High-C content and CO2/Ba ratio of the Earth’s enriched upper mantle

Abstract

One of the fundamental constraints on the carbon content in the Earth’s mantle comes from depleted mid-ocean ridge basalts (D-MORB) that experienced minimal CO2 degassing. The CO2/Ba ratio of 120 ± 20 in these D-MORB have been used to estimate the CO2 content in more enriched MORB (e.g., N- and E-MORB) that have lost CO2 through degassing. However, it has been unclear as to how constant the CO2/Ba ratio is in D-, N-, and E-MORB, which are known to derive from partial melting of a chemically and isotopically heterogeneous upper mantle. In this study, we constrained the CO2/Ba ratio of the E-MORB mantle source in the Pacific by studying the compositions of MORB glass samples from the southern East Pacific Rise (on– and off-axial SEPR and Garrett transform fault) as well as volcanic glass samples from the Pukapuka ridges and Rano Rahi seamounts that extend to the west of the SEPR. Some of the submarine glasses from the Rano Rahi seamounts are D-MORB with highly depleted CO2 and incompatible trace element contents, but with isotopic composition pointing to a long-term enriched mantle source. These lavas were generated by re-melting of the residual Pacific E-MORB mantle source that had undergone a recent melting episode responsible for the Pukapuka ridge magmatism. Some Rano Rahi seamount lavas experienced limited CO2 degassing during eruption, and we consider that their most reliable pre-degassing CO2/Ba ratio is the maximum value of 241 ± 30, measured in the most trace element depleted but isotopically enriched sample. While we show that this high CO2/Ba ratio is in part due to fractionation during the earlier melting event that generated the Pukapuka ridges, the estimated initial CO2/Ba ratio of the Pacific E-MORB mantle source is 177-52+71, still higher on average than that of the D-MORB mantle source (120 ± 20). We use this CO2/Ba ratio for the Pacific E-MORB mantle source to provide an estimate of its CO2 content of 391-180+455 ppm. We speculate that the high CO2/Ba and C-enriched nature of the Pacific E-MORB mantle source could originate from either (1) recycling of oceanic lithosphere at subduction zones or (2) foundering of metasomatized continental lithospheric mantle into the convecting mantle.