upper mantle peridotites.

Subduction-Induced Fractionated Highly Siderophile Element Patterns in Forearc Mantle

Compositions of highly siderophile elements (HSEs) in forearc mantle have been little studied and effects of slab dehydration on their abundances in forearc mantle remains unclear. This study reports two different kinds of HSE patterns for peridotites from a New Caledonia forearc ophiolite. The Group-I samples show relatively flat patterns of Ir-group-platinum-group elements (IPGEs) and enrichment of Pt over Pd. Such patterns imply that interstitial sulfides were significantly removed through melt extraction, whereas sulfides enclosed within silicates were mostly unaffected. Meanwhile, Pt-Fe alloys were generated, resulting in suprachondritic Pt/Pd ratios. In contrast, the Group-II samples display convex HSE patterns and are depleted in all HSEs except for Ru, yielding strongly positive Ru anomalies. This indicates that both enclosed and interstitial sulfides were substantially consumed, whereas chromite was generated to stabilize Ru. Compared to abyssal peridotites, subduction-related peridotites commonly have stronger fractionation in the HSEs. Therefore, the HSE data of mantle peridotites are potentially able to discriminate the tectonic settings of ophiolites.

Empirical constraints on partitioning of platinum group elements between Cr-spinel and primitive terrestrial magmas

Recent experimental studies and in situ LA-ICP-MS analysis on natural Cr-spinel have shown that Rh and IPGEs (Ir-group platinum group elements: Ru, Ir, Os) are enriched in the lattice of Cr-spinel. However, the factors controlling the partitioning behaviour of these elements are not well constrained. In this study, we report the Rh, IPGE, and trace element contents in primitive Cr-spinel, measured by LA-ICP-MS, from nine volcanic suites covering various tectonic settings including island arc picrites, boninites, large igneous province picrites and mid-ocean ridge basalts. The aim is to understand the factors controlling the enrichment of Rh and IPGEs in Cr-spinels, to estimate empirical partition coefficients between Cr-spinel and silicate melts, and to investigate the role of Cr-spinel fractional crystallization on the PGE geochemistry of primitive magmas during the early stages of fractional crystallization.There are systematic differences in trace elements, Rh and IPGEs in Cr-spinels from arc-related magmas (Arc Group Cr-spinel), intraplate magmas (Intraplate Group Cr-spinel), and mid-ocean ridge magmas (MORB Group Cr-spinel). Arc Group Cr-spinels are systematically enriched in Sc, Co and Mn and depleted in Ni compared to the MORB Group Cr-spinels. Intraplate Group Cr-spinels are distinguished from the Arc Group Cr-spinels by their high Ni contents. Both the Arc and Intraplate Group Cr-spinels have total Rh and IPGE contents of 22–689ppb whereas the MORB Group Cr-spinels are depleted in Rh and IPGE (total<20ppb). Palladium and Pt contents are below detection limit for all of the studied Cr-spinels (<1–5ppb). The time-resolved spectra of LA-ICP-MS data for Cr-spinels mostly show constant count rates for trace element and Rh and IPGEs, suggesting homogeneous distribution of these elements in Cr-spinels. The PGE spikes observed in several Cr-spinels were interpreted to be PGE-bearing mineral inclusions and excluded from calculating the PGE contents of the Cr-spinels.On primitive mantle normalized diagrams the Arc Group Cr-spinels are characterized by a fractionated pattern with high Rh and low Os. The Intraplate Group Cr-spinels show flat patterns with positive Ru anomalies. Our results, together with the experimental and empirical data from previous studies, show that PGE patterns of Cr-spinel largely mimic that of the rock in which they are found, and that Rh, Ir and Os contents increase with increasing Fe3+ contents (i.e. magnetite component) in Cr-spinel, although Ru does not. These observations suggest that the enrichment of Rh and IPGEs in Cr-spinel is controlled by a combination of the Rh and IPGE contents in parental melts and the magnetite component of the spinel. Empirical partition coefficients (D) for Rh and IPGEs between Cr-spinels and silicate melts were calculated using the Rh and IPGE contents of the Cr-spinel and their host volcanic rocks after subtracting the accumulation effect of Cr-spinel. The D values for the Intraplate and MORB Group Cr-spinels increase with increasing magnetite component in Cr-spinel and range from 6 to 512, which is consistent with previously reported experimental and empirical values. In contrast, the Arc Group Cr-spinels have significantly higher D values (e.g. up to ∼3700 for Ru) than those of the Intraplate and MORB Group at the same magnetite concentration in the Cr-spinel, suggesting Rh and IPGEs dissolved in silicate melt have stronger affinity for Cr spinel under arc magma conditions than in intraplate magmas. This may be partly attributed to the low temperature of arc magmas relative to intraplate magmas, which leads to the Arc Group Cr-spinels having more octahedral sites at the same magnetite components than the Intraplate Group Cr-spinels. Because of significantly higher D values for the Arc Group Cr-spinels, compared with the Intraplate Group and MORB Group spinels, fractional crystallization of Cr-spinel will more efficiently fractionate Rh and IPGE from Pd and Pt in arc systems than in intraplate and MORB systems, which accounts for the highly fractionated PGE patterns in arc basalts.

