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首页> 外文期刊>Contributions to Mineralogy and Petrology >Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes
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Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes

机译：早变质从劳氏辉石到榴辉岩相的微量元素的重新分布;对深层俯冲带过程的影响

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The transfer of fluid and elements from subducting crust to the overlying mantle wedge is a fundamental process affecting arc magmatism and the chemical differentiation of the Earth. While the production of fluid by breakdown of hydrous minerals is well understood, the liberation of trace elements remains generally unconstrained. In this paper, we evaluate the behaviour of trace elements during prograde metamorphism and dehydration using samples of high-pressure, low-temperature metamorphic rocks from New Caledonia. Samples examined include mafic and pelitic rock-types that range in grade from lawsonite blueschist to eclogite facies, and represent typical lithologies of subducting crust. Under lawsonite blueschist facies conditions, the low temperatures of metamorphism inhibit equilibrium partitioning between metamorphic minerals and allow for the persistence of igneous and detrital minerals. Despite this, the most important hosts for trace-elements include lawsonite, (REE, Pb, Sr), titanite (REE, Nb, Ta), allanite (LREE, U, Th), phengite (LILE) and zircon (Zr, Hf). At epidote blueschist to eclogite facies conditions, trace-element equilibrium may be attained and epidote (REE, Sr, Th, U, Pb), garnet (HREE), rutile (Nb, Ta), phengite (LILE) and zircon (Zr, Hf) are the major trace-element hosts. Chlorite, albite, amphibole and omphacite contain very low concentrations of the investigated trace elements. The comparison of mineral trace-element data and bulk-rock data at different metamorphic grades indicates that trace elements are not liberated in significant quantities by prograde metamorphism up to eclogite facies. Combining our mineral trace-element data with established phase equilibria, we show that the trace elements considered are retained by newly-formed major and accessory minerals during mineral breakdown reactions to depths of up to 150 km. In contrast, significant volumes of fluid are released by dehydration reactions. Therefore, there is a decoupling of fluid release and trace element release in subducting slabs. We suggest that the flux of trace elements from the slab is not simply linked to mineral breakdown, but results from complex fluid-rock interactions and fluid-assisted partial melting in the slab.
机译：流体和元素从俯冲的地壳到上覆的地幔楔的转移是影响电弧岩浆作用和地球化学分化的基本过程。虽然通过水合矿物质分解产生流体是众所周知的，但微量元素的释放通常不受限制。在本文中，我们使用来自新喀里多尼亚的高压，低温变质岩样品评估了痕量元素在顺变质和脱水过程中的行为。所检查的样品包括镁铁质和黄铁质岩石类型，其等级范围从钙钠榴石蓝辉岩到榴辉岩相，代表典型的俯冲壳岩性。在钠钙榴石岩相条件下，变质的低温抑制了变质矿物之间的平衡分配，并允许火成岩和碎屑矿物持续存在。尽管如此，痕量元素最重要的元素包括钠铅矿（REE，Pb，Sr），钛矿（REE，Nb，Ta），尿石（LREE，U，Th），锂铁矿（LILE）和锆石（Zr，Hf ）。在附子蓝色榴辉岩至榴辉岩相的条件下，可能达到微量元素平衡，并产生了附子（REE，Sr，Th，U，Pb），石榴石（HREE），金红石（Nb，Ta），锂铁矿（LILE）和锆石（Zr， Hf）是主要的痕量元素宿主。亚氯酸盐，钠长石，闪石和绿辉石中所含微量元素的含量非常低。通过比较不同变质级别的矿物微量元素数据和块岩数据，可以看出，直到前岩相前，通过变质作用，微量元素不会大量释放。将我们的矿物微量元素数据与已建立的相平衡相结合，我们表明，在矿物分解反应中，直到150 km的深度，所考虑的微量元素仍被新形成的主要和辅助矿物保留。相反，大量的流体通过脱水反应释放。因此，俯冲板中的流体释放与微量元素释放之间存在解耦关系。我们认为，平板中微量元素的通量不仅仅与矿物分解有关，而是平板中复杂的流体-岩石相互作用和流体辅助的部分熔化所导致的。

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