Year

2020

Degree Name

Bachelor of Science (Honours)

Department

School of Earth, Atmospheric and Life Sciences

Advisor(s)

Dr Nicolas Flament

Abstract

Economic diamonds are primarily formed in Archean cratons as their thick lithospheric keels provide appropriate temperature and pressure gradients for diamond formation and stability. Upwelling beneath Archean keels (~ 200 km depth), kimberlitic magmas erupt at geologically high velocities (~ 20 m s-1 ) entrapping mantle xenoliths, including diamonds, and depositing them in surface features called kimberlite pipes. Kimberlite pipes are therefore greatly sought after, being the primary global source of economic diamonds. Through the use of tomographic models, studies have spatially correlated the global kimberlite distribution with fixed deep mantle large low-shear velocity provinces for the last 320 Ma. However, research greatly suggests these basal thermochemical structures to be mobile features, contradicting the use of tomographic models over time. Tomographic models use seismic observations to map present mantle structure, whereas flow models predict the evolution of mantle temperatures based on inputs such as subduction zone evolution and mantle density. Therefore, flow models are able to map basal thermochemical structures as mobile features. This report uses both tomographic models and flow models to investigate the mobility of basal thermochemical structures by comparing each model with the global kimberlite record. Findings have economic application as tomography and flow models that best match the kimberlite record are used in the production of economic kimberlite prospectivity maps. To test the mobility of basal thermochemical structures, flow model and tomography model basal thermochemical structures are superimposed with Archean craton geometries and tectonic reconstructions for the last billion years. Intersections of these two features - prospectivity areas - indicate regions with the potential for diamondiferous kimberlite eruptions. Prospectivity areas are then compared against the global kimberlite distribution, calculating the prospectivity area distances to kimberlites as a metric of success. Flow models prove to have equal or smaller distances to kimberlites than tomographic models. Additionally, flow models better match key periods kimberlite magmatism, such as South Africa and Siberia through the breakup of Pangea - the climax of kimberlite eruptivity. Overall, this report finds flow models to better match the kimberlite record, supporting the theory of mobile basal thermochemical structures.

FoR codes (2008)

040313 Tectonics, 040402 Geodynamics, 840103 Diamond Exploration

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.