Paleoproterozoic Banded Iron Formations Research Articles

How does weathering influence geochemical proxies in Paleoproterozoic banded iron formations? A case study from outcrop samples of 2.46 Ga banded iron formation, Hamersley Basin, Western Australia

Trace metal and rare earth element (REE) abundances in banded iron formations are critical for assessing the chemical composition of ancient seawater and the long-term evolution of the ocean-atmosphere system. Recent studies, however, have highlighted the potential effects of outcrop weathering, raising concerns about whether banded iron formation samples are suitable proxies for ancient redox conditions or if exposure to surficial weathering regimes may have altered key geochemical signals. Here, we present a detailed, high-resolution study of several banded iron formation outcrop samples from the Hamersley Basin, Western Australia, to investigate microscale differences in composition between banded iron formation and weathered surfaces (i.e., weathered crusts). Elemental mapping and bulk-rock geochemical analyses reveal that weathered crust is more enriched in most elements than the banded iron formation, except for silica, which is significantly depleted. There is also a significant loss of redox-sensitive elements (RSEs) in the weathered surface, which suggests that outcrop samples have been affected by higher degrees of chemical leaching than physical erosion. These results are significant, because we clearly show that the geochemical characteristics of the weathered surface—irrespective of how it formed—are distinct from those of the remainder of the sample. This means that with sufficient screening of samples for obvious signs of alteration, banded iron formation outcrop samples may indeed be used as a reliable proxy for the evolution of Earth’s coupled ocean-atmosphere system. This increases the volume of easily accessible Precambrian sample material, so that researchers no longer solely need to rely on core recovered through costly drilling programs.

Antiperthite and Mesoperthite Exsolution Textures in the Zhengjiapo BIF, Changyi Metallogenic Belt, North China Craton: Evidence of UHT Metamorphic Overprint

Paleoproterozoic banded iron formation (BIF) iron ore of the Zhengjiapo region of the Changyi metallogenic belt, Eastern Block of North China Craton contains abundant coexisting antiperthite and mesoperthite textures. The antiperthite and mesoperthite occur in felsic domains of the Zhengjiapo BIF ore and enable derivation of peak temperature metamorphic conditions. Thermodynamic phase modeling shows that equilibrium conditions of corresponding textures, considering the related mineral assemblage of Pl + Qz + Kfs + Mag + Opx + L, are in the range of 870–940 °C and 5.0–6.8 kbar. Ternary feldspar thermometry using reintegrated compositions of antiperthite and mesoperthite in the felsic domain of the studied BIF iron ore reveals even higher peak metamorphic temperatures of 1045–1080 °C. The ultra-high temperature–low pressure conditions of Precambrian BIF have not yet been reported from the North China Craton. The documented ultra-high temperature metamorphism of the Zhengjiapo BIF iron ore in the Changyi metallogenic belt indicates that the BIF was involved in the collision-related tectonic process during Paleoproterozoic to have occurred in the Jiao-Liao-Ji orogenic belt.

Cyclic tidalites and seismites at a submarine hydrothermal system for a 2450 Ma banded iron formation, Hamersley Basin, Western Australia

The Hamersley Group in the Hamersley Basin, Western Australia, contains major deposits of banded iron formation of Neoarchean–Paleoproterozoic age (2629–2449 Ma). The 2450 Ma Weeli Wolli Formation near the top of the group includes banded iron formation with microbands (≤0.05–0.5 mm to rarely 1 mm thick) of chert and iron oxide lamina couplets that display distinctive cycles. Some adjacent, thin microbands have merged with compaction and the diagenetic removal of chert laminae, and maximum counts of cycle period by different studies is preferred, suggesting a period of ∼28 ± 2 microbands. Associated cyclic iron formation shows soft-sediment plastic deformation and brittle fracturing. Rare earth element geochemistry indicates that much of the iron in late Archean–early Paleoproterozoic banded iron formations was derived from submarine hydrothermal systems. Monitoring of hydrothermal vents on mid-ocean ridges and seamounts has identified tidal modulation of the discharge, temperature and dispersal of deep-sea hydrothermal plumes principally by semidiurnal variations in tidal loading and earth tides. Microearthquake swarms are associated with deep-sea hydrothermal systems at the crests of mid-ocean ridges, and tidal triggering of microearthquake activity is indicated for a hydrothermal system on the East Pacific Rise. The microband cycles in the Weeli Wolli Formation are interpreted as tidalites, comprising ∼28 ± 2 semidiurnal microbands per lunar (synodic) fortnightly cycle, related to the tidally modulated activity of a submarine hydrothermal system. The associated plastically deformed and brittle-fractured sediments are viewed as seismites resulting from microearthquakes also related to the hydrothermal system. The Weeli Wolli microband cycles and seismites may represent a rarely identified instance of iron-formation deposition near a submarine hydrothermal system. KEY POINTS Submarine hydrothermal venting was a major source of iron in late Archean–early Paleoproterozoic banded iron formations. Semidiurnal variations in tidal loading and earth tides commonly modulate present-day deep-sea hydrothermal activity, and microearthquake swarms are associated with deep-sea hydrothermal systems. Early Paleoproterozoic banded iron formation from the Hamersley Basin displays cycles with an estimated period of ∼28 ± 2 microbands and associated plastic deformation and brittle fracturing. The microband cycles are interpreted as tidalites comprising ∼28 ± 2 semidiurnal increments grouped in fortnightly cycles at a tidally modulated submarine hydrothermal system, and the deformed sediments as seismites caused by related microearthquake activity.

