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Calibration of the Production Rates of Cosmogenic 36C1 from Potassium

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Calibration of the Production Rates of Cosmogenic 36C1 from Potassium CALIBRATION OF THE PRODUCTION RATES OF COSMOGENIC 36C1 FROM POTASSIUM Jodie Manderson Evans A thesis submitted for the degree of Doctor of Philosophy at The Australian National University July, 2001 daran and Heather Preface This thesis describes the complete calibration of J6C1 produced in-situ from potassium. This project was suggested by Dr John Stone and Dr Keith Fifield, and carried out at the Australian National University (ANU) jointly between the Research School of Earth Sciences (RSES), and the Department of Nuclear Physics, RSPhysSE. The author has taken primary responsibility from the beginning of this project to its conclusion. This has involved collecting the samples, performing mineral separations, preparing them for many different measurements, performing the measurements, and finally, carrying out the analysis and interpreting the data. During this process the author was assisted and advised by many people, as discussed below. With the exception of the samples AT-10, AT-11, AT-12 and AT-13, which were collected by Dr Stone, all the Scottish samples were collected by the author with advice from Dr Colin Ballantyne and assistance from Keith Salt, Matt Franks, Angela Lamb and Dr Greg Lane. The samples for the depth profile from Wyangala were collected by the author with Dr Stone, Dr Fifield, Dr Richard Cresswell, Dr Mariana Di Tada and Dr Kexin Liu. The original trace element measurements were made by ICP-MS at RSES with assistance from Mr Leslie Kinsley. X-ray fluorescence measurements (major element whole rock concentrations and some whole rock uranium and thorium concentrations) were made by Dr Bruce Chappell in the Geology Department at ANU. Later repeat trace element measurements were made by Prompt Gamma Analysis by XRAL in Canada. Potassium concentrations were measured by the author with assistance from Mrs Robyn Maier. Initial calcium measurements were made by the author with the assistance of Mr Mike Shelley. These measurements were then repeated at Ecowise Environmental Laboratories, Australia. All chloride measurements (pyrohydrolysis and isotope dilution) were made by the author following processes developed by Dr Stone. Likewise AgCl samples for the b6Cl analyses were prepared by the author following procedures adapted by Dr Stone from the original work of Conrad et al [Con86] and Zreda et al [Zre91]. AMS measurements were made by the author using the 14UD Pelletron in the Department of Nuclear Physics. Successful 3bCl measurements on the samples relied on the expertise of Dr Fifield and Dr Cresswell, and additional assistance in measuring the samples was provided by Dr Stone, Dr Di Tada and Dr Liu. Acquisition of the multi-parameter data for these measurements employed the software written for the laboratory's data acquisition system by Dr Gordon Foote. The subsequent off-line sorting of these data, which was required to derive the 36C1/Cl values, was carried out by the author using the same software. Analysis and interpretation of the results was performed by the author with initial assistance from Mathematica programs written by Dr Stone and invaluable discussions with Dr Stone, Dr Fifield and Dr Lane. The focus of this work has changed over time. The original intention was to include measurements made on eight samples collected from the Sierra Nevada, USA, by Dr Stone, and three samples collected in the Antarctic by Jeff Klein, but due to uncertainties in their exposure histories that became apparent after this work was underway, they were ultimately omitted from this dissertation. Some of the results obtained on these samples, in addition to four of the Scottish samples included in this work, can, however, be found in the following article: J.M. Evans, J. O. H. Stone, L. K. Fifield and R. G. Cresswell Cosmogenic chlorine-36 production in K-feldspar Nucl. Instr. And Meth. In Phys. Res. B 123, p 334 - 340,1997. No part of this thesis has been submitted for a degree at any other university. Jodie Evans iv Acknowledgemen ts Today is gone. Today was fun. Tomorrow is another one. Everyday, from here to there, funny things are everywhere. Dr Seuss As I put the finishing touches on my thesis I'm amazed at how interested I still am in the subject, and I have a twinge of regret that I won't be continuing the investigation. I don't believe that anyone really enjoys writing up their thesis, but I think I could make a pretty good case that my situation has been more difficult than most! I've gone from being single