Archean Greenstone Belt

1. Archean Greenstone Belt

2. 1. Greenstone Belt Greenstone belts are generally elongate, Archean to Proterozoic terrains comprising intrusive and extrusive mafic to ultramaficigneous rocks, felsic volcanics, and inter-flow or cover sedimentary rocks. Greenstone belts occur sandwiched between regions dominated by granitoids and gneiss. Greenstones are generally of low to moderate metamorphic grade. The term greenstone comes from the green color of many mafic to ultramafic constituents due to an abundance of chlorite. A common igneous rock in greenstones is komatiite. Komatiites are rocks with greater than 18 weight percent magnesium oxide and a well-developed spinifex texture of inter-locking bladed or acicular crystals of olivine or pyroxene.

3. Spinifex texture (named after similarities in crystal shape and pattern to the spinifex grass that grows in South Africa and Western Australia) implies rapid cooling or decompression of the magma. Komatiites formed as volcanic flows and less commonly as intrusive sills. Sedimentary sequences within greenstone belts comprise both clastic (e.g., conglomerate, quartzarenite, shale and graywacke) and chemically precipitated (e.g., banded iron formation and chert) components. Greenstones may also be intruded by syn-to post-tectonic granitoids. Greenstone belts host many major mineral deposits, such as gold and nickel. Greenstone belts were previously often thought to continue to large depths in the crust. Reflection seismic profiles over the Norseman Wiluna Belt of the Yilgarn Craton, Western Australia, however, indicate that this greenstone belt has a relatively shallow (6–9 km) flat-base and overlies a uniformly thick crust.

4. Contrasting models have been proposed for the origins of greenstone belts. Some geologists believe magmatic and tectonic processes during formation of greenstone belts in Archean times were different to present-day plate tectonics. Earth's mantle would have then been far hotter. They cite differences between greenstone belts and Phanerozoic orogens (such as the abundance of komatiitic lavas) and point out that there are no modern analogues to greenstone belts. Opponents to Archean plate tectonics contend that greenstone belts commonly represent a laterally continuous volcano sedimentary sequence (sometimes on a granite-gneiss basement) essentially undeformed prior to late tectonism and may not therefore represent relics of volcanic chains. They consider that Archean tectonics was dominated by mantle plumes and was possibly analogous to the tectonics of Venus. Greenstone belts are interpreted as oceanic plateaus generated by mantle plumes, similar to plume-generated oceanic plateaus in the southern Caribbean. A mantle plume origin is also proposed for neighboring tonalite-trondhjemite-granodiorite sequences.

5. The alternate view is that tectonic processes comparable to present-day plate tectonics were operative during the Late Archean, and possibly were similar to plate tectonics since the Hadean-Archean transition (between 4.0 and 4.2 billion years ago). In a plate tectonic context, greenstones may have formed in volcanic arcs or inter-arc or back-arc basins. Greenstone belts are interpreted to represent collages of oceanic crust, island arcs, accretionary prisms, and possible plateaus. Recent experimental work on the origin of komatiitic magmas indicates that they were hydrous and that temperatures for their formation do not indicate that the Archean upper mantle was significantly hotter than today. Komatiites and similar rocks have also been found in younger orogens. Komatiites may not therefore require different tectonic processes or conditions for their formation, as previously thought.

6. [Supplement] In many granitoid-greenstone terrains, greenstone belts constitute synformal keels between circular to elliptical granitoid bodies. This outcrop pattern is generally thought to be due to deformation resulting solely from the greater density of greenstones compared to underlying granitoid and gneiss. Due to gravitational instability, the underlying, less dense granitoid-gneiss basement domed upwards and rose to form mushroom-shaped bodies called diapirs whilst the denser greenstones sank into the basement. Shear zones were formed along some granite-greenstone contacts due to differential vertical displacement and upright folds developed in the greenstones. This process of either solid-state and/or magmatic diapirism was independent to any tectonic processes that may have acted on margins to granite-greenstone terrains. The formation of granitoid domes in granite-greenstone terrains has also been attributed to crustal extension (producing metamorphic core complexes) or polyphase folding during regional shortening. The more linear form of some greenstone belts is due to subsequent deformation, especially the superposition of regional-scale transcurrent shear zones on early-formed structures.

