TERRESTRIAL AND COSMOCHEMICAL APPLICATIONS OF U-PB ZIRCON, BADDELEYITE, AND PHOSPHATE CHRONOLOGY
Shaulis, Barry Jon 1975-
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Three different geological and cosmochemical problems are explored by the in situ analysis and U-Pb and Pb-Pb dating of zircon, baddeleyite, and phosphates by LA-ICPMS. These are: 1) In situ U-Pb dating of zircons from tuffs of the Stanley Group, Ouachita Mountains range in age from 328-320 Ma. These ages constrain the timing of collision for the Ouachita Orogeny during the formation of Pangaea, constrain the timing of the onset of flysch deposition in the Ouachita Trough at ~323 Ma, and indicate that sedimentation rates over this interval increase from <1 m/My to >300 - 1400 m/My. 2) In situ U-Pb and Pb-Pb dating of baddeleyite and zircon and EPMA analysis of basaltic meteorites from the NWA 773 clan of lunar meteorites show that a) the magnesian and ferroan lithologies in the meteorites crystallized at the same time, 3.1 Ga, b) the magnesian and ferroan lithologies have distinct compositions, c) an older 3.95 Ga clast within the polymict breccia of NWA 773 suggests a proximal relationship between the older and younger components at the impact location on the moon, and d) a 2718 ± 44 Ma U-Th-Pb age of phosphates in NWA 773 is evidence for a younger thermal event, and is in agreement with published Ar-Ar ages of 2.7-2.8 Ga. 3) In situ U-Pb and Pb-Pb dating of 50 zircon and baddeleyites from 9 brecciated, unbrecciated, and polymict eucrites show that a) there are two peaks of magmatism on the eucrite parent body at 4552 Ma and 4501 Ma, b) magmatism lasted 85-90 Ma, ~40 Ma longer than current models, and c) the eucrite parent body was hot enough to form zircon and baddeleyite and recrystallize zircon for ~300 Ma after differentiation of the parent body. 4) Initial methods for the analysis of phosphate minerals were developed by performing a series of round-robin test of various phosphate minerals and zircons. The results of the test show that grouped ages of standards are within 2-5% of their accepted ages, which is comparable to other labs. Through these tests we also found another phosphate, the Bear-Lake Apatite from the Bancroft Terrane in Canada, could also be used as an acceptable standard material in the U-Th-Pb dating of phosphates. An initial test of this technique with an Apollo 17 sample yielded an age of 3936 ± 46 Ma, which is comparable to the Ar-Ar ages between 4030 ± 20 Ma and 3871 ± 40 Ma for this sample.