Dobson, K.J.

Hanna, R.D.

Lee, M.R.

Lindgren, P.

Tomkinson, T.

English

The Murchison CM2 carbonaceous chondrite contains a microfabric as revealed by the preferred orientation of matrix phyllosilicates [1] and the foliation of dark inclusions [2]. Olivine microstructures show that Murchison has a shock stage of S2 [3], and so this shock stage may also be responsible for the microfabric. Here we have sought to understand better the mechanisms of deformation and to assess whether there is evidence for multiple deformation events by combining two techniques: (i) calcite e-twin stress analysis, which is a widely applied method to infer stress orientations in terrestrial rocks [e.g. 4, 5], and high resolution X-ray computed tomography (XCT). A small irregular sample of Murchison was embedded in a block of resin and polished to colloidal silica level on three perpendicular faces. The final size of the block was 9×7×6 mm. For calcite e-twin stress analysis the crystallographic orientations of calcite grains and their e-twins on each of the three polished sides (A, B &amp; C) was determined by electron backscatter diffraction using a FEI Quanta 200F field emission scanning electron microscope following the method of [6]. High resolution (XCT) was also performed on the same sample using a Metris Xtek XTH225 scanner at the Manchester X-ray Imaging Facility. Chondrules were segmented from the tomography volume using the Avizo software. The preferred orientation of the chondrule population was then calculated using the Blob3D software [7]. Calcite e-twin stress analyses of 23 twinned calcite grains from A, B &amp; C show that the greatest intensity of stress was oriented semi-vertically to the plane of side B. The XCT data shows that the chondrules are flattened and aligned with a preferred orientation of tertiary axes (greatest stress) also oriented semi-vertically to side B. Work is ongoing to more precisely constrain the directions of stress related to the e-twins and chondrules. The semi-vertical coincidence of axes of greatest stress as measured by calcite twins and chondrules could be a result of that flattening and calcite twinning took place during the same deformation event, e.g. shallow crustal compaction or impact “gardening”. Impacts are the most likely explanation, since the static pressure required to generate calcite e-twins is unlikely in a shallow crustal setting of the parent body [6

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Understanding the Deformation History of Murchison (CM2): Clues from Calcite Twin Stress Analysis and High Resolution X - ray Computed Tomography

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Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk        Lindgren, P., Lee, M.R. , Dobson, K.J. , Hanna, R.D., and Tomkinson, T. (2012) Understanding the Deformation History of Murchison (CM2): Clues from Calcite Twin Stress Analysis and High Resolution X-ray Computed Tomography. In: 75th Annual Meeting of the Meteoritical Society, 12-17 Aug 2012, Cairns, Australia.  http://eprints.gla.ac.uk/69052/   Deposited on: 4th September 2012   UNDERSTANDING THE DEFORMATION HISTORY OF MURCHISON (CM2): CLUES FROM CALCITE TWIN STRESS ANALYSIS AND HIGH RESOLUTION X-RAY COMPUTED TOMOGRAPHY P. Lindgren1, M. R. Lee1, K. J. Dobson2, R. D. Hanna3 and T. Tomkinson1,4. 1School of Geographical and Earth Sciences, Uni-versity of Glasgow, Glasgow G12 8QQ, UK. E-mail: pau-la.lindgren@glasgow.ac.uk. 2Manchester X-ray Imaging Facility Research Complex at Harwell, University of Manchester OX11 0FA, UK. 3Jackson School of Geological Sciences, University of Texas, Austin TX 78712, USA. 4Scottish Universities Environ-mental Research Centre, East Kilbride E75 0QF, UK.  Introduction: The Murchison CM2 carbonaceous chondrite contains a microfabric as revealed by the preferred orientation of matrix phyllosilicates [1] and the foliation of dark inclusions [2]. Olivine microstructures show that Murchison has a shock stage of S2 [3], and so this shock stage may also be responsible for the microfabric. Here we have sought to understand better the mech-anisms of deformation and to assess whether there is evidence for multiple deformation events by combining two techniques: (i) calcite e-twin stress analysis, which is a widely applied method to infer stress orientations in terrestrial rocks [e.g. 4, 5], and high resolution X-ray computed tomography (XCT). Methods: A small irregular sample of Murchison was em-bedded in a block of resin and polished to colloidal silica level on three perpendicular faces. The final size of the block was 9×7×6 mm. For calcite e-twin stress analysis the crystallographic orien-tations of calcite grains and their e-twins on each of the three pol-ished sides (A, B & C) was determined by electron backscatter diffraction using a FEI Quanta 200F field emission scanning electron microscope following the method of [6]. High resolution (XCT) was also performed on the same sample using a Metris X-tek XTH225 scanner at the Manchester X-ray Imaging Facility. Chondrules were segmented from the tomography volume using the AvizoTM software. The preferred orientation of the chondrule population was then calculated using the Blob3D software [7]. Results and Discussion: Calcite e-twin stress analyses of 23 twinned calcite grains from A, B & C show that the greatest in-tensity of stress was oriented semi-vertically to the plane of side B. The XCT data shows that the chondrules are flattened and aligned with a preferred orientation of tertiary axes (greatest stress) also oriented semi-vertically to side B. Work is ongoing to more precisely constrain the directions of stress related to the e-twins and chondrules. The semi-vertical coincidence of axes of greatest stress as measured by calcite twins and chondrules could be a result of that flattening and calcite twinning took place dur-ing the same deformation event, e.g. shallow crustal compaction or impact “gardening”. Impacts are the most likely explanation, since the static pressure required to generate calcite e-twins is unlikely in a shallow crustal setting of the parent body [6]. Acknowledgements: Peter Chung at the University of Glas-gow for technical support and UK STFC for funding. References: [1] Fujimura A. et al. 1983. EPSL 66:25–32. [2] Hanna R. D. et al. 2012. Abstract #1242. 43rd LPSC. [3] Scott E.R.D. et al. 1992. GCA 56:4281-4293. [4] González-Casado J.M. et al. 2006. J. Struct. Geol. 28:1084–1092. [5] Tourneret C. & Laurent P. 1990. Tectonophys. 180:287–302. [6] Lindgren P. et al. 2011. EPSL 306:289-298. [7] Ketcham R.A. 2005. Geo-sphere 1:32-41. 5280.pdf75th Annual Meteoritical Society Meeting (2012)

Understanding the deformation history of Murchison (CM2): clues from calcite twin stress analysis and high resolution X-ray Computed tomography

Lee, M. R.

Dobson, K. J.

Hanna, R. D.

Tomkinson, T

Understanding the deformation history of Murchison (CM2): clues from calcite twin stress analysis and high resolution x-ray computed tomography

Enlighten: Publications

http://eprints.gla.ac.uk/69052/1/69052.pdf

Understanding the deformation history of Murchison (CM2): clues from calcite twin stress analysis and high resolution X-ray Computed tomography

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The University of Manchester - Institutional Repository

Enlighten: Publications