Water and carbon in the early solar system

Abstract

Hydrogen (H) is the most abundant element in the universe. The original budget and speciation of major volatiles, such as hydrogen and carbon ( C), in the silicate material and atmospheres of rocky bodies in our Solar System was determined during their formation and early evolution. Nearly all the large rocky bodies in the Solar System are thought to have experienced at least one magma ocean phase as an outcome of formation processes. Highly reducing conditions well below the iron-wustite buffer (IW) characterized by a low oxygen fugacity (f02) were likely prevalent during the differentiation stages of rocky bodies. Reducing conditions prevailed during the early stages of planetary evolution and magma ocean solidification, and whilst some bodies like Earth progressively oxidized, others have remained more reducing. The investigation of redox conditions not only has implications for primordial magma oceans, but also for solid terrestrial bodies. Current estimates for the prevalent redox conditions in the silicate reservoirs of Mercury, Venus, the Moon and Mars are more reducing than the Earth. Under highly reducing conditions, the effect of volatile speciation on the mineral/melt partitioning and on H solubility in major silicate minerals such as olivine is largely unknown. After three decades of interest, the influence of pressure, temperature, and composition is fairly well constrained for the storage capacity and solubility of H in olivine. However, considerably less work has been conducted on the effect of f02 on H solubility in olivine, especially at very low /02, and the few available reports on this topic show substantial inconsistency. I present SIMS and Raman data to determine the H content and speciation of C-O-H volatiles in olivine and silicate glasses from experiments at (1-3.5 GPa) and temperature (1435-1750 °C). Using a Cr-Cr2O:3 buffer and hydride-bearing starting material, the /02 is estimated to be - IW-4, which can be directly compared to more oxidi;,;ed experiments run at - IW +4. Results demonstrate the systematic variance of C-O-H speciation, solubility, and partitioning as function of f02, which can be further explored, hut not sufficiently predicted, through C-O-H fluid models. The diminished H contents of olivine under reducing conditions indicates that future work concerning magma ocean crystallization or the reducing deep upper mantle of the Earth must consider the effects of low fO2, high fH2, and the existence of appreciable amounts of C.

Whilst there is a substantial body of literature on the nature and consequences of water incorporation in the Earth, we know comparatively less about the 'volatile' budgets of the other rocky planets. Mars is a particularly interesting body to investigate in this regard as there is evidence for the presence of a hydrosphere during its history, and it is also one of the few bodies we have direct (meteorite) samples from. Understanding the Martian H and C budget is crucial since such dissolved volatiles reduce the Martian mantle solidus, may act as drivers for volcanism, and are vital for the development of habitable surface environments. Concentrations of H and C were measured by SLVIS in experimentally produced replications of synthetic Martian systems provided by Dr. Justin Filiberto with clinopyroxene, orthopyroxene, olivine, and coexisting silicate glass phases. Such nominally anhydrous minerals (NAMs) can contain significant amounts of Hand, therefore, serve as important reservoirs for H within the silicate Earth and Mars. The determined values were used to calculate the mineral/melt partition coefficients (D values) for H and C. Experiments were conducted in nominally anhydrous conditions, and H partitioning values obtained here arc significantly greater than those obtained in previous studies. Comparison ·with literature data indicate that melt H/) content has an important additional control on H mineral/melt partitioning for clinopyroxene and orthopyroxene. Experimental DH<px/melt values also differ from previous models for DH<px/melt that are based solely on clinopyroxene compositions and do not account for the activity of H2O in the melt, especially at low activities. Combined with previous literature data on DH<px/melt, a new empirically calibrated model for DH<px/melt is developed based solely on phase composition which allows estimation of DH""x/mcir as a function of pressure and/or temperature. This model is applied to Martian meteorites, such as the nakhlites, ·which may contain knv water contents. The nakhlitc suite arc augite-rich and minimally shocked meteorites, which arc believed to have formed from lava flows from a single volcano, a shallow intrusion of basaltic magma, or a sill complex on Mars. Using the newly derived DH<px/melt, model, the magmatic water contents of the nakhla parental melt was estimate (and subsequently used to calculate the Martian mantle source H2O content by various equilibrium and fractional (accumulated) melting models. both the parental magmatic and the mantle source H2O contents can be compared to literature estimates for the Miartian mantle derived from other methods. Results suggest the H20 contents of the source regions for the nakhlites is significantly lower than most of Earth's upper mantle, but comparable to the primitive lunar mantle and the Martian depleted and enriched sources. Such limited data from Martian meteorites supports the presence of a partially hydrated Martian mantle, although one with a lower overall 'water' content than Earth, perhaps reflecting marked contrasts in geological processes on the t,vo bodies. Finally, semi-empirical models derived from a thermodynamic starting point are used to further explore the available dataset on H solubility in olivine.

The second half of this thesis similarly concerns H and C at subsurface (elevated pressure and temperature) conditions but focuses on astrobiologically important hydrocarbons and organics. If molecular biotic evidence docs exist on Mars, it is likely preserved within or below the crust where it would be protected from surface radiation and photolytic decomposition. Extended CV exposure is expected to result in the eventual destruct.ion of surface organics and diagnostic biomarkers. Molecular biosignatures may also have an advantage over morphological biosignatures (e.g., microfossils) in the subsurface environment. For instance, while mieromorphology can potentially distinguish biotic from abiotic carbonates, carbonates experience significant loss of primary microfabric and destruction of fossil evidence due to pervasive recrystallization during diagenesis. Experimental studies of the interactions between bio-organics and minerals under conditions simulating the harsh Martian environment provide key insights into possible prebiotic processes and the search for life. In general, organic matter analysed from a geologic environment can fall into three categories: abiotic compounds that are not associated with biological organisms, biogenic compounds produced by biological organisms, and thermogcnic compounds derived from the thermal decomposition of biologically generated compounds undergoing diagenetic processes. It is critical that biosignatures used to evidence pa.st or present life on !vfars arc not only biogenic or thermogenic, but discernible from abiotic compounds despite significant diagenetic or fluid alteration processes. Additionally, assessing the evolution of organic molecules in subsurface environments has significant implications for evaluating plausible scenarios for the origins of life and prebiotic chemistry. Despite protection from CV and oxidative degradation, buried biosignatures may undergo diagenetic processes that decrease the concentration of organic matter, as well as other degradation mechanisms as a result of elevated temperatures, pressures, and mineralorganic interactions. I provide a fuller understanding of preservation potential by considering several variables, including pressure, temperature, and the mineral matrix environment. Results inform future in situ searches for life on :\la.rs as well as the interpretation of organic analyses from past missions.