HR: 14:40h
AN: V53E-05    [Abstracts]
TI: LA-ICPMS Trace- and Rare-Earth Element Zoning in (Ultra)high-Pressure Eclogites of the Western Gneiss Region, Norway
AU: * Kylander-Clark, A R
EM: akylander@umail.ucsb.edu
AF: University of California, Santa Barbara, Bldg. 526, Santa Barbara, CA 93106
AU: Hacker, B R
EM: hacker@geol.ucsb.edu
AF: University of California, Santa Barbara, Bldg. 526, Santa Barbara, CA 93106
AU: Vervoort, J D
EM: vervoort@wsu.edu
AF: Washington State University, Webster Physical Science Building 1228, Pullman, WA 99164
AB: Laser-ablation inductively coupled plasma mass spectrometry was used to determine Lu, Hf, Sm and Nd zoning in garnets and pyroxenes from 5 (ultra)high-pressure eclogites from the Western Gneiss Region, Norway, to determine the significance of Sm/Nd and Lu/Hf ages for the same rocks. Garnets of the 3 lower temperature eclogites (700°C and ~2.0--2.5 GPa) are strongly zoned in Lu, with core concentrations > 3 times that of the rims. Sm shows more subdued zoning, reflecting the reduced partitioning of Sm into garnet compared to Lu. The profiles are bell-shaped, as expected for Rayleigh fractionation, indicating that they formed during growth. Garnets in two of the three samples are unzoned in major elements, reflecting the enhanced diffusivities of these elements compared to rare-earth elements. Clinopyroxenes are unzoned in major elements and Hf; Sm and Nd are unzoned in two of the three samples. The enhanced concentration of Lu in garnet cores---and the more subdued zoning in Sm---indicates that the Lu/Hf ages will be weighted towards the core age and the Sm/Nd ages will represent an `average' age. Geochronologic data obtained at UW, Madison (Kylander-Clark et al., 2005) show that this is indeed the case---the Lu/Hf ages are 10--15 Myr older than the Sm/Nd ages. Garnets and clinopyroxenes of two higher temperature eclogites (700--800°C and ~3 GPa) are unzoned in Lu, Hf, Sm and Nd, and major elements, indicating that they recrystallized at temperatures high enough for homogenization. If so, they should yield cooling ages that are younger than the growth ages of the lower temperature eclogites. Geochronologic data obtained at UW, Madison (Kylander-Clark et al., 2005) show that this also is true---the high-temperature ages are 20--30 Myr younger than the low-temperature ages.
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005