Degassing of reduced carbon from planetary basalts

Abstract

Degassing of planetary interiors through surface volcanism plays an important role in the evolution of planetary bodies and atmospheres. On Earth, carbon dioxide and water are the primary volatile species in magmas. However, little is known about the speciation and degassing of carbon in magmas formed on other planets (i.e., Moon, Mars, Mercury), where the mantle oxidation state [oxygen fugacity (fO 2 )] is different from that of the Earth. Using experiments on a lunar basalt composition, we confirm that carbon dissolves as carbonate at an fO 2 higher than -0.55 relative to the iron wustite oxygen buffer (IW-0.55), whereas at a lower fO 2 , we discover that carbon is present mainly as iron pentacarbonyl and in smaller amounts as methane in the melt. The transition of carbon speciation in mantle-derived melts at fO 2 less than IW-0.55 is associated with a decrease in carbon solubility by a factor of 2. Thus, the fO 2 controls carbon speciation and solubility in mantle-derived melts even more than previous data indicate, and the degassing of reduced carbon from Fe-rich basalts on planetary bodies would produce methane-bearing, CO-rich early atmospheres with a strong greenhouse potential.hydrogen | iron carbonyl | magmatic volatiles | experimental petrology M agmas formed through igneous processes on parent bodies in the early solar system span 12 log units in oxidation state, from 6.3 log units of oxygen fugacity (fO 2 ) below the iron wustite oxygen buffer (IW) to 6 log units above the IW (IW−6.3 to IW+6) (1-4). Over the wide range of basalt fO 2 observed in these planetary bodies, thermodynamic data for the C-O-H gas system indicate that there are significant changes in gas phase speciation (5). These differences should be reflected in how volatiles are dissolved in magmas, but carbon speciation experiments for basaltic systems are limited, particularly at low fO 2 . The developing evidence for volatiles in magmas erupted on the Moon (6, 7) as well as on Earth (3,8), Mars (9-12), and Mercury (13) is a strong incentive to improve our understanding of C-O-H speciation and solubility in planetary basaltic magmas.Previous experimental studies on the speciation of carbon in Fe-free silicate systems under reducing conditions or at fO 2 < IW show carbon is dissolved as methane (14-16). However, under more oxidizing conditions in basaltic melt compositions, carbon is dissolved as carbonate (17)(18)(19), and the transition in speciation from methane to carbonate has long been debated as either occurring at more reducing conditions than IW+1 (20, 21) or at more oxidizing conditions (15). Another study (22) shows that a significant amount of C 4+ (carbonate) can be incorporated at extremely low fO 2 (IW−4.5) if Fe and alkalis are present in the melt. Additional experiments on Fe-bearing basalt are clearly required to resolve the conflicts in existing data. The results are important in understanding the residence time of carbon in the mantle and the rate of carbon degassing during the early evolution of terrestrial planets ...

The effects of sulfur, silicon, water, and oxygen fugacity on carbon solubility and partitioning in Fe-rich alloy and silicate melt systems at 3 GPa and 1600 °C: Implications for core–mantle differentiation and degassing of magma oceans and reduced planetary mantles

“…The bulk water content in this study was similar to lower, compared to previous, similar studies (Wetzel et al, 2013;Dasgupta et al, 2013a;Chi et al, 2014;Stanley et al, 2014). The variation of water content in our silicate melt allowed us to investigate its effect on carbon solubility in silicate melt and partitioning between alloy melt and silicate melt at a fixed temperature and pressure, and also f O 2 in the runs in the Knippa-Fe-Ni-S system (see below).…”

“…Further, these experiments did not extend to very reducing conditions (<IW-1.5) that may be relevant for early accretion of terrestrial planets such as Earth (e.g., Wood et al, 2006).…”

“…The species of C-H-O volatiles in reduced silicate melt, which are mainly a multifunction of fugacities of H 2 O, H 2 , and O 2 , remain unclear, resulting in the carbon species degassed from reduced, solid planetary mantles such as those in Mars and the Moon remaining unclear and debated (Dasgupta, 2013;Wetzel et al, 2013;Stanley et al, 2014).…”

Solubility of water in lunar basalt at low pH2O

“…Recent measurements of dissolved C and H 2 O in lunar glasses and improved understanding of C and H 2 O solubility in lunar melts (e.g., Saal et al 2008;Hauri et al 2011;Wetzel et al 2013;Chen et al 2015;Wetzel et al 2015; this work) warrant a fresh analysis of the major gas species accompanying lunar fire fountaining. In Fig.…”

“…We assume the carbon solubility relationship for reduced lunar glasses (fO 2 <IW−0.55) of Wetzel et al (2013), which is an empirical, linear relationship between dissolved C and total vapor pressure [C (ppm) = 0.2438×P (MPa), where P is pressure]. This relationship predicts that a melt containing 0.36 ppm C (assumed by Wetzel et al (2013) to be dissolved as iron pentacarbonyl and methane)…”