Seismic properties of serpentinites under increasing pressure and temperature conditions

Serpentinites result from the hydration of ultramafic rocks (e.g., peridotite and pyroxenite) that transform primary mineralogical assemblages (olivine-pyroxene) into different serpentine species (lizardite-chrysotile-antigorite) depending on the pressure-temperature (P-T) conditions (Schwartz et al, 2013) related to geodynamic context. Serpentinization mainly occurs from oceanic ridges at LP conditions to subduction zones at HP conditions and its extent needs to be quantified. The Vp/Vs ratio is a tool of choice to distinguish serpentinite from other mafic to ultramafic rocks and thus to investigate the serpentinization extent. The seismic velocity of serpentinites depends on the type and the volume of serpentine species (lizardite/antigorite) present in the rock, as well as the microstructural evolution of the serpentinite, however, these topics remain poorly understood. We selected blueschist and eclogitic serpentinite samples collected in the internal zone of the western Alps. One eclogitic serpentinite sample was also experimentally dehydrated at 700°C in standard pressure conditions. The petrology and (micro)structures of the samples were characterized using both 2D petrological thin sections and 3D X-ray tomography. We performed velocity measurements on the samples at low frequencies (quasi-static stress-strain method: 0.02-1000 Hz) and ultrasonic frequency (wave travel time method: MHz) at different effective pressures (low-frequency experiments: 2-25 MPa; ultrasonic experiments: 0-70 MPa). The description of experimental equipment and methodology can be found in Borgomano et al. (2020). The studied frequency range covers the field geophysical data frequency band and therefore provides better constraints for interpreting the serpentinization extent. As expected, the Vp/Vs ratio changes depending on the mineralogy: lizardite is characterized by higher Vp/Vs (2-2.1) and lower Vp (5200-5700 m/s) than antigorite with lower Vp/Vs (1.75-2) and higher Vp (6000-7000 m/s). Anisotropic structure due to mineral preferred orientation manifest by velocity variation with sample orientation: overall, the Vp and Vs anisotropy degree (Ap, As) of antigorite (Ap:3%-18%; As:1%-16%) are larger than those of lizardite (Ap:3%-9%; As:3%-8%). Furthermore, the seismic velocity of the serpentinite is almost unchanged with pressure and frequency, but pressure- and frequency-dependence of the velocity arise in the dehydrated antigorite sample: 1) in dry condition, with lower Vp/Vs ratio (1.46-1.6) and lower Vp (3500-4600 m/s) in the effective pressure range of 2-25 MPa and almost no frequency dispersion; 2) in water-saturated condition, with higher Vp/Vs ratio (1.87-1.89) and higher Vp (5060-5190 m/s) at ultrasonic frequency, as well as a significant velocity dispersion band linking the results from seismic frequencies to ultrasonic frequency. This study contributes to the calibration of seismic velocity profiles to track the mineralogical transition between lizardite and antigorite, which is related to increasing degrees of metamorphism.   References: Schwartz et al. (2013). Pressure–temperature estimates of the lizardite/antigorite transition in high pressure serpentinites. Lithos, 178, 197-210. Borgomano et al. (2020) An apparatus to measure elastic dispersion and attenuation using hydrostatic-and axial-stress oscillations under undrained conditions. Rev. Sci. Instrument, 91(3).