Osmium Isotope and Highly Siderophile Element Compositions of Lunar Orange and Green Glasses

Introduction: The absolute and relative abundances of the highly siderophile elements (HSE) present in planetary mantles are primarily controlled by: 1) silicate-metal partitioning during core-mantle differentiation, 2) the subsequent addition of HSE to mantles via continued planetary accretion [1-3]. Consequently, constraints on the absolute and relative abundances of the HSE in the lunar mantle will provide unique insights to the formation and late accretionary history of not only the Moon, but also Earth. Determining the HSE content of the lunar mantle, however , has proven difficult, because no bona fide mantle rocks have been collected from the moon. The only materials presently available for constraining mantle abundances are lunar volcanic rocks. Lunar basalts typically have very low concentrations of HSE and highly fractionated HSE patterns [4]. Because of our extremely limited understanding of mantle-melt partitioning of the HSE, even for terrestrial systems, extrapolations to mantle compositions from basaltic compositions are difficult, except possibly for the less compatible HSE Pt and Pd [5]. Primitive, presumably less fractionated materials, such as picritic glasses are potentially more diagnostic of the lunar interior [6]. Here we report Os isotopic composition data and Re, Os, Ir, Ru, Pt and Pd concentration data for green glass (15426,164) and orange glass (74001,1217). As with previous studies utilizing neutron activation analysis, we are examining different size fractions of the spherules to assess the role of surface condensation in the generation of the HSE abundances. Materials: The Apollo 15 green and Apollo 17 orange glasses represent the best and most voluminous examples of the sampled lunar pyroclastic deposits [e.g. 6]. The glasses from both sites are generally spherical in shape and are products of fire-fountaining on the lunar surface. The ca. 3.4 Ga [7] green glasses represent near-primary very low-TiO 2 ba-salts (≈0.2 to 0.7 wt.% TiO 2) with Mg# as high as 0.68. The ca. 3.6 Ga [8] orange glasses represent near-primary high-TiO 2 basalts (≈8.4 to 9.4 wt.% TiO 2) with Mg# as high as 0.54. High pressure experiments and extensive geochemical-isotopic data for both compositions have been interpreted as indicating that these magmas were derived through the melting of lunar magma ocean cumulate assemblages in the deep lunar interior (> 400 km) leaving behind a residuum of oli-vine and orthopyroxene [e.g. 6]. Analytical Methods: Glass samples were purified on a laminar flow bench using a binocular microscope, polystyrene petri dishes, sieves, and clean utensils. Because of …