Ruthenium in komatiitic chromite

Abstract

The distinction between Ru in solid solution and Ru-bearing inclusions is essential for the predictive modeling of platinum-group element (PGE) geochemistry in applications such as the lithogeochemical exploration for magmatic sulfide deposits in komatiites. This study investigates the role of chromite in the fractionation of Ru in ultramafic melts by analyzing chromite grains from sulfide-undersaturated komatiites and a komatiitic basalt from the Yilgarn Craton in Western Australia. In situ analysis using laser ablation ICP-MS yields uniform Ru concentrations in chromites both within-grain and on a sample scale, with concentrations between 220 and 540 ppb. All other platinum-group elements are below the detection limit of the laser ablation ICP-MS analysis. Carius tube digestion isotope dilution ICP-MS analysis of chromite concentrates confirms the accuracy of the in-situ method. Time resolved laser ablation ICP-MS analyses have identified the presence of sub-micron Ir-bearing inclusions in some chromite grains from the komatiitic basalt. However, Ru-bearing inclusions have not been recognized in the analyzed chromites and this combined with the in situ data suggests that Ru exists in solid solution in the crystal lattice of chromite. These results show that chromite can control the fractionation and concentration of Ru in ultramafic systems.

Introduction

Platinum-group elements (PGE: Os, Ir, Ru, Rh, Pt, Pd) are important as petrogenetic tracers in the study of Earth’s accretion history, core–mantle interaction, and mantle differentiation processes (Greenough and Fryer, 1990, Fryer and Greenough, 1992, Rehkämper et al., 1997, Brandon et al., 1999, Brügmann et al., 2000, Puchtel and Humayun, 2000, Righter and Drake, 2000, Brandon and Walker, 2005, Lightfoot and Keays, 2005, Becker et al., 2006, Maier et al., 2009, Savard et al., 2010). They are also potential pathfinders for Ni–Cu mineralization (Brügmann et al., 2000, Lightfoot and Keays, 2005, Fiorentini et al., 2010). However, owing to their low abundances and complex behavior, PGE remain among the least understood elements in geochemistry. As a consequence there is a steadily increasing demand for high quality PGE analyses and for improved knowledge of the thermodynamic properties of PGE in magmatic systems to underpin understanding of their behavior during partial melting and crystallization.

This study investigates the behavior of Ru in rocks of komatiitic affinity, and specifically the role played by chromite in fractionating Ru from the other PGE. The association of Ru, Ir and Os with chromite is well known in ophiolitic complexes (e.g. Augé, 1985, Garuti et al., 1999, El Ghorfi et al., 2008, González-Jiménez et al., 2009, Marchesi et al., 2010) and layered intrusions (e.g. Merkle, 1992, Von Gruenewaldt and Merkle, 1995, Maier and Barnes, 1999, Maier et al., 1999). Chromite-rich rocks in these settings are characteristically enriched in Ru, Ir and Os, which in most cases appear to be hosted in accessory phases such as laurite, arsenides, and alloys. However, there has been a long standing debate about the extent to which these elements can also be present within the chromite lattice (Capobianco and Drake, 1990, Pagé et al., 2009). This issue is important for two main reasons: (1) to underpin quantitative modeling of the behavior of PGE during crystal fractionation, and (2) in the potential application of the PGE chemistry of chromite to geochemical prospecting for magmatic sulfide ores, through geochemical fingerprinting of sulfide-saturated magmas (Fiorentini et al., 2008). Komatiites provide an ideal opportunity to test this potential economic application by studying a restricted sample set of sulfide-free, chromite-bearing olivine cumulate rocks in order to isolate the role of chromite in the fractionation and concentration of PGE.

