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

We have studied the I-Xe system in chondrules and clasts from ordinary chon drites. Cristobalitebearing clasts from Parnallee (LL3.6) closed to Xe loss 1-4 Ma after Bjurbole. Feline (a feldspar- and nepheline-rich clast also f rom 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 t o Bjurbole's within error. A clast from Barwell (L6) has a well-defined ini tial I ratio corresponding to closure 3.62 +/- 0.60 Ma before Bjurbole. Partial disturbance and complete obliteration of the I-Xe system by shock a re revealed in clasts from Julesburg (L3.6) and Quenggouk (1-14), respectiv ely. Partial disturbance by shock is capable of generating anomalously high initial I ratios. In some cases, these could be misinterpreted, yielding e rroneous ages. A macrochondrule from Isoulane-n-Amahar contains concentrati ons of I similar to "ordinary" chondrules but, unlike most ordinary chondru les, contains no radiogenic Xe-129. This requires resetting 50 Ma or more l ater than most chondrules. The earliest chondrule ages in the I-Xe, Mn-Cr, and Al-Mg systems are in re asonable agreement. This, and the frequent lack of evidence for metamorphis m capable of resetting the I-Xe chronometer, leads us to conclude that (at least) the earliest chondrule I-Xe ages represent formation. If so, chondru le formation took place at a time when sizeable parent bodies were present in the solar system.