Isotope analysis of gas from geothermal springs in Zambia could show that a new continental rift is forming, scientists say. Unexpectedly high helium isotope ratios indicate that a weakness in the Earth's crust has broken through to reach the mantle beneath. This rift could eventually become a new tectonic plate boundary. In the meantime, opportunities for geothermal energy could boost local economies.
"The hot springs along the Kafue rift of Zambia have helium isotope signatures which indicate that the springs have a direct connection with the Earth's mantle, which lies between 40 and 160km below the Earth's surface," said Prof Mike Daly of the University of Oxford, an author of the article in Frontiers in Earth Science. "This fluid connection is evidence that the fault boundary of the Kafue Rift is active and therefore the Southwest African Rift Zone is too — and may be an early indication of the break-up of sub-Saharan Africa."
The bubbling gun
The Kafue Rift is part of a 2,500km long zone of rifts that runs from Tanzania to Namibia and may reach the mid-Atlantic ridge. The scientists' attention was drawn to it by topography that suggested a possible new rift, as well as high levels of geothermal anomalies and hot springs. But to confirm a new rift, scientists would need to show that it had broken through the Earth's crust: evidence that fluids had escaped from the liquid mantle to the surface.
"A rift is a large break in the Earth's crust that creates subsidence and associated elastic uplift," said Daly. "A rift may become a plate boundary, but commonly a rift's activity ceases before the point of lithospheric break-up and plate boundary formation."
The scientists visited eight geothermal wells and springs across Zambia: six in the suspected rift zone, and two outside it. They took samples of gas from freely bubbling water, and analyzed these in the laboratory to identify the isotopes of each element present. Isotopes are different forms of an element, which are present in different proportions in the crust and in the mantle. So by testing the isotopes present in the gas, the scientists could detect the presence of gas derived from mantle fluids at the surface. They compared these to readings taken from the East African Rift System, an ancient, well-established rift.
The Earth on the move
The scientists found that the gas from the Kafue Rift, but not the gas from the springs outside the rift, contained a ratio of helium isotopes comparable to samples taken from the East African Rift System. The helium couldn't have come from the atmosphere, because the ratios of helium isotopes weren't consistent with those found in the air, or just from the crust, because there was too much of the mantle-sourced helium isotope present for that. The Kafue Rift samples also contained a proportion of carbon dioxide consistent with carbon dioxide found in mantle fluids. Helium isotopes provide a signal of early-stage rifting: using the East African Rift System as a model, scientists predict that with time, carbon dioxide will become more prominent as volcanic centers develop.
The discovery that the Kafue Rift is active could have important economic implications. Early-stage rifts can provide geothermal energy and access to helium and hydrogen where they are not diluted by the volcanic gases. However, it could have even more significant implications for the future shape of Africa.
"Many of the features of the Great Rift Valley of Kenya offer compelling reasons why East Africa should ultimately become a line of major continental break-up," said Daly. "But the rate of rifting of the East African Rift System is slow. On almost all sides of Africa there are mid-ocean ridges tending to inhibit east-west or north-south extension, so break-up and spreading does seem to struggle to establish itself. The Southwestern African Rift System could be an alternative. It has the required rift-related features, and regional basement fabrics — inherent weaknesses in the crust — favorably aligned to the surrounding mid-ocean ridges and continental geomorphology. This relationship may offer a much lower strength threshold for continental break-up."
"However, this study is based on helium analyses from one general area in the Southwest African Rift System, which is thousands of kilometers long," cautioned Daly. "This early study is being followed by more extensive studies, the next step of which will be completed this year."
