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

Abstract

Primitive lavas and scoriae from the western Mexican Volcanic Belt (MVB) and the Mexican Basin and Range Province (MBRP) display a large range in Os concentrations and Os isotopes, with [Os] ranging from ∼10 to 320 ppt and 187Os/188Os ranging from ∼0.128 to 0.207. Os concentrations and Os isotopes are correlated with abundances of MgO and Ni and with other indices of fractional crystallization, indicating that Os behaved as a compatible element during fractional crystallization. The high oxygen fugacity of subduction-related calc-alkaline and lamprophyric magmas from the western MVB precludes significant sulfide fractionation. Therefore Os must be 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 partitioning between melt with residual sulfide and silicate or oxide phases, and that DOs (sulfide/melt) is less than ∼2 × 105. Three chemically distinct Mexican volcanic suites, calc-alkaline basalts and lamprophyres from the western MVB and intraplate-type alkalic basalts from the MBRP, all display similar ranges in Os isotopes. Assimilation/fractional crystallization modeling suggests that the observed isotopic variations can be generated by minor (<12%) assimilation of Triassic age mafic lower crust. The least radiogenic samples still possess Os isotopes that are elevated relative to normal mantle peridotite. This difference may reflect variable input of radiogenic slab-derived fluids or melts into the source region of the Mexican magmas, with the slab signature strongest in the lamprophyres and calc-alkaline basalts and weakest in the intraplate-type alkalic basalts. Correlations between Os isotopes and Os concentrations in other arc suites and mid-ocean ridge basalts are unlikely to reflect mixing of distinct mantle components but instead reflect pervasive crustal (or seawater) contamination of magmas with low Os abundances.