Trace Elements and Radioactivity in Lunar Rocks: Implications for Meteorite Infall, Solar-Wind Flux, and Formation Conditions of Moon

Abstract

Lunar soil and type C breccias are enriched 3-to 100-fold in Ir, Au, Zn, Cd, Ag, Br, Bi, and Tl, relative to type A, B rocks. Smaller enrichments were found for Co, Cu, Ga, Pd, Rb, and Cs. The solar wind at present intensity can account for only 3 percent of this enrichment; an upper limit to the average proton flux during the last 4.5 x 109 years thus is 8 x 10⁹ cm^-2 yr^-1. The remaining enrichment seems to be due to a 1.5 to 2 percent admixture of carbonaceous-chondritelike material, corresponding to an average influx rate of meteoritic and cometary matter of 2.9 x 10^-9 g cm^-2 yr^-1 at Tranquility Base. This is about one-quarter the terrestrial rate. Type A, B rocks are depleted 10-to 100-fold in Ag, Au, Zn, Cd, In, Tl, and Bi, relative to terrestrial basalts. This suggests loss by high-temperature volatilization, before or after accretion of the moon. Positron activities due mainly to ²²Na and ²⁶Al range from 90 to 220 β⁺ min^-1 kg^-1 in five small rocks or fragments (9 to 29 g). The higher activities presumably indicate surface locations. Th and U contents generally agree with those found by the preliminary examination team.