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.