Low volcanic outgassing rates for a stagnant lid Archean Earth with graphite-saturated magmas

TL;DR

This study models early Earth's volcanic outgassing under stagnant lid conditions, revealing low CO2 emissions due to redox constraints and limited volatile replenishment, impacting early climate conditions.

Contribution

It introduces a coupled numerical model of mantle convection, volatile partitioning, and gas speciation, focusing on graphite-saturated magmas in a stagnant lid regime during the Archean.

Findings

Median CO2 outgassing rates less than 5 Tmol/yr for oxidized mantles

Reduced mantles produce almost no CO2, mainly CO in volcanic gases

Weak outgassing limits early greenhouse effects, affecting climate models

Abstract

Volcanic gases supplied a large part of Earth's early atmosphere, but constraints on their flux are scarce. Here we model how C-O-H outgassing could have evolved through the late Hadean and early Archean, under the conditions that global plate tectonics had not yet initiated, all outgassing was subaerial, and graphite was the stable carbon phase in the melt source regions. The model fully couples numerical mantle convection, partitioning of volatiles into the melt, and chemical speciation in the gas phase. The mantle oxidation state (which may not have reached late Archean values in the Hadean) is the dominant control on individual species' outgassing rates because it affects both the carbon content of basaltic magmas and the speciation of degassed volatiles. Volcanic gas from mantles more reduced than the iron-w\"ustite mineral redox buffer would contain virtually no CO2 because (i) carbonate ions dissolve in magmas only in very limited amounts, and (ii) almost all degassed carbon takes the form of CO instead of CO2. For oxidised mantles near the quartz-fayalite-magnetite buffer, we predict median CO2 outgassing rates of less than approximately 5 Tmol/yr, still lower than the outgassing rates used in many Archean climate studies. Relatively weak outgassing is due in part to the redox-limited CO2 contents of graphite-saturated melts, and also to a stagnant lid regime's inefficient replenishment of upper mantle volatiles. Our results point to certain chemical and geodynamic prerequisites for sustaining a clement climate with a volcanic greenhouse under the Faint Young Sun.