Small-intrusion-hosted Ni-Cu-PGE sulfide deposits in northeastern Vietnam: Perspectives for regional mineral potential

This paper discusses new data on the abundance and distribution of chalcophile elements (Ni, Cu, and PGE) in several small Permian–Triassic-aged sulfide-bearing mafic-ultramafic intrusions, namely the Dong Sang, Bo Ninh, Na Hoan and Ha Tri, belonging to the Cao Bang Complex in the Song Hien domain, Northeastern Vietnam. Mineral potential for Ni-Cu (-PGE) deposits in this domain is poorly understood in contrast to the Song Da domain in northwestern Vietnam, in which the well-known Ban Phuc Cu-Ni-PGE deposit is located. The disseminated sulfides from the Dong Sang, Bo Ninh and Na Hoan intrusions have very similar moderately fractionated chalcophile element abundance patterns, with positive Pd and negative Ru anomalies, and similar ratios of Pd/Ir (1–30) and (Pt+Pd)/(Ir+Ru+Os) (1–23), indicating a genetic link between these sulfides and the previously studied Suoi Cun intrusion, which also belongs to the Cao Bang complex and hosts Ni-Cu-PGE sulfides. The similar Cu/Pd ratios (3100–17100) of the sulfides from these intrusions are close to generally accepted ratios for the mantle range, and this indicates that sulfides were separated from a primary relatively PGE-undepleted magma as a result of a single sulfide segregation event. The sulfur isotopic data (δ34S) are in the range of −2.0 to −3.4‰ and indicate that the mantle is the dominant source of the sulfur, although the native values presumably allow for a small contribution from crustal S. In contrast, the disseminated and massive sulfides from the Ha Tri intrusion have high Pd/Ir (>100) and (Pt+Pd)/Ir (>140) ratios, indicating very high degrees of fractionation of a sulfide liquid, and it also has a wider range of δ34S values between +1.5 and −1.5‰ that overlap with that of the mantle. Mantle-like Cu/Pd ratios (3400–10600) indicate that no interaction has occurred between the sulfide liquid and a large volume of fresh magma in an open system. The results suggest two types of magmatic sulfide mineralization within mafic-ultramafic bodies of the Cao Bang Complex: (I) mineralization related to sulfide-bearing olivine-rich crystal mush and formed by multiple injections of magma into (and through?) the intrusion from the same source (as recorded in the Suoi Cun, Dong Sang, Bo Ninh, Na Hoan intrusions), and (II) mineralization hosted within closed system terminal chambers (as recorded in the Ha Tri intrusion).

Platinum-group element and Re-Os geochemistry of lamprophyres in the Zhenyuan gold deposit, Yunnan Province, China: Implications for petrogenesis and mantle evolution

Cenozoic lamprophyres are widespread along the Ailaoshan suture, SW Yunnan, SW China, where there are also many important gold deposits, especially the Zhenyuan deposit. We have carried out a geochemical investigation of the Zhenyuan lamprophyres in terms of major and trace elements, platinum-group elements (PGE), and Os isotopes. The Zhenyuan lamprophyres can be classified into groups with high or low Os concentrations. The 187Os/188Os ratios, corrected for in situ growth, are highly variable in both types of lamprophyre, ranging from mantle values up to 1.13. The highly radiogenic Os isotopic signatures are interpreted as being due to long-term accumulation of elevated Re/Os in the lithospheric mantle, as a result of subduction-related metasomatism. The highly variable 187Os/188Os ratios of the low-Os lamprophyres might also have resulted from metasomatism of a deeply derived carbonate melt, which did not elevate Os concentrations significantly. The Zhenyuan lamprophyres feature low PGE contents and can be classified into two groups: A and B, based on their primitive-mantle-normalized PGE patterns. Group A is characterized by strongly negative Ru anomalies, while Group B is characterized by slightly negative Ru anomalies and low total PGE contents. The PGE characteristics may be explained by a two-stage model of magma evolution. Very high Cu/Pd and low Pt/Y ratios indicate the first stage involved sulfur saturation and sulfide removal, producing variable total-PGE concentrations, especially in Group B. The second stage involved S-undersaturation and the early crystallization of silicate minerals (e.g., olivine) along with laurite and/or Ru-Ir-Os alloy, fractionating PGEs and producing marked negative Eu anomalies.

