Meteoritics & Planetary Science 36, 1257-1275 (2001) Available online at http://www.uark.edu/rneteor

Siderophile elements in Earth's upper mantle and lunar breccias: Data synthesis suggests manifestations of the same late influx

J. W. MORGAN172*, R. J. WALKER2, A. D. BRANDON2>3 AND M. F. HORAN4

'Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523, USA ZDepartment of Geology, University of Maryland, College Park, Maryland 20742, USA

3Department of Geological Sciences, Northwestern University, Evanston, Illinois 60208-2 150, USA 4Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 2001 5, USA

*Correspondence author's e-mail address: jmorgan@geol.umd.edu

(Received 2000 April 11; accepted in revised form 2001 June 29)

Abstract-The platinum group elements (PGE; Ru, Rh, Pd, Os, Ir, Pt), Re and Au comprise the highly siderophile elements (HSE). We reexamine selected isotopic and abundance data sets for HSE in upper mantle peridotites to resolve a longstanding dichotomy. Re-0s and Pt-0s isotope systematics, and approximately chondritic proportions of PGE in these rocks, suggest the presence in undepleted mantle of a chondrite-like component, which is parsimoniously explained by late influx of large planetisimals after formation of the Earth's core and the Moon. But some suites of xenolithic and orogenic spinel lherzolites, and abyssal peridotites, have a CI-normalized PGE pattern with enhanced Pd that is sometimes termed "non-chondritic". We find that this observation is consistent with other evidence of a late influx of material more closely resembling enstatite, rather than ordinary or carbonaceous, chondrites. Regional variations in HSE patterns may be a consequence of a late influx of very large objects of variable composition.

Studies of many ancient (>3.8 Ga) lunar breccias show regional variations in AdI r and suggest that "graininess" existed during the early bombardment of the Earth and Moon. Reliable Pd values are available only for Apollo 17 breccias 73215 and 73255, however. Differences in HSE patterns between the aphanitic and anorthositic lithologies in these breccias show fractionation between a refractory group (Re, 0 s and Ir) and a normal (Pd, Ni, and Au) group and may reflect the compositions of the impacting bodies. Similar fractionation is apparent between the EH and EL chondrites, whose PGE patterns resemble those of the aphanitic and anorthositic lithologies, respectively.

The striking resemblance of HSE and chalcogen (S, Se) patterns in the Apollo aphanites and high- Pd terrestrial peridotites suggest that the "non-chondritic" abundance ratios in the latter may be reflected in the composition of planetisimals striking the Moon in the first 700 Ma of Earth-Moon history. Most notably, high Pd may be part of a general enhancement of HSE more volatile than Fe suggesting that the Au abundance in at least parts of the upper mantle may be 1.5 to 2x higher than previously estimated.

The early lunar influx may be estimated from observed basin-sized craters. Comparison of relative influx to Earth and Moon suggests that the enrichment of HSE is limited to the upper mantle above 670 km. To infer enrichment of the whole mantle would require several large lunar impacts not yet identified.

INTRODUCTION

The highly siderophile elements (HSE) consist of the platinum group elements (PGE) Ru, Rh, Pd, Os, Ir and Pt, as well as Re and Au. By definition, metal-silicate partition coefficients of HSE are very high, at least at low to moderate pressures and temperatures (Kimura et al., 1974; Borisov et al., 1994; Borisov and Palme, 1995; Holzheid et al., 2000; Righter and Drake, 1997; Ertel et al., 1999,200 1). Therefore,

HSE abundances in the Earth's upper mantle should be vanishingly small after formation of the Earth's metallic Fe-Ni core. In fact, abundances of HSE in the upper mantle are often in the parts-per-billion range and in undepleted, primitive upper mantle (PUM) are in approximately chondritic proportions (Chou, 1978; Jagoutz et al., 1979; Morgan et al., 1981). Re-0s and Pt-0s isotope systematics are both within the chondritic range in PUM; and, in 187Re/1880s and 187Os/188Os, PUM more closely resembles ordinary or enstatite chondrites than

PvelLtde preprint MS#4339 1257

0 Meteoritical Society, 200 I . Printed in USA.

1258 Morgan et al.

carbonaceous chondrites (Meisel et al., 1996,2001; Walker et al., 1997; Brandon et al., 1998).

These observations are best explained by a late influx of planetesimals of broadly chondritic composition after formation of the Earth's core (Kimura et al., 1974; Chou, 1978; Jagoutz et al., 1979; Morgan et al., 1981; Morgan, 1986; Holzheid et al., 2000; Ertel et al., 2001). Many other explanations have been offered, however, and Drake (2000) recently described the rise and demise of the more notable: inefficient core formation (Jones and Drake, 1986), and lowering of metal- silicate partition coefficients by high temperature (Murthy, 199 1) or by a Fe-S-.O liquid composition of the outer core (Brett, 1984). There seems general agreement that the abundances in the mantle of such moderately siderophile elements as Ni were likely established at the base of a deep magma ocean (Li and Agee, 1996). Righter and Drake (1997) proposed that the abundance of Re was set in a similar manner, but this interpretation now appears incorrect since recent work indicates that Re is indeed a HSE (Ertel et al., 2001). Snow and Schmidt (1 998) have suggested that the HSE content of the oceanic mantle may result from contamination by core-mantle interaction. As will be discussed in a subsequent section, isotopic and other evidence from plume-derived materials argue against this view (Walker et al., 1995, 1997; Brandon et al., 1998, 1999).

Many arguments (though not all fatal) may be marshaled against these alternate hypotheses. The most compelling evidence, however, is that the late influx hypothesis requires the broadly chondritic abundance and isotopic ratios of HSE as observed, whereas other hypotheses listed above do not predict these a priori and indeed may have difficulty in explaining them. An exception to this generalization may be the homogeneous accretion model (Azbel et al., 1993; Kramers, 1998). The end product of this mechanism is experimentally almost indistinguishable from the HSE abundance for the late influx model. The homogeneous accretion model will be discussed in detail later.

The contents of PGE in a wide variety of peridotites were recently reviewed (Schmidt et al., 2000). From this and more recent work we have selected for closer review and synthesis several data sets for orogenic and xenolithic spinel lherzolites (Morgan, 1986; Pattou et al., 1996; Rehkamper ef al., 1997; Handler et al., 1997; Handler and Bennett, 1999) and for abyssal peridotites (Snow and Reisberg, 1995; Snow and Schmidt, 1998; Rehkamper et al., 1999; Brandon et al., 2000). These studies raise two important questions. First, in some sample suites, deviations in PGE from CI ratios are noted, in particular a "non-chondritic" PdAr ratio (Morgan et al., 198 1; Morgan, 1986; Pattou, et al., 1996; Snow and Schmidt, 1998). Some deviations may be due to analytical bias and an over-literal interpretation of "chondritic" as meaning identical to CI chondrites. Nevertheless, a demonstration of primordial enhancement of Pd relative to other PGE would place an imuortant constraint on the oriein of HSE in the umer mantle.

Second, other spinel lherzolite suites apparently have chondritic Pd/Ir (Morgan et al., 1981; Morgan, 1986; Rehkamper et al., 1997; Handler el a)., 1997; Handler and Bennett, 1999). Again, while analytical error and sample alteration cannot categorically be excluded, these results suggest there may be significant regional variations in the Earth's upper mantle both in absolute HSE abundances and inter-element ratios. Regional variability ab initio would be a natural consequence of a late influx in which the major amounts of material are added by a few very large objects. In the well- mixed mantle of the homogeneous accretion model (Kramers, 1998), such variability is less likely.

A strong argument in favor of late influx is the clearly visible evidence that the Moon has undergone a severe bombardment by very large objects. The record of these basin-forming impacts is contained in the ancient lunar breccias collected at all lunar highland sites. All but one of these basins- Orientale-formed by -3.8 Ga. Because the Moon was formed in situ (Benz and Cameron, 1990; Cameron, 1997), the Moon and Earth may have sampled the same population of objects (Hartmann, 1976), with the Earth getting the lion's share (Bandermann and Singer, 1973; Chyba, 199 1). Occasionally it has been asked why there was no late influx of siderophiles to the Moon (Taylor and Esat, 1996). In fact, many ancient lunar impact breccias contain significant amounts of siderophile and highly siderophile elements (Hertogen et af., 1977), suggesting that on the tectonically inactive Moon a sampling of the late influx population is still accessible for study.

In this paper, we critically review HSE data for terrestrial peridotites and compare them with those for Apollo 17 lunar breccias and chondrites in order to assess the types of siderophile element fractionation that may be reflected in the terrestrial upper mantle. We estimate the early integrated lunar influx from observed lunar basins and calculate the corresponding HSE enrichment of the Earth's mantle.

TERRESTRIAL UPPER MANTLE

The abundances of HSE in primitive mantle cannot be measured directly, since most available peridotites have suffered depletion by extraction of a basaltic melt to varying degrees. Accordingly, we review here selected data for xenolithic and orogenic spinel lherzolites that have suffered only slight to moderate depletion, and attempt to infer the primitive compositions using 0 s isotopic data and inter- element correlations. For the oceanic mantle we include data for abyssal harzburgites since spinel lherzolite data are sparse.

High-Pd xenoliths are represented by spinel lherzolites from the southwestern USA, which are "slightly depleted" with CdSi > 0.09 (BVSP, 198 1 ; Morgan era)., 198 1 ; Morgan, 1986). More recently, analyses of orogenic spinel lherzolites with similar siderophile composition and degree of depletion from the Pyrenees, France, were reported by Pattou et al. (1996). Xenoliths with atmarenth chondritic PdPGE ratios have been

Siderophile elements in Earth's upper mantle and lunar breccias 1259

analyzed from the Cameroon line, Africa (Rehkamper et af., 1997) and eastern Australia (Morgan et al., 198 1 ; Handler et af., 1997; Handler and Bennett, 1999). HSE data for abyssal peridotites from several localities have also been reported (Snow and Reisberg, 1995; Snow and Schmidt, 1998; Rehkamper et al., 1999; Brandon el af., 2000).

Spinel Lherzolite Xenoliths from Southwestern USA

The upper mantle (UM) suite of 15 spinel lherzolite xenoliths was prepared for siderophile and volatile element analysis for the Basaltic Volcanism Study Project and have been extensively characterized for major, minor and trace elements (BVSP, 198 1). Peridotites originated mainly from localities in the southwestern USA with other examples from Alaska, Hawaii, Mexico and Australia.

