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Archean greenstone belts contain low grade metamorphic volcanic sequences with localised examples of preserved original volcanic textures and relict magmatic minerals, which are invaluable for inferring volcanic processes and by extension magma genesis. A feature observed in many Archean greenstone belts but rare in modern volcanic rocks is variolitic texture with millimetre- to centimetre-sized leucocratic globular structures (varioles), in mafic lavas. Models proposed to explain this texture include rapid undercooling, magma mingling and liquid immiscibility. Here, we investigate samples of Paleoarchean variole-bearing pillow lavas from the ca. 3469 Ma Mount Ada Basalt of the Pilbara Supergroup, Western Australia. The samples contain relict magmatic clinopyroxene and spinel and preserve sharp contacts between varioles and host. No plagioclase is observed in the samples. The clinopyroxene mineral composition is indistinguishable between in the varioles and host. This is despite the variole material having lower MgO and Fe2O3(total) and higher SiO2 and Al2O3 concentrations. However, the variole and host materials have indistinguishable TiO2, Cr2O3, NiO2 and Sm-Nd isotope systematics. These observations are consistent with an immiscibility reaction from a parental melt to a hydrous fluid phase and conjugate mafic liquid, where minerals that formed before the reaction become incorporated into both endmembers where they continued to grow by diffusive exchange between the hydrous fluid and silicate melt. Two lines of evidence indicate that hydrous flux exsolution occurred due to H2O saturation in the precursor magmas, including: (1) the lack of plagioclase, for which crystallisation is supressed at high H2O content; and (2) Nb/Th < 7, which indicates a hydrated mantle source. The variole-bearing pillow lavas are part of the 10-Myr-duration Coongan Subgroup volcanic cycle and occur interspersed with tholeiite basalts and minor komatiites of the Mount Ada Basalt and mafic

This thesis examined how the oldest core of the Australian continent formed more than 3.5 billion years ago. Unraveling the complex tectonic and petrologic history of ancient rocks of the East Pilbara in Western Australia provided important new insights into how and why continents developed on our hot, young planet. A multifaceted methodology of field-based structural geology, uranium-lead (U-Pb) dating of zircon, whole-rock geochemistry, and petrology improved our understanding of early Archaean mass and heat transfer, including the history of the associated planetary surface environment, which hosted some of the earliest life on Earth.

We present a microstructural and Usingle bondPb systematics study comparing pristine, thermally annealed (TA) and chemically abraded (CA) ~ 3500 Ma zircon from a quartz-dioritic gneiss, with the aim to improve pre-analytical workflows for more accurate and precise LA-ICP-MS Usingle bondPb dating of ancient zircon. Four zircon domains are identified: i) low- to medium-U concentric-oscillatory zoned cores, ii-iii) two porous high-U outer alteration domains, and iv) low-U narrow inward growing recrystallization rims. Raman spectroscopy on the pristine zircon reveals a positive correlation between increasing structural damage and U content. The porous high-U outer domains show a drastic increase in non-formula Ca above estimated amorphous fractions of ~ 0.8, which we ascribe to hydrothermal alteration of high-damage zircon characterized by percolating networks of amorphous areas. Upon treatment (TA, CA), hyper-spectral CL and Raman spectroscopy suggest structural recovery of point defects, but not full repair of high-damage amorphous areas. Calculated Raman radiation damage ages suggest that natural annealing affected all domains at ~ 500 Ma, consistent with negligible laser ablation matrix effects comparing pristine and treated cores. We show that reverse discordance in pristine and TA cores is not an analytical artifact. High-U alteration domains are normal discordant and were partially reset at ca. 3410 Ma. Upon CA, Usingle bondPb discordance and scatter are reduced in the cores, yielding intercepts of 3503 ± 14 Ma and 560 ± 550 Ma (MSWD = 1.0). The lower intercept matches the timing of Pb-loss recorded in the alteration domains and the Raman radiation damage age. We argue that short distance Pb redistribution within the cores, which led to reverse discordance, was controlled by Usingle bondPb systematics recording both the crystallization age and the timing of Pb redistribution. Some excess Pb was redistributed from the partially re-set alteration domains into the c