CO2 fluid inclusion barometry in mantle xenoliths from central Mexico: A detailed record of magma ascent

Abstract

Chemical equilibrium and CO2 fluid inclusion barometry are complementary techniques, as they register different geological processes. Chemical equilibrium barometry records the pressure at the site where the xenoliths were incorporated into the transporting magma, but it is not a sensible technique to document magma ascent. CO2 fluid inclusion pore pressure in xenoliths does not register the P–T conditions in the source area, but it allows illustrating a fairly detailed record of different geological processes that occurred during the magma transport to the surface and as the eruption proceeded. Mantle xenoliths from Ventura–Espíritu Santo and Santo Domingo volcanic fields contain dominant CO2 pseudosecondary and secondary fluid inclusions trapped in cpx and ol. Cpx chemical equilibrium pressures indicate a maximum pressure of 10kbar for the source area. Pore pressures obtained in CO2 pseudosecondary and secondary fluid inclusions show a distribution with three maximum peaks at ca. 8, 5–7, and less than 3kbar. A comparison with geophysical models for the area where the xenoliths-bearing volcanoes are located shows that the three peaks in the pore pressures correspond to three physico-chemical transitions within the continental crust. Likewise, the pore pressure suggests that rapid magma ascent is momentarily interrupted by these discontinuities, a fact that allows the formation of new fluid inclusions and the re-equilibration of some of the inclusions already present in the primary minerals of the xenoliths.