These xenoliths were analyzed by radiochemical neutron activation analysis (RNAA) for elements including Os, Re, Ir, Pd, Ni, Au, and Se (Morgan et al., 1981). In this technique, the whole sample (up to 0.5 g) was irradiated at high fluexe (-3 x 1020 neutrons-about one-half a millimole-per cm2). Known amounts of carrier were added and equilibrated with the sample before radiochemical separation of individual elements. Thus, this technique was a type of isotope dilution analysis and results were not sensitive to chemical yield or to chemical blanks. Precision was determined largely by counting statistics of radiochemically pure separates. Accuracy depended on the stoichiometry of the irradiated standards and on the quality of the yield determination.

Of nine "slightly depleted" UM xenoliths with CdSi > 0.09, eight originate from the southwestern USA and are characterized by high S and Se contents, with Pd/Ir higher than chondritic (Fig. 1; data normalized to CI values from Anders and Grevesse, 1989). Petrologically, the samples from the

FIG. I . Comparison of highly siderophile, siderophile and chalcogenic element abundances in "slightly depleted" spinel lherzolite xenoliths from the Basaltic Volcanism Study Project (BVSP, 1981; Morgan et al., 198 1 ; Morgan, 1986) and "very slightly depleted" orogenic spinel lherzolites (Pattou etal., 1996). The CI values used for normalization throughout this work are from Anders and Grevesse (1989).

southwestern USA divide into two groups. The Cochise Crater xenolith (UM 12) and most of those from Kilbourne Hole (UM 3, 4, 6 and 9) have textures that are allotriomorphic- (proto-) granular to porphyroclastic, with coarse average grain sizes between 1 to 3 mm. Three samples (UM 5,7 and 8), however, are fine-grained (-0.5 mm max.) with preferred orientation of tabular grains, and may represent modified wall rock of pyroxenitic dikes (Irving, 1980). Compositionally, the fine- grained samples UM 7 and 8 closely resemble the coarse- grained variety, though UM 7 is markedly enriched in Au (3.0 ppb). The remaining fine-grained sample, UM 5, is anomalous in major element composition (Morgan, 1986). It resembles other Kilbourne Hole samples in S (1 52 ppm) and Se (5 1 ppb), but has chondritic PdIr. UM 5 closely resembles the remaining "slightly depleted" xenolith UM 15 from Mount Quincan, Queensland, Australia. The latter is normal in major element composition, but has S (52 ppb) and Se (7 ppb) much lower than even the lowest Kilbourne Hole peridotite. Data for the coarse-grained spinel lherzolites are summarized in Fig. 1.

As is well-known, the PGE and Ni are compatible in mantle melting processes, whereas Re, Au as well as the chalcogens, S and Se, are mildly incompatible (Morgan et al., 1981; Morgan, 1986). Thus, in the peridotites shown in Fig. 1, these last four elements are variably depleted. As demonstrated by Morgan (1986), the abundances of these elements in primitive mantle may be inferred from 187Re-1870~ isotope systematics. The earlier estimate of 0.26 ppb Re in pristine mantle derived from this data set (Morgan, 1986) was based on osmiridium measurements of 187Os/l88Os = 0.1246 (Alkgre and Luck, 1980), a ratio similar to that of oceanic peridotites (Snow and Reisberg, 1995; Brandon et af., 2000). 187Os/l88Os has been measured directly in a subset ofthe UM suite consisting largely of the Kilbourne Hole samples and results indicate variable long-term Re depletion (Meisel et al., 1996). By extrapolation of these data to Lu or A1203 contents similar to fertile mantle, Meisel et al. (1996) estimated 187Os/l88Os = 0.1290 in undepleted mantle, corresponding to 187Re/l880s in undepleted mantle = 0.427, a Re/Os mass ratio of 0.0887 and a normalized ratio of 1.18 x CI. These values are more similar to ordinary or enstatite chondrites than carbonaceous chondrites. For the most fertile Kilbourne Hole xenoliths, which contain an average of 3.5 ppb Os, the inferred Re/Os ratio yields an undepleted mantle abundance of 0.29 4 0.08 ppb R e - a (8.0 f 2.2) x 10-3 CI, where all uncertainties here and following are 2a of the mean. From this result, S/Re and Se/Re in these rocks yields values for pristine mantle of 254 f 28 ppm S and 84 f 11 ppb Se, or 4.1 f 0.5 and 4.5 2 0.6 CI x 10-3, respectively. (More recently, Meisel et al. (2001) used a much larger data set to determine a global average 187Os/l88Os = 0.1296 ? 0.0008. For our immediate purpose, however, the earlier work (Meisel et al., 1996) may be more appropriate since it relies heavily on many of the same xenoliths discussed here.)

In an earlier analysis of BVSP data (Morgan, 1986), an undepleted mantle value of 1.01 ppb Au was derived from the

1260 Morgan et al.

correlation with Re in grouped data for "slightly depleted" (-7% melt, CdSi > 0.09), "moderately depleted" (-13% melt, 0.09 > Ca/Si > 0.06) and "highly depleted" (-20% melt, 0.06 > CdSi) spinel lherzolite xenoliths. Grouping the data in this way obscured an important relationship in the slightly depleted peridotites where Re and Au from five out of seven of these lie on a hyperbola (Fig. 2). This relation appears to be due to the correlation of Re with S and by extension with degree of depletion or melt fraction, F(Morgan, 1986). Without doing a full analysis, it is evident that

rearranges to

The CI-normalized xenolith data transform to give a linear plot

(3 f [Au] = a /(c - [Re]) f b

where for best fit we find empirically c = 8.40, with corresponding values a = 6.47 t 0.46, b = 2.62 t 0.13 with mean square of weighted deviates (MSWD) = 1.06 (Fig. 2). For a primitive mantle value of Re = 8.0 x 10-3 CI, regression of the transformed data to yield Au = (19 k 5) x 10-3 CI or 2.7 t 0.7 ppb Au. This value is not particularly well determined and, because of the steepening curve as Re increases, it is very sensitive to the choice of the mantle Re concentration. In a later section we will verify the estimate by reference to the HSE pattern in Apollo 17 lunar breccias.

Orogenic Spinel Lherzolites from the Pyrenees, France Since [Re]depleted is related via s to (1 - F), then it is apparent that Eq. (2) has the form of a hyperbola.

-~ Fine-grained

A Coarse- rained V Mount B uincan 0 From re ression + 0 From anorth. AdIr

From ap 8, an. AdIr

A !

0 2 4 6 8

ReICI x lo3 FIG. 2 . Of the seven slightly depleted peridotites from Kilbourne Hole and Cochise Crater (with high Pd/Ir, Re and Au), data for five of these plot on a hyperbola. The data give a linear plot [Au] = a/(c - [Re]) = b, where for best-fit c = 8.40, a = 6.47 f 0.46, b = 2.62 f 0.13 with MSWD = 1.06. For a primitive mantle value of Re = 8.0 x 10-3 CI regression of the transformed data to yield Au =

( I9 f 5) x 10-3 CI or 2.7 2 0.7 ppb Au. Using the same values for b and c, a similar hyperbolic curve through Re and Au data for two lherzolites with chondritic Pd/Ir gives a = 2.43 2 0.53 and a lower Au value in the parental region of the upper mantle of (8.7 f 1.3) x 10-3 CI or 1.2 f 0.2 ppb. The Kilbourne Hole-Cochise Crater data are fit equally well if the regression includes a PUM Au value estimated from the mean AdIr ratio in lunar aphanites. The regression is best fit (MSWD = 1.00) using the following constants; a = 7.27 f 0.30, b = 2.5 14 k 0.10 and c = 8.57 (chosen empirically for best fit). The regression yields a pristine mantle value of ( I 6.2 f 3.4) Au/CI x 103, or 2.3 f 0.5 ppb Au. If the lunar ANT clasts are a good model for the low Pd peridotites, the Au/Ir ratio of (1.45 f 0.07) x C1 give a mantle value of (8.3 5 0.8) x 10-3 CI equivalent to 1.2 f 0.1 ppb Au, a result identical to the regression value.

Pattou et al. (1 996) used NiS fire assay and inductively- coupled plasma mass spectrometry (ICP-MS) to analyze 14 orogenic spinel lherzolites sampled from massifs in the Pyrenees, southwestern France.

In this method, HSE were preconcentrated before analysis by fusion of 15 g samples in a flux containing lithium tetraborate, sodium carbonate, Ni and S. The HSE were extracted almost quantitatively into the resulting NiS bead. The NiS was dissolved and Ru, Rh, Pd, Ir, Pt and Au further concentrated by co-precipitation with Te-metal. After re- dissolution, these elements were measured on a single collector ICP-MS. The large sample size minimized the nugget effect. These were not isotope dilution analyses, thus placing a premium on high and reproducible recoveries (which appear to have been achieved). In addition, blank corrections were required. Replicate analyses suggest that yields were reproducible to a few percent, except for Au ( k 3 0 to 40%).

The Pyrenean massifs occur as small bodies, which were emplaced tectonically, and were sampled over a distance of 200 km. The peridotites are only slightly depleted and appear compositionally identical to the high-Pd, slightly depleted spinel lherzolite xenoliths from Kilbourne Hole, with A1 contents being 1.80 k 0.12% and 1.79 t 0.15%, respectively. Abundances of HSE, Ni and S are extremely similar, too (Fig. l), though the HSE suite determined is largely complementary to that of Morgan ef al. (1 98 1). Of the HSE analyzed by Pattou et al. (1996), four (Ir, Ru, Rh and Pt) plot within the band of mean mantle HSE = 0.71 t 0.8% CI estimated for undepleted mantle (Morgan, 1986). Values for Pd are significantly higher with a weighted mean of (1 0.4 t 0.5) x 10-3 CI and agree well with the value of (10.9 & 0.8) x 10-3 CI in the southwestern USA xenoliths. The high abundance of Pd in some mantle peridotites has been noted previously (Morgan et al., 198 1 ; Morgan, 1986; Pattou et al,, 1996, and references therein). Generally, however, the discussion has been based on high Pd/Ir ratio, which can just as well come from low Ir as

Siderophile elements in Earth's upper mantle and lunar breccias 1261

rn 50.0

high Pd. The systematic differences seen here between other PGE and Pd show that the latter, at least in some parts of the mantle, is overabundant by a factor -1.4.

