A multi-proxy approach to local and regional variations in magma chemistry between 36.5 and 41°S in the Chilean Volcanic Zone

Subduction zone magmatism is the result of complex interactions among subducted slab, mantle wedge, and overlying crust. Separating the relative importance of the various processes requires a multi-proxy approach within the context of a natural experiment where parameters are well controlled. The Chilean southern volcanic zone (SVZ) provides such an opportunity. The convergence rate is roughly constant, while volcano elevation and crustal thickness increase progressively northwards. We have carried out a detailed investigation of the southern part of the SVZ. During field seasons in 2010 and 2011, 250 samples were collected from 13 volcanic centers in the SVZ between 36.5 and 41°S. These samples, as well as about 100 previously collected samples, have all been analyzed for major and trace elements by XRF and ICPMS. 70 of these samples were then selected for Sr-Nd-Pb isotope analysis by MC-ICPMS. Many of the same samples are being analyzed for U-Series, 10Be and oxygen isotopes (e.g. see Cooper et al, this meeting). Though element-element variation diagrams of samples from individual continental arc volcanoes typically reflect multiple processes, a careful and systematic selection of samples from each volcano enables incompatible elements to be independently corrected for the effects of crystal fractionation. Incompatible element concentrations that have been fractionation-corrected back to 6% MgO, such as Na6 and Nb6, were estimated on this basis. Trace element and major element data broadly confirms the results of previous geochemical studies conducted in the SVZ. The Na6, Nb6 and La/Yb ratios generally increase northwards and correlate with crustal thickness and elevation. While these trends at first seem consistent with a decrease in the extent of melting as crustal thickness increases, in detail there is much complexity. In contrast to predictions from simple melting models, Yb6 decreases northwards. Assimilation of low Yb compositions found in some basement samples could contribute to this trend. Such assimilation does not readily account for changes in Na6 and Nb6, however. Careful examination of each individual volcanic center reveals additional complexities, including apparent changes in the composition of the mantle wedge as reflected in high field strength elements. Crustal contamination appears to be most prevalent in the northern section of the study area (Chillan, Antuco, and Llaima), and less so towards the south, particularly at Osorno and Villarica. A composite model where extent of melting decreases and crustal assimilation increases towards the north best accounts for the current data. Forthcoming 10Be and O isotope data will allow us to understand these trends in even greater detail.