7. Isua greenstone belt (Greenland) The Isua Greenstone Belt is an Archean greenstone belt in southwestern Greenland dated at 3.8-3.7 Ga and contains the oldest known, well preserved, metavolcanic (metamorphosedmaficvolcanic), metasedimentary and sedimentary rocks on Earth. It consists of five tectonic domains.

8. Almost all the rocks are deformed and substantially altered by metasomatism, however the transitional stages from the volcanic and sedimentary structures to schists can clearly be seen. The different episodes of metasomatic alterations can also be seen that produced a diversity of metamorphicmineralassemblages from similar protoliths. The metamorphic processes of garnet-growth events span both the early and late Archean. New geological mapping studies are tracing the transitional gradations between the protoliths and their diverse deformed and metasomatised structures. These new mappings show that most of the Isua Greenstone Belt consists of fault bounded rock assemblies (1) derived from basaltand high-magnesiumbasalticpillow lava and pillow lavabreccia, (2) intruded by numerous sheets of tonalite, (3) intercalated with chert-banded iron formations, and a minor component of clasticsedimentary rocks derived from chert and basalticvolcanic rocks.

9. It is thought that the recrystallizedultramafic bodies that occur in the belt are intrusions or komatiitic flows. Studies show that these komatiites are extremely similar to the 3.5 Ga Barbertonbasaltickomatiites of South Africa, and are both Archean equivalents of modern boninites produced by hydrous melting in subduction zones. The Barbertonkomatiites share some of the same geochemical characteristics with modern-day boninites, including petrologic evidence for high magmatic water content. The boniniticgeochemical signatures provide evidence that plate tectonic processes are responsible for the creation of the belt, and that the pillow breccias and basaltic debris indicate that liquid water existed on the surface at the time of their formation. The most common sedimentary rocks are the chertbanded iron formations.

10. [Supplement] The 3.5 Ga Isua basalt-komatiite-chert are parental to the enclosing 2.8 Ga Amitsoq Tonalite-Trondhjemite-Granodiorite (TTG) gneisses suite. Current arguments favor a direct mantle melting to produce the diorites and high magnesian granodiorites found in these Archaean cratons. According to Rapp (1999): "A full continuum of processes can be envisioned from the generation of "pristine" TTG magmas by wet melting of garnet amphibolite/eclogite, to hybridization of these melts by assimilation of peridotite, and the consequent metasomatism of the sub-continental mantle. The Mg-number of Archean granitoids is perhaps the best indicator of TTG lineage, and the extent to which the mantle was involved in early continent formation. The evidence thus far suggests an increasingly important role in the late-Archean, with crustal, oceanic basaltic sources dominating in the early-Archean."

11. [Supplement] Quartz globules from undeformed pillow breccia are associated with a complex system of quartzveins and are interpreted as remnants of a sea-floor hydrothermal system, operating at the time of lava eruption and pillow basalt formation at 3.75 Ga. The quartzglobules are thought to be interpreted as former gas vesicles filled with quartz and carbonate, and are embedded in an altered basalticmatrix now comprising biotite, muscovite and quartz. Silica-filling in the vesicles is thought to have been contemporaneous with the formation of an intricate hydrothermalvein system. During deformation, the strain is thought to have been partitioned into the mica-rich rock matrix, when the vesicles behaved as competent objects, eventually cutting or deforming the thinner veins. Fluid inclusions from quartz in the vesicles resemble present-day sea-floor hydrothermal fluids and are thought to be responsible for the alteration of the pillowed breccia and co-precipitation of quartz and carbonate. Therefore, these carbonates may represent the products of an early sea-floor.