Several explanations have been offered for high Pd. Drawing on chemical similarities between Ni and Pd, Morgan (1 986) suggested that Pd, like Ni, was not completely extracted from the silicate Earth during core formation. Experiments now show, however, that the metaVsilicate partition coefficients for Pd are very high and apparently increase with pressure (Holzheid et al., 2000). As in peridotites from other localities (Jagoutz et al., 1979; Morgan and Baedecker, 1983; Hart and Ravizza, 1996), the HSE in Pyrenean spinel lherzolites are mainly in sulfides (Pattou et al., 1996; Alard et al., 2000) and these latter workers suggest sulfide accumulation may yield high Pd values. Occurrence of PGE in a sulfide phase does not necessarily mean that these elements move with sulfide, however. In fact, since S content is an index of fertility in mantle spinel Iherzolites, such compatible elements as 0 s and Ir are anti-correlated with S, but Re and Au are positively correlated (Morgan, 1986). In both Pyrenean and Kilbourne Hole peridotites, PdIr is constant and Pd abundances, as for those of other PGE, appears to increase slightly with decreasing S content (Fig. 3). Thus, it seems unlikely that S redistribution is responsible for high Pd in these peridotites. From this, we infer that the observed Pd enrichment relative to Ir and other PGE (on a CI- normalized basis) is inherent to the parts of the upper mantle sampled by the xenoliths from the southwestern USA and the Pyrenean orogenic peridotites and is not the result of secondary processes.

Snow & Schmidt A

Rehkamper et al. Lherzolites W Harzburaites

Abyssal Spinel Lherzolites and Harzburgites

Snow and Schmidt (1 998) determined HSE by instrumental neutron activation analysis (INAA) in a suite of three spinel

I SW USA 0 Pd,ppb I PdIr I I Pyrenees v Pd,ppb v PdIr I V 0

0 U

v 0

, . . . . , . . . . , . . . . I . . . .

50 100 150 200 250 300 Sulfur, ppm

FIG. 3. Variation of PdiIr and Pd abundances with S content in spinel lherzolites (Morgan eta/., 1981; Morgan, 1986; Pattou eta/ . , 1996). Pd behaves as a compatible element, as do other PGE, and is anti- correlated with s. The Pd/Ir ratio is constant over the whole range of S. It seems unlikely therefore that the Pd enhancement in these rocks is due to addition of a sulfide component.

lherzolites and six harzburgites from the Atlantic, Pacific and Indian Oceans.

Beginning with 10 g samples, HSE were preconcentrated before irradiation using NiS fire assay as described above. Snow and Schmidt (1998), however, dissolved the resulting NiS bead in concentrated HCI and the insoluble residue containing HSE was filtered off. The filter paper was irradiated and the HSE determined by INAA. The separation is specific for HSE and sensitivity is much greater than in whole sample irradiation (Spettel et al., 1991). Accuracy depends on reproducible and nearly quantitative recoveries, and blank corrections are required.

Four of the six PGE (Ru, Rh, Pt and Pd) in spinel lherzolites plot within uncertainty of the mean mantle value (Fig. 4), with a weighted average of (6.7 2 0.51) x 10-3 CI. Refractory PGE 0 s and Ir in abyssal spinel lherzolites have a lower weighted average abundance of (4.1 f 1.1) x 10-3 CI. The apparent depletion of 0 s and Ir by a factor of 1.6 2 0.5 relative to other PGE seems to be analytical, however, since 0 s isotope dilution results (Snow and Reisberg, 1995) are systematically higher than those reported by INAA in similar abyssal peridotites, or, even the same rock (see Fig. 4, open triangles). These results suggest that yields of 0 s (and probably Ir) may be low in the pre-irradiation separation. It seems likely, there- fore, that abundances of the refractory siderophile elements 0 s and Ir may be similar to those seen in other peridotites, and the ratio of refractory PGE to other PGE may be chondritic.

Abyssal harzburgites and lherzolites (Snow and Schmidt, 1998) have similar PGE patterns (Fig. 4), but abundances are higher in the former due to depletion of harzburgites in the basaltic component and the compatible nature of the PGE. In the harzburgites, four non-refractory PGE have similar abundances with a weighted mean of (9.1 2 0.8) x 10-3 CI. The refractory PGE 0 s and Ir are relatively depleted averaging

x m 8 20.0 s % 10.0

z 3 % 5.0

- 2.0 N .3

8 0 1.0 9 3 0.5

FIG. 4. Highly siderophile element abundances in abyssal spinel lherzolites and harzburgites. Data from Snow and Reisberg (1995) (0s only, open triangles) and Snow and Schmidt (1998); and from Rehkamper ef a/. (1999) and Brandon ef al. (2000) (Re, 0 s and Pt, open squares).

1262 Morgan et al,

(6.1 ? 0.6) x 10-3 CI. The depletion of 0 s and Ir once more may be analytical, since the altered harzburgite A11 107-40-35 has 3.25 ppb 0 s by isotope dilution (Snow and Reisberg, 1995) but only 2.21 ppb 0 s by INAA (Snow and Schmidt, 1998). By INAA, the PGE distribution in harzburgites apparently indicates a smooth increase from 0 s to Pd. As will be discussed later, this has important implications for the interpretation that Snow and Schmidt infer from their data.

More recent isotope dilution results for abyssal peridotites confirm that 0 s and Ir are not significantly depleted relative to most other PGEs. A suite of abyssal harzburgites and dunites from ODP Site 895D, Hess Deep and ODP Holes 920B and D in the MARK area has been analyzed for Ir, Ru, Pt and Pd (Rehkamper et al., 1999) and for Re, 0 s and Pt (Brandon et al., 2000) using slightly different techniques.

Rehkamper et al. (1997,1999) spiked 5-10 g samples with 99Ru, IOSPd, 19lIr and 198Pt and removed silica by treating with HF/HCl. The residue was transferred to a Carius (thick- walled glass) tube, sealed, and digested with aqua regia for 48 h at 230 "C. PGE were separated by anion exchange (Rehkamper et al., 1997) and isotope ratios measured by multicollector ICP-MS.

Brandon et al. (2000) measured 0 s isotopes and Re, 0 s and Pt in 1.5 g samples which were spiked with 185Re, 1900s and 198Pt, and dissolved directly in quartz Carius tubes. Osmium was purified by carbon tetrachloride solvent extraction and by microdistillation. Rhenium and Pt were purified by anion exchange. The Re and 0 s were measured by negative thermal ionization mass spectrometer (NTI-MS). Platinum was measured by multicollector ICP-MS. These should be reliable analyses since isotopic equilibration was carried out in a closed system.

The combined data are illustrated in Fig. 4. Individual PGE abundances in the Rehkamper-Brandon data agree within uncertainty with the harzburgite results of Snow and coworkers, except for 0s . They differ, however, in that four of the five PGE in the newer data (Pd being the exception) have strikingly uniform abundances averaging (8.1 ? 0.7) x 1 @3 CI. The highly precise mean ~86Os/~88Os of 0.1 198362 ? 0.0000010 for these harzburgites also indicates that Pt/Os has always been close to chondritic (Brandon et al., 2000). The mean Pd value of (1 1.1 ? 2.9) x 10-3 CI is just within uncertainty of the other PGE, with a Pd/PGE ratio 1.36 ? 0.28. Because of the heavy depletion (partly compensated by serpentinization) of this suite of Pacific abyssal harzburgites, absolute abundances of the PGE are higher by 12 to 15% than in continental lherzolites. Nevertheless, the Pd/PGE ratio in the abyssal harzburgites is identical within error to the value of 1.45 ? 0.30 in orogenic lherzolites (Pattou et al., 1996; PGE = Ir, Ru, Rh, Pt) and of 1.52 ? 0.16 for lherzolite xenoliths (Morgan, 1986; PGE =

Os, Ir). The three values together give a weighted mean for Pd/PGE of 1.48 ? 0.12.

Before discussing PGE fractionation further, we digress to consider the Re and Au abundances in abyssal peridotites. In

both spinel lherzolites and harzburgites examined by Snow and Reisberg (1995) and Snow and Schmidt (1998), Re is substantially depleted relative to 0 s and other PGE. This suite of abyssal peridotites was derived from variably depleted mantle as shown by the average 187Os/188Os = 0.1246 ? 0.0014 (Snow and Reisberg, 1995) compared to the recent global PUM value of 0.1296 ? 0.0008 (Meisel et al., 2001). In contrast, Au in abyssal spinel lherzolites does not appear to be depleted and the mean abundance of (6.4 ? 3.7) x 10-3 CI is identical to that of the four non-refractory PGE. Gold and Re correlate well in mantle processes (Morgan, 1986), and the apparently undepleted Au abundances in the abyssal spinel lherzolites may be influenced by other effects. One such effect may be weathering, since the most weathered sample studied by Snow and Schmidt (1998) has particularly high Au values. In abyssal harzburgites analyzed by Snow and coworkers, the more usual situation prevails where Au and Re are depleted to the same extent: (2.9 ? 1 .O) x 10-3 CI. These values are not as low as in correspondingly depleted sub-aerial peridotites, however. For example, two highly depleted peridotites from San Carlos, Arizona contain as little as 0.2 and 0.6 x 10-3 CI of Re and Au, respectively (Morgan et al., 1981). In abyssal peridotites it seems possible that there may be a contribution of these elements from seawater.

In the suite of harzburgites and dunites studied by Brandon et al. (2000), the mean Re/Os is chondritic. The average 187Os/188Os = 0.1268 ? 0.0012 is more radiogenic than the value of 0.1246 2 0.0014 measured by Snow and Reisberg (1 995) on their suite of abyssal peridotites. The 187Os/188Os ratio determined by Brandon et al. is still significantly lower than the global PUM value of 0.1296 ? 0.0008 (Meisel et al., 200 l), even though the apparently chondritic 187Re/lgsOs in the abyssal peridotites might lead one to expect them to be similar. Additionally, in the suite of peridotites studied by Brandon et al. (2000), there is no correlation of 187Os/188Os with 187Re/1880s even though there is with other indices of depletion. Clearly, Re and Au abundances in abyssal peridotites are more susceptible to alteration than are the PGE, and offer little guidance to abundances in the upper mantle.

Individual PGE abundances found by Rehkamper et al. (1 999) largely agree within uncertainty with the harzburgite results of Snow and Schmidt (1998). In detail, however, they differ in that the combined results of Rehkamper et al. (1999) and Brandon et al. (2000) show uniform abundance of Os, Ir, Ru and Pt with an enrichment of Pd alone. In contrast, the data of Snow and Schmidt show a steady increase in abundance from 0 s and Ir to Pd, even when the low abundances of 0 s and Ir are replaced by the isotope dilution results of Snow and Reisberg (1 995). (Pd results for both sets of data are identical so that the enrichment of this element is not an issue here.) Thus, although the results of Rehkamper et al. (1 999) and Snow and Schmidt (1998) are in broad agreement, the differences in detail have led to quite divergent inferences concerning the origin of siderophile elements in the oceanic upper mantle.

