Empirical constraints on progressive shock metamorphism of magnetite from the Siljan impact structure, Sweden

Sanna Holm-Alwmark,Aaron J Cavosie,Timmons M Erickson

doi:10.1130/g49498.1

Sanna Holm-Alwmark, Aaron J Cavosie + Show 1 more

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https://doi.org/10.1130/g49498.1

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Abstract

Abstract Little is known about the microstructural behavior of magnetite during hypervelocity impact events, even though it is a widespread accessory mineral and an important magnetic carrier in terrestrial and extraterrestrial rocks. We report systematic electron backscatter diffraction crystallographic analysis of shock features in magnetite from a transect across the 52-km-diameter ca. 380 Ma Siljan impact structure in Sweden. Magnetite grains in granitoid samples contain brittle fracturing, crystal-plasticity, and lamellar twins. Deformation twins along {111} with shear direction of &lt;112&gt; are consistent with spinel-law twins. Inferred bulk shock pressures for the investigated samples, as constrained by planar deformation features (PDFs) in quartz and shock twins in zircon, range from 0 to 20 GPa; onset of shock-induced twinning in magnetite is observed at &gt;5 GPa. These results highlight the utility of magnetite to record shock deformation in rocks that experience shock pressures &gt;5 GPa, which may be useful in quartz-poor samples. Despite significant hydrothermal alteration and the variable transformation of host magnetite to hematite, shock effects are preserved, which demonstrates that magnetite is a reliable mineral for preserving shock deformation over geologic time.

Highlights

Magnetite, with an inverse spinel structure and nominal chemical formula of Fe23+Fe2+O42, is a common accessory mineral in igneous, sedimentary, and metamorphic rocks (e.g., Deer et al, 1992)
We report systematic electron backscatter diffraction crystallographic analysis of shock features in magnetite from a transect across the 52-km-diameter ca. 380 Ma Siljan impact structure in Sweden
Up to 10–20 magnetite grains were surveyed in each sample using backscattered electron (BSE) imaging, and orientation mapping using electron backscatter diffraction (EBSD) was conducted on two to six

Summary

Introduction

With an inverse spinel structure and nominal chemical formula of Fe23+Fe2+O42–, is a common accessory mineral in igneous, sedimentary, and metamorphic rocks (e.g., Deer et al, 1992). Magnetite grains in granitoid samples contain brittle fracturing, crystal-plasticity, and lamellar twins. We used scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analysis to conduct a systematic assessment of shock microstructures in magnetite from the Siljan impact structure.

Results

Conclusion