Browsing by Author "Loehn, Clayton William"
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- Investigation of the monazite chemical dating techniqueLoehn, Clayton William (Virginia Tech, 2011-04-28)In order to evaluate the electron microprobe (EMP) method for chemical dating of monazite, we chemically analyzed selected suites of monazite grains that were previously dated by standard U/Pb isotopic methods at three laboratories each equipped with a sensitive high resolution ion microprobe (SHRIMP). Representing diverse igneous and metamorphic lithologies, these grains yielded conventional isotopic ages ranging in age from Neoarchean to Devonian. Chemical dating was performed at Virginia Tech using a Cameca SX-50 EMP in which the analytical routines and settings were specifically optimized for monazite geochronology, including correction of analytical peaks for all major spectral interferences and correction of peak intensities for local background emission. Placement of cross-grain analytical traverses was based on backscattered electron (BSE) images together with wavelength-dispersive (WD) generated X-ray maps for Y, Th, U, and Ca, which revealed the internal compositional complexity of each grain. Shorter EMP traverses were selected adjacent to each SHRIMP pit in order to provide the best possible comparison of ages obtained by the two dating methods. Synthesis protocol for key elemental measurements (Y, Th, U, and Pb) was developed utilizing the 1Ï elemental errors associated with individual analyses, providing an objective approach for data synthesis. Analytical dates were either accepted or excluded based on analytical and spatial justifications. Isotopic dating techniques utilize three independent age calculations, provided the sample is old enough to have accumulated sufficient 207Pb (i.e., ≥~1000 Ma). Similarly, the chemical dating method can utilize two independent age calculations (i.e., Th/Pb and U/Pb) and a U-Th-Pbtotal centroid age in Th/Pb vs. U/Pb space, verified independently against the calculated Th* or U* CHIME ages. Across the entire 2,200 m.y. age range represented by the sample set, the chemical ages calculated from the EMP data chemical ages are internally consistent (within 2Ï error) with the previously measured SHRIMP isotopic ages, except in one case where bulk mixing of discrete age domains within an ablation pit led to an isotopically discordant apparent age. Overall, this study illustrates that EMP chemical dating (1) represents both an accurate and precise primary method for dating monazite from igneous and polymetamorphic terrains; (2) provides superior spatial resolution for obtaining meaningful ages from small and/or irregular domains of discrete age that may be irresolvable or misinterpreted by other dating techniques that sample larger volumes; and (3) illuminates the geological meaning of isotopically discordant monazite ages obtained using conventional methods with lower spatial resolution (e.g., SHRIMP).
- Monazite Geochronology of the Madison Mylonite Zone and Environs, Southwestern Montana: With Implications for Precambrian Thermotectonic Evolution of the Northern Wyoming ProvinceLoehn, Clayton William (Virginia Tech, 2009-04-03)Neoarchean thermotectonism at in the northern Wyoming province is preserved in metamorphic zircon rims and monazite growth throughout the Snowy shear zone (SSZ) and the Madison mylonite zone (MMZ), South Madison Range, Montana. Comparison of U-Pb and U-Th-Pb ages yielded by monazite grains from both shear zones and zircon rims from SSZ, a new timing for major SE-directed thrusting and formation of the MMZ and SSZ has been identified at ~2550 Ma. The collinearity of these two shears indicates the formation of a much larger single shear zone that extends from the North Snowy block (NE), Beartooth Mountains, through the South Madison range (SW), and is paralleled to the immediate NW by the Mirror Lake and Big Brother shear zones. A detrital zircon study of two quartzites, from the westernmost North Snowy block units, yielded concordant age populations ranging in age from 3556 ± 10 to 2752 ± 9 Ma indicating that these sediments were derived either from older crust located in the Beartooth Mountains or from another source that was relatively close to the region prior to ~2750 Ma. The youngest magmatic zircon core found among these quartzites yielded a U-Pb age of 2690 ± 12 Ma, setting a new maximum age for sandstone deposition, additionally 10 metamorphic zircon rims and one monazite grain provide a new minimum U-Pb age of deposition and metamorphism at 2545 ± 2 Ma. Driving forces behind the ~2550 Ma SE-directed thrusting in the NW Wyoming craton may have been the final stages of supercontinent Kenorland assembly, whereas the ~2450 Ma reactivation, recorded by monazite rim growth, along the SSZ-MMZ may relate to the incipient supercontinent break-up, which has been suggested to have occurred at about this time by other studies.