Lava Origins

Researchers analyze gas samples collected from Tanzanian volcano to determine processes at work in Earth's upper mantle

The local Maasai people in northern Tanzania refer to Oldoinyo Lengai volcano as the Mountain of God. Volcanologists revere it for its carbon-laden lavas, which erupt like motor oil from thorny-shaped hornito cones and leave behind a salty residue that flanks the volcano like snowballs.

Carbon-based lavas, known as carbonatites, are found throughout geological history; however, the Oldoinyo Lengai volcano, located atop the East African Rift, is the only place on Earth where they are actively erupting. These highly unusual lavas erupt at approximately 540°C (1004°F) and contain almost no silica and more than 50 percent carbonate minerals. Most lavas typically contain high levels of silica, which increases their melting point to above 900°C (1652°F) during eruption.

In 2005, a team of scientists from Scripps Institution of Oceanography at UC San Diego, University of New Mexico, and Centre de Recherches Petrographiques et Geochimiques (CRPG) in Nancy, France, obtained pristine samples in an effort to look deep inside the earth's upper mantle. The team reported their findings in the May 11 issue of the journal Nature.

The geochemical analysis reveals that an extremely small degree of partial melting of typical minerals in the earth's upper mantle is the source of the rare carbon-derived lava erupting from Oldoinyo Lengai.

"The gases reveal that the CO₂ is directly sourced from the upper mantle below the East African Rift," said David Hilton, professor of geochemistry at Scripps and coauthor of the paper. "These mantle gases allow us to infer the carbon content of the region's upper mantle to be around 300 parts per million, a concentration that is virtually identical to that measured below mid-ocean ridges."

Mid-ocean ridges are underwater mountain ranges where the seafloor is spreading due to tectonic plates moving away from one another. Rift valleys, such as the one where Oldoinyo Lengai volcano is located, and mid-ocean ridges are considered to be distinct tectonic regions. However, this study has shown that their source chemistries are identical, which led the scientists to suggest that the carbon contents of their mantle sources were not different beneath oceans and continents but due to partial melting of typical minerals located in the earth's mantle.

The geochemical analyses, some of which were conducted at Hilton's geochemical lab at Scripps Oceanography, revealed that magma from the upper mantle below both the oceans and continents is a uniform and well-mixed reservoir of "typical" volcanic gases such as carbon dioxide, nitrogen, argon, and helium.

"These findings are significant because they show that these extremely bizarre lavas and their parent magmas, nephelinites, were produced by melting of a typical upper mantle mineral assemblage without an extreme carbon content in the magma source," said CRPG geochemist Bernard Marty.

—Annie Reisewitz

June 2009

Eruption of carbonatite lava from the hornito of Oldoinyo Lengai volcano

Degassing hornitos and eruption of lava from Oldoinyo Lengai volcano.