Silica metasomatism of peridotite: Insights from the "other half" of forsterite serpentinization

Abstract:

Isochemical hydration of forsterite will produce an equimolar serpentine + brucite assemblage with no net change in fluid pH. However, brucite is often absent within serpentinites due to contemporaneous or later-stage Si metasomatism. To study this “other half” of the serpentinization process, we injected Si-enriched, Mg-free evolved seawater with an initial pH of 7.97 at a Darcy velocity of 1.3 km/yr into a brucite-lined fracture at 150°C for 167 hours. FTIR, XRCT, EDS, XRD, and fluid chemistry data indicate strong Si uptake and lizardite crystallization, particularly when pH of the reacting fluid rises above ~8.5. In this regard, our experimental design offers a unique perspective on the mechanism of serpentinization: our relatively high fluid flux and relatively low temperature push the reaction out of the typical water transport-limited regime into the reaction rate-limited regime. In this regime, it appears that the rates of brucite dissolution outstrip the rates of serpentine precipitation, consistent with literature kinetic data. Thus, when brucite-fluid interfacial area is large, the Si-metasomatic component of serpentinization proceeds by way of a brucite dissolution step rather than direct addition of Si to the brucite structure. However, in later stages of the experiment, brucite reactivity and Si-uptake slow dramatically, consistent with a more diffusive control on reaction progress. The early reaction scheme is in agreement with literature measurements of considerable Mg isotope fractionation during room temperature serpentinization experiments, but the later-stage scheme is in agreement with literature measurements from higher-temperature field systems, which suggest Mg does not fractionate during serpentinization.