Precious metal enrichment at low-redox in terrestrial native Fe-bearing basalts investigated using laser-ablation ICP-MS

Primary native Fe is a rare crystallizing phase from terrestrial basaltic magmas, requiring highly reducing conditions (fO2 <iron-wüstite buffer) in order to form. Reducing conditions in basaltic magmas can be achieved through assimilation of carbonaceous crustal material, which leads to formation of an immiscible, molten, C-rich, native Fe alloy liquid. If this liquid also contains sufficient sulfur, it can undergo further division into conjugate Fe–C-rich and a Fe–S-rich immiscible melts that can effectively scavenge the highly siderophile elements (HSE: Re, Au, and the platinum group elements [PGE], Pd, Pt, Rh, Ru, Ir, Os), as well as Ni and Cu, to economic abundances. Three localities are known globally where native Fe bearing mafic rocks occur: (1) Paleocene basalts of Disko Island, West Greenland; (2) a Miocene lava of the Bühl basalts, Germany; and (3) mafic intrusions associated with the Late Permian Siberian flood basalts. In this contribution, we report major- and minor-element compositions and HSE concentrations for the main alloy phases (FeNi metal and cohenite) and sulfide, for all three known global occurrences of native Fe bearing basalt. Total HSE abundances in metal grains, obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), are lowest in the Bühl basalt, (∼0.05ppm), intermediate in the Disko Island basalts (4–8ppm), and highest the Siberian Khungtukun and Dzhaltul intrusions (10–30ppm). These differences demonstrate that, while native Fe formation is the result of carbonaceous crustal assimilation, HSE enrichment is not ubiquitous during this process. The Siberian occurrences are characterized by Pt PGE (PPGE: Pt, Pd) enrichment relative to the Ir PGE (IPGE: Rh, Ru, Ir, Os), consistent with models of early stage fractionation of olivine, chromite and metallic IPGE in staging magma reservoirs, prior to the addition of C-rich crustal materials in the shallow crust. Relative to Noril’sk Ni–Cu-PGE sulfide ores, the Siberian native Fe basalts are enriched in the PPGE relative to the IPGE, but exhibit Ru enrichments. In contrast, Disko Island native Fe rocks do not show significant fractionation of the PPGE from the IPGE, but have positive Re and Ru anomalies and high Os/Ir ratios. To reconcile these observations, we present a general model where some parental melts experienced early-stage crustal assimilation in staging magma reservoirs, prior to reduction by carbon-rich materials (e.g., Khungtukun, Dzhaltul), whereas basaltic parental melts to Disko Island and Bühl solely experienced interaction with carbonaceous material, resulting in extensive devolatilization, native Fe segregation, and native Fe formation.

Platinum-group element geochemistry of mafic rocks from the Dongchuan area, southwestern China

Mafic intrusions and dykes are well preserved in the Yinmin and Lanniping districts, located within the western margin of the Yangtze Block, SW China. Although these mafic rocks from the two areas formed during different periods, they share similar ranges of PGE concentration. Most of the Yinmin gabbroic dykes contain relatively high PGE concentrations (PGEs = 13.9–87.0 ppb) and low S contents (0.003 %–0.020 %), higher than the maximum PGE concentrations of mafic magmas melting from the mantle. Two exceptional Yinmin samples are characterized by relatively low PGE (PGEs = 0.31–0.37 ppb) and high S (0.114 %–0.257 %) contents. In contrast, most samples from the Lanniping gabbroic intrusion have low PGE concentrations (PGEs = 0.12–1.02 ppb) and high S contents (0.130 %–0.360 %), except that the three samples exhibit relatively high PGE (PGEs = 16.3–34.8 ppb) and low S concentrations (0.014 %–0.070 %). All the Yinmin and Lanniping samples are characterized by the enrichment of PPGE relative to IPGE in the primitive-mantle normalized diagrams, and the high-PGE samples exhibit obvious Ru anomalies. This study suggests that during the ascent of the parental magma, removal of Os–Ir–Ru alloys and/or chromite/spinel leads to high Pd/Ir ratios and Ru anomalies for the Yinmin high-PGE samples and relatively lower Pd/Ir ratios and Ru anomalies for the Lanniping low-PGE samples. We propose that the magmas parental to the Yinmin gabbroic dykes are initially S-unsaturated, and subsequently, minor evolved magma reached sulfur saturation and led to sulfide segregation. Although the Lanniping parental magmas are originally not saturated in S, the high Cu/Pd ratios (3.8 × 104 to 3.2 × 106) for most of the Lanniping samples indicate the S-saturated state and sulfide segregation. A calculation shows that the PGE-poor magmas might have experienced 0.01 %–0.1 % sulfide segregation in the magma chamber. Therefore, our study provides a possible opportunity to discover PGE-enriched sulfide mineralization somewhere near or within the Lanniping mafic intrusion.

