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

Abstract

The finding of abundant aluminium-rich objects within ordinary chondrites1–3 supports the suggestion4 that carbonaceous and type 3 ordinary chondrites, and their components4–7, 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 chondrites9–14. 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 bodies15–17.