Effects of low-pressure igneous processes and subduction on Fe3+/ΣFe and redox state of mantle eclogites from Lace (Kaapvaal craton)
Effects of low-pressure igneous processes and subduction on Fe3+/ΣFe and redox state of mantle eclogites from Lace (Kaapvaal craton)
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DOI：
10.1016/j.epsl.2017.06.030
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发表时间：
2017-09-15
影响因子：
5.3
通讯作者：
Viljoen, K. S.
中科院分区：
文献类型：
--
作者：
Aulbach, S.;Woodland, A. B.;Viljoen, K. S.
重建地幔的氧化还原状态对于讨论大气成分随时间的演变至关重要。金伯利岩地幔榴辉岩捕虏体，通常被解释为代表前洋壳，可能记录的化学和物理状态的对流地幔源，产生其岩浆原岩的太古代。然而，它们的信息通常被一系列原生（火成岩分异）和次生过程（海水蚀变、变质、交代）所掩盖。在这里，我们报告了一系列特征良好的地幔榴辉岩和辉石岩捕虏体中石榴子石中的Fe 3 +/Sigma Fe比和δ O-18，这些捕虏体位于Lace金伯利岩（Kaapvaal克拉通）中，起源于约。石榴石中的Fe 3 +/Sigma Fe（0.01 ~ 0.063，中位数0.02; n = 16）与单斜辉石中的硬玉含量呈负相关，表明在一价阳离子存在下，Fe 3+进入单斜辉石的分配增加，并可与单斜辉石形成偶联置换。硬玉校正的Fe 3 +/Sigma Fe在石榴石中表现出广泛的负趋势与Eu*，与不兼容的行为的Fe 3+在橄榄石斜长石积累的原岩。这一趋势部分被模糊的增加铁3+分配到石榴石沿着一个导电的超声波地热。相比之下，NMORB标准化的Nd/Yb -俯冲洋壳部分熔融损失的替代物（1）-与Fe 3 +/Sigma Fe没有明显的相关性，石榴石8180 vsmow（5.14至6.21千分之一）也没有指向显着的海水蚀变。大块岩石Fe 3 +/Sigma Fe的中位数粗略估计为0.025。这一观察结果与V/Sc系统学相一致，与现代MORB源相比，它们共同指出这些榴辉岩的火成岩原岩是一个减少的西太平洋对流地幔源。相对于铁橄榄石-磁铁矿-石英缓冲层（FMQ），氧辉岩（fO（2））的范围为Δ log fO（2）= FMQ-1.3至FMQ-4.6。在这些还原条件下，通过脱水释放的流体中的碳的溶解度高于接近FMQ的流体中的溶解度。这意味着，太古代的碳运输和沉积过程与现代俯冲带中已知的过程不同，金刚石可能是由富含甲烷的流体形成的。此外，这种还原物质可以驱动氧化还原熔融或冻结后，深循环和迁移的CH 4-轴承流体进入周围地幔。(C)2017爱思唯尔B. V.保留所有权利。
Reconstructing the redox state of the mantle is critical in discussing the evolution of atmospheric composition through time. Kimberlite-borne mantle eclogite xenoliths, commonly interpreted as representing former oceanic crust, may record the chemical and physical state of Archaean and Proterozoic convecting mantle sources that generated their magmatic protoliths. However, their message is generally obscured by a range of primary (igneous differentiation) and secondary processes (seawater alteration, metamorphism, metasomatism). Here, we report the Fe3+/Sigma Fe ratio and delta O-18 in garnet from in a suite of well-characterised mantle eclogite and pyroxenite xenoliths hosted in the Lace kimberlite (Kaapvaal craton), which originated as ca. 3 Ga-old ocean floor.Fe3+/Sigma Fe in garnet (0.01 to 0.063, median 0.02; n = 16) shows a negative correlation with jadeite content in clinopyroxene, suggesting increased partitioning of Fe3+ into clinopyroxene in the presence of monovalent cations with which it can form coupled substitutions. Jadeite-corrected Fe3+/Sigma Fe in garnet shows a broad negative trend with Eu*, consistent with incompatible behaviour of Fe3+ during olivine plagioclase accumulation in the protoliths. This trend is partially obscured by increasing Fe3+ partitioning into garnet along a conductive cratonic geotherm. In contrast, NMORB-normalised Nd/Yb - a proxy of partial melt loss from subducting oceanic crust (1) - shows no obvious correlation with Fe3+/Sigma Fe, nor does garnet 8180vsmow (5.14 to 6.21 parts per thousand) point to significant seawater alteration. Median bulk-rock Fe3+/Sigma Fe is roughly estimated at 0.025. This observation agrees with V/Sc systematics, which collectively point to a reduced Archaean convecting mantle source to the igneous protoliths of these eclogites compared to the modern MORB source.Oxygen fugacites (fO(2)) relative to the fayalite-magnetite-quartz buffer (FMQ) range from Delta log fO(2) = FMQ-1.3 to FMQ-4.6. At those reducing conditions, the solubility of carbon in the fluids released by dehydration is higher than in fluids closer to FMQ. The implication is that Archean processes of C transport and deposition would have differed from those known in modern-style subduction zones, and diamond would have formed from methane-rich fluids. In addition, such reducing material could drive redox melting or freezing upon deep recycling and migration of CH4-bearing fluids into the ambient mantle. (C) 2017 Elsevier B.V. All rights reserved.