HR: 1340h
AN: V23B-1444    [Abstracts]
TI: NanoSIMS ²⁰⁷Pb-²⁰⁶Pb dating of monazite, xenotime and baddeleyite
AU: * Verdel, C
EM: cverdel@gps.caltech.edu
AF: California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, United States
AU: Mahan, K
EM: Kevin.Mahan@colorado.edu
AF: University of Colorado, Boulder, 2200 Colorado Ave, Boulder, CO 80309, United States
AU: Guan, Y
EM: yunbin@gps.caltech.edu
AF: California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, United States
AU: Eiler, J
EM: eiler@gps.caltech.edu
AF: California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, United States
AU: Wernicke, B
EM: brian@gps.caltech.edu
AF: California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, United States
AB: A variety of geochronological problems call for in-situ dating with high spatial resolution (i.e., scales of tens of microns or less). Such techniques are particularly important for studies that focus on the relationships between intracrystalline age variations and petrologic or diagenetic processes. Precise, in-situ geochronology of particularly small grains or domains (ca. 1-5 microns) may be feasible with the Cameca NanoSIMS ion microprobe, which has a spatial resolution as good as tens of nanometers and sufficient mass resolution at high transmission to be useful for geochronologic systems involving isotopes of trace elements. We have used the Cameca NanoSIMS 50L housed in the Caltech Microanalysis Center to determine ²⁰⁷Pb/²⁰⁶Pb ages of monazite, xenotime and baddeleyite standards ranging in age from ~500 to 2000 Ma. Initial efforts focused on Pb-Pb geochronology because it is analytically straightforward: Pb is ionized almost entirely to Pb⁺ and instrumental mass fractionation of Pb isotopes is negligible. We compare our results to previous age determinations made on these standards using TIMS, SHRIMP and electron microprobe techniques. Our NanoSIMS ²⁰⁷Pb/²⁰⁶Pb age of 2047±35 Ma (2σ) for a baddeleyite crystal from the Phalaborwa carbonatite overlaps with the previously determined TIMS and SHRIMP dates of ca. 2060 Ma. We measured an age of 1018±20 Ma on xenotime standard x6413, slightly older than the TIMS ²⁰⁷Pb/²⁰⁶Pb date of 996.7±0.8 Ma. For the high-Th Moacyr monazite standard, variations of common-Pb corrected ages and ²⁰⁶Pb/²⁰⁴Pb ratios for 5 measurements spanning ~100 microns suggest varying amounts of interference from a mass-204 isobar (probably doubly ionized ThNdO₂). The single measurement least-affected by this isobar corresponds to a common-Pb corrected ²⁰⁷Pb/²⁰⁶Pb age of 489±92 Ma, within error of the electron microprobe chemical age of ca. 505 Ma. Spatial variation of the mass-204 isobar intensity suggests that Th may be distributed inhomogeneously within the crystal. These results demonstrate that NanoSIMS Pb isotope data can be used to determine accurate and usefully precise ²⁰⁷Pb/²⁰⁶Pb ages, particularly for Proterozoic and older minerals, and that intracrystalline isotopic variations can be detected at scales of several microns. Future incorporation of U and Th measurements is expected to increase the precision of age determinations, particularly for high-Th and younger minerals.
DE: 1100 GEOCHRONOLOGY
DE: 1115 Radioisotope geochronology
DE: 1194 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting