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.
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