LITHO3 · Quantifying the formation and evolution of the Archaean lithospheric mantle

The formation of the Archean lithosphere was a key event in Earth history, resulting in the construction of the first continents, or cratons. 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 and silicate inclusions in diamond hold the key to constraining these critical parameters. We will reconstruct the original compositions of an extensive collection of exsolved orthopyroxenes and use our recently published thermodynamic model to calculate their formation pressures and temperatures. This innovative approach will reveal the depth extent of Archean melting. We will provide the first constraints on the vertical extent of the lithosphere in the Archean using geothermal gradients calculated from dated diamond inclusions. To achieve this, we will perform cutting edge laser ablation U-Pb dating of garnet inclusions, this challenging application has yielded promising results for xenolithic garnet. Observations will be complemented by new experiments using fertile, depleted and silica-enriched compositions, coupled with with thermodynamic modelling, which will lead to a better understanding of phase relations during peridotite melting. The origin and significance of silica enrichment 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 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. LITHO3 will provide unprecedented insight into the formation and evolution of the cratonic lithosphere.