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

¹ Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195, USA

Excesses of ²²⁶Ra 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 ²²⁶Ra 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 ²²⁶Ra excess. In particular, we found that dehydration melting of amphibolite can produce modeled ²²⁶Ra excesses greater than 300%. Mixtures of such amphibolite dehydration melts with mantle melts will likely retain a ²³⁸U 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 ²²⁶Ra excesses produced by slab dewatering alone.

Key Words: time scales • melting • ²²⁶Ra • ²³⁸U • ²³⁰Th • lower crust

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