Multistage exposure history of the 74261 soil constituents

Abstract

Gray soil 74261 was collected on the rim of Shorty crater in the Taurus Littrow valley. This soil is of particular interest because it may represent old lunar regolith material The cosmic ray exposure history and the isotopic composition of the trapped noble gases of clinopyroxene, plagioclase, basalt, ropy glass, and orange glass separates from the 74261 soil were investigated. Grain size separates from bulk soil were studied in order to determine the isotopic composition of the trapped noble gases. All constituents of this soil experienced an exposure to cosmic rays prior to the excavation by the Shorty crater formation 17 m.y. ago. Except for the orange glasses, all soil constituents were preexposed for about 370 m.y. at an average shielding depth of 75 g/cm2. The orange glasses experienced a preexposure for 90 m.y. at about 60 g/cm2 depth. The time when the preexposure occurred was riot determined because the fission Xe component could not be derived with sufficient precision for 235U‐136Xe dating. The relatively high (40Ar/36Ar)11 ratio of 5.8 to 8.3 indicates, however, that the preexposure to volatiles in the lunar atmosphere and to solar and cosmic radiations occurred early in the history cf the moon, perhaps about 3 aeons ago. In such a scenario the 74261 soil was buried after the preexposure period beneath some shielding material until it was reexcavated by the Shorty crater impact.