Özaydin, Sinan1, Selway, Kate1
1Department of Earth and Environmental Sciences, Macquarie University, Australia
Cratons provide us sparse clues about their composition and evolution through xenoliths and the few geophysical methods that can penetrate the cratonic lithosphere. Alone, these methods do not provide enough information to fully understand cratons so it is vital that they be interpreted together so that the fullest possible understanding of cratons can be developed.
In this talk, we focus on the southern African lithosphere, which is arguably the most well-studied cratonic region on Earth. Comparatively voluminous kimberlite magmatism has produced large quantities of mantle xenoliths and xenocrysts that have been used to interpret the composition and evolution of the southern African cratons. However, questions remain about whether these exhumed mantle rocks are representative of the wider lithospheric mantle.
To address the question of cratonic composition, we compare the southern African mantle xenolith and xenocryst data with new, 3D magnetotelluric (MT) models produced from the SAMTEX dataset. Detailed comparisons between the geochemical and MT data around the Jagersfontein and Kimberley kimberlite pipes show that modal mineral compositions and water content measurements from the xenoliths generally agree with those we would interpret from the geophysical data, subject to uncertainties in the geotherm and experimental constraints. However, the water contents interpreted from the MT data and those from the xenoliths do not agree uniformly, suggesting that the region has experienced some localised metasomatism at the scale of the kimberlite pipes that has not affected the mantle more regionally.
In addition to the detailed comparisons around kimberlite pipes, we also consider the broader implications of the MT model for southern African lithosphere composition. As has been observed for most cratonic regions, the MT model over the southern African cratons shows considerable heterogeneity, showing that cratonic compositions must also be heterogeneous. These conductivity contrasts do not generally follow the surface expressions of tectonic boundaries and are more likely to reflect metasomatic enrichment and melting depletion events than lithospheric architecture related to continental assembly. Outcropping kimberlite pipes avoid the regions of highest mantle conductivity, suggesting an interplay between the processes of mantle metasomatism, which produce the strong mantle conductors, and kimberlite magmatism.
Sinan Özaydin is currently in the final stages of his PhD at Macquarie University. He has developed methods for quantitative interpretation of magnetotelluric data, including publishing the open-source software MATE, and has applied those methods to understanding the southern African lithosphere.