Partitioning of ruthenium, rhodium, and palladium between spinel and silicate melt and implications for platinum group element fractionation trends

@X6-7037/9O/s3.00

GPoehimica t-f Cosmochimica Acla Vol. 54, pp. 869-874 Copyxight 0 1990 Pergamon Press pk. Printi in U.S.A

+ 03

LETTER

Partitioning of ruthenium, rhodium, and palladium between spine1 and silicate melt and implications for platinum group element fractionation trends CHRISTOPHER J. CAFQBIANCOand MICHAELJ. DRAKE Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA (Received November 28, 1989; accepted in revisedform January 23, 1990)

Abstract-High temperature (1450 and 13OO”C),one atmosphere trace element partitioning experiments were made in the CaO-MgO-SiOz-A1203 system to investigate the chemical compatibility of Ru, Rh, and Pd in magnesium aluminate spinels coexisting with Silicate melts. Spinel/melt partition coefficients indicate that Rh and Ru are highly compatible with values near 90 and 20, respectively. These high partition coefficients contrast sharply with partitioning behavior for Pd under similar conditions. Palladium is rather incompatible with partition coefficients of less than 0.02. Platinum group elements, generally considered a geochemically coherent group, may thus be fractionated with respect to one another when spine1 is a magmatic phase. plain Pd/lr trends in the Alexo komatiite. In contrast, BROGMANNet al. (1987) favor true solid solution of some of the noble siderophile elements in silicate and/or oxide crystals to account for the compatible behavior. MITCHELL and KEAYS( I98 1) analyzed mineral separates from mantle nodules and also concluded that a real crystal-chemical control does seem to exist for the distribution of PGEs in mantle phases, even though the bulk of the PGEs are not contained in any major silicate or oxide phase. One undisputed fact in PGE geochemistry is that spine1 phases are consistently found to be concentrators of PGEs (RAZIN and KHOMENKO, 1969; GJJBEJ_.Yet al., 1974; CROCKET, I98 1; PAGE and TALKINGTON, 1984). Chromite, in particular, is noted frequently as a host for PGEs and, especially, the IPGEs (PAGE and TALKINGTON, 1984; AGIORGITISand WOLF, 1978; STOCKMA N and HLAVA, 1984). In many cases PGE alloys and sulfides are included within the chromite grains and lend support to the contentions that compatibility is due more to physio-mechanical circumstances during crystallization than chemical compatibility. This study contains experimental characterization of the magmatic behavior for platinum group elements from both the IPGE (Ru) and PPGE (Rh, Pd) groups. Our new experimental results indicate that chemical control on PGE compatibility, at least for spinels, is possible at magmatic liquidus temperatures.

INTRODUCIION RUTHENIUM,RH, ANDPD are three of the six elements which comprise the platinum group elements, the PGEs. In geochemistry the PGEs are frequently separated into two groups based on systematics from a growing database of geochemical analyses from igneous systems, recently reviewed by BARNES et al. (1985). The iridium-PGEs (IPGEs) are OS, lr, and Ru, and these elements are considered compatible in igneous systems. The palladium-PGEs (PPGEs), Rh, Pt, and Pd comprise the second group, and they are thought to be incompatible in igneous processes. The use of Ir and Pd as group names arises because they are among the more easily analyzed PGEs by neutron activation analysis (NAA) and analysis of these two elements is often thought to be sufficient to characterize the behavior of all the PGEs during igneous evolution. For this reason the Pd/lr ratio is often given as a measure of the fractionation between the two groups (BARNESet al., 1985). In contrast, geochemical data for Ru and Rh are among the least available because of the increased difficulty of the NAA analysis. The assignment of compatibility or incompatibility for the PGEs is decided by whether or not geochemical analyses from igneous systems can be modelled by bulk crystal/liquid partition coefficients (D’s) which are greater or less than unity. This classification is to be distinguished from the case where compatibility is determined from experimentally measured crystal/liquid partitioning behavior. The reason for the operational definition is that experimental studies of PGE partitioning are almost completely lacking. The physio-chemical cause for the compatibility of the PGEs is a subject of some debate (BARNESet al., 1985; CROCKET, 1979). One view is that the inferred compatibility (D > 1) is related in one way or another to the strongly chalcophile and siderophile tendencies of these elements. For instance, nucleation of olivines onto entrained flecks of highly insoluble IPGE metallic alloys has been invoked by BARNESand NALDRETT(1987) to ex-

