Production and Ascent of Magma at Convergent Margins: Information From Small Eruptive Centers in the Central Southern Volcanic Zone, Chile

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9725366 Hickey-Vargas The proposed research is a geochemical study of basaltic volcanic rocks from small eruptive centers in the central Southern Volcanic Zone (CSVZ), Chile. Together with basalts from composite stratovolcanoes in the CSVZ, the fault-aligned small centers define a spatial and geochemical pattern suggesting that they were formed by melting with widely, but systematically varying amounts of hydrous subduction-related fluids. The objective of the research is to test competing hypotheses for melting and ascent of magma from above the subduction plate. Geochemical and isotopic data will be used to determine the relationship between the amount of hydrous fluid component and extent of melting. 10Be and 238U/230Th activity ratios will be determined for selected samples and compared with existing data from the composite stratovolcanoes, to determine whether basalts from the small centers include smaller amounts of recently extracted lithosphere. Petrography and electron microprobe analyses of phenocryst minerals will be used to infer liquid compositions. The results of this research will advance knowledge by contributing to our understanding of convergent margin magmatism, specifically, by identifying some of the controls on the siting of large and small volcanic centers, defining sources of subduction zone magma, and the pathways for cycling some elements between mantle and crust. Further, the research will advance education through the training of one Ph. D. student, and promote collaboration between U.S. and Chilean scientists.