Geochemistry of serpentenites from DSDP Hole 84-566C

We studied a unique chrysotile-antigorite serpentinite, drilled on Deep Sea Drilling Project Leg 84 (Site 566) in the Guatemala forearc. Our in situ major and trace element data provide new constraints on possible reactions and associated trace element mobilisation during shallow serpentinite subduction. Chrysotile of the studied serpentinite, formed by the hydration of an upper mantle peridotite precursor, is partially replaced by antigorite (alone) which also occurs in 0.5 mm wide unoriented veins crosscutting the rock. Based on textural relationships and the P-T-X stability of the rock forming phases, the replacement of chrysotile by antigorite occurred at T < 300 °C, due to interaction between the chrysotile-serpentinite and an aqueous fluid. A comparison of the chemical compositions of reactant and product phases reveals that about 90% of the Cl, more than 80% of the B and about 50% of the Sr hosted originally by chrysotile was lost during fluid-assisted chrysotile-to-antigorite transformation and accompanying partial dehydration, and documents the much lower affinity of antigorite for trace element uptake than that of chrysotile. The fluid-assisted chrysotile-to-antigorite transformation and associated trace element loss documented here can occur in the shallow (< 30 km) region of subduction zones. This transformation decreases notably the Cl and B inventory of subducting serpentinites, which are regarded as one of the most important carriers of these elements into subduction zones. The evolution of serpentinites during initial subduction stages thus appears to be critical in the recycling of specific trace elements such as B or Cl from forearc to subarc depths.我们对产自危地马拉弧前区、深海钻探计划（Deep Sea Drilling Project）第84航次566站位所钻取的一套特殊的纤蛇纹石-叶蛇纹石蛇纹岩开展了研究。我们获得的原位主量与微量元素数据，为浅部蛇纹岩俯冲过程中可能发生的反应及相关微量元素迁移行为提供了新的约束条件。 本研究的蛇纹岩中的纤蛇纹石由上地幔橄榄岩原岩经水化作用形成，其部分被叶蛇纹石单独交代；叶蛇纹石同时以宽约0.5 mm的无定向脉体形式穿切岩石。根据结构构造关系以及造岩矿物的温压-成分（P-T-X）稳定性，纤蛇纹石被叶蛇纹石交代的过程发生于温度低于300℃的条件下，驱动机制为纤蛇纹石蛇纹岩与水溶液的相互作用。通过对比反应相与产物相的化学成分可知，原赋存于纤蛇纹石中的约90%的氯（Cl）、80%以上的硼（B）以及约50%的锶（Sr）在流体辅助的纤蛇纹石向叶蛇纹石转化及伴随的部分脱水过程中发生了流失；该结果证实，叶蛇纹石相较于纤蛇纹石，对微量元素的吸附亲和力要低得多。 本研究记录的流体辅助型纤蛇纹石向叶蛇纹石转化过程及伴随的微量元素流失，可发生于俯冲带浅部（深度小于30 km）区域。该转化过程会显著降低俯冲蛇纹岩的氯、硼储量，而俯冲蛇纹岩被认为是将此类元素运移至俯冲带的最重要载体之一。因此，俯冲初期阶段蛇纹岩的演化过程，对于硼、氯等特定微量元素从弧前区向弧下深度的循环过程而言至关重要。