New research on a mineral called molybdenite published in the journal Earth and Planetary Science Letters provides new insights about the changing chemistry of the Earth as a result of geological and biological processes.
Mineral evolution is an approach to understanding Earth’s changing near-surface geochemistry. All chemical elements were present from the start of our Solar System, but at first they formed comparatively few minerals – perhaps no more than 500 different species in the first billion years. As time passed on the planet, novel combinations of elements led to new minerals.
Molybdenite is the most common ore mineral of the critical metallic element molybdenum.
A team of scientists, led by Dr Robert Hazen of the Carnegie Institution of Washington’s Geophysical Laboratory, analyzed 442 molybdenite samples from 135 locations and ages ranging from 2.91 billion years old to 6.3 million years old. They specifically looked for trace contamination of the element rhenium in the molybdenite, because rhenium can be used to use to gauge historical chemical reactions with oxygen from the environment.
The scientists found that concentrations of rhenium, a trace element that is sensitive to oxidation reactions, increased significantly – by a factor of eight – over the past three billion years. They suggest that this change reflects the increasing near-surface oxidation conditions from the Archean Eon more than 2.5 billion years ago to the Phanerozoic Eon less than 542 million years ago. This oxygen increase was a consequence of what’s called the Great Oxidation Event, when the Earth’s atmospheric oxygen levels skyrocketed as a consequence of oxygen-producing photosynthetic microbes.
In addition, they found that the distribution of molybdenite deposits through time roughly correlates with five periods of supercontinent formation, the assemblies of Kenorland, Nuna, Rodinia, Pannotia, and Pangea.
“Our work continues to demonstrate that a major driving force for mineral evolution is hydrothermal activity associated with colliding continents and the increasing oxygen content of the atmosphere caused by the rise of life on Earth,” Dr Hazen said.
Bibliographic information: Joshua Golden et al. 2013. Rhenium variations in molybdenite (MoS2): Evidence for progressive subsurface oxidation. Earth and Planetary Science Letters, vol. 366, pp. 1–5; doi: 10.1016/j.epsl.2013.01.034
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