低氧逸度结晶分异对上部月幔矿物组成的影响  

Influence of low oxygen fugacity on the mineral composition of crystallization differentiation in the lunar upper mantle

作　　者：吕霖 张驰[1] LÜ Lin;ZHANG Chi(Key Laboratory of Earth and Planetary Physics,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院地球与行星物理重点实验室,中国科学院地质与地球物理研究所,北京100029 [2]中国科学院大学,北京100049

出　　处：《地球物理学进展》2022年第2期530-540,共11页Progress in Geophysics

基　　金：国家自然科学基金(41490633);中国科学院战略性先导科技专项(XDB 41000000)联合资助.

摘　　要：氧逸度是岩浆结晶分异的重要影响因素.低氧逸度会影响岩浆的结晶序列和矿物组成.月球样品指示月球具有比地球和火星更低的氧逸度环境(较铁-方铁矿氧化还原缓冲对低一个对数单位,大约IW-1).现有的月球岩浆结晶分异实验多未考虑低氧逸度对结晶分异的影响,实验中普遍使用石墨样品仓,氧逸度条件约为IW+1.5,显著高于月球的真实氧逸度值,从而影响了月球内部矿物模型的建立和月球探测任务的数据解释.本研究尝试利用活塞圆筒设备,通过改变样品仓的组成,在0.8 GPa、1280℃的条件下,模拟低氧逸度下的上部月幔结晶分异过程.其中,样品仓的选择可以显著改变样品的氧逸度,实验中Pt-Fe双层样品仓可降低氧逸度至约IW,Fe样品仓可降至IW-1.1,BN样品仓可降至IW-2.4,质量百分数为4%的Si粉可降至IW-4.8,但均会对样品成分造成一定的影响.实验结果显示氧逸度对上部月幔熔体结晶的矿物组合和比例有显著的影响.随着氧逸度的降低,结晶矿物中橄榄石的比例降低甚至消失,斜方辉石的比例大幅增加.这一结果与月球遥感数据和嫦娥四号对南极艾特肯盆地的探测结果相吻合,为认识上部月幔组成提供了新的参考依据.Oxygen fugacity plays an important role in the mineral composition of crystallization differentiation in magma.It affects the results of crystallization differentiation including the crystallization sequence and the proportion of minerals.The lunar samples indicate that the moon is crystallized under lower oxygen fugacity condition,which is about 1 log unit below Iron-Wüstite redox buffer(about IW-1),than the Earth and Mars.But few high-temperature highpressure experiments of the lunar magma ocean considered the effect of its low oxygen fugacity,which is significantly lower than the widely used graphite capsule(about IW+1.5)in these experiments.The difference in oxygen fugacity hinders the development of accurate model of lunar interior and the explanation of the results from recent lunar exploration missions.In this study,a series of experiments were carried out at 0.8 GPa and 1280℃,corresponding to about 250 km depth below the lunar surface,in piston-cylinder apparatus to simulate the crystallization differentiation of the lunar upper mantle under reducing conditions.These experiments provide better constraints on the mineral composition of the formation of lunar upper mantle and the South-Pole Aitken basin,which remelts after large impact event and provides unique chance to probe the composition of the lunar interior.Graphite capsule,Fe capsule,Pt-Fe capsule,BN capsule and Si powders in-graphite capsule were used to change the oxygen fugacity of the experiments.Results show that Pt-Fe double capsule,Fe capsule,BN capsule or adding 4%by mass Si powder in graphite capsule change the oxygen fugacity inside the sample to IW,IW-1.1,IW-2.4 and IW-4.8,respectively.Despite the side effect of changing chemical composition,the crystalized mineral assemblages change significantly with oxygen fugacity.The olivine fraction in the crystallized minerals decreases with decreasing oxygen fugacity and it even disappear under extremely reduced environment,meanwhile the relative ratio of orthopyroxene increases.Our results are

关 键 词：低氧逸度 月球岩浆洋 斜方辉石 月幔 南极艾特肯盆地 

分 类 号：P315[天文地球—地震学] P581[天文地球—固体地球物理学]