“Inverted” zircon and apatite (U-Th)/He dates from the Front Range, Colorado: High-damage zircon as a low-temperature (<50 °C) thermochronometer

Zircon (U-Th)/He (ZHe) data were acquired for 23 Proterozoic basement samples from an E-W transect through the Colorado Front Range to evaluate whether metamict zircons yield sensible (U-Th)/He data patterns and useful thermal history information. The 112 ZHe dates vary from 147 to 7 Ma, define positive and negative date-eU correlations, and are younger than titanite (U-Th)/He dates that range from 976 to 614 Ma. At moderate to high alpha dose of 1018-1019α/g, zircons from the range core yield Laramide (52.5±9.6 Ma) dates, whereas those within ∼15 km of the range front yield Miocene (21.6±7.7 Ma) results. The He dates for the high alpha dose zircons are reproducible within each sample suite despite their visibly metamict character. The ∼20 Ma range front ZHe dates are younger than apatite (U-Th)/He (AHe) dates (66.5±9.6 Ma) and published apatite fission-track data (65-45 Ma) for the same and nearby samples. Thermal history simulations can reproduce the first-order range front date-eU patterns and ZHe-AHe date inversion, but the high-damage zircons are more He retentive than predicted by the zircon damage He kinetic model. The ∼20 Ma ZHe dates may be explained by reheating from hydrothermal fluids along range front faults. The results demonstrate the promise of using He data for high-damage zircons to detect low-temperature (<50 °C) events within and below the temperature sensitivity of the AHe system.