Reconstructing mantle carbon and noble gas contents from degassed mid-ocean ridge basalts

Abstract

The fluxes of volatile elements from the mantle have long been used to understand mantle structure and evolution, and are critical controls on Earth's climate stability. Because of the ubiquity of magmatic degassing, inferring pre-degassing volatile concentrations from measured basalts requires the application of a degassing model. Such models, including the commonly-applied equilibrium Rayleigh distillation, typically assume equilibrium or solubility-based partitioning between melt and vapor. Here, we demonstrate that ratios of radiogenic isotopes of He, Ne, Ar and, especially, Xe measured in global mid-ocean ridge basalts (MORBs) are inconsistent with equilibrium degassing models, even when not considering He. We conclude that kinetic disequilibrium is a crucial process affecting volatile abundances during degassing. We present a simple disequilibrium Rayleigh distillation model to reconstruct pre-degassing MORB noble gas and carbon concentrations, which predicts that He and Ne achieve nearly equilibrium partitioning between melt and vesicles, but the slower-diffusing heavier noble gases are strongly affected by disequilibrium, resulting in non-equilibrium fractionation of noble gas elemental ratios, and other volatile element ratios like CO2/He.We apply our model to a large set of MORB data, and find average pre-degassing 3He, 22Ne, and 36Ar concentrations of 4.4 ± 0.9 × 10−10, 6.6 ± 1.4 × 10−11, and 6.8 ± 4.5 × 10−10 ccSTP/g (2σ), with variations of approximately 2 orders of magnitude, similar to other highly incompatible elements. Pre-degassing noble gas concentrations imply a mid-ocean ridge 3He flux of 800 ± 170 mol/yr and upper mantle 3He/22Ne and 3He/36Ar ratios of 6.6 ± 2.0 and 0.64 ± 0.44, but substantial variability in these ratios between samples. Applying our model to CO2, we calculate an average mantle CO2/3He molar ratio of 1.67 ± 0.21 × 109, which, when combined with our estimate of 3He flux, implies an upper mantle CO2 flux of 5.9 ± 1.0 × 1013 g/yr and a CO2 concentration of approximately 110 ppm.Our estimate of the mantle 3He flux is the first determined independently of oceanographic 3He measurements, and consequently represents a time-integrated flux substantially longer than ∼1000 years. And although at the high end of the range of previous estimates, our estimate does not resolve the long-standing heat-helium paradox. Additionally, our requirement for heterogeneous pre-degassing 3He/22Ne and 3He/36Ar ratios between samples is contrary to conclusions of previous applications of disequilibrium degassing models, which advocated uniform ratios. Furthermore, we find that CO2/Ba ratios are highly variable in MORB samples, but still consistent with an average mantle mass ratio of ∼100. However, estimating pre-degassing CO2 concentrations and the mantle CO2 flux depend strongly on the poorly-constrained carbon diffusivity. Consequently, our demonstration of the prevalence of disequilibrium during mid-ocean ridge degassing, and the potential for disequilibrium in other volcanic settings, highlights the need for better characterization of the physical parameters associated with volcanic degassing.