Ion Microprobe Study of Fe-ni System in Ferromagnesian Pyroxene Chondrules in Krymka (ll3.1) by Multicollection

Introduction: Iron-60 decays to 60 Ni with a half-life of 1.5 Ma. Evidence for its presence in the early solar system has been found in chondritic and differentiated meteorites [1-6]. Because it is not synthesized efficiently at all by energetic-particle irradiation, its presence in the early solar system indicates that stellar nucleosynthesis contributed to the inventory of short-lived radionuclides. Our best estimate of the initial 60 Fe/ 56 Fe ratio in the solar system is (0.5-1)×10-6 [ 4]. A recent estimate of the 60 Fe/ 56 Fe ratio in the average interstellar medium, based on the γ-ray observations of [7] (1.4×10-7 [8]) is a factor of >3 lower than our solar system estimate, but is close enough to it that [8] have questioned whether a late stellar source is in fact required. However, both ratios have large uncertainties. The estimate for the interstellar medium depends on assumptions about galactic metallicity and Al/Fe ratio and may be much lower (see [9] for a discussion of this issue). Our current efforts are designed to improve the precision of the estimate for the early solar system. We have been studying 60 Fe-60 Ni systems in ferro-magnesian pyroxene-rich chondrules from two of the least equilibrated ordinary chondrites, Semarkona (LL3.0) and Bishunpur (LL3.1), in which silicates have suffered little or no secondary thermal or aqueous processing [4, 5]. The inferred 60 Fe/ 56 Fe ratios in these chondrules can be translated back to the time of solar system formation by assuming that the chondrules formed 1.5-2.0 Ma after CAIs, an assumption that has apparently been confirmed by measuring the 26 Al/ 26 Mg system in chondrules containing evidence for 60 Fe [10, 11]. The biggest weakness the 60 Fe data from chon-drules is that the precision of the measurements is not high enough to yield precise initial 60 Fe/ 56 Fe ratios or to evaluate the degree to which the isotopic system in the chondrules might have been disturbed. We therefore have been working to develop a multicollection technique for the ims 1280 that will improve the counting statistics by a factor of ~4 relative to our previous measurements. In this abstract, we report our first multicollection 60 Fe-60 Ni data for ferromagnesian py-roxene-rich chondrules from Krymka (LL3.1).