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NEW IXe AGES OF CHONDRULES FRO M THE ORDINARY L 4 CHONDRITE SARATOV

Chondrules from the ordinary chondrite Saratov (L4), were studied as a part of the ongoing investigation of I-Xe and Pb-Pb systems in chondrules from the ordinary chondrites covering different types and methamorphic grades. In the course of this work we analyze and report I-Xe ages for chondrules from Unnamed Antarctic LL3.6 [1, 2], Elenovka L5 [3], NWA 267 H4 [4], and Richardton H5 [5]. Considered together with the previously reported I-Xe ages for Bjurbole L4 [6], Semarkona LL3.0 [7], Chainpur LL3.4 [8, 9], Parnallee [10] these collectively form a consistent picture for each type of ordinary chondrites. The I-Xe ages of NWA267 (H4) and Richardton (H5) chondrules are in good agreement, with the lower metamorphic grade NWA267, slightly older. Ordinary L chondrites Bjurbole (L4) and Elenovka (L5) follow the same trend: I-Xe ages of the lower methamorphic grade Bjurbole (L4) chondrules cluster within 1.5 Ma interval while the I-Xe ages of individual Elenovka (L5) chondrules differ by 27 ± 10 Ma. A similar trend in the I-Xe ages is observed in the LL3.0 – LL3.6 meteorites, with chondrules of higher metamorphic grade being younger and characterized by apparently longer evolution times between closure in highand lowtemperature iodine host phases (Figure 1). With a new calibration for the absolute age of the Shallowater reference based on experimental observations [5] and a broad range of comparisons of I-Xe ages with those provided by other chronometers [11, 12], the absolute I-Xe ages of the oldest chondrules (Semarkona) fall into the time interval for CAI formation 4567.2 ± 0.6 Ma [13], and could reflect chondrule formation, as was suggested by Swindle [7], rather than alteration. In this case the chondrules formed simultaneously with CAI’s or shortly thereafter (uncertainties for old Semarkona ages are rather high). Krot et al. [14] argued the entire range of I-Xe ages in the type 3 ordinary chondrites reflects long aqueous alteration. Clearly, chondrule type should be considered when question of primary or secondary origin of I-Xe ages is addressed. With this in mind, each chondrule intended for the comparison I-Xe – Pb-Pb study was separated into a few fragments. Thus, different fragments from the same chondrule could be analyzed by I-Xe and PbPb chronometers, with one fragment saved for the mineralogical studies and identification of iodine host phases by in situ laser extraction. Fragments designated for I-Xe analyses were irradiated by thermal neutrons in evacuated quartz ampoules. The Xe isotopic composition in each chondrule was measured in step-wise extractions following the same protocol to ensure proper comparison of the results.