On Silica Activity and Serpentinization

Serpentinites have the lowest silica activity of common crustalrocks. At the serpentinization front, where olivine, serpentine,and brucite are present, silica activities (relative to quartz)are of the order of 10^–2·5 to 10^–5, dependingon the temperature. Here we argue that this low silica activityis the critical property that produces the unusual geochemicalenvironments characteristic of serpentinization. The formationof magnetite is driven by the extraction of silica from theFe₃Si₂O₅(OH)₄ component of serpentine, producing extremely reducingconditions as evinced by the rare iron alloys that partiallyserpentinized peridotites contain. The incongruent dissolutionof diopside to form Ca²⁺, serpentine, and silica becomes increasinglyfavored at lower T, producing the alkalic fluids characteristicof serpentinites. The interaction of these fluids with adjacentrocks produces rodingites, and we argue that desilication isalso part of the rodingite-forming process. The low silica activityalso explains the occurrence of low-silica minerals such ashydrogrossular, andradite, jadeite, diaspore, and corundum inserpentinites or rocks adjacent to serpentinites. The tendencyfor silica activity to decrease with decreasing temperaturemeans that the presence of certain minerals in serpentinitescan be used as indicators of the temperature of serpentinization.These include, with decreasing temperature, diopside, andraditeand diaspore. Because the assemblage serpentine + brucite marksthe lowest silica activity reached in most serpentinites, thepresence and distribution of brucite, which commonly is a crypticphase in serpentinites, is critical to interpreting the processesthat lead to the hydration of any given serpentinite. KEY WORDS: serpentinization; serpentinites; silica activity; oxygen fugacity; rodingites; magnetization of serpentinites