Shock metamorphic features in zircon grains from the Mien impact structure : clues to conditions during impact
(2018) In Dissertations in Geology at Lund University GEOR02 20181Department of Geology
- Abstract (Swedish)
- Jämfört med andra himlakroppar i vårt solsystem så finns det få välbevarade nedslagsstrukturer på jorden. Vi vet att jorden har påverkats av nedslag från rymden i lika stor utsträckning som övriga planeter och att dessa nedslag har influerat jordens geologiska och biologiska utveckling. När en krater blivit nederoderad så kvar-står mikroskopiska bevis för att ett nedslag skett på platsen, unika för denna geologiska process. I vissa fall kan de användas för att avgöra vilka tryck- och temperaturförhållanden som rådde när de bildades samt ge information om mineraluppträdande vid extrema förhållanden.
I detta mastersarbete har jag undersökt chockmetamorfa texturer i zirkon från nedslagsstrukturen Mien i Sverige. Impaktiter är bergarter som... (More) - Jämfört med andra himlakroppar i vårt solsystem så finns det få välbevarade nedslagsstrukturer på jorden. Vi vet att jorden har påverkats av nedslag från rymden i lika stor utsträckning som övriga planeter och att dessa nedslag har influerat jordens geologiska och biologiska utveckling. När en krater blivit nederoderad så kvar-står mikroskopiska bevis för att ett nedslag skett på platsen, unika för denna geologiska process. I vissa fall kan de användas för att avgöra vilka tryck- och temperaturförhållanden som rådde när de bildades samt ge information om mineraluppträdande vid extrema förhållanden.
I detta mastersarbete har jag undersökt chockmetamorfa texturer i zirkon från nedslagsstrukturen Mien i Sverige. Impaktiter är bergarter som bildas vid nedslag; tre olika litologier (två smältbergarter och en ”suevit” breccia) har insamlats från platsen och från dessa har zirkon separerats ut. Zirkonkornen har sedan undersökts med svepelektronmikroskop (SEM) för att studera texturer på ytan och inuti kornen. Vidare så har en electron backscatter diffraction (EBSD)-analys utförts på ett korn med granulär textur, i syfte att undersöka eventuella sys-tematiska förhållanden mellan granuler.
Resultaten visar bland annat att 1) de två dominerande texturerna i smältbergarterna är granulär- och porös textur och 2) ett systematiskt förhållande mellan granuler som är indikativt för tidigare förekomst av högtryckspolymorfen reidite (s. k FRIGN zircon). Med hjälp av dessa fynd var det möjligt att upprätta en tryck-och temperaturkarta som visar att smältbergarterna kan ha genomgått tryck uppemot 50 gigapascal och temperaturer runt 1500°C. Få av zir-konkornen i ”suevit” breccian uppvisade granulär eller porös textur, och de bör därför ha upplevt lägre chockförhål-landen än smältbergarterna; detta är i enlighet med den generella stratigrafin för impaktiter där smältbergarterna antas ha formats närmast ”ground zero”. (Less) - Abstract
- Well-preserved impact craters are rare on Earth, especially compared to other celestial bodies in our so-lar system. However, we know that the Earth has been part of the same bombardment and that impact cratering has influenced the Earth’s geological and biological history.
When the morphological evidence of a former impact event has been eradicated, the identification of shock meta-morphic features in minerals could be the crucial step to confirming a suspected impact structure. These microtex-tural characteristics may also provide pressure- and temperature (P-T) constraints and reveal information of mineral behaviour at extreme conditions.
This thesis examines shock metamorphic features in zircon from the Mien impact structure and to... (More) - Well-preserved impact craters are rare on Earth, especially compared to other celestial bodies in our so-lar system. However, we know that the Earth has been part of the same bombardment and that impact cratering has influenced the Earth’s geological and biological history.
When the morphological evidence of a former impact event has been eradicated, the identification of shock meta-morphic features in minerals could be the crucial step to confirming a suspected impact structure. These microtex-tural characteristics may also provide pressure- and temperature (P-T) constraints and reveal information of mineral behaviour at extreme conditions.
