New Mexico Geological Society Annual Spring Meeting — Abstracts

The Carrizozo lava flow, located in south-central New Mexico, is a young (~5000 ka) tholeiitic basalt flow erupted in the Rio Grande rift, which consists of separate upper and lower flows. Whole rock data for six lobes of the lower flow and three lobes of the upper flow indicate a minimal role for fractional crystallization within the flows. Melting of heterogeneous mantle sources or magma mixing/assimilation can account for variations of major and trace elements and ⁸⁷Sr/⁸⁶Sr ratios of whole rocks. Six lobes of the lower flow encompass ~2% SiO₂ variation and < 1% MgO variation and slight K₂O and Al₂O₃ variations, which suggests little, if any, olivine or plagioclase fractionation. ~ 1% magnetite fractionation can account for decreasing Fe₂O₃ contents but cannot account for a 2% increase in SiO₂. These lobes define a linear trend in ⁸⁷Sr/⁸⁶Sr - 1/Sr indicating two component mixing of which one component has higher Sr and lower ⁸⁷Sr/⁸⁶Sr ratios and the other, lower Sr and higher ⁸⁷Sr/⁸⁶Sr ratios. Additionally, both Sr and Zr increase with decreasing SiO₂, which is inconsistent with fractionation.
For the three lobes of the upper flow, ⁸⁷Sr/⁸⁶Sr - 1/Sr does not define a linear correlation, thus mixing may not have occurred. However, 3.5% olivine fractionation can account for observed decreasing MgO contents, although K₂O and Na₂O contents are lower than expected while Fe₂O₃, TiO₂ and SiO₂ are higher than expected. Assimilation of a mafic source, occurring during fractionation, could account for variations in these major elements.