Source Versus Crustal Contamination in Subduction Processes

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ChesleyEAR-0125773Understanding magma genesis in continental arcs is difficult because contamination in the mantle source area and contamination of mantle-derived magmas by assimilation of lithospheric mantle or crustal rocks during ascent and emplacement produce similar geochemical characteristics. We have been studying the trans-Mexican volcanic belt (TMVB) in order to constrain these processes. We feel our previous work has shown that 187Os/188Os is an excellent proxy for assimilation, the geochemistry of samples with low 187Os/188Os (0.13-0.15) is primarily the result of source contamination, whereas geochemistry of samples with high 187Os/188Os (0.15-0.45) is the result of a more complex history that includes assimilation. The ability to decipher different inputs into arc magmas offers an opportunity to couple other elemental and isotopic systems into this framework to help unravel different inputs from source contamination and assimilation. We propose further study, of continental arc volcanism along the Trans-Mexican volcanic belt, whereby we utilize a combined approach of major and trace elements with Os, Pb, Sr and O isotopic analysis on whole rock or olivine and magnetite mineral separates of previously characterized samples, together with new samples from key localities. These data will be used to constrain the relative importance of source vs. crustal contamination for different components and isotopic systems in magma genesis across this arc. Specifically, we will examine: 1) The relative roles of mantle source contamination versus crustal contamination in basaltic magmas within individual fields and across the entire Mexican arc from west to east. 2) Determine whether volcanic fields (Palma Sola) in eastern Mexico are related to subduction of the Cocos plate or are part of the eastern alkalic province, which is unrelated to subduction.