12. [Supplement] In the Southwestern Isua Greenstone Belt, Kyanite has been found in the muscovite-rich schists and was formed by metasomatic reaction. This shows that this part of the belt was subjected to deformation in high strain zones and high grade metamorphism during the late Archaean. According to a study by Frei and Rosing (1999): The kyanite are Al-rich metasomatites formed by transformation of felsicgneisses by fluids derived from ultramaficschists. Peridotite and dunite bodies were transformed into talc-anthophyllite-chlorite-magnesiteschists during prograde reaction with ambient fluids. During amphibolitefaciesmetamorphism and deformation, the ultramaficschist released fluids, which interacted with neighboring lithologies. These fluids were buffered at silica activities...by the talc-anthophyllite-magnesite mineral assemblage. At this low silica activity, chlorite in the ultramaficschists buffered (aAl+)/(aH+)3 activity ratios close to that of corundumsaturation.

13. [Supplement] These fluids caused silica depletion, and high aluminumion activities that intersected kyanitesaturation in the quartz saturated felsic rocks. Progressive leaching in three steps defines a Pb-Pb isochron with an age of 2847 ± 26 Ma. This age is consistent with a Sm-Ndamphibole-plagioclaseisochron of 2849 ± 116 Ma from maficschists at Isua, and a lead (Pb) step leaching age of 2840 ± 49 Ma (MSWD=1.43) on magnetite from Isua Banded Iron Formation (BIF), and with regional granulitefaciesmetamorphism in other parts of the Greenland Archaean block.

14. Belingwe Greenstone Belt(Zimbabwe) Samples taken from the NERCMAR drill hole in the 2.7 Ga Manjeri Formation in the Belingwe Greenstone Belt contain oxide and sulphide ironstones that are indicative of a complex bacteria/archaeaeclogical community. The REE compositions imply an oceanicdeposition similar to that of the late Archean. (Bickle et. al, 1999). The BelingweGreenstoneBelt contains a 7 km succession of mafic and ultramafic lavas and high-level intrusions which overlie a thin sedimentary formation, itself unconformable on a granitic basement. The lavas range in composition from andesites (4 per cent MgO) to peridotitic komatiites (32 per cent MgO). The mineralogy and textures of the most magnesian lavas demonstrate that they were extruded in a completely liquid state. If the source mantle had an MgO content around 40 per cent, then partial melts in the range 35 per cent to 55 per cent would be required to produce the most magnesian liquids observed. Physical constrains on the origin of the mafic and ultrafic lavas Imply a derivation from a depth of >150 km, at temperatures of 1600-2000 oC (Nisbet et al., 1977).

15. A Review of Greenstone Belts in the Superior Province and the Evolution of Archean Tectonic Processes. (Development of Abitibi Greenstone Belt) Kirsty Y. Tomlinson Australian Centre for Astrobiology, Department of Earth and Planetary Sciences, Macquarie University, NSW, 2109, Australia. Introduction The origin and development of Archean greenstone belts continues to be strongly debated, particularly with regard to the roles of subduction, plume magmatism, rifting, diapirism and autochthonous vs allochthonous development (e.g. de Wit, 1998; Hamilton, 2003). It is apparent from studies in the Superior and Slave Provinces of Canada that strongly contrasting tectonic styles may have been in operation at the same time. For example at ca. 2.7 Ga, large diapiric batholiths and synclinal greenstone keels may suggest that diapirism was an important tectonic process in the Slave Province (Bleeker, 2002), whereas in the Superior Province the linear distribution of belts suggests that accretionary tectonics (i.e. plate tectonics) may have dominated (e.g. Stott, 1997). Neither theory precludes the other, and in developing models for Archean tectonic evolution, no one model will be equally applicable to all areas.

17. Greenstone belt types In the Superior Province, greenstone belts are typically collages that contains more than one sequence of rocks. These sequences may have different ages and distinct histories. There are several common types of sequence (or stratigraphic associations) that are found in volcanic dominated greenstone belts, and their features indicate certain environments of formation. These features can also be used to suggest tectono-magmatic settings but are usually open to interpretation.

