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Project Progress

The purpose of this work is to fingerprint late 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 impactors, but in extremely low abundance in the indigenous lunar crust. Towards this end, numerous sub-samples of each of 4 rocks were analyzed this year. We continued work on Apollo 17 impact melt rocks (73215, 73255, 76215) that formed ~ 3.9 billion years ago during generation of the Serenitatis basin. In order to more precisely constrain the HSE characteristics of the Apollo 17 rocks, research professor Puchtel analyzed 20 additional chunks of these rocks in the past year. Further, our 2007 undergraduate GCA student intern, Lorne Loudin, began work on a new Apollo 17 impact melt rock, 76055, and obtained data for three chunks of it. Finally, in the past year Puchtel initiated and completed a study (10 chunks) of a lunar meteorite, NWA 482. The lunar source location for this rock is unknown, but some chemical evidence points to the lunar far side. It too was initially melted at about 3.8 billion years ago. In total, over the three years we have been working on this project, we developed clean techniques to analyze small fractions of lunar melt rocks for the concentrations of six highly siderophile elements and osmium isotopic composition (¹⁸⁷Os/¹⁸⁸Os). A total of fifty-one sub-samples of eight melt rocks have been analyzed.

Results for all rocks show linear variations of HSE when plotted versus Ir abundance (believed to be in very low abundance in the lunar crust). The slopes of the linear regressions of these data should provide accurate elemental ratios of the impactors (i.e., the fingerprints of those bodies). Ratios of Re, Os, and Pt versus Ir yield slopes consistent with chondritic meteorites for all lunar melt rocks. However, ratios of Ru and Pd versus Ir are generally higher than the range of measured chondrites, and the ¹⁸⁷Os/¹⁸⁸Os ratios are also higher in two rocks than in bulk chondrites. This ratio serves as a precise proxy of the long-term Re/Os ratio of these samples (changes resulting from ¹⁸⁷Re→ ¹⁸⁷Os + ^-β, where t_1/2 = 42 billion years). Combined, these results show that impactors that contributed substantial matter to the Moon, and presumably much greater mass to the Earth and Mars, do not all fit within the chemical context of chondrites present in museum collections (at least so far as have been measured) (Figure 1). Of note, recent attempts to better constrain the composition of the Earth’s primitive upper mantle (a hypothetical undepleted, unmodified mantle reflecting primordial composition) suggest that it too may have “suprachondritic” Ru/Ir and Pd/Ir. Therefore, it may be necessary to expand the possible compositions of late accreted materials to the terrestrial planets. Results of this work were most recently presented at the 2007 Lunar and Planetary Sciences Conference (Puchtel et al., 2007a) and a manuscript detailing the results was submitted to Geochimica et Cosmochimica Acta in August 2007 (Puchtel et al., 2007b).

Although a first stage of study is now complete, many issues still remain. Of greatest concern is the possibility that we are still underestimating the effects of the lunar target rocks. It is also possible that some unidentified fractionation process affected Ru, Pd, and Re/Os during creation of the melt rocks. In the coming year, we intend to proceed on two analytical fronts. First will be to analyze some so called “pristine” lunar crustal rocks in order to place constraints on indigenous abundances in typically lunar crustal rocks. We are in the process of adapting our existing chemical separation procedures to enable us to process cleanly these types of materials, some of which are expected to contain very low HSE concentrations. Second, we will continue analysis of lunar impact melt breccias in order to continue to assess the extent of chemical heterogeneity among late accreted materials. We have also begun to examine HSE contained within the components that comprise chondritic meteorites. Our ultimate goal is to link the chemical fingerprints of the HSE to nebular or parent body processes that could provide information regarding the volatile content of the impactors (water?) and their place of origin within the solar system (inner or outer solar system formation?).

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