EXPERIMENTAL AND ANALYTICAL METHODS One atmosphere, high temperature experiments were conducted in either pure oxygen, so as to maximize the amount of oxidized PGE species in the molten silicate system, or in air. Oxide mixes of initial composition CaO = 13.4 wtW: Ma0 = 24.2 wt%: Al,O? = 17.7 wt%; SiOz = 44.7 w-t%,and either Ri, Rh, or Pd me&, were loaded into capsules made from single crystal synthetic spine1 boules available commercially (grown by the flame fusion method, i.e., the Vemeuil process). The oxide mix was fused in the open capsule under an oxyacetylene torch and then suspended in the alumina muffle tube of a gas-mixing, resistance furnace (MO!& elements) at lOOO869

870

C. J. Capobianco and M. J. Drake

I 100°C. The temperature was raised directly to run temperature, and maintained for durations between 3 and 7 days. Samples were quenched by rapid removal from the furnace followed by a blast of air. Polished sections of spine1 crucibles containing the samples were prepared and analyzed using the wavelength dispersive system of the electron microprobe at the University of Arizona, utilizing a I5 kV beam and a sample current of 30 na. Pure metal standards for the PGEs and ZAF correction procedures were used. X-ray intensities in phaseswith low concentrations of PGE were obtained using sample currents of 100 na and counting times on the order of 300-1000 seconds on peak and stepped Scans in the peak region to assess background characteristics. One analysis for Rh in a glass was made using the pPIXE probe at the Los Alamos National Lab (ROGERS et al., 1984) with the help of Dr. P. S. 2. Rogers.

Table 2. Summary of experimental PGE spinel/melt partition coefficients, run conditions, and phase concentrations [in ppm].

RU Rh Pd terror

spine1

glass

atrnospbere

25(9)’

1450

7900(300)2

320(120)*

air

22(9)’

1290

5900(400)*

270(1 1O)2

air

78(G)

1450

4300(460)2

55(9)3

air

Qo(iO)4

1300

50*-73700t

50*-980t

@

<0.02

14.50

25*

I120(100)

02

in phase concentrations

propagated

through

to quotient,

D;

%tanda.rd deviation of electron probe analyses; 3single PIXE analysis and associated counting statistical error; 4standard deviation of D’s calculated from analyses of immediately adjacent phases; *electron probe detection

RESULTS

limit; trange of concent~tions within single ran

The bulk melt com~sition is tem~rature dependent because more of the spine1 capsule dissolves into the sample at higher temperature. The starting capsule material is a very non-stoichiometric spine1 (approximately 90 wt% A1203, L0 wt% MgO) which at run conditions exsolves into corundum and a spine1 with less excess A1203. A recrystallized rind of spinels on the interior wall of the capsule coexists with the silicate melt and may contain the elements of interest. Unattached euhedral spinels within the melt also occur. The bulk compositions of relevant phases are given in Table 1. Table 2 gives a summary of experimentally determined spinellmelt partition coeecients for Pd, Ru, and Rh, the PGE contents of phases of interest, and experimental run conditions. Pd spinel/melt partitioning There is significant solubility of Pd in silicate melts in the CaO-Al*Os-MgO-Si02 (CAMS) system with concentrations over 1000 ppm possible, but we have not detected Pd in the spinels which crystallize in this system. We note that we are unable to confirm the preliminary spinel/melt partitioning data of CAPOBIANCO et al. (1989) which indicated slight compatibility for Pd based on several PIXE (proton induced X-ray emission) microprobe analyses in a synthetic (B-bearing system. Electron probe analysis for the same samples found measurable Pd in the glass but no detectable Pd in any of the spine1 crystals examined. The fact that the PIXE technique causes X-rays to be produced from a sample depth as great as 30 pm could have resulted in apparent compatibility if, for instance, the crystals contained Pd-enriched melt inclusions below the polished surface or if the crystals nucleated on microscopic metallic Pd crystals (see Discussion below).