Komatiites are ultramafic magmas defined by liquid compositions greater than 18 wt.% MgO (when normalized to 100% volatile-free values; Arndt, 2008). Magmas with lower MgO liquid compositions (15–10 wt.% MgO, anhydrous) with demonstrable derivation from komatiitc parents are commonly referred to as komatiitic basalts. Komatiites were erupted as lava flows or emplaced as subvolcanic sills mostly in the Archean and early Proterozoic, and rarely in the Phanerozoic (Arndt, 2008). High temperatures at eruption (1450–1600 °C), high densities (∼2.8 g/cm³), and low calculated viscosities (0.1–10 Pa s) resulted in turbulent flow behavior after eruption (Arndt, 2008), and a tendency for komatiites to assimilate crustal substrates to form magmatic Ni-sulfide ores under particular circumstances (Lesher et al., 1984, Lesher, 1989, Barnes, 2006).

Komatiite magmas are characteristically sulfide-undersaturated on eruption as evident from a lack of PGE depletion (Lesher 1989; Fiorentini et al., 2010), and due to the high solubility of sulfur in komatiites at low pressures (Shima and Naldrett, 1975, O’Neill and Mavrogenes, 2002). Where komatiites became sulfide-saturated as a result of assimilation of crustal sulfur, the behavior of the PGE (and other chalcophile elements such as Ni, Cu, and Au) was influenced by the strong partitioning of the PGE into sulfide melts relative to silicate magmas (Stone et al., 1990, Fleet et al., 1991, Peach and Mathez, 1996, Crocket et al., 1997).

In sulfide-undersaturated komatiite melts, the PPGE (Pd, Pt, and Rh) and the IPGE (Ir, Os, and Ru) are decoupled during fractionation (Keays, 1982, Crocket and MacRae, 1986, Barnes and Picard, 1993, Puchtel et al., 2004, Barnes and Fiorentini, 2008). Platinum and Pd essentially behave as incompatible elements whereas Rh shows an indication of weak compatibility in olivine (Barnes and Fiorentini, 2008). Patterns of Ir variation in komatiites have been explained in terms of saturation with, and accumulation of, a magmatic Ir-rich phase, inferred to be an Os–Ir alloy (Barnes and Fiorentini, 2008). This is consistent with the experimental evidence for extremely low solubilities of Ir in silicate melts (O’Neill et al., 1995) and is furthermore supported by experimental studies that suggest that Ir, Os, and Ru form nuggets attached to chromite (Ballhaus et al., 2006). The fractionation and concentration of Ru also appears to be in part controlled by equilibration with IPGE-dominant phases, but also to a degree by the fractionation of chromite and to a lesser degree olivine (Barnes and Fiorentini, 2008). Bockrath et al. (2004) have shown that laurite (RuS₂) can crystallize directly from a sulfur-bearing, but FeS-undersaturated silicate melt, allowing Ru to be fractionated in S-undersaturated silicate systems.

The role of chromite and olivine in controlling PGE concentrations during fractionation is widely debated and two alternative models exist:

• PGE, particularly IPGE, are predominantly hosted by inclusions or “micro-nuggets” enclosed within olivine and chromite. This can occur either by olivine and chromite nucleating on platinum-group minerals (PGM) at magmatic temperatures and incorporating them as inclusions (Keays, 1982, Melcher et al., 1997, Maier and Barnes, 1999, Ballhaus and Sylvester, 2000, Sattari et al., 2002, Ballhaus et al., 2006, Pagé et al., 2009); or by nucleation of PGM grains in redox boundary layers around growing chromite crystals, as demonstrated experimentally by Finnigan et al., 2008). Evidence for PGM inclusions in chromites from ophiolites (Augé, 1985, Garuti et al., 1999, El Ghorfi et al., 2008, González-Jiménez et al., 2009, Marchesi et al., 2010) and layered intrusions (e.g. Merkle, 1992, Von Gruenewaldt and Merkle, 1995, Maier and Barnes, 1999, Maier et al., 1999) is well documented. Platinum-group element inclusions in olivine have also been used to explain the non-chondritic PGE profiles of komatiites (Keays, 1982, Barnes et al., 1985, Barnes and Fiorentini, 2008). Tredoux et al. (1995) interpret PGE enrichments in olivine-rich rocks and chromitites as the result of the formation and physical accumulation of atomic-scale PGE clusters.

