Cosmogenic tungsten and the origin and earliest differentiation of the Moon

Abstract

The decay of formerly live 182Hf with a half-life of 9 Myr results in variations in the abundance of 182W in early solar system objects. Here we demonstrate that major excesses in 182W in some lunar samples are the results of cosmogenic additions. Apollo 17 high-Ti mare basalts yield high 182W/ 184W of up to ϵ w=+11±1. Even more extreme variations of up to ϵ w=+22±1 are found for mineral separates, although these lavas were erupted more than 500 Myr after the start of the solar system. The measured 182W excess in the separated minerals is correlated with their Ta/W, confirming theoretical models that implicate the 181Ta( n,γ) 182Ta(β −) 182W reaction from cosmic irradiation as the most likely cause. In contrast, olivine–basalt 15555, which has a low cosmic ray exposure age, displays no internal 182W variations and defines an ϵ w of +1.3±0.4. This is consistent with earlier conclusions that the Moon formed about 50 Myr after the start of the solar system. The high-Ti mare basalt source, with very high Hf/W, has a W isotopic composition that is not grossly different, from which a time limit of ∼70 Myr after the start of the solar system can be inferred for the formation of ilmenite-rich layers in the final stages of the lunar magma ocean.