Testing the Processes of Melt Differentiation in the Mexican Volcanic Belt

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This project was co-supported by the Petrology and Geochemistry with the Office of International Science and Engineering. It aims to study processes leading to magma formation ('differentiation') in subduction zones using the Mexican arc as an example. On the Earth's surface, subduction zones form conspicuous chains of volcanoes ('volcanic arcs') parallel to the deep-sea trenches (e.g. the Cascades, Aleutians, Mexico). The formation of arc magmas has intrigued earth scientists ever since the role of subduction zones was recognized, and yet it remains poorly understood. Understanding the process of arc magma formation, however, is essential to quantitatively assess the impact of the arc out flux (including the out flux of volatiles) on the evolution and maintenance of a habitable Earth. This project will test whether recently discovered unusual Ni-rich olivines (up to 0.35-0.53 wt% Ni) that are ubiquitous in primitive alkaline and calc-alkaline magmas of the central Mexican Volcanic Belt may provide a vital clue. These Ni-rich olivines are so unusual since they rule out that arc magmas are simply melts, or mixtures of melts, that either originate from (i) the subducting oceanic crust ('slab'), located approximately ~110-120 km below the volcanic arc, (ii) the Earth peridotite mantle, at 45-110 km depth, or (iii) the pre-existing crust (0-45 km depth) on which the Mexican volcanoes are constructed. Thus, other processes must apply. A possible process may be the formation of a new 'pyroxenite lithology' in the source region of arc magmas. This lithology may form by reaction of silica-rich melts from the slab and the subarc peridotite mantle and generate arc magmas by melting. The research plan involves a detailed study of olivine phenocrysts from Mexican arc volcanic rocks that include: (a) petrographic textures, (b) trace elements in olivines (Co, Mn, Ca, Sc, V, Ni, Ti, Al, B and Li) by laser-ablation methods, (c) volatiles (H2O, CO2, S, F, Cl) and B and B isotopes in their primary melt inclusions by SIMS methods, and (d) He-isotope ratios of olivine separates. Since similar Ni-rich olivines have been recently been found in other arcs, e.g. the Cascades, Japan, Aleutians and Kamchatka, the outcome of this project has broader implications for global arc volcanism. The project promotes scientific partnership between three institutions in the U.S., as well as between the U.S., Mexico and Scotland. It also supports a female career scientist with a disability.