Aluminum-26 in H4 chondrites:: Implications for its production and its usefulness as a fine-scale chronometer for early solar system events

In order to investigate whether or not Al-26 can be used as a fine-scale chronometer for early solar system events we measured, with an ion microprobe, Mg isotopes and Al/Mg ratios in separated plagioclase, olivine, and pyroxene crystals from the H4 chondrites Ste Marguerite (SM), Forest Vale (FV), Beaver Creek and Quenggouk and compared the results with the canonical Al-26/Al-27 ratio for calcium-aluminum-rich inclusions (CAIs). For SM and FV, Pb/Pb and Mn-Cr ages have previously been determined (Gopel et al., 1994; Polnau et al., 2000; Polnau and Lugmair, 2001). Plagioclase grains from these two meteorites show clear excesses of Mg-26. The Al-26/Al-27 ratios inferred from these excesses and from isotopically normal Mg in pyroxene and olivine are (2.87 +/- 0.64) x 10(-7) for SM and (1.52 +/- 0.52) x 10(-7) for FV The differences between these ratios and the ratio of 5 x 10(-5) in CAIs indicate time differences of 5.4 +/- 0.1 Ma and 6.1 +/- 0.2 Ma for SM and FV, respectively. These differences are in agreement with the absolute Pb/Pb ages for CAN and SM and FV phosphates but there are large discrepancies between the U-Pb and Mn-Cr system for the relative ages for CAIs, SM and FV For example, Mn-Cr ages of carbonates from Kaidun are older than the Pb/Pb age of CAIs. However, even if we require that CAIs are older than these carbonates, the time difference between this "adjusted" CAI age and the Mn-Cr ages of SM and FV require that Al-26 was widely distributed in the early solar system at the time of CAI formation and was not mostly present in CAIs, a feature of the X-wind model proposed by Shu and collaborators (Gounelle et al., 2001; Shu et al., 2001). From this we conclude that there was enough Al-26 to melt small planetary bodies as long as they formed within 2 Ma of CAIs, and that Al-26 can serve as a fine-scale chronometer for early solar system events.