Effect of temperature, pressure, and chemical compositions of fluids on the rates of olivine and peridotite serpentinization

Serpentinization, a low-temperature hydrothermal alteration of ultramafic rocks, strongly influences the physical and chemical properties of the oceanic lithosphere. Complete serpentinization significantly decreases the density, yield strength, and seismic velocities of ultramafic rocks, which may result in a great increase in volumes.  Serpentinites are enriched in H₂O, up to 13.5 wt%, and they contain much higher amounts of volatiles (such as carbon, sulfur, and nitrogen) compared to olivine. In particular, serpentine minerals can be stable at great depths, >150 km, suggesting that serpentinization may play an important role for the transferring of H₂O and other volatiles into great depths in subduction zones.

In spite of the significance of serpentinization, the proportions of serpentine in subduction zones still remain poorly constrained. Serpentinization kinetics are important parameters for quantifying the percentage of serpentine in subduction zones. The kinetic data obtained by Martin and Fyfe (1970) have been used to estimate the proportions of serpentine, <5% in subduction zones, which suggests that the distribution of serpentine in subduction zones is strongly affected by serpentinization kinetics. The kinetic data of Martin and Fyfe (1970) were derived from experiments using synthetic forsterite. In recent years, serpentinization experiments were performed using natural olivine, which has rates of serpentinization around 1-2 orders of slower compared to synthetic forsterite (Malvoisin et al., 2012; McCollom et al., 2016).

We have experimentally studied the kinetics of peridotite and olivine serpentinization at temperatures of 300-500 °C and pressures of 3.0-20 kbar. Compared to olivine, peridotite is serpentinized at much faster rates, reflecting the effect of pyroxene and spinel. At relatively high temperatures, e.g., 400-500 °C and 3.0 kbar, the kinetics of olivine serpentinization are sluggish, due to positive Gibbs energies of olivine serpentinization under such T-P conditions. In contrast, the kinetics of peridotite serpentinization under these T-P conditions are much faster. At 500 °C and 20 kbar, complete serpentinization can be achieved within a short period (e.g., 20 days). This suggests that the proportions of serpentine in subduction zones may be mainly controlled by the presence of H₂O rather than the rates of serpentinization. With the presence of enough H₂O, the percentage of serpentine in subduction zones may be much higher than previously proposed.

References:

Huang, R. F. et al. (2017) Journal of Geophysical Research: Solid Earth, 122, doi:10.1002/2017JB014231.

Huang, R. F. et al. (2023). Journal of Geophysical Research: Solid Earth, https:doi.org/10.1029/2022JB025218.

Malvoisin, B. et al. (2012). Journal of Geophysical Research: Solid Earth, 117(B4), B04102, doi: 10.1029/2011JB008842

Martin, B. & Fyfe, W. S. (1970). Chemical Geology, 6, 185‒202.

McCollom, T. M. et al. (2016). Geochimica et Cosmochimica Acta, 181, 175-200.