24 July 2011
On the second day of the Agouron field trip, we piled into the vans and drove out of town, down some rather rugged road (especially for minivans!) and parked next to waste rock from an old mine. But instead of investigating this rock, we set off into the swamp on the other side of the road. After a muggy walk through tailings-stained swamp and tall cattails, along a beaver dam, and than up a rise into the forest, we came to a clearing under some power lines where rocks were exposed.
The rocks in this area are extremely old for the earth – about 2.7 billion years – and a lot happens to rocks in that time, including lots of deformation as multiple generations of continents smash into each other. So lots of the rocks around here have been crumpled and stretched and turned on their sides. By walking along the cleared path beneath the power lines, we were actually able to make our way up the stratigraphic section and get a good look at the types of rocks in a typical greenstone belt.
The first rocks that we encountered were highly deformed and rich in iron and magnesium. These started life as extremely hot lavas bringing material up from deep in the earth, but today have been metamorphosed into chlorite and carbonate minerals. We spent a lot of time at this outcrop, especially because we brought a field-portable spectrometer, so there was a lot of banging on the rocks with hammers and then analyzing with the spectrometer.
In typical greenstone belts, you have repeating packages of rocks that start off magnesium and iron-rich and gradually transition to more silica and aluminum-rich rocks. At the top of this package is often a layer of banded iron formation, deposited in the interval between the volcanic episodes. I should mention that everything seen on this trip was deposited at the bottom of the Archaean ocean, whether it is the volcanic rocks or the banded iron formation sediments.
Banded iron formations are particularly interesting because they can say a lot about the presence of oxygen on the early earth. The origin of banded iron formations is somewhat mysterious, but one leading theory, put forward by Andy Knoll who is on the trip with us, is that you have microbes near the surface of the ocean which release oxygen and oxidize the iron dissolved in the seawater. This oxidized iron then binds to silica dissolved in the water and sinks to the ocean floor, where the respiration of more microbes in the sediments causes most of the iron to go back into solution and the silica to precipitate out along with some iron carbonate.
Nobody knows what governs the thickness of the layers in banded iron formation, but apparently it has been suggested that the finest layers might represent timescales as short as days, as the photosynthetic bacteria-driven oxidation varied with available sunlight. Banded iron formation is stricly an early-earth feature: once the planet became oxygenated thanks to the presence of lots of plan and algae life, the oceans no longer could hold dissolved iron and the iron formations stopped forming.