17 Multidisciplinary Studies of the Rio Grande Rift: Basins, Volcanism, Geophysics, and Hydrogeology I

Thursday, 10 May 2012
8:30 AM - 12:00 PM
Alvarado F&G

IDENTIFYING THE MAGMA SOURCE(S) OF THE CARRIZOZO LAVA FLOW, SOUTH-CENTRAL NEW MEXICO

Paper number: 17-12
Program: Theme Sessions
Time: 11:30 AM – 11:45 AM

Authors

Abstract

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.