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

Abstract

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\circC. 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\sb2O 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