Western Kentucky University TopSCHOLAR® Masters Theses & Specialist Projects Graduate School Spring 2016 Apatite Helium Thermochronology of the Blue Nile Canyon, Ethiopian Plateau Nathaniel C. Blackburn Western Kentucky University, [email protected] Follow this and additional works at: http://digitalcommons.wku.edu/theses Part of the Geochemistry Commons, and the Geophysics and Seismology Commons Recommended Citation Blackburn, Nathaniel C., "Apatite Helium Thermochronology of the Blue Nile Canyon, Ethiopian Plateau" (2016). Masters Theses & Specialist Projects. Paper 1563. http://digitalcommons.wku.edu/theses/1563 This Thesis is brought to you for free and open access by TopSCHOLAR®. It has been accepted for inclusion in Masters Theses & Specialist Projects by an authorized administrator of TopSCHOLAR®. For more information, please contact [email protected]. APATITE HELIUM THERMOCHRONOLOGY OF THE BLUE NILE CANYON, ETHIOPIAN PLATEAU A Thesis Presented to The Faculty of the Department of Geography and Geology Western Kentucky University Bowling Green, Kentucky In Partial Fulfillment of the Requirements for the Degree Master of Science By Nathaniel C. Blackburn May 2016 ACKNOWLEDGMENTS I would like to thank my advisor Dr. Nahid Gani for her guidance, motivation and patiently working with me throughout this research project, and graduate studies. I would also like to thank my committee members, Dr. Aaron Celestian and Dr. Xingang Fan, for their critical comments and suggestions to improve this thesis project. Thanks are due to Dr. Matthijs van Soest for his work and contribution to the apatite helium dating used in this project. I would also like to thank Alexander Falster and Prabhat Neupane of the Earth and Environmental Sciences at the University of New Orleans for providing me the opportunity to use their geochemistry lab facilities for sample preparation. I would like to thank Omni Gipson, Charlye Jackson, Red Blackburn, and Fred Noe for editing assistance and technical support. Kibre Tadese of the Ethiopian Geological Survey is greatly acknowledged for his assistance in the field during sample collection. This project was partially supported by the National Science Foundation Geomorphology and Landuse Dynamics program (grant EAR-0964876), Louisiana Board of Regents (LEQSF (2010-13)-RD-A-17), and Western Kentucky University (WKU) Research and Creative Activities I (RCAP I-14-8056) grants to Dr. Gani. I would also like to thank the WKU Department of Geography and Geology for providing graduate assistantship and travel funding to support this research. Thanks are also due to the Geological Society of America On To the Future (OTF) and WKU Graduate School for travel funding, and Veterans Affairs for additional support. iii CONTENTS ABSTRACT CHAPTER 1: INTRODUCTION ........................................................................................1 1.1. Discussion of Research .....................................................................................1 1.2. Objectives and Broader Significance ................................................................4 CHAPTER 2: GEOLOGIC OVERVIEW ...........................................................................6 2.1. General Tectonic Summary of the East African Rift System ..........................6 2.2. Study Area-The Ethiopian Plateau..................................................................10 CHAPTER 3: METHODS .................................................................................................14 3.1. Sample Collection ...........................................................................................15 3.2. Sample Preparation ........................................................................................16 3.3. Heavy Mineral and Magnetic Mineral Separations ........................................16 3.4. Mineral Picking ..............................................................................................17 3.5. Apatite (U-Th)/He Thermochronologic Dating .............................................17 3.5.1. Overview .........................................................................................18 3.5.2. Factors for Consideration ................................................................21 3.5.3. Apatite Selection and Documentation .............................................23 3.5.4. U, Th, and He Measurement ............................................................24 3.5.5. Age Calculation ...............................................................................24 3.5.6. Alpha Ejection Correction ...............................................................24 3.5.7 Statistical and Graphical Representation of AHe Age Data .............25 3.5.8. Thermal Modeling ...........................................................................26 CHAPTER 4: RESULTS AND INTERPRETATION ......................................................28 4.1. Grain Morphology and Selection ....................................................................29 4.2. (U-Th)/He Age vs. Radius (grain size) ...........................................................29 4.3. (U-Th)/He age vs. Fission Track (FT) Correction ..........................................32 4.4. (U-Th)/He age vs. U, Th and Th/U Concentration .........................................33 4.5. Radius vs. FT Correction .................................................................................34 4.6. Length/Radius vs. Radius ...............................................................................36 4.7. Fission Track correction vs. Volume ..............................................................37 4.8. Minimum Age and Mean Age vs. Elevation ..................................................38 4.9. (U-Th)/He age vs. Frequency .........................................................................39 4.10. Effective Uranium (eU) vs. (U-Th)/He age ..................................................40 4.11. Thermal Modeling ........................................................................................42 CHAPTER 5: DISCUSSION .............................................................................................45 CHAPTER 6: CONCLUSION ..........................................................................................49 CHAPTER 7: REFERENCES ...........................................................................................51 iv LIST OF FIGURES Figure 1.1. Tectonic map of the East African Rift System ..................................................2 Figure 2.1. Tectonic events within the East African Rift System ........................................7 Figure 2.2. Hillshade DEM of the Ethiopian Plateau ..........................................................9 Figure 2.3. Geologic map of the study area .......................................................................11 Figure 2.4. Field photographs of the Blue Nile Canyon. ...................................................12 Figure 3.1. Methodological steps of apatite (U-Th)/He dating ..........................................14 Figure 3.2. Sample profile ................................................................................................15 Figure 3.3. Photographs of samples. ..................................................................................16 Figure 3.4. Canyon incision model ....................................................................................18 Figure 3.5. 238U decay chain ..............................................................................................20 Figure 3.6. Factors of consideration ..................................................................................22 Figure 3.7. Apatite grain images ........................................................................................23 Figure 3.8. Alpha Ejection Correction ...............................................................................25 Figure 3.9. Thermal Modeling flow chart ..........................................................................27 Figure 4.1. Grain Morphology and Selection ....................................................................31 Figure 4.2. (U-Th)/He Age vs. Radius (grain size)............................................................33 Figure 4.3. (U-Th)/He age vs. Fission Track (FT) Correction ...........................................34 Figure 4.4. (U-Th)/He age vs. U, Th and Th/U Concentration ..........................................35 Figure 4.5. Radius vs. FT Correction .................................................................................36 Figure 4.6. Length/Radius vs. Radius ................................................................................37 Figure 4.7. Fission Track correction vs. Volume ...............................................................38 Figure 4.8. (U-Th)/He Age vs. Elevation ..........................................................................39 v Figure 4.9. (U-Th)/He age vs. Frequency ..........................................................................40 Figure 4.10. Effective Uranium (eU) vs. (U-Th)/He age ...................................................41 Figure 4.11. Thermal models of different apatite grains in sample 1 ................................43 Figure 4.12. Thermal models of different apatite grains in sample 3 ................................44 vi LIST OF TABLES Table 3.1. Thermochronometer closure temperatures .......................................................20