INTERFACE-COUPLED DISSOLUTION-PRECIPITATION MECHANISMS IN K FELDSPAR REPLACEMENTS PRODUCED BY LOW T K-METASOMATISM: CREEDE, CO

The coupling of dissolution and precipitation at fluid-mineral interfaces is a widespread occurrence within Earth’s crust. Other studies varying in mineral assemblages have observed the presence of crystallographic controls and the generation of both nano and micro-scale porosity in these replacements, while few have investigated the origin of patchy zoning.

This study focuses on identifying crystallographic controls, the generation of porosity, and the cause for patchy zoning interface-coupled replacements of feldspar-rich igneous ash flow tuffs from Creede, CO by the near-end member variety of K feldspar from naturally occurring, low-temperature K-metasomatism. AFM and SEM were used to image nanoscale porosity on thin sections. SEM was also used to perform correlative EDS and EBSD for compositional analysis and identification of crystallographic controls of the replacements. Microscale porosity was observed in secondary adularia replacements, ranging from a few µm to 20 µm in diameter, and little to no nanoscale porosity. Patches of replacement zoning exhibited compositional differences and an absence of varying crystallographic orientation. Partially replaced samples indicated crystallographic matching of parent and product phases. This suggests that patchy zoning may be caused by an evolving fluid composition through time and that the replacements are crystallographically controlled by the orientation of the parent phase. Additional imaging and chemical and elemental mapping will improve our understanding of the replacement mechanisms in feldspar minerals. Understanding these mechanisms and their involvement in crustal processes such as fluid flow and element release and transport has important implications in the trapping of pollutants, environmental remediation, formation and preservation of building materials, crustal strength and deformation and other global tectonic processes.