Helium is the rarest common element. Out in the Universe, 25% of all matter is helium. Yet on Earth, this abundant element has gone missing. It should be in our air, but it isn’t. Helium is so rare, it is the only element to have been discovered in space before it was known on Earth.
But once you know where to look, it can be found on Earth. It sits in natural gas reservoirs. A little helium is also present within the rocks in the crust and mantle. When you find helium escaping from the ground, something is happening underneath your feet. Tread carefully.
Helium is best known from toy balloons, and more serious floating devices such as blimps and weather balloons. The second familiar application is to make voices go squeaky. The biggest use of helium (30%) is in cryogenics. Helium stays a gas until very low temperatures (4K above absolute zero) making it useful in refrigerators which reach these temperatures. Below 4K it becomes a liquid (never a solid), and liquid helium is a super coolant for superconductors. Elsewhere, helium is used in manufacturing electronics components. Used for breathing purposes, in diving it help avoid decompression problems and allows people to dive deeper. In MRI, breathing in helium helps imaging the inside of lungs (and of course, the MRI itself uses helium for cooling of the instrument). A more mundane modern use is in air bags (together with argon). The use of helium for toy balloons could be seen as a waste of a valuable, non-renewable and increasingly scarce resource.
Capturing the Sun
The word ‘helium’ comes from helios, for the Sun, and refers to its discovery in the Sun where it is abundant. It is present in our atmosphere at a level of 0.00005% which is almost undetectable and certainly unusable. Barring a quick collecting sortie to the Sun, where can we obtain our helium?
The answer was found by accident. Oil wells produce gas as a by-product, and this gas is (wastefully) burned off. An oil well in Kansas (where else?) produced gas that would not burn. The non-flammable gas was send off for analysis. The ‘problem’ turned out to be that the gas contained too little methane to burn. Most of the gas was (inert) nitrogen, but about 2% turned out to be helium. The helium had nothing to do with the unflammability! Yet, because of this accident we now know that helium occurs in natural gas fields.
Helium concentrations in natural gas reservoirs is typically 0.1-0.5%. It is commercially extractable above 0.2%. But the gas fields around the Oklahoma Panhandle region contain ten times higher concentrations. Worldwide production (or rather, mining) of helium is 175 million cubic meters per year, mostly in the US. Annual consumption is in excess of 200 million cubic meter. The difference is made up by the US National Helium Reserve, which is expected to run out in 5 year time. There is indeed a definite need to develop new sources. The true economic cost of the helium in that toy balloon could be as high as 50 pound; the actual price you pay is based on the expectation that new helium resources will become available. This year, a major find of helium in the Tanzania rift valley was announced. The volume mentioned in the press release corresponds to 8 year of worldwide consumption. It is a start.
Helium is a noble gas. If you remember your chemistry, chemical reactions work by using empty locations for electrons, and helium has a filled electron shell – there are no vacancies. So it does not react with other atoms and does not form molecules. Under extremely cold temperatures it can become a liquid, but those temperatures are not reached anywhere outside of specialist laboratories – not even in space. (In space, no one can hear you sqeak.) (Bad pun alarm.) As a noble gas, you would expect it to reside in our atmosphere. Indeed, another noble gas, argon, makes up 1% of our atmosphere. But no helium. Why not? The problem is that it is too light weight. At room temperature, helium atoms fly around at speeds approaching escape velocity. Over time, and not much time, it leaks away into space. The minute amount in our atmosphere, compared to the 25% in space, shows how hard it is to keep hold of something moving at escape velocity.
So where does the helium in natural gas come from? Even a noble gas can get caught within the rocks of the Earth. At the time the Earth formed, there was still helium around, and some of this was caught within the forming Earth. A very small amount only, though. It has been there ever since, stuck within the crust, mantle and core of the Earth, unable to get out, fleeting spirits caught in our planetary prison.
