Physico-chemical control on the REE minerals in chloritoid-grade metasediments from a single outcrop (Central Alps, Switzerland)
Research Highlights
►REE minerals are modified in samples containing veins. ►Fluid enhances REE minerals crystallization during deformation. ►Phosphates can incorporate significant As contents in low-grade metasediments. ►U–Th–Pb ages can record fluid interactions, deformation and/or oxidizing stages.
Introduction
A number of REE minerals (i.e., phases containing rare earth elements) has been reliably used to date metamorphic stages : monazite (e.g., Montel et al., 1996, Williams et al., 1996, Williams et al., 2007), allanite (e.g., Catlos et al., 2000, Gregory et al., 2007, Janots et al., 2009) and xenotime (Hetherington et al., 2008, Rasmussen et al., 2004). Compositional and isotopic zoning is the rule than the exception in these minerals (e.g., Krenn et al., 2009, Pyle & Spear, 2003, Rubatto et al., 2006, Rubatto et al., 2001), and dating is best approached by in situ techniques. Age zoning indicates distinct growth pulses and/or incomplete resetting of the isotopic signature of a precursor phase. In REE minerals, U–Th–Pb closure temperatures are high and thermal resetting by diffusion is insignificant up to at least 800 °C for monazite (Cherniak, 2006, Gardes et al., 2006). This implies that REE mineral ages are more likely to record a (re)crystallization stage than diffusional resetting. In diagenetic and metamorphic rocks, (re)crystallization is enhanced by the presence of fluid. In several geochronological studies, the ages were correlated with a fluid-related event rather than a thermal stage (Bollinger & Janots, 2006, Harlov et al., 2007, Harlov et al., 2008, Janots et al., 2006, Rasmussen et al., 2006, Townsend et al., 2001, Yi & Cho, 2009). Identifying fluid-induced crystallization has economical importance because it can fix time constraints to test ore deposit models (Kempe et al., 2008, Rasmussen et al., 2006) or to constrain the oil window (Evans et al., 2002). It is also crucial to recognize crystallization via fluid, because dissolution/precipitation processes may favour the preferential incorporation or removal of the U, Th and Pb and lead to meaningless total U–Th–Pb ages (Harlov & Hetherington, 2010, Poitrasson et al., 1996, Seydoux-Guillaume et al., 2003;). Actinide mobility during accessory mineral-fluid interaction is also extremely valuable to understand the fate of these elements in the context of nuclear waste storage in geological formations (Ewing, 2001, Oelkers & Poitrasson, 2002, Read et al., 2002).
While numerous petrological studies have addressed the solid reactions between REE minerals (reviews in Gieré & Sorensen, 2004, Spear & Pyle, 2002), much less is known about the conditions, under which REE minerals (re)crystallized via a fluid. Monazite dissolution appears to be strongly dependant on the fluid composition. This effect has experimentally been found to be particularly strong for alkaline or Na2Si2O5-bearing fluids (for T > 600 °C; Harlov & Hetherington, 2010, Hetherington et al., 2010, Seydoux-Guillaume et al., 2002a). Similar behaviour was also observed for xenotime (Hetherington et al., 2010). Various alteration features are distinguished for monazite: cation substitutions, selective element removal, replacement by a secondary phase, dissolution–reprecipitation (Poitrasson et al., 1996, Poitrasson et al., 2000). In the presence of low-grade metamorphic fluids, monazite can alter to a hydrated REE phosphate, such as rhabdophane (Berger et al., 2008), to apatite or allanite (Poitrasson et al., 2000). Several studies report also the replacement of an inherited monazite by a secondary monazite with a different composition in terms of Th, U and REE patterns, (Hetherington & Harlov, 2008, Janots et al., 2008, Rasmussen & Muhling, 2007). The behaviour of allanite in the presence of fluids remains poorly known. Poitrasson (2002) found that REE are preferentially removed during allanite alteration, whereas common lead is high, impeding efforts to date such processes.
