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Project Progress

The purpose of this work is to fingerprint the late additions to the Moon using the relative abundances of the highly-siderophile elements (HSE) that occur in generally high abundance in likely impactors, but extremely low abundance in the indigenous lunar crust. Towards this end, approximately 2g of a variety of Apollo 14 and 17 melt breccias have been obtained from the Johnson Space Center curatorial facilities. These melt rocks are believed to have formed ~ 3.9 Ga ago during the generation of the Serenitatis and Imbrium basins, respectively, and likely sample the impactors that generated these late formed basins. These materials may provide the only direct chemical link to the late accretionary period of the Earth-Moon system. The chemical fingerprints of the HSE in late accreted materials may enable us to ascertain under what conditions and where in the solar system the late accreted materials formed. The ¹⁸⁷Os/¹⁸⁸Os ratios (reflecting long-term Re/Os), coupled with ratios of other HSE, can be diagnostic for identifying the nature of the impactor. A critical issue, however, will be deconvolving the exogenous from indigenous components. In collaboration with Dr. Odette James (USGS retired), a longstanding expert on these rocks, we studied multiple sub-pieces of Apollo 17 poikilitic melt rock 72395, and microbreccia (melt rock) subsamples from Apollo 14 breccia 14321. This year’s work built on the database we generated for Apollo 17 aphanitic melt rocks 73215 and 73255. All rocks were analyzed for Os isotopes and the following HSE: Pt, Pd, Ir, Ru, Re and Os.

Both poikilitic and aphanitic melt rocks were collected from the Apollo 17 site. Poikilitic melt rocks were found primarily at Stations 6 & 7 (North Massif) and HSE data for these rocks have been previously reported. They have a coarser grained melt fraction than the aphanitic rocks of this study. The melt fractions of both types of rock are similar in major- and minor element composition. Aphanitic melt rocks were found primarily at Stations 2 & 3 (South Massif). They have a very fine-grained melt fraction, thus crystallized rapidly. They vary widely in clast population, although on average they are richer in clasts than the poikilitic melt rocks. The Apollo 14 microbreccia consists of clasts of dark melt rocks, of igneous basalts, of vitrophyric impact melts, and of pristine rocks, all in a light-colored matrix of finely comminuted basalt.

Osmium isotopic analysis was accomplished via negative thermal ionization mass spectrometry. The concentrations of the other elements were determined by the isotope dilution method using a multi-collector inductively coupled plasma mass spectrometer. Analytical details were similar to those reported previously with some modifications. Subsamples of 50-100 mg from each rock were digested in Pyrex Carius Tubes at 270°C for 72-96 h. Blanks averaged (pg): Ru 1.6, Pd 24, Re 1.5, Os 2.8, Ir 0.6, and Pt 76. The accuracy of all new concentration data is ±0.5 relative % or better.

The ¹⁸⁷Os/¹⁸⁸Os ratios of the entire suite of twenty A17 aphanitic subsamples average 0.1299±0.0005. Thirteen A17 poikilitic subsamples average 0.1324±3, and six A14 microbreccia subsamples average 0.1341±10 (2σ). There are, thus, resolvable difference in average ¹⁸⁷Os/¹⁸⁸Os ratios between the A17 aphanitic rocks, the A17 poikilitic rocks, and the A14 microbreccias. Because of the relatively large blank contributions to Re, thus relatively large error for this element, concentrations of Re were also calculated from the Re/Os ratio required to evolve from a chondritic initial ¹⁸⁷Os/¹⁸⁸Os, at the time of the meteoritic component formation at 4.56 Ga, to the present isotopic composition. The concentrations of all HSE are generally similar to those measured previously. The A14 microbreccia samples show the largest range of HSE abundances.

In the A17 poikilitic rocks and A14 micro breccias, Ir is well correlated with all HSEs. In the A17 aphanitic rocks, there is a good correlation between Ir and Re, Os, and Ru, and, to a lesser extent, Pt (Fig. 1 – 3).

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From these correlations and assuming that the indigenous lunar component contains negligible Ir, it is calculated to contain (ng/g): Re 0.051±0.005, 0.024±0.005, 0.014±0.001, Ru 1.84±0.07, 0.87±0.09, 0.13±0.01 in 73215-73255, 72395, and 14321, respectively. The Pd content in the indigenous component cannot be estimated for the aphanitic rocks due to poor correlation between Ir and Pd, but is 2.89±0.09 and 0.37±0.01 for 72395 and 14321, respectively. Finally, Os and Pt content in the indigenous component are calculated to be close to zero for all three suites of samples. The relatively accurate determination of the Re content in the indigenous component permits calculation of the ¹⁸⁷Os/¹⁸⁸Os in the three suites of samples corrected for the indigenous contributions from this component to be 0.1268±4, 0.1314±3, and 0.1320±15 for the three suites of samples, respectively. In Fig. 4, ¹⁸⁷Os/¹⁸⁸Os ratios in the three suites of samples are plotted against Pd/Ir and Ru/Ir corrected for the indigenous components. The aphanitic rocks are most similar in these characteristics to carbonaceous chondrites, whereas the A17 poikilitic rocks and A14 micro breccias plot nearest the range for ordinary and enstatite chondrites.

Removal of the effects of indigenous contributions from the HSE of impact-melt rocks is critical to accurately fingerprinting the HSE of the impactors. In the A17 poikilitic rocks and 14321 subsamples, Ir shows good linear correlations with all other HSE, consistent with two-component mixing of a single indigenous component and a single meteoritic component. The new results for poikilitic rock 72395 show evidence for indigenous Pd and Ru, whereas there is no evidence of any indigenous HSE in the 14321 data. When corrected for indigenous components, the putative impactor HSE compositions differ from diagnostic characteristics of the main chondrite groups (Fig. 4), possibly implicating impactors with different nebular histories from anything currently in our sample collections.

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