Siderophile elements in Earth's upper mantle and lunar breccias 1263

Rehkamper et al. (1999) view their rather uniform CI- normalized abundances as hrther good evidence in support of a late influx origin for highly siderophile elements (Kimura et al., 1974; Chou, 1978; Jagoutz et al., 1979; Morgan et al., 1981; Morgan, 1986). They consider the enrichment of Pd as a possible result of terrestrial processes. Detailed petrogenic modeling led Rehkamper er al. to suggest that the Pd enrichment may perhaps be due to addition of sulfides to depleted residues by the action of percolating melts. Insofar as we have already demonstrated that this process for Pd enrichment is not plausible for continental xenoliths and orogenic peridotites, Occam's razor suggests that the postulate may be invalid for abyssal peridotites also.

Let us now examine the hypothesis offered by Snow and Schmidt (1998) to explain enhanced Pd, since it is one some- times raised by other authors (Pattou et al., 1996; Rehkamper et al., 1997). Snow and Schmidt (1 998) rejected the late influx hypothesis based on the mismatch of certain elemental pairs with analyses of meteorites. Instead, they proposed that HSE in the upper mantle were derived, via plumes, from the outer core. Previous work based on iron meteorite data has suggested that crystallization of the inner core indeed can lead to fractionation between the HSE in the outer core in which 0 s is slightly depleted relative to Re, and the refractory siderophiles are substantially depleted relative to Pt, Pd and Au (Morgan et al., 1994, Walker el al., 1995). Re-0s and Pt-0s isotope systematics have been studied in plume-related materials, most notably from the Siberian flood basalts and related ore deposits (Walker et al., 1995, 1997) and Hawaii (Brandon ef al., 1998, 1999). The results have shown systematic and correlated enrichments of 186Os/188Os and 187Os/188Os that are best explained by contamination of the plume source with outer core material in which Re and particularly Pt are enriched relative to 0s. These isotopic enrichments are not seen in abyssal peridotites, however, which are in fact depleted in 187Os/l88Os and have chondritic 1860d188Os (Snow and Reisberg, 1995; Brandon et al., 2000). Thus it seems very unlikely that the enhanced Pd in abyssal peridotites is due to contamination by outer core material as postulated by Snow and Schmidt (1998). It is now generally conceded that the HSE abundances in the upper mantle are most likely the result of late accretion (see, for example, Drake, 2000). Thus, the simplest explanation of high CI-normalized Pd/PGE is as a reflection of the composition of the bodies engaged in late influx.

Spinel Lherzolite Xenoliths from the Cameroon Line, Africa and Eastern Australia

Enhanced Pd is common in mantle peridotites, but not universal. For example, two "slightly depleted" spinel lherzolites from the BVSP suite (UM 05, Kilbourne Hole and UM 15, Mount Quincan, Australia) have PdlIr and other PGE ratios close to chondritic. In addition, the famous sheared garnet lherzolite PHN 161 1 (Thaba Putsoa, Lesotho) that may

represent a sample of the fertile deeper upper mantle has PdIr = 1.03 x CI (Morgan et al., 198 1). Two recent studies provide hrther examples.

Six relatively fertile spinel lherzolite xenoliths from the Cameroon Line were studied by Rehkamper er al. (1 997) using the multicollector ICP-MS technique described above (Rehkamper et al., 1999). These peridotites have flat CI- normalized PGE patterns with Pd/Ir = (0.99 -t 0.19) x CI and are significantly depleted relative to mean mantle with a weighted average of (4.8 5 0.4) x 10-3 CI (Fig. 5).

Analyses were reported in a large suite of peridotite xenoliths from eastern Australia for Re, 0 s (Handler et al., 1997) and Ir, Ru, Rh, Pt, Pd (Handler and Bennett, 1999).

Handler et al. (1 997) made separate direct Carius tube dissolutions of 2-3 g samples for the isotope dilution determination of 0 s and Re in a subset of the peridotites studied by Handler and Bennett (1999). Distillation was used to separate 0 s and the isotopic composition was determined by NTI-MS. Rhenium was purified by anion exchange and analyzed both by NTI-MS and ICP-MS. Handler and Bennett (1999) analyzed peridotites for Ir, Ru, Pt and Pd by isotope dilution, and also for Rh-a mono-isotopic element. Samples (0.7 to 1.2 g) were spiked and dissolved by HF-HNO3 in an open beaker to remove silica, followed by a Carius tube treatment to ensure complete dissolution. PGE were concentrated with Te-metal and after re-dissolving run without further separation on a single collector ICP-MS. Isotope ratios were measured for the spiked elements and Rh was estimated by comparison with standard solutions. The accuracy for Rh determination depends on good, consistent chemical yields. As with the studies by Rehkamper et al. (1 997,1999) described above, the HF-HNO3 attack may not have achieved isotopic equilibrium, which is notoriously difficult for PGE, particularly Ir and Ru. Therefore some chemical yield problems may arise in the steps before the Carius tube treatment.

m

X Morgan et al., 198 1 cn Rehkamper et al., 1997 5 20.0 50*0 1: Handler and Bennett. 1999 z d 10.0 4 7.0

5 .O

a

2 N .3 4 8 2.0 0 F 1.0 5 0.7

FIG. 5 . HSE abundances in Cameroon line (Rehkamper eta]., 1997) "low-Pd" BVSP spinel lherzolites (Morgan ef aZ., 1981; Morgan, 1986), and fertile spinel lherzolites from eastern Australia (Handler and Bennett, 1999).

1264 Morgan et al.

The average Ir value for the four most fertile peridotites (two from Mount Quincan, and one each from Mount Leura and Mount Noorat) agree well with the other results for other PGE shown in Fig. 5 . Four of the elements-Ir, Ru, Pt and Pd-plot within uncertainty of the mean mantle HSE line. The last three of these tend to be systematically higher than (though almost within uncertainty of) the Cameroon line data or that for the Mount Quincan sample UM 15 reported by Morgan et al. (1 98 1). There are major discrepancies with two elements, however: 0 s and Rh. Osmium is variable (0.36 to 3.68 ppb) and systematically lower than other PGE, even in the most fertile samples (Fig. 5 ) . Handler et al. (1999) suggested that the 0 s depletion is correlated with S loss, since the S content of these rocks is very low. It is possible, however, that the source region of these peridotites may have been low in S ab initio. Rhodium is significantly enriched in these samples relative to other PGE. This element is analytically difficult, but most procedural problems would be likely to give low rather than high values. Without hrther analyses, the possibility that this is an actual geochemical effect cannot be discounted. The important point, however, is that there is clear evidence in all ofthese samples that there is no significant enrichment of Pd over other PGE. Regional variations in PGE patterns may reflect heterogeneous trace element distribution in specific areas of the mantle, but the two UM peridotites with chondritic Par-UM 5 (Kilboume Hole, USA) and UM 15 (Mount Quincan, Australia-ffer a cautionary note. The HSE abundances are as close as replicates, though S and Se are markedly different (Morgan et al., 198 1 ; Morgan, 1986). Thus UM 5 has acquired a Mount Quincan-type trace element pattern that clearly is not regionally controlled (but may be by depth?). Compared to other Kilbourne Hole xenoliths, UM5 is compositionally and texturally quite distinct having higher Fe and CdSi (BVSP, 198 1). The REE pattern in UM 5 is flat and closely resembles that of UM 15, but is different from other Kilbourne Hole xenoliths that have uniformly depleted light rare earth elements (REE) (BVSP, 1981).

Although both Rehkamper et al. (1 997) and Handler and Bennett ( 1999) used similar isotope dilution techniques that included removal of silica in an open system before Carius tube equilibration, the agreement in Ir with the radiochemical neutron activation analysis (RNAA) results suggests the general PGE depletion is unlikely to be analytical error.

Uniformity of Iridium in Upper Mantle Lherzolites

The studies reviewed above have looked largely at most or all of the HSE at one or a few localities (Morgan et al., 198 1; Pattou et al., 1996; Rehkamper et al., 1997, 1999; Snow and Reisberg, 1995; Snow and Schmidt, 1998; Brandon et al., 2000; Handler et al., 1997; Handler and Bennett, 1999). In contrast, Spettel et al. (1991) measured Ir in 54 1herzoIites from 15 localities worldwide (Fig. 6).

They irradiated small peridotite samples (-200 mg) whole without pre-irradiation chemical concentration, thereby

Cameroon (g) 1 +w+ I I I Abyssal, 0 s v) E. Australia Vitim, Rus. (a) Pyrenees (4 Abyssal, hz (c) San Carlos (a> f Ts; Potrillo (a) S. Arabia&) I # I

i i # Kilb. Hole (a) Kilb. Hole (b) Mongolia (a) Lesotho (b) i irk Kapfenstein (a) Antarctica@) T , ,I I I ,

2 4 6 8 101214

FIG. 6. Worldwide distribution of Ir in mantle-derived spinel Iherzolites: (a) Spettel et al. (1991); (b) Morgan et al. (1981); (c) Rehkamper et al. (1 999) (harzburgite value multiplied by 0.88 to approximate lherzolite content); (d) Pattou et al. (1996); (e) Handler and Bennett (1999) and Morgan et al. (1 98 I ) ; (9 Snow and Reisberg (1995) (isotope dilution 0 s values used as surrogate for Ir); (g) Rehkamper et al. (1997). Symbols: Filled squares = spinel lherzolite xenoliths; open up triangles = abyssal peridotites; open circle = orogenic peridotites; filled down triangle = sheared garnet lherzolite PHN 161 1.

avoiding problems due to blanks and variable chemical yield. The complex spectrum of induced x-ray and y-ray activity was measured with high-resolution detectors but the HSE analysis generally was limited to Ir and Au, which have high activation cross-sections.

