( 231 Pa/ 235 U)-( 230 Th/ 238 U) of young mafic volcanic rocks from Nicaragua and Costa Rica and the influence of flux melting on U-series systematics of arc lavas

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Geochimica et Cosmochimica Acta, ISSN: 0016-7037, Vol: 66, Issue: 24, Page: 4287-4309

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Rebecca B. Thomas; Marc M. Hirschmann; Hai Cheng; R. Lawrence Edwards; Mark K. Reagan
Elsevier BV
Earth and Planetary Sciences; Journal
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We present U, Th, and Pa isotope data for young lavas from Costa Rica and Nicaragua in the Central American arc. Thorium isotopic ratios for Costa Rica and Nicaragua differ dramatically: Costa Rican lavas are characterized by low ( 230 Th/ 232 Th) (1 to 1.2) and, for four out of five lavas, ( 230 Th/ 238 U) greater than unity. Nicaraguan lavas have high ( 230 Th/ 232 Th) (2.2 to 2.7) and, for five of six samples, ( 230 Th/ 238 U) less than unity. All lavas have ( 231 Pa/ 235 U) greater than unity, with initial values ranging from 1.27 to 1.77, but those from Costa Rica have larger 231 Pa excesses. There is a broad positive correlation between ( 231 Pa/ 235 U) and ( 230 Th/ 238 U) similar to the worldwide trend for arcs outlined by Pickett and Murrell (1997), although many of the Nicaraguan lavas skirt the high end of that trend. In greater detail, the Central American data appear to divide into separate high-( 231 Pa/ 235 U) and low-( 231 Pa/ 235 U) tiers. These tiers may be different because of either different residence times in the crust or different proportions of sedimentary components from the slab.