18. 1) Flood volcanism on submerged (shallow water) continental platforms – these sequences contain thick, laterally extensive tholeiitic mafic flows, pillow basalts, hyaloclastite ± komatiites and minor amounts of felsic tuff, BIF, cherty and clastic sedimentary units. Rarely unconformities are preserved at the base of these sequence, where thin conglomerate-quartzite-arkose-carbonate units overly tonalitic basement. More commonly the base of the sequence is not preserved but detrital zircon ages in sedimentary rocks correspond to the age of nearby granitoid rocks which are inferred to represent basement. The volcanic rocks may also contain xenocrystic zircons, Nd isotopic evidence of older crustal involvement, and geochemical signatures suggesting felsic crustal assimilation. These sequences typically occur early in the development of Archean cratons and are common at 3.0-2.9 Ga in theSuperior Province (North Caribou and Marmion terranes). In the North Caribou terrane evidence of graben development exists in the basement suggesting extension prior to volcanism, and plume-driven rifting has been suggested as the environment of formation. This environment does not have a true modern analogue as the continental flood basalts on the modern Earth are subaerial.

19. 2) Submarine volcanic plains – comprise massive and pillowed tholeiitic mafic flows ± komatiite, BIF, mudstones and rare greywackes. The lavas have juvenile Nd isotopic signatures, lack significantly older zircon inheritance and have primitive mantle normalised REE profiles that are flat to slightly depleted in light REE (but less depleted than modern MORB) or slightly enriched in light REE. The sequences often have tectonic contacts and may be fragments of thicker sequences. They appear to have formed in oceanic environments, and suggested tectonic settings are oceanic plateau, back-arc basin, primitive island arc, oceanic island or possibly mid-ocean ridge. These sequences often flank older continental blocks and may have been accreted to continental margins. Examples are the older (ca. 2.88 Ga?) parts of the Oxford-Stull Lake terrane flanking the northern margin of the North Caribou terrane; the older (ca. 2.78 Ga) parts of the Western Wabigoon terrane flanking the Winnipeg River and Marmion terranes; and parts of the ca. 2.75-2.70 Ga Abitibi subprovince.

20. 3) Diverse volcanic sequences – comprise a variety of submarine to lesser subaerial units, dominated by basalt, with lesser andesite, dacite and rhyolite flows, and dacite-rhyolite pyroclastic units. In some belts komatiites are also present. Both tholeiitic and calcalkaline signatures are often observed, with basalts ranging from slightly light REE depleted (but less depleted than modern MORB) to moderately light REE enriched with negative Nb anomalies (similar to modern arc related basalts). These sequences are commonly associated with syn-volcanic granitoids. Nd isotopic data suggest that some sequences are juvenile while others have experienced minor interaction with older enriched sources (metasomatised mantle or crustal contamination?). They are suggested to represent arc magmatism (both island arc and continental arc on a thin margin), arc/plume interaction, arc rifting or back-arc magmatism. Examples are 2.83 Ga sequences in the Oxford-Stull Lake terrane flanking the northern margin of the North Caribou terrane; various ca. 2.9-2.74 Ga sequences in the Uchi subprovince at the southern margin of the North Caribou terrane; 2.75-2.71 Ga sequences in the Western Wabigoon terrane; and parts of the ca. 2.75-2.70 Ga Abitibi subprovince

21. 4) Continental felsic volcanic centres – comprise thick sequences of massive calc-alkaline dacite-rhyolite flows and pyroclastics with lesser calc-alkaline basalts and andesites and syn-volcanic plutons. They may unconformably overly older tholeiitic to calc-alkaline sequences but basal contacts are usually tectonised or intruded by younger granitoids. Indirect evidence for eruption on older basement occurs in the form of older Nd model ages, zircon inheritance, evolved geochemical signatures and the maturity of the sequences. They are suggested to represent continental arc magmatism and examples are widespread at 2.73-2.71 Ga in the western Superior Province, occurring along both the northern and southern flanks of the North Caribou terrane (in several greenstone belts along a 2000 km long continental margin), and in the eastern part of the Wabigoon subprovince.

