Elsevier

Chemical Geology

Volumes 326–327, 9 October 2012, Pages 61-71
Chemical Geology

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

https://doi.org/10.1016/j.chemgeo.2012.07.014Get rights and content

Abstract

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).

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.

References (47)

  • J. Mullis et al.

    Fluid regimes during late stages of a continental collision — physical, chemical, and stable-isotope measurements of fluid inclusions in fissure quartz from a geotraverse through the Central Alps, Switzerland

    Geochimica et Cosmochimica Acta

    (1994)
  • F. Oberli et al.

    U–Th–Pb and 230Th/238U disequilibrium isotope systematics: precise accessory mineral chronology and melt evolution tracing in the Alpine Bergell intrusion

    Geochimica et Cosmochimica Acta

    (2004)
  • T. Pettke et al.

    Recent developments in element concentration and isotope ratio analysis of individual fluid inclusions by laser ablation single and multiple collector ICP-MS

    Ore Geology Reviews

    (2012)
  • F. Poitrasson et al.

    Electron microprobe and LA-ICP-MS study of monazite hydrothermal alteration: implications for U–Th–Pb geochronology and nuclear ceramics

    Geochimica et Cosmochimica Acta

    (2000)
  • J.M. Pyle

    Temperature–time paths from phosphate accessory phase paragenesis in the Honey Brook Upland and associated cover sequence, SE Pennsylvania, USA

    Lithos

    (2006)
  • R.L. Rudnick et al.

    The composition of the continental crust. In: Rudnick, R.L. (Ed.), The Crust, vol. no. 3

  • U. Schärer

    The effect of initial 230Th disequilibrium on young U–Pb ages: the Makalu case, Himalaya

    Earth and Planetary Science Letters

    (1984)
  • J.S. Stacey et al.

    Approximation of terrestrial lead isotope evolution by a two-stage model

    Earth and Planetary Science Letters

    (1975)
  • R.H. Steiger et al.

    Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology

    Earth and Planetary Science Letters

    (1977)
  • K. Suzuki et al.

    Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites

    Earth and Planetary Science Letters

    (1994)
  • F. Corfu

    Differential response of U–Pb systems in coexisting accessory minerals, Winnipeg River Subprovince, Canadian Shield: implications for Archean crustal growth and stabilization

    Contributions to Mineralogy and Petrology

    (1988)
  • F. Demartin et al.

    Alpine monazite — further data

    The Canadian Mineralogist

    (1991)
  • G.L. England et al.

    SHRIMP U–Pb ages of diagenetic and hydrothermal xenotime from the Archaean Witwatersrand Supergroup of South Africa

    Terra Nova

    (2001)
  • Cited by (0)

    View full text