Serpentinization plays a fundamental role in the biogeochemical and tectonic evolution of the Earth and perhaps many other rocky planetary bodies. Yet, geochemical models still fail to produce accurate predictions of the various modes of serpentinization, which limits our ability to predict a variety of related geological phenomena over many spatial and temporal scales. Here, we use kinetic and reactive transport experiments to parameterize the brucite silicification reaction and provide fundamental constraints on SiO2 transport during serpentinization. We show that, at temperatures characteristic of the sub-seafloor at the serpentinite-hosted Lost City Hydrothermal Field (150 °C), the assembly of Si tetrahedra onto MgOH2 (i.e., brucite) surfaces is a rate-limiting elementary reaction in the production of serpentine and/or talc from olivine. Moreover, this reaction is exponentially dependent on the activity of aqueous silica (aSiO2(aq)), such that it can be calculated according to the rate law: Rate=2.3×10−4aSiO2(aq) 1.5(mol/m2/s). Calculations performed with this rate law demonstrate that both brucite and Si are surprisingly persistent in serpentinizing environments, leading to elevated Si concentrations in fluids that can be transported over comparatively large distances without equilibrating with brucite. Moreover, applying this rate law to an open-system reactive transport experiment indicates that advection, preferential flow pathways, and reactive surface area armoring can diminish the net rate of Si uptake resulting from this reaction even further. Because brucite silicification is a fundamentally rate-limiting elementary reaction for the production of both serpentine and talc from forsterite, our new constraints are applicable across the many environments where serpentinization occurs. The unexpected but highly consequential behavior of this simple reaction emphasizes the need for considering serpentinization and many other hydrothermal processes in a reactive transport framework whereby fluid, solute, and heat transport are intimately coupled to kinetically-controlled reactions.
Bibliographical noteFunding Information:
This research used samples and/or data provided by the International Ocean Discovery Program (IODP). Portions of this research were supported by the United States National Science Foundation under grant number 1426695 . We thank Glenn Hammond for help with the PFLOTRAN simulations, Nicholas Seaton for assistance with SEM analysis, Brian Bagley for assistance with XRCT imaging and processing, and Anna Harrison for providing the brucite used in the open-system reactive transport experiment. The Characterization Facility at the University of Minnesota, where the SEM imaging was performed, receives partial support from NSF through the MRSEC program. The authors wish to thank two anonymous reviewers and Associate Editor Mike Bickle, whose critical evaluation of this manuscript helped to improve its clarity.
- aqueous geochemistry
- reactive transport