Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 10
Presentation Time: 11:30 AM


GLADISH, Eva, Geological Sciences, New Mexico State University, 1255 N Horseshoe St, Gardiner Hall, Las Cruces, NM 88008, RAMOS, Frank C., Department of Geological Sciences, New Mexico State University, Las Cruces, NM 88003, MCMILLAN, Nancy, Department of Geological Sciences, New Mexico State University, Box 30001, MAS 3AB, Las Cruces, NM, NM 88003 and ROWE, Michael C., School of the Environment, Washington State University, 1228 Webster Physical Science Bldg, Pullman, WA 99164-2812,

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.