Source versus crustal processing and the evolution of the mantle wedge in the Trans-Mexican Volcanic Belt: Constraints from Os-O-He isotope systematics in olivine

Abstract

We have integrated Os isotope systematics in olivine phenocrysts with published O and He isotope data from a suite of well-characterized high-Mg olivine-phyric basalts to andesites across the Trans-Mexican Volcanic Belt (TMVB) to address the relative roles of subduction-related crustal input to the mantle source versus shallow fractional crystallization and/or crustal assimilation. Osmium concentrations in the olivines across all the samples range from 7.3 to 2200 pg/g and, with the exception of one anomalous sample with 187Os/188Os(ol) = 0.532, 187Os/188Os(ol) ranges from 0.125 to 0.259. Olivines from the rear-arc samples are relatively unradiogenic in Os (187Os/188Os = 0.122 to 0.136) compared to the arc front olivines (187Os/188Os ≥ 0.130), which are more radiogenic than primitive upper mantle and largely overlap with mantle xenoliths from arc settings. The arc front olivines exhibit distinctly heavier δ18O than those of the rear-arc, but a significant role for crustal assimilation in the evolution of most TMVB magmas can be precluded due to the lack of correlation between 187Os/188Os(ol) or δ18O(ol) with indices of fractionation (e.g., Fo#, Ni(ol), and Mg#(WR)), as well as the mantle-like He isotope signatures of the olivines. This suggests that the radiogenic Os and heavy δ18O are inherited from the mantle source region. A mixing model between mantle and sediment-rich slab-derived components, as proposed previously for other areas of the TMVB, can explain the 87Sr/86Sr(wr) - 206Pb/204Pb(wr) - δ18O(ol) systematics. However, the radiogenic 187Os/188Os(ol) requires an unexpectedly high degree of fluid mobility for Os in this model. Instead, the Os data suggest that serial subduction fluxing and melting of the mantle wedge result in an accumulation of radiogenic Os in the mantle wedge through progressive slab flux, consistent with models from earlier studies based on olivine chemistry and the positive correlation of δ18O(ol) with mantle depletion proxies. The decoupling of 187Os/188Os(ol) and δ18O(ol) may be influenced by the presence of primary and secondary sulfides in the mantle wedge, which control the Os budget.