Evolution of the Non-Uniformitarian Earth

ongoing

The project will use a set of innovative techniques to study the two major events that precede classical, uniformitarian geology. We will search for crystallised remnants of the primordial terrestrial magma ocean and examine the consequences of our observations for mantle dynamics. Crystallisation of the dominant lower mantle phase, bridgmanite, imparts a Mg and Si isotopic fingerprint to cumulates and evolving liquid of the magma ocean. Identifying the relicts of these small but distinctive fractionations requires application of our bespoke, high precision analytical protocols. As an integral part of this work, we will undertake coupled first principles calculations and petrological experiments to provide a benchmark documentation of the isotopic fractionations experienced by these two most abundant cation-forming elements in the silicate Earth. We will also date the global onset of plate tectonics from complementary geochemical signatures in two little-studied detrital minerals; Pb enrichment in continental K-feldspars and Mo depletion in rutiles from exhumed fragments of subducted crust. This work uses ground-breaking methodologies made possible by our unique, tribrid (mass-filter:collision-cell:multicollector) mass-spectrometer, Proteus. We will further develop this technology in collaboration with Thermo Fisher to produce a yet more capable next generation instrument. Our tribrid mass-spectrometer allows us to date single, detrital K-feldspars by laser ablation, using Rb-Sr internal isochrons constructed with analyses of host feldspar and sodic perthite exsolutions. Coupled with in situ Pb isotope analysis, we can calculate the U/Pb of the feldspar’s crustal protolith, which diagnostically tracks contributions from subduction.