Older than the moon

February 06, 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 University of Texas at Austin 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 appear in the journal Nature.

"We used the seismology of the shallow mantle -- the rate at which seismic waves travel through the Earth below its crust -- to make inferences about the deeper mantle," said Jackson, an assistant professor in UCSB's Department of Earth Science. "At 200 km, the shallow mantle has the largest variability of seismic velocities -- more than 6 percent, which is a lot. What's more, that variability, which we hypothesize relates to temperature, correlates with He-3."

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. In Hawaii, the Galapagos Islands, Samoa and Easter Island as well as in Iceland, hotspots had high buoyancy levels, confirming a basic rule of physics: the hotter, the more buoyant.

"We found that the higher the hotspot buoyancy flux, the more melt a hotspot was producing and the more likely it was to have high He-3/4," Jackson said. "Hotter plumes not only have slower seismic velocity and a higher hotspot buoyancy flux, they also are the ones with the highest He-3/4. This all ties together nicely and is the first time that He-3/4 has been correlated with shallow mantle velocities and hotspot buoyancy globally."

Becker noted that correlation does not imply causality, "but it is pretty nifty that we found two strong correlations, which both point to the same physically plausible mechanism: the primordial stuff gets picked up preferentially by the most buoyant thermochemical upwellings."

The authors also wanted to know why only the hottest, most buoyant plumes sample high He-3/4.

"The explanation that we came up with -- which people who do numerical simulations have been suggesting for a long time -- is that whatever this reservoir is with primitive helium, it must be really dense so that only the hottest, most buoyant plumes can entrain some of it to the surface," Jackson said. "That makes sense and it also explains how something so ancient could survive in the chaotically convecting mantle for 4.5 billion years. The density contrast makes it more likely that the ancient helium reservoir is preserved rather than mixed away."

"Since this correlation of geochemistry and seismology now holds from helium isotopes in this work to the compositions we examined in 2012, it appears that overall hotspot geochemical variations will need to be re-examined from the perspective of buoyancy," Konter concluded.
-end-


University of California - Santa Barbara

Related Helium Articles from Brightsurf:

Helium, a little atom for big physics
Helium is the simplest multi-body atom. Its energy levels can be calculated with extremely high precision only relying on a few fundamental physical constants and the quantum electrodynamics (QED) theory.

Why there is no speed limit in the superfluid universe
Physicists from Lancaster University have established why objects moving through superfluid helium-3 lack a speed limit; exotic particles that stick to all surfaces in the superfluid.

First ever observation of 'time crystals' interacting
For the first time ever, scientists have witnessed the interaction of a new phase of matter known as 'time crystals'.

NASA sounding rocket finds helium structures in sun's atmosphere
Helium is the second most abundant element in the universe after hydrogen.

Physicists study mirror nuclei for precision theory test
A precision measurement of helium and hydrogen mirror isotopes reveals new questions in understanding of nuclear structure.

Researchers observe ultrafast processes of single molecules for the first time
Graz University of Technology researchers describe in Physical Review Letters how a molecule moves in the protective environment of a quantum fluid.

Frozen-planet states in exotic helium atoms
In a new study published in EPJ D. Tasko Grozdanov from Serbia and Evgeni Solov'ev from Russia describe the configuration and energy levels of antiprotonic helium that can potentially be produced by colliding slow antiprotons with ordinary helium at CERN.

Nanobubbles in nanodroplets
Freiburg researchers investigate ultrafast reaction of superfluid helium triggered by extreme ultraviolet laser pulses.

Gamma-ray laser moves a step closer to reality
A physicist at the University of California, Riverside, has performed calculations showing hollow spherical bubbles filled with a gas of positronium atoms are stable in liquid helium.

New method for detecting quantum states of electrons
Researchers in the Quantum Dynamics Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) devised a new method -- called image charge detection -- to detect electrons' transitions to quantum states.

Read More: Helium News and Helium Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.