Effect of H2O released by dehydration of serpentine and chlorite on compressional wave velocities of peridotites at 1 GPa and up to 1000 °C

Compressional wave velocities (Vp) of serpentine-bearing wehrlite (0.2 wt.% H2O), serpentinized dunite (4.1 wt.% H2O), chlorite-bearing websterite (1.3 wt.% H2O), and chlorite-bearing dunite (0.6 wt.% H2O) were measured at 1 GPa and up to 1000 °C during heating to and cooling from 1000 °C. During heating, the serpentine-bearing rocks showed a sudden decrease in Vp at 600–700 °C, when serpentine dehydration took place. Similarly, the chlorite-bearing rocks exhibited a sharp change in Vp at 800–900 °C due to chlorite dehydration. During cooling experiments on both serpentine- and chlorite-bearing rocks, Vp showed a linear increase down to room temperature, suggesting that no rehydration reaction took place with decreasing temperature. The decrease in Vp due to dehydration reactions was relatively small (≤1.5%) in rocks with low H2O content (0.2–1.3 wt.%), whereas the dehydration reaction of serpentinized dunite with higher (4.1 wt.%) H2O led to a marked decrease in Vp (−5.1%), which is comparable to the degree of Vp reduction reported from the low-velocity anomaly regions in subducting slabs. The effect of H2O fluid on the Vp of peridotite is evaluated by assessing the ratio of Vp values ( V p / V p 0 ) under both wet and dry conditions. The V p / V p 0 ratio was 0.97–0.99 in rocks with 0.2–1.3 wt.% H2O, but was markedly lower in rocks with 4.1 wt.% H2O (0.87). The run products showed variable geometries of fluid-filled pores, which clearly depended on the H2O content of the rock studied. The run products with 0.2–1.3 wt.% H2O had relatively small and isolated fluid-filled pores, whereas run products with 4.1 wt.% H2O contained relatively large fluid-filled pores that were interconnected. Our results suggest that connected fluid-filled pores cause a significant decrease in Vp, but that isolated fluid-pores have little effect on Vp.