Bridging the shocked monazite gap – Deformation microstructures in natural and laser shock-loaded samples

Abstract

Impact-related damage in minerals and rocks provides key evidence to identify impact structures, and deformation of U-Th-minerals in target rocks, such as monazite, makes possible precise dating and determination of pressure-temperature conditions for impact events. Here a laser-driven shock experiment using a high-energy laser pulse of ns-order duration was carried out on a natural monazite crystal to compare experimentally produced shock-deformation microstructures with those observed in naturally shocked monazite. Deformation microstructures from regions that may have experienced up to ∼50 GPa and 1000°C were characterized using Raman spectroscopy and transmission electron microscopy. Experimental results were compared with nanoscale observations of deformation microstructures found in naturally shocked monazite from the Vredefort impact structure (South Africa). Raman-band broadening observed between unshocked and shocked monazite, responsible for a variation of ∼3 cm−1 in the FWHM, is interpreted to result from the competition between shock-induced distortion of the lattice, and post-shock annealing. At nanoscale, three main plastic deformation structures were found in both naturally and experimentally shocked monazite: deformation twins, mosaïcism, and deformation bands. The element Ca is enriched along host-twin boundaries, which further confirms that the laser shock loading experiment produced both comparable styles of crystal-plastic deformation, and also localized element mobility, as that found in natural shock-deformed monazite. Deformation twins form in the experiment were only along the (001) plane, an orientation which is not considered diagnostic of shock deformation. However, both mosaïcism and deformation, expressed in SAED patterns as streaking of spots, and the presence of extra spots (more or less pronounced), are interpreted as unambiguous nano-scale signatures of shock metamorphism in monazite. Experimentally calibrated deformation features, such as those documented here at TEM-scale, provide new tools for identifying evidence of shock deformation in natural samples.