METAMORPHIC CO2 DEGASSING FROM OROGENIC BELTS

Kerrick and Caldeira (1993, 1994a) concluded that metamorphic CO2 dega ssing in collisional orogens, and especially the Himalayan orogenic be lt, could have been an important factor in enhancing paleoatmospheric CO2 levels and contributing to early Cenozoic global greenhouse warmin g [Kerrick, D.M., Caldeira, K., 1993. Paleoatmospheric consequences of CO2 released during early Cenozoic regional metamorphism in the Tethy an orogen. In: Touret, J.L.R., Thompson, A.B. (Guest-Eds.), Fluid-Rock Intel.action in the Deeper Continental Lithosphere. Chem. Geol. 108, 201-230.] [Kerrick, D.M., Caldeira, K., 1994a. Metamorphic CO2 degassi ng and early Cenozoic paleoclimate. GSA (Geol. Sec. Am.) Today 4, 57-6 5.]. However, our revised CO2 mass loss computations for regional meta morphism in the Himalaya-Karakoram belt incorporating recent geochrono logic data and revised estimates of the proportion of carbonate source rocks indicate that metamorphic CO2 degassing from this orogen cannot explain Early Eocene warmth. Widespread pluton-induced hydrothermal f low occurred during the Eocene in the Cordilleran belt of western Nort h America. Synmetamorphic intrusions, which are common in metamorphic belts, may cause significant regional fluid flow. To obtain a represen tative CO2 flux from such environments, we computed a CO2 flux of 1.5 x 10(12) mol km(-2) Ma(-1) from petrologic and geochemical studies of the Paleozoic plutonic-metamorphic belt in New England (northeastern U nited States). For the 2 X 10(6) km(2) area of Eocene metamorphism in the North American Cordillera, the CO2 fluxes derived from the New Eng land metamorphic belt yield an area-integrated flux of similar to 3 x 10(18) mol Ma(-1). If a significant fi action of this CO2 entered the atmosphere, this degassing flux would alone account for Eocene greenho use global warming. For the Ominica belt within the Cordilleran orogen , a volumetric estimate of the mass of carbonate veins indicates that the consumption of CO2 by precipitation of carbonate veins may not sig nificantly decrease the amount of CO2 in fluids that convect to near-s urface crustal levels. Compared to other Eocene metamorphic belts, the widespread hydrothermal activity in the North American Cordillera may have been the largest, and most climatically significant, source of m etamorphic CO2 to the Eocene atmosphere. CO2 degassing by active metam orphism is most significant in extensional regimes of high heat flow. Extensional tectonism and hydrothermal activity in metamorphic belts m ay have substantially contributed to atmospheric CO2 content throughou t the Phanerozoic. Examples include the Mesozoic circum-Pacific metamo rphic belt, and Oligocene-Miocene regional metamorphism in the Himalay an orogen. (C) 1998 Elsevier Science B.V. All rights reserved.