CLINOPYROXENE, ORTHOPYROXENE, AND OLIVINE CHEMISTRIES IN ULTRAMAFIC TO MAFIC XENOLITHS

We examined two suites of xenoliths, carried to the surface by different mechanisms, to check the hypothesis that mineral compositional zoning records the rate of ascension of the xenoliths. Kilbourne Hole, New Mexico, is a maar volcano where explosive steam explosions brought xenoliths to the surface. Cima Volcanic Field, California, characterized by basaltic cinder cones, is a more traditional volcanic terrane. Many types of xenoliths are found at both locations, including crustal granulites and mantle peridotite/pyroxenite. Most mantle samples are olivine websterite or spinel lherzolite. The samples appear fresh and contain few obvious secondary minerals. Microprobe analyses of olivine, orthopyroxene, clinopyroxene, spinel and plagioclase in lherzolites from both locations showed them to have typical compositions. Equilibria involving these minerals can be used to estimate the temperatures at which the xenoliths equilibrated. There are few equivalent equilibria that can be used to determine pressure. The best pressure estimates are obtained by noting that xenoliths from both regions fall in the spinel peridotite field, suggesting pressures of 10-15 Kb. Application of thermometers based on Ca exchange between opx and cpx, and Fe-Mg exchange between different pairs of mafic minerals, yield 950 to 1000 ºC (Cima) and 900 to 1050 ºC (Kilbourne). Similar temperatures are obtained using thermometers based on the Al content of pyroxene. Ca temperatures were slightly higher than Fe-Mg temperatures, suggesting that there has been some Fe-Mg reequilibration. Compositional zoning is absent or insignificant in most of the mantle xenolith minerals, although some clinopyroxene grains from both sites show Na and Al variation from rim to core. In contrast with the mantle samples, minerals in the crustal xenoliths are highly zoned and altered. Application of thermometers and barometers to the crustal samples is problematic.