Inefficient plagioclase segregation during Lunar Magma Ocean Crystallization can link the mantle sources for High-Alumina and High-Titanium basalts

The nature of the sources and processes involved in the formation of Lunar High-Al and High-Ti basalts are not completely understood. Through petrological experiments designed to study the effects of inefficient plagioclase flotation during late-stage Lunar Magma Ocean (LMO) crystallization, we were able to find a possible mechanism for the Al and Ti enrichment for both types of basalts. Retained plagioclase in late-stage ilmenite-bearing cumulates (IBCs) within the lunar mantle results in low-fraction (1-10%) partial melts that are highly enriched in Al and have a high affinity with the composition of Apollo 14 and Luna 16 high-Al basalts. Additionally, melting of Ti-rich phases ilmenite and ülvospinel, is prominent at higher partial melting fractions (20-60%) resulting in melts that can be enriched in over 20% TiO₂, with high affinity to high-Ti basalts and picritic glasses.

These results imply that both types of magmas may share the same mantle source but formed at different stages of fractional melting processes. The ages of both basalt types may be related to this fractional melting process, with high-Al basalts known to be the oldest Lunar volcanism and youngest known samples, overlapping with the oldest ages for high-Ti basalts at ~3.8 Ga. Our findings can explain multiple aspects of the major element composition of these basalts, but there are other aspects that need to be accounted for. The higher-than-expected Mg contents in both types of basalts, and the presence of olivine and orthopyroxene in the multiple saturation points of high-Ti basalts despite the absence of these minerals in IBCs, indicate that additional processes are involved in the formation of these basalts or even in the formation of mantle cumulates.

The inefficient flotation of plagioclase during LMO crystallization has been proposed as a possible explanation for the lower thickness of the lunar crust compared to experimental determinations [1]. Our findings provide further evidence that flotation inefficiency is a possibility, and its consequences may go beyond crustal thicknesses, but may also affect posterior mantle dynamics and the composition of lunar volcanism.

[1] Charlier, B. et al. (2018), Geochim. Cosmochim. Acta. 234, 50-69.