The Mariana forearc is a unique location for exploring the role serpentinization plays in the marine Si cycle by means of Si stable isotope variations. Here, active mud volcanism transports deep, serpentinized mantle wedge material to the surface and thus offers a natural window to slab dehydration processes in dependence of changing temperature and pressure with depth. Si isotopes were measured in situ by femtosecond laser ablation MC-ICPMS in serpentine within ultramafic clasts from three mud volcanoes (Yinazao, Fantangisña, and Asùt Tesoru) sampled during International Ocean Discovery Program Expedition 366. To corroborate the results, serpentinization of olivine was studied in batch experiments. The Si isotope ratios show large variations between the mud volcanoes and between individual serpentine generations within a given mud volcano. Serpentine that formed early under low water/rock ratios exhibits ${\delta }^{30}$Si of −0.41 ± 0.04‰ (1SD) similar to unaltered olivine which agrees well with experimental findings predicting no significant isotope fractionation during early serpentinization. In contrast, late serpentine veins formed under higher water/rock ratios span a wide range of Si isotope ratios that differ significantly between the individual mud volcanoes. With increasing distance to the trench, ${\delta }^{30}$Si of late veins are −0.10 ± 0.07‰, −1.94 ± 0.13‰, and −0.80 ± 0.22‰ and −0.93 ± 0.21‰. These ${\delta }^{30}$Si values are interpreted to record the isotopic composition of the fluid source, namely subducted biogenic silica and pore fluids, clays, and altered oceanic crust that dehydrate as consequence of rising pressure and temperature with depth. We show that Si isotopes of mantle wedge serpentinites can be used as a reliable new proxy for slab dehydration processes. They may be used in paleo-forearc systems to unravel oceanic sediment and silica biomineralization evolution through geological time.

马里亚纳弧前弧是通过 Si 稳定同位素变化探索蛇纹石化在海洋 Si 循环中的作用的独特位置。在这里，活跃的泥浆火山活动将深层的蛇纹石化地幔楔物质输送到地表，从而为板片脱水过程提供了一个自然窗口，这取决于温度和压力随深度的变化。原位测量Si同位素通过飞秒激光烧蚀 MC-ICPMS 在国际海洋发现计划远征 366 期间采样的三个泥火山（Yinazao、Fantangisña 和 Asùt Tesoru）的超镁铁质碎屑中的蛇纹石中进行。为了证实结果，在批量实验中研究了橄榄石的蛇纹石化。Si同位素比显示出泥火山之间以及给定泥火山内各个蛇纹石世代之间的巨大差异。早期在低水石比下形成的蛇纹石表现出${\delta }^{30}$-0.41 ± 0.04‰ (1SD) 的 Si 类似于未改变的橄榄石，这与预测在早期蛇纹石化过程中没有显着同位素分馏的实验结果非常吻合。相比之下，在较高的水/岩比下形成的晚蛇纹石脉跨越了广泛的 Si 同位素比，这些比在各个泥火山之间存在显着差异。随着与战壕距离的增加，${\delta }^{30}$晚脉Si分别为-0.10±0.07‰、-1.94±0.13‰、-0.80±0.22‰和-0.93±0.21‰。这些${\delta }^{30}$Si 值被解释为记录流体源的同位素组成，即俯冲的生物二氧化硅和孔隙流体、粘土和改变的洋壳，由于压力和温度随着深度的增加而脱水。我们表明，地幔楔蛇纹岩的 Si 同位素可用作板片脱水过程的可靠新代表。它们可用于古弧前系统，以通过地质时间解开海洋沉积物和二氧化硅生物矿化演化。