CH₄ Inclusions in High‐Pressure Metapelite: Revealing the Link Between Fluid Behavior and Redox Mechanisms for Subducting Sedimentary Carbon

Abstract

Abstract(Ultra)high‐pressure metamorphic rocks provide valuable insights into the properties of slab‐derived fluids. Here, we report CH4‐rich fluid inclusions in garnet of a metapelite from the Zermatt‐Saas ophiolite, western Alps. Two types of metapelite, a CH4‐bearing pelitic schist and a calcareous pelitic schist, were investigated to unravel favorable P‐T‐fO2 conditions for preservation of CH4 in high‐pressure metapelite. In the CH4‐bearing pelitic schist, CH4‐rich fluid inclusions exclusively occur in the core of garnet (GrtI) rather than the rim (GrtII). GrtI records P‐T conditions of ∼2.85 GPa and ∼555°C, whereas GrtII records a prograde P‐T path from ∼1.75 GPa at 510°C to ∼2.0 GPa at 530°C. Compositional profile of garnet in the calcareous pelitic schist reflects a prograde metamorphic path from ∼1.9 GPa at 510°C to ∼2.12 GPa at 545°C. CH4‐rich fluid formation may primarily rise from graphite reduction at high‐pressure reduced conditions (ΔFMQ −3.5 to −4, 2.85 GPa, ∼550°C), while graphite and carbonates stabilize in a relatively oxidized environment (ΔFMQ ∼0, 2.12 GPa, 545°C). The initial redox budget of subducted sediments is primarily controlled by the amount of sedimentary carbonate and organic carbon, which plays the most important role in deciding the carbon speciation at different subduction depths. CH4 formation in COH fluids could primarily be attributed to the reduction of graphite. Subducted metasediments act as conduits for transporting non‐oxidized fluids to arc magmas, which provides crucial evidence to support the heterogeneity for slab‐derived COH fluids and offers new insights into the deep carbon cycle.