with no kids, to married with 2 (and it's the best thing that's ever happened to me!). I've moved overseas and back again twice. And I won't even go into how difficult it is to discuss your work with your supervisors when all three of you are living thousands of miles apart. But I will soon be finished and I have many people I would like to thank for helping me get from here to there. First I would like to thank my "tag team" supervisors, John Stone and Keith Fifield. John's enthusiasm for the work has been inspiring, and I'm very grateful for his time and patience in teaching a physicist enough of the geology and chemistry to carry out the project. Keith's expertise with the accelerator, his efforts reading and rereading what I'm sure seemed like endless chapters so promptly, and keeping me focussed when I started stressing out at the end, were a godsend. Thanks to all my official and unofficial advisers during my time here: Garry Allan, Richard Cresswell, Alan Chivas, Trevor Ophel and the person who is really responsible for me finishing, Greg Lane. George Dracoulis has supported me throughout my time at nuclear and I greatly appreciate his efforts at the very end as my default supervisor and critically reading my entire thesis in final form. (I'd also like to thank George for making sure the girls finally got a shower in the refurbishment of the building!). On the RSES side of the road, I'd also like to thank Kurt Lambeck for his support of my Ph.D. I'd like to thank Mariana Di Tada and Kexin Liu for all their special help making my final AMS measurements when I was nine months pregnant with Ciaran. Their support really meant a lot to me. I'd also like to thank Sam King and Phil Day for being great people! As I said, I'm not a geologist, so I have to thank Colin Ballantyne, Keith Salt, Matt Franks and Angela Lamb for all their advice and assistance with collecting samples in Scotland. I'd also like to thank everyone who helped me with various measurements at RSES: Robin Maier, Ian McDougall, John Chappell, Leslie Kinsley, Mike Shelley, John Mya, Shane Paxton and Joe Cali. I'd like to express my appreciation to everyone at nuclear physics for being so supportive over the last 8 years, particularly Gordon Foote and Aidan Byrne, who have been my cheerleaders in the last 6 months, giving me words of encouragement every time they go past my desk to their offices, and Anna Wilson, Nanda Dasgupta and Susan Helyar for reading parts of my thesis. I'd also like to thank Clyde and Susan for being such great friends in Stony Brook! I also really appreciate the help I got from both sets of grandparents when Greg went away for two weeks during the last month of writing up. I'd like to thank my brother for the words of encouragement I'm sure everyone else was thinking, but only he could give ("Just finish the bloody thing!"). I'd like to thank my Dad for his support and doing that last check for typos. And I'd like to thank my Mum for her support and, particularly, for finishing her thesis X years ago with three young kids! Finally, I'd like to thank my two precious children, Ciaran and Heather, for their tolerance of mum disappearing to work all the time for the last couple of weeks, and being stressed and grumpy when she was at home. But mostly I'd like to thank my husband Greg, without whom there is no way I would have finished. He listened to me go on and on about various aspects of my thesis when we were living too far away from my supervisors to talk to them about it properly. He read pages and pages of rubbish I'd written, but most importantly he always believed I'd get there. Thank you! Abstract Chlorine-36 production rates from potassium have been calibrated for the top 10m of the lithosphere, where "6C1 is produced by two main reaction channels: i) spallation, which dominates 3bCl production in the top 2 metres below the earth's surface, and ii) negative muon capture, which dominates at depths of between 2 and ~20 metres below the earth's surface. Rock samples were collected from two sites: i) rock surfaces exposed by the end of the Loch Lomond Readvance glaciation in Scotland 11,600 years ago, and ii) a 6.1m depth profile at a granite quarry in south eastern Australia, for which a steady state J6C1 concentration dictated by a history of constant erosion was assumed. In addition, the 36C1 measurements were made on pure K-feldspar mineral separates to remove the b0Cl contributions from calcium, iron and titanium, and to minimise the 36C1 contribution from chloride. The potassium spallation production rate was deduced from 18 Scottish K- feldspar mineral separates to be 161 (9) atom (gK)'1 y r1 at sea level and high latitude.
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