Characterization and chronology of refractory inclusions (CAIs) in the CV3 chondrite NWA 4502
In this thesis I report the petrographic, elemental and isotopic (oxygen, Al-Mg, Rb-Sr and uranium) compositions as well as U-Pb ages of five coarse-grained igneous inclusions (Type A CAIs 1 and 4a, Type B CAIs 5, 6 and 7) from the CV chondrite North West Africa (NWA) 4502 the second largest CV chondrite so far after Allende. At a mineralogical level, NWA 4502 is rather pristine. In comparison with CAIs from Allende and other CV chondrites, CAIs from the NWA 4502 show many similarities with CAIs from other CV3 chondrites. However, all studied NWA 4502 CAI types have low levels of secondary hydrothermal alteration, thermal and shock metamorphism that affected CAIs in other CV chondrites. Even though NWA 4502 shows pristine parent-body characteristics, complexities discovered in some of the isotope systems especially Rb-Sr indicate significant imprint of terrestrial weathering. Acid leached residues of NWA 4502 CAI fractions yield a Pb-Pb age (4567.40 Â± 0.27 Ma) and initial 87Sr/86Sr (0.698886 Â± 0.000026) and an average initial 26Al/27Al (4.90 Â± 1.51 x 10-5) ratio consistent with values reported for Allende and Efremovka CAIs inspite of the differences in secondary processes between these CV chondrites. This leads to the conclusion that secondary processing did not bias the U-Pb isotopic ages of these CV CAIs. The evaluation of three acid leaching procedures aimed at removing weathering products and terrestrial contamination has shown that U-Pb dating of CAIs using the most radiogenic material can be achieved using mild acid leaching methods on coarse grained pyroxene and melilite-rich fractions. This study enabled us to expand the population of well characterized CV CAIs and provided an insight to better understanding the influence of secondary processes on isotopic systems and the chronology of CAIs.
Refractory inclusions, CV chondrites, Petrography, Chronology, Element and isotope geochemistry
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