Of the seven localities with multiple samples studied by Spettel et al., six overlap with the mantle mean. (A suite from Antarctica (n = 9) averaging 4.9 ? 0.5 ppb Ir, or (1 0.2 ? 1 .O) x 10-3 CI is the exception and will be discussed later.) We have added data for the other peridotite suites discussed above. On a probability plot (not shown) the data points are normally (linear, not log) distributed. From the slope and intercept at 50% probability, we find a mean of (6.7 ? 0.5) x 10-3 CI or ( 3 . 2 ? 0.2) ppb (20mean in both cases). The worldwide

Siderophile elements in Earth's upper mantle and lunar breccias 1265

distribution of Ir therefore agrees reasonably well with the means for several HSE of (7.1 ? 0.8) and (7.4 ? 0.9) x 10-3 CI estimated previously (Morgan, 1986; Chou et al., 1983).

Many ofthe lherzolites discussed in the preceding sections are shown in Fig. 6. Most of these data plot within error of the mantle mean. The Cameroon line xenolith average falls below that of other xenoliths and the mantle average, though the range is large (Rehkamper et al., 1997). The Ir abundances in the Cameroon line xenoliths are bimodal, however, and the three highest abundance samples have Ir contents that agree well with those from other localities, the mean being 3.5 2 0.5 ppb (or (7.2 2 1.0) x 10-3 CI).

Most PGE in abyssal peridotites fall within the mean mantle HSE band, particularly if the 0 s values for abyssal lherzolites from Snow and Reisberg (1995) are plotted as a surrogate for Ir instead of the Snow and Schmidt (1998) measurements. The recent isotope dilution study of PGE in abyssal peridotites included no lherzolites (Rehkamper et al., 1999). Comparison of PGE, excepting 0 s and Ir, between abyssal lherzolite data from Snow and Schmidt (1998) and the more recent isotope dilution data suggests that the combined effects of depletion in the basaltic fraction and subsequent serpentinization rendered the latter results high by -12%. Multiplying the Rehkamper et al. Ir value by 0.88 provides an estimated abyssal lherzolite value that plots within error ofthe mean mantle HSE. Thus it is fair to say that the constancy of Ir values in the upper mantle is remarkable for any natural system, where log normal distributions are the standard rather than the exception.

The Antarctica xenoliths reported by Spettel et al. (1991) have been omitted from the mean since these clearly are very different in their Ir content from those of other localities. The Antarctica samples are relatively depleted, which in part may explain the high Ir values averaging 4.9 ? 0.5 ppb (or (10.2 2 1 .O) x 10-3 CI). But the very high Au values, with a mean of 2.0 2 0.5 ppb (or (14.4 -t 0.3) x 10-3 CI) strongly suggests some secondary enrichment. Kilbourne Hole xenolith UM7 has the same degree of depletion as the Antarctica samples and even higher Ir and Au, with AdIr = 1.7 x CI. Xenolith UM7 is one of the fine-grained variety discussed previously that may be texturally and chemically modified wallrock to an adjacent pyroxenitic dike.

Brief Summary

The abundance distribution of HSE in the Earth's upper mantle is remarkably uniform. In a large number of mantle- derived peridotites, Ir is normally distributed with a mean of (6.7 2 0.5) x 10-3 CI. Spinel lherzolite xenoliths from southwestern USA, orogenic spinel lherzolites from France and abyssal peridotites have similar distribution of PGE in which Pd/Ir is significantly enriched. In contrast, peridotites from West Africa and from eastern Australia have chondritic Pd/Ir. The difference may be due to secondary effects, but more likely reflect regional variations. The incompatible HSEs,

Re and Au, and the chalcogens, S and Se, are variously depleted in all these rocks, but PUM abundances for these elements may be estimated from Re-0s isotope systematics. The PUM abundance of Au is much higher than previous estimates. If mantle HSE originate from a late influx, then regional variations could reflect compositional variations in the large infalling bodies.

EARLY LUNAR INFLUX

Three types of meteoritic material have bombarded the Moon; micrometeorites, crater-forming bodies, and ancient planetesimals (Morgan et al., 1977). Of necessity, the same materials must have fallen to Earth, though the manifestations may differ. The siderophile and volatile element signature of micrometeorite influx is ubiquitous in mature lunar soils. In mare areas where mature soils are relatively uncontaminated from other sources (except for small local impacts and solar wind input), the micrometeorite pattern of siderophile and volatile trace elements shows a uniform enrichment at saturation of 1 to 1.5% CI that has been ascribed to cometary debris (Ganapathy et al., 1970; Morgan et al., 1977). Terrestrially, the same influx must occur globally, but is detectable only in slowly accumulated deep-sea sediments. The HSE in the sediments are of course much less abundant than on the Moon, since the collection time is very much shorter (<I50 Ma) and the higher terrestrial influx is more than compensated by more rapid sedimentation (Barker and Anders, 1968; Morgan, 1968; Peucker-Ehrenbrink and Ravizza, 2000).

The crater-forming populations are well observed on both Earth and the Moon. On the Moon, however, the trace siderophile and volatile signature has always been difficult to recognize, however, because of the widespread presence of the micrometeorite and ancient breccia components (Morgan et al., 1577). On Earth, -150 impact craters are currently known (Grieve and Pesonen, 1996), and a surprising number of these-about half of those reviewed by Palme (1 982)-have little or no indication of meteoritic contamination. The cratering rate on the Moon seems to have decayed by a factor of 2 or 3 between 3 and 1 Ga, but increased again in the last 0.4 Ga (Culler et al., 2000).

Here we are concerned chiefly with the comparatively short- lived population of planetesimals that formed the large lunar basins before -3.8 Ga ago. If these were primary objects of roughly chondritic composition, their very size makes them by far the major contributors of siderophile elements to the lunar surface. Wetherill (198 1) estimated that the mass of the planetesimal that formed the penultimate lunar basin, Imbrium, exceeded that of all subsequent impacts in lunar history. Radiometric ages of ancient lunar impact breccias appear closely grouped, suggesting that the basins associated with the breccias formed within 0.2 to 0.4 Ga in a "terminal cataclysm" (Wasserburg et al., 1977), though the reality of a terminal spike in flux rate has been questioned. For our purpose it matters

1266 Morgan et al.

little since modeling of the early influx rate yields about the same area under the log mass accretion vs. time curve for both continuous and spiked models (e.g., Fig. 2 of Hartmann, 198 1). The important point is that in breccias created during the basin- forming era, the overwhelming mass of siderophile elements was most likely by far to be derived from basin-forming objects, and not from the other two sources.

The siderophile and volatile element distributions in a very large number of ancient highland breccias have been studied by RNAA. These data have been reviewed by Hertogen et al. (1977), and more recently by James (1 994, 1995, 1996). Discriminant analysis of trace element patterns shows regional variations, perhaps related to individual basins (Higuchi and Morgan, 1975). Limiting the discussion to siderophile elements, there is significant fractionation between two groups of siderophile elements; refractory (Re, Os, Ir) and normal ( i e . , with volatilities similar to Fe and Ni, such as Pd and Au). For example, the pattern prevalent in possible Imbrium ejecta is characterized by Ir/Au -0.3 x CI. The interpretation of the siderophile enrichment of ancient lunar breccias has been debated energetically by Ringwood and coworkers, and the Anders group (see, for example, Delano and Ringwood, 1978; Anders, 1978). The Ringwood group held that a component of the siderophile enrichment seen on the Moon-and particularly in the ancient breccias-was similar to that in the Earth's mantle, thereby demonstrating that the Moon was derived from the Earth. Anders argued-we think correctly- that the siderophiles in the ancient breccias were introduced by impact and that regional and lithological variations in siderophile patterns may reflect the compositions of individual planetesimals. Nevertheless, it now appears that parts of the Ringwood argument also had merit, since if the late influx theory is correct, we should indeed see similarities in siderophile patterns between ancient lunar breccias and the Earth's mantle.

Apollo 17 Ancient Breccias

In the study of the terrestrial mantle, the enrichment of Pd relative to Ir is a key issue. In most lunar breccias Pd was not determined, because the analysis for this element by RNAA is more difficult than that for Ir, Au and Re. Two Apollo 17 breccias were studied, however, in the same lab, at about the same time and using the same techniques described above as used for the BVSP suite of peridotites (Morgan et al., 1976a, 1981; Morgan and Petrie, 1979). These rocks, 73215 and 73255, are two of a group of six light-grey breccias from the South Massif at Taurus-Littrow (James et al., 1975, 1978). They were apparently formed by the South Serenitatis impact, with 39ArPAr ages of formation of 3.87 k 0.03 and 3.89 ? 0.03 Ga, respectively (Jessberger et al., 1977; Staudacher et al., 1979). Two lithologies in these breccias contain significant siderophiles; aphanites (fine-grained, fragment-laden melt- rocks) and anorthosite-norite-troctolite (ANT) suite clasts,

mainly anorthositic gabbros. The siderophile abundances are variable in these clasts, as may be illustrated by two normal siderophiles, Ni and Au, that are not fractionated from each other in the two lithologies (Fig. 7). The data for the aphanites are closely grouped-perhaps a reflection of their fine-grained nature-and absolute abundances of Au are only slightly higher than those observed in fertile terrestrial peridotites. The data for the coarser-grained ANT clasts, however, vary by about a factor of 10.

The indigenous lunar component for Au is likely to be very small (Warren et al., 1991), and so the Y intercept in Fig. 7 of 22 2 9 ppm Ni may be a good estimate of the indigenous Ni abundance in lunar highlands material. The low indigenous Ni abundances in lunar breccias contrast markedly with the overabundance in the terrestrial mantle. The excess of Ni in the Earth's upper mantle is due to a decrease in the metal-silicate partition coefficient of Ni with increasing pressure (Li and Agee, 1996). It might be expected that the abundance of Ni in the bulk Moon would be rather similar to that of the Earth's mantle, but this does not appear to be the case. Two mechanisms might deplete Ni in the silicate Moon in general, and in lunar highland rocks in particular. Because of the more reduced nature of the Moon relative to the Earth, a small lunar core could form and extract Ni far more efficiently at low pressure than the terrestrial case. For a pressure of 2 GPa at the base of a lunar magma ocean 400 km deep, the corresponding metal/silicate partition coefficient for Ni would be 316 (Li and Agee, 1996). Thus even a small amount of metal would remove significant Ni. In addition, Ni appears to be fairly well excluded from anorthosites, as illustrated at Fiskenaesett, Greenland (Morgan et al., 1976b).

600

500

400 2

W

0 2 4 6 8 10

Gold, ppb FIG. 7. Nickel-gold correlation in Apollo 17 lunar breccias. The data fall on a single regression line, indicating that these two "normal" siderophile elements do not fractionate from each other between the two lithological types. Absolute abundances of Au in the aphanites are closely grouped and are only slightly higher than those observed in fertile terrestrial peridotites. The intercept approximates the indigenous (= non-meteoritic) abundance of Ni.

