Master of Science (MS)
Geology and Geophysics
We know very little about the tectonic setting present during the Hadean, but based on studies of surviving Hadean zircons, we know that a Hadean protocrust must have been established by at least 4.4 Ga (Kemp et al. 2010), and it must have also been able to accommodate minimum melt conditions (Harrison 2005). Tectonic models for Earth during the Hadean and Archean follow two trends: uniformitarianism and non-uniformitarianism. Uniformitarian models (following the famed geological concept that the present is the key to the past) argue that Hadean zircons and Archean rocks formed via processes akin to modern-style oceanic crust production and subduction. Non-uniformitarian models speculate that a different style of tectonics, likely dominated by magmatism and vertical crustal / lithospheric kinematics, created the Hadean-Archean geological record. In order to help test these models, I studied the East Pilbara craton, an extremely well preserved Archean terrane. The craton consists of eight synclinic granitoid domes surrounded by greenstone belts. This dome-and-keel structure has been interpreted by most workers as the result of vertical tectonics, with prominent research teams arguing that it records buoyancy-driven partial convection within the crust caused by crustal heating by an underlying mantle plume. To examine the conditions present in the East Pilbara Craton and provide a direct point of comparison to the Hadean zircon record, I used titanium-in-zircon geothermometry and zircon U-Pb age dating of zircons from the granitoid domes. These approaches allow a direct comparison with published results from Hadean zircons. The Pilbara zircons show temperatures hotter than Hadean zircons by 400-700° C. The crystallization temperatures estimated from seven Pilbara samples of ages ranging from 3.40-3.21 Ga show a general cooling trend with a slight increase in temperature c. 3.31 Ga, which could reflect cooling after a crustal heating event. The crustal conditions of the Paleoarchean East Pilbara craton are evidently different than those recorded by the preserved Hadean zircons, and thus may represent a tectonic regime change between the Hadean and Paleoarchean.
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Webb, Andrew Kody, "Archean Geodynamic Conditions Using P-T Constraints of the East Pilbara Craton TTGs" (2016). LSU Master's Theses. 4430.
Available for download on Saturday, February 23, 2019