Sm-Nd dating of spatially controlled domains of garnet single crystals: a new method of high-temperature thermochronology

摘要

Ganguly and Tirone [Meteorit. Planet. Sci. 36 (2001) 167-175] 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 mid-crustal magmatic arc exposure 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 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 kbar, 820 ± 30 ℃ [Spear and Parrish, J. Petrol. 37 (1996) 733-765], 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°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 assuming the same peak metamorphic P-T condition. Considering earlier cooling rate data derived from closure temperature vs. age relation of multiple geochronological systems [Spear and Parrish, J. Petrol. 37 (1996) 733-765], a cooling rate of ～15-20°C/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. The Sm-Nd decay system in garnet has relatively high closure temperature (usually > 650 ℃); therefore, the technique developed in this paper fills an important gas in thermochronology, since the commonly used thermochronometers are applicable only at 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 [Granguly et al. , Science 281 (1998) 805-807], along with additional geochronological data, would lead to robust constraints on cooling rates of rocks.