WHOLE-ROCK DATING OF YOUNG EXTRUSIVES BY THE POTASSIUM-ARGON METHOD by AJOY K. BAKSI October 1970 A Thesis submitted in conformity with the requirements for the Degree of Doctor of Philosophy in the University of Toronto Department of Physics {S) Ajoy K. Baksi 1970 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(i) ABSTRACT The feasibility of dating very young extrusives by the wholerock potassium-argon technique has been investigated. Rocks from the Columbia River Basalts, Oregon, Rio Grande de Santiago volcanics, Mexico, San Francisco Volcanic Field, Arizona, Hawaii and Australia have been studied. Some of the results point out the grave difficulties that can arise due to the presence of small quantities of excess argon in such rocks. Comparison on the ages of rocks in the age range 0 to 5 million years strongly suggest that inter-laboratory differences exist especially in the detection and measurement of small quantities of radiogenic argon. Study of a very thick extrusive of Recent age indicates that atmospheric argon may have been incorporated into the edges of the flow as it solidified. The quantity of argon pr~sent and its isotopic compositi~n is not homogeneous throughout the lava flow. Wherever possible,. the data are examined on potassium-argon isochrons. This yields information about the precision of the individual analyses. Lack of total control over the 11 blanks" during the. argon analyses, does not allow UrJequivocal interpretation of the meaning of the "ages" obtained. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(;;) ACKNOWLEDGMENTS I would like to express my gratitude to those whose help and co-operation has been invaluable in the formulation of this thesis. Dr. D. York, Department of Physics, University of Toronto, for his close supervision of the work presented here. Dr. R.M. Farquhar, Department of Physics, University of Toronto, for many profitable discussions particularly those concerned with mass spectrometry. Dr. H.C. Halls, Department of Physics, University of Toronto, for pertinent suggestions regarding the interpretation of some of the results presented in this work and also for critically reading this thesis. Dr. J.J. Fawcett, Department of Geology, University of Toronto, for his co-operation and helpful discussions regarding the petrology of extrusive rocks. Dr. I. McDougall, Department of Geophysics and Geochemistry, Australian National University,.for providing the samples used for interlaboratory comparison. His help was especially appreciated in light of the obvious reluctance of most laboratories to participate in this type. of study. Professor N.D. Watkins, Graduate School of Oceanography, University of Rhode Island, who has permitted me to use his unpublished Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(iii) paleomagnetic results, while also providing samples from the Columbia River Basalts and the Rio Grande de Santiago Volcanics. Professor A.E. Paull, Department of Mathematics, University of Toronto, for helpful discussions about the 11 t 11 tests. Mr. R.J. Doyle, for most of the potassium analyses reported here. He also wrote the computer progranune used for the isochron plots. Mr. W.J. Kenyon, for technical assistance in the argon analysis laboratory. Mr. K.H. Khan for the drafting of the diagrams in this thesis. Mrs. Margaret Hastie for the typing of this thesis. Finally, I would like to express my appreciation to Dr. A. Hayatsu, Department of Geophysics, University of Western. Ontario, for many revealing discussions concerning some of the basic assumptions of the potassium-argon dating method. ' The International Nickel Company of Canada provided financial assistance in the form of a Graduate Fellowship, for the period 1967-69. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(iv) TABLE OF CONTENTS Page No. ABSTRACT ACKNOWLEDGMENTS LIST OF TABLES LIST OF ILLUSTRATIONS CHAPTER l - INTRODUCTION l.l General Comments l. 2 The decay scheme of K 40 1.3 The decay equation 1.4 Isotopic abundances of natural potassium, calcium and argon i.5 The suitabiiity of the potassium-argon dating method 1.6 Source of atmospheric argon and excess argon in rocks 1.7 Scope of this work CHAPTER 2-2. l Introduction I_NSTRUMENTATION AND EXPERIMENTAL TECHNIQUES 2.2 Potassium analysis 2.3 Argon work - introduction 2.4 Argon extraction 1 ine 2.5 Fusion procedure 2.6 Isotope dilution technique 2.7 Mass spectrometer 2.8 Effect of changing ion-repeller voltage 2.Ba Recalibration of Range Factors i ii vii ix l l 4 5 8 10 17 17 20 20 22 25 26 26 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.