Ophiolitic chromitites from the Kızılyüksek area of the Pozantı-Karsantı ophiolite (Adana, southern Turkey): Implication for crystallization from a fractionated boninitic melt

Ophiolitic rocks are widely distributed in Turkey. One type, the Pozantı-Karsantı ophiolite from southern Turkey, contains a large number of chromitite deposits located mostly in the mantle peridotites and close to the Moho transition zone dunite and cumulate dunites. Cr-spinel grains from the chromitites are represented by high Cr# [100×Cr/(Cr+Al)=68–81], and their Mg# [100×Mg/(Mg+Fe2+)] range from 54 to 71. Gallium and Co contents vary between 18 and 32ppm and 185–266ppm, respectively, and they show negative correlation with Cr#. A detailed optical investigation reveals that the Cr-spinel grains contain silicate, platinum-group mineral (PGM) and base metal sulfide (BMS) inclusions. Single phase inclusions of amphibole are the only hydrous silicate phases in the investigated chromitites, and they contain low TiO2 (<0.43wt.%). Olivine, with high Fo (~96) and NiO contents (0.48–0.68wt.%), and clinopyroxene, with low TiO2 (<0.1wt.%), Al2O3 (<2.84wt.%) and Na2O contents (<0.4wt.%) were also observed as primary silicate inclusions. Chromitites contain low concentrations of total platinum-group elements (PGE) ranging between 32 and 162ppb, with an average value of 93ppb. Primitive mantle-normalized PGE diagrams show almost flat to positive slopes from Os to Rh (RhN/OsN=0.99 to 8.5) and negative slope from Rh to Pt and Pd. All samples show marked positive Ru anomalies. Consistent with the geochemical data, Ru, Os, and Ir bearing PGE sulfide (laurite-erlichmanite solid solution series [(Ru, Os)S2–(Os, Ru)S2] phases) are the most common PGM detected in the investigated chromitite samples. They show a narrow range of Os-Ru substitution [Ru#; Ru/(Ru+Os)=0.72–0.97], indicating no erlichmanite in the PGM paragenesis. In addition to the most common PGM laurite, several osmium (Os, Ir), iridium (Ir, Os), irarsite, and one single grain of speryllite (PtAs2) were detected as magmatic inclusions in Cr-spinel. Three unknown PGE/PGE–BME (base metal element) phases were also detected in Cr-spinel grains with compositions that correspond to the chemical formulas of (Os, Ru, Ir, Rh, Fe, Pd)2S5, Ir(Rh,Pt,Ni,Cu)S3, and (Ir, Rh, Ru)2(Ni, Cu)S3, respectively. The high Cr# and low Ti content of Cr-spinel grains and amphibole inclusions with low Ti content as hydrous phases in Cr-spinel grains require a hydrous melt depleted in incompatible trace elements for the formation of investigated chromitites; therefore, we suggest a fore arc tectonic environment for the generation of Kızılyüksek chromitites. The presence of Os-Ir alloys and Ru-rich laurites implies that Cr-spinel crystallization took place at relatively high temperature (1100–1300°C) and low ƒ(S2) (between −1 and −3) conditions. Major and trace element compositional variations of Cr-spinel, wide variation of RhN/OsN ratios of the chromitites and depletion of Os in the chromitites compared to Ir and Ru may imply that Kızılyüksek chromitites crystallized from a variously fractionated boninitic melt.

The Effect of Chromite Crystallization on the Distribution of Osmium, Iridium, Ruthenium and Rhodium in Picritic Magmas: an Example from the Emeishan Large Igneous Province, Southwestern China

The positive correlations between whole-rock concentrations of Cr and Os, Ir and Ru (referred to collectively as iridium-like platinum-group elements, IPGE) in both sulphide-poor plutonic and volcanic rocks suggest that chromite contributes to the collection of these elements during the early stages of sulphide-undersaturated magma differentiation. However, it is not clear whether these correlations are the result of IPGE partitioning into chromite or whether other minerals are involved. Positive correlations between MgO and IPGE have been observed, suggesting that these elements could be incorporated into olivine as well as into chromite. Alternatively, given the siderophile nature of IPGE, they may crystallize as discrete minerals together with chromite, as suggested by the presence of platinum-group minerals in chromite-rich rocks such as mantle podiform and crustal stratiform chromitites. To investigate the effect of chromite crystallization on the distribution of IPGE and Rh in picritic magmas, we have determined the content of these elements in chromites from the Emeishan Large Igneous Province (southwestern China) by in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The time-resolved analysis signals are generally uniform, indicating that the IPGE and Rh are homogeneously distributed within the chromite crystals. The median concentrations are 30 ppb Os, 23 ppb Ir, 248 ppb Ru and 21 ppb Rh. The incorporation of Rh in chromites appears to be influenced by the oxidation state of the magma from which they crystallize because there is a positive correlation between the degree of inversion of the chromite structure [expressed by Fe3+/(Cr + Al + Fe3+)] and the Rh content of the analyzed crystals. In contrast, the enrichments of Os, Ir and Ru in chromites appear to be controlled by other parameters such as temperature or the S content of the magma. Based on empirical calculations, we determined that partition coefficients between chromite and melt are higher for Ru (127) than for Os (23), Ir (27) and Rh (32). Despite these high partition coefficients, mass-balance calculations show that chromite does not account for all the IPGE and Rh concentrations in the rocks, with chromite accounting for maxima of 84, 49, 22 and 20% of the whole-rock Ru, Rh, Ir and Os budgets, respectively. In situ LA-ICP-MS and scanning electron microscopy analyses reveal the presence of micrometric-sized PGE-rich minerals, including laurite (RuS2), Os–Ir ± Ru alloys, sperrylite (PtAs2) and Pt–Fe (± IPGE and Rh) alloys, in association with chromite crystals. The presence of these minerals may account for the balance of IPGE and Rh. Alternatively, if IPGE and Rh are compatible with olivine, the balance may be accounted for by the large amount of olivine crystals found in the Emeishan picrites. Based on numerical modelling, we conclude that chromite, olivine and platinum-group minerals all contribute to the collection of IPGE and Rh during the early stages of picritic magma differentiation. Also, we establish that the preferential incorporation of Ru into chromite is responsible for the negative Ru anomalies observed in the PGE patterns of the Emeishan flood basalts. On the other hand, the relative importance of olivine and platinum-group minerals in controlling IPGE and Rh remains uncertain and is a key subject for further investigation.

PGE geochemical constraints on the origin of the Ni-Cu-PGE sulfide mineralization in the Suoi Cun intrusion, Cao Bang province, Northeastern Vietnam

The Permian (266–262 Ma) Suoi Cun intrusion in the Song Hien Rift Zone (NE Vietnam) consists of a sulfide-bearing mafic-ultramafic unit and a sulfide-free mafic unit. The Cu-Ni-PGE mineralization is represented by disseminated sulfides throughout the sulfide-bearing unit containing ~0.5 wt.% Ni, ~0.05 wt.% Cu, and ~0.2 ppm PGE. The sulfide schlieren have a limited distribution and contain ~2.6 wt.% Ni, ~0.5 wt.% Cu, and ~2.6 ppm PGE. The Suoi Cun rocks containing disseminated sulfides display moderately fractionated mantle-normalized PGE patterns with positive Pd and negative Ru anomalies. In contrast, the sulfide schlieren show enrichment in Ru with lower contents of other PGE except Pd. The low Cu/Pd ratios (1,385–11,529) throughout the intrusion indicate that all sulfides were separated from a PGE-undepleted magma as a result of a single sulfide segregation event. We suggest that sulfides segregated from Mg-rich basaltic magmas in a deep-seated magma chamber due to crustal contamination with country rocks. Then, the sulfide liquid along with early crystallizing olivines and Cr-spinels were pushed out upwards into an upper magma chamber by new pulses of magma. Two processes were important for understanding the PGE distribution: 1) fractionation of the sulfide liquid gave rise to PGE distribution observed in the disseminated ore and, 2) the interaction of oxidized silicate melts with the sulfide liquid was the responsible for the low PGE contents in the sulfide schlieren due to PGE transfer from the oxidized sulfide liquid to the silicate melt.

Geochemistry of the Abulangdang intrusion: Cumulates of high-Ti picritic magmas in the Emeishan large igneous province, SW China

The ~260Ma Abulangdang ultramafic intrusion in the Panxi region, SW China, is part of the Emeishan large igneous province (ELIP). The intrusion is composed of dunite and lherzolite with minor olivine gabbro. Olivine crystals in the dunite have Fo values from 89.0 to 86.5mol%. Chromite grains in both the dunite and lherzolite contain more than 0.5wt.% TiO2 and show a trend of increasing Fe3+ with increasing Fe2+/(Mg+Fe2+). Rocks of the intrusion have γOs(t) values from +0.1 to +1.2 and εNd(t) values from −1.9 to +2.9, similar to the high-Ti picrites, high-Ti flood basalts and Fe–Ti oxide-bearing, gabbroic intrusions of the ELIP. Modeling indicates that the Abulangdang intrusion formed by accumulation of olivine and chromite from a high-Ti picritic magma, whereas the high-Ti flood basalts and Fe–Ti oxide-bearing, gabbroic intrusions formed from derivative, evolved magmas after the early fractionation of minerals from the high-Ti picritic magma. Both the dunite and lherzolite have relatively high PGE concentrations and primitive mantle-normalized chalcophile element patterns with positive Os and Ru anomalies, in contrast to PGE-poor high-Ti flood basalts with negative Os and Ru anomalies. This is consistent with the retention of Ru–Os–Ir trace phases with olivine and chromite in the formation of the Abulangdang intrusion. Using a clinopyroxene geobarometer, the crystallization pressure of the Abulangdang intrusion is calculated to be 7.1 to 8.3kbar, equivalent to a depth of ~21 to 24km, which is deeper than that estimated for the Fe–Ti oxide-bearing, gabbroic layered intrusions of the ELIP (~5kbar). We propose that large volumes of mantle plume-derived high-Ti picritic magma underplating along the Moho boundary underwent early fractionation before eruption, and some high-Ti picritic magma ascended through a fossil conduit where the Abulangdang intrusion formed. Residual, evolved magmas either from the conduits or underplating magma chamber formed Fe–Ti oxide-bearing, gabbroic layered intrusions or high-Ti flood basalts. In this fashion, the Abulangdang intrusion resembles ultramafic portions that are missing in the Fe–Ti oxide-bearing, gabbroic intrusions of the ELIP.