Fossil biomass preserved as graphitic carbon in a late Paleoproterozoic banded iron formation metamorphosed at more than 550°C

Metamorphism is thought to destroy microfossils, partly through devolatilization and graphitization of biogenic organic matter. However, the extent to which there is a loss of molecular, elemental and isotope signatures from biomass during high-temperature metamorphism is not clearly established. We report on graphitic structures inside and coating apatite grains from the c. 1850 Ma Michigamme silicate banded iron formation from Michigan, metamorphosed above 550°C. Traces of N, S, O, H, Ca and Fe are preserved in this graphitic carbon and X-ray spectra show traces of aliphatic groups. Graphitic carbon has an expanded lattice around 3.6 A, forms microscopic concentrically-layered and radiating polygonal flakes and has homogeneous δ13C values around −22‰, identical to bulk analyses. Graphitic carbon inside apatite is associated with nanometre-size ammoniated phyllosilicate. Precursors of these metamorphic minerals and graphitic carbon originated from ferruginous clay-rich sediments with biomass. We conclude that graphite coatings and inclusions in apatite grains indicate fluid remobilization during amphibolite-facies metamorphism of precursor biomass. This new evidence fills in observational gaps of metamorphosed biomass into graphite and supports the existence of biosignatures in the highly metamorphosed iron formation from the Eoarchean Akilia Association, which dates from the beginning of the sedimentary rock record. Supplementary material: Figures illustrating the petrography and analytical results are available at: https://doi.org/10.6084/m9.figshare.c.4430609

Genesis of Huoqiu banded iron formation (BIF), southeastern North China Craton, constraints from geochemical and Hf-O-S isotopic characteristics

The Huoqiu iron deposit is one of the few Superior-type BIF ore fields in China. To clarify the genesis of Huoqiu BIFs, we give out the geochemical and O-S isotopic data of the BIF samples and zircon Hf isotopic data of the wall-rock. The Huoqiu BIFs show dominant composition of SiO2 + Fe2O3T, and varying contents of Al2O3, MgO, CaO, Rb, Sr, Zr and Hf, suggesting their formation mainly through chemical precipitation and incorporation of terrigenous felsic clastics. Shale-normalized rare earth element patterns of the BIF samples show the features characteristic of other Archean and Paleoproterozoic BIFs, with HREE enrichment relative to LREE, positive La and Eu anomalies, and superchondritic Y/Ho ratios comparable to modern seawater. Positive Eu anomalies are attributed to an imprint of high-temperature hydrothermal fluids. None to negative Ce anomaly shown by the BIFs imply the Huoqiu BIFs was precipitated from the mildly oxidized seawater in a nearshore setting. The quartz separated form Huoqiu BIF has δ18OV-SMOW values of 11.8‰–14.8‰, within the extent of the siliceous rocks deposited from high-temperature hydrothermal. The δ34S values of pyrite grains vary from −2.84 to +4.38‰, which is similar as the sulfur derived from mixtures between mantle fluids and sediments; The Δ33S values varied from −0.08‰ to +1.03‰, imply that the Huoqiu BIFs deposited distally from the volcanic center. The Hf model ages of these detrital zircons from the wall rock give three prominent peak at 3.1–3.0 Ga, 2.9–2.7 Ga and 2.5 Ga, respectively, it suggest that there were three episodes of the crust growth in the Huoqiu area. Integrated these data with the pioneering studies on the Huoqiu BIFs and wall rocks, we confirmed that the Huoqiu BIFs were sourced from the high-temperature hydrothermal related with the volcanics and precipitated from the mildly oxidized seawater in a nearshore setting and distally from the volcanics at 2.5 Ga.

