Ertan, Inci Evren, and Leeman, W.P.
Keith Wiess geological Laboratory, MS 126 Rice University, Houston, Texas 77005-1892.
Two different xenolith lithologies were chosen to characterize their Tm and Th by microthermometry. The entrapment pressures and temperatures of these fluid inclusions are calculated by using the MacFlincor program (Brown, 1992). Although fluid entrapment pressures only provide minimum depths for xenolith entrainment, they may be used to estimate the systematic changes in relative depths of origin and lithology. Calculated representative isochores for the lowest and highest density CO2 inclusions suggest the existence of several generations of densities between r = 1.07 - 0.58 g/cc in phlogopite-bearing olivine orthopyroxenite and websterite xenoliths. Assuming equilibration temperatures of being 1000 - 1060oC (calculated from Opx-Cpx thermometers), entrapment pressures of the highest density inclusions are estimated to be ~9 - 9.5 kb. The lowest density secondary CO2 inclusions suggest a pressure of ~2 kb. This range in pressures may simply reflect progressive fluid entrapment over a distance of depths during ascent; sources for the low density secondary inclusions may be either retrapped, decrepitated primary inclusions (e.g., Dromgoole and Pasteris, 1987) or CO2 exsolved from the host lava. Because high pressure fluid inclusions are rarely preserved due to deformation or decrepitation during ascent in hot lava we consider 9 - 9.5 kb to be a minimal depth of entrainment for these xenoliths and thus olivine orthopyroxenite and websterite were probably derived from a subarc mantle domain corresponding to at least ~30 km depth.