Format and philosophy for collecting,compiling, and reporting microprobe monazite ages |
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| Institution: | 1. Institute of Mineralogy and Economic Geology, RWTH Aachen University, Wüllnerstrasse 2, 52062 Aachen, Germany;2. Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa;3. Department of Petrology and Economic Geology, Geological Survey of Denmark and Greenland, DK-1350 Copenhagen K, Denmark;1. Department of Geological Sciences, California State University Northridge, 18111 Nordhoff St, Northridge, CA 91330-8266, USA;2. Department of Earth, Environmental and Planetary Sciences, Brown University, Box 1846, 324 Brook St., Providence, RI 02912, USA;3. Department of Geology and Geophysics, University of Wyoming, 1000 E. University Ave., Dept. 3006, Laramie, WY 82071, USA;1. Geological Survey of Canada, 1 Challenger Dr., Dartmouth, Nova Scotia, B2Y 4A2, Canada;2. Geological Survey of Canada, 601 Booth St., Ottawa, Ontario, K1A 0E8, Canada;3. Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK;1. Institute of Applied Mineralogy and Economic Geology, RWTH Aachen University, Wüllnerstrasse 2, 52062 Aachen, Germany;2. Department of Petrology and Economic Geology, Geological Survey of Denmark and Greenland, DK-1350 Copenhagen K, Denmark;3. Institute of Applied Geosciences, Karlsruhe Institute of Technology, Adenauerring 20b, 76131 Karlsruhe, Germany;4. Department of Earth Sciences, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa;5. Institute of Geology and Mineralogy, Cologne University, Zülpicher Strasse 49b, 50674 Cologne, Germany;1. Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089-0740, USA;2. Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia;3. Institute of Geology and Paleontology, Faculty of Science, Charles University, Albertov 6, Prague, 12843, Czech Republic |
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| Abstract: | Microprobe monazite dating has been increasingly used to constrain the timing of deformation and metamorphism because of the potential to date very small monazite domains (down to 5 μm or less) in structural and petrologic context. This paper presents an analytical strategy, presentation format, and error considerations for microprobe monazite dating. The strategy involves high-resolution compositional mapping to delineate compositional domains within monazite crystals. Then for each compositional domain, a series of Th, U and Pb analyses are made, and a single date and error are calculated. The number of analyses in each domain is determined by the desired statistical precision of the date. Results from several monazite grains are typically combined and, along with textural relationships, are used to build an argument that the dates constrain the age of a deformation or metamorphic event. The total error involves three components: short-term random error (dominated by counting statistical uncertainty), short-term systematic error (uncertainty in background correction, conductive coating variation, and calibration), and long-term systematic error (uncertainty in standard composition, mass absorption factors, decay constants, etc.). In homogeneous compositional domains, short-term random errors (2σ) of less than 10 m.y. can be obtained from five to ten analyses. However, short-term systematic error, mainly background estimation uncertainty, would typically result in a doubling of the magnitude of random error. Microprobe dates are presented as a single Gaussian probability distribution for each domain, along with representative compositional maps. It is recommended that a consistency standard be analyzed during each analytical session and the results be reported along with those from the unknown. This proposed strategy and format are compatible with those of other geochronological techniques; they incorporate analytical limitations associated with trace, as opposed to major element, microprobe analysis, and will allow better comparisons to be made between labs and between different geochronological techniques. |
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