Su Bin, Zhang Di, Chen Yi, Yang Wei, Mao Qian, Li Xian-Hua, Wu Fu-Yuan
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China.
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China.
Sci Bull (Beijing). 2023 Sep 15;68(17):1918-1927. doi: 10.1016/j.scib.2023.07.020. Epub 2023 Jul 15.
Mare basalts returned by the Chang'E-5 (CE5) mission extend the duration of lunar volcanism almost one billion years longer than previously dated. Recent studies demonstrated that the young volcanism was related neither to radiogenic heating nor to hydrous melting. These findings beg the question of how the young lunar volcanism happened. Here we perform high-precision minor element analyses of olivine in the CE5 basalts, focusing on Ni and Co. Our results reveal that the CE5 basalt olivines have overall lower Ni and Co than those in the Apollo low-Ti basalts. The distinctive olivine chemistry with recently reported bulk-rock chemistry carries evidence for more late-stage clinopyroxene-ilmenite cumulates of the lunar magma ocean (LMO) in the CE5 mantle source. The involvement of these Fe-rich cumulates could lower the mantle melting temperature and produce low MgO magma, inhibiting Ni and Co partitioning into the magma during lunar mantle melting and forming low Ni and Co olivines for the CE5 basalts. Moreover, the CE5 olivines show a continuous decrease of Ni and Co with crystallization proceeding. Fractional crystallization modeling indicates that Co decreasing with crystallization resulted from CaO and TiO enrichment (with MgO and SiO depletion) in the CE5 primary magma. This further supports the significant contribution of late-stage LMO cumulates to the CE5 volcanic formation. We suggest that adding easily melted LMO components resulting in mantle melting point depression is a key pathway for driving prolonged lunar volcanism. This study highlights the usefulness of olivine for investigating magmatic processes on the Moon.
嫦娥五号（CE5）任务带回的月海玄武岩将月球火山活动的持续时间延长了近10亿年，比之前测定的时间长得多。最近的研究表明，年轻的火山活动既与放射性加热无关，也与含水熔融无关。这些发现引发了关于年轻的月球火山活动是如何发生的问题。在这里，我们对CE5玄武岩中的橄榄石进行了高精度微量元素分析，重点关注镍和钴。我们的结果表明，CE5玄武岩橄榄石的镍和钴含量总体上低于阿波罗低钛玄武岩中的橄榄石。独特的橄榄石化学性质与最近报道的全岩化学性质为CE5地幔源中月球岩浆海洋（LMO）更多的晚期单斜辉石-钛铁矿堆积物提供了证据。这些富铁堆积物的参与可能会降低地幔熔融温度并产生低氧化镁岩浆，在月球地幔熔融过程中抑制镍和钴向岩浆中的分配，并为CE5玄武岩形成低镍和低钴的橄榄石。此外，CE5橄榄石随着结晶过程镍和钴含量持续下降。分离结晶模拟表明，CE5原始岩浆中氧化钙和二氧化钛的富集（伴随着氧化镁和二氧化硅的亏损）导致了钴随着结晶过程而减少。这进一步支持了晚期LMO堆积物对CE5火山形成的重大贡献。我们认为，添加易于熔融的LMO成分导致地幔熔点降低是驱动月球长期火山活动的关键途径。这项研究突出了橄榄石在研究月球岩浆过程中的作用。
