EVIDENCE OF DISTURBANCE IN THE Al-Mg SYSTEMATICS OF THE EFREMOVKA E60 CAI: IMPLICATIONS FOR THE HIGH-RESOLUTION CHRONOLOGY OF THE EARLY SOLAR SYSTEM

Introduction: The high-resolution Al-Mg extinct chronometer (t1/2 = 0.72 Myr) has been applied extensively towards constraining the chronology of the early Solar System. In particular, there have been many investigations of the Al-Mg systematics in the calciumaluminum-rich inclusions (CAIs), which has been used to infer the age of the Solar System since these objects represent the earliest-formed solids in the solar nebula (e.g., [1-5]). However, this chronometer can only provide relative time constraints and has to be anchored to the high-resolution Pb-Pb chronometer to obtain absolute ages. There are relatively few CAIs for which both internal Al-Mg and Pb-Pb systematics have been determined. One such CAI is E60 from the Efremovka reduced CV3 chondrite, for which an internal Al-Mg isochron corresponding to a Al/Al ratio of 4.63 ± 0.44 × 10 has been determined [6] and a highly precise Pb-Pb age of 4567.11 ± 0.16 Ma has also been reported [7]. However, recent investigations (e.g., [8]) have brought to light discrepancies between the Pb-Pb ages for CAIs and the AlMg systematics in these objects. Additionally, Al-Mg systematics in the angrites D’Orbigny and Sahara 99555 anchored to the E60 CAI yield ages that are ~2 Myr younger than the Pb-Pb ages for these achondrites ([9] and references therein). Therefore, with the goal of assessing the suitability of E60 CAI as a time anchor for the Al-Mg system, we have performed high-precision laser ablation multicollector inductively coupled plasma mass spectrometer (LA-MC-ICPMS) analyses of minerals in rim and the interior of this inclusion. Analytical Methods: The mineralogy and petrography of a polished thick slice of the Efremovka E60 CAI were studied using a Cameca SX-50 electron microprobe and a JEOL JSM-5900LV scanning electron microscope (SEM) equipped with a Thermo Electron energy dispersive spectrometer. Magnesium isotopes were measured with a Thermo-Finnigan Neptune MCICPMS instrument and a Photon Machines fast excimer laser ablation system (producing 193 nm wavelength and 4 ns pulse length) at Arizona State University using methods similar (unless noted here) to those described in [10]. Analyses were conducted in medium resolution mode to resolve any interferences on the Mg masses. Individual measurements typically consisted of 25 integration cycles, each of 8.4 sec duration. The laser spot size was 4090 μm, with a repeat rate of 4 Hz and a laser energy density of roughly 2.8 J/cm. Signal intensity for Mg ranged from 0.5 to 3.0 × 10 A. All measurements employed sample-standard bracketing, using isotopically well-characterized synthetic glasses from evaporation experiments having CAI-like compositions [11]. Standard glass compositions were chosen that approached the compositions of the analyzed phases. Based on repeated measurements of these glasses, typical external reproducibilities (2σ) for the Al/Mg ratios and the δMg* values (defined as the non mass-dependent deviations in the Mg/Mg ratio relative to the DSM3 standard) are ±5% and ±0.15‰, respectively.