The Earth has a second source of helium. Radioactive decay of uranium and thorium produces helium as one of its decay products. (Alpha particles emitted by radioactive elements are in fact helium atoms.) The radioactive elements are part of the Earth and their decay has slowly added helium to the Earth’s interior. In Australia’s Koongarra region where uranium deposits exists, the ground water near the uranium is strongly enriched in helium, arising from the nuclear decay.
The helium captured within the rocks can be released when these rocks fracture or melt. Once released, the very buoyant gas begins to diffuse upward. Two common rock types are impermeable to helium: anhydrite and salt, and the helium collects below layers of these. The same layers can also capture natural gas. Helium being lighter than natural gas, collects in the upper levels of these gas reservoirs.
If not intercepted by these layers, the gas eventually comes to the surface. This happens above regions where the Earth is disturbed and active: in rift zones, in earthquake regions and in places of active volcanism. An example is Yellowstone, which leaks an amazing 60 tons of helium per year. That is a lot and suggests that the Yellowstone hotspot has picked up helium from a reservoir in the crust. The heat of the hotspot released the helium, probably beginning about 2 million years ago. (Do the geysers sound perhaps a little bit squeaky?) The Tanzanian helium reservoir was also found from the high helium (and nitrogen) emission from the ground.
This release of helium is a common feature around volcanoes. Take for instance the eruption near La Restinga on El Hierro, in 2011. (This volcano was named Bob on VolcanoCafe, for reasons lost in history.) Extensive gas measurements were taken on the island in the air and in ground water before and during the eruption (although analyzed only later): these showed that helium emissions began to increase one month before the eruption began.
Helium occurs in two isotopes. The normal one is He-4, consisting of 2 protons and 2 neutrons. The rare one is He-3, with 2 protons but only 1 neutron. In the Earth’s atmosphere, He-3 is extremely rare: it accounts for only 0.0001% of all helium, i.e. 0.0001% of 0.00005%.
The rarity of He-3 is a pity, as it is even more useful than He-4. Below 3 Kelvin, where both are liquids, the two isotopes behave dramatically different. He-4 becomes a superfluid, capable of flowing without friction, climbing out of containers, carrying current without resistance (‘resistance is futile’), conducting away large amounts of heat (which makes it so useful as a coolant) and doing all kinds of other things belonging to the realm of the physics of magic (‘Harry Potter and the deathly helium’ is expected any day). He-3 is a more normal liquid, and the two don’t mix well: they positively dislike and repel each other. Dilution refrigerators using this conflict to reach astronomically low temperatures of a few milli-Kelvin. (‘Astronomically’ should here be taken as a figure of speech. Nowhere in space gets that cold, but some detectors used by astronomers do require that kind of cooling.)
He-3 is so useful and so rare, it is one of the most expensive substances on Earth. As so little can be mined, it is made in nuclear reactors and obtained from decommissioning nuclear bombs. About 10 kg is produced each year, and it sells for more than a million dollars per gram!
The helium emitted by Yellowstone contains 10 times the fraction of He-3 of the atmosphere. Within the 60 tons of annual Yellowstone helium, there is 0.5 kg of He-3. This is a billion dollars going up in smoke each year! It could pay for all scientific research into volcanoes in the world, with enough left to pay for dinner for all VolcanoCafe readers. And volcano research would still turn a profit. If only we could catch it.
Radioactive decay in the Earth produces new He-4, but no He-3. At the same time, helium continues to be lost to outgassing into the atmosphere. The result is that the ratio of He-3/He-4 in the Earth goes down over time. In places where the original helium has been lost, all the helium is radiogenic (i.e. produced later by radioactivity) and there is little or no He-3 left. Where there is more primordial helium, left over from the Earth’s formation, there is a higher fraction of He-3.
In regions where the crust is very stable, such as Britain, the helium from the ground is almost entirely radiogenic: less than 1 per cent is original. Continental plates are highly processed. In Greece, more tectonically active and prone to magma movement, more of the helium is original. In spreading ridges, the fraction of He-3 is quite high and much of the helium is primordial. About 70% of global He-3 emissions come from mid-oceanic ridges, where mantle material comes to the surface. Clearly, He-3 comes from the mantle.
