Mantle helium arrived by meteorite

The isotopic composition of volcanic gases suggests the importance of early meteoritic contributions to the geochemistry of Earth’s mantle, reports Sarah Day from the BA.

Geoscientist Online 10 September 2008

Researchers at the University of Manchester have identified a neon isotope signature from within the Earth that may redefine our ideas about how the gases, dissolved in mantle rocks and found today emerging at mid-ocean ridges, actually got there.

The results, presented at the BA today by Professor Christopher Ballentine, support the theory that a late bombardment, probably after the creation of the Moon, played the most significant role.
The isotope ³He is known to emanate from mid ocean ridges, and is used as a tracer by scientists studying the circulation patterns of the ocean. But where did it come from? ³He is an isotope normally found only in space; somehow it has became trapped inside our planet but how?

Scientists have argued that to put enough ³He into the deep Earth to explain the volcanic emissions seen today, the early molten Earth had to have been surrounded by a dense atmosphere like that around Jupiter - with an isotopic composition similar to the Solar Wind with respect to ³He - and that this gradually dissolved in the rocks under immense pressure.

It was not possible to decide between this and the competing meteorite theory on He isotopes alone and so isotopes of neon have been used to constrain the He results. Samples of fossilized volcanic gas, uncontaminated by the atmosphere, were obtained from rocks at Bravo Dome (New Mexico), and analysed to determine their neon isotope ratios. These results were then compared with known ratios of neon isotopes obtained from gas-rich meteorites, and the Solar Wind. The neon isotope fingerprint of these primitive mantle-derived gas samples matched those of gases found within meteorites.

On impact, the meteorites – with their cargo of dissolved gases – were accreted into the Earth, creating the deep reservoir of ³He that feeds today’s mid-ocean ridges. The Manchester researchers are now using a new instrument to try to find this ‘meteorite signature’ in other trace gases as well as neon, in order to confirm their results.

Ballentyne also told the BA that the Mantle’s gases show a distinct “oceanic” signature. “In addition to the gases trapped during accretion, the very act of plate tectonics and subduction is slowly adding volatile elements to the deep Earth” said Ballentyne. “Despite a widely held belief by geologists that planetary degassing is a one-way and outward process (the "subduction barrier model"), we estimate that subducting seawater could account for about half of the water in the Earth's mantle today.”

Professor Ballentyne said that the effect of adding water to the mantle was to increase rock plasticity and so to increase the speed of plate tectonic processes. However, this effect might be counteracted over time by gradual Earth cooling as radioactive elements decay away to stable isotopic products.

Ballentyne was speaking at the session “Isotopes – silent witnesses to Earth history”, co-organised by the Society.