The Mariana forearc provides a unique natural laboratory to study slab dehydration in an active subduction zone by its deep-rooted mud volcanism. To test if mantle wedge serpentinites would record the source fluid composition and thus the dehydration reactions in the slab, we investigated silicon (Si) isotopic compositions (δ30Si) in serpentine veins by in-situ femtosecond laser ablation ICP mass spectrometry. Our samples were recovered during IODP Expedition 366 and originate from three mud volcanoes that root in different depths, so that the pressure/temperature conditions in their source regions vary.
The δ30Si values differ strongly between the mud volcanoes but also between different serpentine generations within individual samples. Serpentine that formed under low water/rock ratios has δ30Si similar to pristine olivine. In contrast, serpentine veins that formed under higher water/rock ratios show large ranges in δ30Si that vary significantly but systematically between the mud volcanoes and thus with the metamorphic grade at depth. Average δ30Si of such serpentine veins are ‑0.10 ‰, ‑1.94 ‰, and ‑0.80 ‰ to ‑0.93 ‰ with increasing depth-to-slab. We interpret these across-forearc changes to record the Si isotopic compositions of the fluid sources, that are at shallow depth (inferred slab temperatures of ~80°C) the dehydration of (biogenic) opal and release of pore fluids, at intermediate depth (~150°C) clay mineral breakdown, and at the deepest point (>250°C) decomposition of clay minerals and altered oceanic crust. These data imply that Si isotope signatures of wedge serpentinites can be used as a reliable proxy for slab dehydration processes.