Siderophile elements in Earth’s upper mantle and lunar breccias 1267

Any contribution of the impactor is superimposed on an indigenous lunar component. Except for Ni, for which we make a correction, the indigenous abundances of HSE are generally agreed to be very low (Warren el al., 1991, and references therein). Nevertheless, we further minimize the effect by choosing samples with more than 5 ppb 0 s at which level the HSE indigenous correction may be neglected. Data for three ANT clasts and four aphanites were doubly normalized to CI and Ni since this element is well determined in the breccias. The fractionation of the siderophile elements is between two groups; the refractory siderophiles Re, 0 s and Ir, and the “normal” siderophiles Pd, Ni and Au (Fig. 8). The coherence of these groups strongly suggests a cosmochemical effect, and not a non-equilibrium process that took place during the impact. The siderophile pattern in the ANT clasts is almost unfractionated relative to CI, except for an enrichment of Au. In the aphanites, however, the refractory siderophiles are significantly depleted, while still maintaining the same relative proportions within the refractory and normal siderophile subgroups. It has been noted previously, most recently by James (1 995) and Norman et al. (200 l), that the siderophile element pattern in the aphanites resembles that seen in EH chondrites. But perhaps we should not be looking to small recently-fallen meteorites in our museum collection for a perfect match to a very large body that fell 3.86 Ga ago. The chondrites chronicle the processes that took place in the early solar system and it is that record, rather than the actual abundances of any given group of chondrites, that can improve our understanding of the bodies impacting the Earth and Moon. Even so, the similarity between some E chondrites and the Apollo 17 breccia clasts is astonishingly close, as recent isotope dilution analyses for PGE show (Horan and Walker, 2000).

Horan and Walker (2000) analyzed whole-rock chondrites for Re, Os, Ir, Pt and Pd by isotope dilution. About 100 mg of

sample was spiked and digested for at least 24 h with reverse aqua regia in quartz Carius tubes, without any preliminary open system removal of silica. 0 s was separated from other HSE by solvent extraction into CC14 and purified by microdistillation. Anion exchange was used to separate a Re fraction, a Pt-Ir fraction, and a Pd fraction. Re and 0 s were measured by NTI-MS. Isotope ratios of Pt and Ir were measured simultaneously and Pd separately by rnulticollector ICP-MS. In this work, isotopic equilibration is established at the outset of the analysis and blank corrections were small to negligible.

The aphanite siderophiles are essentially identical to those in the EH4 chondrites Kota-Kota and Indarch (Fig. 9); and, remarkably, the ANT siderophile pattern duplicates that of the EL6 chondrites Daniel’s Kuil and Yilmia (Fig. 10). The E chondrites were depleted in refractories-both lithophile (Larimer and Anders, 1967) and siderophile (Larimer and Wasson, 1988)-and the same fractionation effects seem to have influenced the bodies accreted by the Earth and Moon in the last stages. Thus, the signature in the lunar breccias, limited as it is in the present case, provides us with a direct frame of reference against which to judge the range of compositions to be expected in the terrestrial mantle. From the relative influx to Earth and Moon detailed in a later section (see Fig. 14), we may calculate that the surface density of basin-sized impacts on Earth was -4x as great as on the Moon. Therefore, the mixing of ejecta that we see in bulk ancient breccias must have occurred on Earth. The Apollo 17 analyses were carried out on carefully separated clasts and so the range of composition is greater than that in the bulk breccias. By the same token, however, Au and Ir data for breccias from other Apollo sites (Hertogen et al., 1977; James, 1994,1995,1996) suggests that the siderophile signatures in the ANT suite clasts and aphanites at Apollo 17 are close to the middle of the compositional range of basin-forming objects. Therefore, the siderophile patterns seen in Apollo 17 breccias may not be significantly different

0 Anorthositic Gabbro a

0

# 1

Refractory

Re 0 s Ir

E z” 0.5 1

Pd I Ni I Au FIG. 8. Nickel-normalized siderophile element abundances in Apollo 17 breccias. The siderophile elements in lunar breccias appear to fractionate as two groups; refractory siderophiles Re, 0s and Ir, and normal siderophiles Pd, Ni and Au.

& U

2 a

E; 1.5

s

% 0 u

.r( N l H

2 E Om7

0.5

V Kota-Kota EH4

Normal I Pd I Ni I Au

FIG. 9. Comparison of HSE in Apollo 17 aphanites with those in EH chondrites Kota-Kota and Indarch. Data are normalized to CI and Ir.

Morgan et al. 1268

k U

a 2 s

z 5 1.5

0 - 1

0.7 N

I I I I I 1 0 Anorthosite V Daniel's Kuil EL6

Q

0

7

Normal I

FIG. 10. Comparison of HSE in Apollo 17 anorthositic gabbros with those in EL chondrites Daniel's Kuil and Yilmia. Dataare normalized to CI and Ir.

from the compositions expected in complex mixtures of more fractionated types.

SYNTHESIS

Comparison of Siderophile Signatures in the Early Influx to Earth and Moon

The fractionation of refractory and normal elements in the Apollo 17 aphanites resembles the Ir-Pd fractionation in slightly depleted Kilbourne Hole and Cochise crater xenoliths, and in the Pyrenean orogenic peridotites (Fig. 1 1). The lunar aphanites and the slightly depleted xenoliths were analysed by similar RNAA techniques (Keays et al., 1974; Anders et al., 1988),

5

& Y

- 0.5

z" 0.2

0 Apollo 17 aphanites W Xenolithic lherzolites

FIG. 11. Comparison of siderophile elements in Apollo 17 aphanites with those in BVSP xenoliths (Morgan et al., 1981; Morgan, 1986), orogenic spinel lherzolites (Pattou et al., 1996) and abyssal peridotites (Rehkamper et al., 1999; Brandon et al., 2000). Open squares represent the estimated fertile mantle abundances for Re and Au.

and this comparison is least likely to suffer from analytical bias. It is clear that the fractionation between the refractory elements as represented by Ir and Os, and the normal elements Pd and Ni (plotted here as a surrogate HSE) are very similar. In particular, Ni, with its small uncertainty, agrees almost identically with Pd in the xenoliths. Also, Se in aphanites, and S and Se in slightly depleted xenoliths, are very similar. Recent isotope dilution results for abyssal harzburgites (Rehkamper et al., 1999; Brandon et al., 2000) match Apollo 17 aphanite data remarkably well. The Ir-normalized Pd is slightly higher than in the lunar breccias, but within uncertainty. Similarly, the more precise Ni value in aphanites is lower than Pd in abyssal peridotites, but within the larger Pd uncertainties. The earlier INAA results for abyssal harzburgites (not shown in Fig. 11, but see Fig. 4) are broadly similar to the isotope dilution data, though the Ir-normalized Pd is higher because of the low INAA Ir (Snow and Reisberg, 1995; Snow and Schmidt, 1998).

In the case of the orogenic peridotites (Pattou et al., 1996), there arises the possibility of analytical differences between RNAA and the pre-concentration ICP-MS technique. In these peridotites, Pd falls within the range of the lunar breccias, but is significantly higher than Ni. In absolute terms Pd is not atypically high in the Pyrenean orogenic peridotites, and is in fact identical to the abundances in the "slightly depleted" xenoliths. Thus, high Pd in the orogenic massive peridotites may be due to the need to normalize to Ir (Fig. 1 1). As a whole, the "higher than chondritic" Pd/Ir ratio observed in these xenolithic, orogenic and abyssal peridotites may in fact reflect the composition of an impacting population that differs from CI composition, having undergone refractory siderophile- normal siderophile cosmochemical fractionation. The S content of the Pyrenean peridotites is within error of the normalized aphanite Se value. Thus, explanation by secondary terrestrial effects may not be required for these observations, even though it cannot be categorically excluded.

The peridotites from the Cameroon line and eastern Australia have essentially chondritic Pd/Ir ratios that match that of the lunar ANT clasts, and are in agreement also with the Ni/Ir ratio in these Apollo 17 samples (Fig. 12). The overlap in elements between the lunar ANT clasts and the Cameroon suite is sparse, however, and values close to chondritic are not uniquely diagnostic. The eastern Australian peridotites have an unusual depletion of 0 s relative to Ir that is clearly a strong mismatch with the Apollo 17 ANT clast pattern. Recently, Handler et al. (1 999) have shown that in these peridotites Ir/Os and Cu/S are correlated, and they suggest that S and 0 s depletions were the result of secondary near-surface processes. There is no correlation of Pd/Ir with either Cu/S or Os/Ir and such ratios as Ru/Ir, Pt/Ir and Pd/Ir may be representative of the regional mantle. The low S and Se (in fertile spinel lherzolite UM 15) in the eastern Australian peridotites appears to be primary, if the match to the lunar ANT clast data is valid.

If HSEs in the upper mantle and lunar breccias are of common origin, as the evidence here seems to indicate, then

Siderophile elements in Earth's upper mantle and lunar breccias 1269

3 k c1

Td

U u 0 c,

1 0.7 0.5 0.3

0.1 0.07 0.05 0.03

Eastern Australia

FIG. 12. Comparison ofsiderophile elements in Apollo 17 anorthositic gabbro clasts with those in spinel lherzolites from the Cameroon line (Rehkamper et al., 1997) and Eastern Australia (Handler and Bennett, 1999). The open square represents the estimated Au content for the Mt Quincan xenolith UM 15.

the high mantle abundance of Au suggested in a previous section seems strongly supported by the lunar data. In fact, the well-determined Au/Ir ratios observed in the lunar breccias may provide a better estimate of mantle Au than the hyperbolic extrapolation of the xenolith data, where errors become large as we approach the fertile mantle intercept almost asymptotically. If we group the aphanite datum with those for high P a r xenoliths from the southwestern USA, the regression is best fit (MSWD =1.00) using the following constants; a = 7.27 f 0.30, b = 2.5 14 r 0.10 and c = 8.57 (chosen empirically for best fit). The regression yields a pristine mantle value of (16.2 2 3.4) AulCI x 103, or 2.3 -+ 0.5 ppb Au (Fig. 2). These results are much higher than the value of 1 ppb Au previously proposed for a chondritic mantle, but may apply only to the mantle source of high PdIr peridotites.

