Mapping of CO2 fluxes along the Mid-Ocean Ridge system: A Global Petrological Approach

The measurements of the volume per cent vesicles in basalts are important because they can be linked quantitatively to magma degassing, a key parameter to understand gas transfer from the Earth interior to the hydrosphere and the atmosphere. Unfortunately, such measurements are rare and data limited preventing any global approach. In order to circumvent this problem, vesicularity has been determined on 50 samples (Atlantic, Pacific and Indian oceans) by image analysis on thin sections of the outermost 1-1.5 cm of pillows. Logarithmic vesicularity relative to depth range for the 120 samples (70 from the literature) is highly scattered suggesting that the primary control on the vesicularity is not the depth of eruption. The spread in the data is clearly related to spreading rate and to a lesser extent to compositional variations. These aspects of the data suggest a multiple linear regression of the data with log vesicularity as the dependent variable and spreading rate and K2O/TiO2 as independent variables. The correlation allows the determination of the vesicularities for any portion of oceanic ridges provided the axial depth, spreading rate and K2O/TiO2 is determined. This empirical equation allows us to calculate the extent of magma degassing and the CO2 fluxes at segment scale all along the Mid-Ocean-Ridge system assuming that the gas contained in vesicles is exclusively CO2 and that it follows a perfect Henry's law. Magma degassing is low in the Pacific Ocean and high in the Atlantic and Indian oceans because vesicularity is reversely correlated to spreading rate. The total CO2 flux rate for the global ridge system is ~(5.8± 1.8) 1011 mol/yr and the CO2 mantle abundance is ppm. Thus our estimates are at the low end of what has been suggested in the literature. However, the principal strength of this new approach is indeed a global coverage of oceanic ridges but also at the scale of ocean basins. Importantly, our method does not require the mantle carbon content determination as a priori information.