The role of deformation in the formation of banded iron formation-hosted high-grade iron ore deposits, Hamersley Province (Australia)

The Hamersley Province (Western Australia) hosts some of the world’s largest iron ore deposits but despite decades of research, their genesis is still extensively debated. Many iron ore deposits are hosted in complexly deformed Archean to Paleoproterozoic banded iron formations, comprising thin chert and iron oxide bands interlayered with silicate-rich shales and carbonates. Current iron ore genesis models have identified a strong structural control on ore formation linked to extensive hypogene and supergene fluid circulation along fault structures. These fluid pathways facilitate the removal of vast amounts of gangue minerals, leading to enrichment of the iron oxide residue to iron ore. However, the evolution of the associated structures has not yet been considered as a key element in ore genesis.Here we show through multiscale structural analyses that deformation not only forms suitable fluid channels, but that folding and shearing also result in significant synkinematic removal of gangue minerals. Our multidisciplinary investigation of the structural evolution of the Mount Tom Price deposit combines microtectonic, field geology and 3D implicit modelling techniques to establish a link between deformation structures at various scales. Microscale shear bands and outcrop-scale asymmetric parasitic folds share striking similarities in their evolution and their controlling mechanisms. Both features record substantial non-coaxial deformation accompanied by volume changes due to stress-induced silica remobilisation. The closely spaced layering of rheologically different lithologies within Hamersley Province strata plays a crucial role in complex multilayer deformation, which resulted in extensive strain partitioning.Our study suggests that deformation was of major significance in the upgrading of banded iron formation to iron ore and was active from the early stages of banded iron formation during diagenesis. Deformation structures also established a micro- to deposit-scale lateral and vertical fluid network, which enabled infiltration by hypogene and supergene fluids during or after deformation. These new insights have important implications for iron ore genesis models, structural applications in the mine environment, and for understanding complex multilayer deformation with volume loss.

Decoupled sources of the 2.3–2.2 Ga Yuanjiacun banded iron formation: Implications for the Nd cycle in Earth’s early oceans

The recognized worldwide gap in BIF deposition between 2.4 and 2.0 billion years ago has long been considered as an obstacle to fully determining the geochemical composition of seawater at that time. However, the recently dated 2.3–2.2Ga Yuanjiacun banded iron formation (BIF) in the North China Craton (NCC) offers a possibility to redress these uncertainties. Shale-normalized rare earth element-yttrium (REE+Y) patterns of the BIF and interlayered meta-chert samples show features characteristic of other Archean and Paleoproterozoic BIFs, with HREE enrichment relative to LREE, positive La and Eu anomalies, and superchondritic Y/Ho ratios comparable to modern seawater. Very low Al2O3 (<0.5wt%) and high field strength elements (HFSE) concentrations (<10ppm) indicate an essentially detritus-free depositional setting, while positive Eu anomalies are attributed to an imprint of high-temperature hydrothermal fluids. Sm–Nd isotopic features further point to two periodically interacting water masses controlling the deposition of the BIF. The first is seafloor-vented hydrothermal fluids (εNd(t)∼+3.5) derived from interaction with a depleted mantle source and associated with high Fe fluxes. The second is ambient surface seawater (εNd(t)∼−2.4), which obtained its signature through weathering of the nearby landmasses and associated with high Si fluxes. Our findings suggest that the REE budget of the oceans prior to 2.3Ga was generally dominated by hydrothermal circulation of seawater through depleted mantle-derived source rocks. However, where evolved local continental crust and/or an enriched mantle source were present, this positive mantle Nd signal became discernible in the BIFs. By comparing the Nd isotopic features of similar aged BIFs and marine carbonates, it is concluded that similar to modern oceans, the early Precambrian ocean was not well-mixed with respect to its Nd isotopic composition.