22. 5) Late alkaline-shoshonitic sequences – occur locally in the Superior Province associated with late transpressional faults (at ca. 2.71-2.68 Ga, younging from north to south), following the major N-S shortening event. They comprise alkaline volcanic rocks with evolved Nd isotopic signatures and geochemistry, and continent-derived alluvial-fluvial sedimentary rocks with a large diversity of detrital zircon ages. The sequences are thought to have formed in late pull-apart basins and well-developed examples include 2.71-2.70 Ga sequences along the north and south margins of the North Caribou terrane (in the Oxford-Stull Lake terrane and the Uchi subprovince); and 2.69-2.68 Ga sequences in the Abitibi subprovince where the type locality of this “Timiskaming” sequence occurs.

23. Plate tectonics in the Archean It is clear from the Superior Province that 3.0-2.7 Ga greenstone belts formed in both oceanic and continental environments. Archean “oceanic” sequences may not represent true oceanic crust generated at mid-ocean ridges but an abundance of other oceanic environments appears to be represented by the diverse rock record (e.g. primitive island arc, back-arc and oceanic island sequences). Continental sequences also appear to represent both divergent and convergent plate settings, related to rifting or hot-spot magmatism and subduction-zone magmatism. The diversity of greenstone belt sequences requires a diversity of tectono-magmatic processes to generate them, and is most consistent with the operation of plate tectonics (or something resembling it) in the Archean.

24. [Supplement] It has been suggested from the secular evolution of Archean granitoid rocks (e.g. Smithies et al., 2003; Martin and Moyen, 2002), that plate tectonics may have evolved through the Archean to become more similar to present day plate tectonics. They suggest that prior to 3.1 Ga the angle of the down-going oceanic plate in subduction zones may have been shallow or even flat and hence excluded development of a mantle wedge. Tonalites were generated by melting of the subducted oceanic plate, but the magmas did not interact with the mantle wedge prior to their emplacement in the crust. After 3.1 Ga a systematic increase of Cr and Ni in tonalite suggests increasing interaction of slab melts with a mantle wedge, indicating a gradually thicker mantle wedge and hence steeper angles of subduction. Mantle wedge processes in subduction zones therefore became increasingly important after 3.1 Ga which is consistent with the increasing presence of mantle wedge-derived rocks (such as calc-alkaline basalts, sanukitoids and Nbenriched basalts) in the mid to late Archean record.

25. 2. Komatiite Komatiites are ultramafic mantle-derived volcanic rocks. They have low SiO2, low K2O, low Al2O3, and high to extremely high MgO. Komatiites were named for their type locality along the Komati River in South Africa. True komatiites are very rare and essentially restricted to rocks of Archaean age and most are greater than two billion years old, restricted in distribution to the Archaean shield areas. Komatiites occur with other ultramafic and high-magnesian mafic volcanic rocks in Archaean greenstone belts.

26. Komatiites are restricted to the Archaean, with few Proterozoic and few Mesozoic or Phanerozoic komatiites known (although high-magnesian lamprophyres are known from the Mesozoic). This restriction in age is thought to be due to secular cooling of the mantle, which may have been up to 500 °C hotter during the early to middle Archaean (4.5 to 2.6 Ga). The early Earth had much higher heat production, because of the greater abundance of radioactive elements, as elements with a relatively short half-life, such as the uraniumisotope with mass 235, have appreciably diminished in abundance by radioactive decay. The youngest komatiites are from the island of Gorgona on the Caribbean oceanic plateau.

27. Komatiite core

28. Komatiite

29. Thin section through the coarse-bladed olivine spinifex zone in the slowly cooled interior of a komatiite unit. Samples are ~0.5 m from upper chill margin. The long, dark stripes in the thin section outline the shapes of olivine (now altered to serpentine and magnetite) blades that reach 15 cm in length in this portion of the unit. The lighter-colored triangles and quadrilaterals contain tremolite, chlorite and magnetite and represent regions where spinel and high-Ca pyroxene crystallized after the early olivine blades.

