Platinum Group Element (PGE) Geochemistry to

JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.77, April 2011, pp.295-302

Platinum Group Element (PGE) Geochemistry to understand the Chemical Evolution of the Earth’s Mantle SISIR K. MONDAL Department of Geological Sciences, Jadavpur University, 188 Raja S.C. Mullick Road, Kolkata - 700 032, India Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West@79th Street, New York City, NY-10024, USA Email: [email protected] Abstract: Platinum group elements (PGE: Os, Ir, Ru, Rh, Pt, Pd) are important geochemical and cosmochemical tracers. Depending on physical and chemical behaviour the PGEs are divided into two subgroups: IPGE (Ir, Os, Ru) and PPGE (Pd, Pt, Rh). Platinum group elements show strong siderophile and chalcophile affinity. Base metal sulfides control the PGE budget of the Earth’s mantle. Mantle xenoliths contain two types of sulfide populations: (1) enclosed within silicate minerals, and (2) interstitial to the silicate minerals. In terms of PGE characters the included variety shows IPGE enriched patterns – similar to the melt-depleted mantle harzburgite, whereas the interstitial variety shows PPGE enriched patterns – resembling the fractionated PGE patterns of the basalt. These PGE characters of the mantle sulfides have been interpreted to be representative of multi-stages melting process of the mantle that helped to shape the chemical evolution of the Earth. Keywords: Mantle xenoliths, Base metal sulfides, Platinum group element (PGE), Os isotope, Mantle heterogeneity

INTRODUCTION

Platinum group elements (PGE: Pt, Pd, Rh, Ir, Ru and Os) are similar to one another in some respects but their behavior differs markedly from their congeners in Group 8b of the periodic table, the transition metals Fe, Ni and Co (Westland, 1981). The PGEs together with Au constitute a coherent group of siderophile and chalcophile elements having a strong repulsion to form oxygenated compound the so-called ‘noble’ character. Within the range of oxygen and sulfur fugacities prevalent in the crust and upper mantle, the PGEs commonly exhibit chalcophile behaviour (Naldrett and Duke, 1980). The PGEs are much important as they have distinctive geochemical behaviour that helps to reconstruct some evolutionary aspects of the Earth. In addition, they have a great economical relevance and represent an important target in ore exploration. In this article I present a short review on geochemical aspects of the platinum group elements. In addition, the subject of the mantle heterogeneity has been discussed with respect to the PGE and Os isotopic compositions of the mantle. PLATINUM GROUP ELEMENTS: GENERAL ASPECTS

Geochemistry of the PGEs helps to understand the

chemical evolution of the Earth’s mantle. The PGEs can be subdivided in two groups with contrasting characters: (a) Ir-PGE, containing Os, Ir and Ru; and (b) Pd-PGE containing Pd, Pt and Rh (Barnes et al. 1985). The members of the former group are more siderophile and refractory, whereas the latter are more chalcophile and volatile. Table 1 summarizes some physical, geochemical and mineralogical characteristics of the PGEs. It is evident from the table that the ‘refractory’ PGEs (Os, Ir and Ru) have very high fusion temperature, and the ‘fusible’ PGEs (Rh, Pt and Pd) have lower fusion temperature. Buchanan (1988) described the PGEs as (a) light triad: Ru, Rh and Pd, and the (b) heavy triad: Os, Ir and Pt. The light elements have only half the density of the platinum triad; Os, Ir and Pt have extraordinary high densities (Table 1). Mineralogically, the PGEs form about 150 mineral species, including native metals, alloys, arsenides, tellurides, selenides, antimonides and oxides, and they also enter into solid solution in various base metal sulfides (e.g., Cabri, 1981; 2002). Platinum group elements are most commonly found in sulfides of mafic-ultramafic rocks (e.g., Maier, 2005; Mondal and Zhou, 2010). Two types of deposit are distinguished: primary deposits in rock, and secondary deposits in alluvium. Alluvial deposits include modern

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SISIR K. MONDAL Table 1. Some physical, geochemical and mineralogical characteristics of the PGEs (Source: Cabri, 1981; 2002; Buchanan, 1988) Element Ruthenium (Ru)

