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首页> 外文期刊>Contributions to Mineralogy and Petrology >Cs–Rb–Ba systematics in phengite and amphibole: an assessment of fluid mobility at 2.0 GPa in eclogites from Trescolmen, Central Alps
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Cs–Rb–Ba systematics in phengite and amphibole: an assessment of fluid mobility at 2.0 GPa in eclogites from Trescolmen, Central Alps

机译：Cs–Rb–Ba系统在辉石和闪石中的应用：中阿尔卑斯山特雷斯科门的榴辉岩中2.0 GPa流体迁移率的评估

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摘要

Eclogites from Trescolmen that contain abundant hydrous minerals (phengite, amphibole, paragonite, zoisite, talc, apatite) show petrographic evidence for fluid infiltration under conditions of 2.0 to 1.8 GPa, 650 °C. Large ion lithophile elements (LILE, e.g. Cs, Rb, Ba and Sr) were analysed by in-situ techniques in all eclogite mineral phases in order to characterize the behaviour of fluid-mobile elements at high pressure. In-situ analysis of carefully-chosen metamorphic assemblages circumvents the problem of partial late-stage alteration, which can severely influence the calculated element budgets of whole-rock samples. Phengite is the dominant host for Cs, Rb, and Ba in both eclogite and adjacent garnet mica schist samples, and incorporates >90% of the budgets of these elements in whole rocks. LILE contents of phengites in phengite-rich rocks are likely to record the Cs/Rb and Ba/Rb ratios of their host rock protoliths. The LILE patterns of eclogite are consistent with protoliths derived from basalt that underwent seafloor alteration, whereas those of mica schist are almost identical to average upper continental crust. In contrast, LILE patterns of eclogite samples that lack phengite, but do contain amphibole, are unlike any plausible protolith, but are identical to those of amphibole in phengite-bearing samples. This observation points to homogenization of the LILE in different lithologies, which we correlate with petrographic evidence for fluid infiltration. Because phengite in garnet mica schist has a strong capacity to buffer the fluid with respect to Cs, Rb, and Ba, homogenization of amphiboles is best explained by fluid infiltration from the surrounding metapelites into eclogite bodies, implying at least metre-scale fluid mobility. The amphibole homogenization can be most easily modelled by a pervasive open-system fluid flux through the eclogites, possibly facilitated by ductile deformation during the early stages of uplift. Simple calculations give minimum fluid-rock ratios of ~0.001 to 0.004. Demonstration of the mobility of very small volumes of fluid through eclogite is an important prerequisite of many subduction zone models that try to explain across-arc variations in trace element geochemistry. The low fluid-rock ratios from this study are not in contrast with oxygen isotope heterogeneities reported from other eclogite localities. Fluid mobile elements such as Cs, Rb and Ba are more sensitive indicators of small volume, fluid-rock interaction and are therefore potentially valuable for understanding fluid infiltration processes in systems where oxygen isotope shifts are not large enough to be detectable.
机译：Trescolmen的榴辉岩​​中含有丰富的含水矿物（辉石，闪石，方钠石，膨润土，滑石，磷灰石），岩石学证据表明在650°C和2.0至1.8 GPa的条件下流体渗透。通过现场技术在所有榴辉岩矿物相中分析了大型离子亲石元素（LILE，例如Cs，Rb，Ba和Sr），以表征在高压下流体活动元素的行为。精心选择的变质组合的原位分析规避了部分后期蚀变的问题，这可能严重影响整个岩石样品的计算元素预算。榴辉岩是榴辉岩和相邻的石榴石云母片岩样品中Cs，Rb和Ba的主要寄主，并且在整个岩石中占这些元素预算的> 90％。富含铅矿的岩石中锂铁矿的LILE含量可能会记录其宿主岩石原岩的Cs / Rb和Ba / Rb比。榴辉岩的LILE模式与经历了海底蚀变的玄武岩原生岩相一致，而云母片岩的原生岩与普通上陆壳几乎相同。相比之下，缺少亚辉石但确实含有角闪石的榴辉岩样品的LILE模式与任何看起来可行的原生石不同，但与带有亚辉石的样品中的角闪石相同。该观察结果表明LILE在不同岩性中均质化，这与岩石渗透流体的证据相关。由于石榴石云母片岩中的亚辉石对Cs，Rb和Ba具有很强的缓冲流体的能力，因此，闪石的均质化最好用流体从周围的变质岩渗入榴辉岩体内来解释，这至少说明了米级的流体流动性。闪石的均质化最容易通过流经榴辉岩的开放系统流体通量来模拟，这可能是由于隆升早期的韧性变形而促进的。简单计算得出的最小流体岩石比约为0.001至0.004。证明极少量液体通过榴辉岩的流动性是许多俯冲带模型的重要前提，这些模型试图解释微量元素地球化学中的弧形变化。这项研究的低流体-岩石比与其他榴辉岩地区报道的氧同位素非均质性形成对比。诸如Cs，Rb和Ba之类的流体可移动元素是较小体积，流体-岩石相互作用的较敏感指示剂，因此对于理解氧同位素位移不足以至于无法检测的系统中的流体渗透过程具有潜在的价值。

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