Shock-darkening in ordinary chondrites: Mesoscale modelling of the shock process and comparison with shock-recovery experiments

Research output: ThesisDoctoral Thesis

Abstract

Ordinary chondrites are primitive materials of the solar system; they were subject to thermal and shock metamorphism during asteroid accretion and collision history.

Shock-darkening is a shock metamorphic process which occurs in ordinary chondrites where iron sulphides and metals form a network of tiny melt veins, optically darkening the lithology. Together space weathering and shock-darkening can be a major factor in alteration of reflectance spectra, suppressing the 1 and 2 micron silicate absorption bands. S-complex asteroids, hosting ordinary chondrites, display silicate absorption bands. C/X-complex asteroids are either devoid of 1 and 2 micron silicate absorption bands or presenting a weak silicate absorption band at 1 micron. If shock-darkening can alter the spectra of S-complex asteroids, they can appear like C/X-complex asteroids and induce a mismatch in the asteroid distribution.

This thesis provides an in-depth study of shock-darkening in order to determine the pressure-temperature conditions for shock melting of both iron sulphides and metals, in ordinary chondrites. In order to perform this study the following actions were required:
I. observing shock wave interactions in heterogeneous mediums composed of silicates, metals, and iron sulphides, the principal components of ordinary chondrites
II. quantifying post-shock heating and melting of the individual phases
III. comparing my results with observations of shock metamorphism in ordinary chondrites
IV. investigating on the best conditions to reproduce shock-darkening in shock-recovery experiments.

In contrast to shock-recovery experiments, I adopted a numerical modelling method which calculated the post-shock heating and melting of individual phases and provided observation of shock wave interactions in heterogeneous mediums. The shock physics code iSALE was used on a mesoscale to study shock compression of ordinary chondrites. Using complex models, the numerical study lead to the following results:
A) 40−60 GPa is the likely range for shock-darkening, dominated by melting of iron sulphides.
B) Heterogeneous distribution of peak shock pressures and post-shock heating is caused by strong impedance contrasts between phases (with strong pressure increases through reflections from high density phases to lower density phases, e.g. metals to silicates).
C) Special conditions, such as eutectic melting, hotspots from convergence of shock waves, or pore crushing, are necessary to melt metals.
D) Porosity and pre-heating are important boundary conditions affecting shock metamorphism.
E) Results from the mesoscale models are compatible to observations of shock metamorphism in ordinary chondrites.

Finally, simulations of shock-recovery experiments showed that the reverberation technique may prevent shock-darkening from occurring. Compared to a single pressure load, the reverberation technique reduces the rise in entropy from super-imposing pressures, thus, if sufficient pressure for shock-darkening (40–60 GPa) is achieved, melting of iron sulphides or metals may not occur. Alternatively, I showed that spherical shock-recovery experiments, which use spherically induced shock waves to shock spherical samples, are ideal to study shock-darkening because the rise in entropy is directly related to the peak-shock pressure in the sample.

With my results, a more in depth quantitative study of the volume of shock-induced darkened materials during asteroid collisions is now possible.
Original languageEnglish
Supervisors/Advisors
  • Kohout, Tomas, Supervisor
  • Wünnemann, Kai, Supervisor, External person
  • Kukkonen, Ilmo, Supervisor
Place of PublicationHelsinki
Publisher
Print ISBNs978-951-51-3995-5
Electronic ISBNs978-951-51-3996-2
Publication statusPublished - Mar 2019
MoE publication typeG5 Doctoral dissertation (article)

