Terrestrial Alteration and/or Contamination
Terrestrial crustal materials and fluids can have relatively high 187Os/188Os ratios of greater than 1, high Re/Os ratios of greater than 8, and low total Os concentrations of around 0.05 ppb (e.g., Esser and Turekian 1993). Meteorites, especially those with low-HSE abundances, can be susceptible to disturbance of primary signatures by terrestrial alteration processes that add or remove HSE (e.g., Huber et al. 2006; Brandon et al. 2012). This is especially true for aubrites, which contain minerals such as oldhamite (CaS) or alabandite (MnS) that are altered easily under the oxidized conditions in the terrestrial atmosphere. As most aubrite samples in this study display some evidence for terrestrial oxidation in the form of brown Fe-Oxide coating, some of the samples with measured 187Os/188Os ratios of >0.14 (Table 1) could reflect influence of terrestrial weathering instead of primary signatures. Mixing models of meteoritic Re and Os in the typical range of aubrites from this study (1, 10, and 100 ppb Os; Re/Os = 11; 187Os/188Os = 0.12) with terrestrial Re and Os (0.05 ppb Os, 0.4 ppb Re, 187Os/188Os = 1) shows that some of the variation in 187Os/188Os, especially the elevated values in samples like Bustee, can potentially be attributed to terrestrial contamination (Fig. 4; Table 1). These relationships also hold true for elevated Re/Os, indicating potential addition of not only radiogenic Os, but also of Re. The most extreme Re-Os signatures as seen in Bustee require >70% of Os and Re to be added, while less radiogenic signatures, such as seen in Bishopville, require around 30–50% (Fig. 4).
Figure 4. 187Os/188Os versus a) Re/Os, b) Os (ppb) c) 1/Os (ppb) d) Pd/Ir. To evaluate potential sample contamination by terrestrial alteration, mixing between terrestrial crustal contaminant (0.05 ppb Os, 0.39 ppb Re, 187Os/188Os = 1) with meteoritic material similar to typical aubrites (solid line: 1 ppb Os, 0.08 ppb Re, 187Os/188Os =0.12; dashed line: 10 ppb Os, 0.8 ppb Re, 187Os/188Os = 0.12) was modeled. Horizontal error bars in panel a) show error on the Re/Os ratio introduced by blank variability for low concentration samples.
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However, there are several arguments that suggest that most, or possibly even all, of the observed Os signatures are primary. All aubrites other than the Antarctic samples, Mt. Egerton, Pesyanoe, and Shallowater are observed falls, minimizing the length of exposure time within the terrestrial environment (Foshag 1940; Lonsdale 1947; Beck and LaPaz 1951; Keil 1989). With the exception of ALHA78113, all the finds have high Os concentration (>10 ppb for Os), limiting the impact of terrestrial alteration from material with Os concentration below 0.05 ppb (Esser and Turekian 1993; Huber et al. 2006). Rhenium concentrations are as low as 0.18 ppb in ALHA78113, making Re more susceptible to terrestrial contamination, in addition to the issue of blank contribution to these values. Weathered Antarctic chondrites have no resolvable shift in Os isotopic composition compared with less weathered falls and finds, suggesting a minor impact of terrestrial alteration on the Re-Os isotopic system in meteorites (Huber et al. 2006).
In a recent study of Martian meteorites, Brandon et al. (2012) found significant addition of terrestrial Re and Os (up to 48% Re and 2% Os, with radiogenic 187Os/188Os as high as 0.34) in arid desert finds. We were not able to perform similar leaching experiments as were carried out in Brandon et al. (2012) to evaluate the magnitude of terrestrial contamination on HSE because of the unstable behavior of the exotic sulfides. Day et al. (2012a) suggested open-system behavior of Re during terrestrial alteration based on large variability of Re/Os compared to Ir/Os; resulting in mobilization and redistribution of Re within the brachinites and brachinite-like achondrites they examined (e.g., Northwest Africa 5400), rather than addition of Re or Os from external terrestrial sources. As in the case for these meteorites examined by Day et al. (2012a), the limited range in Ir/Os and 187Os/188Os compared with Re/Os in aubrites indicates mobility of Re rather than Os during terrestrial weathering. While the latter process is conceivable for finds with longer, unknown, residence times on Earth relative to falls, it seems unlikely for falls, some of which have been recovered within days or weeks (Lonsdale 1947; Beck and LaPaz 1951). However, because of their exotic mineral composition, aubrites may be more susceptible to terrestrial alteration than less reduced meteorites.
