Temporal Constraints on Archean Crustal Geodynamics and Neoproterozoic Glaciation

• Main Author: MacLennan, Scott Angus
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2019
• Subjects: Geochemistry; Geology
• ISBN: 1392398274; 9781392398272

Secular cooling of Earth's mantle today is achieved via convection, and conductive heat loss through the lithosphere. The formation of the oceanic lithosphere and its eventual demise through subduction is critical for the generation of new continental crust and Earth's climate stability over long timescales through the silicate weathering feedback. However, there are open questions about whether mantle cooling has always resulted in modern-style plate tectonics and whether there are periods in Earth history where negative feedbacks on climate fail to stop catastrophic change. This thesis addresses both questions using time as the primary variable.The Pilbara craton in northwest Australia contains a sequence of well preserved mafic and felsic volcanic rocks that envelop granitoid-gneiss domes and form a structural pattern that is ubiquitous in Archean (4 - 2.5 Ga) crust. These domal structures are hypothesized to be evidence that the due to a hotter Archean mantle, the planet was stuck in a stagnant lid configuration, with mantle convection below a globally continuous outer lithosphere. A large U-Pb thermochronologic dataset from the Paleoarchean eastern Pilbara craton shows that these domal structures record finite strain over ca. 500 Ma, with prolonged residence in the upper crust, at odds with models that propose primarily vertical motion within a horizontally stagnant lithosphere.The third chapter focuses on the Neoproterozoic Era (1000—541 Ma), where a total breakdown of the silicate weathering feedback is hypothesized based on observations that imply the entire planet was covered in ice (Snowball Earth). A key prediction of the Snowball Earth hypothesis— onset of glacial conditions in the tropics should be globally synchronous— is tested by acquiring high-precision age constraints on glacial diamictite in the Arabian Nubian Shield in northern Ethiopia. These dates show synchronicity of tropical glaciation at a ca. 1 Ma level of precision, supporting the Snowball Earth hypothesis. The fourth chapter tests whether all Neoproterozoic glacial diamictites are related to Snowball Earth events. High-precision dates on diamictites from southwest Virginia show this is not the case and require tropical glaciers at unconstrained altitude ca. 30 Ma prior to the first Snowball Earth, implying “icehouse” Earth conditions at ca. 751 Ma.