Sam Hall ’23 Looks At Evidence of Interface-Coupled Dissolution-Precipitation in K Feldspars

Sam Hall ’23 is a Geology major working with thesis advisor Jim Rougvie. He studies electron backscatter diffraction evidence of interface-coupled dissolution-precipitation in K feldspars.

Interface-coupled dissolution-precipitation (ICDP) is a model for mineral replacement that involves precipitation along a fluid interface. Unlike in the conventional two-step model of mineral replacement in which the parent mineral is entirely dissolved before the replacement precipitates, during ICDP dissolution and precipitation occur simultaneously. ICDP is a mechanism for mineral replacement during metasomatism, a process by which rocks and minerals are altered via interaction with a reactive fluid. ICDP is potentially a fundamental process for widespread mineral replacement throughout the Earth’s crust.

This project aims to identify and understand the reaction mechanisms that took place during low-T K metasomatism of ash flow tuffs in Creede, CO. These samples contain feldspar crystals that display a variety of distinct zoning patterns. Some of these patterns consist of alternations between original and replacement mineral, while others display variation in cathodoluminescence (CL) color of replacement material that is thought to reflect changing compositions of the metasomatizing fluid.

The goal of this research is to determine the mineral replacement mechanisms that were responsible for the feldspar replacements at Creede and whether feldspar zoning patterns retain information about the continuous vs. discntinuous nature of the replacement process. Additionally I aim to identify criteria can be used to distinguish ICDP from overgrowth or two-stage replacement histories. In order to address these questions, a number of different imaging techniques will be used. Electron backscatter diffraction (EBSD) will be performed using scanning electron microscopy at Northwestern University. This will be compared with existing CL images in order to determine crystallographic orientations within zoning patterns. This information will allow us to gain a better understanding of ICDP and how it progresses with time.

By: Sam Hall'23
December 20, 2022

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