Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138.
Proc Natl Acad Sci U S A. 2019 Jan 2;116(1):73-78. doi: 10.1073/pnas.1811377115. Epub 2018 Dec 17.
We report high-precision Mg isotopic analyses of different types of lunar samples including two pristine Mg-suite rocks (72415 and 76535), basalts, anorthosites, breccias, mineral separates, and lunar meteorites. The Mg isotopic composition of the dunite 72415 (δMg = -0.140 ± 0.010‰, δMg = -0.291 ± 0.018‰), the most Mg-rich and possibly the oldest lunar sample, may provide the best estimate of the Mg isotopic composition of the bulk silicate Moon (BSM). This δMg value of the Moon is similar to those of the Earth and chondrites and reflects both the relative homogeneity of Mg isotopes in the solar system and the lack of Mg isotope fractionation by the Moon-forming giant impact. In contrast to the behavior of Mg isotopes in terrestrial basalts and mantle rocks, Mg isotopic data on lunar samples show isotopic variations among the basalts and pristine anorthositic rocks reflecting isotopic fractionation during the early lunar magma ocean (LMO) differentiation. Calculated evolutions of δMg values during the LMO differentiation are consistent with the observed δMg variations in lunar samples, implying that Mg isotope variations in lunar basalts are consistent with their origin by remelting of distinct LMO cumulates.
我们报告了不同类型的月球样本的高精度镁同位素分析结果，包括两种原始镁质岩石（72415 和 76535）、玄武岩、斜长岩、角砾岩、矿物分离物和月球陨石。最富镁的、可能也是最古老的月球样本辉长岩 72415 的镁同位素组成（δMg = -0.140 ± 0.010‰，δMg = -0.291 ± 0.018‰），可能提供了月球整体硅酸盐（BSM）镁同位素组成的最佳估计值。月球的这个 δMg 值与地球和球粒陨石的相似，反映了太阳系中镁同位素的相对均一性，以及月球形成的巨撞击过程中缺乏镁同位素分馏。与地球玄武岩和地幔岩石中镁同位素的行为不同，月球样本的镁同位素数据显示出玄武岩和原始斜长岩之间的同位素分馏，反映了早期月球岩浆海洋（LMO）分化过程中的同位素分馏。在 LMO 分化过程中计算出的 δMg 值演化与月球样本中观察到的 δMg 变化一致，这意味着月球玄武岩中的镁同位素变化与其通过不同的 LMO 堆积物再熔化的起源一致。
