L13- Banded Iron Formations and other minerals

What do the Banded Iron Formations consists of?

Layers alternating with:

• Magnetite (Fe3O4)

• Hematite (Fe2O3)

• Chert (SiO2)

When is the oldest BIF?

Around 3.8 Ga

Why are BIF’s important?

Major source of iron ore

What event are BIF’s related to and what years did it last?

• Great Oxidation Event (GOE)

• 2.4-2Ga

• O2 enters Earth’s system

When were BIF’s abundant?

2.4-1.8 Ga

When was there no dissolved Oxygen?

Archean Eon (4-2.5 Ga)

What was sea water like in the Early Archean?

• No O2

• Rich in dissovled Fe 2+ and SiO2

How did Fe 2+ enter the ocean?

• Erosion of continents

• Spreading ridges

How is Fe 3+ made?

• Oxidation of Fe2+ in seawater

• Green Sulfur Bacteria

What do the BIF represent?

• seasonal temp fluctuations

• Blooms of Green Sulfur Bacteria

How is free O2 produced?

Through the first Cyanobacteria (Photosynthetic) in Stromatolites

When were the first stromatolites?

Around 3.5 Ga

Where is free O2 used

Used to oxidize Fe 2+ to form BIF bands

At what time was there excess O2 that could escape to the atmosphere?

2.2 Ga - Cyanobacteria O2 oxidize all Fe

What caused the production of BIF’s to stop?

• Oxygenated surface waters extend into deeper water - Green Sulfur Bacteria like anoxic environments

• Bacteria are pushed down where it is too dark to photosynthesize

Why does pyrite begin to form?

• sulfate (SO4 2-) produced is washed into the ocean

• Sulfate-reducing bacteria in the deep ocean produce sulfide (H2S), which turns into pyrite (FeS2)

in what form does sulfide accumulate?

in the form of pyrite (H2S)

What are some characteristics of chert?

• Conchoidal fracture

• May occur as nodules or thick banded deposits

• Formation is poorly understood

What are some characteristics of Coal?

• Concentration plant material - not oxidized

• Volatiles driven off from coals

• Very common in the Carboniferous of the Northern Hemisphere

What are Volcaniclastic Sediments?

Clastic rocks composed mostly of volcanic materials that have been transported by wind and water.

What are pyroclastic rocks?

Generated by explosive volcanic activity. Deposits include ash, lapilli, and bombs or blocks.

What are Hyaloclastites?

• Volcanic breccias containing a high proportion of black volcanic glass fragments.

How do Hyaloclastites form?

When volcanic eruptions interact with water. Fragmentation formed by force or by thermal shock.

What are Lahars?

Mudflow or debris flow containing a high proportion of volcanic debris.

How are Lahars triggered?

• Volcanic eruptions

• Earthquakes

• Wet conditions (Rainfall, or Melting of snow or glaciers)