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Subduction Zone Magmatism

The Geodynamics of the Andean Southern Volcanic Zone (Argentina-Chile), a Geochemical Approach

We have received funding from NSF-Earth Sciences to establish the budget and distribution the volatile contents of lavas from Chile-Argentina Andes between Lat. 35°-46° S. Knowledge of the distribution and composition of the chemical components associated with convergent margin magmatism is a fundamental requirement for understanding the origin, composition and evolution of the continental crust and the heterogeneity of the mantle. Establishing the budget and distribution of volatiles in the mantle at convergent margins is a necessary step to understand the processes responsible for the generation of arc volcanoes. Volatiles influence mantle melting, magma crystallization and volcanic eruption, and their abundances and spatial distribution provide important constraints on models of mantle flow, slab dehydration and crustal recycling. Over the last 30 years of funded research, the Andes Southern Volcanic Zone (SVZ) has been extensively investigated, with geochemical, petrological, geophysical, tectonic and geological studies providing important information on the composition of the magmatic arc, and the structure and evolution of the crust and mantle. However, data on volatile concentration and stable isotopic composition of lavas from the SVZ are virtually non-existent. This is a glaring omission from the geochemical data set given the important role volatile elements and mantle heterogeneity play in the generation and composition of arc magmas. The focus of our research will be to examine the regional variations in volatile contents and isotopic composition in SVZ basalts. The geochemical data collected will be used, once shallow processes are accounted for, to discriminate between slab dehydration, subduction of sediments and lower crustal interaction, and to test models of slab-flux melting versus adiabatic decompression.

Interpretations of the geochemical and petrological information will be carried out in conjunction with temperature and mantle-fluid (both hydrous and melt) flow models. These models will have a twofold purpose. First, they will stress the influence of a pressure-, temperature- and water-dependent mantle rheology on the temperature and the solid flow in the mantle wedge. The resulting solid flow fields will then enable us to evaluate its effect on the water and melting distribution within the mantle wedge and provide a link between geophysical and geochemical observations that would otherwise not be possible. The models of melt migration and mantle flow will predict, in particular, the distribution of melt delivered to the overriding plate and the proportion of the slab component contributing to the mantle melt as a function of the subduction angle, age of the descending oceanic lithosphere, thickness of the overriding lithosphere, obliquity of the convergence and the convergence rate; all parameters that have been previously determined for the SVZ. The systematic volatile data gathered from basalts of the SVZ combined with melt production behaviors from the models will allow us to constrain element and mass fluxes at this subduction zone. Such an approach will yield a clearer picture of arc lava evolution across the arc and may explain the observed chemical diversity in lavas erupting closely both in space and time. This combination of flow models and arc lava geochemistry will therefore allow for a better understanding of transport mechanisms in the mantle and how this can be shaped into a geochemically and geodynamically consistent picture of mantle flow, melt production, migration, and mantle heterogeneity (slab modified mantle) both in general and, more specifically, beneath the Andes Southern Volcanic Zone (SVZ).

Related Publications:
Cagnioncle, A.M. Kelemen, P. K., Parmentier, E.M. and Saal A.E. - (2008) - The Aleutians: a case study for fluid migration and melt production models. Geology, submitted.

Cagnioncle, A., Parmentier, E., Saal, A.E. and Kelemen, P. - (2007) - Simple Models of Melting and Trace Element Transport in Subduction Zones. EOS Trans. AGU 88, (52), Fall Meet. Suppl., Abstract V33F-04.

Brown faculty collaborators:
Marc Parmentier

Other project collaborators:
E. Hauri (Carnegie Institution of Washington); K. Hoernle (Christian-Albrechts University of Kiel, Germany); E. Nakamura (Okayama University, Japan); E. Baldo (Universidad de Cordoba, Argentina); L. Lopez-Escobar (Universidad de Concepcion, Chile); J. van Orman (Case Western Reserve University); R. Hickey-Vargas (Florida Internationa University); S. Kay (Cornell University).

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