PhD defence. Austin Jarl Boyd. Geochemical Dynamics of the Isua Supracrustal Belt and the Eoarchean Earth

Candidate

Austin Jarl Boyd

Title

Geochemical Dynamics of the Isua Supracrustal Belt and the Eoarchean Earth

Academic Advisor

Minik T. Rosing, Natural History Museum of Denmark

Assessment Committee

Professor Lars Stemmerik (chairman), Natural History Museum of Denmark

Directeur de Recherche Maud Boyet, CNRS,The French National Centre for Scientific Research

Associate professor Thorsten Nagel, Department of Geoscience, Aarhus University

Abstract

The Isua Supracrustal Belt (ISB) contains rocks that were formed within the crust of the Eoarchean Earth. Petrological and geochemical analysis of these rocks are used to make inferences about the geodynamic setting of their formation and of the prevailing conditions of Earth. Highly deformed and compositionally altered felsic rocks comprise a large portion of the exposed rocks, but their origin has remained enigmatic. Newly discovered granular felsic rocks that have only experienced minor deformation, contain structures that are consistent with mafic magma infiltration into a felsic magma. Zircons that were recrystallized during mafic magma infiltration record a U-Pb age of 3808 ± 1.2 Ma (2σ). Light oxygen isotopic compositions of zircons from the ISB felsic rocks are consistent with melts being derived from hydrothermally altered lower crust. Hafnium isotopic composition of these zircons indicate that their source melts were formed from fusion of rocks with a CHUR-like composition, but that Hf incorporated into late-stage zircons was partially derived from coexisting mafic magma that was derived from depleted mantle. The data is consistent with the predominantly highly deformed felsic rocks of the outermost ISB consisting of one suite with a protolith that is analogous to the felsic rocks sampled from the magma mingling association. This suggests that felsic and mafic magma formation was generally concurrent during the formation of the ISB. Major and trace elemental compositions and Nd- and Hf-isotopic compositions indicate that felsic melts were formed from rocks that are unrelated to the mantle source of the comingling mafic melts and point towards a tectonic setup involving subduction of hydrated mafic rocks. Elsewhere within the ISB, mafic pillow basalts contain amygdules with mineral assemblages indicating two-stage seafloor-alteration consistent with their formation in a divergent tectonic setting. A mobile crust is also indicated by positive Δ33S measured in sulfides, which are most likely derived from subduction of sediments. Some sulfides also contain sulfur isotopic compositions consistent with biological fractionation of sulfur.