olelog

Different isotopes are used in studies of the origin of volcanic rocks or magma. Today I shall concentrate on Helium isotopes.

There are eight known isotopes of helium, but only helium-3 and helium-4 are stable. Helium is unusual in that its isotopic abundance varies greatly depending on its origin. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to study the origin of such rocks.

Helium-3 (He-3) is a light, non-radioactive isotope of helium with two protons and one neutron, and is very rare on Earth. Helium-4 is also a non-radioactive isotope of helium. It is by far the most abundant of the two naturally-occurring isotopes of helium, making up about 99.99986% of the helium on earth.

Radioactive decay of uranium and thorium produces 4He, whereas 3He in the Earth's mantle is not produced by radioactive decay and was only incorporated during the accretion of the Earth around 4500 million years ago. 3He/4He ratios in many ocean-island basalts that erupt at hotspot volcanoes, such as Hawaii and Iceland, can be up to sixfold higher than in mid-ocean ridge basalts.

The high 3He/4He ratio seems to indicate that the lava making up oceanic islands like Hawaii is in part derived from the Earth’s mantle and has been unchanged since the formation of the Earth (primordial Earth). On the other hand the Hawaiian lava has an extremely low concentration of helium, which paradoxically seems to indicate that the part of the mantle that melts to produce the oceanic island has been previously melted, which would let helium gas escape. This would indicate that the lavas making up oceanic islands like Hawaii have been recycled, going through a process of melting and solidifying and melting again, like lavas that erupt in the mid-ocean ridges.

In a report in the journal Nature of 25 October, Gonnermann and Mukhopadhyay explain that the low concentration of helium in island magma doesn’t have to mean that it has been recycled. The two showed that helium would be lost from the island magma as it moved to the surface for the first time and as the enormous pressure it was under decreased. As the pressure declines, gases such as helium and carbon dioxide dissolved in the magma form bubbles, much like bubbles in a soda bottle when the top is popped.

The presence of a larger amount of carbon dioxide in the ocean island lavas compared with mid-ocean ridge lavas is the key, because it forms bubbles and provides a place for helium gas to cross into from the liquid magma. Once the magma reaches the Earth’s surface, the carbon dioxide and helium are lost to the air or water where it emerges.

It is largely believed that a slow circulation (convection) within the mantle — the layer between the crust and the core — coupled with the movement of the continental plates bringing material to the surface and back down again, have recycled the entire Earth over billions of years, leaving no material from the primordial Earth to be studied.

The authors believe that the geochemical data from Hawaii are consistent with parts of the mantle not having melted over Earth history, but if this lava is a remnant of the primordial Earth, it will require rethinking mantle convection theories to allow certain parts of the Earth to remain untouched in their original state. If that is true we have to come up with scenarios or models of mantle convection that leave parts of the mantle untouched. Maybe a layer is hidden somewhere in the lower mantle that is out of the main circulation or there may be pockets of primordial material scattered throughout.

* http://www.news.harvard.edu/gazette/2007/11.01/07-earlyisland.html
* http://www.physorg.com/news113234685.html