HR: 16:30h
AN: V12I-03    [PDF]
TI: Sm-Nd Dating of Spatially Controlled Domains of Garnet Single Crystals
AU: * Ducea, M
EM: ducea@geo.arizona.edu
AF: University of Arizona, Dept of Geosciences, Tucson, AZ 85721 United States
AU: Ganguly, J
EM: ganguly@geo.arizona.edu
AF: University of Arizona, Dept of Geosciences, Tucson, AZ 85721 United States
AU: Rosenberg, E
EM: Erin.J.Rosenberg@Dartmouth.EDU
AF: Dartmouth College, Dept of Earth Sciences, Hanover, NH 03755 United States
AB: Ganguly and Tirone have recently presented a method of determining the cooling rates of rocks from the difference between the core- and bulk-ages of a crystal as determined by a single decay system. We present the first application and successful test of the method using the core and bulk ages of garnet single crystals, according to the Sm-Nd decay system, in two rock samples with contrasting cooling rates, which can be constrained independently. The samples belong to the metamorphic core complex, Valhalla, British Columbia, and the granulite facies mid-crustal magmatic arc of the Salinian terrane, California. We have micro-sampled the garnet crystals over specific radial dimensions, and measured the Nd isotopes of these small sample masses, as NdO+ via conventional solid source mass spectrometry, to determine the Sm-Nd age difference between the core and bulk crystals. Using a peak metamorphic P-T condition of 8 kb, 820 C, the core (67.3 Ma) and bulk (60.9 Ma) ages of the British Columbian garnet sample yield a cooling rate of 8 C/Myr, which is in very good agreement with the cooling rates that we have derived by modeling the retrograde Fe-Mg zoning in the same garnet, and resetting of the bulk Sm-Nd garnet cooling age with respect to the U-Pb zircon age. Propagating the uncertainties of the inferred peak metamorphic condition, the three independent methods yield a cooling rate of 4-8 C/Myr for the British Columbian sample. The Salinian sample, on the other hand, yields indistinguishable core (78.2 ñ 2.7 Ma) and bulk (77.9 ñ 2.9 Ma) ages, as expected from its fast cooling history, which can be constrained by the results of earlier studies. Since the Sm-Nd decay system in garnet has relatively high closure temperature (usually more than 650 C), the technique developed in this paper fills an important gap in thermochronology since the commonly used thermo-chronometers are applicable only at significantly lower temperatures.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting