Banded Iron Formation and Destruction
Transcript of Banded Iron Formation and Destruction
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