Fig. 3.

Example ²⁷Al/²⁴Mg ratios versus cycle number for (a) the anorthite glass running standard with 1 wt. % MgO, and for (b-d) chondrule plagioclase. Increasing cycle numbers represent greater sputtering depth into samples. The shape of the ²⁷Al/²⁴Mg versus cycle number trend is likely related to the SIMS sputtering behavior of the sample, while differences in ²⁷Al/²⁴Mg ranges among samples represent variability in plagioclase compositions. BSE images of SIMS pits associated with (b-d) are also shown, where scale bars are 10 μm. In (b) and (c), chondrule ²⁷Al/²⁴Mg versus cycle number patterns approximate that of the anorthite running standard, suggesting the analyses were not compromised by inadvertent measurement of pyroxene grains. The image affiliated with (c) demonstrates that overlap with thin strands of an Fe-rich phase does not compromise data quality. In (d) the significant ²⁷Al/²⁴Mg decrease after the first 150 cycles indicates partial analysis of an unwanted phase. SEM imaging and EDS of the subsequent SIMS pit confirmed the presence of a small high-Ca pyroxene (HPx) grain. Therefore, only the first 150 cycles of this analysis were used to calculate the Al-Mg isotope systematics.