The Role Of Hydrogen In Cratonic Growth And Stability: Lessons From The Tanzanian Lithosphere

Hydrogen strongly influences mantle rheology and is therefore likely to be an important factor in the growth and stability of cratons. Hydrogen also strongly affects electrical conductivity so, through using magnetotelluric data, it is possible to infer the hydrogen content of the lithospheric mantle in-situ and test models of craton formation. Tanzania is an ideal natural laboratory to test hypotheses on lithospheric hydrogen content since it contains regions representative of very different tectonic regimes. These include the stable Tanzanian Craton and the East Africa Rift that is currently reworking lithosphere that has been previously deformed in the East African Orogeny. A mantle plume underlies the Tanzanian Craton and is thought to be responsible for the rifting. The Tanzanian lithosphere is well sampled by voluminous xenoliths which allow lithospheric composition and the geotherm to be constrained. With these factors constrained, magnetotelluric data can be used to determine hydrogen content with more confidence than is possible in areas without xenolith controls. Hydrogen contents were calculated for two locations in Tanzania: the first was in the stable central Tanzanian Craton and the second was on the eastern margin of the craton where incipient rifting is occurring. The central Tanzanian Craton was found to have a high lithospheric mantle water content of ~10-2 wt% which is comparable to that of the oceanic asthenosphere and is hard to reconcile with the long-term survival of the craton. It is possible that the water was introduced into the lithosphere recently by kimberlite volcanism or that, if the lithosphere has had a high water content throughout its history, the central craton has been shielded from deformation by weaker orogens that surround it. The eastern margin of the craton has a water content of 10-3 to 10-4 wt% throughout much of the lithospheric mantle that decreases to 10-4 to 10-5 wt% at the base of the lithosphere and at depths corresponding to the uppermost plume head. Xenolith data show evidence for partial melting of the plume head and the base of the lithosphere in this dehydrated region. The partial melting and dehydration of a plume head beneath a craton is a present-day observation of the processes that may have formed cratonic roots.