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

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. Approxi- mately 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 signifi- cantly 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 TiO(2 )content in most pyroxenes ultimately resulted in high Ti# values, but the CE5 pyroxenes still record magmatic differentiation from a low-Ti melt.

Automated summary

Determining the types of lunar mare basalts is crucial for understanding the chemical composition of the lunar mantle and the processes involved in their formation. Previous analyses classified the Chang'e-5 basalts as low-Ti type based on bulk-rock analysis and Ti abundances in olivine and plagioclase, while pyroxene composition suggested an affinity to the high-Ti type. However, a reassessment combining pyroxene composition and newly obtained data demonstrated that the CE5 basalts are a result of magmatic differentiation from a low-Ti source, with relatively lower Cr2O3 content and higher TiO2 content compared to both low-Ti and high-Ti basalts.