Abstract
Low-temperature chronometers offer potential to gain insights into the temporal evolution of hydrothermal systems. The long-lived fault-bound Grimsel pass hydrothermal system (including a fossil and an active part) in the Central European Alps serves here as a key site to test such an application. Zircon and apatite grains were separated from samples collected along a fault transect. The resulting zircon (U-Th)/He ages are homogenous along the profile at 8–9 Ma and thus record the regional cooling evolution, remaining unaffected by the younger hydrothermal activity. In contrast, the apatite (U-Th)/He ages show three age groups: One Group (1) of ca. 5 Ma inside and outside the hydrothermal zone matches the low-temperature part of the regional cooling trend, while group (2) with ages as young as 1–2 Ma occurs in a central narrow zone associated with hydrothermal activity. One sample (group 3) displays older apparent ages compared to the regional cooling trend. Group (2) apatite samples reveal a different cathodoluminescence texture and trace-element chemistry, which we interpret together with the young age as apatite growth or re-crystallization within the hydrothermal system. Forward 1D modelling of He diffusion indicates that apatite (U-Th)/He ages should always be reset when exposed to hot thermal waters (up to ~140 °C) present over ka timescales or to intermediate temperature waters (~90 °C) over Ma timescales. Combining our measured apatite (U-Th)/He ages with forward modelling results highlight that, besides regional cooling trends, local heat anomalies within hydrothermal zones are very variable in space and time. Combined trace-element geochemistry and (U-Th)/He dating shows local occurrence of newly-formed apatites crystals, which are best described as geochronometers rather than thermochronometers. Such information is important to explore the longevity of hydrothermal systems and associated spatial distributions of heat anomalies.
Highlights
Rock cooling/exhumation signals can be well determined by lowtemperature thermochronology methods such as the apatite and zircon fission-track (AFT and ZFT) or (U-Th)/He (AHe and ZHe) systems (Reiners and Brandon, 2006; Reiners et al, 2018, and references therein)
Recent studies on hydrothermal systems associated with active tectonic faults have shown that AHe ages far away from faults record regional cooling, but closer to the fault, AHe ages may be perturbed (Whipp and Ehlers, 2007; Gorynski et al, 2014; Milesi et al, 2019, 2020)
Along hydrothermally-active faults, different AHe-age behaviour can be observed, leading to three general groups of AHe age data: (1) grains which do not record the thermal overprint imposed bythermal fluids, (2) AHe ages younger than regional cooling age patterns related to new mineral precipitation/ growth with no or very minor diffusional He loss, (3) macroscopically non-visible but frequent fracturing of apatite grains facilitating gain or loss of He resulting in apparent AHe ages that are older or rejuvenated
Summary
Rock cooling/exhumation signals can be well determined by lowtemperature thermochronology methods such as the apatite and zircon fission-track (AFT and ZFT) or (U-Th)/He (AHe and ZHe) systems (Reiners and Brandon, 2006; Reiners et al, 2018, and references therein). Hydrothermal systems can modify the heat distribution in a rock mass and thereby disturb the above mentioned cooling/exhumation signal determined by low-temperature thermo chronometers (Gorynski et al, 2014; Reiners et al, 2018, and references therein). Recent studies on hydrothermal systems associated with active tectonic faults have shown that AHe ages far away from faults record regional cooling, but closer to the fault, AHe ages may be perturbed (Whipp and Ehlers, 2007; Gorynski et al, 2014; Milesi et al, 2019, 2020). AHe ages may be (i) younger compared to the regional trend
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