Additionally, fractionation of Re and Os by terrestrial alteration would require largely different relative mobilities of Re and Os, because both are primarily hosted in NiFe metal in aubrites (van Acken et al. 2012). The exotic sulfides do not significantly contribute to the HSE budget of aubrites (van Acken et al. 2012), and hence can only have limited involvement in terrestrial alteration. Fractionation of Re from Os by weathering would have to occur over a period of months for the observed fall samples, which is likely an unreasonably short time for removing both elements from their metal host or adding them from the surroundings, and fractionating Re from Os in the magnitude observed. Furthermore, terrestrial alteration removes Re more efficiently than Os (e.g., Walker et al. 2002; Fischer-Goedde et al. 2010) and should result in lower Re/Os, in contrast to the Re enrichments observed in aubrites. Osmium appears unaffected, as excellent correlations with Ir, Ru, and Pt are maintained (Fig. 2). To take potential terrestrial contamination and uncertainty introduced by large blank contributions into account, the Re/Os and 187Os/188Os values in low concentration samples (ALHA78113, Bishopville, Bustee, Khor Temiki, LAP 03719, LAR 04316, Norton County, and Peña Blanca Spring) are considered with caution during the following discussion.
Another potential cause for the observed enrichment in Re may be enrichment in 187Re by neutron capture of 186W, as outlined for lunar crustal rocks by Day et al. (2010). If this process is operating, the isotopic composition of “common” Re used during the isotope dilution calculation may be wrong and Re concentrations would be overestimated. The potential for such an interaction of aubrites with cosmic neutron radiation is significant. Aubrites have cosmic ray exposure (CRE) ages among the longest for all known meteorites (Eberhardt et al. 1965; Lorenzetti et al. 2003; Herzog et al. 2011), and neutron capture by 149Sm and 157Gd, which have large neutron capture cross-sections, has been shown to occur in aubrites, resulting in anomalous Sm and Gd isotopic composition comparable to those in lunar regolith (Hidaka et al. 2006). However, to develop excess in 187Re sufficient to disturb measured Re signatures from neutron capture of 186W, a W/Re >17,000 is required, as observed in lunar soils (Day et al. 2010). While no bulk W data for aubrites exist, Re concentrations determined independently by neutron activation (Wolf et al. 1983) would require a bulk W concentration around 10 ppm for these W/Re ratios to be achieved. This bulk W concentration for aubrites is unreasonably high, given the W concentrations of 0.3–0.7 ppm and W/Re ratios of less than 200 in aubrite metals, the primary host for both elements (van Acken et al. 2012).
Sample Heterogeneity and HSE fractionation
The excellent correlations between concentrations of HSE (Fig. 2) are consistent with all HSE being hosted in the same phase, presumably FeNi metal (Wolf et al. 1983; Casanova et al. 1993; van Acken et al. 2012). Highly siderophile element concentrations and 187Os/188Os ratios of multiple aliquots of the same sample can yield poor reproducibility (Table 1) perhaps resulting from primary sample heterogeneity, physical mixing during brecciation, and/or nugget effects due to the heterogeneous distribution of metal (Casanova et al. 1993; van Acken et al. 2012). Mechanical mixing of fragments during brecciation is a plausible explanation because most aubrites are breccias, and some have been identified as regolith breccias (Keil 1989). However, unless metal grains in aubrites represent different populations from sources with different Re/Os and 187Os/188Os, this “nugget effect” does not account for the large differences between 0.1331 and 0.2263 in measured 187Os/188Os ratios between some sample duplicates (Table 1). These duplicates may represent mixtures of different breccia fragments in various proportions. However, the mm-to-cm sizes of the fragments from which the analyzed powders were prepared are smaller than the several cm average sizes of breccia fragments such that this is unlikely to explain the large 187Os/188Os differences (e.g., Lonsdale 1947; Neal and Lipschutz 1981). The chondritic interelement HSE ratios and 187Os/188Os and isotopic compositions of two aubrites (ALH 84007-009, LAP 03716) suggest a relatively minor influence of aubrite parent body(ies) processes on HSE in these samples.
