HR: 15:25h
AN: T23C-07    [Abstracts]
TI: Apatite geochronology: Applications to exhumation histories in orogenic settings
AU: * Barfod, G H
EM: Barfod@geology.ucdavis.edu
AF: Department of Geology, UCDavis, One Shields Avenue, Davis, Ca 95616 United States
AU: Albarede, F
EM: albarede@ens-lyon.fr
AF: Ecole Normale Superieure de Lyon, Allee d'Italie, Lyon, 69364 France
AU: Krogstad, E J
EM: ejk@dlc.ku.dk
AF: Danish Lithosphere Centre, Oster Voldgade 10, Copenhagen, 1350 Denmark
AU: Frei, R
EM: robertf@geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oster Voldgade 10, Copenhagen, 1350 Denmark
AB: The Lu-Hf system in apatite represents a robust dating tool for assessing the extent, timing and duration of high-grade metamorphic processes. Until recently, garnet has been the only common phase in high-grade terranes suitable for Lu-Hf geochronology. However, apatite has elevated $^{176}$Lu/$^{177}$Hf ratios compared to garnet (up to 90 versus less than 30 [e.g., 1]). The $^{176}$Lu/$^{177}$Hf ratios are heterogeneous on a mm-scale allowing, in some cases, for dating of single apatite crystals [2]. Moreover, mineral inclusions in apatite can be largely eliminated by dissolution of the host apatite in highly dilute acid [2]; whereas one of the major challenges in garnet geochronology is assessing the influence from possible inherited inclusions [e.g., 3]. Apatite from three Proterozoic terranes yields Lu-Hf ages that are consistently older than their respective Pb step leaching (PbSL) ages. Isotopic closure for the Lu-Hf system, therefore, occurs prior to U-Pb closure in this mineral. In the Adirondack Lowlands, where H$_{2}$O activity was low, Lu-Hf ages of 1270 Ma and 1230 Ma are obtained for 3 cm and 0.3 cm diameter apatite crystals, respectively. These ages significantly predate the peak metamorphism conditions during the Elzevirian orogeny in the Adirondack Lowlands that reached upper amphibolite facies at 1170-1130 Ma. In contrast, apatite from the fluid-rich Otter Lake area and Black Hills record surprisingly young Lu-Hf and PbSL ages, presumably reflecting late exchange facilitated by fluids. The Lu-Hf ages for metamorphic apatites are shown to be controlled by temperature and grain size, and, in areas where fluid activity was high during peak metamorphism and exhumation, by fluid composition. [1] Anczkiewicz et al., 2004, EPSL, 255, 147-161; [2] Barfod et al., in press, GCA; [3] Scherer et al., 2000, GCA, 64, 3413-3432.
DE: 3660 Metamorphic petrology
DE: 3694 Instruments and techniques
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting