Synthetic fluid inclusions XXIII. Effect of temperature and fluid composition on rates of serpentinization of olivine

Serpentinization, i.e. the hydrothermal alteration of ultramafic rocks, is an important and ubiquitous geologic process that occurs at slowand ultraslow-spreading mid-ocean ridges, magma-poor passive margins, and subduction zones. While serpentinization occurs over a wide temperature range and involves diverse fluid compositions, few experimental studies systematically examined the effects of temperature and fluid composition on the kinetics of serpentinization of olivine, and published rates diverge greatly. We present results of an experimental study using synthetic fluid inclusions in Mg-rich olivine as microbatch reactors to monitor the effects of temperature (100-350 degrees C), fluid composition (H2O-MgCl-NaCl, H2O-NaCl, H2O-MgCl), and total salinity on serpentinization rates of olivine under closed system conditions. Petrographic observations and Raman analyses of the experimental run products revealed the alteration of olivine to pro duce serpentine minerals, brucite, and magnetite. The salinity of the aqueous fluid in the inclusions increased as H2O was removed from the solution and incorporated into the hydrous product phases and served as a proxy for reaction progress. The fastest serpentinization rates were found at 250 degrees C in the presence of a seawater-like aqueous solution. This result further suggests that serpentinization of olivine is fastest at lower temperatures (-250 degrees C) and shallower depths in the oceanic lithosphere than suggested by previous studies (similar to 300 degrees C). Moreover, our experiments show that serpentinization rates decrease by several orders of magnitude as salinity and the concentration of dissolved Mg increase. These effects may reconcile some of the differences observed when comparing results of previously published rates obtained from experiments involving fluids of different compositions. We constructed a quantitative model based on the principle of detailed balance to predict variations in serpentinization rates (J) from low temperatures to 320 degrees C. It suggests that at 25 degrees C, serpentinization rates are 4 to 5 orders of magnitude slower than at 250 degrees C. Moreover, the temperature dependence model has been coupled with the kinetic effects derived from experiments involving different fluid composition to calculate the amount of time required for serpentinization of olivine having different reactive surface areas. The model shows that between 200 and 300 degrees C serpentinization is fast on geologic timescales when rocks with large reactive surface areas interact with a seawater-like aqueous solution (complete serpentinization in days to decades). (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Serpentinization, the hydrothermal alteration of ultramafic rocks, is a significant geological process that occurs in slow and ultraslow-spreading mid-ocean ridges, magma-poor passive margins, and subduction zones. Experimental studies have shown that temperature and fluid composition greatly impact the kinetics of serpentinization of olivine, with increases in salinity and dissolved Mg concentration leading to decreased rates.