Abstract
We report here technical details of 26Al–26Mg dating of single plagioclase grains located in a chondrule of the Efremovka CV3 chondrite using a high spatial resolution secondary ion mass spectrometer (NanoSIMS). A ca. 500 pA mass filtered 16O– primary beam was used to sputter 5–7 μm diameter craters. Secondary positive beams were extracted for mass analysis. We detected 27Al++ (at mass 13.5), 24Mg+, 25Mg+, and 26Mg+ simultaneously under a static magnetic field for the first session. To obtain accurate Mg isotopic ratios but at the expense of a longer analytical time, we have also measured Mg isotopes (24Mg, 25Mg, 26Mg) using a single detector with a magnet scanning mode. Apparent 26Mg excesses were observed in small plagioclase grains (#1 and #4) of the chondrule. A positive correlation between the Al/Mg ratio and the excesses 26Mg engender a 26Al–26Mg isochron with an initial 26Al/27Al ratio of (3.3 ± 1.3) × 10–6 for #1 and (1.7 ± 1.4) × 10–6 for #4. In contrast, large plagioclase grains (#2 and #3) show no resolvable excesses 26Mg. The initial 26Al/27Al ratios of −(0.13 ± 0.71) × 10–6 (#2) and −(0.12 ± 1.26) × 10–6 (#3) are consistent with zero. A substantial difference exists among the initial 26Al/27Al ratios of individual grains in the same Efremovka chondrule, which may be attributable to parent body alteration of the meteorite. Careful treatment is necessary to calculate the formation age combining the data from different grains, even in a single chondrule.
