Magnesium stable isotopes support the lunar magma ocean cumulate remelting model for mare basalts

Abstract

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 (δ25Mg = −0.140 ± 0.010‰, δ26Mg = −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 δ26Mg 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 δ26Mg values during the LMO differentiation are consistent with the observed δ26Mg variations in lunar samples, implying that Mg isotope variations in lunar basalts are consistent with their origin by remelting of distinct LMO cumulates.

A reassessment of the iron isotope composition of the Moon and its implications for the accretion and differentiation of terrestrial planets

“…This was also reported to bias the oxygen isotope compositions of basalts 75075 and 15555 (Spicuzza et al, 2007;Liu et al, 2010), and, more recently, the silicon isotope composition of ferroan anorthosites 65035 and 60015 (Poitrasson and Zambardi, 2015), when powders were produced from rock chips weighing less than ~100 mg (Armytage et al, 2012). Moreover, for basalt 15555, an anomalously light Mg isotope signature reported recently (Sedaghatpour and Jacobsen, 2019), combined with significant Fe-Mg inter-mineral diffusion affecting Mg isotope systematics in olivine crystals of 15555 (Richter et al, 2016), provide an additional clue for possible sample Fe isotope heterogeneity issue (see Table 1).…”

“…This partly results from analytical difficulties to determine accurate 26 Mg values of silicate rock samples. Furthermore, lunar samples appear to yield heterogeneous 26 Mg values, with high-Ti basalts tending towards lighter isotopic compositions according to Sedaghatpour et al (2013) and Sedaghatpour and Jacobsen (2019) whereas Wiechert and Halliday (2007) found the opposite. Given these controversies, it is still difficult at this point to use Mg isotope compositions to discuss planet formation and differentiation processes.…”

“…As a result, Liu et al (2010) concluded that: 1) partial melting in the lunar mantle probably did not produce Fe isotope fractionation, and 2) Fe isotope compositions of low-Ti and high-Ti mare basalts directly mirror those of their corresponding lunar mantle sources. These conclusions have recently been challenged by Sedaghatpour and Jacobsen (2019) on the basis of a fractional crystallization model. However, these models are frequently underconstrained, with many of the unknown parameters adjusted to fit the observed bulk-rock analyses (see review by Dauphas et al, 2017).…”

“…There is no consensus yet as to whether the majority of terrestrial bodies have a chondritic 26 Mg signature (see recent review by Teng, 2017, and see also Sedaghatpour and Jacobsen, 2019), or if the Earth is isotopically heavier (Wiechert and Halliday, 2007;Hin et al, 2017). This partly results from analytical difficulties to determine accurate 26 Mg values of silicate rock samples.…”

Mare basalt meteorites, magnesian-suite rocks and KREEP reveal loss of zinc during and after lunar formation

“…Lunar materials from the Apollo missions and as meteorites have been shown to form from the same silicate reservoirs based on stable isotopes of refractory major rock-forming elements (e.g., O, Mg, Si, Fe; Spicuzza et al, 2007: Liu et al, 2010Armytage et al, 2012;Sossi and Moynier, 2017;Sedaghatpour and Jacobsen, 2019). For moderately volatile elements like zinc, the isotopic compositions of lunar crustal samples reflect effects of evaporation, condensation, mixing and contamination at the lunar surface (Moynier et al, 2006;Herzog et al, 2009;Paniello et al, 2012a;Kato et al, 2015;Day et al, 2017a), as well as the Zn-bearing phases present in samples.…”