A breath of light air

Geoscience has benefited from being an economically useful pursuit for a very long time. Some of the justification for Strata Smith’s mapping of England was to be able to predict where coal deposits could be found.  In the U.S., state surveys were initially to locate mineral resources.  Many western educational enterprises started as mining schools, helping to locate and exploit precious metals.  In the twentieth century, there was a shift into energy, with the oil and gas industry supporting institutions educating its employees. As a result, geologists have had access to rich databases of their own literature far longer than most other sciences. Conversely, extractive industries have benefited from the insights of academic pursuits like plate tectonics as the relationship of resources to other earth phenomena made searching for resources less erratic. Although earth science has provided many other instructive insights, it was the search for resources that largely allowed geology to be pursued at a grand scale.  As the desire to reduce carbon emissions has grown and as hard rock mining has dialed in to a pretty well-known set of deposits, it seemed that the practical side of the field was withering.  Academic appointments in fields like mining geology have been fading for years, and petroleum geology has looked more and more like petroleum engineering the last 20 years or so.

So it was kind of a blast from the past when researchers at Durham and Oxford announced that they think they know how to find deposits of helium. This drew attention from the BBC and Ars Technica as helium reserves have been plummeting, in no small part because of some short-sighted actions from the U.S. Congress. Helium for most of us is kind of a balloon filling gas, but the increased use of liquid helium in various activities (most notably in MRI scanners) has made it of far greater value than something to be allowed to float into the sky and vanish.

Up to now, helium has pretty much been an accidental discovery associated with oil and gas exploration.  The new work more or less argues that economic deposits are things we can anticipate in some areas, making the prospect of searching for helium a plausible one.  The basic logic of this work is in some ways tangled up with academic interest in helium for other, very different reasons.  So it might be worth a moment to contemplate helium.

For starters, there are two isotopes: helium 3 and helium 4.  He-3 is primordial or extraterrestrial; He-4 is mainly the product of radioactive decay (mainly from uranium and thorium).  The ratio between the two in gases in hot springs and volcanoes is sometimes thought to reflect on the presence of primordial materials deep in the earth. One of the easiest attempts to use radioactivity to date rocks tried to exploit that origin of He-4: in the early days of the twentieth century, Ernest Rutherford tried to date a rock using the helium produced by radioactive decay; this failed because helium will diffuse out of rocks.  This approach though has been resurrected as U-Th/He dating, which exploits that diffusion, which only occurs at high enough temperatures, so the date related to the amount of helium is related to the time a rock cooled enough to hold on to its helium.

Now the helium will accumulate as the minerals get older and older while remaining cold enough to not allow for diffusion.  If these rocks are later reheated, the helium will diffuse out and rise. So a place where a lot of helium might be being produced is one where old rocks are being reheated by new volcanic activity.  And this pretty well describes the proposed helium resource the new work has identified, namely areas in the East African Rift where old African shield rocks are being heated by the young volcanoes along the rift.

The other end of the trick is one familiar to the oil and gas industry: finding traps where such gas will accumulate.  For helium, this has traditionally been under layers of salt, which is fairly impermeable to helium.  Some shales have also retarded helium’s rise out of the earth fairly effectively. So if helium is now a valuable enough gas to justify pursuit using common exploration technology, there are now some decent targets to anticipate: places with salt deposits with old basement underneath that have experienced recent reheating of the crust.  Fortunately for the supply of helium, such locations are not terrifically rare, but many are implausible locations for oil and natural gas and so probably have not been explored like classic petroleum provinces.

If a helium industry emerges, we might learn more about this element.  At the present, there are a number of issues with helium that affect our view of how Earth works.  For instance, the notion that volcanic centers with high ratios of He-3 to He-4 must be tapping deep, un-degassed parts of the earth indicates that such material exists, but an alternative is that these sites are simply sampling areas that have been depleted in the radiogenic He-4, perhaps by fairly recent thermal events. In contrast, other areas with low He-3 to He-4 ratios might be places where the radiogenic gas is now being released from olde reservoirs by active tectonics.  That all this matters in understanding ages gained from U-Th/He dating techniques means that we might soon be seeing this old problem get attacked from multiple new angles.

That this business of finding a new resource almost seems to be coming out of the blue is a reminder of the benefits of a robust research program into many facets of earth science.  You can not be too sure where the next useful discovery will come from.



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