Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H2O/Ce, δD, CO2/Nb) in the North Atlantic mantle (14° N and 34° N)

In order to better characterise mantle CO2/Nb-variability, we obtained and compiled major and trace elements, content and isotope composition of both CO2 and water on two series of mid-ocean ridge basalt (MORB) samples dredged at ∼ 14° N (n = 6) and 34° N (n = 11) on the mid-Atlantic ridge. All samples are carbon-saturated. One, the so-called popping rock 2ΠD43 kept its vesicles, the initial (pre-degassing) C-contents of the 16 other samples being reconstructed from their assumed degassing history. For water, the samples show large variations, from 1300 to 6900 ppm and from 1900 to 7900 ppm with associated δD-values ranging from − 55 to − 79‰ and from − 55 to − 88‰ for samples at 14° N and 34° N respectively. For carbon, the inferred initial predegassing contents vary greatly, from 660 to 14,700 ppmCO2 and from 1400 to 57,600 ppmCO2 for samples at 14° N and 34° N respectively. Measured Nb-contents range from 4.5 to 29.6 ppm show both good agreement with previously published data and positive correlations with reconstructed initial CO2-contents. The mean CO2/Nb range from ∼ 570 to ∼ 730 at 14° N and 34° N respectively. CO2 and Nb data for the two undegassed samples available so far (i.e. the popping rock of the present study and the basaltic glasses from the Siqueiros transform fault from the study of Saal et al., 2002) show significant variations in CO2/Nb over a factor of 2 and thus questions the constant CO2/Nb previously emphasised for these two samples, this view being supported by CO2/Nb-ratios of samples whose initial C-contents were reconstructed. For incompatible elements such as Ce, K and including water, a comparison of the geochemical characteristics of transform fault basaltic magmatism with other MORB systems shows magma transform fault magmatism to be unrepresentative of mantle compositions. Assuming a more appropriate average MORB CO2/Nb-ratio of ∼ 530 and a mean MORB Nb-content of 3.31−1.8+3.99, we computed a mantle carbon flux of 2.3−1.3+2.7 × 1012 mol/yr, a value actually consistent with that derived from C/3He systematics.