The present study aims to characterize the role of physico-chemical conditions on REE mineral crystallization, with a special emphasis on the fluid control, in the Central Alps. This orogen is ideal for such a study, because its tectono-metamorphic history and fluid regime are relatively well known (e.g., Engi et al., 2004, Frey & Ferreiro Mählmann, 1999, Mullis, 1996). Furthermore, REE mineralogy has been previously investigated in metapelites along the metamorphic profile of the northern Central Alps (Janots et al., 2008). In the present study, we extended the sample collection of Janots et al. (2008) to a largest variety of lithologies, some including veins, at a single outcrop of Mesozoic (mono-metamorphic) metasediments that reached greenschist facies conditions.
Section snippets
Geological background
Samples were taken at a single locality, Garvera, in the external part of the Central Alps (Switzerland). This part of the Alpine orogen results from a Tertiary subduction followed by the collision, leading to the well established Barrovian overprint. In the Central Alps, a progressive metamorphic gradient is found along a north–south geotraverse, from diagenetic conditions (around Lake Lucerne) up to partial melting (around Bellinzona). The Garvera outcrop is located in the Urseren Zone, which
Methods
The REE minerals were identified in polished sections using optical microscopy and backscattered images from the electron microprobe (EMP, Jeol JXA8200, University of Bern). Quantitative EMP analyses were obtained using the analytical scheme described by Scherrer et al. (2000). Kα X-ray lines were used for Si, Fe, Ca, P and Al, Lα lines for Y, Ce, La and Er, Lβ lines for Pr, Dy, Sm, Ho, Gd, and Mα lines for Th and Mβ line for U and Pb. For Nd the most intense Lα line was preferred to the Lβ
Samples
The investigated samples (13 in total) are pelitic to marly metasediments; they include samples from the collection of the University of Basel (samples from M. Frey). Pelites consist of a matrix of μm-scale quartz, chlorite, white mica and rutile with abundant chloritoid porphyroblasts and sporadic apatite. Apatite commonly has a core rich in quartz and rutile inclusions, with an inclusion-free rim. It shares straight grain boundaries with chlorite and chloritoid. Metamarls are typically
REE mineralogy in the metasediments devoid of veins
Ten of the eleven metasediment samples contain allanite as dominant LREE-carriers (Table 1). Allanite grains are tiny, but their size tends to increase with the carbonate content in the sample. The typical grain sizes are ~ 10 μm in metapelites to ~ 40 μm in the metamarls. Allanite is often elongated along the main foliation (Fig. 3a). Commonly, basal sections are surrounded by symmetrical pressure shadows of quartz (Fig. 3b). Allanite can be found as inclusions in chloritoid porphyroblasts or
Discussion
Detailed analysis of 13 samples from the same outcrop allows five populations of LREE minerals on the basis of chemical ages, compositional data, and textural arguments, to be distinguished (Mnz1, Mnz2, Mnz3, Aln1 and Aln2). As the investigated outcrop shows no evidence of any major tectonic discontinuity, the sample suite is thought to have experienced the same metamorphic evolution in terms of P–T. Discrepancies in the observed REE minerals thus are attributed to variations in the original
Conclusions and perspectives
In chloritoid-grade metasediments of the Central Alps, five populations of LREE minerals have been distinguished. Based on geochronological, mineral chemical, and textural arguments, the relative temporal sequence deduced for LREE minerals is associated with the deformation and fluid/rock interactions (Fig. 10). From the oldest to the youngest one, the phases appear as follows: Mnz1, Mnz2, Aln1, Aln2, and Mnz3.
We cannot preclude the existence of some other LREE phases in the sedimentary
Acknowledgements
This research project was financially supported by the Swiss National Fond (Nos. 20020-101826/1 and 200021-103479) and the WWU Münster. The paper has been greatly improved by the critical and constructive comments of D. Harlov and C. Hetherington, supervised by the editor I. Buick. We are grateful to E. Gnos for his assistance with the electron microprobe; S. Brechbühl and V. Jacob for their excellent polished sections. This paper benefited from numerous fruitful discussions with C. Putnis, A.
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