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

We have performed INAA, petrographie, and noble gas analyses on seventeen chondrules from the Semarkona meteorite (LL3.0) primarily to study the relationship of the I-Xe system to other measured properties. We observe a range of ≥10 Ma in apparent I-Xe ages. The three latest apparent ages fall in a cluster, suggesting the possibility of a common event. The initial ¹²⁹I/¹²⁷I ratio (R₀) is apparently related to chondrule type and/or mineralogy, with nonporphyritic and pyroxene-rich chondrules showing evidence for lower R₀'S (later apparent I-Xe ages) than porphyritic and olivine-rich chondrules. In addition, chondrules with sulfides on or near the surface have lower R₀S than other chondrules. The ¹²⁹Xe/¹³²Xe ratio in the trapped Xe component anticorrelates with R₀, consistent with evolution of a chronometer in a closed system or in multiple similar systems. On the basis of these correlations, we conclude that the variations in R₀ represent variations in ages, and that later event(s), possibly aqueous alteration, preferentially affected chondrules with nonporphyritic textures and/or sulfide-rich exteriors about 10 Ma after the formation of the chondrules.