Fields of Science

  • 1171 Geosciences

Cite this

@phdthesis{7bc8023eb5904b769a1a784e03e6fbb2,
title = "Shock-darkening in ordinary chondrites: Mesoscale modelling of the shock process and comparison with shock-recovery experiments",
abstract = "Ordinary chondrites are primitive materials of the solar system; they were subject to thermal and shock metamorphism during asteroid accretion and collision history.Shock-darkening is a shock metamorphic process which occurs in ordinary chondrites where iron sulphides and metals form a network of tiny melt veins, optically darkening the lithology. Together space weathering and shock-darkening can be a major factor in alteration of reflectance spectra, suppressing the 1 and 2 micron silicate absorption bands. S-complex asteroids, hosting ordinary chondrites, display silicate absorption bands. C/X-complex asteroids are either devoid of 1 and 2 micron silicate absorption bands or presenting a weak silicate absorption band at 1 micron. If shock-darkening can alter the spectra of S-complex asteroids, they can appear like C/X-complex asteroids and induce a mismatch in the asteroid distribution.This thesis provides an in-depth study of shock-darkening in order to determine the pressure-temperature conditions for shock melting of both iron sulphides and metals, in ordinary chondrites. In order to perform this study the following actions were required:I. observing shock wave interactions in heterogeneous mediums composed of silicates, metals, and iron sulphides, the principal components of ordinary chondritesII. quantifying post-shock heating and melting of the individual phasesIII. comparing my results with observations of shock metamorphism in ordinary chondritesIV. investigating on the best conditions to reproduce shock-darkening in shock-recovery experiments.In contrast to shock-recovery experiments, I adopted a numerical modelling method which calculated the post-shock heating and melting of individual phases and provided observation of shock wave interactions in heterogeneous mediums. The shock physics code iSALE was used on a mesoscale to study shock compression of ordinary chondrites. Using complex models, the numerical study lead to the following results:A) 40−60 GPa is the likely range for shock-darkening, dominated by melting of iron sulphides.B) Heterogeneous distribution of peak shock pressures and post-shock heating is caused by strong impedance contrasts between phases (with strong pressure increases through reflections from high density phases to lower density phases, e.g. metals to silicates).C) Special conditions, such as eutectic melting, hotspots from convergence of shock waves, or pore crushing, are necessary to melt metals.D) Porosity and pre-heating are important boundary conditions affecting shock metamorphism.E) Results from the mesoscale models are compatible to observations of shock metamorphism in ordinary chondrites.Finally, simulations of shock-recovery experiments showed that the reverberation technique may prevent shock-darkening from occurring. Compared to a single pressure load, the reverberation technique reduces the rise in entropy from super-imposing pressures, thus, if sufficient pressure for shock-darkening (40–60 GPa) is achieved, melting of iron sulphides or metals may not occur. Alternatively, I showed that spherical shock-recovery experiments, which use spherically induced shock waves to shock spherical samples, are ideal to study shock-darkening because the rise in entropy is directly related to the peak-shock pressure in the sample.With my results, a more in depth quantitative study of the volume of shock-induced darkened materials during asteroid collisions is now possible.",
keywords = "1171 Geosciences",
author = "Juulia-Gabrielle Moreau",
year = "2019",
month = "3",
language = "English",
isbn = "978-951-51-3995-5",
series = "DEPARTMENT OF GEOSCIENCES AND GEOGRAPHY A",
publisher = "Helsingin yliopisto",
address = "Finland",

}

TY - THES

T1 - Shock-darkening in ordinary chondrites

T2 - Mesoscale modelling of the shock process and comparison with shock-recovery experiments

AU - Moreau, Juulia-Gabrielle

PY - 2019/3

Y1 - 2019/3

N2 - Ordinary chondrites are primitive materials of the solar system; they were subject to thermal and shock metamorphism during asteroid accretion and collision history.Shock-darkening is a shock metamorphic process which occurs in ordinary chondrites where iron sulphides and metals form a network of tiny melt veins, optically darkening the lithology. Together space weathering and shock-darkening can be a major factor in alteration of reflectance spectra, suppressing the 1 and 2 micron silicate absorption bands. S-complex asteroids, hosting ordinary chondrites, display silicate absorption bands. C/X-complex asteroids are either devoid of 1 and 2 micron silicate absorption bands or presenting a weak silicate absorption band at 1 micron. If shock-darkening can alter the spectra of S-complex asteroids, they can appear like C/X-complex asteroids and induce a mismatch in the asteroid distribution.This thesis provides an in-depth study of shock-darkening in order to determine the pressure-temperature conditions for shock melting of both iron sulphides and metals, in ordinary chondrites. In order to perform this study the following actions were required:I. observing shock wave interactions in heterogeneous mediums composed of silicates, metals, and iron sulphides, the principal components of ordinary chondritesII. quantifying post-shock heating and melting of the individual phasesIII. comparing my results with observations of shock metamorphism in ordinary chondritesIV. investigating on the best conditions to reproduce shock-darkening in shock-recovery experiments.In contrast to shock-recovery experiments, I adopted a numerical modelling method which calculated the post-shock heating and melting of individual phases and provided observation of shock wave interactions in heterogeneous mediums. The shock physics code iSALE was used on a mesoscale to study shock compression of ordinary chondrites. Using complex models, the numerical study lead to the following results:A) 40−60 GPa is the likely range for shock-darkening, dominated by melting of iron sulphides.B) Heterogeneous distribution of peak shock pressures and post-shock heating is caused by strong impedance contrasts between phases (with strong pressure increases through reflections from high density phases to lower density phases, e.g. metals to silicates).C) Special conditions, such as eutectic melting, hotspots from convergence of shock waves, or pore crushing, are necessary to melt metals.D) Porosity and pre-heating are important boundary conditions affecting shock metamorphism.E) Results from the mesoscale models are compatible to observations of shock metamorphism in ordinary chondrites.Finally, simulations of shock-recovery experiments showed that the reverberation technique may prevent shock-darkening from occurring. Compared to a single pressure load, the reverberation technique reduces the rise in entropy from super-imposing pressures, thus, if sufficient pressure for shock-darkening (40–60 GPa) is achieved, melting of iron sulphides or metals may not occur. Alternatively, I showed that spherical shock-recovery experiments, which use spherically induced shock waves to shock spherical samples, are ideal to study shock-darkening because the rise in entropy is directly related to the peak-shock pressure in the sample.With my results, a more in depth quantitative study of the volume of shock-induced darkened materials during asteroid collisions is now possible.