Table 1. Representative bulk phase compositions phase

T(Y)

h%@

spine1

1300

27.8

72.6

spine1

1450

25.9

73.9

-

-

glass

1300

15.5

22.9

48.2

14.0

glass

1450

12.2

31.8

45.5

10.0

A]203

Si@

CaO

-

Table 2 shows that Pd is rather incompatible partition coefficients less than 0.02.

in spine1 with

Ru spinel/me~t partitioning An oxide of ruthenium, RuOz, is present in the 1300°C runs, while at 1450°C only Ru as metal was found in the run products. The amount of Ru in the spine1 was not particularly sensitive to temperature and was fairly uniform in the unattached euhedral spinels. The lack of zonation is presumably attributable to the continuous replenishment of Ru in the melt by the coexistin& unit activity, Ru-phase. Electron microprobe analyses for Ru in the glass have fairly large relative errors giving partition coefficients of dRu = 22 or 25 (29). Nevertheless, it is inescapable that Ru is significantly compatible in spine1 under our experimental conditions. An important observation in the Ru experiments concerns the character of the luminescence of the crystals under the electron beam. The exsolved spinels within the interior of the capsule walls and in the recrystallization rind glow a dull green, except in the region close to the melt interface where there was no luminescence. Nor is there luminescence for detached euhedral spinels within the bulk of the melt. The absence of luminescence distinguishes crystais containing measureable concentrations of Ru from those that do not contain measureable Ru. Thus, although it is not uncommon to find that some crystals had grown around either Ru oxide or metal and were presumably nudeated by the Ru, included Ru phases are not the source of the Ru X-rays for the spine& we measured. The Ru must be present in a dissolved state within the spine1 structure to be able to extinguish the luminescence of Ru-bearing spinels. Loss of luminescence would not be expected from metallic or oxide inclusions. We have also excluded the possibility that some other element is responsible for “quenching” the luminescence because luminescence is found in spine]-bearing experiments without Ru. For instance, all spinels in the Pd-bearing runs luminesce. Rh spine//melt partitioning Table 2 shows that the partition coefficients for Rh are even larger than those for Ru, with vafues appro8ching 100.

Fractionation of P&group elements The 1300°C Rh-beating runs produced some remarkable run products, yielding interesting data concerning the crystal chemistry of PGEs in high temperature synthetic spinel. In all experiments Rh wire was used as the source of Rh; however, in 1300°C runs the wire became encrusted with yellow transparent spine1 crystals. These crystals partially isolated the Rh source from the rest of the charge so that crystals throughout the charge had different Rh contents. Furthermore, many of the crystals exhibited prominent zonation, unlike the Ru-spinels, because the source of Rh was effectively cut off by the encrustation of crystals. Sample-wide equilibrium with respect to Rh distribution was obviously not achieved in these cases. Nevertheless, analyses of crystals and adjacent glasses yielded similar partition coefficients, suggesting local equilibrium. The surprising result is the large amount of Rh measured in some spinels (over 9 wt% Rh203) for several crystals attached to the wire. Figure 1 plots wt% RhZO) versus wt% Al203 based on electron microprobe analyses for spinels in the 1300°C run products. The reference line is for a stoichiometric spinel, Mg(RhXAl,_x)204, which is plotted for x between 0.05 and 0.0. Only spinels without Rh fall off this trend because they are slightly non-stoichiometric. The clustering of compositional points around the stoichiometric trend shows that Rh is substituting for Al. DISCUSSION

Platinum group element spinels and crystal chemical considerations

To understand the geochemical behaviors of the PGEs with respect to natural spinels and spinel/melt partitioning in magmatic systems, it is valuable to consider the crystal chemical role these elements play in synthetic systems in which they are major components. Although a literature search has not produced any prior reports of the terminal solid solution of the MgRhzOQcomponent in MgA1204 spinel, the Rh endmember is known to form a normal 2,3 spine1

wt. % A&O, FIG. 1. Electron microprobe analyses of Rh-bearirig spinels in 1300°C experiment. Solid line is a reference line for the solid solution Mg(RhXAll_J204 and is plotted for x between 0.05 and 0.0.

