THE STABILITY OF OLIVINE-ORTHOPYROXENE-CLINOPYROXENE-SPINEL ASSEMBLAGES AND APPLICATION TO XENOLITHS FROM KILBOURNE HOLE, NEW MEXICO

In peridotites, olivine, clinopyroxene, and orthopyroxene are complex solid solutions with wide stability fields. Depending mostly on bulk composition and pressure, these minerals may be accompanied by plagioclase (low pressure), spinel (moderate pressure), or garnet (high pressure). Although a particular mineral assemblage is stable over a range of P-T, the compositions of the individual minerals vary with changing conditions.

Application of standard geothermobarometers to Ol-Cpx-Opx-Sp peridotites is problematic. An alternative approach is to use a bulk rock composition to calculate equilibrium phase diagrams to determine the conditions under which the 4-phase assemblage is stable. This requires consideration of the system SiO2-Al2O3-Cr2O3-FeO-MgO-CaO-Na2O, internally consistent thermodynamic data for end members, and reliable mixing models for all mineral solutions. Using experimental studies in simpler systems as constraints, we derived multicomponent thermodynamic models and applied them to xenoliths from Kilbourne Hole. This approach constrains P and T of equilibration and is less sensitive to mineral compositional variations, or uncertainty in activity models, than standard thermobarometery. The ultramafic samples from Kilbourne Hole consist of different lithologies: porphyroclastic olivine-rich peridotite (PORP), porphyroclastic lherzolite (PL), fine-grained equigranular lherzolite (FGEL), and pyroxenite (PX). Calculations indicate that PORP, PL, and FGEL equilibrated at about 14Kb-1125C, 12Kb-1030C, 11Kb-980C, respectively, placing them in the uppermost mantle of the Rio Grande Rift.

These temperatures are similar to Ca in Opx calculations (Brey and Kohler, 1990). Our modeling provides the first P estimates, which place the xenoliths in the spinel stability field at depths that correspond to North American lithosphere. Sub-Moho seismic velocities require the temperatures of 1000 to 1150 to be a regional phenomenon rather than restricted simply to the vertical column sampled by Kilbourne Hole. These temperatures are probably in excess of a steady state conductive geotherm.