ultramafic rocks

April Editorial      


Hematites hold proof of early Oxygen on Earth


Until recently a popular model for Earth's atmospheric evolution postulated that until about 2.5 billion years ago the planet had an anoxic atmosphere (i.e. an atmosphere low in or lacking oxygen). There is a common belief that a bacterium known as Cyanobacteria changed this when it made its appearance as the very first life form to use water and the sun for photosynthesis. This process which produced free oxygen as a waste product which collected in the atmosphere. As we reported in our September 2008 editorial (to see this article click here) 'At this time the sea was not as we know it now. It was rich in iron, which gave the ocean a distinctly green colour. Free oxygen released by cyanobacteria reacted with the iron molecules in the water, which resulted in iron oxidation. The iron oxide was much heavier and settled on the bottom of the ocean, slowly depleting the water of iron." This iron oxide gave rise to so called oxide-rich Banded Iron Formation (BIF), which typically consists of alternating bands containing haematite [Fe2 3+ O3], magnetite [Fe2+ Fe2 3+ O4] and iron silicate.

But recent research, just published in Nature Geoscience (1), suggests that oxygen was present in the atmosphere as early as 3.46 billion years ago. This latest research was jointly supported by the NASA Astrobiology Institute, Kagoshima University, University of Tokyo and the Department of Geosciences, and the Pennslvania State University. Masamichi Hoashi and his co-workers looked at the hematite-rich chert (chert is a brittle sedimentary rock) obtained from the a deep drill core in the Pilbara Craton of Western Australia. Hematite is produced through oxidation process, so dating ancient sources of hematite can be used to discover when oxygen was present on Earth.

A problem with using hematite to date the early oxygenation of the Earth is that of reliably estimating the time when the hematite was actually produced. For example, it is generally assumed that hematite found in ancient rocks is actually quite modern, formed by the oxidation of siderite (a mineral composed of iron carbonate FeCO3 which is commonly found in hydrothermal veins) in the modern atmosphere. To avoid this 'contamination' the samples had to be collected from rocks which have not been exposed to a relatively modern atmosphere. So the searchers drilled to the base of a hill in the Pilbara Craton, deep below the water table to collect their samples. Analysis of those samples revealed two generations of hematite. The first generation of hematite occurs as the major component (~6wt%) in the ferruginous (iron-rich) chert and the second generation occurs as patches and veins in the volcanic rock. Using rhenium-osmium (Re-Os) isotopic measurements the scientists found that the iron-rich chert was about 2.76 billion years old. But the hematite (red jasper in this case) recovered from the volcanic rocks was estimated to be much older (about 700 million years older) dating these rocks to around 3.4 billion years.

But one more possible artifact had to be excluded. Iron compounds exposed to ultra-violet light can formtiny particles of ferric hydroxide, which can be converted to hematite at temperatures of at least 60oC. The investigating team argued that if the hematite recovered from volcanic veins was formed from ferric hydroxide which settled at the bottom of the ocean, the mineral structure would have been different. That is it would be in a pattern of aggregates which combine together to produce a large crystal. A characteristic feature of those aggregates is the presence of large empty spaces between crystals, something which can be easily seen with electron microscopy. Thus the team could show by electron microscopy that the samples they had recovered were not aggregates. Indeed, the red jasper recovered from the deep volcanic veins was formed from a single crystal which grew in the presence of iron-rich and oxygen rich waters, slowly converting the iron compounds into hematite using oxygen dissolved in the deep ocean water.

If this theory is correct it would explain why this ancient hematite can only be found in areas with active submarine volcanism. It would also suggest that oxygen was present on Earth much earlier than previously thought, which also suggests that there must have been some life form at that time which could release oxygen through photosynthesis. Therefore cyanobacteria may be much older that previously thought, or another as yet unidentified organism was also capable of photosynthesis.

1. Masamichi Hoashi, David C. Bevacqua, Tsubasa Otake, Yumiko Watanabe, Arthur H. Hickman, Satoshi Utsunomiya and Hiroshi Ohmoto. (2009) Primary haematite formation in an oxygenated sea 3.46 billion years ago. Nature Geoscience 2, 301 - 306.


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