Browsing by Author "Sharton-Bierig, Eytan"
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Item Widespread Disequilibrium of Phosphorus in Olivine(2020-04-09) Sharton-Bierig, Eytan; Lee, Cin-Ty AThere are significant inconsistencies in the literature regarding the partition coefficient of phosphorus in olivine. This indicates that there may be a problem with the methods for determining this value. In this study, olivine phenocrysts from the Sequeiros Transform Zone were measured for phosphorus to determine the issues and constrain a more accurate value. The measured values range over several orders of magnitude. Estimates of phosphorus diffusivity in a basaltic melt are slower than some estimates of olivine growth rate, and this may mean that this spread is in some part due to kinetic effects during crystallization. As it is difficult to separate these contributions from the true equilibrium value, an alternative approach to constrain this value is needed. Using a statistical approach, we find a maximum value of D=0.027 and a minimum value of D=0.0033.Item Widespread phosphorous excess in olivine, rapid crystal growth, and implications for magma dynamics(Presses universitaires de Strasbourg, 2022) Lee, Cin-Ty; Sun, Chenguang; Sharton-Bierig, Eytan; Phelps, Patrick; Borchardt, Jackson; Liu, Boda; Costin, Gelu; Johnston, A. DanaTrace element zoning is often used to unravel the crystallization history of phenocrysts in magmatic systems, but interpretation requires quantifying the relative importance of equilibrium versus disequilibrium. Published partition coefficients for phosphorous (P) in olivine vary by more than a factor of ten. After considering kinetic effects, a new equilibrium partition coefficient was extrapolated from a re-examination of natural and experimental systems, indicating that P partition coefficients in olivine are significantly over-estimated. These new partitioning constraints allow us to establish a theoretical P Equilibrium Fractionation Array (PEFA) for mid-ocean ridge basalts (MORBs), revealing that most olivines from MORBs have excess P (2–15 times PEFA) and are thus in disequilibrium. Using an independent case study of natural dendritic olivines, we show that such P enrichments can be explained by diffusion-limited incorporation of P during rapid crystal growth. If growth rate can be related to cooling, the rapid growth rates of olivines have implications for magma system dynamics, such as the size of magma bodies or where crystallization occurs within the body.