Preferential Orthopyroxene Serpentinization and Implications for Seismic Velocity Interpretation in the Fore‐Arc Mantle

Abstract

For better interpretation of seismic tomography data of the hydrated fore-arc mantle, we examined conditions of nonstoichiometric hydration of peridotite reported by a previous experimental study (i.e., preferential orthopyroxene (Opx) serpentinization and resultant talc formation) at 500-580°C and under 0.8-1.8 GPa. The corresponding seismic velocity was calculated by considering the talc production. Talc formed under all the experimental conditions even in the olivine-dominated system, in which the olivine to Opx ratio was 7:3 by weight. The amount of talc produced was comparable to that of serpentine at high temperature and low pressure and increased with increasing temperature and decreasing pressure due to decreasing contribution of olivine in the reaction. We also present a natural case of preferential Opx reaction in the Horoman peridotite complex, Japan, on the basis of its strong similarity in textures and mineral phases to our experimental products obtained at 0.8-1.3 GPa. Calculations on seismic velocities of hydrated harzburgite using the Voigt-Reuss-Hill averaging scheme showed that the formation of talc along with antigorite leads to a decrease in P wave (V p ) and S wave velocities (V s ) but no remarkable increase in V p /V s due to the lower V p /V s value of talc (~1.60) compared with that of antigorite (~1.77). Such a seismic anomaly has been observed in the shallow mantle wedge of the southern Western Alps subduction zone, where suppressed water supply from the subducted continental lithosphere might allow the persistence of talc produced by the preferential Opx reaction.

The Northern Terminus of Cascadia Subduction

“…Locally, SiO 2 metasomatism could further increase the abundance of talc. The abundance of talc may also be greater than equilibrium calculations suggested, owing to the preferential reaction of orthopyroxene during mantle hydration (Nakatani & Nakamura, 2019). Ab initio calculations by Mainprice et al (2008) indicate that isotropic talc possesses unusually low V P / V S < 1.6 at pressures <1.5 GPa.…”

Weakening of Peridotite Sheared at Hydrothermal Conditions

“…The calculated free energy of hydrolysis at 50 MPa is also lower for enstatite than olivine at temperatures above 250°C (Allen & Seyfried, 2003). Orthopyroxene hydration reactions have been documented in experiments at 400°C (Allen & Seyfried, 2003) and 580°C (Nakatani & Nakamura, 2016, 2019). Thus, olivine dissolution is the major factor in peridotite weakening at temperatures ≤250°C, whereas orthopyroxene reactions may dominate at higher temperatures.…”