Effect of H2O released by dehydration of serpentine and chlorite on compressional wave velocities of peridotites at 1GPa 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 degrees C during heating to and cooling from 1000 degrees C. During heating, the serpentine-bearing rocks showed a sudden decrease in V, at 600-700 degrees C when serpentine dehydration took place. Similarly, the chlorite-bearing rocks exhibited a sharp change in V, at 800-900 degrees 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 V, clue 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 V-p (-5.1%), which is comparable to the degree of V, reduction reported from the low-velocity anomaly regions in subducting slabs. The effect of (H)2O fluid on the V, of peridotite is evaluated by assessing the ratio of V-p 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 V-p. (c) 2007 Elsevier B.V. All rights reserved.