首页|Apatite low-temperature chronometry and microstructures across a hydrothermally active fault zone
Apatite low-temperature chronometry and microstructures across a hydrothermally active fault zone
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NSTL
Elsevier
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 similar to 140 degrees C) present over ka timescales or to intermediate temperature waters (similar to 90 degrees 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.
NSTL
Elsevier
