Format and philosophy for collecting, compiling, and reporting microprobe monazite ages

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.

[1]  M. Jercinovic,et al.  Age mapping and dating of monazite on the electron microprobe: Deconvoluting multistage tectonic histories , 1999 .

[2]  M. Hamilton,et al.  Electron probe (Ultrachron) microchronometry of metamorphic monazite: Unraveling the timing of polyphase thermotectonism in the easternmost Wyoming Craton (Black Hills, South Dakota) , 2005 .

[3]  Kazuhiro Suzuki,et al.  Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the South Kitakami terrane, Northeast Japan, revealed by the chemical Th-U-total Pb isochron ages of monazite, zircon and xenotime , 1991 .

[4]  F. Spear,et al.  Apatite, Monazite, and Xenotime in Metamorphic Rocks , 2002 .

[5]  G. Lawes,et al.  Scanning Electron Microscopy and X-Ray Microanalysis , 1987 .

[6]  P. Goncalves,et al.  Electron-microprobe age mapping of monazite , 2004 .

[7]  R. Parrish.  U–Pb dating of monazite and its application to geological problems , 1990 .

[8]  M. Jercinovic,et al.  Microprobe monazite geochronology: putting absolute time into microstructural analysis , 2002 .

[9]  P. Goncalves,et al.  Petrology and in situ U–Th–Pb Monazite Geochronology of Ultrahigh-Temperature Metamorphism from the Andriamena Mafic Unit, North–Central Madagascar. Significance of a Petrographical P–T Path in a Polymetamorphic Context , 2004 .

[10]  M. Jercinovic,et al.  Monazite geochronology of UHP and HP metamorphism, deformation, and exhumation, Nordøyane, Western Gneiss Region, Norway , 2000 .

[11]  A. Provost,et al.  Electron microprobe dating of monazite , 1996 .

[12]  K. Ludwig.  Mathematical–Statistical Treatment of Data and Errors for 230Th/U Geochronology , 2003 .

[13]  J. Hibbard,et al.  Smith River allochthon: A southern Appalachian peri-Gondwanan terrane emplaced directly on Laurentia? , 2003 .

[14]  Michael J. Jercinovic,et al.  Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects , 2005 .

[15]  F. Spear,et al.  Yttrium zoning in garnet: Coupling of major and accessory phases during metamorphic reactions , 2003 .

[16]  E. Watson,et al.  Pb diffusion in monazite: a combined RBS/SIMS study , 2004 .

[17]  L. Hartmann,et al.  Electron microprobe dating of monazite substantiates ages of major geological events in the southern Brazilian shield , 2004 .

[18]  M. Horstwoodc,et al.  The generation of prograde P – T – t points and paths ; a textural , compositional , and chronological study of metamorphic monazite , 2004 .

[19]  K. Karlstrom,et al.  Electron-microprobe monazite dating of ca. 1.71-1.63 Ga and ca. 1.45-1.38 Ga deformation in the Homestake shear zone, Colorado: Origin and early evolution of a persistent intracontinental tectonic zone , 2001 .

[20]  I. Wendt,et al.  The statistical distribution of the mean squared weighted deviation , 1992 .

[21]  Suzuki Kazuhiro,et al.  MONAZITE AGES BY THE CHEMICAL Th-U-TOTAL Pb ISOCHRON METHOD FOR PELITIC GNEISSES FROM THE EASTERN SeR RONDANE MOUNTAINS, EAST ANTARCTICA , 1996 .

[22]  F. Spear,et al.  Four generations of accessory-phase growth in low-pressure migmatites from SW New Hampshire , 2003 .

[23]  M. Searle,et al.  Two episodes of monazite crystallization during metamorphism and crustal melting in the Everest region of the Nepalese Himalaya , 2000 .

[24]  G. Wasserburg,et al.  U-Th-Pb systematics in three Apollo 14 basalts and the problem of initial Pb in lunar rocks , 1972 .

[25]  G. Foster,et al.  The significance of monazite U–Th–Pb age data in metamorphic assemblages; a combined study of monazite and garnet chronometry , 2000 .

[26]  R. Wirth,et al.  Transmission electron microscope study of polyphase and discordant monazites: Site-specific specimen preparation using the focused ion beam technique , 2003 .

[27]  Mark D. Semon,et al.  POSTUSE REVIEW: An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements , 1982 .

[28]  A. Berger,et al.  Nondestructive chemical dating of young monazite using XRF 2. Context sensitive microanalysis and comparison with Th-Pb laser-ablation mass spectrometric data , 2002 .

[29]  A. Cheburkin,et al.  Nondestructive chemical dating of young monazite using XRF: 1. Design of a mini-probe, age data for samples from the Central Alps, and comparison to U–Pb (TIMS) data , 2002 .

[30]  A. Savitzky,et al.  Smoothing and Differentiation of Data by Simplified Least Squares Procedures. , 1964 .

[31]  Kazuhiro Suzuki,et al.  Chemical Th–U–total Pb dating by electron microprobe analysis of monazite, xenotime and zircon from the Archean Napier Complex, East Antarctica: evidence for ultra-high-temperature metamorphism at 2400 Ma , 2002 .

[32]  S. J. B. Reed,et al.  Electron Microprobe Analysis , 1975 .

[33]  W. Heinrich,et al.  Experimental resetting of the U–Th–Pb systems in monazite , 2002 .

[34]  Christiane Wagner,et al.  Trace element analysis with the electron microprobe: New data and perspectives , 1999 .

[35]  G. Jeong,et al.  Early Permian peak metamorphism recorded in U–Pb system of black slates from the Ogcheon metamorphic belt, South Korea, and its tectonic implication , 2003 .

[36]  Richard L. Brown,et al.  Correlations between chemical and age domains in monazite, and metamorphic reactions involving major pelitic phases: an integration of ID-TIMS and SHRIMP geochronology with Y?Th?U X-ray mapping , 2004 .

[37]  D. Rhede,et al.  A three-dimensional method for calculating independent chemical U/Pb- and Th/Pb-ages of accessory minerals , 1996 .

[38]  Kazuhiro Suzuki,et al.  Denudation history of the high T/P Ryoke metamorphic belt, southwest Japan: constraints from CHIME monazite ages of gneisses and granitoids , 1998 .

[39]  Frank S. Spear,et al.  Contributions to precision and accuracy of monazite microprobe ages , 2005 .

[40]  E. Ghent,et al.  An electron microprobe study of the U–Th–Pb systematics of metamorphosed monazite: the role of Pb diffusion versus overgrowth and recrystallization , 1999 .

[41]  F. Spear,et al.  Contributions to Precision and Accuracy of Chemical Ages of Monazite , 2022 .

[42]  E. R. Cohen.  An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements , 1998 .

[43]  F. Albarède,et al.  An improved U-Th-Pb age calculation for electron microprobe dating of monazite , 2001 .

[44]  O. Legendre,et al.  Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total lead determination: implications for lead behaviour in monazite , 1998 .

[45]  Montel.,et al.  Preservation of old U–Th–Pb ages in shielded monazite: example from the Beni Bousera Hercynian kinzigites (Morocco) , 2000 .

[46]  M. Hamilton,et al.  Comparative isotopic and chemical geochronometry of monazite, with implications for U-Th-Pb dating by electron microprobe: An example from metamorphic rocks of the eastern Wyoming Craton (U.S.A.) , 2005 .