Magmatic CO2 and saline melts from the Sybille Monzosyenite, Laramie Anorthosite Complex, Wyoming

Abstract

There are three populations of fluid inclusions in quartz from the Sybille Monzosyenite: early CO2, secondary CO2, and rare secondary brines. The oldest consist of low density CO2 (ρ≅0.70) inclusions that appear to be co-magmatic. The densities of these inclusions are consistent with the inferred crystallization conditions of the Sybille Monzosyenite, namely 3 kilobars and 950–1000° C. The other types of inclusions are secondary; they contain CO2 (ρ≅0.50) and secondary brine inclusions that form trains radiating out from a decrepitated inclusion. The sites of these decrepitated inclusions are now marked by irregularly shaped fluid inclusions and solid inclusions of salt and carbonate. Rather than fluid inclusions, feldspar contain abundant solid inclusions. These consist of magmatic minerals, hedenbergite, hornblende, ilmenite, apatite, and graphite, intimately associated with K, Na chlorides. We interpret these relations as follows: The Sybille Monzosyenite formed from a magma that contained immiscible droplets of a halide-rich melt along with a CO2 vapor phase. The salt was trapped along with the other obvious magmatic minerals during growth of the feldspars. CO2 may have also been included in the feldspars but it probably leaked later during exsolution of the feldspars and was not preserved. Both the saline melt and the CO2 vapor were trapped in the quartz. The melt inclusions in the quartz later decrepitated, perhaps due to progressive exsolution of fluids, to produce the secondary H2O and CO2 inclusions. These observations indicate that the Sybille Monzosyenite, which is a markedly anhydrous rock, was actually vapor-saturated. Rather than being H2O, however, the vapor was CO2-rich and possibly related to an immiscible chloride-rich melt.