Source contamination versus assimilation : an example from the Trans-Mexican Volcanic Arc John Chesley a; *, Joaquin Ruiz a , Kevin Righter b , Luca Ferrari c , Arturo Gomez-Tuena d a Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA b Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA c Unidad de Ciencias de la Tierra, Instituto de Geolog| ¨a, Universidad Nacional Auto ¨noma de Me ¨xico, Campus Juriquilla, Apdo. Postal 1-742, C.P. 76001, Quere ¨taro, Quere ¨taro, Mexico d Instituto De Geologia, Universidad Nacional Automa De Mexico, Ciudad Universitaria, Mexico City, Mexico Received 16 May 2001; received in revised form 9 July 2001; accepted 9 November 2001 Abstract Volcanic samples representing a wide range of lithologies and compositions were collected from the Miocene to Quaternary age Michoacan-Guanajuato volcanic field (MGVF) in the Trans-Mexican volcanic belt (TMVB). The samples were analyzed for major and trace elements, and 87 Sr/ 86 Sr and 187 Os/ 188 Os in an effort differentiate the importance of source contamination and assimilation in continental arc magmatism. Re concentrations in the MGVF samples range from 0.03 to 0.13 ppb and Os concentrations range from 0.05 to 0.001 ppb. The 87 Sr/ 86 Sr of the samples vary little, ranging from 0.7037 to 0.7047, despite a wide range in major element composition. However, the 187 Os/ 188 Os vary greatly, from 0.135 to 0.410. Decreasing Os concentration and increasing 187 Os/ 188 Os show a clear relation with indicators of fractionation such as MgO or Ni. A plot of 187 Os/ 188 Os versus Ba/Nb for all samples from the MGVF show two distinct trends: (1) a wide variation in Ba/Nb (50^200) associated with minor variations in 187 Os/ 188 Os (V0.135^0.145), and (2) increasing 187 Os/ 188 Os (0.145^0.40) associated with restricted Ba/Nb (35^70). These trends are best explained through a dynamic multi-component process. Fluids are released from the subducting slab, resulting in melting of the overlying asthenospheric wedge. The pristine fluids have high Ba and low Re and Os concentrations. The resulting melts have variable Ba/Nb, but unradiogenic 187 Os/ 188 Os. Superimposed upon these melts are both assimilation and fractional crystallization processes, which affect both the Ba/Nb and 187 Os/ 188 Os systems as they ascend into the lower crust. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: Re/Os; subduction; island arcs; volcanism; metasomatism; assimilation 1. Introduction Understanding magma genesis in continental arcs is di¤cult because contamination in the man- tle source area by £uids, melts, subducted sedi- ments and contamination of asthenospheric man- tle-derived magmas by assimilation of lithospheric 0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0012-821X(01)00580-5 * Corresponding author. Tel.: +1-520-621-9639; Fax: +1-520-621-2672. E-mail address: jchesley@geo.arizona.edu (J. Chesley). Earth and Planetary Science Letters 195 (2002) 211^221 www.elsevier.com/locate/epsl