A “mode effect,” nonrepresentative sampling of the whole rock resulting in increased variability of chemical measurements, has been suggested for lunar samples (Spicuzza et al. 2007) and brachinites (Day et al. 2012). Given the very coarse-grained nature of aubrites (Foshag 1940; Lonsdale 1947; Beck and LaPaz 1951; Keil 1989, 2010), nonrepresentative sampling is likely to also have contributed to the poor reproducibility in some samples (Peña Blanca Spring, Norton County, Mt. Egerton).
In addition to variations in HSE contents within samples, twelve samples (ALHA78113, ALH 84008, ALH 84009, Bishopville, Bustee, Khor Temiki, LAP 03719, LAR 04316, Mayo Belwa, Mt. Egerton, Norton County, Pesyanoe) display fractionated HSE signatures relative to average chondrites (Table 1; Figs. 1 and 2). Deviations from chondritic HSE/Ir ratios are especially present in the more incompatible HSE such as Pd and Re (Figs. 1b and 2; Table S2). The samples which show the most fractionated, nonchondritic HSE patterns (Fig. 1) with enriched Re and Pd also show indications of disturbed Ar-Ar and Rb-Sr isotope systems, which give young ages of 3.5 to 4.5 Ga (Bogard et al. 2010). Norton County, which has Pd/IrN ratios of 4.02 to 11.98 and Re/IrN of 0.93 to 3.61 shows Ar-Ar ages between 1.3 and 4.5 Ga (Bogard et al. 2010), and a Rb-Sr age of 4.47 Ga (Minster and Allegre 1976). Bishopville has Pd/IrN of 1.18 to 4.41 and Re/IrN of 1.91 to 50.8 (Table S2), with reported Ar-Ar ages of 1.5 to 4.2 Ga (Bogard et al. 2010) and Rb-Sr ages of 3.5 to 3.9 Ga (Compston et al. 1965). Shallowater has an Ar-Ar age of 4.53 ± 0.05 Ga (McCoy et al. 1995) and a I-Xe age of 4.562.8 ± 0.3 Ga (Gilmour et al. 2006). Assuming 4.563 ± 0.001 Ga as the formation age of Shallowater (Gilmour et al. 2006) and using this age as a reference point, the calculated initial 187Os/188Os ratios in aubrites are between –0.7475 and 0.1036. Calculations of initial values in low-HSE concentration samples such as Bustee and Norton County are significantly affected by Re blank uncertainty, which can result in spurious initial 187Os/188Os values different by as much as 0.1 (Fig. 5). Even taking the uncertainties introduced by Re blank contribution into account, these impossible initial values support disturbance in the Re-Os isotope system and other isotope systems (Ar-Ar, Rb-Sr) in samples such as Bustee, Mayo Belwa, Norton County, or Bishopville (Table 1; Fig. 5).
Figure 5. Temporal evolution of aubrites, using measured 187Os/188Os as present-day composition and measured Re/Os to calculate 187Os/188Os back to solar system formation. Red lines: enstatite chondrite (Walker et al. 2002), black lines: aubrites with approximately enstatite chondritic development; gray lines: aubrites with disturbance of the Re-Os isotope system, quickly developing unreasonably low 187Os/188Os. Dashed gray lines: minimum and maximum calculated development lines for low-Re samples Norton County and Bustee considering uncertainty caused by blank variability.
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The enrichment in Pd and Re by a factor of up to 12, excluding the high, nonreplicable values in Mayo Belwa and Bishopville, seen in several samples (e.g., Norton County, Bustee), is commonly interpreted as a partial melt signature in the absence of metal in the terrestrial mantle (e.g., Morgan 1986). Whether this interpretation can be applied to a smaller, much more reduced parent body is questionable. The relative incompatibility of Re, Pd, and, to a lesser extent, Pt in the terrestrial mantle is governed by their partitioning in sulfides and silicates (Ballhaus et al. 2006; Fonseca et al. 2007; Mallmann and O’Neill 2007), and the relative compatibility in sulfide and silicate melts during partial melting (Ballhaus et al. 2006; Mallmann and O’Neill 2007). In most aubrite sulfides, HSE concentrations are below detection limits for LA-ICP-MS (<1–5 ppb, Casanova et al. 1993; van Acken et al. 2012), rendering them insignificant as a HSE carrier phase.
