Intellectual Merit: A fundamental breakthrough in understanding the solid Earth geochemical cycle was the recognition of slab recycling in arcs. Quantifying the magnitude of the slab flux, however, is a complex task that requires understanding the petrogenesis of arc magmas, and in particular the causes of their high average SiO2 (>50-65 wt%) by which they differ globally from magmas produced at mid-ocean ridges and intraplate settings. The purpose of this project is to investigate the provenance of high-Si andesites and dacites (=?silicic magmas?) in the central Mexican Volcanic Belt (MVB) from mantle vs. crustal sources. The presence of equilibrium olivines with high, mantle-type 3He/4He (=7-8 Ra) in coeval basaltic and andesitic MVB magmas suggests the possibility that the high-SiO2 melts may have a mantle origin. Studies of silicic volcanic rocks from Quaternary composite volcanoes Malinche,Popocateptl and Nevada de Toluca and from coeval silicic monogenetic volcanoes of The interjacent Sierra Chichinautzin Volcanic Field are proposed to test this idea. Comprehensive major and trace element and isotopic data (incl. Sr-Nd-Pb-Hf isotopes) will be measured in bulk rocks and phenocrysts to evaluate crust vs. mantle contributions to formation of these rocks, and specifically whether or not the silicic magmas are principally(i) crustal partial melts, vs. (ii) melts generated in a subarc mantle infiltrated by reactive silicic fluids and partial melts from the subducting slab. A crustal origin of the silicic magmas would emphasize the importance of intracrustal processing and recycling at the expense of the recycled crustal flux from slab. Confirming the existence of broader spectrum of basaltic to dacitic initial mantle melts in the central MVB, however, has transformative potential as to models of arc magma genesis, arc and continental crustal growth, arc-trench connectivity and hence the overall impact of arc fluxes on the broader geochemical cycles.
Broader Impacts: The project furthers the careers of two female scientists (Straub, with a disability, and Bolge). Participation of undergraduate and graduate students will contribute to the training of the next generation. Scientific interaction between US scientists at Lamont and the Oregon State University and international colleagues in Mexico, Taiwan and Scotland will be furthered. The project proposed is of interest to the broader science community, as it addresses questions on the forefront of the GeoPRISMS Initiative and of the New Science Plan of the post-2013 International Ocean Discovery Program (IODP).