Atomic number

Atomic weight

Melting T °C

Density

Main platinum group minerals

44

101

2334

12.4

Laurite (RuS2)

Rhodium (Rh)

45

102

1966

12.4

Hollingworthite (RhAsS)

Palladium (Pd)

46

107

1552

12.0

Kotulskite (PdTe), Braggite (Pt,Pd)S, Merenskyite (PdTe2)

Osmium (Os)

76

190

2700

22.7

Erlichmanite (OsS2)

Iridium (Ir)

77

193

2454

22.6

Irarsite (IrAsS)

Platinum (Pt)

78

195

1769

21.5

Cooperite (PtS), Braggite (Pt,Pd)S, Sperrylite (PtAs2), Moncheite (PtTe2)

placers, which commonly show an association with ultramafic/mafic complexes such as ‘Alaskan-’ or ‘Alpinetype’ intrusions and paleoplacers. The Witwatersrand is the only known example of the ‘paleoplacer’ that is sufficiently rich in the platinum group elements to permit recovery (Macdonald, 1987). The orthomagmatic class contains all those deposits that form solely within the magmatic environment (Naldrett, 2004; Mondal, 2008; Mungall and Naldrett, 2008 for review). Almost all production of the platinum group metals comes from South Africa (supplies in 2009: Pt = 4.53 million ounces, Pd = 2.37 million ounces and Rh = 0.663 million ounces) - mostly from the Bushveld Complex, and Russia – where most production comes from Norilšk in Siberia, from where PGEs are extracted with Cu and Ni (supplies in 2009: Pt = 0.785 million ounces, Pd = 2.675 million ounces and Rh = 0.07 million ounces; Data source: ‘Platinum, 2010’). The platinum group elements have many industrial uses, which are essentially based on their very high fusion temperature, the absence of chemical reactivity and exceptional catalytic properties. The main end-uses of PGE lie in electronics: Pt-Rh thermocouples are extensively used in the measurement of high temperature in furnace; Pd has widespread use in telephone exchange still operating on electro-mechanical switching; the car industry (manufacturing catalytic exhausts), jewelry (the main use of Pt), dentistry (Pd), chemical (Pt and Rh for making glass fibers) and the petroleum industry (Pt). PLATINUM GROUP ELEMENTS AS GEOCHEMICAL TRACER

Geochemistry of the platinum group elements provides unique clues to the early origins of our planet (e.g., Carlson

et al. 2008). The platinum group elements are particularly useful as tracers of the impacting extra-planetary materials in the strongly PGE-depleted crust of the Earth and other planets (e.g., Palme, 2008). The distribution of PGEs within the Earth is a subject of intense debate (Crocket, 2002; Lorand et al. 2008). Platinum group elements have a strong siderophile affinity (Chou, 1978). Laboratory experiments provide the evidence that the PGEs have more than 105 times greater preference for liquid metals than for the silicate magmas (e.g., Righter, 2003). Therefore, during core-mantle differentiation, the PGEs should have preferentially partitioned into the core-forming metal, leaving the mantle depleted in these elements relative to their original abundances. However, the measured or estimated PGE contents of planetary mantles are much higher than predicted by these partition coefficients. Calculation shows that if the Earth’s mantle was in equilibrium with its core, the mantle would contain three orders of magnitude less of the PGE than is observed, supporting a late addition of PGE components (Drake and Righter, 2002). The most common explanation put forward to account for this disparity is that the last 1% of the Earth’s accretion occurred after the iron-rich core had separated from the mantle. According to this model the Earth’s mantle underwent whole-sale bulk depletion in PGE during core formation (at about 4.55 Gyr ago), followed by progressive re-enrichment with PGE in response to addition of cosmic material from the Hadean to Early Archean (4.5-3.8 Gyr ago) through heavy meteorite bombardment - the so-called ‘late veneer’ (Chou, 1978). Another viable mechanism is based on different crystallo-chemical properties of the elements at high pressure and temperature. It has been suggested that the distribution coefficients of the siderophile elements (especially Pt, Pd and Au) between metal and JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011

