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

Janots, E., Brunet, F., Goffe, B., Poinssot, C., Burchard, M. and Cemic, L. (2007) Thermochemistry of monazite-(La) and dissakisite-(La): Implications for monazite and allanite stability in metapelites Contributions to Mineralogy and Petrology, 154 (1). pp. 1-14. DOI 10.1007/s00410-006-0176-2.

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Thermochemical properties have been either measured or estimated for synthetic monazite, LaPO4, and dissakisite, CaLaMgAl2(SiO4)(3)OH, the Mg-equivalent of allanite. A dissakisite formation enthalpy of -6,976.5 +/- 10.0 kJ mol(-1) was derived from high-temperature drop-solution measurements in lead borate at 975 K. A third-law entropy value of 104.9 +/- 1.6 J mol(-1) K-1 was retrieved from low-temperature heat capacity (C-p) measured on synthetic LaPO4 with an adiabatic calorimeter in the 30-300 K range. The C-p values of lanthanum phases were measured in the 143-723 K range by differential scanning calorimetry. In this study, La(OH)(3) appeared as suitable for drop solution in lead borate and represents an attractive alternative to La2O3. Pseudo-sections were calculated with the THERIAK-DOMINO software using the thermochemical data retrieved here for a simplified metapelitic composition (La = Sigma REE + Y) and considering monazite and Fe-free epidotes along the dissakisite-clinozoisite join, as the only REE-bearing minerals. Calculation shows a stability window for dissakisite-clinozoisite epidotes (T between 250 and 550 degrees C and P between 1 and 16 kbar), included in a wide monazite field. The P-T extension of this stability window depends on the bulk-rock Ca-content. Assuming that synthetic LaPO4 and dissakisite-(La) are good analogues of natural monazite and allanite, these results are consistent with the REE-mineralogy sequence observed in metapelites, where (1) monazite is found to be stable below 250 degrees C, (2) around 250-450 degrees C, depending on the pressure, allanite forms at the expense of monazite and (3) towards amphibolite conditions, monazite reappears at the expense of allanite.

Document Type: Article
Keywords: monazite allanite calorimetry thermodynamic properties phase diagrams U-Th-Pb geochronology lanthanum differential scanning calorimetry molal thermodynamic properties rare-earth-elements u-th-pb heat-capacity system na2o-k2o-cao-mgo-feo-fe2o3-al2o3-sio2-tio2-h2o-co2 phosphate minerals grade metamorphism phase-equilibria eastern alps
Research affiliation: Kiel University
DOI etc.: 10.1007/s00410-006-0176-2
ISSN: 0010-7999
Date Deposited: 12 Jan 2012 05:44
Last Modified: 08 Oct 2012 10:20

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