There are insufficient data to estimate independently the initial Au content of peridotites with chondritic PdIr. The two BVSP spinel lherzolites UM5 and UM15 plot below the hyperbolic curve for high Pd/Ir peridotites, suggesting a lower initial Au abundance. The data are too close together to suggest any trend, but transforming the data with previous values for b and c gives a mantle Au value for the low Pd/Ir peridotites of (8.7 f 1.3) x 10-3 CI or 1.2 2 0.2 ppb Au. If the lunar ANT clasts are a good model for the low Pd peridotites, the AdIr ratio of (1.45 ? 0.07) x CI give a mantle value of (8.3 r 0.8) x 10-3 CI equivalent to 1.2 ? 0.1 ppb Au, a result identical to the regression value (Fig. 2).

Relative Influx to Earth and Moon

During the Moon's early heavy bombardment, the Earth received a larger share of the influx due to its greater mass and gravitational radius. The ratio of influx rates for the Earth and

Moon is influenced by the Earth-Moon distance, and particularly by the geocentric velocity of the incoming body at infinity (Bandermann and Singer, 1973). Bills and Ray (1 999) recently summarized the evolution of the lunar orbit. Tentative extrapolation of their results suggests that the Earth-Moon separation between 3700 and 4400 Ga intersects flat portions of the Bandermann-Singer velocity curves and would not have a major influence on the Earth/Moon influx ratio. Accordingly, we use in this section the simpler formulation of Chyba (1 99 1) that neglects the minor effect of Earth-Moon separation on the relative influx rates. In some respects, the geocentric velocity ofthe incoming object is a free parameter, though it is most likely 110 km s-1 (Morgan et al., 1977). The velocity directly affects the relationship of lunar basin size to the mass of the impacting body, however, since the empirical scaling laws require an approximate energy dependence, particularly in very large craters (Wetherill, 1981; Melosh, 1989; Chyba, 1991).

It is clear now that the late influx was not of CI composition (Meisel et al., 1996,2001). Nevertheless, rather than abandon this useful frame of reference let us assume a volatile-free CI composition ( i e . , -70% of the mass of CI). The mass of the present upper mantle is about one-fourth of the total mantle mass of 4.11 x 1024 kg. If the average upper mantle HSE content of -7 x 10-3 CI is uniformly distributed, then the upper mantle contains -0.7 x 7 x 10-3 = 5 x 10-3 of volatile-free late influx material for a total mass of -5 x 1021 kg. The total influx represented by the lunar basins is more difficult to estimate. Morgan et al. (1977) derived a cumulative mass for large impactors of 1.5 x 1020 kg. This result yields an Earth/ Moon influx ratio (AEIAM) = 35, agreeing well with the value of 33 estimated independently by Hartmann (1976). Both estimates suggest a reasonable geocentric velocity of -8 km s-1. More recently, Chyba (1 991) examined the relative influxes to Earth and Moon with a view "to estimate exogenous volatile and prebiotic organic delivery". Accordingly, his interest was not limited to the ancient basin-forming episode, but included impact events up to the present day and all those that produced craters larger than 4 km diameter. His calculation was based on an analytical fit of craters vs. time and an assumed lunar impact velocity of 12 km s-1. Our following discussion differs in that we limit ourselves to integrated effects of the basin- forming impacts on the Moon before -3.8 Ga, and we independently estimate the average impact velocity.

A realistic estimate of the lunar influx may be best derived from the large basins actually characterized. Spudis (1993) lists 45 lunar basins and large craters of Imbrian or pre-Imbrian age that have apparent diameters, D, 2 250 km (Fig. 13). If these basins were formed from the same population of objects, then impact velocities may have been similar. In that event, the mass of the incoming body, 8, would be roughly proportional to the amount material excavated, which in turn would be proportional to some function of D. For example, Wetherill (1 98 1) used energy scaling to derive the relationship that reduces to:

1270 Morgan et at.

E South Pole-Aitken PI

20.7 x Imbrium

E r/l r/l

10 / I

.i

M z 5

E' s o 0 500 1000 1500 2000 2500

Basin diameter, km FIG. 13. Cumulative mass influx for 45 ancient lunar basins and large craters with diameters, D, > 250 km (Spudis, 1993). Assuming that impact velocities were similar for this early population of impacting objects, the mass ofthe incoming body, 0, would be roughly proportional to 03 .36 . The masses of the impactors are scaled to that ofthe body forming the lmbrium basin. Note that almost three-fourths of the total influx is derived from one very large basin: South Pole- Aitken.

where 6' is in kilograms and D is in meters.

a similar dependence on D More recently, Chyba ( 1 99 1 ) developed an expression with

(9) 6 = 4.54 x 104 v-1.67 D3.36

The odd exponent for v arises because the true cratering relation likely lies somewhere between energy and momentum scaling. The two expressions in Eqs. (8) and (9) give quite similar results, but since we intend to follow Chyba (1991) for calculating the relative influx to Earth and Moon, we will also use his cratering equation for consistency.

The total influx to the Moon corresponding to the Earth late veneer may be estimated as a function of impact velocity from the basin-forming cratering record. We normalize crater diameter, D, to that of Imbrium (=l I60 km) and calculate the cumulative value of (D/Dlrn)3.36 for the 45 basins listed by Spudis (1993). Then, for any given impact velocity, w, the total lunar influx is 2 0 . 6 8 ~ the mass 61, of the Imbrium projectile (Fig. 13).

The impact velocity v is related to the geocentric velocity at infinity, c, and to the lunar escape velocity, OM, of 2.38 km s-1 by

Thus, for any given geocentric velocity we can derive the mass of the Imbrium projectile, and hence, the total late influx

to the Moon. This result divided into the estimated integrated late influx in the Earth's upper mantle (=5 x 1021 kg) gives an Earth/Moon mass influx ratio. In Fig. 14, we compare this result with that derived independently from the gravitational considerations (Chyba, 1991; his Eqs. (16) and (17)). The lines intersect at a geocentric velocity, c, of 5.96 km s-1 and an EartWMoon mass influx ratio 52.5. (This result agrees well with an earlier estimate-c = 6 km s-1 and Earthmoon influx ratio of 50 (Morgan et al., 2001)-based on Eq. (8) (Wetherill, 1981) and an Earth/Moon influx ratio derived from Bandermann and Singer (1973), using an Earth-Moon distance of 34 Earth radii corresponding to the 3.84 Ga age of the Imbrium impact (Bills and Ray, 1999).) The new value for geocentric velocity is much lower than those of present-day meteorites of -15 km s-I, and is slightly lower than those derived above from earlier calculations of lunar influx rates (Hartmann, 1976; Morgan et al., 1977). Impact velocities for lunar basin formation have been estimated from HSE contents of lunar breccias, assuming that these reflected the ratio of excavated mass to impacting mass (Morgan et al., 1977). The impact velocities generally ranged from less than the lunar escape velocity of 2.38 to 10 km s-1. The result from Fig. 14 yields an average lunar impact velocity of 6.4 km s-1 that falls within this range and is much more closely constrained than the earlier estimate. The mass of the Imbrium object corresponding to this impact velocity is 4.7 x 1018 kg and that of the largest impactor-for South Pole-Aitken-is 7.1 x 1019 kg. These values are in good agreement with previous estimates of 4 x 1018 and 4 x 1019 kg for the Imbrium object and the largest lunar impactor, respectively (Wetherill, 198 1). Chyba (1991), using the same equations as here, but quite different assumptions, derives a mass of 1.4 x 1019 kg for the South

0 -3 z 200

% 70 50

b

2 100 .d

d 0 g 20 c. 5 10 w 7

5

. Gravity j Cratering i \

influx ratio' . .

f velocity c i =6000 m.sec-' I 0 3000 6000 9000 12000

Geocentric velocity, m.sec" FIG. 14. The variation of the EartMMoon influx rate ratio can be determined independently from the relative gravitational cross- sections (curve marked "Gravity") and from calibration of the lunar influx rate by empirical cratering mechanics (curve marked "Cratering"). The intersection of the curves yields a mutually satisfactory solution for the two variables.

Siderophile elements in Earth’s upper mantle and lunar breccias 1271

Pole-Aitken impactor. This smaller value is due to the high average impact velocity of 12 km s-1 chosen by Chyba and the smaller diameter of 2200 km he selected for this largest lunar basin. We estimate the total lunar influx to be 9.7 x 1019 kg, a value slightly lower, but more defensible, than that of 1.5 x 1020 kg estimated by Morgan et al. (1977). Our result agrees very well with a total lunar influx of 1 x 1020 calculated by Chyba (1991).

The results from Fig. 14 indicate the enrichment of the Earth mantle in HSE most likely is limited to the upper mantle above 670 km. Granting our assumptions, enrichment of the whole mantle cannot be easily accommodated. The total mass of the mantle is -4x that of the mantle above 670 km. In order to enrich the whole mantle with siderophiles to the extent observed in the upper mantle, an EartWMoon mass influx ratio of 4 x 52.5 = 2 10 would be required. The resulting ratio implies an unrealistically low geocentric velocity of -1.5 km s-1. Restriction of late influx enrichment to the upper mantle above the 670 km discontinuity has important implications. It may be the absence of significant HSE in the lower mantle that allows us to observe in plume basalts an 0 s isotopic signature possibly due to a small addition of outer core material to the source region (Walker et al., 1995, 1997; Brandon et al., 1998, 1999). Similarly, this result may explain the low content of -0.35 ppb Re in ocean island basalts (OIB) derived from deep- seated plumes when compared to -0.93 ppb Re in mid-ocean ridge basalts (MORl3) originating in the upper mantle (Hauri and Hart, 1997). In OIB, even the small Re abundance may be due predominantly to core-mantle interaction.

Missing Basins and Very Large Impactors

The restriction of HSE enrichment to the upper mantle is an important result. The skeptic therefore may legitimately ask if we have underestimated the terrestrial influx, particularly since one of the estimates by Chyba (1991) of 1.5 x 1022 kg would be sufficient to enrich 75% of the total mantle. Chyba calculated the total mass of the influx to Earth using two methods. In the first, which he based on the total crater count over the whole history of the Moon, he estimates that the Earth received 24x the lunar influx-a mass of 2.4 x 1021 kg. The difference between this result and the value of 5 x 1021 kg we chose may be almost entirely ascribed to the high value of 12 km s-1 he selected for c, the geocentric velocity at infinity. In a second estimate, he used statistical arguments (Sleep et al., 1989) to conclude that 17 objects larger than the one forming South Pole-Aitken on the Moon may have struck the Earth. Assuming then that even these very large objects have a number distribution corresponding to the Dl.8 dependency for lunar craters, Chyba calculates that the largest of these may have had a mass as large as 9.5 x 1021 kg and the total influx mass to the Earth is significantly increased to becomes 1.5 x 1022 kg.

