Computer Science
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....7064h&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #7064
Computer Science
Scientific paper
The absolute and relative abundances of the highly siderophile elements (HSE) present in planetary mantles are primarily controlled by silicate-metal partitioning during core-mantle differentiation and by later planetary accretion. Constraints on HSE distributions in the lunar mantle will provide insights to the formation and late accretionary history of not only the Moon, but also Earth. Because no samples of the lunar mantle have been collected, the only materials presently available for constraining mantle abundances are lunar volcanic rocks. The Apollo 15 green and Apollo 17 orange glasses are spherules derived from fire-fountain eruptions of mare basalts. These glasses are more primitive than the mare basalts, and probably were derived by the melting of cumulate assemblages from the lunar magma ocean in the deep interior (> 400 km). Despite our limited understanding of mantle-melt partitioning of the HSE, even for terrestrial systems, these glasses provide the best hope for constraining the HSE composition of the lunar interior. We report Os isotopic composition data and Re, Os, Ir, Ru, Pt and Pd isotope dilution concentration data for green glass (15426,164) and orange glass (74001,1217). Absolute and relative abundances of HSE are variable. The green glass has elemental ratios of the HSE and an Os isotopic composition within the range defined by chondritic meteorites, suggesting that the HSE were dominated by a meteoritic contaminant. This sample probably provides little information about the lunar interior, but may help constrain the types of meteorites impacting the lunar surface. Two size fractions (180-325 mesh and 75-180 mesh) of the orange glass were analyzed. HSE patterns for both size fractions are fractionated, having higher chondrite-normalized abundances of Pt and Pd, compared with Os and Ir. The supra-chondritic 187Os/188Os ratio (0.1339) of the finer fraction indicates modestly fractionated Re/Os relative to chondritic. The coarser size fraction has greater depletions in Os and Ir, and more radiogenic 187Os/188Os, indicating substantially fractionated Re/Os since its formation. These features of the coarser orange glass most likely reflect a signature dominantly from the deep lunar mantle. Abundances of Pt and Pd of both size fractions of orange glass are factors of 3-10 less than terrestrial rocks with similar MgO and may indi-cate somewhat lower abundances of HSE in the lunar mantle relative to the terrestrial mantle.
Horan Mary
Papike James J.
Shearer C.
Walker Raymond
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