The Role of Flux Melting on U-series Systematics of Young Lavas from Costa Rica and Nicaragua

We have acquired U, Th and Pa isotope data for young lavas that derive from source regions affected by slab fluid addition that ranges in magnitude from minor beneath Costa Rica to substantial beneath Nicaragua. Four of the five Costa Rican samples have (²³¹Pa/²³⁵U) and (²³⁰Th/²³⁸U)>1 five out of six Nicaraguan lavas have (²³⁰Th/²³⁸U)<1 and (²³¹Pa/²³⁵U)>1, but (²³¹Pa/²³⁵U) ranges to lower values in Nicaragua. On a (²³¹Pa/²³⁵U) vs. (²³⁰Th/²³⁸U) diagram, the data, though limited in number, seem to define two trends with decreasing (²³⁰Th/²³⁸U), one with a slope of ~2.5 and the other with a slope of ~0.7. Trace element ratios indicate that the latter were affected more by addition of hemipelagic sediment, suggesting a link between subduction-fluid source and Pa-Th-U systematics. The combined ²³¹Pa-²³⁵U and ²³²Th-²³⁸U-²³⁰Th data provide constraints on the timing and mechanisms of fluid addition and partial melting beneath the Central American arc. Significant (²³¹Pa/²³⁵U) excesses (>1.5) in both Costa Rica and Nicaragua require a melting process that allows for ingrowth, as simple batch or fractional melting cannot explain the excesses at melt fractions large enough to explain trace element abundances and ratios. We propose a flux-ingrowth melting model in which the mantle wedge flows downward with the subducting slab and partially melts as fluid is added to regions with suitably hot temperature. We assume critical melting at low porosity ( ~10^-3) and that melt extraction and transport are rapid enough (< 8 kyr) to preserve observed ²²⁶Ra excesses. Because solid mantle may traverse the melting region over 10⁵-10⁶ yrs., ²³¹Pa and ²³⁰Th ingrow from U retained in the matrix. Magmas are aggregated instantaneously from all parts of the melting regime. This flux-ingrowth model matches a wide range of U-series and trace element data from Costa Rican and Nicaraguan lavas, with required average extents of melting of ~1% and 8-10%, respectively. Integration of melts from regions that have experienced extensive fluid addition, partial melting and U-daughter ingrowth with those from incipiently fluxed and melted regions yields liquids with elevated (²³¹Pa/²³⁵U) even after extensive fluid addition. The model produces linear arrays on Th isotope equiline plots which resemble isochrons, but which have no age significance. Upwelling and/or extensive melt-rock reaction is not required by U-series data from Central America or other arcs. Finally, the flux-ingrowth model is broadly consistent with substantial ²²⁶Ra excesses in Nicaragua without requiring the action of a distinct late slab fluid.