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

Holm-Alwmark, Sanna LU ; Erickson, Timmons M. and Cavosie, Aaron J. (2022) In Geology 50(3). p.377-382
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 <112> are consistent with spinel-law twins. Inferred bulk shock pressures for the investigated samples, as constrained by planar deformation features... (More)

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 <112> 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 >5 GPa. These results highlight the utility of magnetite to record shock deformation in rocks that experience shock pressures >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.

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Contribution to journal
publication status
published
subject
in
Geology
volume
50
issue
3
pages
6 pages
publisher
Geological Society of America
external identifiers
  • scopus:85125519423
ISSN
0091-7613
DOI
10.1130/G49498.1
language
English
LU publication?
yes
id
acf2afea-627d-437f-ba66-f6d063065bc0
date added to LUP
2022-06-17 15:09:57
date last changed
2022-06-17 15:09:57
@article{acf2afea-627d-437f-ba66-f6d063065bc0,
  abstract     = {{<p>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.</p>}},
  author       = {{Holm-Alwmark, Sanna and Erickson, Timmons M. and Cavosie, Aaron J.}},
  issn         = {{0091-7613}},
  language     = {{eng}},
  number       = {{3}},
  pages        = {{377--382}},
  publisher    = {{Geological Society of America}},
  series       = {{Geology}},
  title        = {{Empirical constraints on progressive shock metamorphism of magnetite from the Siljan impact structure, Sweden}},
  url          = {{http://dx.doi.org/10.1130/G49498.1}},
  doi          = {{10.1130/G49498.1}},
  volume       = {{50}},
  year         = {{2022}},
}