Low temperature magnetic properties of the Late Archean Boolgeeda iron formation (Hamersley Group, Western Australia): environmental implications

The origin of the iron oxides in Archean and Paleoproterozoic Banded Iron Formations is still a debated question. We report low and high temperature magnetic properties, susceptibility and saturation magnetization results joined with scanning microscope observations within a 35 meters section of the Late Archean Boolgeeda Iron Formation of the Hamersley Group, Western Australia. With the exception of two volcanoclastic intervals characterized by low susceptibility and magnetization, nearly pure magnetite is identified as the main magnetic carrier in all iron-rich layers including hematite-rich jasper beds. Two populations of magnetically distinct magnetites are reported from a 2 meter-thick interval within the section. Each population shows a specific Verwey transition temperature: one around 120-124 K and the other in the range of 105-110 K. This temperature difference is interpreted to reflect two distinct stoichiometry and likely two episodes of crystallization. The 120-124K transition is attributed to nearly pure stoichiometric magnetite, SEM and microprobe observations suggest that the lower temperature transition is related to chemically impure silician magnetite. Microbial-induced partial substitution of iron by silicon is suggested here. This is supported by an increase in Total Organic Carbon (TOC) in the same interval.

Flower-like apatite recording microbial processes through deep geological time and its implication to the search for mineral records of life on Mars

The biological cycling of phosphorus on Earth could be as early as the origin of life in early Archean. However, because of the low abundance and fine particle size, phosphate related to microbial ecophysiological activities in early sedimentary rocks, especially those deposited before the Great Oxidation Event (GOE, ca. 2.45-2.32 Ma), is still poorly addressed. It is not until recently that certain petrographic and mineralogical features of apatite in the early Precambrian sedimentary rocks were found related to microbial activities. In this study, we report high-resolution electron microscopic investigations on apatite from the Neoarchean to early Paleoproterozoic banded iron formations (BIFs), Mesoproterozoic to Lower Cambrian black shale and phosphorites, and Pliocene sediments. Apatite in BIFs occurs as 4-8 mm radial flowers with “petals” made of apatite rods. Their mineralogical and petrologic features are highly similar to those in the younger sedimentary rocks in which biomass have been confirmed to play an important role in the mineralization of phosphate. We suggest that these sedimentary rocks or sediments have experienced similar phosphogenetic processes mediated by biomass that led to the mineralization of phosphorus. The formation and preservation of phosphate (apatite) with conspicuous recognizable features in association with biological activities from Late Archean to Pliocene implies its universal significance in recording microbial processes through deep geological evolution. With mild dynamic processes, the martian (sub)surface has better preservation conditions than Earth, and the micro-structure of phosphate formed in environments mediated by microorganisms could be recognized by high-resolution observations on the surface of Mars or returned samples, if microbial life ever developed on Mars.

Banded iron formation (?) at Grunehogna Craton, East Antarctica – Constraints from aeromagnetic data

We report a strong magnetic high over the Giæverryggen at the Grunehogna craton in western Dronning Maud Land, East Antarctica. The anomaly was coarsely mapped by previous surveys. Recently acquired magnetic data (2012/13) better constrain its spatial extent and permit the estimate of a theoretical amplitude value of +4000 to +4200nT at bedrock surface. From the magnetic data, we estimated a minimum magnetic susceptibility in a spherical source body with a midpoint depth of ∼5.6km to be greater than 193×10−3SI, in the absence of relevant remanent magnetization. An exemplary 2.75-D forward modeling is carried out. Comparisons with neighboring magnetic highs in a plate tectonic reconstruction of western Dronning Maud Land and southern Africa reveal clear similarities to the magnetic signature of the Paleoproterozoic banded iron formation in the Mhlapitsi fold and thrust belt near Polokwane (formerly Pietersburg) in South Africa, suggesting a similar source for the Giæver Magnetic Anomaly and allow the interpretation that both regions were juxtaposed in Archaean and Paleoproterozoic times.

