Zoisite-aqueous fluid trace element partitioning with implications for subduction zone fluid composition
Zoisite-aqueous fluid trace element partitioning with implications for subduction zone fluid composition
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DOI：
10.1016/j.chemgeo.2007.01.008
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发表时间：
2007-04
期刊：
影响因子：
3.9
通讯作者：
M. Feineman;F. Ryerson;D. DePaolo;T. Plank
中科院分区：
文献类型：
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作者：
M. Feineman;F. Ryerson;D. DePaolo;T. Plank
已经确定了带状石英-流体微量元素分配系数，并用于模拟俯冲板片中含水相破裂过程中产生的流体。在750-900 °C和2.0 GPa下测定了Rb、Ba、Th、U、Nb、Ta、Pb、Sr和9种稀土元素（REE）的分配系数。结果表明，相对于其它高压相（DSr>100，DPb>10），锆石中Sr和Pb的相容性非常好。的zoite流体分配系数与先前确定的榴辉岩相矿物（石榴石，cpx，金红石，多硅白云母）的分配系数相结合，以产生特定的反应分配系数，以及大量榴辉岩流体分配系数。批平衡模型，然后使用近似的条件下，在板的流体形成。模拟了两种流体：1）由特定的带状榴辉岩分解反应产生的流体，2）由各种连续和不连续反应产生的组合流体，这些反应有机会与残余的含带状榴辉岩的榴辉岩组合平衡。我们发现，个别反应过程中产生的流体有更极端的微量元素富集比那些已经平衡的榴辉岩板作为一个整体。特别是，流体中所产生的黝帘石分解反应往往是强烈富集铅和锶。流体的极端组合物从个别脱水反应可能会被保存为流体包裹体和脉中的折返榴辉岩。平衡的流体具有更温和的微量元素富集模式，这是更一致的流体成分采样的岛弧玄武岩（IAB）在火山前下方的熔融区。我们建议，流体中产生的板，这可能最初有极端的微量元素富集和分馏模式，并可能显示极端的变化在一个小的空间尺度上，最终至少部分平衡与散装榴辉岩。随后与沉积层的部分熔融物混合，并在前往熔融区的途中与地幔楔交换，进一步使流体均匀化。因此，我们预计，“变质”俯冲带流体记录的榴辉岩中的包裹体和静脉将非常显着不同的成分从“交代”俯冲带流体，有助于岛弧玄武岩的形成，即使他们的起源可能是相同的。
Zoisite-fluid trace element partition coefficients have been determined and are used to model fluids generated during the breakdown of hydrous phases in a subducting slab. Partition coefficients were determined for Rb, Ba, Th, U, Nb, Ta, Pb, Sr, and nine rare earth elements (REE) at 750–900 °C and 2.0 GPa. Our results show that Sr and Pb are extremely compatible in zoisite relative to other high-pressure phases (DSr>100, DPb>10). The zoisite-fluid partition coefficients are combined with previously determined partition coefficients for eclogite-facies minerals (garnet, cpx, rutile, phengite) to generate reaction-specific partition coefficients as well as bulk eclogite-fluid partition coefficients. Batch equilibrium models are then used to approximate the conditions of fluid formation in the slab. Two kinds of fluids are modeled — 1) fluids resulting from specific zoisite breakdown reactions, and 2) combined fluids from a variety of continuous and discontinuous reactions that have had the opportunity to equilibrate with a residual zoisite-bearing eclogite assemblage. We find that fluids generated during individual reactions have much more extreme trace element enrichments than those that have equilibrated with the eclogitic slab as a whole. In particular, fluids generated during zoisite-breakdown reactions tend to be strongly enriched in Pb and Sr. Fluids of extreme composition from individual dehydration reactions may be preserved as fluid inclusions and veins in exhumed eclogites. Fluids that have equilibrated with zoisite-bearing eclogite have a more moderate trace element enrichment pattern that is more consistent with the fluid component sampled by island arc basalts (IAB) in the zone of melting beneath the volcanic front. We propose that fluids generated in the slab, which may initially have extreme patterns of trace element enrichment and fractionation, and may show extreme variation on a small spatial scale, are ultimately at least partially equilibrated with the bulk eclogite. Subsequent mixing with partial melts of the sediment layer and exchange with the mantle wedge en route to the melting region serve to further homogenize the fluids. Therefore, we expect that “metamorphic” subduction zone fluids recorded by inclusions and veins in eclogite will differ quite markedly in composition from “metasomatic” subduction zone fluids that contribute to the formation of island arc basalts, even though their origin may be the same.