The distribution of lunar basins shown in Fig. 14 does not align well with the DI.8 dependency, however. Using South

Pole-Aitken as a datum point, the D l . 8 curve would suggest that decades of study of the lunar surface have failed to reveal three large basins with diameters between 1200 and 2600 km. To be sure, additional large basins have been suggested such as the proposed -2300 km Procellarum, or Gargantuan, basin (Cadogan, 1974; Whitaker, 1981). But Spudis (1993) is categorical; “Procellarum basin does not exist”. Conversely, the log of cumulative crater counts for 45 observed ancient lunar basins and large craters with diameters, D, > 250 km (Spudis 1993) vs. crater diameter D in meters yields a linear and highly correlated plot (r2 = 0.987) with a slope of-1.87 x 10-6 and a log intercept of 2.282. (Fig. 15). This correlation suggests that South Pole-Aitken may be an atypically very large basin. The log N vs. D plot of lunar basins predicts one object large enough to excavate a crater >1200 km diameter, as observed, but there will only be 4 x 10-3 objects available to form a crater with D > 2500 km. Although this low probability event clearly occurred, it does not seem a firm basis for using South Pole-Aitken to predict extremely large impacts on Earth (Chyba, 1991). If we use the size distribution estimated from lunar basins in Fig. 15, we find that there would be 57 objects larger than the Imbrium impactor striking the Earth, and the largest would be about half the mass of the South Pole-Aitken body. Since we have already used South Pole-Aitken in our estimate of lunar influx mass, it seems unlikely that the terrestrial influx has been grossly underestimated.

The Homogeneous Accretion Model

As mentioned in the Introduction, the homogeneous accretion model can yield abundances in the mantle ofthe HSE

Lunar Craters, D < 250km Slope = - 1.872 x lo”

c g 20 u

10 -

South Pole - Aitker

5 -

2 .

1

500 1000 1500 2000 2500

Crater Diameter, D, km FIG. 15. Log of cumulative crater counts for 45 ancient lunar basins and large craters with diameters, D, > 250 km (Spudis 1993), plotted vs. crater diameter D in meters. The linear and highly correlated plot suggests that South Pole-Aitken is an atypically very large basin. Alternatively, ifthe D1.8 relation is valid for these very large craters, three or four craters larger than Imbrium still remain to be discovered.

1272 Morgan et al.

Re, Ir and Au that are approximately chondritic, if its rather specific assumptions are granted. For skillfully chosen inputs, the results would be difficult to resolve analytically from those predicted by the late influx model (Azbel eta& 1993; Kramers, 1998). 0 s isotopic evidence has not been addressed specifically in Kramers' model, but adjustment of free parameters (e.g., relative amounts of liquid and solid metal and liquid and solid silicates) probably could be tailored to meet this constraint.

In the homogeneous accretion model, the initial formation of the core is not a run-away process that essentially turns the proto-Earth inside out (Davies, 1990). Instead, core formation is gradual, beginning at -20 Ma with a homogeneous proto- Earth and sustained at varying rates as accretion continues for the first 150 Ma of Earth's history. As in "Phase 3" of the thermal model of Davies (1990), the mantle is assumed to undergo vigorous convection and is always well mixed. Localized melting occurs due to impact and particularly to decompression melting at the convective upwelling regions. In each melting event, metal (90% liquid, 10% solid) separates from silicate (30% liquid, 70% solid) at the bottom ofthe melt zone, and eventually drains away to the core as an inverse diapir. During the melting process there is no interaction with the rest of the upper mantle, but once the metal has been removed, the metal-free silicate mush is uniformly mixed into the mantle prior to the next cycle. Thus, the newly accreting metal is removed, and the metal accumulated in the upper mantle prior to the onset of core formation is successively diluted by each melting episode. At 80 to 100 Ma, the core addition rate is decreased to equal the influx rate so that HSE abundances in the mantle stay constant. (This step appears to optimize the W isotopic result.) At 100 Ma, when the influx rate is <lo% of the peak rate, core formation is then allowed to occur until the "target" ( i e . , the observed) abundances in the mantle are attained. If the very specific premises of this model were accepted, then the results for HSE would be analytically almost indistinguishable from the late influx model.

Objections may be raised to the plausibility of any model that requires the removal of more than 99% of HSE without significant fractionation, though Kramers ( 1998) does adroitly circumvent problems encountered with the rather similar inefficient core formation model of Jones and Drake (1986). But a critical difficulty with the homogeneous accretion model lies in the selective use of the thermal model of Davies (1990). The highly convective mantle in Davies' "Phase 3" is a key element in Kramers' model. The heat to drive convection and promote decompression melting, however, is provided in Davies' "Phase 2" as gravitational energy released by an initial runaway core segregation in a proto-Earth of perhaps half the present radius. The same event would of course simultaneously remove siderophiles and HSE from the silicate mantle. Thus, the chondritic HSE and siderophile abundances that are required as an initial condition of Kramers' model would not be present. And without "Phase 2" core segregation there would no highly convective mantle. A further difficulty may arise

with the highly convective regime due to fluid dynamic considerations (Stevenson, 1990). In Kramers' model, the input of metal to the core takes place primarily from areas of decompression melting at the top of the uprising element of convection cells. Since these regions by definition are hot and close to melting, it is quite likely that the separating metal will preferentially descend in such regions and "rain down" in droplets (Stevenson, 1990) rather than forming an inverse diapir. Ultimately, the added density of the metal droplets may exceed the thermal density contrast in the deep mantle that drives convection and may halt the process.

The homogeneous accretion model also has difficulty accommodating the giant impact thought to form the Moon (Cameron, 1997). This highly energetic event, placed at 60 Ma by Kramers (1998), of necessity would completely melt the mantle and quantitatively remove any metal in the mantle to the core. In Kramers' modified model, core formation is stopped immediately after the giant impact. Because the influx rate is still significant at 60 Ma (-5.9 x 1016 kg/year), the siderophile abundances soon recover to levels similar to those at -80 Ma in the case without the giant impact. Then core segregation is recommenced and the modified model is allowed to run its course. Not surprisingly, results are similar to those of the original model without a giant impact. Even allowing the particular assumptions in this model, it seems unlikely that core addition, which had been proceeding steadily for 40 Ma, would turn off after a major heating event. It is true that vigorous convection may cease soon after the impact due to superheating at the surface and at the core-mantle boundary (Benz and Cameron, 1990). Thus the particular model of decompression heating espoused by Kramers and coworkers would not be applicable. Nevertheless, the mantle as a whole would be molten by impact superheating and would not retain metal as required by the modified homogeneous accretion model. Further, the metallic phase of a continuing influx would be swiftly removed from the molten mantle and added to the core. Thus, there could be no rapid build-up of siderophile elements in the mantle as in Kramers' (1998) model. Only after the Earth had cooled sufficiently would a metal phase be again retained in the mantle. In this respect, the homogeneous accretion model appears to converge to the late influx model.

CONCLUSIONS

The abundance distribution of HSE in the Earth's upper mantle is remarkably uniform. A large number of Ir measure- ments in mantle-derived peridotites are normally distributed with a mean of (6.7 r 0.5) x 10-3 CI.

Spinel lherzolite xenoliths from southwestern USA, orogenic spinel lherzolites from France and abyssal peridotites have similar distributions of PGE in which P a r is significantly enriched. In contrast, peridotites from West Africa and from eastern Australia have chondritic Pd/Ir. The difference may be due to secondary effects, but also may reflect regional variations.

Siderophile elements in Earth's upper mantle and lunar breccias 1273

The incompatible HSE and the chalcogens S and Se are variously depleted in all these rocks, but abundances may be corrected to PUM values using Re-0s isotope systematics.

If the late influx origin for mantle HSE is accepted, then regional variations could reflect compositional variations in large infalling bodies and should be reflected in the HSE patterns of ancient (>3.7 Ga) lunar breccias. In Apollo 17 breccias, aphanites have HSE and chalcogen patterns closely matching those o f high Pd/Ir spinel Iherzolites. The corresponding distribution in ANT clasts is similar to that in peridotites with chondritic Pd/Ir. The closeness of the agreement may be coincidental, but suggests that new studies of lunar breccias might elucidate the HSE composition and regional variation in the Earth's upper mantle.

Comparison of relative influx to Earth and Moon suggests that the enrichment of HSE may be limited to the upper mantle above 670 km. If the lunar breccia analogy is correct, the Au abundance in the upper mantle may be considerably higher than previously thought and reexamination of the xenolith data admits this possibility.

Abundances of HSE and chalcogens in the PUM sampled by the high PdIr peridotites are as follows; 0.29 2 0.08 ppb Re, 3.5 ? 0.8 ppb Os, 3.5 f 0.6 ppb Ir, 5.8 f 0.6 ppb Ru, 6.5 f 0.5 ppb Pt, 5.7 -+ 0.5 ppb Pd, 2.3 k 0.5 ppb Au, 63 5 9 ppb Se and 210 f 20 ppm S. The HSE Re through Pt average (7.3 2 0.7) CI x 10-3, a value that agrees within uncertainty with the world-wide distribution of Ir. Other normalized values are (10.2 k 0.9) Pd, (16.4 2 3.6) Au and (3.4 f 0.5) Se and S.

There is as yet insufficient information to derive a fertile mantle composition for the chondritic PdAr group if indeed such a group is real. We would expect, however, a broadly chondritic HSE distribution with Au/Ir z 1.5 x CI and (S, Se)/Ir z 0.1.

Considerable effort is now being spent on the study of HSE in the Earth's mantle and often concern is expressed when the results fail to match chondrite values. The study ofchondrites is valuable to define the processes of cosmochemical fraction, but the kilogram-sized chondrites falling in the last few years are unlikely to be exact samples of the material that enriched the Earth's mantle in HSE. The identity of these objects can be found only in the ancient lunar breccias created at that time.

Acknowledgements-This work was supported in part by NSF Grants EAR 971 1454 and 9706815, and NASA grant NAG 57634. This paper was stimulated by the memories ofthe vigorous debates between Ed Anders and Ted Ringwood on the origin of siderophiles in the lunar breccias and their relation to planetisimal influx and the terrestrial mantle. We thank an anonymous reviewer and particularly Jan Kramers for trenchant comments and suggestions that have greatly improved this paper.

Editorial handling: U. Krahenbiihl

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