Volcanic origin for nickel ore deposits 21 November 2009Posted by admin in volcanoes.
Tags: current research, iron-nickel sulphides, undersea volcanism, volcanoes and the atmosphere
Some 10% of the world’s nickel production comes from iron-nickel sulphide (or, if you prefer, sulfide) deposits laid down on seafloors between 2.5 and 3 billion years ago and found in those interesting ultramafic volcanic rocks called komatiites. How these deposits came to be there, however, has always been something of a mystery, for the ore requires sulphur to form and neither the magmas hosting the ore nor the seawater contained sufficient sulphur for the process. Clearly, if the sulphur doesn’t come from the magma or the seawater it must come from the substrate: because of the very high temperatures at which the magma was erupted (greater than 1500oC) it was able thermo-mechanically to erode its substrate, thus acquiring sulphur in areas where the substrate was sulphur-rich. But how did the sulphur come to be there in the first place?
In a paper entitled ‘Atmospheric sulfur in Archean komatiite-hosted nickel deposits’ (abstract) in the new issue of Science a team of scientists from the Carnegie Institution, Woods Hole Oceanographic Institution and the universities of Manitoba and Western Australia argue that it all starts with volcanoes. Sulphur dioxide was erupted by volcanoes into an anoxic atmosphere, where high levels of UV sunlight broke down the gas and allowed sulphur to descend via rainfall and accumulate on the seabed. There geothermal action formed it into sulphide which was combined with nickel in magmas to produce the iron-nickel sulphides found in komatiites. The unusual isotope sulphur-33, produced by the atmospheric breaking-down of the volcanic sulphur dioxide, has turned up in rocks found in Western Australia, providing scientists with the key to recreating the sulphur’s complex ancient journey.
- Andrey Bekker, Mark E. Barley, Marco L. Fiorentini, Olivier J. Rouxel, Douglas Rumble, Stephen W. Beresford, ‘Atmospheric sulfur in Archean komatiite-hosted nickel deposits’, Science, Vol. 326. no. 5956 (20 Nov 2009), pp. 1086 – 1089. DOI: 10.1126/science.1177742. (abstract)