¹Jintuan Wang, ²Hongluo L. Zhang, ¹Le Zhang, ¹Yonghua Cao, ²Zhendun Qi, ¹Pengli He, ¹Mang Lin,¹Yi-Gang Xu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2026.01.046]
¹State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
²State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
Copyright Elsevier

Lunar basalts are much more reduced than their terrestrial counterparts and exhibit more than three orders of magnitude variability in oxygen fugacity (fO2). However, the origin of this large fO2 variation remains enigmatic. The Chang’e-5 (CE-5) basalt, derived from a pyroxene-bearing mantle source, provides a unprecedented opportunity to decipher the redox variation of lunar samples. The mineral/melt partitioning behaviors of vanadium (V) and europium (Eu) are sensitive to fO2, and thus capable of evaluating the fO2 of rocks. However, previous oxybarometers based on the partitioning behaviours of V and Eu are not applicable to CE-5 basalt due to the difference in composition and formation P−T conditions. Here we performed experiments at 1120−1140 °C and fO2 range of IW −1.2 to IW+3.3 (IW, iron-wüstite buffer) on a synthesized CE-5 whole rock (WR) composition and calibrated oxybarometers (olivine/melt and spinel/melt V and plagioclase/melt Eu partitioning) pertinent to the CE-5 lunar basalt. Applying the calibrated oxybarometers to CE-5 basalt, we estimated the fO2 of the CE-5 basalt to be
, which is generally more oxidized than most lunar basalts. To further investigate the cause for the high fO2 of CE-5 basalt, we performed crystallization modeling of the lunar magma ocean. The results reveal that the lunar mantle is stratified with Fe3+/FeT and fO2, with the shallower regions being more oxidized, suggesting that the oxidized nature of CE-5 basalt likely induced by the involvement of oxidized shallow mantle reservoirs. Moreover, the results found a unified framework to explain the large fO2 variation in lunar samples.