In this study, we present new Ga isotope data from a suite of 28 mare basalts and lunar highland rocks. The delta Ga-71 values of these samples range from -0.10 to +0.66 parts per thousand (where delta Ga-71 is the relative difference between the Ga-71/Ga-69 ratio of a sample and the Ga-IPGP standard), which is an order of magnitude more heterogeneous than delta Ga-71 values in terrestrial magmatic rocks. The cause of this isotopic heterogeneity must be established to estimate the bulk delta Ga-71 value of the Moon. In general, low-Ti basalts and ferroan anorthosite suite (FAS) rocks have delta Ga-71 values that are lower than high-Ti basalts and KREEP-rich rocks. The observation that rocks derived from later forming LMO cumulates have higher delta Ga-71 values suggests that Ga isotopes are fractionated by processes that operate within the chemically evolving LMO, rather than localized degassing or volatile redistribution. Correlations between indices of plagioclase removal from the LMO (e.g. Eu/Eu*) with Ga isotope ratios suggest that a Delta Ga-71(Plagioclase-melt) of -0.3 parts per thousand, (where Delta Ga-71(plagioclase-melt) is the isotopic fractionation associated with crystallization of plagioclase from a melt), could drive the observed isotopic fractionation in high-Ti mare basalts and KREEP-rich rocks. This would be consistent with the observation that FAS rocks have delta Ga-71 values that are lower than mare basalts. However, the addition of KREEP-like material into the mare basalt source regions would not contribute enough Ga to perturb the isotopic composition outside of analytical uncertainty. Thus, basalts derived from early formed LMO cumulates such as those from Apollo 15, would preserve light Ga isotopic compositions despite containing modest amounts of urKREEP. We estimate that the delta Ga-71 value of the LMO was similar to 0.14 parts per thousand prior to the onset of plagioclase crystallization and extraction. Whether this delta Ga-71 value is representative of the initial BSM cannot be ascertained from the current dataset. It remains plausible that the Moon accreted with a heavier Ga isotopic composition than the Earth. Alternatively, the Moon and Earth could have accreted with similar isotopic compositions (BSE = 0.00 +/- 0.06 parts per thousand, Kato et al., 2017) and volatile loss drove the LMO to higher delta Ga-71 values prior to formation of the lunar crust. (C) 2021 Elsevier B.V. All rights reserved.

Rocks from the lunar interior are depleted in moderately volatile elements (MVEs) compared to terrestrial rocks. Most MVEs are also enriched in their heavier isotopes compared to those in terrestrial rocks. Such elemental depletion and heavy isotope enrichments have been attributed to liquid-vapor exchange and vapor loss from the protolunar disk, incomplete accretion of MVEs during condensation of the Moon, and degassing of MVEs during lunar magma ocean crystallization. New Monte Carlo simulation results suggest that the lunar MVE depletion is consistent with evaporative loss at 1,670 +/- 129 K and an oxygen fugacity +2.3 +/- 2.1 log units above the fayalite-magnetite-quartz buffer. Here, we propose that these chemical and isotopic features could have resulted from the formation of the putative Procellarum basin early in the Moon's history, during which nearside magma ocean melts would have been exposed at the surface, allowing equilibration with any primitive atmosphere together with MVE loss and isotopic fractionation.

Isotopic compositions of reservoirs in the Moon can be constrained from analysis of rocks generated during lunar magmatic differentiation. Mare basalts sample the largest lunar mantle volume, from olivine- and pyroxene-rich cumulates, whereas ferroan anorthosites and magnesian-suite rocks represent early crustal materials. Incompatible element enriched rocks, known as 'KREEP,' probably preserve evidence for the last highly differentiated melts. Here we show that mare basalts, including Apollo samples and meteorites, have remarkably consistent delta Zn-66 values (+1.4 +/- 0.2 parts per thousand) and Zn abundances (1.5 +/- 0.4 ppm). Analyses of magnesian-suite rocks show them to be characterized by even heavier delta Zn-66 values (2.5 to 9.3 parts per thousand) and low Zn concentrations. KREEP-rich impact melt breccia Sayh al Uhaymir 169 has a nearly identical Zn composition to mare basalts (delta Zn-66 =1.3 parts per thousand) and a low Zn abundance (0.5 ppm). Much of this variation can be explained through progressive depletion of Zn and preferential loss of the light isotopes in response to evaporative fractionation processes during a lunar magma ocean. Samples with isotopically light Zn can be explained by either direct condensation or mixing and contamination processes at the lunar surface. The delta Zn-66 of Sayh al Uhaymir 169 is probably compromised by mixing processes of KREEP with mafic components. Correlations of Zn with Cl isotopes suggest that the urKREEP reservoir should be isotopically heavy with respect to Zn, like magnesian-suite rocks. Current models to explain how and when Zn and other volatile elements were lost from the Moon include nebular processes, prior to lunar formation, and planetary processes, either during giant impact, or magmatic differentiation. Our results provide unambiguous evidence for the latter process. Notwithstanding, with the currently available volatile stable isotope datasets, it is difficult to discount if the Moon lost its volatiles relative to Earth either during giant impact or exclusively from later magmatic differentiation. If the Moon did begin initially volatile-depleted, then the mare basalt delta Zn-66 value likely preserves the signature, and the Moon lost 96% of its Zn inventory relative to Earth and was also characterized by isotopically heavy Cl (delta Cl-37 = >= 8 parts per thousand). Alternative loss mechanisms, including erosive impact removing a steam atmosphere need to be examined in detail, but nebular processes of volatile loss do not appear necessary to explain lunar and terrestrial volatile inventories. (C) 2019 Elsevier B.V. All rights reserved.

月海玄武岩主要产于月球近边的盆地中,覆盖面积为月球表面的1%,其形成年龄多在39～31亿年之间,是各类月岩中最年轻的。与地球玄武岩相似,月海玄武岩由斜长石、辉石和橄榄石组成,但它们比地球玄武岩具有更低的Mg#、Al2O3、K和Na含量,高的FeO含量(大于16%)和变化范围大的TiO2含量(小于1%到大于13%)。根据TiO2含量的变化,月海玄武岩分成高Ti(>6%),低Ti(1.5%<6%)以及极低Ti(<1.5%)三类。所有月海玄武岩都具有Eu负异常,并亏损挥发性元素和亲铁元素。月海玄武岩的同位素特征指示其至少为三个组分混合的产物:(1)高238U/204Pb、高87Sr/86Sr和负εNd组分,可能是岩浆海分异的残余岩浆即KREEP;(2)低238U/204Pb、低87Sr/86Sr和正εNd组分,来源于原始月幔,其熔融产物为低Ti玄武岩;(3)中等87Sr/86Sr和εNd组分,位于月幔的顶部,经历了岩浆海(洋)过程中形成的堆晶物质的再熔融,还可能受到了陨击事件的影响,其熔融产物是高Ti玄武岩。月海玄武岩的元素和同位素地球化学性质支持岩浆海的假...

月球KREEP岩石形成于壳幔的分界层中,天然放射性元素Th是指示KREEP岩分布的主要元素之一。在月球正面,Th元素含量高的区域主要分布在西部的月海区,包括雨海和风暴洋及其附近地区,而在月球背面Th含量高的地区为雨海对峙区以及南极爱特肯地区。高地的Th含量相对较低。现代月球表面Th的分布特征与3.85Ga年前的雨海事件有重要的关系,且在月球正面还发现了椭圆形的高Th含量区域,是月球上特殊的地球化学省,与月球初期的化学成分分布的不均一性有关。