Colloquium: Bruno Reynard
"Geophysical properties of serpentine and water cycle in subduction zones"
Abstract: Water is recycled in the Earth's interior when hydrothermally altered oceanic lithosphere and sediments are subducted. Serpentinization occurs in the cold mantle wedge above the dehydrating subducting plate. I present experimental measurements of serpentine physical properties that allow discussing water recycling in the mantle.
Low seismic velocities in the shallow mantle (200 km) near subduction can give an image of extensive hydration and serpentinization within the stability of serpentine below 700°C. We found that antigorite and deformed serpentinites have a very high seismic anisotropy and remarkably low velocities along particular directions. VP and VS anisotropies of serpentinites can reach 40 and 50%, respectively. These properties can be used to detect serpentinite, to quantify the amount of serpentinization, and to discuss relationships between seismic anisotropy and deformation in the mantle wedge.
The mantle wedge is generally very serpentinized (50-100%) down to 60 km in hot subductions while it is often not significantly serpentinized in the coldest subductions. Low seismic velocities in the lower seismicity plane located 30 km below the slab surface are better explained by seismic anisotropy of anhydrous deformed peridotites than by serpentinization. The contribution of the subducting lithosphere mantle to the water budget of subduction zones is thus likely limited to the first 2-3 kilometers beneath oceanic crust.
In the mantle wedge, the high electrical-conductivity bodies observed in hot areas (> 700°C) are explained by melting of the mantle under volcanic arcs. In the cold (<700°C) melt-free fore-arc mantle wedge, high conductivities can reflect the actual concentrations of fluids, provided the conductivity contrast is high between the fluid and serpentinized rocks. Using complex impedance measurements, we found that serpentinites have low electrical conductivities (< 10-4 S.m-1) similar to those of dry mantle minerals below 700°C. Thus, electrical conductivity is only sensitive to the fluid content and salinity in the hydrated mantle wedge, not to serpentinization.
A small fraction (ca. 1% in volume) of connective high-salinity fluids accounts for the highest observed conductivities. In the hydrated mantle wedge, the low-salinity fluids (? 0.1 m) released by slab dehydration evolve towards high-salinity (? 1 m) fluids during progressive serpentinization. These fluids can mix with arc magmas at depths and account for high-chlorine melt inclusions in arc lavas.
Bruno Reynard, Director of Research CNRS