The co-evolution of Earth's crust and hydrosphere : a silicon isotope perspective

Abstract

The silicon isotope system is a well-established geochemical tool for tracing processes influenced by the interplay of Earth’s present-day crust and hydrosphere. While there is good understanding of stable silicon (Si) isotope systematics for high- and low-temperature processes in the Phanerozoic eon, these are poorly constrained for early Earth processes. Particularly, the Si isotope composition of Archaean and Proterozoic crustal materials is a scarce record mainly consisting of silica precipitates. However, recent investigations of Si isotopes in Archaean granitoids linked heavy isotopic signatures to seawater-derived sources, invoking the hydrosphere in forming Earth's earliest continental crust.

Motivated by recent research gaps, this thesis aims to explore crust-hydrosphere interactions by establishing the Si isotope compositions of Archaean granitoids, specifically tonalite-trondhjemite-granodiorites (TTGs), and other lithologies representative of more than one-third of Earth history. This thesis presents outcomes from five Si isotope studies on globally diverse, ancient silicates. Here we find Eoarchaean igneous rocks from Greenland were influenced by supracrustal fluids, necessitating a primeval hydrosphere in forming early continents. We explore Si isotope behaviour during ancient partial melting from an Archaean migmatite from Ontario and find that TTG sources were likely seawater-silicified.

This thesis records a secular homogenising of Si isotopes in the ancient upper continental crust from global glacial diamictites, supporting craton stabilisation after ~3.0 Ga. We also show Archaean –Proterozoic Fennoscandian weathering crusts imply no significant Si isotope trends at the Great Oxidation Event and highlight local controls instead. Finally, this thesis explores the Archaean marine silica cycle with a model for the Si isotope evolution of seawater, aligning with data that require a heavy Si isotope signature in the early oceans.

In total, this thesis contributes a more robust temporal Si isotope record of ancient crustal materials and provides a greater understanding of the connection between Earth’s earliest crust and hydrosphere.