Big Ideas in Volcanology #08:


#08.3 _ No free oxygen was part of the Earth’s primordial atmosphere

Water vapor is the most common volcanic gas today, and also CO2, SO2, CO, HCl, N are expelled. Furthermore, O2 has never been observed to be expelled to the atmosphere by an eruption. If volcanism is unrelated to it, how has it been added to the atmosphere?

Free oxygen in atmosphere has been biologically induced ≈ 2.4 billions of years ago with the so-called Great Oxygenation Event (GOE).

Oxygen was emitted by photosynthetic organisms as a waste product even before the GOE, but organic matter and dissolved iron chemically captured any free oxygen at that time. The GOE is the point when these minerals became saturated and could not capture any more oxygen. Indeed, the excess free oxygen started to accumulate in the atmosphere fluctuating until today’s levels. The red and green lines in the figure represent the range of estimates of O2 pressure whereas the red dashed line the current volume of O2 in the atmosphere.


Left, O2 build-up in the Earth's atmosphere; right, O2 content of the atmosphere over the last billion years


During stage 1 (≈ 3.85–2.45 Ga) basically no O2 was in the atmosphere. Stage 2 (≈ 2.45–1.85 Ga) describes a phase of production of O2, but absorbed in oceans and seabed rocks. In stage 3 (≈ 1.85–0.85 Ga) O2 starts to gas out of the oceans but is absorbed by land surfaces. Finally, during stages 4 (≈ 0.85–0.54 Ga) and 5 (≈ 0.54 Ga–present) O2 absorbers fill and the gas accumulates in the atmosphere.

The oxygen has been quickly removed from the atmosphere in stages 2 and 3 by the weathering of reduced minerals, mostly iron. This 'mass rusting' led to the deposition of Banded-Iron Formations (BIFs). These formations consist of repeated thin layers of iron oxides, such as magnetite  (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shale and chert.

BIFs are abundant around the time of the GOE and become less common after ≈ 1.80 Ga. The total amount of oxygen locked up in the banded iron beds is estimated to be approximately 20 times the volume of oxygen present in the modern atmosphere. Important banded iron beds are in the Animikie group (Minnesota, USA) and the Pilbara region (Western Australia), exploited for the extraction of iron ore.

BIFs rich in hematite


Earth’s hydrosphere comes from Volcanic Degassing

#08.1                    #08.2                    #08.3                    #08.4                    #08.5                    #08.6