Cooling rate of clinopyroxene reveals the thickness and effusion volume of Chang'E-5 basaltic flow units

Abstract

China's Chang'E-5 mission landed in the northern Oceanus Procellarum and sampled the youngest mare basaltic regolith (1.9–2.0 Ga) to date. The returned samples enable us to gain new insights into the thickness and eruption volume of the basaltic flow unit, i.e., Em4, the fourth Eratosthenian-aged mare unit at the landing region. In this study, 1D bottom-up and top-down cooling and top-down-only cooling thermophysical models have been constructed for the emplacement of the Chang'E-5 flow unit in order to constrain its minimum thickness through the cooling rates. Based on the zoning profiles of Mg–Fe diffusion in clinopyroxene, the cooling rate of the coarsest-grained sample is constrained to be 0.000025–0.00055 K/h, corresponding to a flow unit with a minimum vertical dimension of 21–118 m. The minimum effusion volume of a single flow unit at the Em4 area is calculated to be 777–3626 km3, which indicates that the Em4 region where Chang'E-5 landed is mainly formed in one major effusion event. The large volume of the Chang'E-5 mare flow suggests that the mantle source of Chang'E-5 basalts in the Procellarum KREEP Terrane was warm enough to produce significant amounts of melt.