PETROLOGY AND GEOCHEMISTRY OF THE SERVILLETA BASALTS, TAOS PLATEAU, NM, USA

Understanding potential sites for CO2 capture and sequestration through carbon mineralization in the US requires detailed petrologic surveys of mafic magmas. We conducted a petrologic study on the Servilleta tholeiitic basalts from the Taos Plateau to characterize its mineralogy and chemistry as a resource assessment and to understand the petrogenesis of the basalts. While previous work on the Servilleta basalts suggests they were generated from an EMORB source, there is limited information on their petrology and no estimates of pre-eruptive conditions. Whole rock analyses were conducted on 10 basalts; six were selected for electron probe analyses based on the extent of weathering. Samples contain olivine (~0.5-5.5%), clinopyroxenes (~1.5-8%), plagioclase (5-20%), and spinels in low abundance. Olivines are generally elongate, subhedral to anhedral crystals with cores that range in Mg#s (XMgO / (XMgO + FeO) * 100) from ~67-82, with an average of ~80. Clinopyroxenes occur either as a groundmass phase or as subhedral phenocrysts. Clinopyroxenes have Mg#s ranging from ~57-81. Plagioclase occurs as large, elongate, tabular crystals and makes up the majority of the groundmass. Large plagioclases span a range of ~44-64 mol% anorthite. Spinels are present in the groundmass of all samples and are also included in olivine and/or pyroxene. Groundmass spinels tend to be enriched in Ti, whereas spinel inclusions in olivine are enriched in Al and Cr. Temperatures at the time of olivine and clinopyroxene growth range from 1136-1250°C and 1129-1222°C, respectively. We incorporated olivine + Cr-spinel pairs and whole rock compositions into the new oxygen barometer of Bell et al. (2024) to obtain sample fO2 values, as a function of the fayalite-magnetite-quartz (FMQ) buffer. Oxygen fugacities for three basalts with olivine-spinel pairs that passed equilibrium tests range from -0.2 (±0.15) to -0.2 ∆FMQ. Trace element ratios were also used to determine elemental ratios in the mantle source. Source concentrations of Th/Nb and La/Ba are -0.04 and 0.41, respectively, and correspond to an EMORB source. Our work suggests the Servilleta basalts were produced by melting of enriched, reduced (~FMQ) mantle beneath the Rio Grande Rift between 3.5 to 5 Ma pointing towards an asthenospheric source.