The formation of the Archean lithospheric mantle was a key event in Earth history, resulting in the construction of the first continents, termed cratons, and laying the foundations of our habitable planet. Today, the cratonic lithosphere forms a cool, mechanically strong keel of depleted peridotite that extends 200-250 km below the surface. The lithosphere formed by extensive mantle melting, however, there are conflicting models for the environment in which melting took place. Efforts to understand the formation of the cratonic lithosphere are hampered by a lack of quantitative information on the depth of mantle melting and the original thickness of the Archean lithosphere.
Exsolved orthopyroxenes within peridotite xenoliths hold the key to constraining these critical parameters. We will reconstruct the original compositions of an extensive collection of exsolved orthopyroxenes and will use thermodynamic modelling to calculate their formation pressures and temperatures. This innovative approach will reveal the depth extent of Archean melting. Observations will be complemented by new experiments using fertile, depleted and silica-rich peridotite compositions, coupled with thermodynamic modelling, which will lead to a better understanding of phase relations during peridotite melting. In many locations, primary melting signatures are obscured by silica enrichment, the origin and significance of which is poorly understood. We will conduct melt-rock reaction experiments to test the hypothesis that silica addition occurred via interaction with ascending komatiite melt.
We will provide the first constraints on the vertical extent of the lithosphere in the Archean using geothermal gradients calculated from dated diamond inclusions, and of changes in lithospheric thickness using geothermal gradients calculated from garnet xenocrysts entrained in Proterozoic kimberlites. To achieve this we will perform cutting edge laser ablation U-Pb dating of garnet inclusions and will develop a new machine learning single crystal garnet geothermobarometer.
We will thus address several fundamental issues: the depth of Archean mantle melting; the origin of silica enrichment; and the link between cratonic peridotite and komatiite magma, providing key insight into the formation and evolution of the cratonic lithosphere.