Siliciclastic associated banded iron formation from the 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa

Most models proposed for banded iron formation (BIF) deposition are based on observations of well-preserved Late Archean and Paleoproterozoic BIF. Efforts to push the understanding gained from younger successions deeper in time have been hampered by the high metamorphic grades that characterize Early Archean BIF. This study focuses on a unique occurrence of well-preserved and contextualized BIF from the Early Archean (∼3.2Ga) Moodies Group, in the Barberton Greenstone Belt, South Africa. The Moodies BIF occurs thinly interbedded with fine-grained and cross-stratified sandstones, indicating deposition during times of decreased clastic sediment supply. In the Moodies BIF, chert is present as concretions, and is never observed in direct contact with the siliciclastic material but is always associated with iron minerals. This observation suggests that the processes leading to the formation of both chert and iron minerals were coupled. The dominant iron-rich minerals within unweathered Moodies BIF are hematite and magnetite, with less common occurrences of Fe–carbonate phases (mainly ankerite). Petrographic textures reveal that hematite constitutes an early mineral phase, while magnetite and ankerite display textures indicative of a late diagenetic or metamorphic origin. Carbonaceous particles are present in close association with the magnetite crystals. These C-bearing phases may be the preserved organic matter of microbes involved in the production of the ferric iron precursor phases, though it is difficult to rule out an origin from abiotic processes involving thermal decomposition of siderite to magnetite and organic carbon compounds. Nonetheless, the range of textures, mineralogies, and valence states supports the view that diagenetically-stabilized BIF mineralogies reflect the interaction of ferric iron phases with reducing fluids during diagenesis. These patterns are commonly observed in younger Archean and Paleoproterozoic iron formations, and imply a continuity of processes operating in the iron and silica cycles across both a range of paleoenvironments and long intervals of Archean time.

SIMS analyses of silicon and oxygen isotope ratios for quartz from Archean and Paleoproterozoic banded iron formations

Banded iron formations (BIFs) are chemical marine sediments dominantly composed of alternating iron-rich (oxide, carbonate, sulfide) and silicon-rich (chert, jasper) layers. Isotope ratios of iron, carbon, and sulfur in BIF iron-bearing minerals are biosignatures that reflect microbial cycling for these elements in BIFs. While much attention has focused on iron, banded iron formations are equally banded silica formations. Thus, silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in BIFs. BIFs are banded by definition, and microlaminae, or sub-mm banding, are characteristic of many BIFs. In situ microanalysis including secondary ion mass spectrometry is well-suited for analyzing such small features. In this study we used a CAMECA IMS-1280 ion microprobe to obtain highly accurate (±0.3‰) and spatially resolved (∼10 μm spot size) analyses of silicon and oxygen isotope ratios for quartz from several well known BIFs: Isua, southwest Greenland (∼3.8 Ga); Hamersley Group, Western Australia (∼2.5 Ga); Transvaal Group, South Africa (∼2.5 Ga); and Biwabik Iron Formation, Minnesota, USA (∼1.9 Ga). Values of δ 18O range from +7.9‰ to +27.5‰ and include the highest reported δ 18O values for BIF quartz. Values of δ 30Si have a range of ∼5‰ from −3.7‰ to +1.2‰ and extend to the lowest δ 30Si values for Precambrian cherts. Isua BIF samples are homogeneous in δ 18O to ±0.3‰ at mm- to cm-scale, but are heterogeneous in δ 30Si up to 3‰, similar to the range in δ 30Si found in BIFs that have not experienced high temperature metamorphism (up to 300 °C). Values of δ 30Si for quartz are homogeneous to ±0.3‰ in individual sub-mm laminae, but vary by up to 3‰ between multiple laminae over mm-to-cm of vertical banding. The scale of exchange for Si in quartz in BIFs is thus limited to the size of microlaminae, or less than ∼1 mm. We interpret differences in δ 30Si between microlaminae as preserved from primary deposition. Silicon in BIF quartz is mostly of marine hydrothermal origin (δ 30Si < −0.5‰) but silicon from continental weathering (δ 30Si ∼ 1‰) was an important source as early as 3.8 Ga.

Fe isotope and trace element geochemistry of the Neoproterozoic syn-glacial Rapitan iron formation

We have measured the iron isotope compositions and trace element concentrations of a suite of iron formation (IF) samples from the Neoproterozoic Rapitan Group, which was deposited during the older of two glacial episodes recorded in the Windermere Supergroup of the northern Canadian Cordillera. Like most other Neoproterozoic examples, iron in the Rapitan IF resides almost exclusively as hematite. This mineralogical simplicity compared to Archean and Paleoproterozoic banded iron formations is attributed to a limited supply of organic carbon to the Rapitan glacial ocean that inhibited diagenetic production of reduced iron phases. Sedimentological considerations indicate that the Rapitan IF was deposited during a rise in relative sea level related to a period of glacial advance and isostatic subsidence. Trace element data, including rare earth element plus yttrium (REE + Y) patterns, suggest an anoxic deep ocean dominated by low-temperature hydrothermal input and capped by a weakly oxic surface ocean. The iron isotope data show a trend of increasing δ 57Fe (versus IRMM-14) up-section from ~−0.7‰ to 1.2‰, corresponding to a shift from a muddy IF facies to a dominantly jaspilitic IF facies. This distinct isotopic pattern likely records a steep isotopic gradient across the iron chemocline in Rapitan seawater.