871

and several other Rh-oxyspinels are known with a 2+ cation from the lirst series transition metals (see GREENWOOD,1968, and BERTAUT and DULAC, 196 1). BERTAUT and DULAC (196 1) report crystal chemical &morphism between Rh3+ and Cr3+ for several classes of oxides, including spinels containing other transition metals. Rhodium, like Cr, generally prefers octahedra1 coordination in these materials. Our data indicating Rh substitution for Al suggest that Rh3+ is the stable oxidation state in the spinels of our experiments and, probably, in the melt as well. The higher maximum solubility of Rh attainable in spinels grown at 1300°C compared to 1450°C is probably due to the increased stability of oxidized Rh over metallic Rh, rather than any structural changes in the host phase (although the spinels do become more stoichiometric at the lower temperatures). Ruthenium is also known to form spine1 structure compounds (e.g., KRUTZSCHand KEMMLER-SACK,1983). However, the oxidation state of Ru in these crystals is not as unambiguous as the case for Rh. The apparently stable presence of the rutile structure oxide, RuO?, at run conditions might suggest that Ru4+ should be the prevalent oxidized species. Yet attempts to synthesize the hypothetical 4,2 spinel, Mg,Ru04, by DULAC( 1969) were not successful. Rutheniumdoped ferrite spinels have been synthesized wherein both Ru3+ and Ru4+ ions are thought to coexist (KRISHNAN, 1971). Moreover, Co2Ru04 prepared by DULAC(1969) was thought to contain Ru3+, Co3+, and Co’+ cation oxidation states. We do not know the oxidation state of Ru in our experiments, in part because the limited range of solution of the Ru component does not permit convincing chemical variations to be constructed, in contrast to the case for Rh. Both Ru4+ and Ru3+ are plausible species and it is possible that both oxidation states are present in significant concentrations. On the other hand, the more limited extent of the solid solution of Ru in MgA1204 may imply that the Ru ion does not prefer the predominantly normal 2,3 structure of the host phase. Perhaps the Ru spine1 solid solution is more akin to the MgA1204 - Mg2Ti04 system (with Ru acting similarly to Ti, i.e., a 4,2 spinel) and where a corresponding limited miscibility is found. Platinum group element oxyspinel compounds are also known for Pt, Ir, and even Pd (MULLER and ROY, 1971, 1969; KRUTZSCHand KEMMLER-SACK,1983). Most of these materials can only be synthesized under very oxidizing conditions, sometimes involving kilobars of oxygen pressure. In the case of Pd, a poorly crystallized inverse spinel, Mg,Pd04, has been synthesized by MULLERand ROY ( 197 1) under 200 atm of pure oxygen at 900°C. The relative stabilities of various PGE spinels described above help to rationalize the dramatically contrasting partitioning behavior we find for Rh and Ru compared to Pd. Since all the PGEs considered form oxyspinel compounds, at least under some conditions, the issue of their relative compatibilities depends on which oxidized PGE species are present and, to first order, how their ionic radii compare with the most suitable substituent. Because the tetravalent oxidation state for Pd, as in the compound Mg,Pd04, requires a very high oxygen potential and because the trivalent species for Pd is virtually unknown (COTTON and WILKINSON,1972), the predominant species in our experiments is likely to be

872

c.

