Zircon (U-Th)/He Closure Temperature Lower Than Apatite Thermochronometric Systems: Reconciliation of a Paradox

Here, we present seven new zircon (U-Th)/He (ZHe) ages and three new zircon fission track (ZFT) ages analyzed from an age-elevation profile (Machu Picchu, Peru). ZFT data present ages older than those obtained with other thermochronological data, whereas the ZHe data interestingly present ages similar to those obtained with apatite (U-Th)/He (AHe). It has been proposed that He retention in zircon is linked to the damage dose, with an evolution of the closure temperature from low values associated with a low alpha-dose (< 10(16) alpha/g), subsequently increasing before decreasing again at a very high alpha-dose (> 10(18) alpha/g). Studies have focused on He diffusion behavior at high alpha-dose, but little is known at low doses. We propose that the ZHe closure temperature at alpha-dose ranging from 6 x 10(15) to 4 x 10(16) alpha/g is in the range of similar to 60-80 degrees C. This value is lower than that proposed in the current damage model ZRDAAM and demonstrates that the ZHe and AHe methods could have similar closure temperatures at low alpha-dose (i.e., similar ages). These new data strengthen our previous geological conclusions and even highlight a cooling rate approximately twice as important as that deduced from AHe and apatite fission track data alone at Machu Picchu.

Automated summary

In this study, new zircon (U-Th)/He (ZHe) ages and zircon fission track (ZFT) ages were analyzed from an age-elevation profile in Machu Picchu, Peru. The ZFT data showed older ages compared to other thermochronological data, while the ZHe data interestingly showed ages similar to apatite (U-Th)/He (AHe). It was proposed that He retention in zircon is linked to the damage dose, with a closure temperature that varies with alpha-dose. The study suggests that the closure temperature of ZHe at low alpha-dose ranges from 6 x 10(15) to 4 x 10(16) alpha/g, and is lower than previously thought. These findings strengthen previous geological conclusions and highlight a cooling rate at Machu Picchu that is approximately twice as important as deduced from AHe and apatite fission track data alone.