Ancient graphite in the Eoarchean quartz-pyroxene rocks from Akilia in southern West Greenland II: Isotopic and chemical compositions and comparison with Paleoproterozoic banded iron formations

We present detailed petrographic surveys of apatite grains in association with carbonaceous material (CM) in two banded iron formations (BIFs) from the Paleoproterozoic of Uruguay and Michigan for comparison with similar mineral associations in the highly debated Akilia Quartz-pyroxene (Qp) rock. Petrographic and Raman spectroscopic surveys of these Paleoproterozoic BIFs show that apatite grains typically occur in bands parallel to bedding and are more often associated with CM when concentrations of organic matter are high. Carbonaceous material in the Vichadero BIF from Uruguay is generally well-crystallized graphite and occurs in concentrations around 0.01 wt% with an average δ 13C gra value of −28.6 ± 4.4‰ (1 σ). In this BIF, only about 5% of apatite grains are associated with graphite. In comparison, CM in the Bijiki BIF from Michigan is also graphitic, but occurs in concentrations around 2.4 wt% with δ 13C gra values around −24.0 ± 0.3‰ (1 σ). In the Bijiki BIF, more than 78% of apatite grains are associated with CM. Given the geologic context and high levels of CM in the Bijiki BIF, the significantly higher proportion of apatite grains associated with CM in this rock is interpreted to represent diagenetically altered biomass and shows that such diagenetic mineral associations can survive metamorphism up to the amphibolite facies. Isotope compositions of CM in muffled acidified whole-rock powders from the Akilia Qp rock have average δ 13C gra values of −17.5 ± 2.5‰ (1 σ), while δ 13C carb values in whole-rock powders average −4.0 ± 1.0‰ (1 σ). Carbon isotope compositions of graphite associated with apatite and other minerals in the Akilia Qp rock were also measured with the NanoSIMS to have similar ranges of δ 13C gra values averaging −13.8 ± 5.6‰ (1 σ). The NanoSIMS was also used to semi-quantitatively map the distributions of H, N, O, P, and S in graphite from the Akilia Qp rock, and relative abundances were found to be similar for graphite associated with apatite or with hornblende, calcite, and sulfides. These analyses revealed generally lower abundances of trace elements in the Akilia graphite compared to graphite associated with apatite from Paleoproterozoic BIFs. Graphite associated with hornblende, calcite, and sulfides in the Akilia Qp rock was fluid-deposited at high-temperature from carbon-bearing fluids, and since this graphite has similar ranges of δ 13C gra values and of trace elements compared to graphite associated with apatite, we conclude that the Akilia graphite in different mineral associations formed from the same source(s) of CM. Collectively our results do not exclude a biogenic origin of the carbon in the Akilia graphite, but because some observations can not exclude graphitization of abiogenic carbon from CO 2- and CH 4-bearing mantle fluids, there remain ambiguities with respect to the exact origin of carbon in this ancient metasedimentary rock. Accordingly, there may have been several generations of graphite formation along with possibly varying mixtures of CO 2- and CH 4-bearing fluids that may have resulted in large ranges of δ 13C gra values. The possibility of fluid-deposited graphite associated with apatite should be a focus of future investigations as this may prove to be an alternative pathway of graphitization from phosphate-bearing fluids. Correlated micro-analytical approaches tested on terrestrial rocks in this work provide insights into the origin of carbon in ancient graphite and will pave the way for the search for life on other ancient planetary surfaces.

Oxygen Isotope Study of Paleoproterozoic Banded Iron Formation, Hamersley Basin, Western Australia

AbstractA submillimeter‐scale variation of δ18O in quartz was identified in chert and Fe‐oxide mesobands of the Brockman Iron Formation, Western Australia, using an in situ CO2‐laser fluorination technique. The total range of variation is 11.2–23.0‰, with &gt;5‰ variations within a single mesoband of approximately 2 cm thickness. These data contradict most previous works, which have suggested that banded iron formations are isotopically homogeneous. The present sample was obtained outside of the iron mining area, and as such is considered to have been less altered by the hydrothermal event recently shown to be responsible for the formation of the iron ore. The data suggest that the largest Paleoproterozoic banded iron formations may have formed not in a stable, quiescent sea, but instead as a result of increased influx of iron‐ and silica‐rich solutions during periods of increased magmatism and submarine hydrothermal activity in a rift basin environment.

