Rheology of the East Africa Rift is not controlled by mantle hydrogen content

Abstract:

Plate tectonics on Earth requires strong lithospheric plates, weak underlying convecting asthenospheric mantle and weak zones in the lithosphere where rifting occurs. For several decades, hydrogen has been considered to be an important control on the strength of mantle minerals. However, recent experimental work has questioned this relationship and in addition, xenolith and magnetotelluric data from several cratonic regions have shown them to be surprisingly rich in hydrogen. In order to test the impact of hydrogen content on rheology, magnetotelluric data were collected over the East Africa Rift and the adjacent un-rifted Tanzanian Craton. The East Africa Rift is the best exposed continental rift on Earth and its contrasting behaviour with the stable Tanzanian Craton makes it an ideal location to test models of mantle rheology. The conductivity of mantle minerals is strongly controlled by their hydrogen contents so magnetotellurics is the ideal tool to measure mantle hydrogen contents in-situ. Results show that the Tanzanian Craton is extremely rich in hydrogen while the East Africa Rift is anhydrous both in magmatically rich and poor regions. Therefore, hydrogen is not an important control on plate strength in East Africa. The main contrast between the craton and the regions undergoing rifting is grain size, which was reduced in the rifted regions during Proterozoic deformation. Therefore, the localization of rifting appears to be controlled by grain-size. These results show that continued reactivation of deformed regions on Earth is likely not due to high hydrogen contents but to small grain size. The results should encourage further experimental work to reconcile the relationship between hydrogen content, grain-size and rheology.