Shock recovery with decaying compressive pulses: Shock effects in calcite (CaCO$_3$) around the Hugoniot elastic limit

TL;DR

This study investigates shock effects in calcite minerals using impact experiments with decaying compressive pulses, revealing deformation mechanisms and proposing a method to estimate the depth of calcite in meteorite parent bodies.

Contribution

It introduces a novel impact experiment approach with decaying pulses to analyze shock metamorphism in calcite, providing insights into deformation and a strategy for depth reconstruction.

Findings

Over half of calcite grains show undulatory extinction above 3 GPa

Shock pressure exceeds the elastic limit but causes plastic deformation in calcite

Method allows for analyzing different metamorphic degrees in a single experiment

Abstract

Shock metamorphism of minerals in meteorites provides insights into the ancient Solar System. Calcite is an abundant aqueous alteration mineral in carbonaceous chondrites. Return samples from the asteroids Ryugu and Bennu are expected to contain calcite-group minerals. Although shock metamorphism in silicates has been well studied, such data for aqueous alteration minerals are limited. Here, we investigated the shock effects in calcite with marble using impact experiments at the Planetary Exploration Research Center of Chiba Institute of Technology. We produced decaying compressive pulses with a smaller projectile than the target. A metal container facilitates recovery of a sample that retains its pre-impact stratigraphy. We estimated the peak pressure distributions in the samples with the iSALE shock physics code. The capability of this method to produce shocked grains that have experienced different degrees of metamorphism from a single experiment is an advantage over conventional uniaxial shock recovery experiments. The shocked samples were investigated by polarizing microscopy and X-ray diffraction analysis. We found that more than half of calcite grains exhibit undulatory extinction when peak pressure exceeds 3 GPa. This shock pressure is one order of magnitude higher than the Hugoniot elastic limit (HEL) of marble, but it is close to the HEL of a calcite crystal, suggesting that the undulatory extinction records dislocation-induced plastic deformation in the crystal. Finally, we propose a strategy to re-construct the maximum depth of calcite grains in a meteorite parent body, if shocked calcite grains are identified in chondrites and/or return samples from Ryugu and Bennu.