MISBEHAVING MINERALS: EXPLORING EXTRACTION METHODS FOR TERRESTRIAL SHOCKED ZIRCONS

Shocked zircons found in asteroid impact ejecta can provide key constraints on the impact conditions and pressure-temperature histories of impact ejecta. This study is focused on zircons from Chicxulub distal ejecta (i.e. K/Pg boundary) localities in Southern Colorado and Northern New Mexico. Previous studies have identified zircons from these impact materials but there is no published methodology for extracting zircons from these complicated materials. We experimented with varying known mineral extraction methods to develop a revised procedure for zircon extraction from this clay-rich and barite-rich layer.

Our revised protocol is: (1) Collected samples are crushed with a mortar and pestle in ethanol, (2) Powders are then panned to remove fine-grained clays (wash), (3) Density separation is performed on the non-wash materials with methylene iodide (MI) diluted to lithium metatungstate (LMT) density (MI is preferred due to reactions of LMT with clays), (4) The MI sink (more dense) are sieved to 150µm and, (5) Grains are picked using an optical microscope. Since shocked zircons are not always euhedral, all grains were picked that exhibited characteristics of zircon, i.e. high relief, high birefringence, crystal habit, etc. Picked minerals were verified using energy dispersive spectroscopy in a variable pressure scanning electron microscope. Most picked minerals were barite, but 10s of zircons were identified. The steps that contributed the most to high zircon yields and reduction in processing time were the panning, MI, and sieving procedures. This project will continue after the conclusion of the EarthScope Consortium’s Geo-Launchpad internship, during which the zircons will be characterized via electron backscatter diffraction for shock microtextures and dated via U-Pb using secondary ionization mass spectrometry.