HR: 1340h
AN: V13A-0517    [Abstracts]
TI: Direct Comparison of Monazite Ages Obtained By in situ Techniques: Ion-Probe Isotopic Ages Versus Electron Microprobe Chemical Ages
AU: * Tracy, R J
EM: rtracy@vt.edu
AF: Virginia Tech Geosciences, 4044 Derring Hall Virginia Tech, Blacksburg, VA 24061-0420 United States
AU: Loehn, C W
EM: cloehn@vt.edu
AF: Virginia Tech Geosciences, 4044 Derring Hall Virginia Tech, Blacksburg, VA 24061-0420 United States
AU: Dahl, P S
EM: pdahl@kent.edu
AF: Kent State University Geology, 212 McGilvrey Hall Kent State U., Kent, OH 44242 United States
AU: Aleinikoff, J N
EM: jaleinikoff@usgs.gov
AF: U.S. Geological Survey, USGS Denver, Denver, CO 80225 United States
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: SHRIMP Lab Stanford/USGS, Green Earth SCiences Building Stanford Univ., Stanford, CA 94305-2115 United States
AU: Hamilton, M
EM: mahamilton@geology.utoronto.ca
AF: Geochronology Lab, University of Toronto, Dept. of Geology Univ. of Toronto, Toronto, ON M5S 3B1 Canada
AU: Mazdab, F
EM: fmazdab@usgs.gov
AF: SHRIMP Lab Stanford/USGS, Green Earth SCiences Building Stanford Univ., Stanford, CA 94305-2115 United States
AU: Jones, C
EM: cjones1@kent.edu
AF: Kent State University Geology, 212 McGilvrey Hall Kent State U., Kent, OH 44242 United States
AB: In situ analytical techniques for geochronology are rapidly becoming the method of choice for characterizing compositionally and chronologically complex minerals, including monazite (mnz) and zircon (zrc). Two such techniques include Ion Microprobe (IMP) (esp. SHRIMP) for both mnz and zrc, and electron microprobe (EMP) for mnz. Debate remains concerning comparability of ages obtained by the two different techniques: U-Pb isotopic dating (IMP) versus Th-total U-total Pb (EMP). The IMP has an advantage in analytical precision whereas the EMP has an advantage in spatial resolution. We report 6 examples of individual monazite grains that have been dated by both techniques, covering a range of ages from 300 to 2850 Ma. Three of our examples are grains that have been used as IMP standards at the Geological Survey of Canada (GSC) or at the USGS-Stanford SHRIMP lab; the other grains are from research samples. Most grains display complex zoning in Y, Th and U, both in BSE images and in compositional maps, reflecting complex growth and recrystallization histories. In all cases, the ages obtained by the two techniques agree within their 2-sigma associated error, except where IMP ablation pits cross age boundaries and resolve a mixed age of the two domains, or where EMP spots fall near cracks or pits in the grain surface. One prominent example is a 100-micron mnz from the Tobacco Root Mountains, Montana. It contains a low-Th older core (ca. 2.85 Ga), a higher-Th mantle domain of about 2.45 Ga, and a low-Th rim of 1.78 Ga. This grain has 6 IMP spots that range in age from 1880 Ma (near-rim) to 2785 Ma (core). Only two IMP pits fall totally within a single chemical and age zone delineated by EMP analyses or compositional maps (the medial age zone): 2451 (+/-4) and 2432 (+/-10). The weighted mean EMP age of this domain is 2452 (+/-6). IMP spots aimed at the older core are 2619 (+/-11) and 2785 (+/-9); the weighted mean core age from EMP analyses is 2859 (+/-14). This suggests that the improved spatial resolution of the EMP has better sampled the core domain. Other direct comparison of IMP and EMP ages include: (1) a grain of Wendell mnz; 3 IMP ages of 311 (+/-42), 280 (+/-20) and 294 (+/-20); the weighted mean EMP age is 318 (+/-4). (2) Former USGS IMP standard TM-1: ID-TIMS age of 1768 (+/-1.4); weighted mean EMP age of 1736 (+/-10). (3) 3 grains of Black Mountain (VT) Granite: grain 5, IMP age of 368 (+/-12) and EMP age of 358 (+/-5) ; grain 6, IMP age of 376 (+/-12), EMP age of 365 (+/-6); and grain 12, IMP age of 386 (+/-12), EMP age of 382 (+/-2).
DE: 1115 Radioisotope geochronology
DE: 1135 Correlative geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005