U-Th-Pb systematics in lunar highland samples from the Luna 20 and Apollo 16 missions

Abstract

Luna 20 and Apollo 16 soils which represent two highland sites contain Pb which has ²⁰⁶Pb/²⁰⁴Pb of from 53 to 277 and radiogenic ²⁰⁷Pb/²⁰⁶Pb of from 0.78 to 0.83. The soils yield very discordant ages which range from 4.91 up to 6.08 AE. A few of the results on soils can be discussed in terms of a simple two-stage model in which the lunar crust is generated by the addition of magmatic material enriched in Pb relative to both U and Th (without U-Th fractionation). The fractionation factor (Pb/U)_(magma)/(Pb/U)_(source) is commonly ∼ 2.5 or more. In no way can the soil model ages give a direct determination of the age of the Moon. The highland anorthosite 60025 gave a Pb concentration of 6.9 ppb and appears distinctly radiogenic with ²⁰⁶Pb/²⁰⁴Pb= 25.4. The ²⁰⁷Pb-²⁰⁶Pb age is 6.64 AE but the U concentration of 0.79 ppb is sufficient to account for the radiogenic Pb. This result is distinctly different from ancient and highly radiogenic initial Pb in 14310, 14053 and 15415. The extent to which various rock types of different ages and undetermined exotic materials contribute unsupported initial Pb to lunar soil is not yet established. The complexity of the Pb-U-Th data on the soils is remarkable in comparison with the simple behavior of the Rb-Sr results.