Evidence for a dominantly reducing Archaean ambient mantle from two redox proxies, and low oxygen fugacity of deeply subducted oceanic crust
Evidence for a dominantly reducing Archaean ambient mantle from two redox proxies, and low oxygen fugacity of deeply subducted oceanic crust
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DOI：
10.1038/s41598-019-55743-1
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发表时间：
2019-12-27
影响因子：
4.6
通讯作者：
Viljoen, K. S.
中科院分区：
文献类型：
--
作者：
Aulbach, Sonja;Woodland, Alan B.;Viljoen, K. S.
氧逸度（fO（2））是一个强烈的变量，涉及到一系列的过程，塑造了地球系统，但有争议的时间和速度的最上层对流地幔氧化到目前的fO（2）周围的铁橄榄石磁铁矿石英氧缓冲。在这里，我们报告的Fe 3 +/σ Fe和fO（2）的古榴辉岩捕虏体与大洋地壳原岩，采样同时代的环境对流地幔。使用新的和已发表的数据，我们表明，在这些榴辉岩，两个氧化还原代理，V/Sc和Fe 3 +/Sigma Fe，表现同情，尽管不同的反应，他们的原岩分化和形成后脱气，海水蚀变，脱挥发分和部分熔融，证明了一个意想不到的鲁棒性Fe 3 +/Sigma Fe。因此，这些过程虽然造成了显著的散射，但并没有完全消除底层的对流地幔信号。仅考虑未交代的样品与非累积和小分化的原岩，V/Sc和Fe 3 +/Sigma Fe在两个大洋榴辉岩套房显着低于现代洋中脊玄武岩（MORB），而第三套的比值类似于现代MORB，表明氧化还原不均匀性。另一个主要发现是地幔深处榴辉岩的fO（2）估计值虽然很低，但仍有变化，这不允许CO2为主的流体或纯碳酸盐岩熔体的稳定。相反，低fO（2）榴辉岩可能导致古代俯冲板片其他部分产生的流体和熔体中的CO2有效减少，这与榴辉岩金刚石形成年龄、榴辉岩金刚石相对于榴辉岩丰度的不成比例的频率一致。地幔岩石圈中榴辉岩的丰度以及地幔榴辉岩中普遍缺乏碳酸盐。这表明碳循环至少达到金刚石稳定性的深度，并且可能代表了随着时间的推移碳进入的重要途径。
Oxygen fugacity (fO(2)) is an intensive variable implicated in a range of processes that have shaped the Earth system, but there is controversy on the timing and rate of oxidation of the uppermost convecting mantle to its present fO(2) around the fayalite-magnetite-quartz oxygen buffer. Here, we report Fe3+/Sigma Fe and fO(2) for ancient eclogite xenoliths with oceanic crustal protoliths that sampled the coeval ambient convecting mantle. Using new and published data, we demonstrate that in these eclogites, two redox proxies, V/Sc and Fe3+/Sigma Fe, behave sympathetically, despite different responses of their protoliths to differentiation and post-formation degassing, seawater alteration, devolatilisation and partial melting, testifying to an unexpected robustness of Fe3+/Sigma Fe. Therefore, these processes, while causing significant scatter, did not completely obliterate the underlying convecting mantle signal. Considering only unmetasomatised samples with non-cumulate and little-differentiated protoliths, V/Sc and Fe3+/Sigma Fe in two Archaean eclogite suites are significantly lower than those of modern mid-ocean ridge basalts (MORB), while a third suite has ratios similar to modern MORB, indicating redox heterogeneity. Another major finding is the predominantly low though variable estimated fO(2) of eclogite at mantle depths, which does not permit stabilisation of CO2-dominated fluids or pure carbonatite melts. Conversely, low-fO(2) eclogite may have caused efficient reduction of CO2 in fluids and melts generated in other portions of ancient subducting slabs, consistent with eclogitic diamond formation ages, the disproportionate frequency of eclogitic diamonds relative to the subordinate abundance of eclogite in the mantle lithosphere and the general absence of carbonate in mantle eclogite. This indicates carbon recycling at least to depths of diamond stability and may have represented a significant pathway for carbon ingassing through time.