Enrichments of Pd and Re can potentially be explained by a number of processes. As Pd and Re enrichments do not always occur in the same sample, different processes may be the underlying cause. Palladium enrichments could be explained by less siderophile behavior of Pd during metal segregation (Mann et al. 2012), leaving the silicate residue with elevated Pd/Os. Addition of fractionated material, as seen in some metal grains from chondritic inclusions in the Cumberland Falls aubrite (Pd/Os up to 60, Re/Os up to 0.25) or troilite grains from Shallowater (Pd/Os and Re/Os of up to 3; van Acken et al. 2012), would produce small enrichments in Pd and Re. In addition to minor Re addition by terrestrial processes (Fig. 4a), addition of metal and sulfide with fractionated HSE signature could serve to explain the enrichments, although the amount of material needed to reproduce the bulk aubrites HSE signatures might well exceed the amount of inclusion material available (see discussion about chondrite addition below).
Core Formation and Remixing in the Aubrite Parent Body(ies)
Excluding the anomalous metal-rich samples, Shallowater and Mt. Egerton, which may have an origin on reduced bodies different from the aubrite parent body(ies) (Keil 1989, 2010; Keil et al. 1989), all aubrites have HSE concentrations at least an order of magnitude lower than is typical for chondrites. Metal/silicate partition coefficients of HSE (Dmetal/silicate) are typically greater than 106 (e.g., Kimura et al. 1974; O’Neill et al. 1995; Righter 2003; Brenan and McDonough 2009; Mann et al. 2012); hence, HSE are segregated into metal cores. Because of the reduced oxidation state of aubrites, HSE are exclusively hosted in metal phases (Casanova et al. 1993; van Acken et al. 2012) and the abundance of metal from traces to 3.7 wt% (Watters and Prinz 1979) thus controls their bulk HSE concentrations. If representative of the silicate portions of the aubrite parent body(ies), these HSE concentrations are orders of magnitude too high to result from metal/silicate equilibrium partitioning during core formation, which would predict HSE concentrations of 3 ppt or less in the silicate portion. Another explanation for the high and variable HSE concentrations in aubrites is thus required.
Keil et al. (1989) advocated a complex history of impact-induced breakup and reassembly for the parent body of the anomalous Shallowater aubrite based on the petrology and trace element contents of sulfides and silicates. In this case, elevated HSE concentrations could be the result of remixed core material during reassembly with the silicate portions of the body after initial differentiation and parent body destruction. Alternatively, impact-induced shearing stress may draw core material back into the mantle (Rushmer et al. 2005). Different scenarios of remixing of core material into previously depleted silicate can be envisioned: remixing of bulk core metal, either liquid or solid from a differentiated core, or a mixture of the latter two in proportions different from the bulk core. To evaluate these possibilities, the composition of core forming metal on the aubrite parent body(ies) needs to be constrained. Siderophile element partitioning between silicate, liquid and solid metal during metal segregation and differentiation is controlled by the temperature, pressure, and the minor element content of the metal, most notably S, C, Si, and P (e.g., Chabot and Jones 2003).
For modeling, either EL or EH chondrites were assumed as a proxy for undifferentiated starting materials, forming a perfectly segregated metal core making up 40% of the total mass (metal content in enstatite chondrites; Mason 1966), which leads to HSE concentrations in the silicate portion of the resulting differentiated body between 0.1 and 3 ppt. Measured HSE concentrations in aubrites as representative of the silicate portion of the aubrite parent body(ies) are 2–5 orders of magnitude higher than HSE concentrations in the silicate portion after core formation (Table 1; Fig. 1); and planetary processes in addition to accretion and core formation are required to explain HSE systematics in aubrites.
If a differentiated core is taken into account, the influence of minor element concentrations in metal on HSE partitioning behavior between solid and liquid metal becomes important (e.g., Jones and Drake 1983; Chabot and Drake 2000; Chabot et al. 2003, 2006, 2008, 2010; Corrigan et al. 2009; Hayden et al. 2011). For high S content around 25%, all HSE with the exception of Pd partition into solid metal over S-rich melts (Chabot et al. 2003). For lower S concentrations around 5%, all elements are moderately compatible in solid metal over metal liquid. The low S concentrations in aubrite metals below 0.15% support removal of an S-rich melt phase (van Acken et al. 2012). However, bulk HSE abundances (Table 1; Fig. 1) argue against removal of S-rich melt as a dominant process, as Pd is not or only marginally depleted. The effect of C, P, and Si on HSE solid metal/liquid metal partition coefficients is small (Chabot and Drake 2000; Chabot et al. 2006, 2010; Corrigan et al. 2009), although Ru shows greater affinity for P-rich metal. Effects of minor elements on HSE partitioning between solid and liquid metal are negligible over the range of minor element content (Si, S, C, P) encountered in aubrite metals (Casanova et al. 1993; van Acken et al. 2012). The exceptions are high contents of S (approximately 25%), which lead all HSE except Pd to accumulate in metal solid, and C (approximately 4%), which results in fractionation of Ru from the other HSE (Chabot et al. 2006, 2008).