Platinum-group element geochemistry of the Zhuqing Fe-Ti-V oxide ore-bearing mafic intrusions in western Yangtze Block, SW China: control of platinum-group elements by magnetite

Platinum-group element (PGE) geochemistry combined with elemental geochemistry and magnetite compositions are reported for the Mesoproterozoic Zhuqing Fe–Ti–V oxide ore-bearing mafic intrusions in the western Yangtze Block, SW China. All the Zhuqing gabbros display extremely low concentrations of chalcophile elements and PGEs. The oxide-rich gabbros contain relatively higher contents of Cr, Ni, Ir, Ru, Rh, and lower contents of Pt and Pd than the oxide-poor gabbros. The abundances of whole-rock concentrations of Ni, Ir, Ru, and Rh correlate well with V contents in the Zhuqing gabbros, implying that the distributions of these elements are controlled by magnetite. The fractionation between Ir–Ru–Rh and Pt–Pd in the Zhuqing gabbros is mainly attributed to fractional crystallization of chromite and magnetite, whereas Ru anomalies are mainly due to variable degrees of compatibility of PGE in magnetite. The order of relative incompatibility of PGEs is calculated to be Pd < Pt < Rh < Ir < Ru. The very low PGE contents and Cu/Zr ratios and high Cu/Pd ratios suggest initially S-saturated magma parents that were highly depleted in PGE, which mainly formed due to low degrees of partial melting leaving sulfides concentrating PGEs behind in the mantle. Moreover, the low MgO, Ni, Ir and Ru contents and high Cu/Ni and Pd/Ir ratios for the gabbros suggest a highly evolved parental magma. Fe–Ti oxides fractionally crystallized from the highly evolved magma and subsequently settled in the lower sections of the magma chamber, where they concentrated and formed Fe–Ti–V oxide ore layers at the base of the lower and upper cycles. Multiple episodes of magma replenishment in the magma chamber may have been involved in the formation of the Zhuqing intrusions.

Platinum‐Group Element Geochemical Characteristics of the Picrites and High‐Ti Basalts in the Binchuan Area, Yunnan Province

AbstractThe Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block. In the present study, the Wuguiqing profile in thickness of about 1440 m is mainly composed of high‐Ti basalts, with minor picrites in the lower part and andesites, trachytes, and rhyolites in the upper part The picrites have relatively higher platinum‐group element (PGE) contents (ΣPGE=16.3–28.2 ppb), with high Cu/Zr and Pd/Zr ratios, and low S contents (5.03–16.9 ppm), indicating the parental magma is S‐unsaturated and generated by high degree of partial melting of the Emeishan large igneous province (ELIP) mantle source. The slightly high Cu/Pd ratios (11 000–24 000) relative to that of the primitive mantle suggest that 0.007% sulfides have been retained in the mantle source. The PGE contents of the high‐Ti basalts exhibit a wider range (ΣPGE=0.517–30.8 ppb). The samples in the middle and upper parts are depleted in PGE and have εNd (260 Ma) ratios ranging from −2.8 to −2.2, suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%–0.554% sulfides, and the sulfide segregation process may also provide the ore‐forming material for the magmatic Cu‐Ni‐PGE sulfide deposits close to the studied basalts. The samples in this area show Pt‐Pd type primitive mantle‐normalized PGE patterns, and the Pd/Ir ratios are higher than that of the primitive mantle (Pd/Ir=1), indicating that the obvious differentiation between Ir‐group platinum‐group elements (IPGE) and Pd‐group platinum‐group elements (PPGE) are mainly controlled by olivine or chromites fractionation during magma evolution. The Pd/Pt ratios of most samples are higher than the average ratio of mantle (Pd/Pt=0.55), showing that the differentiation happened between Pt and Pd. The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt‐alloy phase in the mantle; whereas the differentiation in the high‐Ti basalts is probably associated with the fractionation of Fe‐Pt alloys, coprecipitating with Ir‐Ru‐Os alloys. Some high‐Ti basalt samples exhibit negative Ru anomalies, possibly due to removal of laurite collected by the early crystallized chromites.

New evidence for a volcanic arc on the western margin of a rifting Rodinia from ultramafic intrusions in the Andriamena region, north-central Madagascar

