Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites

Abstract

The finding of abundant aluminium-rich objects within ordinary chondrites^1–3 supports the suggestion⁴ that carbonaceous and type 3 ordinary chondrites, and their components^4–7, share common origins. However, oxygen isotope systematics⁸ demonstrate that the chondrules present within the two meteorite types have isotopically distinct solid precursors, so while they may have a similar mode of formation, the chondrules come from isotopically separate reservoirs (a later reaction with a similar ambient gas is a possibility⁸). ²⁶Mg excesses generated by the in situ decay of ²⁶Al have previously only been found within Ca–Al-rich inclusions (CAIs) from carbonaceous chondrites^9–14. In a search for ²⁶Mg excesses generated by ²⁶Al decay we analysed four Al-rich objects from the type 3 ordinary chondrites using an ion microprobe. We report here the presence of ²⁶Mg excesses of up to 100% in an unusually pure hibonite clast from the Dhajala chondrite; this ²⁶Mg excess is the first to be found in an ordinary chondrite. Despite the large ²⁶Mg excesses, the initial ²⁶Al/²⁷Al ratio defined (8.4 × 10⁻⁶) is some five times lower than that commonly observed in calcium–aluminium-rich inclusions from the carbonaceous chondrites. No ²⁶Mg excesses were observed in the three plagioclase-bearing chondrules analysed. The presence of ²⁶Mg excesses within both unequilibrated ordinary and carbonaceous chondrites supports a common origin for components of these two meteorite types; however, ²⁶Al may not have been sufficiently plentiful to act as a major heat source in condensed Solar System bodies^15–17.