Meteoritics & Planetary Science 44, Nr 11, 1787–1796 (2009) Abstract available online at http://meteoritics.org Interpreting the I-Xe system in individual silicate grains from Toluca IAB O. V. PRAVDIVTSEVA1∗, A. P. MESHIK1, C. M. HOHENBERG1, and M. PETAEV2 1Washington University, Box 1105, One Brookings Drive, Saint Louis, Missouri 63117, USA 2Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Hoffman 208 Cambridge, Massachusetts 02138, USA *Corresponding author. E-mail:
[email protected] (Received 24 March 2009; revision accepted 25 August 2009) Abstract–Detailed isotopic and mineralogical studies of silicate inclusions separated from a troilite nodule of the Toluca IAB iron meteorite reveal the presence of radiogenic 129Xe in chlorapatite, plagioclase, perryite, and pyroxene grains. Subsequent I-Xe studies of 32 neutron-irradiated pyroxene grains indicate that high-Mg and low-Mg pyroxenes have distinctive I-Xe signatures. The I-Xe system in high-Mg pyroxenes closed at 4560.5 ± 2.4 Ma, probably reflecting exsolution of silicates from the melt, while the low-Mg pyroxenes closed at 4552.0 ± 3.7 Ma, 8.5 Ma later, providing a means for determining the cooling rate at the time of exsolution. If the host Toluca graphite-troilite- rich inclusion formed after the breakup and reassembly of the IAB parent body as has been suggested, the I-Xe ages of the high-Mg pyroxenes separated from this inclusions indicate that this catastrophic impact occurred not later than 4560.5 Ma, 6.7 Ma after formation of CAIs. The cooling rate at the time of silicates exsolution in Toluca is 14.5 ± 10.0 °C/Ma.