The effect of oxygen fugacity on the equilibrium partitioning of lithium between olivine and clinopyroxene

Open Access
- Author:
- Deane, James Anthony
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 21, 2013
- Committee Members:
- Maureen Feineman, Thesis Advisor/Co-Advisor
- Keywords:
- Lithium
Petrology
Experimental petrology
oxygen fugacity
equilibrium partitioning
mantle chemistry
xenolith - Abstract:
- Mantle olivines and clinopyroxenes from xenoliths sampled worldwide commonly display disequilibrium distributions of lithium and its isotopes. While many theories of open- and closed- system redistribution of lithium have been proposed, none has been adequately demonstrated experimentally. Closed system redistribution of lithium between olivine and clinopyroxene has been proposed to occur in response to changing equilibrium partition coefficients driven by changes in temperature or oxygen fugacity. Previous experiments (Yakob et al., EPSL. 330, 11-21, 2012) have shown that there is no temperature dependence on Li partitioning between 700-1100°C. The goal of this thesis is to determine the potential effect of oxygen fugacity on lithium partitioning between olivine and clinopyroxene. To this end, a series of piston cylinder experiments was designed to fix the oxygen fugacity at various mantle values while natural olivine and diopside starting materials were equilibrated with a Li-bearing aqueous fluid. Oxygen fugacity was buffered using the solid state fO2 buffers Fayalite-Magnetite-Quartz, Magnetite-Wüstite, and Mo-MoO2. A silver capsule was employed to minimize the solubility of Fe in the capsule, thus keeping it in the experimental charge. Iron retention is critical because the mechanism being tested is coupled substitution of Li+ and Fe3+ for Mg2+ or Fe3+ in the olivine structure. Results from these experiments show there is no apparent relationship between oxygen fugacity and equilibrium partitioning of Li between olivine and clinopyroxene at upper mantle conditions. Over the range of fugacities investigated, DLiol/di was 1.7±0.4 regardless of fO2. The lack of a relationship between Li partitioning and oxygen fugacity, coupled with the previously determined lack of a relationship with temperature, suggests that closed system redistribution of Li between mantle phases is unlikely to explain the isotopic variability we see in xenoliths worldwide. Instead, open system interaction with metasomatic fluids or melts could be the cause. This implies that Li isotopes in mantle xenoliths are not necessarily representative of the ambient mantle from which they are sourced, and therefore limits the use of Li isotopes as a passive tracer of recycled crustal material in the mantle. However, Li isotopic fractionation during open-system processes related to xenolith entrainment and eruption is likely to be useful as a geospeedometer to determine the rates of these processes.