Uranium-thorium-lead isotopes in some Tranquillity Base samples and their implications for lunar history

Abstract

CT-Th-Pb isotopic studies of four rocks, a breccia, and lunar fines yield: (1) U (ppm) in 4 rocks average 0·53; breccia, 0·66; fines, 0·55. (2) Th (ppm) in rocks average 1·90; breccia, 2·02; fines, 2·17. (3) Pb (ppm) in rocks average 1-11; breccia, 1·86; fines, 1·46. (4) Th/U for rocks is 3·6; breccia, 3·05; fines, 3·9. U²³⁸/Pb²⁰⁴ ranges from 65 to 912, and implies extreme Pb²⁰⁴ depletion on the moon compared to earth and meteorites. All leads are very radiogenic, 75 per cent to more than 95 per cent, and well-defined radiogenic daughter-parent relations exist. Apparent Pb²⁰⁷ /Pb²⁰⁶ ages for all four rocks range from 4·13-4·22 billion years and Pb-U ages are nearly concordant. U-Th-Pb systematics in the breccia and fines yield nearly concordant apparent "ages" at 4·60-4·63 billion years. These ages have no real time significance but indicate the presence of old radiogenic lead components in the composite debris. Acid leaching studies have demonstrated that the leads in the fines and breccia are mixtures of extraordinarily heterogeneous components having Pb²⁰⁷ /Pb²⁰⁶ ratios which vary by more than 50 per cent. Two ancient components, tentatively identified, are (1) old radiogenic rock leads with associated parents and (2) old parentless radiogenic leads. Model calculations which remove lead isotope components of Tranquillity Base volcanic age from the regolith, indicate an excess of radiogenic lead with a composite Pb²⁰⁷ /Pb²⁰⁶ ratio of l ·96 ± 0· 14. This yields an apparent age of 4·95 ± 0· 10 billion years which would be a minimum age for the oldest debris in the regolith and, presumably, for the moon. Volatile transfer is believed responsible for the parentless leads and is suggested as a major lunar geological process.

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