J. Capobianco and M. J. Drake

I’d’+. However, the & electronic configuration of Pd2* leads to stabilization of the square planar coordination (due to crystal field effects) such as is found in PdO (the only stable oxide of Pdf. The preference of Pd2+ for square planar coordination is probably a major factor determining the incompatibility of Pd in spinel. Nevertheless, if any Pd is to substitute into spine1 the least unfavorable site would probably be the octahedral site (square planar sites can be thought of as extremely tetragonally distorted octahedral sites), and, for the sake of comparison, the Pd’+ ionic radius for octahedral coordination is used below. Considering the ionic radii ( 100, 76, 80.5, and 67.5 pm) for octahedral coordination for the most probable predominant cations (Pd’+, Ru“+, Rh”+, and Al”, respectively; HUHEEY, 1978) and PGE cationic valences. it is reasonable that the order of compatibility should be Rh > Ru > Pd and experimentally we find Rh > Ru $ Pd. We note that this is not the order of compatibility deduced from analytical data on igneous rocks in which Ru > Rh. However, our system is a very simple one and other unexamined variables, notably spine1 composition. may alter the Rh, Ru sequence. Ceochemical implications Our new experimental results do not conclusively settle the debate on the mechanism of PGE fractionation: chemical solubility in oxides and/or silicates versus mechanical fractionation by inclusion. However, the large c~st~/liquid partition coefficients we find for Ru and Rh between spinet and melt are very suggestive of the former mechanism being important. What is perhaps even more significant geochemicaIly is the difference in partitioning behavior between Rh and Ru on the one hand and Pd on the other. Bearing in mind that the ex~~ments we report were made in a very simple system, CAMS, and that we have not yet explored potentially significant variables such as the effect of transition metals, we are necessarily cautious in the application of our data. However, it seems fairly safe to conclude that. if spine1 is a liquidus phase in a sulfur-undersaturated stage of magmatic evolution, the fate of that spine1 could decide whether the subsequent rock is PGE enriched or depleted, and whether the PGE pattern is fractionated. We note that our experiments were made under oxygen fugacities which are not at all similar to igneous systems, but this should be completely inconsequential if compatibility in spine1 is governed by oxidized PGE species. If this is the case the high .f& has only served the experimental need to have PGE concentrations in regimes where microbeam techniques are useful. In the natural concentration range (i.e., parts per billion) we expect the relative magnitudes of the spinel/melt partition coefficients to be similar to our results. For instance, in the ppb concentration range crystal defect sites might accomodate foreign atoms in greater numbers than regular c~~~l~phic sites, resulting in larger partition coefficients. An appeal to defect siting for Pd in spine1 is, therefore, a mechanism to alter the partition coefficient for Pd, however, this mechanism would not have much effect on cationic species which are thermodynamically stabilized on regular crystallographic sites, as Rh and Ru seem to be. Fu~he~ore, with partition

coefficients on the order of 20-90 for Ru and Rh and less than 0.02 for Pd, the defect equilibria must provide dramatically stabilized sites for Pd compared to the liquid in order to override the factor > 1000 needed to Ievel differences between Pd and Ru, Rh. Such increases in D for Pd seem unlikely given that Pd is apparently able to find suitable silicate melt sites. as indicated by the larger concentration of dissolved Pd compared to either Ru or Rh the melt concentrations. The mechanism of PGE cotlcentration by incorporation of refractory and insoluble PGE metallic flecks into oxides and silicates has merits of its own. In some of our experiments we have seen PGEs in metallic form within liquidus crystals. It is apparent that these particles can and do serve well as sites for heterogeneous nucleation. In fact, the only previous ex~~mentally measured PGE compatibility, reported for Ir in magmatic phases by MALVINet al. (1986), may have been due to this nucleation phenomenon. (This conclusion was drawn after we were unable to measure the very insoluble elemental Ir in silicate systems and from reexamination of run products from the reconnaissance study of MALVIN et al., 1986.) Platinum crystals are also known to facilitate nucleation in glasses, sometimes greatly reducing the activation energy for crystallization (RINDONE, 1962). Moreover, PGE crystal nuclei have also been implicated in other surface energy related phenomena such as inducing liquid-liquid phase separation in an otherwise stable sodium phosphate melt (RINDONE and RYDER, 19.57). Ruthenium has also been shown previously to be an effective nucIeating agent for ferrite spinels in nuclear waste glasses (BICKFORD and JANTZEN, 1986). We note in passing that the Ru and Rh spinels from the walls of our spine1 capsules did not require nucleation because of the preexisting spine1 substrate of capsule wall. There are, however, some complications with the hypothesis that platinum group element alloys play a role in the fractionation of PGE patterns. First, and most important, the solubility of the PGE alloys must be extremely low for terrestrial magmas to be saturated in PGEs at the natural concentration levels (i.e., ppb’s). Nevertheless, if a PGE, say Ir, one of the least soluble (AMOSS et al., I987), reaches saturation and precipitates as a metallic phase, the activities in the silicate for the rest of the PGEs would also be lowered by partitioning into the Jr metal flecks. Would not the other PGEs in the magma then be scavenged by the metallic phase. thereby precluding PGE fractionation? In the magmatic temperature regime, there are extensive binary solid solutions between Ir and the rest of the PGEs, with one impo~ant exception (HANSEN, 1958). Palladium forms only a limited alloy solution with Ir and would only be efficiently scavenged if the Pd/Ir ratio was quite low to begin with. On the other hand, if the relative amounts of lr and Pd arc originally comparable. or Pd > Ir, the removal from the magma of silicate or oxide crystals which were nucleated by insoluble Ir precipitates could indeed fractionate the Pd/Ir ratio of the magma, but the mechanism would not necessarily affect the other PGE/Ir ratios. Gold, though not a PGE, is also very insoluble in Ir and consequently would also be strongly hctionated. Smoothly trending fractionated PGE patterns would not be expected from the insoluble precipitate mechanism if PGE alloys are formed during or after the Ir precipitation.

