MacPherson et al Al-Mg isotope data for condensate inclusions

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Al-Mg isotope data, collected by ion microprobe, from aggregate structured refractory inclusions. From the (2025) GCA article entitled "HIGH-PRECISION SIMS ANALYSES OF INITIAL 26AL/27AL IN UN-MELTED REFRACTORY INCLUSIONS: THE SEARCH FOR MULTIPLE CONDENSATION EPISODES." by Glenn J. MacPherson, Alexander N. Krot, Kazuhide Nagashima, and Marina Ivanova

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Aluminum- and magnesium-isotope compositions were measured in situ with the UH Cameca ims-1280 ion microprobe using protocols similar to those in Makide et al. (2009) and Park et al. (2016). Minerals with high 27Al/24Mg ratios (gehlenitic melilite and hibonite) were analyzed using an 16O− primary beam with ~80 or 100 pA. A fully focused primary beam was rastered over 2×2 µm in order to make pits shallower, resulting overall spot size of ~4-5 µm. Secondary 24Mg+, 25Mg+, and 26Mg+ ions were measured with the axial electron multiplier (EM) in peak jumping mode, and 27Al+ ions were measured with a multicollection Faraday Cup (FC), simultaneously with 25Mg+ measurements. The mass resolving power was ~3700, sufficient to separate interference ions. Spinel grains were analyzed with an 16O− primary beam diameter of ~10 μm. The primary current was set to ~4 or 6 nA. Four multicollection FCs were used to detect 24Mg+, 25Mg+, 26Mg+, and 27Al+, simultaneously. The mass resolving power (MRP) was set to ~2400 with exit slits of 500 μm. Although this setting does not allow a complete separation of 25Mg+ from 24MgH+ interference, the contribution of 24MgH+ onto 25Mg+ was estimated to be less than 1 ppm. Mass fractionation was corrected using experimentally determined empirical fractionation factors (β). This empirical factor combines the instrumental and intrinsic fractionation into a single fractionation law. For each measurement setup, the measured Mg-isotope ratios from different standards plot on a single linear array in 25Mg' vs. 26Mg' [logarithmic form of delta values defined by 25,26Mg' = 1000 × ln(25,26Mg/1000 + 1)] as shown in Figs. S1–S4. Excess 26Mg (26Mg*') were calculated as the deviations from the empirical fractionation line (i.e., 26Mg*' = 26Mg' - (1/β) × 25Mg'). Then 26Mg*' values were converted to linear delta values to report as 26Mg*. The measured 27Al/24Mg ratios were corrected using relative sensitivity factors (RSFs; defined as 27Al/24Mg [SIMS] / 27Al/24Mg [True], where “True” is the value determined by electron microprobe analysis corrected for 24Mg/total Mg) determined from measurements on standards, including synthetic melilite and Madagascar hibonite.

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