Banded Iron Formation and Destruction

Iron formation – Stratigraphic Unit composed of 15-85% FeIronstones –Phanerozoic iron-rich rocksBIF – iron formations with alternating bands of chert

low alumina content - <1%GIF – granular iron formations

terrigenous content with ooliths

Alternating bands of Fe rich and poor layersChert often found as Fe poor layersLayers mm to cm in widthExtremely fine grainedNo detrital materialsArchaen and Paleoproterozoicorigins common

Fewer found in Mesoproterozoic and PaleozoicNew-school formations Rapitan and Lahn–Dill

Important stores of oxygenUp to 20x that of present atmosphere

Important stores of FeAlthough many BIFs are thought to have been subducted already

Under debate for many yearsMechanisms▪ Volcanics▪ Replacement▪ Sedimentary originsLocations▪ Deep vs. shallow seas▪ Terrestrial

Purpose of Lascelles paperUniformitarian explanation of BIFs

Hematite bands rather than chertNo evidence of chert formation/replacementAustraliaHelped to start the

uniformitarian modelOld processes occur as

present processes

Fig. 2. Unweathered chert-free magnetite BIF, Mt. Gibson. Start of coring at bottom left. The first two trays are nearly pure magnetite with abundant iron silicate in third and fourth trays giving lighter gray color. Chert bands arrows) are only present in the last three rows of core at the extreme right, length of tray 1 m

BIFs are used in models attempting to predict

Paleoenvironments▪ [CO2]▪ [O2]▪ [Fe]▪ [Si]

Unless their formation is understood these models are irrelevent

Hydrothermal vents & seafloor spreading centersDeep water environmentsDistributed with currents – slumping moundsFe is supersaturated in anoxic conditions

Precipitate when oxygenated env’t evolves as insoluble iron oxides (FeO) formFalls out of solution around vent areasColloidal suspensions

Today’s oceans and atm is different

Used evidence from Fe formations todayApplied processes to Archaen conditions

Black smokersSpreading centersSites of volcanism

Ferrous Iron reacts with dissolved silicaForms hydrous Al-poor, iron-rich silicatesAdditions of Fe from pre-O2 terrestrial and atmosphere Fe saturation occurred and precipitates (especially) around vents were commonDensity currents distributed Fe deposits

Silicate replacment leads to chert-free BIFsSilicate/water reactions lead to chert BIFs

Chert content suggested to be linked with silicate-bearing organisms“Snowball-Earth” implications

Other Questions?nai.nasa.gov

ocw.mit.edu

Destruction of BIFsBIF explaination for ultralow-velocity zones (ULVZs)

•P-wave velocity used – distance a plume extends into mantle•5- to 40-km-thick region at the base of the mantle •P-wave velocities depressed up to 10% from overlying mantle•This area is termed the ultra-low-velocity zone (ULVZ)

Must be negatively buoyant in mantleConstituents must stick to CMB Must persist at CMB without dissociation for 3GyrPhysical properties consistent with ULVZsVolume of subducted BIFs must be similar to ULVZs

Chemical changes may occur… butBased on angle of subduction and time taken to enter the CMB….Melting temp of FeO is 5000K so only solid-state Rx would destroy BIFs…No first-hand measurement data available Depending on the sound velocity model used –FeO has bulk sound velocity of about 9.3 km/s Consistent with properties of ULVZs…

Although not all of CMB has been observedBIF formation properties appear to be within range of ULVZs

Additional mapping of CMBNot all regions documented

Determine proper sound velocity for FeOUsed MgO and physical properties to make current assumptions