Abstract

While much work on weathering and alteration processes has focused on granitoid compositions that represent typical continental crust, the subaerial weathering of volcanic rocks is an important part of the geochemical cycle. The alteration of volcanics is a strong sink for CO2 in part because of their compositions, hydraulic properties and chemical reactivity. On a global scale weathering of volcanics contributes disproportionately to CO2 consumption fluxes, and appears to depend strongly on temperature. Ocean island and arc settings are particularly important because they continuously generate new substrate, are geomorphically active, and are commonly wet. The coupled geochemical-hydrological evolution of the surface of volcanic islands differs substantially from most continental settings, is poorly understand, and can lead to substantial underestimates of weathering fluxes. The processes that control basalt alteration in a soil profile are also incompletely understood, in part because of the presence of complex amorphous phases in both primary and altered materials. We have used reactive transport modeling with a chronosequence of basaltic soils to better understand the roles of affinity, water saturation, acidity and ligand complexation in the weathering of basalt under humid conditions. Finally, the major province for volcanic alteration is the oceanic crust, where oxidative alteration is a major part of the global redox cycle. An interesting question is how the oceanic crust was coupled to the global redox cycle under conditions of low seawater sulfate and oxygen in Deep Time.