[S-CG54] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG54-01] Pervasive microcracking and damage evidenced by P-wave velocities monitoring during antigorite dehydration up to 2.5GPa

★Invited Papers

*Alexandre SCHUBNEL^1,3,2, Arefeh Moarefvand^2,1,5, Julien Gasc², Loïc Labrousse⁴ (1.CNRS , 2.ENS Paris - PSL University, 3.JSPS - University of Tokyo, 4.ISTEP Sorbonne University, 5.Total Pau France)

Keywords:serpentine, dehydration, fracture, wave velocities

The evolution of P-wave velocities was measured during pure antigorite dehydration experiments at hydrostatic pressure and temperature representative of subduction zone conditions (from 1 to 2.5 GPa and 600 to 700°C). In all experiments, P-wave velocity decreased dramatically at the onset of dehydration. This drop in P-wave velocity decreased in amplitude with increasing pressure, but remained noticeable, even at 2.5 GPa, a pressure at which the total reaction volume change is expected to become negative. Compared to experiments performed at 650 °C, the drop in P-wave velocity at 700 °C, occurred over a shorter time interval and was more pronounced, due to faster dehydration kinetics. Recovered samples, analyzed under scanning electron microscopy, reveal a transition from the dehydration reaction taking place along fractures, close to equilibrium, to the presence of a dehydration reaction front, when the reaction was overstepped.

Our results demonstrate that antigorite dehydration is systematically accompanied by microcracking, even at high pressure. Computing elastic properties of the dehydrating mineral assemblage using effective medium theory modelling, we show that: i) the reaction progress can be retrieved from the in-situ P-wave velocity measurements; ii) extrapolation of our results at subduction zone conditions are compatible with seismological observations of high Vp/Vs ratios. The observed softening of elastic properties in our experiments can be related to (hydro)fracturing processes at grain scale generated by water release upon dehydration even above 2 GPa, which supports dehydration stress transfer as being a reasonable model for intermediate depth earthquake triggering.