Scapolite phase equilibria and carbon isotope variations in high grade rocks: Tests of the carbon-dioxide-flooding hypothesis of granulite genesis.

Scapolite decarbonation reactions and carbon isotope analysis of CO$\sb2$ extracted from scapolite are used to determine the presence, composition, and source of fluid components in high grade rocks. Scapolite-plagioclase-garnet-quartz assemblages, common to many lithologies in high grade terranes, monitors CO$\sb2$ activity ($a$CO$\sb2$) by the reaction 2 Meionite + Quarts = 5 Anorthite + Grossular + 2 CO$\sb2$. The P-T-X location of this reaction was calculated using an internally consistent thermodynamic data set for meionite and phases in the CASCH system. Activity-composition relations for meionite in scapolite were calculated from the thermodynamic data set and compositional data on natural scapolite-plagioclase-calcite assemblages. Equilibration pressures of scapolite assemblages were calculated from clinopyroxene-garnet-plagioclass-quartz barometers calibrated for this study. The $a$CO$\sb2$ was calculated for a variety of high grade gneisses from the southwestern Grenville Province and other terranes. Granulites typically yield low to moderate values of $a$CO$\sb2$ (less than 0.5). Calc-silicates and meta-anorthosite yield moderate $a$CO$\sb2$. Deep crustal xenoliths yield a range of $a$CO$\sb2$. CO$\sb2$ for carbon isotope analysis was extracted from scapolite in mineral separates and whole rock gneisses with phosphoric acid at 75$\sp\circ$C. Scapolite meta-anorthosite and calcite from adjacent marble have the same carbon isotope composition indicating marble was a local source of fluids leading to $\sp{13}$C enrichment of the anorthosite. $\delta\sp{13}$C of granulites and mafic gneisses from the southwestern Grenville range from $-3$ to $-10$ per mil, consistent with a variety of carbon sources and a lack of regional homogenization of carbon in the gneisses via a pervasive fluid. The fluid calculations and isotope data are not consistent with pervasive CO$\sb2$ flooding in the southwestern Grenville Province. Locally, the effects of interaction of rocks with a CO$\sb2$ rich vapor phase have been documented and serve as a model to evaluate CO$\sb2$-rock interaction in other terranes. The calculations are also consistent with the vapor absent granulite facies metamorphism in the Furua Complex of Tanzania, and either vapor absent metamorphism or a mixed CO$\sb2$-H$\sb2$O fluid in the Sargur Terrane (southern India), Bergen Arcs (Norway), and many lower crustal xenoliths.Ph.D.Earth SciencesGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/128275/2/8907106.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/128275/5/Moecher-DP_DeepBluepermissions_agreement-CCBY.pdfDescription of Moecher-DP_DeepBluepermissions_agreement-CCBY.pdf : Hidden Deep Blue OA and CC license agreemen