This thesis examines shock metamorphic features in zircon from the Mien impact structure and to what extent these can be used to constrain P-T conditions. This has been done by sampling of impactites of three different lithologies from Mien: two impact melt rocks with different degrees of clast-content and one “suevite” breccia; from these rocks, zircon grains were separated. Exterior and interior textures in these grains were documented using scanning electron microscopy (SEM)-imaging, and electron backscatter diffraction (EBSD)-analysis was conducted on a completely granular grain in order to study orientation relationships between granules. Further, Raman spectrosco-py was conducted on grains with granular– and microporous texture to reveal potential phase variations.
The main results are 1) a textural study with grains classified on the basis of notable features, revealing granular- and microporous texture as two dominating features in impact melt rocks and 2) the construction of an orientation map with corresponding pole figures, showing a systematic relationship between granules that is indicative of the transformation of and reversion from reidite to zircon. This relationship has just recently been established in studies from other impact structures. I have suggested a P-T-path for the impactites based on observations in this study; the impact melt rocks could possibly have experienced pressures as high as >50 GPa and temperatures ~1500°C. The rarity of granular – and microporous texture in zircon from the “suevite” breccia suggests that these have been sub-jected to lower pressures and temperatures than the impact melt rocks, which is in accordance with the expected stratigraphy of impactites where the impact melt occur closest to “ground zero”. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/8948695
- author
- Martell, Josefin LU
- supervisor
-
- Carl Alwmark LU
- Paula Lindgren LU
- organization
- course
- GEOR02 20181
- year
- 2018
- type
- H2 - Master's Degree (Two Years)
- subject
- keywords
- Zircon, shock metamorphism, Mien, FRGN zircon, granular texture
- publication/series
- Dissertations in Geology at Lund University
- report number
- 542
- language
- English
- id
- 8948695
- date added to LUP
- 2018-06-16 12:28:39
- date last changed
- 2018-06-16 12:28:39
@misc{8948695,
abstract = {{Well-preserved impact craters are rare on Earth, especially compared to other celestial bodies in our so-lar system. However, we know that the Earth has been part of the same bombardment and that impact cratering has influenced the Earth’s geological and biological history.
When the morphological evidence of a former impact event has been eradicated, the identification of shock meta-morphic features in minerals could be the crucial step to confirming a suspected impact structure. These microtex-tural characteristics may also provide pressure- and temperature (P-T) constraints and reveal information of mineral behaviour at extreme conditions.
This thesis examines shock metamorphic features in zircon from the Mien impact structure and to what extent these can be used to constrain P-T conditions. This has been done by sampling of impactites of three different lithologies from Mien: two impact melt rocks with different degrees of clast-content and one “suevite” breccia; from these rocks, zircon grains were separated. Exterior and interior textures in these grains were documented using scanning electron microscopy (SEM)-imaging, and electron backscatter diffraction (EBSD)-analysis was conducted on a completely granular grain in order to study orientation relationships between granules. Further, Raman spectrosco-py was conducted on grains with granular– and microporous texture to reveal potential phase variations.
The main results are 1) a textural study with grains classified on the basis of notable features, revealing granular- and microporous texture as two dominating features in impact melt rocks and 2) the construction of an orientation map with corresponding pole figures, showing a systematic relationship between granules that is indicative of the transformation of and reversion from reidite to zircon. This relationship has just recently been established in studies from other impact structures. I have suggested a P-T-path for the impactites based on observations in this study; the impact melt rocks could possibly have experienced pressures as high as >50 GPa and temperatures ~1500°C. The rarity of granular – and microporous texture in zircon from the “suevite” breccia suggests that these have been sub-jected to lower pressures and temperatures than the impact melt rocks, which is in accordance with the expected stratigraphy of impactites where the impact melt occur closest to “ground zero”.}},
author = {{Martell, Josefin}},
language = {{eng}},
note = {{Student Paper}},
series = {{Dissertations in Geology at Lund University}},
title = {{Shock metamorphic features in zircon grains from the Mien impact structure : clues to conditions during impact}},
year = {{2018}},
}