Scientists say a previously little known form of sulphur, which only occurs at extreme temperatures and pressures, may be the most common form of the element on Earth.
Dr Gleb Pokrovski from the Université Paul Sabatier in Toulouse, France, and Professor Leonid Dubrovinsky from the Universität Bayreuth in Germany hypothesised that sulphur's ability to transform into different species (chemical forms) could produce interesting results under extremes of temperature and pressure.
To test their hypothesis they performed experiments with sulphur rich fluids at high temperatures and high pressures.
They unexpectedly discoved that a form of sulphur called trisulphur anion or S3- was the dominant stable form of the element in geological fluids at temperatures above 250 degrees Celsius and pressures greater than 0.5 gigapascals.
Writing in the journal Science, they conclude this trisulphur anion could be the most common form of the element in places like subduction zones (where one tectonic plate slides beneath another at the plate boundary) and in magma brewing deep beneath volcanoes.
Sulphur is distributed throughout the mantle and crust mainly in sulphide and sulphate minerals.
Sulphur-rich brines, called hydrothermal fluids, transport precious metals, such as gold, copper and platinum deep underground, forming them into ore deposits.
Underground gold transport
In a perspective accompanying the main article, Dr Craig Manning from Earth & Space Sciences at the University of California in Los Angeles says the study gives scientists a better understanding of how hydrothermal fluids work.
Manning says different forms of sulphur bind to different ore metals in water in different ways.
"This helps our understanding of how metals like gold are transported and concentrated in ore bodies deep underground."
"Trisulphur increases the capability of these fluids, more than we previously thought, to transport metals like gold in subduction zones or deep under mountain ranges like the Himalayas."
Manning says if we could shine a light on these deep geologic fluids, the apparent prevalence of trisulphur means they should be deep ultramarine blue, "just like the valuable mineral lazurite which is coloured by its interaction with the trisulphur anion, and which has been used to decorate jewelry boxes since before King Tut."
The Mount Pinatubo puzzle
Manning believes the findings could also answer one of the great mysteries surrounding the 1991 eruption of Mount Pinatubo in the Philippines.
Sulphur released during the eruption caused average global surface temperatures to drop by about half a degree Celsius for several years.
Manning says the levels of sulphur found in the lava didn't match the levels ejected into the atmosphere.
"Numerous scientific papers were published to try and explain the disparity, sparking significant scientific debate.
"One theory suggests that in addition to the lava being ejected into the atmosphere, additional subsurface [sulphur] gases emitted by the lava were also entrained in the eruption and ejected high into the atmosphere."
"If Pokrovski and Dubrovinsky are right, the gases could have included trisulphur."
According to Manning, because trisulphur exists in such extreme conditions, scientists didn't look for it at the time. Pokrovski and Dubrovinsky believe that because trisulphur anion only forms above certain temperatures and pressures there wouldn't be any evidence of it under other conditions.
Manning says the study isn't a new exploration tool, but should lead to new thinking on how metals like gold and copper get transported and ultimately find their way into ore deposits at the Earth's surface.