Drawing from a Primordial Reservoir Deep in the Earth

October 9, 2017

The Earth’s mantle — the layer between the crust and the outer core — is home to a primordial soup even older than the moon. Among the main ingredients is helium-3 (He-3), a vestige of the Big Bang and nuclear fusion reactions in stars. And the mantle is its only terrestrial source.

Scientists studying volcanic hotspots have strong evidence of
this, finding high helium-3 relative to helium-4 in some plumes, the
upwellings from the Earth’s deep mantle. Primordial reservoirs in the deep Earth, sampled by a small number of volcanic hotspots globally, have this ancient He-3/4 signature.

Inspired by a 2012 paper that proposed a correlation between such hotspots and the velocity of seismic waves moving through the Earth’s interior, UC Santa Barbara geochemist Matthew Jackson teamed with the authors of the original paper — Thorsten Becker of the Jackson School of Geosciences and Jasper Konter of the University of Hawaii — to show that only the hottest hotspots with the slowest wave velocity draw from the primitive reservoir formed early in the planet’s history.

Their findings appeared in February 2017 in the journal Nature.
For their study, the researchers used the latest seismic models of the Earth’s velocity structure and 35 years of helium data. When they compared oceanic hotspots with high levels of He-3/4 to seismic wave velocities, they found that these represent the hottest hotspots, with seismic waves that move more slowly than they do in cooler areas. They also analyzed hotspot buoyancy flux, which can be used to measure how much melt a particular hotspot produces.