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²³¹Pa excesses in arc volcanic rocks: Constraint on melting rates at convergent margins

¹ Department of Geology, University of Illinois at Urbana-Champaign, 1301 West Green Street, Urbana, Illinois 61801, USA

The ²³¹Pa-²³⁵U disequilibria provide greater ability to constrain the rate of mantle melting in convergent margin settings than the more often analyzed ²³⁸U-²³⁰Th-²²⁶Ra systems, which are strongly affected by fluid-addition processes from the subducting slab. Here we present new ²³¹Pa-²³⁵U data for 12 samples from the Kick'em Jenny (KEJ) submarine volcano in the Southern Lesser Antilles to define the melting rate at a subduction zone with one of the lowest convergence rates. The KEJ samples have the highest average (²³¹Pa)/(²³⁵U) yet measured in global arcs, consistent with other studies of the Southern Lesser Antilles lavas. These results reinforce the previously noted negative correlation between average (²³¹Pa)/(²³⁵U) and convergence rate in all arc settings globally.

We develop a model to explain this negative correlation and to better constrain melting rates at convergent margins. Assuming that the corner flow velocity is coupled to and equal to the subducting slab velocity, the melting rate, directly reflecting the flux of water added, becomes a linear function of subduction rate. This physical model is then coupled to three different melting models previously developed for calculating U-series disequilibria (reactive porous flow, dynamic, and flux melting). All three models reproduce the globally observed negative correlation between subduction rate and ²³¹Pa excess. Although the style of melting cannot be easily discriminated, the good correspondence between models and observation provides an example of how geochemical and geophysical models can be linked to provide a self-consistent model of melt generation in convergent margin settings.

Key Words: ²³¹Pa-²³⁵U • melting process • subduction rate • convergent margin