As with addition of chondritic material, addition of small amounts (0.1–1%) of segregated metal of either a differentiated or undifferentiated core back into an HSE-depleted silicate portion of the aubrite parent body(ies) can match the observed HSE concentrations in aubrites reasonably well (Fig. 6). However, the high Dmetal/silicate and uniform Dliquid metal/solid metal values do not cause significant fractionation of HSE from each other, with the possible exception of Pd in S-rich systems, and remixing of core material with a silicate portion of the aubrite parent body(ies) stripped of its HSE cannot account for fractionation of HSE. While remixing of <0.1% to 3% of core material is consistent with the HSE concentrations observed (Fig. 6) and with the broadly chondritic HSE ratios for the more compatible HSE Os, Ir, Ru, and Pt (Fig. 1a), the enrichments in Pd and Re observed in some samples require additional processes to operate during the formation of aubrites (Fig. 1b).
Figure 6. a–h) Highly siderophile element (Pt, Ru, Pd, Re) versus Os. Solid lines denote mixing of core material. Parts (e–h) represent details of parts (a–d). Core composition was calculated from an EL starting composition (Horan et al. 2003; Brandon et al. 2005b; Fischer-Goedde et al. 2010; van Acken et al. 2011), assuming a core comprising 40% of the body mass, corresponding to metal content in EL meteorites. Models were calculated with two sets of Dmetal/silicate parameters: For the first set, metal/silicate partitioning coefficients were set to 106 for all HSE (solid lines with long tick marks); for the second set, the high-pressure parameters from Mann et al. (2012) were used (experiment Z538, dashed line with short tick marks). Core remixing models were calculated for both remixing of bulk core material and mixtures of core material having undergone differentiation into a liquid and a solid portion, which yield no resolvable difference.
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Aubrite metals in Cumberland Falls, Mt. Egerton, and Aubres show fractionated HSE patterns, with a marked depletion in refractory HSE (Os, Ir, Ru, Pt), which is interpreted to be a quenched metal liquid signature (van Acken et al. 2012). However, the bulk HSE concentrations in Aubres and Mt. Egerton do not reflect depletion of Os, Ir, Ru, and Pt compared with Pd and Re (Fig. 1), raising the need for a complementary HSE-rich phase, potentially heterogeneously distributed HSE alloys, as seen in the metal phases of chondrites (e.g., Rambaldi et al. 1983; Campbell and Humayun 2003; Horan et al. 2009). The presence of these alloys, so far undetected by in situ studies, would also serve to explain the strong nugget effect of over two orders of magnitude, as observed in samples like Mt. Egerton and Norton County.
