The iodine-xenon system in clasts and chondrules from ordinary chondrites: Implications for early solar system chronology

Abstract

We have studied the I-Xe system in chondrules and clasts from ordinary chondrites. Cristobalite-bearing clasts from Parnallee (LL3.6) closed to Xe loss 1-4 Ma after Bjurbole. Feline (a feldspar- and nepheline-rich clast also from Parnallee) closed at 7.04 ± 0.15 Ma. Two out of three chondrules from Parnallee that yielded well-defined initial I ratios gave ages identical to Bjurbole's within error. A clast from Barwell (L6) has a well-defined initial I ratio corresponding to closure 3.62 ± 0.60 Ma before Bjurbole. Partial disturbance and complete obliteration of the I-Xe system by shock are revealed in clasts from Julesburg (L3.6) and Quenggouk (H4), respectively. Partial disturbance by shock is capable of generating anomalously high initial I ratios. In some cases, these could be misinterpreted, yielding erroneous ages. A macrochondrule from Isoulane-n-Amahar contains concentrations of I similar to 'ordinary' chondrules but, unlike most ordinary chondrules, contains no radiogenic 129Xe. This requires 50 Ma or more later than most chondrules. The earliest chondrule ages in the I-Xe, Mn-Cr, and Al-Mg systems are in reasonable agreement. This, and the frequent lack of evidence for metamorphism capable of resetting the I-Xe chronometer, leads us to conclude that (at least) the earliest chondrule I-Xe ages represent formation. If so, chondrule formation took place at a time when sizeable parent bodies were present in the solar system.