Those spare nickels in your pocket might not be there without the help of ancient volcanoes that blasted sulfur dioxide into the sky billions of years ago. The discovery solves a mystery that has dogged researchers for decades, says geochemist Edward Ripley of Indiana University, Bloomington, who was not affiliated with the study.
The nickel in ore deposits is actually nickel sulfide, a compound that is rich in sulfur. The sulfur is "critically important," says geochemist Douglas Rumble of the Carnegie Institution of Washington in Washington, D.C. It concentrates nickel into a form that can be mined commercially. But no one knew where the sulfur came from. Neither the ancient seawater under which the nickel ore deposits were once buried, nor the molten magmas that welled up from Earth's mantle and originally deposited the nickel, contained very much sulfur.
Rumble, geochemist Andrey Bekker of the University of Manitoba in Winnipeg, Canada, and several other colleagues hit upon a clue to the sulfur's origins in ancient rocks from Western Australia. The rocks contained unusual ratios of two versions, or isotopes, of sulfur known as sulfur-33 and sulfur-32. Only ultraviolet (UV) light from the sun can create such an effect, Rumble says, in the course of breaking down sulfur dioxide gas. When the scientists examined nickel-bearing ore samples, also from Western Australia as well as from Canada, they found the same ratio of the sulfur isotopes.
Armed with this information and further analyses, Rumble, lead author Bekker, and colleagues propose the following scenario today in Science. Early in Earth's history, volcanic eruptions spewed massive amounts of sulfur dioxide into the atmosphere, where UV sunlight broke down the gas and created the odd sulfur isotope ratios. The sulfur descended with the rain and accumulated into sedimentary beds on the sea floor. Once there, superheated water from geothermal vents at various locations on the sea floor cooked the sulfur into sulfide. Finally, nickel-bearing magma welled up from Earth's mantle, combining with the sulfide to form nickel sulfide and encasing the compound inside volcanic rock called komatiite.
In terms of geological time scales, the whole process was lightning-fast. It may have taken as little as a few million years, Bekker says. And as soon as the sediments were assimilated by the magma, he says, it might have taken only "several decades to form the mineral deposit."