Stable Isotope Fractionation in Carbonatites from Proterozoic to Recent: Tracking the Mantle Evolution of CO2

Carbonatites are an unusual form of igneous rock that consist of >50 modal % carbonate minerals. While their exact method of formation is still debated, experimental petrology and radiogenic isotope analyses indicate that they have their origin in the deep mantle, most likely from extremely low-fractional (>1%) melting of carbonated peridotite. Carbonatites typically occur as intrusive bodies, associated with alkaline rock provinces and rift zones, and may be co-genetic with diamond-bearing kimberlites. No carbonatites have been found older than 3.0 Ga (Tappe et al., 2011), and carbonatite activity appears to have increased throughout time.

The deep-mantle origin of carbonatites suggests that they may have incorporated elements of subducted oceanic crust that descended to various depths within the mantle, perhaps as deep as the core-mantle boundary. The subducted elements include remnants of continent-derived sediments, oceanic basalt, and lithospheric mantle, all of which have distinct isotopic compositions of carbon and oxygen resulting from differing types of interaction and fractionation with physical (e.g. atmospheric, oceanic) and biogenic sources. Since there has been an overall increase in the levels of biogenic carbon throughout time, a careful analysis should show a trend towards increasing biogenic isotope fractionation in carbonatites from Archaean to modern time.

This pilot study examines the carbon and oxygen isotope ratios of a variety of carbonatites of different ages, from early Proterozoic to Recent, in a preliminary effort to assess sources and evolution of carbon dioxide from the earth's mantle over time.