Chronology of the early Solar System from chondrule-bearing calcium-aluminium-rich inclusions

Chondrules and Ca-Al-rich inclusions (CAIs) are high-temperature components of meteorites that formed during transient heating events in the early Solar System. A major unresolved issue is the relative timing of CAI and chondrule formation(1-4). From the presence of chondrule fragments in an igneous CAI, it was concluded that some chondrules formed before CAIs (ref. 5). This conclusion is contrary to the presence of relict CAIs inside chondrules(6-10), as well as to the higher abundance of Al-26 in CAIs(11); both observations indicate that CAIs predate chondrules by 1-3 million years (Myr). Here we report that relict chondrule material in the Allende meteorite, composed of olivine and low-calcium pyroxene, occurs in the outer portions of two CAIs and is O-16-poor (Delta O-17 approximate to -1 parts per thousand to -5 parts per thousand). Spinel and diopside in the CAI cores are O-16-rich (Delta O-17 up to -20 parts per thousand), whereas diopside in their outer zones, as well as melilite and anorthite, are O-16-depleted (Delta O-17 = -8 parts per thousand to 2 parts per thousand). Both chondrule-bearing CAIs are Al-26-poor with initial Al-26/Al-27 ratios of (4.7 +/- 1.4) x 10(-6) and <1.2 x 10(-6). We conclude that these CAIs had chondrule material added to them during a re-melting episode similar to 2 Myr after formation of CAIs with the canonical Al-26/Al-27 ratio of 5 x 10(-5).