Apatite low-temperature chronometry and microstructures across a hydrothermally active fault zone

• Published in: Chemical geology Vol. 588; p. 120633
• Main Authors: Berger, Alfons, Egli, Daniel, Glotzbach, Christoph, Valla, Pierre G., Pettke, Thomas, Herwegh, Marco
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20.01.2022; Elsevier
• Subjects: Apatite (U-Th)/He ages; Earth Sciences; Fault zones; Geochemistry; Hydrothermal systems; Microstructures; Sciences of the Universe; Trace element geochemistry; Hydrothermal systems; Fault zones; Microstructures; Trace element geochemistry; Apatite (U-Th)/He ages
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2021.120633

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.