Element distribution in size fractions of Apollo-16 soils: Evidence for element mobility during regolith processes |
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| Authors: | WV Boynton C-L Chou RW Bild PA Baedecker JT Wasson |
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| Institution: | Institute of Geophysics and Planetary Physics, Departments of Chemistry and Geophysics and Space Physics, University of California, Los Angeles, Calif. USA |
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| Abstract: | Three Apollo-16 soils, 61220, 63500 and 65500, having diverse properties were separated into six size fractions and analyzed for 8 volatiles and siderophiles. Relative concentrations of an additional 20 elements were determined in 61220 and 63500. The volatile elements Cd, Zn, In and Ga increase in concentration with decreasing grain size; in the finest fractions the increase is roughly parallel to the increase in specific surface area, and a surface correlation is inferred. The total increase from coarsest (177–500 μm) to finest (<5 μm) fraction is by factors of 10–20 for Zn and Cd in soils 61220 and 63500. Concentrations of elements that are good indicators of KREEP (Sm), mare basalts (Fe) and anorthosites (Ca) show nearly no dependence on size. The preservation of the observed surface correlation throughout regolith evolution suggests that the volatile elements are labile on the lunar surface.Concentration-size distributions of siderophiles show peaks in the 80–300 μm range for each soil, independent of whether they are dominantly extralunar (Ni, Ge, Au, Ir) or lunar (Co) in origin. If this peak results from agglutinate formation, a viable mechanism must allow for incorporation of the extralunar siderophiles. Alternatively, the peak may result from a continuous growth of metal grain size during the evolution of the regolith. |
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