Reassessing the classification of Chang'e-5 basalts using pyroxene composition

Abstract

Determining the types of lunar mare basalts (i.e., very low-Ti, low-Ti, and high-Ti) is important for constraining the chemical composition of the lunar mantle and the possible dynamic processes during their formation. To date, bulk-rock analysis and inversion of Ti abundances in olivine and plagioclase indicate the Chang'e-5 (CE5) basalts as the low-Ti type. In contrast, the chemical composition of pyroxenes, which is also used to distinguish basalt types, shows an affinity to the high-Ti type as inferred from the Fe#-Ti# relationship. To investigate this discrepancy, we reassessed the classification of CE5 basalts by combining previously available pyroxene composition and the newly obtained data for CE5 basalts and a lunar basalt meteorite (NWA 4898). Our results demonstrate a consistent decrease in Cr2O3 content within pyroxenes from ∼0.8 wt% to nearly zero with magmatic differentiation, diverging from high-Ti basalts but resembling the Apollo low-Ti basalts. Approximately half of the CE5 pyroxenes with Mg# < 45 have lower Cr2O3 content compared to both the Apollo low- and high-Ti basalts. In contrast, TiO2 content in CE5 pyroxenes is higher than in most low-Ti basalts, but similar to high-Ti basalts, especially at Mg# between 60 and 40. However, such a high Ti signature is unlikely originated from a high-Ti source but more likely evolved from a low-Ti source wherein the Ti content in the melt significantly increased before eruption. This interpretation is supported by the increase-decrease of TiO2 trend in pyroxene grains from lunar meteorites NWA 4898, NWA 4734, and LAP group. Thus, the relatively lower Cr2O3 content in pyroxenes at Mg# < 45 and the relatively higher TiO2 content in most pyroxenes ultimately resulted in high Ti# values, but the CE5 pyroxenes still record magmatic differentiation from a low-Ti melt.