Elsevier

Chemical Geology

Volume 279, Issues 1–2, 3 December 2010, Pages 55-62
Chemical Geology

Research paper
Monazite–allanite phase relations in metapelites

https://doi.org/10.1016/j.chemgeo.2010.10.004Get rights and content

Abstract

Calculations of the transition from allanite to monazite-bearing assemblages in typical pelitic bulk compositions have been made using thermodynamic data estimated from oxide sums and inferred from natural parageneses. Calculations in the CFASHPCe and MnNCKFMASHPCe systems place the allanite to monazite transition in the middle amphibolite facies (525–600 °C) for a bulk composition similar to Shaw's average pelite. The temperature of the transition is pressure dependent, and strongly dependent on the bulk rock CaO content, consistent with inferences from natural parageneses. The transition is also a function of the bulk Al2O3 content, although the calculated result is opposite to that inferred from natural samples. Comparison with published results in the La–Mg system suggest that the nature of the REE phosphate does not greatly influence the conditions of the transition.

Research Highlights

► The monazite to allanite phase transition has been calculated from newly derived thermodynamic data. ► The monazite to allanite phase transition is a continuous reaction with a positive dP/dT. ► The transition occurs in the amphibolite facies for typical pelitic bulk compositions. ► The transition is a strong function of bulk CaO content and a lesser function of Al2O3 content.

Introduction

The quantitative relationship among pressure, temperature and the relative stabilities of allanite and monazite is crucial for the interpretation of the ages of these minerals. A number of studies have examined progressive metamorphic sequences, with different conclusions depending on the suites examined. In general, it has been concluded that in rocks of little or no calcium (and sufficient LREE and P), monazite is formed by at least the low greenschist facies and persists throughout metamorphism (e.g., Overstreet, 1967). In rocks of sufficient CaO and LREE, allanite forms at low grades and reacts out at conditions of around the staurolite and/or kyanite isograd (e.g., Smith and Barreiro, 1990, Kingsbury et al., 1993, Wing et al., 2003, Kohn and Malloy, 2004, Fitzsimons et al., 2005, Rasmussen et al., 2006, Janots et al., 2006, Janots et al., 2008, Tomkins and Pattison, 2007, Corrie and Kohn, 2008; see discussion in Spear and Pyle, 2002). Indeed, several studies (e.g., Smith and Barreiro, 1990, Wing et al., 2003, Janots et al., 2006, Janots et al., 2008; and the experimental study of Janots et al., 2007) have even reported the formation of low-grade monazite (low greenschist facies) that gives rise to allanite and is eventually replaced by a new generation of monazite around the conditions of the staurolite or Al2SiO5 isograd. However, the relationships between these minerals are rather complex and the reactions by which the transition occurs, and quantification of the relationship between allanite stability and bulk rock composition (notably CaO content), have not been resolved in detail.

This paper explores the relationship between allanite (CaCeFeAl2(SiO4)3OH) and monazite (CePO4) during progressive metamorphism by examining both the chemographic relations and through thermodynamic calculations. A similar set of calculations was undertaken by Janots et al. (2007) based on new thermodynamic data in the system SiO2–Al2O3–FeO–Fe2O3–MgO–CaO–Na2O–K2O–P2O5–La2O3–CO2–H2O and equilibrium assemblage diagrams were presented for a system containing LaPO4 (monazite) and CaLaMgAl2(SiO4)3OH (dissakisite). The results from this and the present study substantiate the inferences of earlier workers and make it possible, within the errors of the thermodynamic data, to calculate the monazite isograd in rocks of any pelitic bulk composition.

Section snippets

Chemographic relations

The simplest chemical system in which to represent the chemographic relations between monazite and allanite is SiO2–Al2O3–FeO–CaO–H2O–P2O5–Ce2O3 (CFASHPCe). Schists in such a system will contain quartz and fluid (H2O) in excess. Additionally, phases such as chlorite, garnet, staurolite, chloritoid, pyrophyllite, kyanite, sillimanite, andalusite are typical. To simplify representation, this paper will first consider rocks in which an Al2SiO5 polymorph is in excess. Projection from quartz, H2O

Thermodynamic model

Quantification of the allanite–monazite phase relations requires thermodynamic data, which are not available for all phases of interest. In this study, data for the silicates are taken from the compilations of Berman (1988), with modifications described by Pattison et al., 1999, Spear and Kohn, 1996, Spear and Wark, 2009, Spear and Pyle, 2010. Values of entropy, volume and heat capacity for monazite and apatite have been taken from the compilations of Robie et al., 1978, Robie and Hemingway,

Equilibrium assemblage diagrams

An equilibrium assemblage diagram (EAD, also referred to as a “pseudosection”) for the system CFASHPCe is presented in Fig. 5 that delineates the P–T fields for assemblages that contain allanite, monazite, or both. The bulk composition has elemental proportions that are similar to that of Shaw's average pelite (Shaw, 1956; also used by Symmes and Ferry, 1992) and is listed in Table 3. The assemblage garnet + allanite + apatite is stable up to around 700 °C at a pressure around 5 kbar. Monazite then

Discussion

The calculations presented in this study are consistent with most inferences about the allanite-to-monazite transition in metapelitic bulk compositions. Smith and Barreiro (1990) and later Wing et al., 2003, Kohn and Malloy, 2004, Corrie and Kohn, 2008, all report the major appearance of monazite at P–T conditions near the staurolite and/or Al2SiO5 isograd. Kohn and Malloy (2004) did not believe that allanite was the precursor but further study on the same rocks reported by Corrie and Kohn

Conclusions

The results presented here should not be applied uncritically. Rather, they should serve as a template for further calculations on specific bulk compositions of interest. Further refinement of the thermodynamic data of accessory phases, especially allanite, monazite, and apatite are critical requirements for future improvements. In addition, it is essential that a more thorough assessment of the relative concentrations of the various trace elements in all silicates be made. Even though the

Acknowledgments

This work was supported by grants from the National Science Foundation EAR-106738, EAR-337413, and EAR-409622 and the Edward P. Hamilton Distinguished Chair for Science Education at Rensselaer Polytechnic Institute. Constructive reviews by D. Harlov and an anonymous reviewer served to clarify the text and highlight additional studies of importance.

References (29)

  • E. Janots et al.

    Thermochemistry of monazite-(La) and dissakisite-(La): implications for monazite and allanite stability in metapelites

    Contributions to Mineralogy and Petrology

    (2007)
  • E. Janots et al.

    Prograde metamorphic sequence of REE minerals in pelitic rocks of the Central Alps: implications for allanite–monazite–xenotime phase relations from 250 to 610 °C

    Journal of Metamorphic Geology

    (2008)
  • D.E. Kelsey et al.

    Thermobarometric modeling of zircon and monazite growth in melt-bearing systems: examples using model metapelitic and metapsammitic granulites

    Journal of Metamorphic Geology

    (2008)
  • J.A. Kingsbury et al.

    Monazite paragenesis and U–Pb systematics in rocks of the eastern Mojave Desert, California, U.S.A.: implications for thermochronometry

    Chemical Geology

    (1993)
  • Cited by (173)

    View all citing articles on Scopus
    View full text