An examination of the reactivity of CO2 with Mg-silicates at pressures and temperatures of the earth's mantle and implications for deep carbon storage

Advisors: Mark R. Frank.Committee members: Nicole D. LaDue; Jim Walker.The deep carbon cycle can be better constrained by understanding the reactivity of CO2 with common mantle minerals. Surface reservoirs exchange carbon to the mantle via subduction zones, which carry carbonate minerals into the mantle. It is commonly accepted that there is a carbon deficiency with regards to degassing and the mantle may be the largest potential reservoir for terrestrial carbon. Carbon is released from decarbonation reactions as a volatile (i.e. CO2) and may react with common mantle minerals including forsterite (end member Mg-olivine) to produce a carbonate mineral, magnesite, which has been shown to have exceptional stability under high pressure and temperature conditions. The purpose of this study is to demonstrate a magnesite forming reaction between forsterite and CO2 using a high pressure diamond anvil cell with in-situ laser heating. Samples analyzed using micro X-ray diffraction definitively show the presence of magnesite at pressures ranging from 2.43 to 21.1 GPa and temperatures of up to 1590 K. These results indicate that magnesite can be produced through a forward reaction in the upper mantle and into the transition zone, and that carbon prefers a crystalline solid at these conditions over CO2. Previous studies conducted at pressures less than 3 GPa suggest magnesite (MgCO3) and enstatite (Mg2Si 2O6) as reaction products. This data show that at pressures from 2.43 to 15.8 GPa, magnesite forms in addition to a Mg2Si 2O6 phase and an SiO2 phase. At pressures higher than 15 GPa, Mg2Si2O6 is no longer observed and magnesite and SiO2 are the favored products. The results from this study indicate a complex reaction pathway as a function of structural changes in both products and reactants with increasing pressure. However, the presence of magnesite as a reaction product at the conditions of the upper mantle and transition zone show this phase is important as a deep carbon reservoir and may be a key factor in melt production at the core-mantle boundary and in the generation of carbonatite and kimberlitic magmas.M.S. (Master of Science