* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
Quaternary Dating with Thermochronology Tim Randol November 3, 2015 Quaternary Geology Thermochronology Greek root “thermôs”— warm, heat. Termo-Man Greek root “chrónos”– time Geothermal Gradient • Increasing temperature with depth. • Rate changes with regions. Thermochronology Useful for Dating: • Intrusion/eruption ages (cooling of magma/lava) • Hydrothermal activity • Metamorphism of rocks • Exhumation (bringing rocks to the surface) What Is Thermochronology? Dating the temperature-time (cooling) history of a rock. Radiometric dating • U-Th, Ar-Ar, U-Pb, etc. Closure temperatures • The last time the crystal reached high enough temperatures for radioactive isotopes to enter or leave the crystal system. Thermochronometers • Minerals with known closure temperatures. (adapted from P. Fitzgerald, S. Baldwin, G. Gehrels, P. Reiners, and M. Ducea) High temperature: Thermochronometers • U-Pb in zircon • Th-Pb in Monazite Useful for old rocks • Unlikely for reset temperature to occur (David Schneider) Nunavut, Canada Low temperature: • Fission tracks in apatite (AFT) • U-Th/He in apatite (Ahe) Thermochronometers In Zeitler et al., 1987 • Helium interpreted as lowtemperature cooling ages of the sample. Most powerful for investigating interactions between topogrphy and exhumation of the crust. Apatite Fission Track How Does It Work? (after D’ALESSIO, M., BLYTHE, A., BURGMANN, R., AGU 2000) • Fission tracks develop in a crystal over time as uranium (238U) spontaneously decays. • Age calculated using ratio of # of FT’s to amount of 238U present. • Partial anneal Zone (PAZ) between ~80-110C (from University of Leicester) U-Th/He How Does It Work? • Once the rock cools, 4He starts accumulating as alpha particles. • This 4He is measured and compared with the U and/or Th within the sample. U-Th/He How Does It Work? U-Th/He Apatite from granite in western Utah Procedures • 4-5 grains are picked for sampling. • A lazer is used to extract the He that is trapped in the crystal. • An isotopic ratio is measured for the extracted 4He and a known amount of 3He. • With the measured amounts of U, Th, and He, an age is calculated. • Interpretation of the age can be the most complicated part. ( from apparentdip.wordpress) Interpretaton of the Age Exhumation Rates McQuarrie, N., 2014 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.