30. Diamonds in volcaniclastic komatiite from French Guiana Capdevila et al. (1999) Nature The world's main sources of non-alluvial diamonds are found in ultrapotassic kimberlite1 or lamproite2 diatremes (pipes filled with explosive volcanic debris), most of which have Phanerozoic ages and are located in stable Precambrian cratons. Diamond exploration has therefore tended to focus on such deposits. Microdiamonds are known to occur in metamorphic rocks such as gneiss3 and eclogite4 that have equilibrated deep in the mantle and were then tectonically transported to the surface, but such deposits are thought to have little commercial potential. Here we report a new type of diamond occurrence from the Dachine region in French Guiana for which the host rock is volcaniclastic komatiite—an unusual type of volcanic rock whose composition and origin are quite unlike those of kimberlite and lamproite. These komatiites form part of a Proterozoic island-arc sequence, a tectonic setting distinct from that of all other currently exploited diamond deposits. The discovery of diamonds in volcaniclastic komatiite has implications not only for diamond exploration, but also provides strong evidence that these komatiite magmas originated at depths of 250 km or greater within the Earth.

32. 3. Banded iron formation (BIF) Banded iron formation is iron rich chert (cryptocrystalline silica, SiO2). The banded colors, usually on a cm scale are due to differing amounts and oxidation states of Fe-containing minerals: hematite, magnetite, grunerite, siderite and sometimes pyrite. BIF is not forming today and although it can be found in the Archean, most deposits of BIF were formed around 2 billion years ago. Banded iron formations are a distinctive type of rock often found in old sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite or hematite, alternating with bands of iron-poor shale and chert. Some of the oldest known rock formations, formed around three thousand million years before present (3 Ga), include banded iron layers, and the banded layers are a common feature in sediments for much of the Earth's early history.

33. Banded Iron Formations are composed of alternating layers of iron-rich material (commonly magnetite) and silica (chert). Each layer is relatively thin, varying in thickness from a millimeter or so up to several centimeters.

34. BANDED IRON-FORMATIONS (BIFS) WORLDWIDE. BIFs are the diagenetic product of a chemical precipitate in a very iron-rich-system: SiO2-FeO-Fe2O3-CaO-MgO-CO2-H2O. They occur in the geologic record from 3.8 Ga (Isua, West Greenland) to about 1.8 Ga with a maximal abundance at about 2.5 Ga, and a reoccurrence in Neoproterozoic time (from about 0.8 and 0.6 Ga). Many of the 3.8 to 1.8 Ga BIFs have very similar average chemistries and the late diagenetic assemblages consist mainly of chert, magnetite, hematite, stilpnomelane and carbonates (siderite, dolomite to ankerite, calcite). Regional metamorphism results in assemblages rich in various amphiboles, and at higher grades, various pyroxenes. Several major BIFs in Brazil with an age of about 2.4 Ga are much richer in Fe3+ (almost all hematite-rich) than normal, possibly as a result of deep weathering and secondary oxidation. The Neoproterozoic BIFs are chemically distinctly different from most others because about 95% of their total iron is Fe3+.

35. 3.8 Ga Isua BIF SW Greenland

36. Archean Folded BIF: Ord Range, W Australia Dimension: 15 cm in vertical scale Light: chert jasper bands Dark: magnetite- rich bands

37. Stromatolite, Ord Range, W Australia Stromatolite: An organosedimentary structure produced by sediment trapping, binding, and/or precipitation as a result of growth and metabolic activity of micro- organism principally cyanophytes (blue/green algae).

38. All BIFs between 3.8 and 1.8 Ga show REE patterns with pronounced positive Eu anomalies, negative Ce anomalies and depletion in the light REE. These patterns are the result of chemical precipitation from solutions that represent mixtures of seawater and hydrothermal input (of Fe and Si) from spreading centers in oceanic crust. The Neoproterozoic BIFs (e.g., Rapitan iron-formation, Yukon, Canada) display a lack of the Eu anomaly, and their overall REE pattern is very similar to that of modern ocean water at 100 m. This suggests that the hydrothermal input was highly diluted by ocean water at this late Precambrian time. The Neoproterozoic iron-formations commonly show a close association with glaciogenic deposits. The Rapitan BIF is interpreted as having been deposited during a major transgressive event with a rapid sea-level rise during an interglacial period, after earlier buildup of ferrous iron in solution in deeper water during a glacial period.

