Regional Heterogeneity of Kreep: Impact Melts from Apollo

Introduction: Impact melt breccias from Apollo 16 and 17 show subtle but resolvable differences in trace element composition between the two sites, possibly indicating regional heterogeneity in the KREEP component contributing to these breccias. The compositional differences are most apparent in the high-field strength elements (HFSE; e.g., Zr, Hf, Nb, Ta), which are fractionated in KREEP relative to the bulk Moon. Compatible trace elements such as Cr, V, and Sc further document compositional differences in target lithologies. These trace element signatures appear to reflect multiple impact events into compositionally distinct crustal terranes during the Nectarian (3.8-3.9 Ga) episode of intense bombardment. Samples and Methods: Samples of Apollo 16 impact melt breccias weighing approximately 3 g each were crushed to a fine powder using a steel piston and agate mortar and pestle. 30 mg aliquots of the powder were dissolved in distilled HF-HCL-HNO3, and analysed for trace element compositions by quadrupole ICP-MS using solution nebulization. Samples analysed for this study include 60315, 60335, 62235, 65055, 66095, 67235, 68415, and 68416. These samples span most of the range of bulk compositions observed in Apollo 16 impact melts, representing Groups 1 (60315, 62235), 2 (60335, 66095), and 3 (68415, 68416, 65055) of Korotev [1]. The results are compared with data from our lab for Apollo 17 poikilitic LKFM impact melts [2; excluding 77035] and noritic anorthositic rocks 62236 and 67215 [3] in order to minimize inter-laboratory bias. Replicate analyses of USGS rock standards analysed with the Apollo 16 breccias demonstrate a precision of <2% RSD (1σ, n = 3-6) for the REE and HFSE concentrations and trace element ratios considered here. Results: Incompatible lithophile element abundances in the Apollo 16 melt breccias span about an order of magnitude, e.g., Zr 90-960 ppm, Nb 6-56 ppm, Hf 2-19 ppm, Ta 0.3 – 2.2 ppm, Ce 17-160 ppm, and show excellent linear correlations among the REE and HFSE (Figs. 1, 2). Compared to the Apollo 16 impact melt suite, Apollo 17 poikilitic breccias have intermediate lithophile element abundances and a more restricted range of compositions (Figs. 1, 2).