Al-Mg ISOTOPE SYSTEMATICS IN THE EFREMOVKA

Introduction: In recent years, the calcium-aluminum-rich inclusion (CAI) E60 from the Efremovka reduced CV3 chondrite has been used as an age anchor for the Al-Mg extinct chronome-ter (t 1/2 ~0.73 Ma) since an internal Al-Mg isochron corresponding to a 26 Al/ 27 Al ratio of (4.63±0.44)×10-5 has been determined for it [1] and a highly precise 207 Pb-206 Pb age of 4567.11±0.16 Ma has also been reported [2]. However, recent investigations have noted discrepancies between 207 Pb-206 Pb ages and 26 Al-26 Mg systematics in several meteoritic materials. For example, Al-Mg systematics in the angrites D'Orbigny and Sahara 99555 anchored to the E60 CAI yield ages that are ~2 Ma younger than the Pb-Pb ages for these achondrites ([3] and references therein). Therefore, with the goal of assessing the suitability of the E60 CAI as a time anchor for the 26 Al-26 Mg system, we initiated a high-precision laser ablation multicollector inductively coupled plasma mass spectrometer (LA-MC-ICPMS) study of internal 26 Al-26 Mg isotope systematics in this inclusion. We have previously presented some preliminary results from this investigation [4]. Here, we report the results of further analyses that demonstrate evidence for re-equilibration of the Al-Mg system in E60. Analytical: A polished thick slice of the Efremovka E60 CAI was studied at UH using a Cameca SX-50 electron micro-probe and a JEOL JSM-5900LV SEM equipped with a Thermo Electron energy dispersive spectrometer. Magnesium isotopes were measured with a Thermo-Finnigan Neptune MC-ICPMS instrument and a Photon Machines fast excimer laser ablation system (producing 193 nm wavelength and 4 ns pulse length) at ASU using methods similar to those described earlier [4]. Results and Discussion: Previously we reported analyses of a diopside-rich rim (1 spot), diopside in the interior (2 spots), melilites in the rim and in the interior (3 spots each), and anor-thites in the rim (3 spots) of the E60 CAI [4]. Since anorthites in E60 are typically fine-grained and we used beam spots up to ~90 μm, we were unable to get clean analyses of this phase, and the highest 27 Al/ 24 Mg ratios reported in [4] were only up to ~16. The majority of these preliminary data defined a slope corresponding to a 26 Al/ 27 Al ratio of (2.9±0.5)×10-5 and an initial 26 Mg/ 24 Mg ratio of 0.23±0.09 ‰ (MSWD=0.9); the exceptions were the melilite rim analyses which fell above this isochron. We have now made …