Silica in BIFs

Following close on the heels of the hypothesis that iron in Precambrian banded iron formations was precipitated by bacteria (see BIFs and bacteria in February 2003 issue of Earth Pages News) is an account of the origin of silica that makes up roughly half the banding (Hamade, T. and 4 others 2003.  Using Ge/Si ratios to decouple iron and silica fluxes in Precambrian banded iron formation.  Geology, v. 31, p. 35-38).  The rare-earth elements and Nd isotopes in the iron-rich layers suggests that they probably originate from ocean-floor hydrothermal activity.  How their cherty layers formed has largely been overlooked.  Before the Cambrian Explosion there were no organisms that secreted silica in their skeletons.  Consequently, the dissolved silica content of Precambrian oceans was probably much higher than now.  Because silica becomes highly soluble only under very alkaline conditions, it may have been close to saturation in Precambrian seawater.  Quite small changes in seawater chemistry would result in its precipitation as fine-grained chert.  But the main issue is where the dissolved silica came from.

Hamade et al. examined the amount of germanium in the cherts, because it is in the same group of elements and acts as if it were a heavy isotope of silicon.  So it follows Si very closely in its distribution.  Alteration of mafic rocks by sea-floor hydrothermal activity dissolves Si, and so does weathering of continental materials; there is a dual source of seawater Si.  However, basalts have more than 10 times as much Ge as do granitic rocks, and the Ge/Si ratio is a good guide to the dominant source of Si.  Cherts in the BIFs from the famous Hamersley basin in Western Australia have Ge/Si ratios that increase with the amount of iron.  The most silica-rich BIFs seem to have formed from waters derived from continental areas, whereas the iron-rich varieties have a sea-floor hydrothermal signature.  The authors conclude that these BIFs formed on a continental shelf subject to regular, periodic upwellings of deep ocean water.

One thought on “Silica in BIFs

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s