A pristine low-Ti cumulate source for Chang’e 5 basalts revealed by Sr-Nd-Hf isotopes

The Chinese Chang’e 5 mission has returned the youngest mare basalts (∼2.0 Ga) identified thus far. However, their mantle source remains highly debated. Here, we present an integrated study of the mineralogy, chemical composition, Pb–Pb age, and whole rock Sr–Nd–Hf isotopic compositions of one Chang’e 5 (CE5) basalt clast. The indistinguishable age (1.96 ± 0.09 Ga), similar mineral chemical compositions and Sr–Nd isotopic compositions to those of previously reported CE5 basalts indicate that they had a common mantle source. After correction for cosmic ray effects, the whole rock εNd-εHf values plot exactly on the regression line defined by Apollo low-Ti basalts (R2 = 0.95), revealing their source affinity from a depleted mantle cumulate, consistent with their depleted Sr isotopic features. Combined with its deep negative Eu anomaly (Eu/Eu* of 0.44), the modelling calculations imply that the CE5 basalts were derived from a moderate-stage olivine(±orthopyroxene)+pigeonite+plagioclase cumulate with removal of most plagioclase. Furthermore, combining the CE5 basalts and Apollo 12 and 15 low-Ti mare basalts could be best interpreted as having this common, depleted pristine low-Ti cumulate source with variable TIRL (trapped instantaneous residual liquid). Our observations emphasise a pristine low-Ti cumulate source formed by the lunar magma ocean (LMO) for the CE5 basalts, leaving the melting mechanism a mystery.