Melting Systematics in Mid-ocean Ridge Basalts and Implications for Global CO2 Fluxes

Abstract:

We present a new model for anhydrous melting in the spinel and plagioclase stability fields that provides enhanced predictive capabilities for the major element compositional variability found in mid-ocean ridge basalts (MORBs). The melting model is coupled to geodynamic simulations of mantle flow and mid-ocean ridge temperature structure to investigate global variations in MORB chemistry and crustal thickness as a function of mantle potential temperature, spreading rate, mantle composition, and the pattern(s) of melt migration. To constrain global variations in mantle melting parameters we incorporate evidence from both MORB major element compositions and seismically determined crustal thicknesses. Specifically, we show that to explain the global data set of crustal thickness, Na₈, Fe₈, Si₈, Ca₈/Al₈, and K₈/Ti₈ (oxides normalized to 8 wt% MgO) requires a relatively narrow zone over which melts are pooled to the ridge axis. In all cases, our preferred model involves melt transport to the ridge axis over relatively short horizontal length scales (~25 km), implying that although melting occurs over a wide region, up to 20–40% of the total melt volume is not extracted, and will eventually refreeze and refertilize the lithosphere. We further incorporate constraints from melt inclusion datasets to constrain the global mid-ocean ridge CO₂ degassing flux. Our estimates indicate that ~3.4 x10¹⁴ g/yr of CO₂ are released by MORB melting, however, more than half of this CO₂ may remain trapped in the lithospheric mantle.