873

Fractionation of F’t-groupelements Furthermore, other less siderophile but more abundant elements in the magma, such as Fe, would be expected to dissolve into the alloy as well and perhaps diminish or even eliminate the known immiscibilities of the Ir-Pd and Ir-Au binary alloys. If this did occur then even the potential for fractionating Au and Pd from Ir would be removed. A problem with fitting the new experimental data into interpretations of PGE abundances is the comparative lack of Ru and Rh analyses mentioned in our Introduction. The compatibility of Rh is not predicted by the classification into IPGE and PPGE (Rh is a PPGE). But it seems now from the experimentally measured spinel/melt compatibility that Rh could behave more like an IPGE than like a PPGE. The fact that the half-life for the Rh isotope measured in NAA is only 4.4 minutes and, therefore, requires special techniques for measurement has undoubtedly contributed to the scarcity of Rh analyses and perhaps also to its inclusion into the incompatible PPGE group. Notwithstanding our discussion of the results of MALVIN et al. (1986), and in view of the present results and of the known enrichment of IPGEs in natural spinels (PAGE and TALKINGTON, 1984), we suspect that Ir and OS will be found to be chemically compatible in spinel, i.e., preferentially incorporated in a dissolved state into the spine1 crystal structure. Platinum group element abundances are commonly presented in plots which are analogous to chondrite-normalized REE plots. In the case of PGEs, however, the ordering scheme for the abscissa which is commonly adopted is that of decreasing melting temperature (NALDRETTet al., 1979), giving the sequence OS, Ir, Ru, Rh, Pt, and Pd, which nicely distinguishes IPGEs from PPGEs. Our data indicate that negative slopes on such plots would be expected for rocks which have accumulated spine1 phases, and such is the case in some chromitite bodies in ophiolite complexes (PAGE and TALKINGTON, 1984). But overemphasizing such facile agreement may be very dangerous given the fact that often such chromites contain platinum group minerals which host the majority of the PGEs. However, our main point is that we can no longer simply neglect the differential chemical solution of PGEs in spinels. Another example where PGE interpretations might be simplified using the present results is found in KEAYS (1982), who finds low Pd/Ir ratios in dunitic komatiites from Western Australia which also contain high Cr contents. One of Keays suggestions (he rejects chemical compatibility in olivine, in contrast to the conclusions of BRUGMANNet al., 1987) is that perhaps the low Pd/Ir is due to an Ir-0s alloy which in the mantle lay along the olivine grain boundaries but during diapiric rise became incorporated in the olivine crystals while the Pd is taken into the melt as a sulfide. A simpler interpretation might become plausible if any spine1 phases could be found as inclusions in the Cr-rich olivine. Then the decreased Pd/Ir ratios might be, at least in part, attributed to spinel/silicate melt partitioning.

results suggest a mechanism for fractionation of the PGEs, and question their accepted division into IPGEs and PPGEs. Acknowledgments--The

original manuscript benefitted from thoughtful official reviews by John Jones, Gordon McKay, Denis Shaw, and Sarah-Jane Barnes. Thanks to Pam Rogers at Los Alamos for the PIXE analysis. This work was supported by NSF Grant

# EAR86-18266. Editorial handling: G. Faure

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