This shows that the mantle still has a lot of material that is original, which has not mixed with descending continental plates or been at the surface before. The high fraction of original helium in the mantle also suggests that there is rather little uranium and thorium there. Most of Earth’s radioactivity may be in the crust.
An application of He-3 comes from the African rift valley. The He-3/He-4 ration measured in ground water in the Afar region of Djibouti, where the hotspot hit 25 million year ago, is 10 times higher than that in the atmosphere, showing that the hotspot brought up volatiles from the mantle. The same ratio is found in the nearby stretch of the ridge valley in Ethiopia. But further away, in Kenya, the He-3/He-4 ratio is close to that in the atmosphere, and here the helium comes from the crust. The helium in the rift valley in northern Kenya, close to Ethiopia, is intermediate in composition. The African rift is different in different places: in the north it is driven by mantle heat, but in central Africa it is crustal extension without involvement from the mantle.
Helium and volcanoes
Let’s go back to Yellowstone. It emits a lot of helium with a lot of He-3. What does this tell us? The large amount of helium suggests there is a captured helium reservoir underneath Yellowstone. This must have formed a long time ago, well before the hotspot arrived. The amount of He-3 shows that the helium in the reservoir came from the mantle, and not from the crust. How long has it been there? In theory we could date it from the He-3/He-4 ratio, but in practice this depends on too many uncertainties. It may have been there a long time, perhaps a billion year, but take any precise age with a large grain of salt.
How about helium and El Hierro? In 2003, long before any volcanic unrest, the island emitted 9 kg per day of helium. In July 2011, just after the deep earthquakes began, it had increased slightly, to 11 kg/day, with a three times higher fraction of He-3 than in 2003. In mid September, helium emission increased to 24 kg/day. Two days later, the seismic crisis started and the on-going earthquake activity migrated to the south of the island. The eruption began on October 12. Helium emissions peaked in November, before slowly declining again.
What happened? The increasing pressure of the inflating magma reservoir caused small cracks to appear in the rocks. It was a type of volcanic, self-induced fracking. The cracks allowed the helium to escape and make its way to the surface. CO2 can do the same and it also used to measure ground emissions, but helium can diffuse more rapidly and is not affected by biology: it is the more powerful tracer. The isotopic ratios show that the helium which heralded the El Hierro eruption had a mantle origin.
A final explosive example is Ontake, Japan, which unexpectedly erupted in September 2014 killing a number of tourists. It was a phreatic explosion, without any precursors or warnings. But in the beginning was helium. From June 2003 until November 2014, helium emission close to Ontake, but not at more distant sites, showed an increasing fraction of He-3. In hindsight, this indicated a re-activation by newly arrived magma over a decade, which increased gas pressure near the surface, culminating in the sudden explosion. There may be more to come.
Long Valley caldera is also monitored for helium. During 1987, He-3/He-4 ratios went up by 25%, indicating the arrival of new magma. It stayed constant in 1988 and went down again later. Here, He-3 allows one to distinguish ground level changes from magma intrusions from those
which are purely caused by moving water.
But not all helium refugees foreshadow impending volcanic eruptions. The Oberpfalz in Germany is not an active volcanic region, however the helium here has a higher than usual He-3 fraction, which is different from the rest of Europe. Mantle helium is managing to find its way to the surface. The region, around the Bavarian forest, is part of an ancient mountain belt, an old stitch running across Europe, where two parts of the continent came together. Is this region about to be reactivated? Or was mantle helium trapped here around the time of the continental collision? The latter seems more likely.
Not far away is Joachimsthaler, a town in a valley with old mines. It was an important source of valuable metals; the name has left imprints in our language. Coins made out of its metal became known as ‘thaler’, a word that survived in Europe in the names of various types of currency. Before the euro, it remained in use in the Netherlands for a 2.5 guilder coin. In the US, ‘thaler’ became ‘dollar’; in this form, Joachimsthaler still rules the world. Helium does more than just announcing impending volcanoes. It also makes money.