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These pockets, lying roughly 2,900 kilometers (1,800 miles) below the surface, have been known for many years but previously lacked an explanation of how they formed.
The relatively small rock bodies are termed “ultra-low velocity zones” because seismic waves greatly slow down as they pass through them. Geoscientists have thought the zones are partially molten, yet the pockets are puzzling because many are observed in cooler regions of the deep mantle.
“These small regions have been assumed to be a partially molten version of the rock that surrounds them,” said Mingming Li, lead author of the study, which was published today in the journal Nature Communications. “But their global distribution and large variations of density, shape and size suggest that they have a composition different from the mantle.”
Li joined ASU’s School of Earth and Space Exploration this month as an assistant professor. He was a graduate student of former ASU Associate Professor Allen McNamara, also a co-author on the paper; McNamara is now at Michigan State’s Department of Earth and Environmental Sciences. The additional co-authors are ASU Professor Edward Garnero and his doctoral student Shule Yu.
“We don’t know what ultra-low velocity zones are,” said McNamara. “They are either hot, partially molten portions of otherwise normal mantle, or they are something else entirely, some other composition.”
Because seismic evidence allows both possibilities, he said, “We decided to model mantle convection by computer to investigate whether their shapes and positions can answer the question.”
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Image via Edward Garnero, Arizona State University
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