Thirteen of 16 aubrites samples from this study (excluding LAR 04316, Mt. Egerton, and Shallowater) are breccias. Thus, past research has put a special emphasis on various inclusions and clasts, especially in ALHA78113, Cumberland Falls, and Khor Temiki (Neal and Lipschutz 1981; Verkouteren and Lipschutz 1983; Lipschutz et al. 1988; Newsom et al. 1996; Keil et al. 2011). Some of these clasts have been described as chondritic (Neal and Lipschutz 1981; Verkouteren and Lipschutz 1983; Lipschutz et al. 1988), and have been shown to have chondritic HSE concentrations (Wolf et al. 1983). Some aubrites (ALH 84007–84009, LAP 03719, Aubres, Peña Blanca Spring) show broadly chondritic HSE ratios (Table S2), consistent with a primordial origin of the HSE-bearing phases in these samples. The HSE concentrations in these samples are less than chondritic by one to two orders of magnitude (Table 1; Wolf et al. 1983). Brecciated aubrites could thus be regarded as a mixture between the chondritic, HSE-rich inclusions and an aubritic, almost HSE-free matrix. The resulting HSE patterns would then represent “diluted” chondrite patterns and the absolute HSE abundances would be determined by how much of each of the two components are within each sample fraction measured. For example, to explain the Pt and Os concentrations of the aubrite samples, and assuming chondritic concentrations of HSE for the inclusions (Horan et al. 2003; Brandon et al. 2005; Fischer-Goedde et al. 2010; van Acken et al. 2011) and no HSE contents in the orthopyroxene-dominated matrices, between 0.1 and 7.5% by total weight needs to be made up of chondritic inclusion fragments (Fig. 7). For ALHA78113, in which chondritic inclusions are present (Lipschutz et al. 1988), less than 0.5% of chondritic material is sufficient to produce the observed concentrations of 1–2 ppb Os, 2.1–2.2 ppb Pd, and 2.2–3.9 ppb Pt (Table 1; Fig. 7). The percentage of inclusions present within ALHA78113 is difficult to estimate and has not been determined. Three inclusions were studied by Lipschutz et al. (1988) for a total sample mass of 299g for ALHA78113; inclusions were both larger and more abundant in Cumberland Falls (Neal and Lipschutz 1981; Verkouteren and Lipschutz 1983; Lipschutz et al. 1988), where they make up to 10% of the total sample in some fragments.
Figure 7. a, c) Pt (ppb) versus Os (ppb); b, d) Pd (ppb) versus Os (ppb); (c and d) represent shaded area in (a and b), respectively. Line denotes mixing of chondritic material (Horan et al. 2003; Fischer-Goedde et al. 2010), as seen in chondritic inclusions in aubrites (Neal and Lipschutz 1981; Rubin 2010) with aubrite silicate mantle material after formation of a core on the aubrite parent body(ies). Tick marks represent the percentage of chondritic material added. Concentration of aubrite silicate calculated using Dmetal/silicateHSE of 106 (e.g., Righter 2003).
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The type(s) of chondritic clasts found in aubrites cannot be resolved here due to the similarity of HSE concentrations for the different chondrite groups (Horan et al. 2003; Brandon et al. 2005; Fischer-Goedde et al. 2010; van Acken et al. 2011). While some clasts have been described as ordinary chondrites with variable degrees of equilibration with the aubrite matrix (Rubin 2010), other clasts have been labeled “forsterite chondrites,” with an affiliation to K chondrites, winonaites, and IAB irons (Neal and Lipschutz 1981). It is also noteworthy that metals, which are the primary HSE carriers in aubrites, have unfractionated HSE signatures outside of the chondritic inclusions, while displaying fractionated signatures within, which has been attributed to solid metal/liquid metal partitioning (van Acken et al. 2012). Addition of these metal grains to some aubrites could produce a slightly fractionated HSE pattern, as seen in bulk samples for these elements. In summary, addition of metal from chondritic inclusions to an aubritic matrix can explain some of the HSE concentrations observed in aubrites, and potentially provide an explanation for the fractionated patterns, in particular, the Pd enrichments over chondrites (Mayo Belwa, Bustee, Bishopville, Norton County). However, the bulk inclusions show chondritic Pd/Os and subchondritic Re/Os (Wolf et al. 1983), so whether the metal grains with fractionated patterns can be considered representative for the chondrite inclusions remains an open question.
Late accretion, the addition of chondritic material to a planetary body after core formation, but prior to crystallization of the crust, is thought of as the process governing HSE concentrations in the silicate portions of terrestrial planets (e.g., Kimura et al. 1974). Recent studies show that smaller, differentiated bodies may have been affected by late accretion of chondritic material, albeit to lesser extent than larger bodies (Dale et al. 2012; Day et al. 2012b). The predicted effect of addition of a late chondritic accretion on the bulk HSE systematics of meteorites originating from the silicate portion of such a planetary body is indistinguishable from addition of chondritic fragments by late impact processes as discussed above.
Comparison with Re-Os Isotopes and HSE in Meteorites from Other Small, Differentiated Bodies
To understand differentiation of small planetary bodies such as the aubrite parent body(ies), the results from aubrites need to be put in context with results from other groups of meteorites. A large body of HSE and Os isotope data exists for chondrites (Walker et al. 2002; Horan et al. 2003; Brandon et al. 2005; Fischer-Goedde et al. 2010; van Acken et al. 2011); ureilites (Rankenburg et al. 2007, 2008); howardites, eucrites, and diogenites (HEDs, Dale et al. 2012; Day et al. 2012b); shergottites (Brandon et al. 2000, 2012); angrites (Dale et al. 2012; Riches et al. 2012); and brachinites (Day et al. 2012a). Although all of these groups are significantly different from aubrites with respect to mineral assemblage, oxidation state, and oxygen isotopic composition, they are all thought to have experienced differentiation during the earliest stages of the solar system (e.g., Mittlefehldt 2003).
