Mantle-derived Gases Found in Zambian Hot Springs

Gases collected from boiling mineral springs in Zambia contain the chemical signature of having come directly from the Earth’s mantle. The finding provides geochemical evidence that mantle fluids are reaching the surface through faults in the Kafue Rift zone. Researchers sampled gases from hot springs lying in the green thickets of the Kafue Rift southern boundary fault zone.

Rūta Karolytė at the University of Oxford and her colleagues analysed gas samples collected from five hot springs and three geothermal wells in central Zambia. Helium and carbon isotope ratios in the gases from the Zambian hot springs and wells are the same as those that exist deep beneath Earth’s crust.

Fluids from the mantle up to 190 kilometres below the springs are making their way to the surface.

“What our data confirms is that this system is currently ‘awake’ and geologically active,” said Rūta Karolytė. The Kafue Rift in Zambia is part of a 2500-kilometre-long rift zone stretching from Tanzania to Namibia and possibly out into the Atlantic Ocean. Geologists observed geography, low-gravity anomalies, high sub-surface temperatures and low-level seismicity in the Kafue Rift area.

The earliest stages of continental rifting release gases that have accumulated in the rocks for millions of years, among which is helium. “It’s hard to find these tectonic conditions that are just right to concentrate and release helium in a way that it can be captured,” said Rūta Karolytė.

Karolytė noted that while the rift is active, it is uncertain whether it will evolve into a new ocean basin, a process that would take more than 100 million years if it occurs at all. Patrice Rey at the University of Sydney, Australia, said that despite the absence of active volcanoes and significant seismicity, there are many signs in the region that the landscape is tectonically active.

The new geochemical evidence from the hot springs reveals that the Kafue Rift is an early-stage continental rift where mantle fluids rich in primordial helium-3 are rising through faults. “It is reasonable to think that the Kafue Rift may evolve into a plate boundary sometime in the future,” said Patrice Rey. 1799564.