Micro to Nanoscale Investigation of Volcanic Glass Aqueous Alteration: The Cases of Rare Earth Mineralization and Remobilization in Texas Bentonite Deposits and Early-Stage Alteration of Galápagos Coastal Basalt

Abstract

In this dissertation, I apply a suite of electron microscopy techniques to investigate aspects of aqueous alteration of volcanic glass (ash and basalt), in coastal settings. The first chapter investigates the fate of rare earth elements and yttrium (REE) released during the alteration of rhyolitic volcanic ash into bentonite, in one deposits of the Eocene Texas coastal plains, near Gonzales, Texas where REE enrichment is comparable to that of North American weathering deposits under consideration for extraction. Scanning electron microscopy (SEM) show that REEs occur as phosphate nanoparticles dispersed in the montmorillonite matrix or aggregated at the boundary of partially altered phenocrysts, or in narrow void space separating stacks of montmorillonite. Light REE phosphates are the most common and were identified as monazite by transmission electron microscopy (TEM); a second group composed of heavy REEs (xenotime) was identified as epitaxial on zircon. Textural evidence indicates that monazite is syngenetic with montmorillonite and accumulated due to the permeability reduction that followed alteration. The second chapter examines the occurrence of REEs in manganese oxide pods at the base of a Gonzales bentonite deposit where REE concentration increases by an order of magnitude. SEM and TEM analysis revealed the presence of globular aggregates, up to several microns in diameter, of xenotime nanoparticles, also hosting trace concentration of Zr, Dy, Er, Yb and Ce. Light REEs precipitated in smaller aggregates of rhabdophane nanoparticles, with a range of composition. The properties of the REE minerals indicate they were remobilized from the bentonite by an oxidizing fluid circulating through fractures and precipitated with Manganese oxides at the layer boundary. The third chapter investigates the early stages of alteration of coastal basalt exposed to seawater from Isabela and Fernandina Islands, Galápagos. TEM analysis identified a less dense, amorphous alteration layer on quenched glass and plagioclase exposed to the surface with distinct chemical features that indicate ion exchanges between basalt and seawater. Further, TEM analysis also identified surface regions covered with a passivating amorphous Mg, Fe silicate precipitate that provides a substrate for the build-up of carbonate cement.