A SHORT DURATION OF CHONDRULE FORMATION IN THE SOLAR NEBULA: EVIDENCE FROM 26AL IN SEMARKONA FERROMAGNESIAN CHONDRULES

Abstract

The 26Al–26Mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite Semarkona (LL3.0) were studied using a secondary ion mass spectrometer. Their glass or plagioclase portions contain excesses of 26Mg, and in two chondrules the 26Mg excesses are well correlated with 27Al/24Mg, which demonstrates the in-situ decay of 26Al. The initial 26Al/27Al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 × 10−6 and 9 × 10−6, indicating their short formation duration of less than 1 My. If the solar nebula was initially homogeneous in Al isotopes, the chondrule formation ages are ∼2 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 recycling process. It further suggests that the young apparent ages (3 to >5 My after CAIs) for chondrules in type 3.4 UOCs are due to the disturbance of the 26Al–26Mg system, possibly during parent body metamorphism. The result is not consistent with the extended nebular time scale of >5 My and the chondrule formation by planetary processes. The Ni isotopic analysis of the FeO-rich olivines in a type II chondrule in Semarkona did not show any detectable excess 60Ni in spite of their high Fe/Ni ratios. The upper limit of the initial 60Fe/56Fe ratio of the solar system was estimated to be 3.4 × 10−7, which is consistent with the previous estimate (0.2–1.9 × 10−7) from eucrites. This result confirms that the 60Ni excess previously observed from CAIs was not due to the decay of the short-lived nuclide 60Fe, but a Ni isotopic anomaly of nucleosynthetic origin.