DEVELOPING NEW TOOLS TO ASSESS REDOX DIVERSITY IN BASALTS AND THEIR MANTLE SOURCES: A CASE STUDY IN THE RIO GRANDE RIFT

The redox record preserved in primitive basaltic magmas is an accessible, albeit indirect, archive of redox heterogeneity among various geochemical reservoirs within Earth’s mantle. Quantitative redox studies basalts can provide important new insights into the spatial and temporal changes in the oxidation state of various geochemical reservoirs in the mantle which are driven by the cycling of oxygen between surficial lithospheric material and the mantle. Primitive, olivine-phyric basalts from the Rio Grande Rift are well suited to addressing questions about the processes and intensive variables that are important for generating magmatic redox heterogeneity and how these variables relate to mantle processes, as they originate from a range of depths and mantle sources. To access the redox record in the non-glassy, RGR basalts, we have calibrated two thermodynamic oxybarometers (olivine-a_SiO2-spinel and Cr Valence in Olivine) and applied them to a suite of Quaternary RGR basalts. Our new models yield oxygen fugacities that range from ∆QFM –0.5 to +1.2 for RGR basalts in the Albuequrque basin and Jemez Lineament. The southernmost basalt from the Potrillo volcanic field has the most reducing signature (∆QFM–0.5); basalts become nearly an order of magnitude more oxidizing northward through the rift (~∆QFM+0.5). Redox variation in basalts from at least one volcanic field (Cat Hills) span nearly an order of magnitude (∆QFM+0.6 to +1.3), despite close geographic proximity, suggesting a locally heterogeneous mantle source. Samples with elevated oxidation states are correlated with Nd isotopic signatures associated with the lithospheric mantle and lower crust; those samples with low fO₂ values reflect an asthenospheric Nd isotopic signature. We postulate that the oxidized signatures of the RGR basalts are a product of melting a variably metasomatized mantle, residual from Farallon subduction, and that any metasomatized mantle in the southern part of the RGR has been exhausted.