39. Earlier banded iron formations The conventional concept is that the banded iron layers were formed in water as the result of oxygen released by photosyntheticcyanobacteria, combining with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the substrate, which may have been anoxic mud (forming shale and chert). Each band is similar to a varve. The banding is assumed to result from cyclic variations in available oxygen. It is unclear whether these banded formations were seasonal or followed some other cycle. It is assumed that initially the Earth started out with vast amounts of iron dissolved in the world's acidic seas. Eventually, as photosynthetic organisms generated oxygen, the available iron in the Earth's oceans was precipitated out as iron oxides.

40. It is theorized that the Earth's primitive atmosphere had little or no free oxygen. In addition, Proterozoic rocks exposed at the surface had a high level of iron, which was released at the surface upon weathering. Since there wasn't any oxygen to combine with it at the surface, the iron entered the ocean as iron ions. At the same time, primitive photosynthetic blue/green algae was beginning to proliferate in the near surface waters. As the algae would produce O2 as a waste product of photosynthesis, the free oxygen would combine with the iron ions to form magnetite (Fe3O4). This cleansed the algae's environment. As the biomass expanded beyond the capacity for the available iron to neutralize the waste O2 the oxygen content of the sea water rose to toxic levels. This eventually resulted in large-scale extinction of the algae population, and led to the accumulation of an iron poor layer of silica on the sea floor. As time passed and algae populations re-established themselves, a new iron-rich layer began to accumulate. Unfortunately, the algae would again proliferate beyond the capacity of the iron ions to clean up their waste products, and the cycle would repeat. This went on for approximately 800,000,000 years!

41. Later banded iron formations Until fairly recently, it was assumed that the rare later banded iron deposits represent unusual conditions where oxygen was depleted locally and iron-rich waters could form then come into contact with oxygenated water. An alternate explanation of these later rare deposits is undergoing much research as part of the Snowball Earth hypothesis — wherein it is believed that an early equatorial supercontinent (Rodinia) was totally covered in an ice age (implying the whole planet was frozen at the surface to a depth of several kilometers) which corresponds to evidence that the earth's free oxygen may have been nearly or totally depleted during a severe ice age circa 750 to 580 Ma prior to the Ediacaran wherein the earliest multicellular lifeforms appear. Alternatively, some geochemists suggest that BIFs could form by direct oxydation of iron by autotrophic (non-photosynthetic) microbes. The total amount of oxygen locked up in the banded iron beds is estimated to be perhaps twenty times the volume of oxygen present in the modern atmosphere. Banded iron beds are an important commercial source of iron ore.

42. BIF and oxygen state in the Archean

43. Addition of O2 to the Atmosphere • Today, the atmosphere is ~21% free oxygen. How did oxygen reach these levels in the atmosphere? Revisit the oxygen cycle: • Oxygen Production • 。Photochemicaldissociation - breakup of water molecules by ultraviolet • Produced O2 levels approx. 1-2% current levels • At these levels O3 (Ozone) can form to shield Earth surface from UV • 。Photosynthesis - CO2 + H2O + sunlight = organic compounds + O2 - produced by cyanobacteria, and eventually higher plants - supplied the rest of O2 to atmosphere. • Oxygen Consumers • 。Chemical Weathering - through oxidation of surface materials (early consumer) • 。Animal Respiration (much later) • 。Burning of Fossil Fuels (much, much later)

44. Cyanobacteria Throughout the Archean there was little to no free oxygen in the atmosphere (<1% of presence levels). What little was produced by cyanobacteria, was probably consumed by the weathering process. Once rocks at the surface were sufficiently oxidized, more oxygen could remain free in the atmosphere. During the Proterozoic the amount of free O2 in the atmosphere rose from 1 - 10 %. Most of this was released by cyanobacteria, which increase in abundance in the fossil record 2.3 Ga. Present levels of O2 were probably not achieved until ~400 Ma. Evidence from the Rock Record ‧Iron (Fe) is extremely reactive with oxygen. If we look at the oxidation state of Fe in the rock record, we can infer a great deal about atmospheric evolution. ‧Archean - Find occurrence of minerals that only form in non-oxidizing environments in Archean sediments: Pyrite (FeS2), Uraninite (UO2). These minerals are easily dissolved out of rocks under present atmospheric conditions.

