Anomalous Sound Velocities of Antigorite at High Pressure and Implications for Detecting Serpentinization at Mantle Wedges

Abstract

Elastic properties of antigorite are important for interpretation of seismic mapping of serpentinization in the mantle wedge above subducting slabs. The compressional (VP) and shear (VS) wave velocities in pure antigorite aggregates were measured simultaneously up to 8.4 GPa by ultrasonic interferometry. We found that VP increases monotonically with pressure while VS increases with pressure up to about 3 GPa but undergoes a negative pressure dependence above 4 GPa. Compared to other mantle minerals, antigorite exhibits significantly lower P and S wave velocities as well as a higher VP/VS ratio at upper mantle pressures. We modeled velocity reductions manifesting through the formation of antigorite in mantle peridotite and provide compelling evidence that seismic anomalies with low‐velocity and high VP/VS ratios is caused by varying degrees of serpentinization in subduction zones.

Geneses of Two Contrasting Antigorite Crystal Preferred Orientations and Their Implications for Seismic Anisotropy in the Forearc Mantle

“…In a subduction zone setting, forearc mantle wedge is characterized by low Vp and Vs values and high Vp / Vs ratios, which are commonly ascribed to the seismic signatures of variably hydrated or serpentinized peridotites that are formed by fluids released from the subducted slab (e.g., Bostock et al, 2002; Brocher et al, 2003; Christensen, 2004; DeShon & Schwartz, 2004; Hyndman & Peacock, 2003; Kamiya & Kobayashi, 2000; Kawakatsu & Watada, 2007; Nakajima, Tsuji, & Hasegawa, 2009; Nakajima, Tsuji, Hasegawa, Kita, et al, 2009; Tibi et al, 2008; Wang et al, 2019). The mantle wedge serpentinization is spatially heterogeneous, being strongest on the top of the subducted slab and weakening toward the inside of mantle wedge, owing to the gradients of fluid content, shear strain and temperature, and anisotropic permeability of foliated serpentinite (Angiboust et al, 2012; Guillot et al, 2015; Hilairet & Reynard, 2009; Kawano et al, 2011).…”

The Anomalous Seismic Behavior of Aqueous Fluids Released during Dehydration of Chlorite in Subduction Zones

“…The progressive decrease in velocity ratio in chlorite with increasing temperature prior to the dehydration may be linked to the decrease in the principal elastic moduli C33 component, due to the development of preferred orientation in chlorite perpendicular to the principal compression axis. Low elastic wave velocities and a high Vp/Vs ratio have also been reported for antigorite [23], another principal hydrous mineral in subduction zones.…”

“…Minerals 2021, 11, x 7 of 12 linked to the decrease in the principal elastic moduli C33 component, due to the development of preferred orientation in chlorite perpendicular to the principal compression axis. Low elastic wave velocities and a high Vp/Vs ratio have also been reported for antigorite [23], another principal hydrous mineral in subduction zones. While the velocity ratio of major mantle minerals varies in a narrow range, the parameter has been considered as a reliable seismic tool to distinguish liquid phases from solid minerals.…”

“…The prior knowledge of the elastic properties of major mantle mineral phases provides the basic framework for the interpretation of seismic parameters [17] obtained through inversion methods. The available criteria allow the identification of solid from liquid phases [18][19][20][21][22] or distinguish hydrous minerals from nominally anhydrous minerals [15,[22][23][24]. However, there are no robust seismic parameters to identify aqueous fluids.…”

“…The Vp/Vs of major mantle phases, olivine, garnet, clinopyroxene (cpx), orthopyroxene (opx), are shown for comparison[47]. Diamond symbols indicate antigorite[23] at 1.8 GPa (red) and 3.8 GPa (blue) at 300 K. The blue shaded line indicates the Vp/Vs of basaltic melt[18] for increasing melt fraction. The faded blue area corresponds to the dehydrating fluid.…”