New form of sulfur discovered in geological fluids

These findings are published in the Feb. 25, 2011 issue of the journal Science.

Until now, geochemists believed that inside Earth, only two forms of molecules contained sulfur: sulfides (based on H₂S or S^2-) and sulfates (based on H₂SO₄ or SO₄^2-). Yet they had no way of directly plunging a probe into the hydrothermal fluids² that flow through rocks to verify this theory. To get round this problem and test their hypothesis, the French-German team first created fluids similar to those in Earth's crust and mantle, i.e. aqueous solutions containing elementary sulfur (S) and thiosulfates (molecules containing the S₂O₃²⁻ ion). They then used a diamond anvil cell to bring the fluids to the temperatures and pressures found at depths of several kilometers (several hundred degrees and tens of thousands of atmospheres).

The researchers used an optical method known as Raman spectroscopy to identify the chemical species, and they were astounded to discover not two, but three forms of sulfur, the third being the trisulfur ion S₃^-. This was a double surprise: although S₃^- was already known to chemists (it is found in sulfur-containing silicate glass and ultramarine pigments for instance), it had never been observed in an aqueous solution.

The detection of S₃^- during these experiments means that sulfur must be considerably more mobile in hydrothermal fluids in Earth's crust than was previously thought. This is because, unlike sulfides and sulfates, which attach to minerals as soon as they appear in fluids, S₃^- proves to be extremely stable in the aqueous phase. In other words, below ground these ions must flow for long distances in dissolved form, taking with them the noble metals to which they may be bound. This chemical species may therefore be the main metal transporting agent in two major types of gold and copper deposits: Archaean greenstone belts³ and subduction zone magmas.

This discovery could provide additional indicators in the search for new deposits, by helping geologists to identify the pathways along which metals travel prior to forming veins. In addition, the presence of S₃^- in hydrothermal fluids could affect sulfur isotope fractionation models (a sort of equivalent to the carbon-14 dating technique), which until now have taken no account of this chemical species. These new findings could for instance help scientists to find out more about the geological conditions in  Earth's crust and on its surface shortly after the appearance of life.

Notes

1. Laboratoire 'Géosciences Environnement Toulouse' (CNRS/Université Paul Sabatier/IRD) and Bayerisches Geoinstitut/University of Bayreuth.
2. A hydrothermal fluid is a natural hot, aqueous fluid whose temperature usually exceeds 100°C.
3. These rocks formed during the Archaean era, between -4 and -2.5 billion years ago.