20-EPN-050: 26-Aluminium-26-Magnesium systematics of chondrules and clasts in unequilibriated ordinary chondrites

In-person visit by Audrey Bouvier (Universität Bayreuth, Germany) to TA2.9 Ion probe facility (IPF), CRPG (France).
Dates of visit: 7-11 June 2021

Chondrules are a major component of chondritic meteorites whose time and mechanism of formation are still debated. Inconsistencies in formation ages of chondrules have been found between ages determined by the absolute Pb-Pb chronometer or using the relative ²⁶Al-²⁶Mg chronometer. While the Pb-Pb ages suggest that chondrules formed continuously for about 4 Ma from the time of CAI formation, the ²⁶Al-²⁶Mg data show evidence that chondrules did not form until about 1.8 Ma after CAIs. One possible explanation could be a heterogeneous distribution of ²⁶Al in the solar nebula.

To evaluate this hypothesis, we used secondary ionization mass spectrometry (SIMS) to date chondrules and clasts from unequilibrated ordinary chondrites with the ²⁶Al-²⁶Mg chronometer. Three chondrules from ordinary chondrites show resolvable excesses in ²⁶Mg due to the decay of ²⁶Al and formed around 2 Ma after CAI formation, consistent with previous studies. Analysis of a large igneous inclusion from Paposo 004 supports a formation age within 1 Ma after CAI. The presence of a relict olivine chondrule in this inclusion provides contextual evidence that chondrule formation must have taken place prior to this time.

---