Degassing of organic carbon during regional metamorphism of pelites, Wepawaug Schist, Connecticut, USA


A comprehensive understanding of the degassing systematics of organic carbon (OC) during regional metamorphism is necessary to evaluate the role that metamorphism plays in the global carbon cycle. In this study, weight percentages and δ13C values of OC were measured in 70 samples of metapelites from the Wepawaug Schist, Connecticut, where classic Barrovian metamorphism occurred and graphitic OC is widespread. Relative to low-grade chlorite + biotite zone rocks, our mass balance analysis shows that OC in the metapelites underwent progressive loss from −14% (−0.06 g OC per 100 g rock) in the garnet zone, through −21% (−0.09 g/100 g) in the staurolite zone, to −26% (−0.11 g/100 g) in the kyanite zone. The average δ13C values in different metamorphic zones (ranging from −14.74‰ to −16.24‰) are all much higher than normal organic material in marine sediments, and increase slightly from the chlorite + biotite zone to the garnet zone and decrease slightly at higher metamorphic grades. Organic carbon degassing in the form of CH4 during the late stage of diagenesis or in the earliest stages of metamorphism could produce this significant 13C enrichment. Under the assumption that the 13C enrichment is caused by graphite degassing during the lowest-grade metamorphism (chlorite zone or lower), the degassing profile of OC during the regional metamorphism is reconstructed by combining the δ13C and OC mass change data. The computed results indicate that graphitic OC in the Wepawaug Schist probably underwent considerable loss at lowest-grade metamorphic conditions, ranging from ~−40% to ~−90% (or from −0.23 g OC per 100 g rock to −2.8 g OC per 100 g rock), and remained relatively inert at higher grades. Based on the mass balance analysis, δ13C systematics, and exploratory modeling results, this study argues that the lowest-grade or pre-metamorphic stages would be the more efficient OC liberators, and that the degassing potential of OC in the major stages of Barrovian metamorphism appears to be much more restricted. Additional independent studies are required to decipher the early degassing of OC after the deposition of organic matter, which could in turn help better constrain the degassing of OC during regional metamorphism.

Chemical Geology