Al-zoning of serpentine aggregates in mesh texture induced by metasomatic replacement reactions
Al-zoning of serpentine aggregates in mesh texture induced by metasomatic replacement reactions
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交代置换反应诱导的网状结构中蛇纹石聚集体的铝分区
DOI：
10.1093/petrology/egy039
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发表时间：
2018
影响因子：
3.9
通讯作者：
Noriyoshi Tsuchiya
中科院分区：
文献类型：
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作者：
Ryosuke Oyanagi;Atsushi Okamoto;Yumiko Harigane;Noriyoshi Tsuchiya
海洋岩石圈的蛇纹石化通常随着网格结构的发展而进行。对蛇纹石化的方辉橄榄岩和含斜长石的韦尔长岩的检查显示，蛇纹石网状结构中存在明显的铝分带，具有富铝核心和贫铝边缘，以及富铝脉，表明在蛇纹石化过程中，铝从斜长石和辉石中局部迁移。为了揭示Al对蛇纹石化过程中反应机制和结构发展的影响，我们在橄榄石（Ol）-斜长石（Pl）-H2O体系中进行了水热实验，并分析了橄榄石替代物的矿物学和微观结构随距Ol-Pl边界距离的变化。 Al-Si 交代带发育于靠近 Ol-Pl 边界的 Ol 托管区，形成 Al-蛇纹石和少量 Ca-皂石。远离 Ol-Pl 边界，形成了无铝蛇纹石、水镁石和磁铁矿，表明“等化学”反应（与水分离）的进展。交代带中橄榄石之后的铝蛇纹石聚集体表现出铝的特征分带。微观结构证据表明，分带是由于铝交代锋的迁移而产生的，涉及无铝溶液的早期蛇纹石化，以及随后的橄榄石假象替换，以及随着溶液中铝含量的增加同时出现过度生长。含铝水溶液导致橄榄石假晶的形成，这与等化区中原始橄榄石轮廓缺乏保存形成对比。天然和实验产生的网状纹理的分区比较表明，在蛇纹石化过程开始时，网状纹理中的贫铝边缘形成，随后是富铝网状核心和富铝脉的耦合形成。我们的实验结果表明，网格纹理中的铝分区代表了蛇纹石化过程中从封闭系统到开放系统的转变。
Serpentinization of oceanic lithosphere commonly proceeds with the development of mesh texture. Examination of a serpentinized harzburgite and a plagioclase-bearing wehrlite revealed conspicuous zoning of Al in a serpentine mesh texture, with Al-rich cores and Al-poor rims, as well as Al-rich veins, indicating local transport of Al from plagioclase and pyroxene during serpentinization. To reveal the influences of Al on the reaction mechanisms and textural development during serpentinization, we conducted hydrothermal experiments in the olivine (Ol)–plagioclase (Pl)–H2O system and analyzed the variations in mineralogy and microtexture of olivine replacements as a function of distance from the Ol–Pl boundary. The Al–Si metasomatic zone, where Al-serpentine and a minor amount of Ca-saponite were formed, was developed in the Ol-hosted region close to the Ol–Pl boundary. Far from the Ol–Pl boundary, Al-free serpentine, brucite, and magnetite were formed, indicating the progress of an ‘isochemical’ reaction (separate from water). The aggregates of Al-serpentine after olivine in the metasomatic zone showed a characteristic zoning of Al. Microtextural evidence indicates that the zoning was produced in response to the migration of an Al metasomatic front that involved an early-stage of serpentinization with an Al-free solution, and the subsequent pseudomorphic replacement of olivine and simultaneous development of overgrowths as the amount of Al increased in the solution. The Al-bearing aqueous solution caused the formation of olivine pseudomorphs and this contrasts with the lack of preservation of original olivine outlines in the isochemical zones. Comparisons of zoning in natural and experimentally produced mesh textures suggest that Al-poor rims in the mesh texture form at the start of the serpentinization process, followed by the coupled formation of Al-rich mesh cores and Al-rich veins. Our experimental results indicate that Al-zoning in the mesh texture represent the transition from a closed to an open system during serpentinization.