Sulfur in Archean komatiite implies early subduction of oceanic lithosphere
Sulfur in Archean komatiite implies early subduction of oceanic lithosphere
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DOI：
10.1016/j.epsl.2022.117826
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发表时间：
2022-11
影响因子：
5.3
通讯作者：
Y. Kubota;F. Matsu'ura;K. Shimizu;A. Ishikawa;Y. Ueno
中科院分区：
文献类型：
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作者：
Y. Kubota;F. Matsu'ura;K. Shimizu;A. Ishikawa;Y. Ueno
科马提岩是一种富含镁的火山岩，几乎只在太古宙时期喷发，需要地球上地幔的高度部分熔融才能形成。因此，太古宙科马提岩为早期地球的地幔地球化学提供了最可靠的信息。然而，目前尚不清楚科马提石的成分是否只反映了早期深部地幔的残留物，或者可能提供了俯冲成分再循环的证据。硫的质量非依赖分馏(S-MIF)可能有助于区分这两种情况，因为S-MIF只在缺氧大气中通过光化学产生。本文报道了2.7Ga贝灵圭科马提岩及其相关火山岩的多重硫同位素分析。一些贝灵圭科马提岩显示出明显的S-MIF特征(Δ33 S从−0.20‰到−0.12‰)。岩石学、微量元素化学和锶-钕-铅同位素组成表明，样品中含有贫镍的原始火成岩硫化物，地壳污染极小。如果科马提岩源区与早期亏损的浅地幔有关，并通过海底蚀变从海水中获得S-MIF(Δ33 S<；0‰)，则观测到的S和初始Nd值的同位素值是一致的。我们认为，贝灵圭科马提岩至少部分来源于过去俯冲的大洋岩石圈，该岩石圈从太古代到现在在地幔中停滞不前。因此，太古宙科马提岩中的硫同位素为俯冲历史和地壳物质的再循环提供了新的制约条件。
Komatiite is a magnesium-rich volcanic rock that erupted almost exclusively during the Archean era, requiring high partial melting of Earth's upper mantle to form. Archean komatiites, therefore, provide the most reliable information on the mantle geochemistry of the early Earth. However, it remains unclear whether komatiite compositions reflect only remnants of an early deep mantle or may provide evidence of recycling of subducted components. Sulfur mass-independent fractionation (S-MIF) could be useful to distinguish the two scenarios because S-MIF is produced only by photochemistry in anoxic atmospheres. We report here multiple sulfur isotope analyses of the 2.7 Ga Belingwe komatiites and related volcanics. Some Belingwe komatiites show clear S-MIF signatures (Δ 33 S from− 0.20‰ to− 0.12‰). Petrology, trace element chemistry, and Sr-Nd-Pb isotope compositions demonstrate that the samples contain Ni-poor pristine igneous sulfides and crustal contamination is minimal. The observed S and initial Nd isotope covariation can be reconciled if the komatiite source involved an early depleted shallow mantle which acquired S-MIF from seawater (Δ 33 S< 0‰) via seafloor alteration. We suggest that the Belingwe komatiites were derived at least partly from a past subducted oceanic lithosphere that stagnated in the mantle from the Archean era to the present. Therefore, the sulfur isotopes in Archean komatiite provide new constraints on the history of subduction and the recycling of crustal material.