Carbon stable isotope constraints on CO2 degassing models of ridge, hotspot and arc magmas

Abstract

Carbon dioxide emissions from volcanoes are important parameters to constrain in order to fully understand the Earth system, especially the effect of volcanic forcing on climate. The characterization of carbon concentration in magmas has been used to constrain volcanic fluxes. Because of low CO2 solubility in silicate melts, however, CO2 is significantly degassed from magmas due to decompression during transfer to the surface. The measurement of the carbon stable isotope ratio (13C/12C expressed as δ13C-values) in natural submarine glasses has been a helpful geochemical tool to study magma degassing. Carbon stable isotope fractionation at magmatic temperature between CO2 in vesicles and carbonate ions dissolved in the melt is still large enough to cause variations in δ13C-values. This variability can be used to deduce the mode of degassing (open vs. closed system, equilibrium vs. kinetic) operating in a given magmatic system.In this study, I present a review of the existing carbon isotope data for magmas of three different settings (ridge, hotspot and arc). This review allows to (1) investigate the diversity of degassing modes operating within a given setting and (2) compare the prevailing degassing mode between these three settings.Except for rare undersaturated samples and for volatile-rich, vesicular popping rocks, Mid-Ocean Ridge Basalts (MORBs) are predominantly extensively degassed and supersaturated in CO2 reflecting incomplete degassing during their last degassing step. Such a behavior is also reflected in their vesicle-dissolved carbon isotopic fractionations that are generally smaller than equilibrium values stemming from kinetic/diffusive effects. By contrast, the CO2 + H2O gas phase in hotspot and arc magmas is predominantly in chemical equilibrium with the melt because of volatile-rich initial conditions (and thus larger vesicularity) enhancing vapor-melt chemical and isotopic equilibrium. This larger initial volatile content is responsible for the extensive open-system (Rayleigh distillation) degassing generally observed in hotspot and arc magmas.This review highlights the fact that one single degassing model cannot explain the geochemical evolution of all magmas. Also, no single model can be assigned to one specific type of magma (i.e., a significant diversity of degassing mode exists within a given type of magma, notably in MORBs). It is important to take into account these observations when degassing corrections are applied on the basis of noble gas (He/Ne or He/Ar) ratios. It appears helpful to characterize the carbon concentration and stable isotope ratios in vesicles and dissolved in the glass in order to (1) identify the mode of degassing operating in a given magmatic system and (2) apply the most appropriate degassing correction to these magmas. It is only after this critical correcting step has been achieved that an assessment of initial magmatic CO2 contents can be reasonably undertaken.