Ultrahigh‐pressure metabasaltic garnets as probes into deep subduction zone chemical cycling
Ultrahigh‐pressure metabasaltic garnets as probes into deep subduction zone chemical cycling
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DOI：
10.1029/2004gc000746
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发表时间：
2004-12
期刊：
影响因子：
3.7
通讯作者：
R. L. King;G. Bebout;K. Kobayashi;E. Nakamura;Sebastiaan N. G. C. van der Klauw
中科院分区：
文献类型：
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作者：
R. L. King;G. Bebout;K. Kobayashi;E. Nakamura;Sebastiaan N. G. C. van der Klauw
我们展示了一种方法来检查俯冲洋壳的变质历史，可以补充记录的俯冲带化学循环来自火山弧产生的火成岩的研究。通过合并利用石榴石分带建立原位高分辨率微量元素地球化学的榴辉岩反应历史的方法，并应用于代表俯冲到弧下深处的含柯石英基性榴辉岩，我们能够直接识别仅在火山弧或理论变质模型研究中推断的俯冲板片中有助于元素流动性的反应的地球化学表现。具体而言，我们确定了一个反相变质反应，仅基于地球化学和矿物包裹体内石榴石的分区，并推断这些功能是一个记录的共存的斜锆石+钛铁矿和可能的微量元素负载流体的故障从岩石反相变质作用过程中解放。然后，我们能够通过计算这些榴辉岩反应的P-T依赖性并与已发表的P-T轨迹进行比较，为反应指定一个特定的深度间隔。由于石榴石对岩石学和地球化学过程的记录有很强的保存能力，这种方法特别适合于超高压（UHP）榴辉岩的研究，在这种榴辉岩中，严重的退变质作用（通常与折返作用有关）通常会掩盖退变质的历史。
We demonstrate an approach to examining the metamorphic history of subducting oceanic crust that can complement records of subduction zone chemical cycling derived from studies of igneous rocks produced at volcanic arcs. By merging methods utilizing garnet zoning to establish prograde reaction histories with in situ high‐resolution trace element geochemistry, and application to coesite‐bearing mafic eclogites representing subduction to depths beneath arcs, we are able to directly identify geochemical manifestations of reactions contributing to element mobility in the subducting slab that are only inferred in studies of volcanic arcs or theoretical metamorphic models. Specifically, we identify a prograde metamorphic reaction, based solely on the zoning of geochemistry and mineral inclusions within garnet, and infer that these features are a record of the breakdown of coexisting clinozoisite + titanite and probable liberation of trace element–laden fluid from the rock during prograde metamorphism. We are then able to assign a specific depth interval for the reaction through calculation of the P‐T dependence of the reaction for these eclogites and comparison with a published P‐T trajectory. Because of the robust preservation of records of petrologic and geochemical processes by garnet, this methodology is particularly suited for study of ultrahigh‐pressure (UHP) eclogites, in which severe retrograde alteration (generally related to exhumation) commonly obscures prograde history.