• Olivine and chromite host PGE in solid solution (Puchtel and Humayun, 2001, Brenan et al., 2003, Fiorentini et al., 2004, Righter et al., 2004). Puchtel and Humayun (2001) have claimed that Ru, Os, and Ir are slightly compatible to moderately incompatible in olivine with distribution coefficients (D) of D_Ru = 1.7, D_Os = 1.2, and D_Ir = 0.77, and that Pt and Pd are highly incompatible in olivine (D_Pt = 0.08, D_Pd = 0.03). They also claim that all PGE are compatible in chromite with distribution coefficients of D_Ru = 151, D_Os = 153, D_Ir = 100, D_Pt = 3.3, and D_Pd = 1.6. However, it is noted that these values are based on bulk rock samples and mineral separates. Therefore it is not possible to rule out the presence of PGE nuggets which can affect the calculation of the distribution coefficients. High temperature experiments by Righter et al. (2004), show that Ru, Ir, and Rh can be highly compatible in Cr-rich spinel, with distribution coefficients of D_Rh = 41–530, D_Ru = 76–1143, and D_Ir = 5–22,000. These experiments were performed at oxidation states 2–3 orders of magnitude above the quartz–fayalite–magnetite (QFM) buffer, well outside the range of most mantle derived mafic and ultramafic magmas, and hence do not necessarily reflect natural behavior. Moreover, the wide range of D_Ir = 5–22,000 in chromites grown under near-identical conditions suggests the presence of Ir nuggets in (or attached to) chromite, similar to the observations made by Ballhaus et al. (2006). Experimental studies also indicate that Ru and Rh have ionic radii and charges that theoretically allow them to substitute for Cr³⁺ and Fe³⁺ within the spinel structure (Capobianco et al., 1994, Capobianco, 1998, Righter and Downs, 2001). This hypothesis is also supported by a recent experimental study by Brenan et al. (2011) who demonstrate that the compatibility of Ru in chromite is strongly related to fO₂ in the way that fO₂ controls the speciation of Ru in the melt and the ferric-iron content of chromite. If Ru is predominately present as trivalent species, it can substitute for Fe³⁺ and Cr³⁺ in the chromite lattice. As a consequence, ferric-iron-rich chromite can accommodate more Ru than chromite that crystallized at more reduced conditions.

Although these studies suggest that some of the PGE can exist in solid solution in chromite and olivine, the evidence is commonly based on indirect observations. As outlined above, this includes (1) the absence of detectable PGE-bearing inclusions; (2) experimentally determined PGE partition coefficients under unnatural conditions at high oxidation states; and (3) the analysis of mineral concentrates and calculated PGE partition coefficients. In-situ studies to determine if PGE are present in solid solution in chromite (or olivine) are extremely rare in the literature. Pagé et al. (2009) analyzed several chromites from podiform chromitite deposits, stratiform chromitite seams and boninitic lavas by laser ablation ICP-MS, but did not detect any PGE apart from PGM inclusions in chromite. A subsequent study of chromites from the Alexo komatiite flow (Canada) yielded Ru concentrations of 450 ± 57 ppb (Pagé et al., 2011). Park et al. (2010) analyzed chromites from the highly oxidized Ambae lava (Vanuatu Arc) and report concentrations of Ru = 92.6 ppb, Os = 36.8 ppb, Ir = 52.1 ppb and Rh = 75.8 ppb (with Pd, Pt, and Au below detection). In both studies the Ru is interpreted to be in solid solution based on the time-resolved laser ablation ICP-MS spectra.

