MS Defense Talk


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This is my defense talk from the University of Utah from May 2009. It is about 30-40 minutes in length.

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  • WI Lab Photos
  • Point out that complicated textures emerge in seemingly uniform grains, and that for the most part Temp correlates positively with CL
  • MS Defense Talk

    1. 1. Oxygen Isotopes, TitaniQ, and Cathodoluminescence analyses in the Alta Stock, UT: Insights into pluton assembly and early history Benjamin W. Johnson M.S. Candidate May 2009
    2. 2. Outline <ul><li>Motivation-Who cares about granites </li></ul><ul><li>Geology of Alta </li></ul><ul><li>Background/methods </li></ul><ul><ul><li>Oxygen Isotopes </li></ul></ul><ul><ul><li>Cathodoluminescence (CL) </li></ul></ul><ul><ul><li>TitaniQ temperatures </li></ul></ul><ul><li>Results/Discussion </li></ul><ul><ul><li>Oxygen Isotopes </li></ul></ul><ul><ul><li>CL/TitaniQ temperatures </li></ul></ul><ul><li>Conclusions </li></ul>
    3. 3. Motivation <ul><li>Granite is what makes continents </li></ul>Some picture of granites here Keith Beisner
    4. 4. <ul><li>Traditional emplacement model has been put into question </li></ul>A significant difficulty in the incremental emplacement model is that it is difficult to map individual increments. Figures from Glazer et al., 2004 It is possible that magma increments leave behind traces of themselves geochemically/texturally
    5. 5. Geology of the Alta Stock BP BP CP BP CP
    6. 6. Two end-member emplacement models Incrementally in an extensional duplex Instantaneously (<5,000yrs) Vogel et al. (2001) Cook et al. (1997)
    7. 7. Sampling strategy <ul><li>We wanted to try and detect variation at a variety of scales </li></ul>Hand sample/thin section Outcrop Stock-wide
    8. 8. Techniques utilized <ul><li>Oxygen Isotopes </li></ul><ul><li>Cathodoluminescence (CL) </li></ul><ul><li>TitaniQ </li></ul>
    9. 9. Oxygen Isotopes  18 O=[( 18 O/ 16 O sm - 18 O/ 16 O std )/ 18 O/ 16 O std ] x 1000=X‰ <ul><li>Isotopes, including oxygen, primarily </li></ul><ul><li>reflect variations in the source of magma </li></ul><ul><li>The size of fractionations are functions of temperature, so under igneous conditions (high temps), fractionations will be small </li></ul>Delta notation:
    10. 10. Precision of ± 0.1‰
    11. 11. What is CL? Weak visible light emitted by a semiconductor (minerals) when excited by an electron beam. (From D’Lemos et al., 1997)
    12. 12. TitaniQ Geothermometry Calibrated by Wark and Watson (2006) <ul><li>Ti in quartz varies as a function of the activity of Ti and temperature. </li></ul><ul><li>Calibrated in the presence of rutile </li></ul><ul><li>The Alta stock has no rutile, but it does have titanite. </li></ul><ul><li>Previous work has found that an activity of 0.6 is reasonable for a titanite-bearing rock. </li></ul>Changes in Ti concentration can be interpreted as temperature changes.
    13. 13. CL/TitaniQ CL intensity is a function of Ti concentration. This thermometer is very sensitive, our sampling technique has an uncertainty of ±10ºC. Quartz from the Vinalhaven Granite, ME (Wiebe et al., 2007) A A’ A’ A B B’ B B’
    14. 14. Oxygen Isotopes Similar WR but Qtz varies in granodiorite from the border and central phases
    15. 15. Minerals have exchanged with each other to sub-solidus temperatures of 550-650°C. The solidus is estimated at 675 °C (Hanson) Solidus
    16. 16. Additional plots suggest that all minerals analyzed have equilibrated to 550-650°C Solidus
    17. 17. Whole rock oxygen isotope values are ~consistent, but quartz isotope values vary by 1.1‰