Ultramafic–mafic complexes that intruded into Archaean ortho- and paragneisses in the Andriamena region of north-central Madagascar fall within the domain of a 900-km-long belt consisting of gabbroic and granitoid plutons that has been interpreted as the roots of a Neoproterozoic (700–800 Ma) continental arc. This Andean-type arc is reported to extend from north-central Madagascar through the Seychelles into India. The ultramafic bodies (∼ 2 km × 1 km) are crudely zoned, with dunite forming the cores surrounded by harzburgite, lherzolite, pyroxenite, pegmatoidal pyroxenite and fine-grained gabbroic rocks on the margins of the intrusions. One sampled contact between an ultramafic entity and the basement is chilled and sharp, indicating an intrusive rather than a sheared contact; this argues against allochthonous emplacement as in ophiolite suites. The composition of this chilled basaltic margin is similar to volcanic lithologies from well-known continental Andean-type arcs. PGE data from dunites, harzburgites, lherzolites and websterites of the Lavatrafo body show enrichment in the Pd-group PGEs with significant Pt-enrichment and prominent negative Ru anomalies. [Ru/Ru*] N ranges from 0.037 to 0.31 with average [Ru/Ir] N = 0.17. These features are characteristic of zoned Alaskan–Uralian-type complexes ([Ru/Ru*] N = 0.029–0.15), which generally form the roots of volcanic arcs. Dunites, forming the cores of some intrusions, show enrichment in the LREE, are depleted with respect to chondrites (0.1× chondrite) and display negative Eu anomalies possibly caused by orthopyroxene fractionation. The Andriamena complexes are suggested to represent the roots or feeder conduits of a continental Andean-type arc and provide new evidence, in the form of ultramafic intrusions, for a convergent margin on the western edge of the Rodinia supercontinent. This tectonic setting is consistent with published data suggesting eastward subduction, on the eastern edge of the Mozambique Ocean, underneath the continental terrains of Madagascar, the Seychelles and NW India.

Controls on the PGE distribution of Permian Emeishan alkaline and peralkaline volcanic rocks in Longzhoushan, Sichuan Province, SW China

The ~ 260-Ma Emeishan large igneous province in SW China and northern Vietnam contains low-Ti tholeiitic basalts, high-Ti alkaline basalts and minor peralkaline volcanic rocks. The flood basalts in Longzhoushan, Sichuan Province, SW China, comprise four units, each of which has volcanic breccia at the base. From the base upward, rocks of Units 1 and 2 are high-Ti basalts with relatively high MgO (7.6–11.0 wt.%), TiO 2 (1.7–4.5 wt.%) and Ti/Y ratios (420–790). Unit 3 is composed of tephrites with lower MgO (0.87–6.2 wt.%), TiO 2 (0.96–2.14 wt.%) and Ti/Y ratios (190–530), whereas Unit 4 consists of basaltic andesites with moderate MgO (3.5–4.8 wt.%), TiO 2 (2.2–3.1 wt.%) and Ti/Y ratios (383–606). The high-Ti basalts may have been derived from an OIB-like mantle source and the basaltic andesites were probably generated by crustal contamination and fractionation of high-Ti basaltic magmas. The tephrites show high Nb/Y (1.2 to 2.7) and Th/Y (0.31 to 0.55) ratios, variable Zr/Hf ratios (40 to 49) and relatively low εNd( t) (− 0.7 to + 0.28) values, suggesting an asthenospheric mantle contribution modified by carbonate-rich fluids. The high-Ti basalts contain relatively high and variable PGE and their Cu/Pd ratios are relatively constant (compared to the tephrites) and vary from 8,500 to 86,000. We attribute such relative constancy of the Cu/Pd ratios to the presence of residual sulphide in the source of the high-Ti rocks and subsequent crystallization from S-undersaturated melts. The basaltic andesites have relatively high and constant Cu/Pd ratios (130,000–370,492) and the lowest PGE contents (average 0.12 ppb for Pd, 0.20 ppb for Pt), suggesting an earlier removal of sulfide due to crustal contamination before emplacement. The tephrites have PGE abundances (0.057 to 1.89 ppb for Pt and 0.11 to 2.10 ppb for Pd) higher than those for basaltic andesites, however, the Cu/Pd ratios (33,300 to 429,000) of the rocks are more variable than those for the basaltic andesites, suggesting that the tephrites may have undergone both S-saturated fractionation during emplacement of the magmas and weak removal of sulfides from the source magmas. Early fractionation of chromite together with removal of laurite and/or Os–Ir–Ru alloys from the parental magmas may be responsible for the negative Ru anomalies of the tephrites, high-Ti basalts and basaltic andesites.

Platinum-group elemental and Sr–Nd–Os isotopic geochemistry of Permian Emeishan flood basalts in Guizhou Province, SW China

The 260 Ma Emeishan continental flood basalts (ECFB) occur in SW China and northern Vietnam. A 550-m-thick sequence of ECFB in Guizhou Province (SW China) is composed of 12 basaltic lava flows, each ranging from 10 to 140 m in thickness. All samples belong to the high-Ti basalts (TiO 2 = 1.9–4.4 wt.% and Ti/Y = 400–700). Relative to the upper flows (Flows 9–12), those from the lower part of the section (Flows 1–8) are depleted in Nb, Ta, P and Sr and enriched in Zr, La and Th, consistent with higher degree of crustal contamination. Overall, the concentrations of Os, Ir and Ru range from 0.01 to 0.23 ppb and are low relative to those of Rh, Pt and Pd, which range from 0.2 to 1.1, 6 to 17 and 5 to 18 ppb, respectively. Different lava flows have different primitive mantle-normalized PGE patterns, and samples from the same flow show similar patterns. Fractionations between PPGE and IPGE are mainly controlled by olivine and chromite fractionation. The negative correlations between Pd/Ir and Pt/Pd, and Ni/Pd and Pd/Cr are consistent with crystal fractionation. Flows 6–8 and 10–12 show obvious negative Ru anomalies, explained by fractionation of laurite (Ru, Os, Ir)S 2 or Os–Ir–Ru alloys in the primary magma, or by coprecipitation of laurite with chromite or olivine. Flows 9–12 are Os-poor and have higher initial 187Os/ 188Os ratios than Flows 1–8, indicating it is easy to alter the Os isotopic compositions of basaltic magmas with lower Os concentrations.

Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinites in the Lomié ultramafic complex, South-East Cameroon

The behaviour of major and trace elements have been studied along two serpentinite weathering profiles located in the Kongo-Nkamouna and Mang North sites of the Lomié ultramafic complex. The serpentinites are characterized by high SiO 2 and MgO contents, very low trace, rare earth and platinum-group element contents. Lanthanide and PGE contents are higher in the Nkamouna sample than in Mang North. Normalized REE patterns according to the CI chondrites reveal that: (i) all REE are below chondrites abundances in the Mang North sample; (ii) the (La/Yb) N ratio value is higher in the Nkamouna sample (23.72) than in the Mang one (1.78), this confirms the slightly more weathered nature of the Nkamouna sample. Normalized PGE patterns according to the same CI chondrites reveal a negative Pt anomaly in the Mang sample. The Nkamouna sample is characterized by a flat normalized PGE pattern. All element contents increase highly from the parent rock to the coarse saprolite. In the weathering profiles, Fe 2O 3 contents decrease from the bottom to the top contrarily to Al 2O 3, SiO 2 and TiO 2. The contents of alkali and alkaline oxides are under detection limit. Concerning trace elements, Cr, Ni, Co, Cu, Zn and Sc decrease considerably from the bottom to the top while Zr, Th, U, Be, Sb, Sn, W, Ta, Sr, Rb, Hf, Y, Li, Ga, Nb and Pb increase towards the clayey surface soil. Chromium, Ni and Co contents are high in the weathered materials in particular in the saprolite zone and in the nodules. REE contents are high in the weathered materials, particularly in Nkamouna. Their concentrations decrease along both profiles. Light REE are more abundant than heavy REE. Normalized REE patterns according to the parent rock reveal positive Ce anomalies in all the weathered materials and negative Eu anomalies only at the bottom of the coarse saprolite (Nkamouna site). Positive Ce anomalies are higher in the nodular horizon of both profiles. An additional calculation method of lanthanide anomalies, using NASC data, confirms positive Ce anomalies ([Ce/Ce ∗] NASC = 1.15 to 60.68) in several weathered materials except in nodules ([Ce/Ce ∗] NASC = 0.76) of the upper nodular horizon (Nkamouna profile). The (La/Yb) N ratios values are lower in the Nkamouna profile than in Mang site. PGE are more abundant in the weathered materials than in the parent rock. The highest contents are obtained in the coarse saprolite and in the nodules. The elements with high contents along both profiles are Pt (63–70 ppb), Ru (49–52 ppb) and Ir (41 ppb). Normalized PGE patterns show positive Pt anomalies and negative Ru anomalies. The mass balance evaluation, using thorium as immobile element, reveals that: – major elements have been depleted along the weathering profile, except for Fe, Mn and Ti that have been enriched even only in the coarse saprolite; – all the trace elements have been depleted along both profiles, except for Cr, Co, Zn, Sc, Cu, Ba, Y, Ga, U and Nb that have been enriched in the coarse saprolite; – rare earth elements have been abundantly accumulated in the coarse saprolite, before their depletion towards the top of the profiles; – platinum-group elements have been abundantly accumulated in the coarse saprolite but have been depleted towards the clayey surface soil. Moreover, from a pedogenetical point of view, this study shows that the weathering profiles are autochtonous, except in the upper part of the soils where some allochtonous materials are revealed by the presence of zircon grains.

Mass‐Independent Isotope Fractionation of Molybdenum and Ruthenium and the Origin of Isotopic Anomalies in Murchison

Dauphas et al.'s model for the nucleosynthetic origin of Mo and Ru anomalies in meteorites leaves the case of Murchison (CM2) unexplained. We explore the possibility that such a discrepancy is due to mass-independent effects controlled by nuclear field shift with, in particular, staggering between odd and even masses. We first demonstrate the existence of such mass-independent fractionation of Mo and Ru isotopes by chemical exchange of Mo and Ru between DC18C6 crown ether and aqueous solutions. Our results fit the nuclear field shift theory of Bigeleisen. We then review the correlation between the mean-square charge radius (which controls the nuclear field shift) and the isotopic anomalies found in an Allende CAI and in Murchison. Although Mo and Ru in the Allende CAI show a clear indication of nucleosynthetic components, the mass-independent anomalies observed in Murchison show a strong correlation with the nuclear charge distribution. We therefore argue that some isotopic anomalies observed in meteorites may be due to nuclear field shift rather than nucleosynthetic processes. Such effects are temperature dependent and may represent either genuine nebular processes or analytical artifacts. This new interpretation may help assess the existence of anomalies due to the extinct isotopes 97Tc and 98Tc.