Concentrations of HSE in these meteorite groups vary over more than seven orders of magnitude from <1ppt in diogenite Meteorite Hills 00424 (Day et al. 2012b) to several 1000 ppb (brachinite NWA 5400, Day et al. 2012a; Fig 8a). Aubrites have Os concentrations comparable to shergottites, HEDs, and angrites between 0.1 and a few tens of ppb, with the anomalous samples Shallowater and Mt. Egerton showing values of a few 100 ppb, in the range of chondrites, ureilites, and brachinites. Concentrations in angrites, HEDs, and shergottites extend up to three orders of magnitude below those seen in aubrites to below 1 ppt (Fig. 8a). This range of concentrations underscores the differences between aubrites and ureilites, which have been suggested to have undergone loss of metallic liquid and not have formed a core (Rankenburg et al. 2008), and brachinites and brachinite-like achondrites, which are interpreted as residues from partial melting of an undifferentiated volatile-rich body, with the anomalous meteorites Graves Nunatak 06128 and 06129 forming complementary felsic crusts (Day et al. 2009, 2012a), while pointing to similarities in the differentiation histories of aubrites, angrites, HEDs, and shergottites.
Figure 8. Comparison of HSE and 187Os/188Os signatures in aubrites with other meteorite groups. a) Pt/Os versus Os (ppb), b–c) Pt/Os versus Pd/Os, panel c) represents shaded area in panel b, d) 187Os/188Os versus Re/Ir. Data sources: aubrites: this study, chondrites: Walker et al. (2002), Horan et al. (2003), Brandon et al. (2005), Fischer-Goedde et al. (2010), van Acken et al. (2011); angrites: Dale et al. (2012), Riches et al. (2012); shergottites: Brandon et al. (2012); brachinites and brachinite-like achondrites: Day et al. (2009, 2012a); HEDs: Dale et al. (2012), Day et al. (2012b). Single samples with extreme values were excluded for scale reasons (e.g., shergottite Los Angeles, 187Os/188Os = 0.74088, Pd/Os = 2919, Pt/Os = 1603, shergottite Zagami, Pd/Os = 155, Pt/Os = 197, Brandon et al. ; diogenite Talampaya, Pt/Os = 42, Dale et al. ).
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As absolute concentrations in HSE are dependent on the amount of metal present and hence highly variable in samples with inhomogeneous distribution of metal (e.g., Mt. Egerton, Table 1), interelement HSE ratios are a more suitable tool to study planetary differentiation processes. Because Pt, Pd, and Re behave more incompatibly than Os, Ir, and Ru during partial melting of sulfide-bearing silicate lithologies, ratios such as Pt/Os, Pt/Ir, Re/Ir, and Re/Os (approximated by 187Os/188Os) are useful tracers for magmatic differentiation. Elevated Pt/Os, Pt/Ir, Re/Ir, and 187Os/188Os compared with chondritic reference values (Pt/Os: 1.61–2.20, Pd/Os: 0.90–1.49, Re/Ir: 0.07–0.11; Horan et al. 2003; Brandon et al. 2005; Fischer-Goedde et al. 2010; van Acken et al. 2011) are interpreted as melt signatures, whereas lower values reflect melt residues (e.g., Morgan 1986). Aubrites cover a range in Pt/Os (0.76–5.87), Pd/Os (0.56–14), and Re/Ir (0.03–4.7, see Table S2) comparable to angrites (Pt/Os: 0.79–39, Pd/Os: 0.07–7.9, Re/Ir: 0.03–16.7; Dale et al. 2012; Riches et al. 2012), but narrower than shergottites (Pt/Os: 0.30–22.7, Pd/Os: 0.28–15.8, Re/Ir: 0.02–50, excluding Los Angeles and Zagami; Brandon et al. 2012) and HEDs (Pt/Os: 0.74–42, Pd/Os: 0.07–55, Re/Ir: 0.03–8; Dale et al. 2012; Day et al. 2012b), and wider than both ureilites (Pt/Os: 0.83–1.86, Pd/Os: 0.10–0.98, Re/Ir: 0.03–0.11; Rankenburg et al. 2008) and brachinites (Pt/Os: 0.17–2.56, Pd/Os: 0.06–0.79, Re/Ir: 0.03–0.25; Day et al. 2009, 2012a; Figs. 8b–d). The very low Pd/Os and Pt/Os values below 0.5 as seen in achondrites from all other