A short duration of chondrule formation in the solar nebula: Evidence fromAl-26 in Semarkona ferromagnesian chondrules

The Al-26-Mg-26 systems of five ferromagnesian chondrules from the least me tamorphosed ordinary chondrite Semarkona (LL3.0) were studied using a secon dary ion mass spectrometer. Their glass or plagioclase portions contain exc esses of Mg-26, and in two chondrules the Mg-26 excesses are well correlate d with Al-27/Mg-24, which demonstrates the in-situ decay of Al-26. The init ial Al-26/Al-27 ratios in these chondrules obtained from the slope of isoch rons show a narrow range of between 6 X 10(-6) and 9 X 10(-6), indicating t heir short formation duration of less than 1 My. If the solar nebula was in itially homogeneous in Al isotopes, the chondrule formation ages are simila r to2 My younger than those of CAIs. Our results based on the study of the least metamorphosed UOC are consistent with the previous studies on Al-rich chondrules that the chondrule formation started at least 2 My after CAIs. Alternatively, the older records before 2 My were erased by chondrule recyc ling process. It further suggests that the young apparent ages (3 to >5 My after CATs) for chondrules in type 3.4 UOCs are due to the disturbance of t he Al-26-Mg-26 system, possibly during parent body metamorphism. The result is not consistent with the extended nebular time scale of >5 My and the ch ondrule formation by planetary processes. The Ni isotopic analysis of the F eO-rich olivines in a type IT chondrule in Semarkona did not show any detec table excess Ni-60 in spite of their high Fe/Ni ratios. The upper limit of the initial Fe-60/Fe-56 ratio of the solar system was estimated to be 3.4 x 10(-7), which is consistent with the previous estimate (0.2-1.9 X 10(-7)) from eucrites. This result confirms that the 60Ni excess previously observe d from CATs was not due to the decay of the short-lived nuclide Fe-60, but a Ni isotopic anomaly of nucleosynthetic origin. Copyright (C) 2000 Elsevie r Science Ltd.