The role of C-O-H volatiles in the martian mantle and the production of the martian atmosphere.

Abstract

Evidence suggests that liquid water was once eroding the martian surface at rates comparable to many climates on present-day Earth. However, the thin modern martian atmosphere does not support liquid water. The fundamental variable in the evolution of the martian atmosphere is the storage of C-O-H volatiles in the interior, and the processes and fluxes leading to ventilation of those volatiles to the atmosphere. A key constraint on the likely CO2 fluxes accompanying martian magmatism is that much of the martian mantle is thought to be sufficiently reduced, between the iron-wüstite buffer (IW) and one log unit above IW (IW+1), such that carbon resides principally as graphite. In a reduced, graphite-saturated mantle there is a simple relationship between CO2 solubility and oxygen fugacity (fO2) which shows that an order of magnitude increase in oxygen fugacity changes the amount of CO2 dissolved in the melt by one order of magnitude. This thesis presents experimental investigations of the solubility of CO2, and other C-O-H species, in martian basalts and the implications for martian atmospheric evolution through three sets of laboratory-based experiments. In Chapter 2, experimental carbonate solubility is determined in a synthetic melt based on the Adirondack-class Humphrey basalt at 1-2.5 GPa, and 1400-1650 ºC. Experimentally determined CO2 solubilities are used to model the production of an early martian greenhouse. For the Humphrey source region, constrained by phase equilibria to be near 1350 ºC and 1.2 GPa, the resulting CO2 contents are 51 ppm at the IW, and 510 ppm at IW+1. However, solubilities are expected to be greater for depolymerized partial melts similar to primitive shergottite Yamato 980459 (Y 980459) which are investigated in Chapter 3. Similar experiments are performed on a synthetic starting material based on Y 980459. Despite large differences in FeO* (Fe2O3+FeO) and MgO contents, the CO2 solubilities in Y 980459 are similar to those in a less primitive Humphrey rock and a Hawaiian tholeiite. The small sensitivity of CO2 solubility to compositional variations among martian and tholeiitic basalts means that the experimentally determined solubilities may be applicable to a wide spectrum of martian magmatic products. In Chapter 4, the extraction of C-O-H volatiles from the Martian mantle is determined using the dissolved concentrations of C-O-H volatiles as a function of oxygen fugacity in synthetic martian magmas coexisting with graphite. CO2 solubilities change by one order of magnitude with an order of magnitude change in oxygen fugacity, as predicted by previous work. Other reduced species, such as Fe-carbonyls and amides, are detected in reduced graphite-saturated martian basalts. An atmosphere produced by degassing of magmas similar to this study would be richer in C-O-H species than previously modeled using only CO2 and could create a much warmer climate that stabilizes liquid water on the ancient martian surface.