groups are not seen in aubrites (Fig. 8c). Aubrites do show extreme Re/Ir as high as 4.7, as also seen in single shergottites and HEDs, which are interpreted as products of igneous differentiation and formation of different mantle reservoirs on Mars and 4 Vesta, respectively. Aubrites show a wider range in 187Os/188Os, reflective of long-term development of Re/Os, than chondrites (0.123–0.131; Walker et al. 2002; Horan et al. 2003; Brandon et al. 2005; Fischer-Goedde et al. 2010; van Acken et al. 2011; Fig. 3), ureilites (0.121–0.131; Rankenburg et al. 2007), and brachinites (0.120–0.131; Day et al. 2009, 2012a), and comparable to angrites (0.106–0.212; Dale et al. 2012; Riches et al. 2012), HEDs (0.117–0.206; Dale et al. 2012; Day et al. 2012b), and shergottites (0.116–0.246, with the exception of Los Angeles, Brandon et al. 2012; Fig. 8d).
The similar range of Pt/Os and Pd/Os seen in angrites, which crystallized from basaltic magma under oxidizing conditions (IW + 1 – IW + 2; Mittlefehldt et al. 1998), compared to aubrites, albeit with a few samples with Pd/Os as low as 0.07 (Riches et al. 2012), indicates similar igneous processing operating on both the aubrites and angrite parent bodies, despite the large difference in oxidation state. The lesser range in Pt/Os and Pd/Os in aubrites compared with shergottites and HEDs can be interpreted as a lesser degree of magmatic differentiation occurring within the aubrite parent body(ies) compared with Mars and the HED parent body, possibly reflecting faster cooling due to smaller body size. The onset of planetary differentiation and formation of different mantle reservoirs of Mars has been placed around 4.5 Ga (Brandon et al. 2000; Kleine et al. 2002; Debaille et al. 2007; Dauphas and Pourmand 2011). Younger crystallization ages of shergottites and ALH84001 indicate a prolonged period of igneous activity in Mars from 4.1–0.15 Ga (Lapen et al. 2010; Brandon et al. 2012; and references therein). In contrast, aubrite ages much younger than 4.45 Ga (Compston et al. 1965; Bogard et al. 2010) are considered disturbed and without geological meaning. Superchondritic 187Os/188Os from 0.136 to 0.163 in some mafic shergottites (e.g., Dar al Ghani 476, NWA1068, NWA5789; Brandon et al. 2012) reflects long-term elevated Re/Os of the shergottite mantle sources resulting from early differentiation around 4.5 Ga, while similarly superchondritic values in aubrites (up to 0.226 in Bustee; Fig. 8d) probably cannot be attributed to formation of distinct mantle reservoir because of the smaller parent body size, and hence faster cooling time of the aubrite parent body compared with Mars (Zellner et al. 1977).
In summary, while aubrites have HSE characteristics that are distinct from primitive meteorites such as chondrites, their HSE signatures are by no means unique among evolved achondrites. Similarities in the degree and range of HSE depletion are seen in a significant range of redox states, from extremely reduced (IW-6 to IW-8, aubrites) to oxidized (IW+1 to IW+2, angrites) and thus apply to planetary bodies forming and differentiating at various distances from the Sun (e.g., Wasson and Kallemeyn 1988). The closest comparison of aubrites is to angrites and HEDs, which share the wide range in 187Os/188Os ratios, HSE concentrations and ratios seen in aubrites (Fig. 8) (Dale et al. 2012; Day et al. 2012b; Riches et al. 2012). Despite a number of overlapping HSE characteristics with other achondrite groups, aubrites maintain distinct HSE signatures lacking the extreme depletion in HSE concentration seen in angrites. HEDs, and shergottites, and the low Pt/Os and Pd/Os ratios of brachinites and ureilites, point to a unique history of the aubrite parent body(ies).