AB - Ordinary chondrites are primitive materials of the solar system; they were subject to thermal and shock metamorphism during asteroid accretion and collision history.Shock-darkening is a shock metamorphic process which occurs in ordinary chondrites where iron sulphides and metals form a network of tiny melt veins, optically darkening the lithology. Together space weathering and shock-darkening can be a major factor in alteration of reflectance spectra, suppressing the 1 and 2 micron silicate absorption bands. S-complex asteroids, hosting ordinary chondrites, display silicate absorption bands. C/X-complex asteroids are either devoid of 1 and 2 micron silicate absorption bands or presenting a weak silicate absorption band at 1 micron. If shock-darkening can alter the spectra of S-complex asteroids, they can appear like C/X-complex asteroids and induce a mismatch in the asteroid distribution.This thesis provides an in-depth study of shock-darkening in order to determine the pressure-temperature conditions for shock melting of both iron sulphides and metals, in ordinary chondrites. In order to perform this study the following actions were required:I. observing shock wave interactions in heterogeneous mediums composed of silicates, metals, and iron sulphides, the principal components of ordinary chondritesII. quantifying post-shock heating and melting of the individual phasesIII. comparing my results with observations of shock metamorphism in ordinary chondritesIV. investigating on the best conditions to reproduce shock-darkening in shock-recovery experiments.In contrast to shock-recovery experiments, I adopted a numerical modelling method which calculated the post-shock heating and melting of individual phases and provided observation of shock wave interactions in heterogeneous mediums. The shock physics code iSALE was used on a mesoscale to study shock compression of ordinary chondrites. Using complex models, the numerical study lead to the following results:A) 40−60 GPa is the likely range for shock-darkening, dominated by melting of iron sulphides.B) Heterogeneous distribution of peak shock pressures and post-shock heating is caused by strong impedance contrasts between phases (with strong pressure increases through reflections from high density phases to lower density phases, e.g. metals to silicates).C) Special conditions, such as eutectic melting, hotspots from convergence of shock waves, or pore crushing, are necessary to melt metals.D) Porosity and pre-heating are important boundary conditions affecting shock metamorphism.E) Results from the mesoscale models are compatible to observations of shock metamorphism in ordinary chondrites.Finally, simulations of shock-recovery experiments showed that the reverberation technique may prevent shock-darkening from occurring. Compared to a single pressure load, the reverberation technique reduces the rise in entropy from super-imposing pressures, thus, if sufficient pressure for shock-darkening (40–60 GPa) is achieved, melting of iron sulphides or metals may not occur. Alternatively, I showed that spherical shock-recovery experiments, which use spherically induced shock waves to shock spherical samples, are ideal to study shock-darkening because the rise in entropy is directly related to the peak-shock pressure in the sample.With my results, a more in depth quantitative study of the volume of shock-induced darkened materials during asteroid collisions is now possible.

KW - 1171 Geosciences

M3 - Doctoral Thesis

SN - 978-951-51-3995-5

T3 - DEPARTMENT OF GEOSCIENCES AND GEOGRAPHY A

PB - Helsingin yliopisto

CY - Helsinki

ER -