Deformation and recrystallization mechanisms in glacier salt: Evolution of microstructures inferred from EBSD and microstructural analyses (Kuh-e-Namak diapir (Dashti, Iran))

Abstract

This study presents a detailed microstructural analysis of salt samples collected from the exposed mountain glacier in Iran, the Kuh-e-Namak (Dashti). Our goal is to pinpoint the different deformation mechanisms leading to grain size reduction and, how these, together with the influx of rainwater and development of porosity, affect the creep of the polycrystalline halite. We investigated 17 gamma-irradiated thin sections by transmitted and reflected light microscopy, quantitative grain and subgrain statistics, and crystallographic-preferred orientation (CPO) mapping using Electron Back Scattered Diffraction (EBSD). The microstructural evidence suggests a combination of solution-precipitation accompanied by grain-boundary sliding and dynamic recrystallization. The grain size decreases by subgrain rotation recrystallization, microcracking, and grain boundary migration by three different mechanisms: 1) grain boundary bulging into grains, 2) nucleation of new grains and, 3) segmentation of porphyroclasts by planar domains of dynamic recrystallization. The mean grain size ranges from 118 to 508 μm and subgrain sizes from 14 to 99 μm from which differential stresses between 1.9 and 10.2 MPa were calculated. Inferred strain rates for the glacier are in the order of magnitude of 10⁻¹⁰-10⁻⁸ s⁻¹. The increasing shape-preferred orientation of halite grains from the crestal domal part of the diapir towards the frontal parts of extrusive glaciers is interpreted as a result of dominant solution-precipitation creep and salt flow. Rainwater influx rendering this important deformation mechanism switch is attributed to the development of porosity along microcracks and grain boundaries.