Low abundances of highly siderophile elements in the lunar mantle: evidence for prolonged late accretion

摘要

Abundances of the highly siderophile elements (HSE) Re, Ir, Os, Ru, Pt and Pd were determined for etchates and residues of highly purified lunar orange (74220 and 74001,1217 and 1218) and green (15421,65 and 15426,164) volcanic glasses. Most etchates have >100 pg/g Os and Ir, and in some instances have HSE concentrations that approach chondritic relative abundances. Some etchates also have ~(187)Os/~(188)Os within the range of chondritic meteorites. In contrast, most residues have Os concentrations of <50 pg/g and have considerably suprachondritic ~(187)Os/~(188)Os ratios. The disparities in Os isotopic compositions between etchates and residues reflect the presence of at least two components in each of the glass fractions. The components are interpreted to be the indigenous lunar glasses and a meteoritic contaminant. Mixing models suggest the indigenous components in orange and green glasses have Os concentrations ≤25 pg/g. Compared to terrestrial rocks with comparable MgO contents, the concentration estimates suggest the lunar mantle sources of the volcanic glasses were depleted in HSE by at least a factor of 20 relative to the terrestrial mantle, consistent with Os and Ir concentrations in the lunar mantle of ≤0.15 ng/g. The HSE depletion in the lunar mantle indicates that it may be missing a substantial share of late accreted materials. The depletion is best explained as a consequence of the early formation of the lunar crust and subsequent isolation of the lunar mantle from late accreted materials. Given that the lunar crust was largely formed by 4.4 Ga, this suggests a preponderance of late accreted materials were added to the Earth–Moon system subsequent to 4.4 Ga, and requires a substantial influx of mass into the inner solar system more than 150 Ma after the formation of the solar system.