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silicate phases can be changed under high pressure and magmas, have high Re/Os ratios compared to the mantle temperature conditions, owing to which the lower mantle rocks. The advantage of using the Os isotope system is materials may contain higher amount of siderophile elements that this system develops large parent-daughter contrasts (e.g., Cottrell and Walker, 2006; Righter et al. 2008; Brenan among magma sources, it records prior melt depletion effects and McDonough, 2009). Another idea, that the PGE-rich in mantle sources, and also helps to identify the effects of liquid outer core material can be transported back into the any secondary event over primary magmatic events. In mantle, as trace elements in plumes, rooted at the core-mantle addition, the 187Re/188Os isotope system (where the decay boundary has also received a great deal of attention (e.g., constant, α, for 187Re=1.666×10-11 year-1, Smoliar et al. Brandon and Walker, 2005). 1996) is particularly useful for discriminating between The relative effects of these processes can be assessed long-term melt-depleted reservoirs, including the subusing PGE abundance patterns as well as Re-Os isotopic continental lithospheric mantle (SCLM; Walker et al. 1989; systematics of the mantle materials (e.g., Shirey and Pearson, 1999; Mondal et al. 2007). Walker, 1998). Conventional radiogenic isotope studies of Base metal sulfide minerals are common in mantle mantle differentiation traditionally involve elements (Sr, Nd, rocks (Fig. 1), but are not widely characterized in detail for Hf, Pb) that strongly prefer melt over residual solid (e.g., the purpose to look and to examine the processes shaping Faure, 1986). Therefore, mantle residues after melt the chemical evolution of the Earth’s mantle. It has been extraction are difficult to study because they have very documented that sulfide minerals in mantle peridotites low concentrations of these elements. Unlike any other control the PGE budget of mantle rocks along with their element in a radioisotope system, Os is a compatible element behaviour during mantle melting. Hence, the sulfide minerals which stays in the residue during melting in the mantle. As in mantle rocks can be used to trace the petrogenetic a result, most magmas, and the crust formed from)LJXUH such processes responsible for the differentiation of the Earth

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MP Fig.1. SEM image of a kimberlitic mantle xenolith (mantle peridotite) from west Greenland. Sample showing base metal sulfides as inclusion within olivine (bms-1) and at the interstitial spaces of olivine (bms-2). SEM imaging conducted by the author at the EPMA laboratory, University of Copenhagen. Sample courtesy Minik Rosing, Natural History Museum of Copenhagen, University of Copenhagen. JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011

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(Mitchell and Keays, 1981; Morgan, 1986; Guo et al. 1999; Aulbach et al. 2009). Because at equilibrium, the concentration of all of the PGEs is at least 10,000 times higher in sulfide melt than in coexisting silicate melt, sulfide is an extremely potent agent for the collection and segregation of PGE (e.g., Fleet et al. 1999; Sattari et al. 2002; Brenan and McDonough, 2005). Major research has been focused on immiscible sulfides in mantle derived magmas; however, very little is known about the origin of these mantle sulfides and studies in this direction are also scarce. OSMIUM ISOTOPIC CONSTRAINTS ON MANTLE HETEROGENEITY

It has been shown that in terms of Os isotopic compositions, the Earth’s convecting upper mantle is chondritic (e.g., Meisel et al. 2001; Walker et al. 2002). However, there are strong evidences for mantle heterogeneity in terms of siderophile elements, particularly with respect to Os isotopic signatures (Frei et al. 2006; Bernstein et al. 2006; Ahmed et al. 2006). Peridotites that have recently been removed from the old sub-continetal lithospheric mantle (SCLM) are observed to have Os isotopic compositions that are much less radiogenic than chondritic meteorites (e.g., Walker et al. 1989; Pearson et al. 1995). The depleted, subchondritic Os isotopic compositions in these rocks have been attributed to longterm Re depletion, which evidently reflects ancient melt removal, coupled with the moderate incompatibility of Re but compatibility of Os, during mantle melting (e.g., Walker and Shirey, 1998; Mondal et al. 2007 and references therein). Similarly less radiogenic Os compositions were found in orogenic peridotites, which are thought to represent Proterozoic SCLM (Reisberg and Lorand, 1995). Walker and Stone (2001) reported significantly subchondritic Os isotopic compositions for an Archean high-Mg volcanic rock, the Boston Creek flow of the Abitibi greenstone belt (Ontario, Canada), suggesting derivation from ancient SCLM. In addition to SCLM, 187Os depleted materials have also been identified in oceanic rocks (e.g., Harvey et al. 2006). The depleted compositions were interpreted to reflect either derivation from long-term Re-depleted SCLM that had been delaminated from the craton, or survival of highly Re-depleted reservoirs for more than 1 Ga in the convecting mantle. The observations from the mantle xenoliths confirm that the SCLM is geochemically distinct from the convective mantle, and has remained stable as residue of extensive