    18. 18. What is the cause of variation in quartz? 9.31 9.45 9.16 9.19 9.20 9.37 9.52 9.11 9.17 9.36 7.82 7.72 7.75 7.38
    19. 19. Model granodiorite as evolution from a parent magma. “Amount” effect 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00  18 O Qtz  Qtz May account for 0.4‰ BP CP
    20. 20. “ Temperature” Effect Can account for 0.25‰  Qtz-Bi  18 O Qtz BP CP
    21. 21. “ Hydrothermal” Effect <ul><li>Hard to quantify. </li></ul><ul><li>It is possible that the variation </li></ul><ul><li>in WR represents the alteration due </li></ul><ul><li>to hydrothermal activity. </li></ul><ul><li>If so, the “hydrothermal” effect could </li></ul><ul><li>be ~0.3‰ </li></ul>
    22. 22. Oxygen Isotope Conclusions <ul><li>Minerals have exchanged oxygen isotopes to sub-solidus temperatures (550-650°C) </li></ul><ul><li>Quartz varies by 1.1‰ </li></ul><ul><li>Up to 0.9‰ may be due to amount, temperature, and/or hydrothermal effects </li></ul><ul><li>Remaining variations (0.2‰) cannot be clearly ascribed to distinct magma increments </li></ul><ul><li>Oxygen isotope data do not clearly identify magma increments, but do not disprove the existence of increments </li></ul>
    23. 23. CL/Ti temperatures <ul><li>Some are quite complex! </li></ul><ul><li>Correlates positively (Bright CL with High Ti) </li></ul><ul><li>White bars are 200  m </li></ul>BSE CL
    24. 24. Cathodoluminescence/TitaniQ temperatures Most phenocrysts display CR zoning in CL with higher temperatures in the core of the grain, decreasing temperatures towards the edge, and a bright CL, hot Ti thermal rejuvenation rim.
    25. 25. Some grains in the border phase show relic CR zoning and a thermally rejuvenated edge. Core Rim
    26. 26. Border phase samples show a variety of textures suggesting grain replacement and synkinematic emplacement
    27. 27. The central phase has higher maximum temperatures than the border phase.
    28. 29. CL/Ti Conclusions <ul><li>CL images reveal a complex history of crystal growth/alteration not visible by standard microscopic techniques </li></ul><ul><li>CL shows three main textures: CR zoning (growth zoning), bright CL rims (thermal rejuvenation), and tile-like patterns (annealed cracks) </li></ul><ul><li>The presence of bright CL, high Ti temperature rims in phenocrysts from the central phase shows evidence for either one large scale thermal rejuvenation or many local rejuvenations from multiple emplacements </li></ul>
    29. 30. Summary <ul><li>Minerals have exchanged oxygen isotopes to sub-solidus temperatures (550-650°C) </li></ul><ul><li>Variations in  18 O of quartz cannot clearly identify discreet magma increments </li></ul><ul><li>CL textures support synkinematic emplacement </li></ul><ul><li>Phenocrysts throughout the central phase record widespread thermal rejuvenation either from one large-scale event or emplacement of individual magma aliquots </li></ul><ul><li>Both CL and Ti temperatures are consistent with, but do not prove, the growth of the Alta stock from discreet magma increments </li></ul>
    30. 31. Acknowledgments <ul><li>Committee: John Bowman, John Bartley, Barb Nash </li></ul><ul><li>Field Work: Jared Singer, Will Gallin, Keith Christianson, </li></ul><ul><li>Melinda Hilber, Neil Lareau, Krysia Skorko </li></ul><ul><li>Lab Work: Henny Cathey, John Valley, Mike Spicuzza, Mike DePhanger, Mike Spilde, Quintan Sahratian, Eric Thomas, Erich Peterson </li></ul><ul><li>Mental Support: Matt Heumann, Michelle Mary, Mike Stearns, Kit Clemmons, Keith Beisner, Jared Gooley, Ian Semple </li></ul><ul><li>Family: Dad, Mom, Tom and Emily! </li></ul>
    31. 32. Thanks and questions?