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Antarctic Alteration of Meteorites
Thesis
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Steer, Elisabeth (2016). Antarctic Alteration of Meteorites. PhD thesis The Open University.
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(cid:13)c 2016 Elisabeth Steer
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A A M
NTARCTIC LTERATION OF ETEORITES

Thesis submitted for the degree of Doctor of Philosophy
Department of Physical Sciences
The Open University

By
Elisabeth Dawn Steer, MGeol.

21st March 2016

ABSTRACT
The discovery of large accumulations of meteorites in Antarctica is comparably recent,
with annual collection expeditions beginning in 1974. Since then, over 50,000 meteorites
have been recovered from the icefields of Antarctica. Many of those meteorites have had
long residence times of up to 2 Ma mostly encased in ice; during which many of the
meteorites have undergone considerable alteration. Understanding the paths of alteration
allows reconstruction of original features, but additionally it gives a unique opportunity to
examine a natural laboratory of cold, water-restricted alteration environments similar to
those on Mars today and in its history.
To fully understand the weathering of meteorites in these environments, six L6 chondrites
in a variety of weathering states have been examined and characterised petrologically,
chemically, and magnetically. Chemical analyses undertaken are major and trace elements
measured in bulk and in spatially resolved analysis and bulk oxygen isotopes.
Petrology has proven to control the weathering patterns and alteration state of the
meteorite. A chemical weathering index has been developed to characterise the state of
weathering using bulk chemical data, which also links with the petrological findings. This
is especially evident in micro features created by shock, which promote rapid mineral
breakdown and acidification of alteration fluids, which, fundamentally changes the nature
and speed of the alteration. Shock generated features have created increased vulnerability
to weathering and so areas that have undergone significant shock on Mars are more
vulnerable to weathering and breakdown. However, the heterogeneity that is inherent to
alteration environments of low water to rock ratios results in short transport distances for
elements, resulting in little bulk chemical change with significant mineralogical alteration.
i
ACKNOWLEDGMENTS
There is no way I could ever manage to thank everyone who has helped me in some way over the last
few years, as there have been so many, but I would like to thank as many as I can below in no
particular order. I hope I haven’t forgotten anyone, but it’s been a long haul and there are many
people to thank.
Firstly I want to thank my supervisors Susanne, Monica and Ian without whom I would not have been
able to do my PhD. I want to thank Susanne in particular for all the days, nights and weekends she has
put in gradually moulding me into a functioning researcher. I really couldn’t have done it without the
constant support and encouragement.
I couldn’t have done my analytics without the technical help and advice from Sam Hammond, who
put up with me bothering her a lot. Also thanks to Andy Tindle, Jenny Gibson, Richard Greenwood,
Jérôme Gattacceca, the team from CEREGE, Diane Johnson and John Watson. Conversations with
many people helped shape my thesis; in particular discussion with Richard Greenwood and Jérôme
Gattacceca has been invaluable. I thank Sarah Sherlock and Rhian Jones for being my viva examiners
and Axel Hagermann for organising and chairing the viva.
Thanks to my family, for always reminding me that there was a life outside of my thesis and always
being incredulous at me working over holidays.
My friends have provided me with a lot of emotional support over the years and have listened to an
awful lot of moaning, so thanks to James Mortimer, Dan Dawson, Mohit Melwani-Daswani, Ed
Allanwood, Erika Kaufmann, Jean-David Bodénan, Jess Barnes, Feargus Abernethy, Alice Griffiths,
the DPS houseband, Wolverton Town band, Tom Barratt, Roy Adkin, Joe Rushton and everyone in
the PhD student office.
Finally I have to thank Mark, without whom I would’ve undoubtable been a blubbering wreck and
never would have finished.
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Table of Contents
Abstract ....................................................................................................................................... i
Acknowledgments......................................................................................................................ii
Contents ................................................................................................................................... iii
List of Figures ........................................................................................................................... xi
List of Tables ........................................................................................................................... xx
Chapter One: Introduction ......................................................................................................... 1
1.1. Discovery and Accumulation method of meteorites in Antarctica ................................. 1
1.2. The Blue Icefields ........................................................................................................... 4
1.3 Timing of weathering processes ....................................................................................... 6
1.4 Mineralogical Changes ..................................................................................................... 9
1.4.1 Weathering Scales ..................................................................................................... 9
1.4.2 Weathering of Fe-Ni metals and sulphides .............................................................. 12
1.4.3 Silicate weathering .................................................................................................. 14
1.4.4 Evaporites ................................................................................................................ 17
1.4.5 Antarctic Varnish ..................................................................................................... 19
1.5. Magnetic properties of altered meteorites ..................................................................... 21
1.6. Trace Element Changes in Weathered Antarctic Meteorites ........................................ 21
1.7. Halogen Additions......................................................................................................... 23
1.8. Oxygen .......................................................................................................................... 29
iii
1.9. Antarctica as a Martian analogue .................................................................................. 31
1.10. Alteration Minerals on Mars ....................................................................................... 35
1.11. Sample selection .......................................................................................................... 37
1.12. Research justification and objectives………………………………...………………38
Chapter Two: Methods ............................................................................................................ 40
2.1. Samples and preparation ............................................................................................... 40
2.1.1. Sample preparation for solution ICP-MS and ICP-OES ........................................ 42
2.2. Optical Microscope ....................................................................................................... 45
2.3. Scanning Electron Microscope (SEM) – Energy Dispersive X-ray Spectroscopy ....... 45
2.3.1. Instrumentation ....................................................................................................... 45
2.3.2. Background ............................................................................................................. 46
2.3.3. Analytical method, mapping and modal counts ..................................................... 49
2.4. Electron Microprobe Analysis (EMPA) ........................................................................ 55
2.4.1. Instrumentation ....................................................................................................... 55
2.4.2. Background ............................................................................................................. 55
2.4.3. Analytical Conditions ............................................................................................. 55
2.5. Solution ICP-MS and ICP-OES .................................................................................... 56
