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