FUN with PANURGE: High mass resolution ion microprobe measurements of Mg in Allende inclusions

The performance characteristics of PANURGE, a modified CAMECA IMS3F ion microprobe, have been studied at a mass resolving power of 5000 for the purpose of determining isotopic ratios at a precision level approaching that of counting statistics using beam switching. The techniques used for this type of measurement are described. Using this approach, the isotopic composition of Mg and Si and the atomic ratio of $\text{Al}\text{Mg}$ in minerals from the Allende inclusion WA and the Allende FUN inclusion Cl have been measured with the ion microprobe at high mass resolving power. Enrichments in ²⁶Mg of up to 260%. have been found. Mg and $\text{Al}\text{Mg}$ measurements on cogenetic spinel inclusions and host plagioclase crystals yield Mg-Al isochrons in excellent agreement with precise mineral isochrons determined by thermal emission mass spectrometry. The measurements confirm the presence of substantial excess ²⁶Mg in WA (${}^{26}\text{Mg}{}^1{}^{27}\text{Al = 5 × 10}{}^{\mathrm{?5}}$) and its near absence in Cl (${}^{26}\text{Mg}{}^1{}^{27}\text{Al}\text{< 4 × 10}{}^{\mathrm{?6}}$). In WA plagioclase, data for which ${}^{27}\text{Al}{}^{24}\text{Mg}\text{= 300 to 1000}$ define a linear array with ${}^{26}\text{Mg}{}^1{}^{27}\text{Al}\text{= 3 × 10}{}^5$ and with initial ${}^{26}\text{Mg}{}^{24}\text{Mg}$ composition 30%. greater than in high Mg phases. This suggests a metamorphic reequilibration of Mg in Allende plagioclase at least 0.6 my after WA formation. There were no variations in detected ${}^{26}\text{Mg}{}^1{}^{27}\text{Al}$ in WA plagioclase associated with concentration of ²⁶Mg¹ into isolated clusters. We have confirmed by ion probe measurements that the Mg composition in Allende Cl is highly fractionated and is uniform among pyroxene, melilite, plagioclase, spinel crystals and spinel included in melilite and plagioclase crystals. Likewise, the Si composition is mass fractionated and is the same in pyroxene, melilite and plagioclase.