Osmium abundance and isotope variations in mafic Mexican volcanic rocks: Evidence for crustal contamination and constraints on the geochemical behavior of osmium during partial melting and fractional crystallization

Primitive lavas and scoriae from the western Mexican Volcanic Belt (MVB) an d the Mexican Basin and Range Province (MBRP) display a large range in Os c oncentrations and Os isotopes, with [Os] ranging from similar to 10 to 320 ppt and Os-187/Os-188 ranging from similar to0.128 to 0.207. Os concentrati ons and Os isotopes are correlated with abundances of MgO and Ni and with o ther indices of fractional crystallization, indicating that Os behaved as a compatible element during fractional crystallization. The high oxygen fuga city of subduction-related calc-alkaline and lamprophyric magmas from the w estern MVB precludes significant sulfide fractionation. Therefore Os must b e compatible in at least one phase other than sulfide (e.g., olivine and/or Cr-spinel). The similarity of Os abundance trends in arc and nonarc suites suggests that Os concentrations are controlled primarily by equilibrium pa rtitioning between melt with residual sulfide and silicate or oxide phases, and that D-Os (sulfide/melt) is less than similar to 2x10(5). Three chemic ally distinct Mexican volcanic suites, calc-alkaline basalts and lamprophyr es from the western MVB and intraplate-type alkalic basalts from the MBRP, all display similar ranges in Os isotopes. Assimilation/fractional crystall ization modeling suggests that the observed isotopic variations can be gene rated by minor (<12%) assimilation of Triassic age mafic lower crust. The l east radiogenic samples still possess Os isotopes that are elevated relativ e to normal mantle peridotite. This difference may reflect variable input o f radiogenic slab-derived fluids or melts into the source region of the Mex ican magmas, with the slab signature strongest in the lamprophyres and calc -alkaline basalts and weakest in the intraplate-type alkalic basalts. Corre lations between Os isotopes and Os concentrations in other arc suites and m id-ocean ridge basalts are unlikely to reflect mixing of distinct mantle co mponents but instead reflect pervasive crustal (or seawater) contamination of magmas with low Os abundances.