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Project Progress

GCA Co-I Prof. Richard Walker and colleagues at the University of Maryland (Dept. of Geology) have completed the measurement of Os isotopic compositions and some highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances in 60 subsamples of seven lunar breccias. These are: Apollo 17 poikilitic melt breccias 72395, 76215 and 76055; Apollo 17 aphanitic melt breccias 73215 and 73255; Apollo 14 polymict breccia 14321; and lunar meteorite NWA 482, a crystallized impact melt rock. The purpose of this work is to fingerprint late (4.5 — 3.8 Ga) additions to the Moon (and presumably the Earth) using the relative abundances of highly-siderophile elements (HSE) that occur in generally high abundance in likely lunar impactors, but in extremely low abundance in the indigenous lunar crust.

Plots of Ir vs. other HSE define excellent linear correlations, indicating that all data sets represent dominantly two-component mixtures of a low-HSE indigenous target component and a high-HSE exogenous impactor component. Linear regressions yield intercepts that are statistically indistinguishable from zero for all HSE, except for Ru and Pd in two samples. The slopes of the linear regressions are presumed to be insensitive to target contributions of Ru and Pd of the magnitude observed; thus, the trendline slopes approximate the elemental ratios in the impactor components.

The ¹⁸⁷Os/¹⁸⁸Os and regression-derived elemental ratios in the Apollo 17 aphanitic melt breccias and the lunar meteorite indicate that the impactor components in these samples have close affinities to chondritic meteorites. The impactor component in the Apollo 17 aphanitic melt breccias is likely dominated by HSE-rich granulitic-breccia clasts incorporated in the impact melt, whereas the high HSE component in the lunar meteorite probably represents the impactor. The impactor components in the Apollo 17 poikilitic melt breccias and in the Apollo 14 breccia have higher ¹⁸⁷Os/¹⁸⁸Os, Pt/Ir, and Ru/Ir and lower Os/Ir than is currently known in chondrites. The compositions of these components suggest that the impactors they sample were distinct from known chondrites, and perhaps represent a type of primitive material not currently delivered to Earth as meteorites. Some characteristics of these materials mimic estimates for relative abundances of some HSE in Earth’s primitive upper mantle (undifferentiated mantle). Thus, the population of impactors involved in the formation of the giant lunar basins may be similar to late accreted materials to Earth that may have established HSE abundances in the terrestrial mantle. During 2007-2008 these results were incorporated into a manuscript that was recently published Geochimica et Cosmochimca Acta (Puchtel et al., 2008).

Because of the possible complexities indigenous HSE could add to our interpretation of lunar impact melt rocks, during 2007-2008 we also began study of HSE in so-called “pristine” lunar crustal rocks. Pristine lunar crust is that portion of the Moon’s crust that retains its magmatic character and has not suffered later impactor additions. Much previous work on the HSE Au and Ir has been conducted on these types of rocks, but little is known about the inventory of other HSE in this material. The acquisition of further data is essential for two reasons. First, analysis of mare basalts and pyroclastic glasses suggests that lunar mantle has, on average, ~20 × lower HSE abundances than the terrestrial mantle. Therefore, lunar crust, derived from melt segregation of the mantle, should have ultra-low HSE abundances to be consistent with this hypothesis. Second, lunar impact melt breccias have variable and elevated HSE inventories as a function of distinct chondritic impactor populations striking the surface of the Moon between 4.4 to 3.8 Ga. As noted above, regressions of HSE data for some melt rocks indicate that initial HSE inventories of some pre-impact rocks, presumably pristine lunar crust, may have been substantial.

We have nearly completed analysis of 3 ferroan anorthosites (FAN), 3 norites and the ‘classic’ lunar troctolite, 76535, for their Os, Ir, Ru, Pt, Pd and Re abundances and Os isotope compositions. These rocks were studied because they are representative of FAN and high-Mg suite rocks in the Apollo collections. We have also analyzed two Apollo 12 mare basalts, lunar mare basalt meteorites MIL 05035 and LAP 04841 and Apollo 15555 and 70135, analyzed previously.
Because of the anticipated, ultra-low abundances of pristine crustal rocks, two analytical challenges had to be circumvented. First, there is difficulty of maintaining low and constant blank contributions. Second, because of the limited availability of this material, proper addition of enriched-isotopic tracers for isotope dilution concentration measurements are required to avoid inadequate ¹⁸⁷Os/¹⁸⁸Os isotope analysis and error magnification issues. Thus, we have performed a two-stage analytical campaign, first analyzing aliquots of around 0.13 to 0.3g and then measuring larger aliquots of between 0.5-1.2g, which are optimally spiked.

Results for the initial aliquots are shown in Figure 1. These results, presented at the 2008 Lunar and Planetary Science Conference, clearly demonstrate that the pristine lunar crust has ultra-low abundances of the HSE and that there is no evidence (in the suite examined) for extreme fractionation. Further, pristine lunar crustal rocks have nearly chondritic measured ¹⁸⁷Os/¹⁸⁸Os, implying long-term Re/Os evolution for the entire silicate Moon that is similar to chondritic meteorites. Thus, pristine lunar crust has an inventory consistent with an HSE-depleted silicate Moon and requires that the initial compositions calculated from lunar impact melt breccias may require an alternate explanation. This might include: 1) prior impactor contributions masked by later HSE addition; 2) analytical artifacts; and 3) a geographically inhomogeneous spread of HSE impactor materials on the lunar surface. These possibilities can be tested with further HSE analysis of Apollo rocks and new lunar meteorite finds. Results of this work are currently being incorporated in a manuscript that we anticipate will be submitted for review during Fall 2008.

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