Compositional control on volcanic arc elevation and implications for magmatic differentiation

Abstract:

Examining the controls on compositional variability in volcanic arcs is central to understanding the bulk geochemical evolution of the Earth and provides constraints for examining other planets. Here we explore the relationship between elevation and arc lava compositions in island and continental arcs. We show, using globally compiled major and trace element data for active subaerial volcanic arcs and a series of digital elevation models, that lavas become, on average, more evolved with increasing elevation. This corroborates work relating crustal thickness to melt evolution, since elevation scales with crustal thickness and can be measured with greater certainty. At low elevations, lava compositions range from basalt to rhyolite, and as elevation increases, lavas become more andesitic to dacitic on average, with fewer basalts. Trace elements also show systematic behavior with changes in elevation. Incompatible element concentrations generally increase with elevation while compatible element concentrations simultaneously decrease. In particular, we note that the average Mn, Y, Sc, and Yb content in lavas decreases with increasing elevation while La/Yb and Sr/Y ratios increase. This trend is suggestive of garnet and/or amphibole fractionation at depth, given that the above elements are highly compatible in both minerals and both phases are stable at high pressures. Thus, we suggest that evolved magmas may be produced in elevated arcs due to more extensive differentiation in thick crust or as a result of the formation of garnet or amphibole-bearing cumulates at depth.