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 similar to 14 degrees N (n = 6) and 34 degrees N (n = 11) on the mid-Atlantic ridge. All samples are carbon-saturated. One, the so-called popping rock 2 Pi D43 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 delta D-values ranging from -55 to -79 parts per thousand and from -55 to -88 parts per thousand for samples at 14 degrees N and 34 degrees N respectively. For carbon, the inferred initial predegassing contents vary greatly, from 660 to 14,700 ppmCO(2) and from 1400 to 57,600 ppmCO(2) for samples at 14 degrees N and 34 degrees 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 similar to 570 to similar to 730 at 14 degrees N and 34 degrees 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 similar to 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) x 10(12) mol/yr, a value actually consistent with that derived from C/He-3 systematics. (C) 2007 Elsevier B.V. All rights reserved.