Image copyright Juerg Matter Image caption The experiment injected 220 tonnes of carbon dioxide several hundred metres underground

Scientists think they have found a smart way to constrain carbon dioxide emissions - just turn them to stone.

The researchers report an experiment in Iceland where they have pumped CO2 and water underground into volcanic rock.

Reactions with the minerals in the deep basalts convert the carbon dioxide to a stable, immobile chalky solid.

Even more encouraging, the team writes in Science magazine, is the speed at which this process occurs: on the order of months.

"Of our 220 tonnes of injected CO2, 95% was converted to limestone in less than two years," said lead author Juerg Matter from Southampton University, UK.

"It was a huge surprise to all the scientists involved in the project, and we thought, 'Wow! This is really fast'," he recalled on the BBC's Science In Action programme.

Image copyright Lamont-Doherty Earth Observatory Image caption The reactions in the volcanic rock produced immobile carbonates (white deposits)

With carbon dioxide concentrations in the atmosphere marching ever upwards and warming the planet, researchers are keen to investigate so called "carbon capture and storage" (CCS) solutions.

Previous experiments have seen pure CO2 injected into sandstone, or deep, salty aquifers. Chosen sites - which have included disused oil and gas wells - have relied on layers of impermeable capping rocks to hold down the carbon dioxide. But the fear is always that the CO2 could find a way to leak back out into the atmosphere.

Image copyright Juerg Matter Image caption Cores were drilled to check on the progress of the subterranean reactions

The Carbfix project on Iceland, on the other hand, seeks to solidify the unwanted carbon in place.

Working with the Hellisheidi geothermal power plant outside Reykjavik, it combined the waste CO2 with water to make a slightly acidic liquid that was then sent hundreds of metres down into the volcanic basalts that make up so much of the North Atlantic island.

The low pH water (3.2) worked to dissolve the calcium and magnesium ions in the basalts, which then reacted with the carbon dioxide to make calcium and magnesium carbonates. Cores drilled into the experimental site pulled up rock with the tell-tale white carbonates occupying the pore spaces.

The researchers also tagged the CO2 with carbon-14, a radioactive form of the element. In this way, they were able to tell if any of the injected CO2 was leaking back to the surface or finding its way out through a distant watercourse. No such escape was detected.

"This means that we can pump down large amounts of CO2 and store it in a very safe way over a very short period of time," said study co-author Martin Stute from Columbia University's Lamont-Doherty Earth Observatory, US.

"In the future, we could think of using this for power plants in places where there's a lot of basalt - and there are many such places."

Dr Matter added: "You can find basalts on every continent and, certainly, you can find them offshore because all the oceanic crust - so below the seafloor - is all basaltic rocks. In terms of the availability of basaltic rocks to take care of CO2 emissions globally - no problem."

Image copyright Thinkstock Image caption Iceland has abundant basalts, but these rocks are not found everywhere

Image copyright P.HUEY/SCIENCE/AAAS Image caption Some CCS experiments have pumped pure CO2 into sedimentary rocks (L) where the gas is trapped below an impermeable caprock. In CarbFix (R), the CO2 is dissolved in water and chemical reactions at depth then ensure nothing leaks back to the surface

There is, however, the issue of cost. Capture of the CO2 at power stations and other industrial complexes is expensive, and without incentives it is currently deemed to be uneconomic. The infrastructure needed to pump the gas to the burial site also has to be considered.

And in the case of the Carbfix approach, a substantial amount of water is required. Only something like 5% of the mass sent underground is CO2.

Christopher Rochelle is an expert on CCS at the British Geological Survey and was not involved in the Iceland experiment.

He said Carbfix underlined the importance of moving beyond modelling and lab studies to real-world demonstrations. Only by doing this can the technology readiness be properly assessed.

"We need to do more field-scale tests, like this one in Iceland, to better understand the types of processes that are ongoing and how fast they work," he told BBC News.

"Here, they injected into reactive rocks and the minerals precipitated relatively quickly and are then unable to migrate anywhere. That's great, but the rocks under Iceland are different to those under the North Sea, for example. So the approach that is taken is going to have to vary depending on where you are. We are going to need a portfolio of techniques."

The Hellisheidi geothermal power station has now moved beyond the experiment reported in Science magazine and is routinely injecting CO2 into the subsurface in larger quantities. The company is also burying hydrogen sulphide - another byproduct from the plant. This benefits the locals who have had to suffer the occasional waft of rotten eggs coming over their properties.

Image copyright Lamont-Doherty Earth Observatory Image caption The Hellisheidi geothermal power station continues to pump CO2 under Iceland

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos