PETROLOGIC AND GEOTHERMAL PROFILE OF THE LITHOSPHERE BENEATH KILBOURNE HOLE BASED ON GEOCHEMICAL ANALYSIS OF A MANTLE AND CRUSTAL XENOLITH

This study describes the geochemistry of an upper mantle and mid-crustal xenolith, both collected at Kilbourne Hole New Mexico. The mantle xenolith phase assemblage consists of forsterite, diopside, spinel, and enstatite. The mid-crustal quartz diorite xenolith phase assemblage is anorthite, quartz, diopside, and enstatite. The mantle xenolith exhibits grain and thin-section scale chemical variations which are interpreted as metasomatic alteration due to interaction with clinopyroxenite melt, in which chromium is being removed from spinel proximal to the vein. The magnesium number (Mg# (Mg number = 100 * [mols Mg /( mols Mg + mols Fe)) of olivine is 85.5 on the right side of the vein and 87 on the left. The spinel chromium number (Cr number = 100 * [mols of Cr/(mols of Al + mols of Cr)] exhibits a negative correlation with titanium on the left side of the vein in contrast to both the right side of the vein and within the vein which show no correlation. Temperatures obtained using the Brey and Köhler (1990) two-pyroxene thermometer are on average 1149°C on the left of the clinopyroxenite vein and 1236°C on the right and are corroborated by calcium in orthopyroxene and calcium in olivine thermometry (Köhler and Brey, 1990) which record temperatures of 1132°C. The temperatures were used to constrain the diffusion of calcium in olivine, which in turn yielded a timescale of 0.5-10 years in which the clinopyroxenite material interacted with the olivine. The clinopyroxenite is related to the basanite lava that erupted at KBH. This basanite likely infiltrated the upper mantle that contained the xenolith, causing the reaction rims in the clinopyroxene and diffusing calcium in olivine along the way generating the elevation of calcium concentration at the rims. The calcium-in-olivine signature indicated that the xenoliths were directly related to the rift extension, and a timescale for the disaggregation of the mantle could be constrained. The calculated equilibrium temperatures from both xenoliths coupled with the pressures derived from the calcium in olivine thermobarometer were used to calculate the depth and derive a geothermal gradient which is steeper than previously believed.