On the hydration of olivine in ultramafic rocks: Implications from Fe isotopes in serpentinites
On the hydration of olivine in ultramafic rocks: Implications from Fe isotopes in serpentinites
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DOI：
10.1016/j.gca.2017.07.011
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发表时间：
2017-10
影响因子：
5
通讯作者：
S. Scott;K. Sims;B. Frost;P. Kelemen;K. Evans;S. Swapp
中科院分区：
文献类型：
--
作者：
S. Scott;K. Sims;B. Frost;P. Kelemen;K. Evans;S. Swapp
铁在蛇纹石化过程中的行为在很大程度上控制了蛇纹石宿主微生物群落的氧化还原反应和能量收支。本文对新喀里多尼亚部分蛇纹石化的石灰岩矿物分离物进行了铁同位素数据分析，以了解铁在蛇纹石化过程中的行为。我们新的矿物分离铁同位素数据与现有的蛇纹岩全岩研究数据进行了比较，结果表明，在蛇纹岩化过程中，铁的迁移仅限于俯冲带蛇纹岩化的最高温度。对新喀里多尼亚矿物分离物的测量显示出显著的铁同位素分馏，其中蛇纹岩-水镁石混合物的δ56Fe ~−0.35‰最低，磁铁矿的δ56Fe ~ +0.75‰最高。橄榄石、正辉石及整个岩石组成δ56Fe = 0.00‰的误差范围内。矿物相之间的铁同位素测温揭示了两个不同的平衡温度，一个是地幔橄榄石和辉石（~ 1325°C），另一个是蛇纹岩组合的低得多的温度（~ 335°C）。同位素、矿物学和地球化学综合资料表明，在蛇纹石成矿的磁铁矿形成阶段，随着蛇纹石成矿的进行，与矿物学组合平衡的孔隙流体演化出较高的铁浓度。当该孔隙流体从蛇纹石化环境中移除时，孔隙流体中从岩石中移除的总铁丰度远低于岩石整体铁丰度，对岩石整体组成的影响最小。
The behavior of Fe during serpentinization largely controls the potential for oxidation-reduction reactions and energy budget for serpentinite-hosted microbial communities. We present Fe isotope data for mineral separates from a partially serpentinized dunite from New Caledonia to understand the behavior of Fe during serpentinization processes. Our new Fe isotope data in mineral separates is compared to existing data from whole rock studies of serpentinites, which have generally concluded that Fe mobility during serpentinization is restricted to the highest temperatures of serpentinization in subduction zones. Measurements of mineral separates from New Caledonia show significant Fe isotope fractionations, with serpentine-brucite mixtures having the lowest δ56Fe ∼ −0.35‰ and magnetite having the highest δ56Fe ∼ +0.75‰. Olivine, orthopyroxene, and the whole rock composition are all within error of δ56Fe = 0.00‰. Fe isotope thermometry between mineral phases reveals two distinct temperatures of equilibration, one for the mantle olivine and pyroxene (∼1325 °C), and a second, much lower temperature (∼335 °C) for the serpentinite assemblage. The combined isotopic, mineralogical and geochemical data indicate that during the magnetite-forming stage of serpentinization, a pore fluid in equilibrium with the mineralogical assemblage evolves to higher Fe concentrations as serpentinization proceeds. When this pore fluid is removed from the serpentinizing environment, the total abundance of Fe removed from the rock in the pore fluid is much less than the bulk rock Fe and has a minimal effect on the overall rock composition.