HR: 1340h
AN: V23C-1549    [Abstracts]
TI: Assessment of the rutile (U-Th)/He thermochronometry on the KTB drill hole, Germany
AU: * Wolfe, M R
EM: mrwolfe@ku.edu
AF: Dept. of Geology, University of Kansas, 1475 Jayhawk Blvd., 120 Lindley Hall, Lawrence, KS 66049, United States
AU: Stockli, D F
EM: stockli@ku.edu
AF: Dept. of Geology, University of Kansas, 1475 Jayhawk Blvd., 120 Lindley Hall, Lawrence, KS 66049, United States
AU: Shuster, D L
EM: dshuster@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, United States
AU: Walker, J D
EM: jdwalker@ku.edu
AF: Dept. of Geology, University of Kansas, 1475 Jayhawk Blvd., 120 Lindley Hall, Lawrence, KS 66049, United States
AU: Macpherson, G L
EM: glmac@ku.edu
AF: Dept. of Geology, University of Kansas, 1475 Jayhawk Blvd., 120 Lindley Hall, Lawrence, KS 66049, United States
AB: (U-Th)/He dating of rutile, a common accessory mineral in HT and HP metamorphic rocks, such as eclogite, blueschist, and granulite, has to the potential to constrain cooling histories of rocks that have traditionally been problematic to date using established radiometric techniques. This study presents new rutile (U-Th)/He age and diffusion data from the German Continental Deep Drilling Project (KTB) to quantify the position of the rutile He partial retention zone (HePRZ) and thermal history of the KTB borehole. The down-hole rutile (U-Th)/He age profile allows for the empirical determination of He diffusion kinetics in rutile over geological time scales and validation of laboratory-derived He diffusion kinetics. Initial step-heating experiments of rutile have established a closure temperature of ~220-235°C. The KTB borehole is ideally suited to perform in-situ calibrations of rutile as a (U-Th)/He thermochronometer due to its rutile-bearing lithologies, depth and temperature range (up to 260°C), and previous fission track, (U-Th)/He, and ⁴⁰Ar/³⁹Ar thermochronometry studies, establishing a detailed thermal history for the KTB and adjacent vicinity. The 9 km deep KTB drill hole penetrates a series of tectonically-stacked fault blocks of metapelitic paragneisses and amphibolites along the western margin of the Bohemian Massif. Similar to published titanite (U-Th)/He data, rutile and zircon (U-Th)/He ages systematically vary from ~72 Ma to 0 Ma below the base of the partial retention zones. Low parent nuclide concentrations in rutile (<1 ppm) from KTB amphibolites require TIMS or MC-ICP analysis to obtain reliable age data. In addition, we are experimentally quantifying He diffusion kinetics on selected samples from the KTB drill hole. However, low U and Th concentrations and subsequent ⁴He ingrowth limit ⁴He diffusion experiments and require ³He diffusion experiments on proton-irradiated samples. This approach also allows quantification of diffusion kinetics on totally reset samples below the rutile HePRZ. The down-hole rutile (U-Th)/He age profile compliments the laboratory determined He diffusion kinetics of rutile by providing a natural quantification of He-diffusion over geologic time scales. Our new zircon and rutile (U-Th)/He analyses complement preexisting thermochronometric data and further constrain the thermal history of the KTB drill hole and surrounding Variscan tectonic zones.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1115 Radioisotope geochronology
DE: 1140 Thermochronology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting