Search

International audience; 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.

The timing and duration of hydrothermal activity during orogenesis are difficult to constrain, because such systems are open and multistage. Using mm-sized monazite crystals from two Alpine clefts from the Central Alps (Switzerland), we demonstrate that combined U–Th–Pb isotopic systematics of hydrothermal monazite can constrain both timing and fluid evolution during crystallization. Our data highlight four major characteristics of the cleft monazites: (i) extreme Th/U ratios (ii) significant common Pb in the U system (up to 39% 206 Pb), (iii) excess 206 Pb (up to 54%), and (iv) precise and reliable Th–Pb ages. Comparison of our results with literature data indicates that Th/U in monazite is a remarkable discriminant of geological conditions, capable of distinguishing metasedimentary, magmatic and hydrothermal origin. Hydrothermal monazite crystals are characterized by Th/U ratios up to 629, among the highest values ever reported. Extreme Th/U ratios in monazite enhance incorporation of 230 Th, a short-lived intermediate product in the 238 U– 206 Pb decay chain, generating 206 Pb excess which, if not accounted for, results in 206 Pb/ 238 U ages that are too old, for example in the samples investigated here by as much as 300%. For the two zoned monazite crystals studied in detail, 208 Pb/ 232 Th ages are reliable for the different compositional domains visible in back-scattered electron (BSE) images. 232 Th/ 208 Pb ages for the older domains (15.2 ± 0.3 Ma and 14.1 ± 0.3 Ma, respectively) are consistent with the structural relationships of the hydrothermal veins, indicating early retrograde genesis. The youngest rim age at 13.5 ± 0.4 Ma for the Blauberg crystal marks termination of monazite precipitation. The resolvable age difference is interpreted to reflect pulsed monazite growth over an extended period of hydrothermal activity. In both crystals, the outer rims have the highest 206 Pb excess , confirming a two-stage crystallization. Two scenarios are envisaged to account for the 206 Pb excess evolution. In the first, increasing 206 Pb excess is caused by a modification of the Th/U fractionation between monazite and fluid due to an evolution of the fluid towards more oxidizing conditions that favor partitioning of hexavalent U into the fluid. Alternatively, it is possible that 230 Th increased with time in the fluid. In this second case, monazite growth would have occurred in a closed system from a fluid that was initially in disequilibrium with the 238 U– 206 Pb decay chain and progressively equilibrated (t

A newly developed XRF-microprobe at the Institute of Mineralogy and Petrology, University of Bern, Switzerland has been applied for precise chemical Th–U–Pb dating of individual monazite grains separated from Pb-free polished petrographic thin sections. The nondestructive nature of the XRF-measurement permitted a comparative study of dating methods by sequentially applying chemical dating by electron microprobe analysis (EMPA), chemical dating by XRF-microprobe analysis, and isotopic 208Pb/232Th dating by Laser Ablation Plasma Ionisation Multi-collector Mass Spectrometry (LA-PIMMS) analysis. As an example, the 2σ precision achieved with the XRF-microprobe for well characterised reference material, monazite FC-1 (TIMS age 54.3±1 Ma; μ-XRF age 55.3±2.6 Ma), doubly polished to 30 μm in thickness, is below 5% after 90 min integration time (50 kV; 30 mA) at a spatial resolution of 90 μm. At 38-μm spatial resolution, the uncertainty is 35% for the same integration time. The sample characteristics are 200–300 ppm of Pb (μ-XRF), 3.8–5.1 wt.% of Th (EMPA), and 0.4–1.4 wt.% U (EMPA). Combined with an electron microprobe and conventional optical microscopy, the XRF-microprobe is thus a competitive low-cost and nondestructive alternative to more costly isotopic methods. The XRF-microprobe is easy to use and maintain.