Decoupling photochemical Fe(II) oxidation from shallow-water BIF deposition

Oxidized Fe minerals in Archean–Paleoproterozoic banded iron formations (BIFs) are commonly taken to indicate the presence of biogenic O 2 or photosynthetic Fe(II)-oxidizing bacteria in the oceans' photic zone. However, at least one viable abiogenic oxidation mechanism has been proposed. Prior to the rise of atmospheric oxygen and the development of a protective ozone layer, the Earth's surface was subjected to high levels of ultraviolet radiation. Bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved Fe(II). Under such conditions, dissolved ferrous iron species, such as Fe 2+ and Fe(OH) +, would have absorbed radiation in the 200–400 nm range, leading to the formation of dissolved ferric iron [Fe(III)], which in turn, would have hydrolyzed to form ferric hydroxide [Fe(OH) 3] at circumneutral pH [Cairns-Smith, A.G., 1978, Precambrian solution photochemistry, inverse segregation, and banded iron formations. Nature 76, 807–808; Braterman, P.S., Cairns-Smith, A.G., and Sloper, R.W., 1983, Photo-oxidation of hydrated Fe 2-Significance for banded iron formations. Nature 303, 163–164]. This process has been invoked to account for BIF deposition without need for biology [François, L.M., 1986, Extensive deposition of banded iron formations was possible without photosynthesis. Nature 320, 352–354]. Here, we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models. The experiments simulate the chemistry of ambient Precambrian seawater mixing with Fe(II)-rich hydrothermal fluids with, and without, UV irradiation. We find that if Fe(II) was effused from relatively shallow seamount-type vent systems directly into an anoxic photic zone, the photochemical contribution to solid-phase precipitation would have been negligible. Instead, most of the Fe(II) would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite ((Fe) 3Si 2O 5(OH) 4), and/or the ferrous carbonate, siderite (FeCO 3), depending on different simulated atmospheric pCO 2 levels. Conversely, in experiments where Fe(II) was exposed either to phototrophic Fe(II)-oxidizing bacteria or to O 2, ferric hydroxide formed rapidly, and the precipitation of ferrous iron phases was not observed. If, as suggested on mass balance grounds, BIF deposition requires that Fe be sourced from shallow seamount-type systems, then we are driven to conclude that oxide-facies BIF are the product of a rapid, non-photochemical oxidative process, the most likely candidates being direct or indirect biological oxidation, and that a significant fraction of BIF could have initially been deposited as ferrous minerals.

Phase equilibria in rocks of the paleoproterozoic banded iron formation (BIF) of the Lebedinskoe deposit, Kursk Magnetic Anomaly, and the petrogenesis of BIF with alkali amphiboles

BIF with alkali amphibole at the Lebedinskoe iron deposits, the largest in Russia, were metamorphosed at 550°C and 2–3 kbar and contain ferriwinchite, riebeckite, actinolite, grunerite, and aegirine-augite. All reaction textures observed in the rocks were produced during the prograde metamorphic stage and represent the following succession of mineral replacements: Gru → Rbk, Act → Win → Rbk. Data obtained on the textural relations and compositional variations of Ca, Ca-Na, and Na Al-free amphiboles point to the complete miscibility in the actinolite-ferriwinchite and ferriwinchite-riebeckite isomorphic series. Riebeckite is formed in BIF during the prograde metamorphic stage, with the participation of a fluid insignificantly enriched in Na+ and at increasing oxygen fugacity. The critical factors controlling the development of alkali amphiboles and Ca-Na pyroxenes in carbonate-bearing BIF is the oxygen activity and the presence of at least low concentrations of Na+ ions in the fluid. The minerals contain Fe3+, and all reactions producing them are oxidation reactions. The origin of riebeckite late in the course of the mineral-forming process is caused by the Ca2+Mg2+ → Na+Fe3+ heterovalent isomorphic replacement in calcic and calcic-sodic amphiboles and by the oxidation of grunerite in the presence of a fluid enriched in Na ions.