Constraints on fluid evolution during metamorphism from U–Th–Pb systematics in Alpine hydrothermal monazite
Highlights
► 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.
Introduction
With its three radioactive decay series, U–Th–Pb dating is a powerful tool to evaluate whether the isotope system has remained closed after mineral growth. This is commonly the case for phosphate minerals which suffer negligible resetting by diffusion up to high temperature (Cherniak et al., 2004). Hence, several phosphate chronometers including monazite, (LREE,Th)PO4, have been used to reliably date mineralization (England et al., 2001, Tallarico et al., 2005, Rasmussen et al., 2006, Lobato et al., 2007, Kempe et al., 2008, Sarma et al., 2008).
Alpine clefts are voids filled by crystals that precipitated from aqueous fluid during late stage Alpine metamorphism (Mullis et al., 1994, Mullis, 1996). Hot fluid (300–500 °C; Mullis et al., 1994, Mullis, 1996) interacts with the wall rock, leading to dissolution of minerals in the alteration halo around the clefts and mineral precipitation in the voids and altered rock mass. Dating such mineralization has proven difficult because suitable mineral-chronometer pairs are often absent and/or because later overprinting along with multiple stages of fluid activity have occurred (Purdy and Stalder, 1973). Alpine clefts have long been known to produce well-developed monazite crystals in some metasediments and metagranitoids (Niggli et al., 1940), but it is only recently that some of these have been dated (Gasquet et al., 2010). For the Lauzière massif (French Alps) crystals, Th–Pb ages form two groups at 5–7 Ma and 10–11 Ma. In some samples, the 207Pb/235U and 208Pb/232Th ages obtained deviate by as much as 55%, and the authors state that 207Pb/235U ages are imprecise (low 207Pb), while the 206Pb/238U ages are meaningless due to 206Pbexcess (Gasquet et al., 2010). One sample from the Pelvoux massif yielded a 208Pb/232Th age of 17.6 ± 0.3 Ma. Remarkably, this age is, within error, identical to a cleft xenotime U–Pb age of 18.0 ± 1.0 Ma obtained by Köppel and Grünenfelder (1975) on a sample from the Gotthard massif in the Central Swiss Alps (~ 250 km distant from the Pelvoux massif).
In this study, we investigate hydrothermal monazites from two Alpine clefts from the Central Alps (Griesserental and Blauberg, Aar Massif and Gotthard Massif, Switzerland). In situ SIMS U–Th–Pb data are combined with textural observations (back-scattered electron images and X-ray mapping) to obtain the initial age and growth duration of the two studied hydrothermal monazites.
Section snippets
Analytical techniques
U–Th–Pb analyses of monazite were performed using a Cameca IMS1280 SIMS instrument at the Swedish Museum of Natural History (Nordsims facility). Analytical methods closely follow those described by Harrison et al. (1995) and Kirkland et al. (2009), using a − 13 kV O2− primary beam of ca. 6 nA and nominal 15 μm diameter. The mass spectrometer was operated at + 10 kV and a mass resolution of ca. 4300 (M/ΔM, at 10% peak height) with data collected in peak hopping mode using an ion-counting electron
Sample description
The two studied monazite grains are from clefts located in the Griesserental, Aar Massif, Switzerland (46°45.56′N; 08°45.20′E) and the Blauberg area, Gotthard Massif, Switzerland (46°34.57′N; 08°35.34′E). In both cases, the clefts are oriented approximately horizontal, and perpendicular to the steeply-dipping rock foliation.
The yellow monazite from the Griesserental (Fig. 1a) occurs in clefts hosted in lower greenschist facies, strongly foliated gneisses of the Aar massif crystalline basement.
Monazite texture and composition
Back-scattered electron (BSE) images of the Griesserental crystal reveal a zoned domain (referred to as Griesserental-1), visible at its upper side in Fig. 2a. This zoned domain is surrounded by fairly homogeneous monazite comprising the main part of the grain (Griesserental-2). Monazite zoning is attributed to Th variations that are well distinguished on the Th map (Fig. 3): Griesserental-1 domain corresponds to the Th-rich irregular zone surrounded and penetrated by the Th-poor zone (Table 1,
Lead isotope disturbance in Alpine hydrothermal monazite
A considerable challenge to in-situ U–Th–Pb dating is to discriminate common from radiogenic Pb. By comparison with conventional dating by isotope dilution TIMS, measurement of the non-radiogenic isotope 204Pb by SIMS suffers from high uncertainties due to its low abundance and possible isobaric interferences (Williams, 1998, Fletcher et al., 2010). In addition, the composition of any common Pb may be difficult to determine and commonly must be assumed. Several studies have suggested that
Conclusions
In this study, a large set of high-resolution spot analyses was necessary to distinguish different growth stages and to address the growth duration of hydrothermal monazite. Robust Th–Pb ages were obtained for the different domains in the two crystals studied, indicating episodic fast growth of individual domains (below SIMS age resolution). Despite the occurrence of the clefts in rocks of different metamorphic grade, the similarity of the ages obtained may indicate a link between cleft
Acknowledgments
The Nordsims ion microprobe facility is operated by the research funding agencies of Denmark, Iceland, Norway, and Sweden, the Geological Survey of Finland and the Swedish Museum of Natural History. This is Nordsims contribution #320. Kerstin Lindén is thanked for careful preparation of samples. The authors thank the reviewers and editor for their useful comments, especially for the data treatment.
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