In Africa, a rift has developed and it could be enough to split the continent in two.
It’s a nearly invisible battle of massive tectonic forces, but for scientists like Scripps Institution of Oceanography, UC San Diego geochemist David Hilton, it is possible to watch the action thanks to the lighter-than-air gas helium.
Helium comes in two isotopic forms. Helium-4 is continuously produced by radioactive decay of elements like uranium and thorium. In contrast, helium-3 is not produced in the solid Earth and its inventory reflects the amount originally incorporated into the earth when the planet was formed 4.5 billion years ago. The planet’s supply of helium is continuously declining as it moves slowly from the interior mantle of the earth to the crust, then into the atmosphere before it is lost to space. However, the ratio of helium-3 to helium-4 in the earth is constantly falling because helium-4 is always being created.
The different origins of these two helium isotopes are why geochemists like Hilton find it so useful. Higher 3He/4He ratios suggest that the helium is from the Earth’s mantle, which traps a greater proportion of 3He-rich primordial gas. Lower ratios suggest that helium comes from the Earth’s crust where uranium and thorium are concentrated. Hilton takes advantage of this isotopic contrast to learn the origins of rock and geothermal gases all over the planet.
Most recently, Hilton’s team traveled to East Africa and used helium isotopes to solve a puzzle about how the mountains there were created.
“The rift valley is tearing East Africa apart,” said Hilton. “In another 50 million years, we’ll have another ocean there.”
The East African Rift valley bisects two high plateaus that are the size of entire countries: the Ethiopia Dome and the Kenya Dome. Geologists thought that two mantle plumes, deep within the earth, pushed up these two plateaus. But in the 1970s, Scripps Oceanography scientist Harmon Craig discovered something puzzling: there was a great deal of helium-3 only in the Ethiopia Dome. If these two plateaus have the same origin, they should have similarly high helium isotope ratios. But the lower 3He/4He ratio of the Kenya Dome is inconsistent with the presence of a mantle plume. So what is the origin of this huge plateau in the southern part of the East African Rift valley?
There are two ways to measure the helium ratios in magma. Hilton and his colleagues can collect volcanic rocks if they’re available. Otherwise, he measures helium in the gas bubbling from geothermal vents and hot springs.
To collect rocks from a volcano, first he checks whether the volcano is likely to erupt any time soon.
“If there's been any activity in the last few months,” Hilton said, “forget it.”
The Oldoinyo Inyo Lengai Volcano in Tanzania, site of Hilton’s recent trip, is a six-hour climb to the top.
“We have dinner and then a nap, then start climbing around midnight so we can summit the volcano at dawn. Then we have a whole day to work. This avoids climbing in the peak equatorial heat of the early afternoon,” Hilton said, adding that the return is even more exciting. “Sometimes you can almost ski downhill on your boots if there is enough scoria [tiny volcanic pebbles].”
Sampling geothermal vents is a different matter. Where CO2 emerges from geothermal vents, it can collect in depressions because it is heavier than air. These mazukus, Swahili for ‘evil wind’, are lethal areas that kill birds and mammals. Sometimes, though, the mazukus are exactly where Hilton wishes to sample. How does he get sampling equipment inside a deadly mazuku?
“Very carefully,” Hilton said. “We wear gas masks, of course.”
After two separate expeditions to the East African Rift, climbing volcanoes at midnight and venturing near lethal geothermal vents, Hilton thinks he has an answer about the formation of the two plateaus along the East African Rift. By examining small olivine crystals that form in magmas, his group discovered elevated 3He/4He ratios in the Kenya Dome similar to those found in the Ethiopia Dome, supporting the hypothesis that these two plateaus are products of the same thing: a huge African superplume underlying the entire East African Rift.
Previous researchers only measured helium at geothermal vents and hot springs, so they missed the answers that Hilton unlocked by examining the rocks. “Olivine crystals are like bottles for volcanic gases,” said Hilton.
They trap the helium signature until an inquisitive and adventurous geochemist comes along to unlock the secrets.
– Jill Harris is a fifth-year graduate student in the laboratory of marine biologist Jennifer Smith