In search of lost time: Raman thermochronology of FC-1 zircon

• Published in: Contributions to mineralogy and petrology 2024, Vol.179 (1), p.2, Article 2
• Main Authors: Härtel, Birk, Enkelmann, Eva, Jonckheere, Raymond, Ludwig, Thomas, Krause, Joachim, Ratschbacher, Lothar
• Format: Article
• Language: English
• Subjects: Age; Annealing; Apatite; Earth and Environmental Science; Earth Sciences; Geology; Low temperature; Mineral Resources; Mineralogy; Original Paper; Petrology; Radiation; Radiation damage; Temperature; Thorium; Zircon
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-023-02083-z

Translating thermochronological ages to geological models requires knowledge of the thermal sensitivity of the applied thermochronometer, i.e. the closure temperature or the partial annealing/retention zone. Zircon Raman dating is a thermochronometer that uses radiation-damage measurements and matched analyses of actinide contents in zircon. Experimental work placed its closure temperature at 330–370 °C for the internal ν 2 (SiO 4 ) and ν 3 (SiO 4 ) Raman bands at 439 and 1008 cm −1 , and 260–310 °C for the external rotation (ER) band at 356 cm −1 . However, experimental annealing models also predict partial radiation-damage annealing over a broad temperature range (> 500 °C). We test these closure temperatures by dating zircon from the U–Pb reference material FC-1. We matched Raman-based radiation-damage measurements with U and Th concentrations measured with a secondary ion mass spectrometer. The zircon Raman ages for the ν 2 , ν 3 , and ER bands are 942 ± 23 Ma (2 s), 978 ± 38 Ma, and 1033 ± 32 Ma. This is lower than the expected range of 1040–1080 Ma, between the apatite U–Pb (~ 490 °C) and zircon (U-Th)/He (ZHe; ~ 200 °C) ages. We discuss long-term, low-temperature annealing of radiation damage during protracted cooling of the sample as a cause of age reduction. This explanation fits both, experimental annealing models and zircon Raman data from other geological settings. Long-term, low-temperature annealing complicates the interpretation of standalone zircon Raman data. However, exploiting this effect by combining zircon Raman and ZHe dating on the same sample provides thermal-history information beyond that revealed by either of them.