Scapolite phase equilibria and carbon isotopes: constraints on the nature and distribution of CO₂ in the lower continental crust

Recent applications of scapolite phase equilibria and carbon isotopes are reviewed, in order to evaluate the role played by CO₂ in the genesis of granulites in the lowermost crust. Scapolite occurs mainly in mafic granulites from exposed high grade terranes, most of which have had a prior upper crustal history before equilibration at 9-10 kbar, and in high pressure (11-15 kbar) xenoliths and orthogneisses which likely represent mantle derived material that has not interacted with supracrustal lithologies. Calculated equilibrium CO₂ activities are sufficiently high (> 0.7) to be consistent with the presence of a CO₂-rich fluid phase accounting for the low H₂O activities in scapolite-bearing granulites and some xenoliths. CO₂ activities less than ca. 0.5 or the absence of scapolite are consistent with fluid absent metamorphism. The measured isotopic composition of carbon (δ¹³C_PDB) in scapolite is only a tight constraint on the source of carbon in scapolite for mafic xenoliths and high pressure orthogneisses. Carbon isotope values for these lithologies (- 7 to - 2‰) overlap the range of values for CO₂ dissolved in or exsolved from basalts, and are consistent with a magmatic origin for the CO₂. The carbon isotope composition of scapolite in granulites from exposed terranes (- 10 to - 4‰) is more equivocal, and may instead be related to pregranulite facies carbonate present in the rock. Minimum values of volumetric fluid/rock, calculated to account for the modal abundance and composition of scapolite in granulites (on the order of 0.001 to 0.1), are much lower than those required for the desiccation of amphibolites to granulites (0.1 to 0.5). Thus scapolite should form in response to infiltration of such large volumes of CO₂, if P, T, and bulk compositional constraints are met. However only seven of twenty granulite occurrences contain primary carbonate-bearing scapolite in mafic granulites. It appears that CO₂ occurs in the lower crust only locally in a CO₂-rich fluid phase or dissolved in mafic melts, and not on a pervasive scale or at the high volumetric fluid/rock required the model of carbonic metamorphism.