CO₂ solubility in Martian basalts and Martian atmospheric evolution

To understand possible volcanogenic fluxes of CO₂ to the Martian atmosphere, we investigated experimentally carbonate solubility in a synthetic melt based on the Adirondack-class Humphrey basalt at 1–2.5GPa and 1400–1625°C. Starting materials included both oxidized and reduced compositions, allowing a test of the effect of iron oxidation state on CO₂ solubility. CO₂ contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and Fe³⁺/Feᵀ was measured by Mössbauer spectroscopy. The CO₂ contents of glasses show no dependence on Fe³⁺/Feᵀ and range from 0.34 to 2.12wt.%. For Humphrey basalt, analysis of glasses with gravimetrically-determined CO₂ contents allowed calibration of an integrated molar absorptivity of 81,500±1500Lmol⁻¹cm⁻² for the integrated area under the carbonate doublet at 1430 and 1520cm⁻¹. The experimentally determined CO₂ solubilities allow calibration of the thermodynamic parameters governing dissolution of CO₂ vapor as carbonate in silicate melt, KII, (Stolper and Holloway, 1988) as follows: lnKII ⁰=-15.42±0.20, ΔV⁰=20.85±0.91cm³mol⁻¹, and ΔH⁰=−17.96±10.2kJmol⁻¹. This relation, combined with the known thermodynamics of graphite oxidation, facilitates calculation of the CO₂ dissolved in magmas derived from graphite-saturated Martian basalt source regions as a function of P, T, and [Formula: see text] . For the source region for Humphrey, constrained by phase equilibria to be near 1350°C and 1.2GPa, the resulting CO₂ contents are 51ppm at the iron–wüstite buffer (IW), and 510ppm at one order of magnitude above IW (IW+1). However, solubilities are expected to be greater for depolymerized partial melts similar to primitive shergottite Yamato 980459 (Y 980459). This, combined with hotter source temperatures (1540°C and 1.2GPa) could allow hot plume-like magmas similar to Y 980459 to dissolve 240ppm CO₂ at IW and 0.24wt.% of CO₂ at IW+1. For expected magmatic fluxes over the last 4.5Ga of Martian history, magmas similar to Humphrey would only produce 0.03 and 0.26bars from sources at IW and IW+1, respectively. On the other hand, more primitive magmas like Y 980459 could plausibly produce 0.12 and 1.2bars at IW and IW+1, respectively. Thus, if typical Martian volcanic activity was reduced and the melting conditions cool, then degassing of CO₂ to the atmosphere may not be sufficient to create greenhouse conditions required by observations of liquid surface water. However, if a significant fraction of Martian magmas derive from hot and primitive sources, as may have been true during the formation of Tharsis in the late Noachian, that are also slightly oxidized (IW+1.2), then significant contribution of volcanogenic CO₂ to an early Martian greenhouse is plausible.