Autori: Ducea, M.N., Ganguly, J., Rosenberg, E., Patchett, P.J., Cheng, W., Isachsen, C.

Editorial: Earth and Planetary Science Letters, vol. 213 no 1-2, p.31-42, 2003.

Rezumat:

Ganguly and Tirone [1] 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. Here we present the first application 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 ?1 kb, 820 ?30 oC [2], the core (67.3?2.3 Ma) and bulk (60.9?2.1 Ma) ages of the British Columbian garnet sample yield a cooling rate of 2-13 0C/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 assuming the same peak metamorphic P-T condition. Considering earlier cooling rate data derived from closure temperature vs age relation of multiple Geochronological systems [2], a cooling rate of ~ 15-20 oC/Myr seems most reasonable for the Valhalla complex. Diffusion kinetic analysis shows that the Sm-Nd core age of the selected garnet crystal could not have been disturbed during cooling. Consequently, the core age of the garnet crystal, 67.3?2.3 Ma, corresponds to the peak metamorphic age of the Valhalla complex. The Salinian sample, on the other hand, yields indistinguishable core (78.2?2.7 Ma) and bulk (77.9?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 > 650 oC), 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. Simultaneous modeling of the retrograde Fe-Mg zoning in garnet, spatially resolved Sm-Nd ages of garnet single crystals, and resetting of the bulk garnet Sm-Nd age from the peak metamorphic age [3], along with additional geochronological data, would lead to robust constraints on cooling rates of rocks.

Cuvinte cheie: Sm-Nd geochronology, garnet, exhumation rates