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

Abstract

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°C [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°C); therefore, the technique developed in this paper fills an important gap 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 [Ganguly et al., Science 281 (1998) 805–807], along with additional geochronological data, would lead to robust constraints on cooling rates of rocks.