Dehydration of Serpentinites from Mantle Wedge and Subducted Slabs Evidenced by Magnesium Isotopes of Arc Lavas

Serpentinites serve as the primary source for water cycling in subduction zones, which triggers earthquakes and arc volcanism. The dehydration of serpentinites from mantle wedge and subducted slabs at different depths was predicted by thermodynamic simulations, but verifiable geochemical evidence is lacking. Here, we investigate Mg isotopic compositions of volcanic rocks across the Tonga-Kermadec and Ryukyu subduction zones, which document significant variations in slab-depth, to trace serpentinite dehydration. Volcanic rocks from both arc and back-arc regions of these two subduction zones exhibit elevated δ26Mg (−0.31 to −0.08‰) compared to mantle (−0.29 to −0.21‰). The heavy Mg isotopic signatures in arc lavas reflect fluids from mantle wedge serpentinites, which dehydrate at subarc depths. In contrast, elevated δ26Mg in back-arc lavas are attributed to fluids from slab serpentinites, which are cooler and thus dehydrate at greater depths. This study highlights the complementary roles of two types of serpentinites in Earth’s water cycling.

蛇纹岩（serpentinites）是俯冲带水循环的首要物质来源，该过程可引发地震与弧火山作用。不同深度的地幔楔与俯冲板片中的蛇纹岩脱水过程已通过热力学模拟得到预测，但目前仍缺乏可验证的地球化学证据。本研究对汤加-克马德克（Tonga-Kermadec）与琉球（Ryukyu）两条俯冲带内的火山岩开展镁同位素组成分析，以其记录的板片深度方向的显著差异为约束，追踪蛇纹岩的脱水过程。两条俯冲带弧区与弧后区的火山岩，其δ26Mg值（范围为−0.31‰~−0.08‰）均高于地幔端元值（−0.29‰~−0.21‰）。弧区熔岩呈现的重镁同位素特征，反映了亚弧深度发生脱水的地幔楔蛇纹岩所释放的流体。与之相对，弧后区熔岩的偏高δ26Mg值，则归因于温度更低、因此在更大深度发生脱水的俯冲板片蛇纹岩所释放的流体。本研究阐明了两类蛇纹岩在地球水循环中的互补作用。