Many geologic events within the earth's crust are greatly influenced by volatile components such as water and fluorine. Most minerals cannot incorporate these volatiles, and therefore do not record these events. However, titanite, amphibole, and mica are common minerals that host small fractions of hydroxyl and fluorine. These minerals potentially preserve information about the concentrations of volatiles in the transient melts and fluids present during geologic events, like metamorphism and volcanic eruptions. This study will empirically evaluate both these minerals' incorporation and retention of hydroxyl and fluorine as a function of temperature and pressure, thus providing fundamental information about mineral chemistry and producing a tool for evaluating the nature of volatiles in the development of the earth and in modern geologic processes. The study will specifically apply the resulting data to two areas with complicated yet distinctly different volatile histories, the Serido Fold Belt, Brazil, and the Southern Oklahoma Aulacogen, USA.
This study will subject examples of each mineral, titanite, pargasite amphibole, and biotite mica, to excess fluorine or hydroxyl at temperatures and pressures equivalent to those found in the earth's crust. These components move into and out of minerals by diffusion. The incorporation of the excess component will be greatest at the exposed surface of the mineral and decrease with depth as a function of temperature, pressure, and exposure time. We will characterize the concentration profile of these components as a function of depth, largely using particle accelerator techniques, and determine their diffusivities from this profile. The diffusivity of each component is a fundamental property of each mineral, useful in evaluating the concentration of these components in natural specimens. In the case of the Serido Fold Belt, it is anticipated that diffusivities of fluorine and hydroxyl in biotites and amphiboles within the region's Neoproterozoic metamorphic schists will determine the influence of the 523 million year old metamorphism and alteration events. In the case of the Southern Oklahoma Aulacogen, the diffusivities in all three minerals will be useful in constraining the magmatic concentration of fluorine and water during the development of this 540 million year old rift system. In addition to these two areas, the data from this study will undoubtedly find application in other work on metamorphism, plutonism, volcanism, the hydrothermal alteration events common to ore deposits, and quite possibly to synthetic crystallization processes studied in the material sciences.
