226Ra/230Th excess generated in the lower crust: Implications for magma transport and storage time scales

Abstract

Excesses of 226 Ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs, followed by fast ascent rates of magma from slab to surface (up to 1000 m/yr). However, we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle-derived 226 Ra excesses, and so reduce inferred vertical transport rates. We developed an incongruent, continuous melting model, and both the incongruent melting reaction and ingrowth effects contribute to the 226 Ra excess. In particular, we found that dehydration melting of amphibolite can produce modeled 226 Ra excesses greater than 300%. Mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 U excess (subducted slab) signature. This amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements, similar to those observed in several arc settings, which may distinguish these magmas from those with 226 Ra excesses produced by slab dewatering alone.