In Situ Titanium Isotopic Compositions of Ilmenite in Chang’e-5 Basalts Reveal Contributions from Ilmenite-Bearing Cumulate

Abstract Ilmenite-bearing cumulate (IBC), formed during the late stages of lunar magma ocean (LMO) crystallization, is widely considered as the principal reservoir supplying titanium (Ti) to lunar basalts. Chang’e-5 (CE-5) basalts differ compositionally from Apollo and Luna samples and exhibit intermediate Ti contents, complicating evaluation of the ilmenite contribution to their mantle sources. Here, we present major and trace element compositions and in situ Ti isotope measurements of ilmenite from CE-5 basalts to constrain the extent of IBC involvement in their mantle regions. Petrogenetic modeling suggests that mixing of 86% per cent crystallized solid (PCS) and 95% PCS derived from the LMO in an ~8:2 ratio best reproduces the primary mantle source of the CE-5 basalts. Trace-element modeling of pyroxene further requires incorporation of IBC material into the source region to account for the observed basalt compositions. Ilmenite grains exhibit δ49Ti values ranging from −0.09 ± 0.06 to +0.23 ± 0.07‰. Ilmenites with higher Mg# generally exhibit heavier Ti isotopic compositions, whereas those with lower Mg# display greater variability in δ49Ti, providing direct evidence for differential involvement of IBC-derived components in the CE-5 mantle source and explaining the elevated Ti contents of these basalts. These Ti-isotope signatures further suggest stratification within the IBC layer, in which late-stage IBC material that sank into the deeper mantle possessed heavier isotopic compositions, whereas lighter Ti was introduced during subsequent magma evolution—potentially accounting for the heavier Ti-isotope signatures observed in Apollo high-Ti basalts. Residual IBC material that failed to sink may have been preserved in the upper mantle beneath the Procellarum KREEP Terrane, potentially sustaining prolonged basaltic magmatism on the lunar nearside and contributing to the relatively young eruption ages of the CE-5 basalts.