QUANTIFYING MINERAL REACTION RATES OF LOW-TEMPERATURE SERPENTINIZATION USING SILICON ISOTOPE DOPING TECHNIQUE

Olivine minerals are unstable in the presence of water and CO₂ and undergo serpentinization and carbonations at low temperatures. Constraining the reaction rates of this process can give us a better understanding of the viability of using ultramafic mantle peridotites for mineral-based carbon sequestration and storage at large scale. Although many studies have measured serpentinization rates under higher temperature-pressure conditions, previous attempts to quantify the slow kinetics of this reaction near-equilibrium (below ~150°C) were unsuccessful due to the high uncertainty involved in measuring such small chemical changes using traditional analytical techniques. Here we use a novel ²⁹Si doping method to create serpentinization reactions at low temperatures (<100°C) and analyze changes over time to the experimental fluid using ICP-MS.

We present results from two sets of experiments to monitor the fluid-mineral interaction between olivine grains and fluid doped with ~400 ppb ²⁹Si. One set ran for approximately three months at ~25°C and atmospheric pressure, using olivine grains <53 μm. The other set ran for approximately three months at 85°C and atmospheric pressure, using olivine grains >150 μm and <250 μm. Time series for both sets for ²⁸Si and ²⁹Si show reaction between mineral and fluids over this time scale. Preliminary analysis by ICP-MS using ²⁸Si and ²⁹Si standard solutions indicate that small concentrations down to ~1 ppb are measurable for both isotopes. These time-series results should enable us to calculate the reaction rates of very slow serpentinization.