Constraints on fluid evolution during metamorphism from U–Th–Pb systematics in Alpine hydrothermal monazite

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% 206Pb), (iii) excess 206Pb (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 230Th, a short-lived intermediate product in the 238U–206Pb decay chain, generating 206Pbexcess which, if not accounted for, results in 206Pb/238U 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, 208Pb/232Th ages are reliable for the different compositional domains visible in back-scattered electron (BSE) images. 232Th/208Pb 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 206Pbexcess, confirming a two-stage crystallization. Two scenarios are envisaged to account for the 206Pbexcess evolution. In the first, increasing 206Pbexcess 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 230Th 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 238U–206Pb decay chain and progressively equilibrated (t < 0.5 Ma).

► Th–Pb dating provides precise and reliable ages for late-Alpine hydrothermal monazite. ► Monazite dating indicates short episodic growth over 1–2 My during Alpine hydrothermal activity. ► Hydrothermal monazite shows extreme Th/U ratio up to 629. ► 206Pbexcess may reach up to 54% of 206Pb in hydrothermal monazite. ► 206Pbexcess offers information on the evolution of fluid conditions during monazite growth.