Iodine-xenon, chemical, and petrographic studies of Semarkona chondrules - Evidence for the timing of aqueous alteration

The relationship of the I-Xe system of the Semarkona meteorite to other measured properties is investigated via INAA, petrographic, and noble-gas analyses on 17 chondrules from the meteorite. A range of not less than 10 Ma in apparent I-Xe ages is observed. The three latest apparent ages fall in a cluster, suggesting the possibility of a common event. It is argued that the initial I-129/I-127 ratio (R0) is related to chondrule type and/or mineralogy, with nonporphyritic and pyroxene-rich chondrules showing evidence for lower R0s than porphyritic and olivine-rich chondrules. Chondrules with sulfides on or near the surface have lower R0s than other chondrules. The He-129/Xe-132 ratio in the trapped Xe component anticorrelates with R0, consistent with the evolution of a chronometer in a closed system or in multiple systems. It is concluded that the variations in R0 represent variations in ages, and that later events, possibly aqueous alteration, preferentially affected chondrules with nonporphyritic textures and/or sulfide-rich exteriors about 10 Ma after the formation of the chondrules.