Multiple reservoirs of volatiles in the Moon revealed by the isotopic composition of chlorine in lunar basalts
Multiple reservoirs of volatiles in the Moon revealed by the isotopic composition of chlorine in lunar basalts
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DOI：
10.1016/j.gca.2018.12.032
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发表时间：
2019-12-01
影响因子：
5
通讯作者：
Anand, Mahesh
中科院分区：
文献类型：
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作者：
Barnes, Jessica J.;Franchi, Ian A.;Anand, Mahesh
与大多数其他太阳系物质相比，月球样品中氯的同位素(氯-37和氯-35)的分馏程度很高。最近，月球岩石的氯同位素特征被归因于岩浆海洋存在期间发生的大规模脱气过程。在这项研究中，我们调查了一套月球玄武岩，其中大多数以前没有被分析过，与以前的模型符合得如何。所记录的氯同位素组成(增量氯-37(千分之几)=[(氯-37/氯-35(样品)/氯-37/氯-35(最低氯含量))-1]x1000，其中氯-37/氯-35(最低氯含量)指的是标准平均海洋氯化物)，其范围类似于+7至+14(阿波罗15)、+10至+19(阿波罗12)、+9至+15(70017)、+4至+8(最低氯含量05035)和+15至+22(卡拉哈里009)。本研究的氯同位素数据支持Boyce等人以前报告的混合趋势。(2015)和Barnes等人。(2016)，低钛玄武岩(包括低钛玄武岩和克里普玄武岩)中磷灰石的氯同位素组成与块体岩石不相容的微量元素丰度呈正相关。这一趋势被解释为月球岩浆海洋的urKREEP残液而不是地幔堆积物中集中了不相容的微量元素，包括Cl在内的证据，以及urKREEP Cl具有高度分馏的同位素组成。因此，玄武岩的源区是由地幔(贫氯和相对不分馏的)和urKREEP之间的可变混合形成的。高钛玄武岩的氯同位素测定结果具有较大的变异性，且散布在低钛玄武岩形成的趋势附近。关于月球陨石的大部分数据也符合其来源中挥发分的混合，但Kalahari 009高度贫化不相容的微量元素，其含有严重分馏的氯同位素组成的磷灰石。鉴于Kalahari 009是最古老的月球玄武岩之一，应该来自非常早期形成的地幔堆积体，重氯同位素特征很可能与其地幔来源无关，而更有可能与岩浆或次生蚀变作用有关，可能是通过撞击驱动的月壳蒸汽交代作用。(C)2019年提交人。爱思唯尔有限公司出版。
The isotopes of chlorine (Cl-37 and Cl-35) are highly fractionated in lunar samples compared to most other Solar System materials. Recently, the chlorine isotope signatures of lunar rocks have been attributed to large-scale degassing processes that occurred during the existence of a magma ocean. In this study we investigated how well a suite of lunar basalts, most of which have not previously been analyzed, conform to previous models. The Cl isotope compositions (delta Cl-37 (parts per thousand) = [(Cl-37/Cl-35(sample)/Cl-37/Cl-35(SMOC)) - 1] x 1000, where SMOC refers to standard mean ocean chloride) recorded range from similar to+7 to +14 parts per thousand (Apollo 15), +10 to +19 parts per thousand (Apollo 12), +9 to +15 parts per thousand (70017), +4 to +8 parts per thousand (MIL 05035), and +15 to +22 parts per thousand (Kalahari 009). The Cl isotopic data from the present study support the mixing trends previously reported by Boyce et al. (2015) and Barnes et al. (2016), as the Cl isotopic composition of apatites are positively correlated with bulk-rock incompatible trace element abundances in the low-Ti basalts, inclusive of low-Ti and KREEP basalts. This trend has been interpreted as evidence that incompatible trace elements, including Cl, were concentrated in the urKREEP residual liquid of the lunar magma ocean, rather than the mantle cumulates, and that urKREEP Cl had a highly fractionated isotopic composition. The source regions for the basalts were thus created by variable mixing between the mantle (Cl-poor and relatively unfractionated) and urKREEP. The high-Ti basalts show much more variability in measured Cl isotope ratios and scatter around the trend formed by the low-Ti basalts. Most of the data for lunar meteorites also fits the mixing of volatiles in their sources, but Kalahari 009, which is highly depleted in incompatible trace elements, contains apatites with heavily fractionated Cl isotopic compositions. Given that Kalahari 009 is one of the oldest lunar basalts and ought to have been derived from very early-formed mantle cumulates, a heavy Cl isotopic signature is likely not related to its mantle source, but more likely to magmatic or secondary alteration processes, perhaps via impact-driven vapor metasomatism of the lunar crust. (C) 2019 The Authors. Published by Elsevier Ltd.