The general model for arc-type basalt genesis is that subducted plates reach a certain depth and release their fluids in a series of dehydration reactions. The fluid migrates into the overlying mantle wedge and initiates melting. High 187Os/188Os ratios in some arc magmas have been attributed to this fluid. Although this interpretation follows logically from the general model for arc-type basalt genesis, it is not supported by experimental data or by Re, Os, or Cl data for arc-type basalt, eclogites, and peridotites. Our new data for basaltic rocks from the Mexican volcanic belt together with data from several other arcs lead us to propose that subduction fluids contain only minimal Re, Os, and Cl, and therefore that 187Os/188Os is not increased significantly in the melts above 0.145. The observed low Re concentrations are best explained by initiation of arc-basalt melting in the garnet stability field. Retention of Re by garnet in the source will deplete arc-type basalts in Re relative to oceanic-island basalt and mid-ocean-ridge basalt. Radiogenic Os measured in many arc-type basalts is most likely due to crustal interaction rather than a subduction fluid with high and radiogenic Os concentrations.

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