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% Pb-206), (iii) excess Pb-206 (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 Th-230, a short-lived intermediate product in the U-238-Pb-206 decay chain, generating Pb-206(excess) which, if not accounted for, results in Pb-206/U-238 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, Pb-208/Th-232 ages are reliable for the different compositional domains visible in back-scattered electron (BSE) images. Th-232/Pb-208 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 Pb-206(excess), confirming a two-stage crystallization. Two scenarios are envisaged to account for the Pb-206(excess) evolution. In the first, increasing Pb-206(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 Th-230 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 U-238-Pb-206 decay chain and progressively equilibrated ( t<0.5 Ma). (C) 2012 Elsevier B.V. All rights reserved.