1. Quaternary Dating with Thermochronology Tim Randol November 3, 2015 Quaternary Geology

2. Thermochronology Greek root “thermôs”— warm, heat. Greek root “chrónos”– time Termo-Man

3. Geothermal Gradient Increasing temperature with depth. Rate changes with regions.

4. Useful for Dating: Intrusion/eruption ages (cooling of magma/lava) Hydrothermal activity Metamorphism of rocks Exhumation (bringing rocks to the surface) Thermochronology

5. What Is Thermochronology? Dating the temperature-time (cooling) history of a rock. Closure temperatures The last time the crystal reached high enough temperatures for radioactive isotopes to enter or leave the crystal system. Thermochronometers Radiometric dating U-Th, Ar-Ar, U-Pb, etc. Minerals with known closure temperatures. (adapted from P. Fitzgerald, S. Baldwin, G. Gehrels, P. Reiners, and M. Ducea)

6. Thermochronometers High temperature: U-Pb in zircon Th-Pb in Monazite Useful for old rocks Nunavut, Canada(David Schneider) Unlikely for reset temperature to occur

7. Thermochronometers Low temperature: Fission tracks in apatite (AFT) U-Th/He in apatite (Ahe) Most powerful for investigating interactions between topogrphy and exhumation of the crust. In Zeitler et al., 1987 Helium interpreted as low- temperature cooling ages of the sample.

8. How Does It Work? (after D’ALESSIO, M., BLYTHE, A., BURGMANN, R., AGU 2000) Fission tracks develop in a crystal over time as uranium ( 238 U) spontaneously decays. Age calculated using ratio of # of FT’s to amount of 238 U present. Partial anneal Zone (PAZ) between ~80-110C Apatite Fission Track (from University of Leicester)

9. How Does It Work? U-Th/He Once the rock cools, 4 He starts accumulating as alpha particles. This 4 He is measured and compared with the U and/or Th within the sample.

10. How Does It Work? U-Th/He

11. 4-5 grains are picked for sampling. A lazer is used to extract the He that is trapped in the crystal. Apatite from granite in western Utah U-Th/He Procedures ( from apparentdip.wordpress) An isotopic ratio is measured for the extracted 4 He and a known amount of 3 He. With the measured amounts of U, Th, and He, an age is calculated. Interpretation of the age can be the most complicated part.

12. Interpretaton of the Age Exhumation Rates McQuarrie, N., 2014

13. Complications of (U-Th/He) Calculated (U-Th)/He ages of a crystal can be strongly dependent on variations in fractional He loss caused by crystal size variations.