Tracing elemental pathway and paleo-redox conditions of serpentinization in upper mantle section of the Spongtang ophiolite, Ladakh, NW Himalaya

Mineral chemistry revealed exchange of Mg, Si, Al, and Fe between olivines, pyroxenes, and the serpentinized matrix. The lizardites and chrysotiles are stable at sub-greenschist (p < 0.4 GPa; T⁓100–300°C) to low-T blueschist (p > 0.4 GPa; T⁓150–280°C) facies. The stability field of antigorite started at 320°C which generated mixed lizardite-antigorite phases. Later, antigorite completely replaces lizardite to form pure antigorite at high-T blueschist (T > 400°C; p > 0.8 GPa) and eclogite facies (T > 420°C; p > 1.1 GPa) at shallow mantle wedge region in a reduced condition during intra-oceanic subduction. The upper mantle section of the Spongtang ophiolite has undergone serpentinization in three stages, starting from a sub-greenschist to blueschist and eclogite facies in evolving plate tectonics from a spreading ridge to a subduction setting. The presence of sulphide mineral assemblages such as awaruite and pyrrhotite is indicative of a reducing condition during serpentinization. The serpentinites show similar Rare Earth Elements (REE) patterns with peridotites but higher concentrations of Ba, U, Sr, As and Pb in serpentinites indicate a significant Large Ion Lithophile Elements (LILE) uptake during serpentinization. Serpentinites exhibit molybdenum depletion with enrichment of arsenic and lead relative to fresh peridotites. These findings reveal an open-system seafloor alteration overprinted by subduction-related fluids during Neo-Tethyan closure.

矿物化学分析结果显示，橄榄石（olivines）、辉石（pyroxenes）与蛇纹岩化基质（serpentinized matrix）之间发生了镁（Mg）、硅（Si）、铝（Al）及铁（Fe）的元素交换。利蛇纹石（lizardites）与纤蛇纹石（chrysotiles）的稳定赋存范围为亚绿片岩相（sub-greenschist facies，压力p<0.4 GPa，温度T≈100~300℃）至低温蓝片岩相（blueschist facies，p>0.4 GPa，T≈150~280℃）环境。叶蛇纹石（antigorite）的稳定起始温度为320℃，此时会形成利蛇纹石-叶蛇纹石混合相。在洋内俯冲（intra-oceanic subduction）的还原环境下，于浅部地幔楔（mantle wedge）区域的高温蓝片岩相（T>400℃，p>0.8 GPa）及榴辉岩相（eclogite facies，T>420℃，p>1.1 GPa）环境中，叶蛇纹石会完全取代利蛇纹石，形成纯叶蛇纹石集合体。斯邦塘蛇绿岩（Spongtang ophiolite）的上地幔剖面经历了三期蛇纹岩化作用，其构造演化背景从扩张脊（spreading ridge）逐步转变为俯冲环境（subduction setting），赋存的变质相带也从亚绿片岩相依次过渡至蓝片岩相、榴辉岩相。镍铁矿（awaruite）、磁黄铁矿（pyrrhotite）等硫化物矿物组合的存在，指示蛇纹岩化作用发生于还原环境中。蛇纹岩（serpentinites）与橄榄岩（peridotites）具有相似的稀土元素（Rare Earth Elements，REE）配分模式，但蛇纹岩中钡（Ba）、铀（U）、锶（Sr）、砷（As）及铅（Pb）的含量更高，表明蛇纹岩化过程中发生了显著的大离子亲石元素（Large Ion Lithophile Elements，LILE）富集作用。相较于新鲜橄榄岩，蛇纹岩表现出钼（Mo）的亏损以及砷、铅的富集特征。上述研究结果表明，在新特提斯洋闭合（Neo-Tethyan closure）过程中，洋底发生了开放体系蚀变作用，且该蚀变作用被俯冲带流体叠加改造。