Abstract
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 α-dose (<1016 α/g), subsequently increasing before decreasing again at a very high α-dose (>1018 α/g). Studies have focused on He diffusion behavior at high α-dose, but little is known at low doses. We propose that the ZHe closure temperature at α-dose ranging from 6 × 1015 to 4 × 1016 α/g is in the range of ~60–80 °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 α-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.
Highlights
To quantitatively unravel the tectonics and/or relief evolution of a given region, lowtemperature thermochronology methods such as (U-Th)/He and fission track dating of apatite (AHe and AFT, respectively) or zircon (ZHe and zircon fission track (ZFT), respectively) are often used together e.g., [1,2]
For over a decade, methodological studies have highlighted that He diffusion in apatites and zircons is strongly dependent on the radiation damage dose, increasing the range of the closure temperature, as important age dispersion with positive apatite (U-Th)/He (AHe)-age, and positive and negative zircon (U-Th)/He (ZHe)-ages correlations are often observed e.g., [8,9] for zircons, e.g., [10–14]
1), we show that in the case of low α-doses, ZHe closure temperatures are closer to AHe closure temperatures than previously proposed, and present are closerages to AHe temperatures proposed, present younger younger thanclosure
Summary
To quantitatively unravel the tectonics and/or relief evolution of a given region, lowtemperature thermochronology methods such as (U-Th)/He and fission track dating of apatite (AHe and AFT, respectively) or zircon (ZHe and ZFT, respectively) are often used together e.g., [1,2]. The ZFT and ZHe methods are generally known to record higher temperatures or deeper processes than the AFT and AHe methods [3,4] because of their higher closure temperatures [5–8]. For a given mineral, (U-Th)/He thermochronometry is generally considered more sensitive to lower temperatures than fission track thermochronology [1,5]. Today, these techniques are routinely applied, and numerous studies are published each year for exhumation quantification purposes. For over a decade, methodological studies have highlighted that He diffusion in apatites and zircons is strongly dependent on the radiation damage dose, increasing the range of the closure temperature, as important age dispersion with positive AHe-age, and positive and negative ZHe-ages correlations are often observed e.g., [8,9] for zircons, e.g., [10–14]
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