The distinction between solid solution and PGM inclusions is essential for the predictive modeling of PGE geochemistry in applications such as lithogeochemical exploration for nickel sulfide deposits. Komatiites host some of the world’s largest magmatic nickel-sulfide deposits and the development of reliable lithogeochemical indicators to guide exploration has been a long-standing goal. Lithogeochemistry of komatiites is currently applied in nickel-sulfide exploration in two distinct ways: indirectly, through identification of favorable geological environments, and more directly, through identification of signatures that record sulfide liquid segregation, by ascribing PGE depletion to sulfide segregation (e.g. Brügmann et al., 1993, Barnes et al., 1995, Lesher et al., 2001, Sproule et al., 2005, Fiorentini et al., 2010). Zhang et al. (2008) have compared basalts from large igneous provinces and suggest that it is also possible to discriminate mineralized from barren systems using isotopic and PGE data. However, PGE content and distribution in melts are the result of a complex range of factors, e.g. composition of the parental magma (especially the degree of sulfide saturation or undersaturation), oxygen fugacity, and theoretically also wall-rock contamination. Therefore, whole-rock PGE signatures are generally ambiguous and difficult to interpret. Moreover, a whole-rock approach may not be able to discriminate superimposed effects such as metamorphism and serpentinization from primary magmatic PGE signatures.

Fiorentini et al. (2008) conclude that contrasting trends of Ru vs. Cr are evident in mineralized (sulfide-saturated) and unmineralized (sulfide-undersaturated) komatiitic cumulates, and that these trends can be used in the exploration for nickel sulfide ores. A plot of whole-rock Ru-Cr concentrations in S-poor (<0.2 wt.%) dunitic komatiite samples from the Yilgarn Craton in Western Australia is shown in Fig. 1. Three trends can be discerned. The first trend is characterized by high Ru variability with concentrations up to 16 ppb at low Cr values around 1000–1500 ppm (Fig. 1-A). Due to the strongly chalcophile behavior of Ru, this trend is commonly interpreted to be the result of sulfide accumulation in the magma system. The second trend is in the opposite direction (merged with the sulfide accumulation trend) and reflects sulfide extraction (Fiorentini et al., 2008). The third trend displays a correlation between Cr and Ru (Fig. 1-A and B). Using the Cr concentrations as a rough indicator of the chromite content of chromite-saturated and sulfide-undersaturated komatiites (Barnes, 1998), and assuming that Ru is compatible in chromite, it can be inferred that the fractionation and concentration of Ru along this trend is controlled by the crystallization of chromite (Fiorentini et al., 2008).

However, the interpretation of Ru–Cr whole-rock data is often complicated by the fact that samples plot in overlapping fields and/or trends, which can make the whole-rock approach ambiguous and unreliable. Moreover, two other trends shown in Fig. 1-A display an uncommon behavior that cannot be explained using existing models for Ru fractionation in komatiitic systems. One trend, seen in the Mount Clifford data set, is characterized by strongly depleted Ru concentrations (<1 ppb) compared to other Yilgarn komatiites but has strongly varying Cr concentrations between 1000 and 10,500 ppm Cr. The other trend, as in data from the Horn, displays an anomalous Ru variation (∼2–14 ppb) at constant high Cr concentrations of around 6000–6500 ppb. This trend cannot be explained by the presence of a sulfide source, and requires new understanding of the relationship between chromite crystallization and the concentration of Ru and Cr in magmas and cumulus assemblages.

This study tests the concept developed by Fiorentini et al. (2008) by investigating the Ru content of chromite itself, with the ultimate goal of using detrital chromite as a resistate indicator mineral, analogous to the use of garnet in diamond exploration (Griffin and Ryan, 1995). The approach taken in this study combines (1) in situ laser ablation ICP-MS analysis of chromite grains to determine Ru concentrations at low ppb levels and the spatial distribution of Ru within the grains; and (2) Carius tube digestion isotope dilution ICP-MS analysis of chromite concentrates to test the accuracy of the in situ studies. The research goal is to test the hypothesis that the fractionation and concentration of Ru in sulfide undersaturated komatiitic melts is controlled by the crystallization of chromite and accordingly to determine whether Ru is present in solid solution in chromite.