45. ‧Banded Iron Formation (BIF)- Deep water deposits in which layers of iron-rich minerals alternate with iron-poor layers, primarily chert. Iron minerals include iron oxide, iron carbonate, iron silicate, iron sulfide. BIF's are a major source of iron ore, they contain magnetite (Fe3O4) which has a higher iron-to-oxygen ratio than hematite. These are common in rocks 2.0 - 2.8 B.y. old, but do not form today. ‧Red beds(continental siliciclastic deposits) are never found in rocks older than 2.3 B. y., but are common during Phanerozoic time. Red beds are red because of the highly oxidized mineral hematite (Fe2O3), that probably forms secondarily by oxidation of other Fe minerals that have accumulated in the sediment. Conclusion - amount of O2 in the atmosphere has increased with time.

46. Supplement-1 Evolution of the Hydrosphere and the Atmosphere The atmosphere and the oceans both arose from volcanic degassing very early in earth history. The lighter fraction made up the atmosphere while the heavier fraction made up the oceans. Also the presence of water served as transporter of soluble solids and gases between the land, sea and atmosphere. Early Degassing : Loss of Noble Gases Helium, argon and xenon in low concentration when compared to their cosmic abundance. It must have been lost from the earth at high rates, early on (i.e., the first 50 million years of earth history) in order to be so low now. If degassing from volcanoes has always has the same composition, the major gasses would be water vapor and CO2 while the minor gasses would be H2S, CO, H2, N2, CH4, NH3, HF, HCl, and Ar. But no oxygen.

47. The Evolution of Oxygen Oxygen is toxic to living systems. Oxygen-mediating enzymes had to evolve to handle the toxic properties of O2 Different segments of the solar spectrum are capable of creating O2. 1. Ultra-violet (1500 and 2100 angstroms causes the photodissociation of H20 in the upper atmosphere. H2 drifts into outer space, leaving O2 behind. This this mechanism accounts for only 0.1% of the present atmospheric level i.e., PAL). 2. Visible light, used by photoautotrophs, creates far more O2 from CO2 and H2O and photosynthetic enzymes (e.g., chlorophyll). The carbon released by photosynthesis was held by these organisms by the creation of organic chemicals used in growth. Stages of Oxygen Development through Geologic Time

48. Oceanic Oxygen • 1. 4.0-3.2 Billion Years Ago • a. Oxygen held in mantle and oceanic crust. Little outgassed • b. That which did was trapped in evaporite precipitation and the oxidation of CO to CO2. • c. Fe+2 and Mn+2 accumulated in the sea from hydrothermal vents and the leaching of volcanics and immature sediments. These two are soluble in their +2 state. The lack of O2 kept them reduced and soluble.

49. 3.2-2.6 Billion Years Ago Early photoautotrophic organisms release small concentrations of O2 that oxidized Fe+2 creating insoluble Fe+3 precipitates in Banded Iron Formations (i.e., BIF) in island arc (i.e., Algoma type BIF) and shallow shelf environments (i.e., Superior type BIF) as a result of oceanic upwellings. • 2. 2.6-2.0 Billion Years Ago • a. Development of extensive Superior-type BIF and manganese deposits capturing O2 released by the photoautotrophs. • b. Stromatolites in limestones expand rapidly both providing O2 to the ocean while capturing carbon at the same time. • c. The presence of oxygen cleared the sea of soluble iron and manganese so that little further oxygen was needed for BIF and manganese deposit formation.

50. B. Atmospheric Oxygen. Oxygen produced by photoautotrophs was used in BIF and manganese deposits. Once O2 production increased beyond that needed by BIF and manganese deposit formations, could it accumulate in the ocean and in the atmosphere. Free oxygen began to accumulate in the atmosphere between 2.4 and 1.9 billion years ago. Evidence for this includes the following. 1. BIF deposits iron-leaching from paleosols disappear after 1.9 billion years. The presence of oxygen rendered iron and manganese insoluble, hence they would not leach. Leaching could only occur if iron and manganese were in the +2 state and therefore soluble and mobile.