ASGP (2015), vol. 85: 405–425
STABILITY OF MONAZITE AND DISTURBANCE OF THE Th-U-Pb SYSTEM UNDER EXPERIMENTAL CONDITIONS OF 250–350 °C AND 200–400 MPa
Bartosz BUDZYŃ (1, 2), Patrik KONEČNÝ (3) & Gabriela A. KOZUB-BUDZYŃ (4)
1) Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Kraków ING PAN, Senacka 1, PL-31002 Kraków, Poland; e-mail: ndbudzyn at cyf-kr.edu.pl
2) Institute of Geological Sciences, Jagiellonian University, Oleandry 2a, PL-30063 Kraków, Poland
3) Dionýz Štúr State Geological Institute, Mlynská dolina 1, SK-81704 Bratislava, Slovak Republic;
e-mail: patrik.konecny at geology.sk
4) AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Mickiewicza 30, PL-30059 Krakow, Poland; e-mail: lato at agh.edu.pl
Budzyń, B., Konečný, P. & Kozub-Budzyń, G. A., 2015. Stability of monazite and disturbance of the Th-U-Pb system under experimental conditions of 250–350 °C and 200–400 MPa. Annales Societatis Geologorum Poloniae, 85: 405–425.
Abstract: This experimental study provides important data filling the gap in our knowledge on monazite stability under conditions of fluid-mediated low-temperature metamorphic alteration and post-magmatic hydrothermal alterations. The stability of monazite and maintenance of original Th-U-total Pb ages were tested experimentally under P-T conditions of 250–350 °C and 200–400 MPa over 20–40 days. The starting materials included the Burnet monazite + K-feldspar ± albite ± labradorite + muscovite + biotite + SiO2 + CaF2 and 2M Ca(OH)2 or Na2Si2O5 + H2O fluid. In the runs with 2M Ca(OH)2, monazite was unaltered. REE-enriched apatite formed at 350 °C and 400 MPa. The presence of the Na2Si2O5 + H2O fluid promoted the strong alteration of monazite, the formation of secondary REE-enriched apatite to fluorcalciobritholite, and the formation of REE-rich steacyite. Monazite alteration included the newly developed porosity, patchy zoning, and partial replacement by REE-rich steacyite. The unaltered domains of monazite maintained the composition of the Burnet monazite and its age of (or close to) ca. 1072 Ma, while the altered domains showed random dates in the intervals of 375–771 Ma (250 °C, 200 MPa run), 82–253 Ma (350 °C, 200 MPa), and 95–635 Ma (350 °C, 400 MPa). The compositional alteration and disturbance of the Th-U-Pb system resulted from fluid-mediated coupled dissolution-reprecipitation. In nature, such age disturbance in monazite can be attributed to post-magmatic alteration in granitic rocks or to metasomatic alteration during metamorphism. Recognition of potentially altered domains (dark patches in high-contrast BSE-imaging, developed porosity or inclusions of secondary minerals) is crucial to the application of Th-U-Pb geochronology.
Manuscript received 12 November 2014, accepted 25 May 2015