partial melt extraction in the Archean (Boyd, 1989; Bernstein et al. 1998; Griffin et al. 2003; Herzberg, 2004; Carlson et al. 2005). Some peridotite xenoliths removed from the SCLM underlying Archean portions of the cratons e.g., Greenland, Siberian, Kaapvaal, Wyoming and Tanzania cratons, indicate melt depletion as much as 3.6 Ga ago (e.g., Mondal et al. 2007 and references therein). Osmium isotopic studies of unaltered chromites from the Nuasahi and Sukinda ultramafic-mafic complexes, in the Singhbhum craton of the Indian shield, indicate the existence of a SCLM domain beneath the Singhbhum craton that started to form in 3.7 Ga ago by depletion of a primitive mantle (Mondal et al. 2007). The average initial 187Os/188Os compositions of unaltered chromites of massive chromitite layers from two igneous complexes are subchondritic: 0.1031±0.0007 (Nuasahi) and 0.1029±0.0005 (Sukinda), with negative γOs values of -1.78±0.65 and -2.04±0.43, respectively. The subchondritic signatures of Os in the Singhbhum chromites are comparable with results of age-constrained chromites from ultramafic intrusions in the Zimbabwe craton (e.g., Nägler et al. 1997). They indicate the sub-chondritic nature of the source mantle, and are more compatible with subcontinental lithospheric mantle extraction characteristics. KNOWLEDGE GATHERED FROM THE STUDIES OF SULFIDE MINERALS IN MANTLE XENOLITHS

Within the peridotitic mantle xenoliths, sulfide minerals are mostly present as inclusions as well as at the interstitial spaces of olivine, garnet, clinopyroxene and orthopyroxene (Fig. 1). The PGE abundances and Os isotopic characters of these sulfide minerals, have been used as a tracer of geochemical evolution of the source mantle, from where these xenoliths were sampled by the kimberlitic magma or a basaltic magma. Together with petrological information, PGE geochemical and Os isotopic data of the sulfide minerals can help to assess the importance of magmatic processes (e.g., mantle melting) versus primary differentiation process responsible for the chemical character of the mantle. In particular, one can determine whether or not the source of the xenoliths is part of a long term Redepleted cratonic mantle reservoir (i.e. SCLM), the timing of the depletion of the mantle, and what the various Os isotopes as well as variation in PGE contents among the included and interstitial base metal sulfides tell us about how metals were redistributed in subsolidus environments. When the upper mantle peridotite undergoes partial melting, its abundances of siderophile elements are fractionated. If plotted in order of decreasing melting temperature, PGE abundances in magmas (e.g., basalt) JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011


JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011

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produce positively sloped patterns, while restitic mantle rocks (e.g., harzburgite) show smooth negative slopes (Fig. 2). The mechanism of fractionation is not known in detail, but likely involves the selective uptake of Os, Ir and Ru, relative to Pt and Pd, in residual olivine, Cr-spinel and crystalline monosulfide solid solution (Righter et al. 2004; Brenan et al. 2005; Ballhaus et al. 2006 and references therein). From a detailed study of the mantle xenoliths Alard et al. (2000) described two texturally and compositionally distinct types of sulfides in mantle-derived peridotites: (1) rounded monosulfide solid solution enclosed in silicates, usually in depleted, S-poor samples, and (2) interstitial sulfides, often rich in Ni or Cu. Both types, commonly, occur in a single sample. Figure 2 shows comparison of the abundances of selected PGEs, measured in-situ using LAICP-MS by Alard et al. (2000) in sulfide grains from the mantle lherzolite (Fig. 2a), and the abundances in residual harzburgite and basalt (Fig. 2b). The PGE signature of the sulfides enclosed in olivine is almost identical to that of the residual harzburgite. With the exception of platinum, the PGE in the interstitial sulfides found between the grains, display a signature similar to that of the basalt. So, the two sulfides have the complementary patterns that are expected for a melt and its mantle residue. From these data, Alard et al. (2000) argue that the unusual patterns of PGE abundances seen in mantle peridotites can be explained by ordinary melting processes. They interpreted the silicate-enclosed sulfides as the residues of melting processes, and the interstitial sulfides as the crystallization products of sulfidebearing (metasomatic) fluids. These strong differences between monosulfide solid solution and interstitial sulfides agree with experimental data that suggest a strong affinity of the IPGE (Ir, Os, Ru) for the monosulfide solid solution, while Pt and Pd partition into the coexisting sulfide liquids (Bockrath et al. 2004). Using LA-ICP-MS, Pearson et al. (2002) determined Os isotopic compositions of sulfides in kimberlitic mantle xenoliths, and showed that sulfide inclusions in silicates preserve significantly less radiogenic compositions than interstitial sulfides and accordingly produce significantly older and more realistic Re-Os age information than bulk rock analysis. The in-situ analyses using LA-ICP-MS by other workers (e.g., Alard et al. 2002; Griffin et al. 2004) also showed that the two generations of sulfides are not contemporaneous: ‘basaltic’ intergranular sulfides are 1-2 billion years younger than ‘restitic’ sulfides, suggesting that the interstitial sulfides are likely metasomatically introduced phases in the melt-depleted peridotites. Therefore, it seems very likely that the abundances of PGE measured in modern

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Fig.2. Comparison of the chondrite-normalized PGE patterns of the included (e-ol MSS) and and interstitial sulfides (i-Curich Pn; i-Pn) from the mantle lherzolite (data source: Alard et al. 2000). The PGE patterns of the included sulfides show similarity with the chondrite-normalized PGE patterns of the melt depleted mantle harzburgite, whereas the PGE patterns for the interstitial sulfides are similar to the basalt. Mantle harzburgite xenolith (East African Rift) data from Lorand et al. (2003); Basalt (Southern and Middle Kolbeinsey Ridge) data from Rehkämper et al. (1999). Chondrite data from McDonough and Sun (1995). Figure is based on Alard et al. 2000; Rehkämper, 2000).

mantle rocks were overprinted by the magmatic processes, especially melt removal and refertilization. Evidences from the Os isotopic studies of peridotitic sulfide inclusions in diamond from the SCLM even suggest metasomatic signatures in 3.5 Ga Slave craton, and favours subduction process can be the key to forming the earliest cratonic nuclei (e.g., Shirey et al. 2004). In this regard, apart from understanding the initial differentiation of the planet, it is important to also evaluate the magmatic processes that contributed in the evolution of the Earth’s mantle regions, in terms of material addition and material recycling through time.

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Acknowledgements: I am dedicating this article to honour the vast contributions of B. P. Radhakrishna to the Indian geology. This review article is based on a collaborative project on the kimberlitic mantle xenoliths from west Greenland. I am thankful to Steve Shirey (Department of Terrestrial Magnetism, Carnegie Institution, Washington), Stefan Bernstein (Natural History Museum of Denmark, University of Copenhagen), Minik Rosing (Natural History Museum of Denmark, University of Copenhagen) and Ed Mathez (American Museum of Natural

History, New York City for their collaboration to conduct this project through a support from the Nordic Center for Earth Evolution (NordCEE), Copenhagen. My research scholars Sarifa Khatun and Ria Mukherjee are thankfully acknowledged for their informal comments and discussion on this article. I am grateful to B. Mahabaleshwar for his encouragement to write this article for the Journal of Geological Society of India. Rajesh Srivastava is thankfully acknowledged for his thoughtful comments on this article.

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(Received: 21 December 2010; Revised form accepted: 28 January 2011)
