Abyssal peridotites reveal the near-chondritic Fe isotopic composition of the Earth
Abyssal peridotites reveal the near-chondritic Fe isotopic composition of the Earth
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DOI：
10.1016/j.epsl.2013.01.011
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发表时间：
2013-03-01
影响因子：
5.3
通讯作者：
Dauphas, Nicolas
中科院分区：
文献类型：
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作者：
Craddock, Paul R.;Warren, Jessica M.;Dauphas, Nicolas
相对于原始的、未分化的陨石（球粒陨石），陆相、海洋和大陆玄武岩的Fe-56/Fe-54比值约为+0.1‰。陆地玄武岩的δ Fe-56值也与火星和灶神星的玄武岩不同，后者具有球粒状的铁同位素组成。陆生玄武岩同位素富集的过程存在争议，部分原因是陆生玄武岩地幔源的铁同位素组成尚不清楚。在这里，我们报告了深海橄榄岩的铁同位素测量，这些橄榄岩是洋脊有限熔融的残留物，因此是地幔对流部分组成的最佳代表。我们的数据显示，深海橄榄岩的平均δ Fe-56值为+0.010 +/- 0.007（相对于IRMM-014），与球粒陨石难以区分。在校正了海底风化和地幔融化的数据后，我们估计地球地幔的平均铁同位素组成为δ Fe-56 = +0.025 +/- 0.025‰，在目前的分析精度范围内，这与球粒陨石也无法区分。我们确定在部分地幔熔融过程中，三角洲Fe-56中橄榄岩残留物的最大位移为0.01‰。我们的研究结果反对核-幔分异过程中的同位素分异或形成月球的巨大撞击过程中的铁汽化，因为这两个过程都会产生非球粒质的大块地幔三角洲铁-56值。此外，我们的结果表明，钙钛矿和Fe-0金属中的地幔Fe2+-Fe3+歧化以及金属向核心的偏析可能不是硅酸盐地幔中Fe同位素分馏的驱动因素。相反，深海橄榄岩和morb的不同铁同位素组成支持熔体铁同位素分馏的证据，而不是海洋和大陆地壳形成过程中的残留物。(C) 2013 Elsevier B.V.版权所有
Terrestrial oceanic and continental basalts are enriched by approximately +0.1 parts per thousand in Fe-56/Fe-54 ratio relative to primitive, undifferentiated meteorites (chondrites). The delta Fe-56 values of terrestrial basalts are also distinct from those of basalts from Mars and asteroid Vesta, which have chondritic Fe isotopic compositions. The processes responsible for the isotopic enrichment of terrestrial basalts are debated, in part because the Fe isotopic composition of the mantle source of terrestrial basalts is unknown. Here we report Fe isotopic measurements of abyssal peridotites, which are the residues of limited melting at oceanic ridges and are thus the best proxies for the composition of the convective portion of the mantle. Our data show that abyssal peridotites have a mean delta Fe-56 value of +0.010 +/- 0.007 parts per thousand (relative to IRMM-014), which is indistinguishable from chondrites. After correcting this data for seafloor weathering and mantle melting, we estimate the average Fe isotopic composition of the terrestrial mantle to be delta Fe-56 = +0.025 +/- 0.025 parts per thousand, which is also indistinguishable from chondrites, within current analytical precision. We determine that the maximum shift in delta Fe-56 for peridotite residues during partial mantle melting is 0.01 parts per thousand. Our results argue against isotopic fractionation during core-mantle differentiation or iron vaporization during the Moon-forming giant impact, because both processes would yield a bulk mantle delta Fe-56 value that is non-chondritic. In addition, our results suggest that disproportionation of mantle Fe2+-Fe3+ in perovskite and Fe-0 metal and segregation of metal to the core could not have been a driver for Fe isotopic fractionation in the silicate mantle. Instead, the different iron isotopic compositions of abyssal peridotites and MORBs support mounting evidence for iron isotopic fractionation of melts but not residues during the formation of oceanic and continental crust. (C) 2013 Elsevier B.V. All rights reserved.