Source contamination versus assimilation: An example from the Trans-Mexican volcanic arc

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 ⁸⁷Sr/⁸⁶Sr and ¹⁸⁷Os/1⁸⁸Os 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 ⁸⁷Sr/⁸⁶Sr of the samples vary little, ranging from 0.7037 to 0.7047, despite a wide range in major element composition. However, the ¹⁸⁷Os/¹⁸⁸Os vary greatly, from 0.135 to 0.410. Decreasing Os concentration and increasing ¹⁸⁷Os/¹⁸⁸Os show a clear relation with indicators of fractionation such as MgO or Ni. A plot of ¹⁸⁷Os/¹⁸⁸Os 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 ¹⁸⁷Os/¹⁸⁸Os (~0.135-0.145), and (2) increasing ¹⁸⁷Os/¹⁸⁸Os (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 ¹⁸⁷Os/¹⁸⁸Os. Superimposed upon these melts are both assimilation and fractional crystallization processes, which affect both the Ba/Nb and ¹⁸⁷Os/¹⁸⁸Os systems as they ascend into the lower crust.