by Mary Caperton Morton Thursday, August 10, 2017
During the Archean, between 4 billion and 2.5 billion years ago, Earth’s super-heated young mantle produced a unique type of lava known as komatiite. In a new study published in Nature Geoscience, researchers looking at 90-million-year-old komatiites in Costa Rica — by far the youngest komatiites ever found — suggest the modern mantle may still harbor pockets of intense heat reminiscent of early Earth.
Today, the ambient temperature of the mantle is about 1,300 degrees Celsius, with hotter and cooler regions varying between 1,500 and 1,100 degrees. Past work found that komatiite rocks, such as those found in the Tortugal terrain in Costa Rica, may have originated in mantle regions as hot as 1,700 degrees Celsius, which are thought to have existed in the Archean. Komatiites themselves aren’t all that rare, but the age of the new samples is, says Oliver Shorttle, a geologist at the University of Cambridge in England who was not involved in the new study. “Earth has routinely erupted rocks from sources this hot in the distant past, but it’s unusual to see them in such a young setting,” he says.
When the Tortugal rocks were first characterized 20 years ago in a study in Geology, researchers noted that they were unusually ultramafic — low in silica and high in magnesium — for the setting. This volcanism in Costa Rica is thought to be linked to the hot spot fueling the Galapagos volcanoes 1,400 kilometers to the southwest. In the new study, lead author Jarek Trela, a geoscience graduate student at Virginia Tech, and his colleagues identified distinctive spiny and crisscrossed crystals of olivine in the lava. The composition of the olivine in the Costa Rican komatiite samples helped the team determine the temperature at which the magma melted in the mantle.
“Olivine can be used as a thermometer to measure how hot these lavas were when they began to cool,” says Esteban Gazel, a geologist also at Virginia Tech and a co-author of the study. “At higher temperatures, olivine will incorporate more aluminum into its structure, and spinel will incorporate more chromium. If you know how much of these elements are present in each mineral, then you know the temperature at which they crystallized,” he says. Using this method, the team found that the Tortugal komatiites crystallized at temperatures higher than 1,600 degrees Celsius, making this the hottest lava younger than 2.5 billion years ever found.
“Komatiites are basically superhot versions of Hawaiian-style lava flows,” Gazel says. But, he says, komatiites are so hot that they would have “glowed white instead of red.” Lava flows in Hawaii typically originate from melt at temperatures of about 1,100 degrees Celsius.
The findings suggest that Earth’s mantle may still host portions of very hot temperatures similar to those found on early Earth that can produce komatiite-like melts in modern times, Gazel says. If true, these are “pockets of mantle that have survived eons of circulation and are still as hot as the primordial Earth,” he says. “Some models indicate that everything in the mantle convects and mixes, but then we keep finding really strong isotopic evidence that there are areas along the core-mantle boundary that are untouched by circulation.”
The next step will be to take a closer look at isotopic signatures in the Tortugal komatiites, concentrating on isotopes of tungsten and osmium that can reveal clues about the temperature and composition of the source material of the lava, although pinpointing the location or depth of the source will likely prove impossible, says Shorttle, who wrote an accompanying commentary in Nature Geoscience. “It’ll be interesting to see what avenues of inquiry this group and others take to investigate the provenance of the source region of these lavas,” Shorttle says. “It certainly has some implications for the evolution of Earth’s mantle as we know it.”
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