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

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 ν2(SiO4) and ν3(SiO4) 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 ν2, ν3, 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.