A PETROGRAPHIC INVESTIGATION OF BASALTIC ROCKS FROM THE UPPER CARRIZOZO LAVA FLOW, VALLEY OF FIRES, NEW MEXICO

The Valley of Fires, NM is a 75-kilometer-long basaltic flow that erupted from a small cinder cone volcano known as Little Black Peak, and is located on the eastern edge of the Rio Grande Rift, approximately 2.4 kilometers northwest of Carrizozo, New Mexico. The lava flow sits upon an area of crustal weakness known as the Capitan Lineament. There have been three distinct episodes of eruptions identified, however, the time of these eruptions has been poorly constrained, with estimates ranging between 700 years and 5000 years. The focus of this investigation was to constrain the eruption temperature between lava flows of the Valley of Fires by determining the Anorthite CaAl₂Si₂O₈ (An) – Albite NaSi₃O₈ (Ab) content of the plagioclase. Three lava flows were delineated based on satellite imagery and distance from Little Black Peak. Samples from the three flows (A, B, and C units) were collected along highway US-380 where vertical sections of the lava flows are best exposed. A total of eighteen thin-sections were made from collected samples – seven from flow A, five from flow B, and six from flow C. Nine of the samples were prepared for XRD analysis. Six thin-sections, two from each unit, were selected based on the size of plagioclase phenocrysts suitable for An-Ab compositional analysis. Michael-Levy determination of mineral composition of 208 plagioclase phenocrysts from thin-sections indicates that unit-A shows an elevated anorthite content relative to units-B and –C. One hundred and four analyses from Unit A give a composition of An_78-86. Fifty-two analyses of Unit B samples were determined to be univariant at An₆₀. Unit C has a composition of An₆₅. These data suggest that Unit A may be distinct eruptive event of higher temperature lava. Further analysis of the data is being conducted at this time, however, preliminary analysis displays a correlation between the Michael-Levy composition and the composition found in XRD analysis. SEM-EDS elemental maps of olivine phenocrysts are planned to compare with temperature data from petrographic and XRD methods.