2.5.1. Solution preparation ............................................................................................... 56
2.5.2. Instrumentation ....................................................................................................... 57
2.5.3. Background ............................................................................................................. 58
2.6. LA-ICP-MS ................................................................................................................... 59
iv
2.6.1. Instrumentation ....................................................................................................... 60
2.6.2. Analytical method................................................................................................... 60
2.7. Magnetic Measurements ............................................................................................... 61
2.7.1. Instrumentation ....................................................................................................... 61
2.7.2. Background ............................................................................................................. 62
2.8. Oxygen Isotope Measurement ....................................................................................... 62
2.8.1. Instrumentation ....................................................................................................... 62
Chapter Three: Petrography ..................................................................................................... 64
3.1 Sample Overviews.......................................................................................................... 64
QUE 99022 ....................................................................................................................... 67
QUE 94214 ....................................................................................................................... 67
ALH 78130 ....................................................................................................................... 69
ALH 84056 ....................................................................................................................... 70
ALH 84058 ....................................................................................................................... 70
ALH 85017 ....................................................................................................................... 73
3.2. Mineralogy .................................................................................................................... 74
3.2.1. Olivine .................................................................................................................... 74
3.2.2. Pyroxene ................................................................................................................. 80
3.2.3. Plagioclase .............................................................................................................. 81
3.2.4. Troilite .................................................................................................................... 84
3.2.5. Kamacite ................................................................................................................. 89
v
3.2.6. Taenite .................................................................................................................... 94
3.2.7. Chromite ................................................................................................................. 96
3.2.8. Phosphates .............................................................................................................. 98
3.2.9. Melt Products/pockets .......................................................................................... 100
3.2.10. Large pockets of alteration product .................................................................... 102
3.3. Oxidation percentages ................................................................................................. 104
3.4. Fracture patterns .......................................................................................................... 107
3.5. Antarctic Varnish ........................................................................................................ 113
3.6. Museum Weathering ................................................................................................... 115
3.7. Discussion: The effect of shock on weathering patterns ............................................. 118
3.8. Discussion: Sample bias; thin vs thick section ........................................................... 124
Chapter Four: Spatially resolved data .................................................................................... 127
4.1. EMPA data .................................................................................................................. 127
4.1.1 Olivine ................................................................................................................... 127
4.1.2. Pyroxene ............................................................................................................... 137
4.1.3. Feldspars ............................................................................................................... 150
4.1.4. Alteration product ................................................................................................. 150
4.1.5. Troilite .................................................................................................................. 156
4.1.6. Kamacite and Taenite ........................................................................................... 160
4.1.7. Chromite ............................................................................................................... 160
4.2. LA-ICP-MS ................................................................................................................. 161
vi
4.2.1. Olivine .................................................................................................................. 161
4.2.2. Pyroxenes.............................................................................................................. 167
4.2.3. Feldspars ............................................................................................................... 171
4.2.4. Troilite .................................................................................................................. 171
4.2.5. Phosphates ............................................................................................................ 173
4.2.6. Kamacite and Taenite ........................................................................................... 175
4.2.7. Chromite ............................................................................................................... 176
4.2.8. Pockets of Alteration Products ............................................................................. 176
4.2.9 Discussion of LA-ICP-MS data ............................................................................. 183
4.3. Summary ..................................................................................................................... 192
Chapter Five: Bulk rock analysis ........................................................................................... 194
5.1. ICP-MS analysis .......................................................................................................... 194
5.1.1. Trace element analysis ............................................................................................. 195
Lithium ........................................................................................................................... 195
Boron .............................................................................................................................. 199
Phosphorous.................................................................................................................... 199
Scandium ........................................................................................................................ 200
Titanium.......................................................................................................................... 201
Vanadium........................................................................................................................ 202
Chromium ....................................................................................................................... 204
Manganese ...................................................................................................................... 204
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