This is a projected chain of actions that it is thought took place. That it is tortuous makes it somewhat unconvincing. That is unfortunate. It appears that early atmospheric process beneficiated the sulfur and concentrated enough in the sea bed at least, if not saturating the crust to some depth. This provided a reaction horizon that then captured nickel and led to the creation of nickel sulphide deposits unique to the geological era. Other minerals were also involved.
This is a unique mineralogy out of the Precambrian that has not occurred since.
I wonder if this reaction horizon could also explain the prevalence of banded iron deposits in ancient rocks. The sulphur would fix the iron in place, yet be reduced later in the presence of oxygen.
Rich Ore Deposits Linked To Ancient Atmosphere
The key evidence came from a form of sulfur known as sulfur-33, an isotope in which atoms contain one more neutron than "normal" sulfur (sulfur-32). Both isotopes act the same in most chemical reactions, but reactions in the atmosphere in which sulfur dioxide gas molecules are split by ultraviolet light (UV) rays cause the isotopes to be sorted or "fractionated" into different reaction products, creating isotopic anomalies.
by Staff Writers
Much of our planet's mineral wealth was deposited billions of years ago when Earth'schemical cycles were different from today's. Using geochemical clues from rocks nearly 3 billion years old, a group of scientists including Andrey Bekker and Doug Rumble from the Carnegie Institution have made the surprising discovery that the creation of economically important nickel ore deposits was linked to sulfur in the ancient oxygen-poor atmosphere.
These ancient ores - specifically iron-nickel sulfide deposits - yield 10% of the world's annual nickel production. They formed for the most part between two and three billion years ago when hot magmas erupted on the ocean floor. Yet scientists have puzzled over the origin of the rich deposits. The ore minerals require sulfur to form, but neither seawater nor the magmas hosting the ores were thought to be rich enough in sulfur for this to happen.
"These nickel deposits have sulfur in them arising from an atmospheric cycle in ancient times. The isotopic signal is of an anoxic atmosphere," says Rumble of Carnegie's Geophysical Laboratory, a co-author of the paper appearing in the November 20 issue of Science.
Rumble, with lead author Andrey Bekker (formerly Carnegie Fellow and now at the University of Manitoba ), and four other colleagues used advanced geochemical techniques to analyze rock samples from major ore deposits in Australia and Canada
The key evidence came from a form of sulfur known as sulfur-33, an isotope in which atoms contain one more neutron than "normal" sulfur (sulfur-32). Both isotopes act the same in most chemical reactions, but reactions in the atmosphere in which sulfur dioxide gas molecules are split by ultraviolet light (UV) rays cause the isotopes to be sorted or "fractionated" into different reaction products, creating isotopic anomalies.
"If there is too much oxygen in the atmosphere then not enough UV gets through and these reactions can't happen," says Rumble. "So if you find these sulfur isotope anomalies in rocks of a certain age, you have information about the oxygen level in the atmosphere."
By linking the rich nickel ores with the ancient atmosphere, the anomalies in the rock samples also answer the long-standing question regarding the source of the sulfur in the ore minerals. Knowing this will help geologists track down new ore deposits, says Rumble, because the presence of sulfur and other chemical factors determine whether or not a deposit will form.
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