PACROFI VI - Electronic Program

Fluid Inclusion Studies in Xenoliths of the Cascades Subarc Mantle

Ertan, Inci Evren, and Leeman, W.P.

Keith Wiess geological Laboratory, MS 126 Rice University, Houston, Texas 77005-1892.

Mantle xenoliths consisting of spinel lherzolite, dunite, harzburgite, websterite and orthopyroxenite occur in basaltic lavas of the Simcoe volcanic field, Cascades arc, Washington. Pure CO2 inclusions with or without vapor bubbles up to 50 microns in length occur as secondary or pseudosecondary inclusions mostly in ortho- and clinopyroxene in websterite- and orthopyroxenite-, and rarely in olivine in dunite xenoliths. These xenoliths are invaded by alkali- and silica-rich glasses (phonolithic-trachytic) that are not related to the host lavas. Almost all of the secondary inclusions can be traced back to glass veins which percolated along the grain boundaries. Necked-down and vermicular melt inclusions (some vitrified glass) in websterites are also common features. In some very coarse grained webterites dark, negative crystal-shaped secondary CO2 inclusions lie parallel to exsolution planes of Opx-Cpx lamellae, blades of spinels, melt inclusions, along with trains of sulfide blebs. CO2 inclusions are particularly abundant and diverse in a rare phlogopite-bearing olivine orthopyroxenite and usually occur in the healed fractures or cleavage planes of orthopyroxenes along with metasomatizing melts and newly formed phlogopites.

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.