a two radius circular array method inferring phase velocities of love waves using microtremor records 1 we present a new method to estimate phase velocities of love waves from horizontal motion seismograms of microtremors obtained with seismic sensors placed around two circumferences of different radii under the assumption that the fundamental mode dominates we applied our new method to field data acquired at a test site where the shallow underground seismic velocity profile is known and have confirmed that the phase velocities of the fundamental mode love waves could be inferred with sufficient precision within certain wavelength ranges outside the wavelength ranges suitable for analysis the spectral ratio an intermediary quantity calculated from measurement records prior to phase velocity evaluation deviated from theoretical expectations the pattern of deviations could satisfactorily be accounted for by considering three factors theoretically the presence of higher mode surface waves directional aliasing related to the finite number of sensors and the presence of incoherent noise

dust to dust evidence for the formation of primary hematite dust in banded iron formations via oxidation of iron silicate nanoparticles abstract conventional models for the deposition of banded iron formations bifs envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide hydroxide particles in surface waters that settled to form laterally extensive layers of iron rich sediment a fundamental tenet of this model is that fine grained hematite so called dusty hematite in least altered bifs represents the dehydration product of original oxide hydroxide precipitates however this premise has never been proven we have investigated the origin of the earliest formed iron oxides in chert in well preserved bifs of the 2 63 2 45 billion year old hamersley group australia we find that laminated chert in bifs show progressive stages of in situ alteration from gray green chert containing iron silicate nanoparticles to red chert with abundant hematite dust analysis of textures by transmission electron microscopy of samples from the transition zone between gray green and red chert reveals that dusty hematite formed after partial dissolution of iron silicate nanoparticles by the precipitation of iron oxides in resulting cavities these observations suggest that hematite dust is not a relict of an original seawater precipitate but the end product of post depositional oxidation our observations are consistent with paleomagnetic results from the hamersley group which record two major phases of magnetic remanence carried by hematite that post date deposition by more than 200 million years our results may provide an alternative explanation for the origin of jasper in bifs deposited before the start of the great oxidation event about 2 4 billion years ago if correct it follows that hematite dust is not a reliable proxy for paleoenvironmental conditions or biological processes in early precambrian seawater furthermore our results suggest that the primary iron precipitate in bifs was iron silicate mud that was silicified at or just below the sediment water interface a hypothesis that requires neither dissolved oxygen nor photosynthetic life but was an inorganic chemical process reflecting anoxic oceans enriched in iron and silica

decoupling photochemical fe ii oxidation from shallow water bif deposition abstract 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 francois 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 3 si 2 o 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

evidence from laminated chert in banded iron formations for deposition by gravitational settling of iron silicate muds the deposition of clay sized particles by settling from suspension within a water column typically forms highly porous 80 90 water muds dominated by uniformly distributed and randomly oriented particles the depositional microfabric of muds is rapidly destroyed during burial resulting in mudstones with a strong preferred particle orientation here we show that laminated chert in banded iron formations from the hamersley group australia and ghaap group south africa preserve laminae of nanometer sized particles of greenalite with textures that resemble the fabric of freshly deposited muds the oldest such fabrics in the rock record the preservation of the textures implies that the nanoparticles were frozen by silica cement on or just below the seafloor an interpretation supported by their presence in intraformational chert clasts and nodules with strong differential compaction the co occurrence of stacks of plates forming thicker aggregates with multiple face to edge contacts indicates that the greenalite may have been deposited as loosely formed flocs the common occurrence in certain horizons of iron silicate microgranules dispersed in the greenalite mud resembles aggregates of clay particles documented in modern muds these textures support models for active seawater precipitation of the iron rich phases and point to collision and flocculation of particles during sediment transport upon deposition amorphous silica rapidly occluded the available porosity and entombed the precipitates in a chemically inert cement that limited burial related compaction and hindered diagenetic reactions providing a taphonomic view of the earliest mineralogy and texture of iron formations

in situ high temperature esr measurements for kerogen maturation abstract kerogen samples from middle miocene mudstone cores in two japanese exploratory wells have been investigated by an in situ high temperature esr technique anhydrous pyrolysis of the kerogens results in four distinctstemperature stages in spin concentration ns ns increasessin the first and third at end of catagenesis is characterized only by two stages third and fourth these data are based on the formation and recombination of free radicals and have been used to derive parameters related to kerogen maturation and hydrocarbon generation

an iron shuttle for deepwater silica in late archean and early paleoproterozoic iron formation iron formations are typically thinly bedded or laminated sedimentary rocks containing 15 or more of iron and a large proportion of silica commonly 40 in the ca 2590 2460 ma campbellrand kuruman complex transvaal supergroup south africa iron formation occurs as a sediment starved deepwater facies distal to carbonates and shales iron minerals primarily siderite define the lamination the silica primarily occurs as thin beds and nodules of diagenetic chert now microcrystalline quartz filling pore space and replacing iron formation minerals and co occurring deepwater lithologies mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production anoxygenic photosynthesis abiotic photochemistry and chemoautotrophy using fe ii as an electron donor the origin and mechanism of silica precipitation in these deposits have received less attention here we present a conceptual model of iron formation that offers insight into the deposition of silica the model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides which precipitate fe oh 3 binds silica and sinks from the surface ocean along with organic matter shuttling silica to basinal waters and sediments fe iii respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters driving precipitation of siderite from fe2 and respiration influenced co2 silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases this model explains many of the mineralogical textural and environmental features of late archean and earliest paleo proterozoic iron formation

epr study of kerogens from three alaskan north slope wells abstract whole rock and isolated kerogens from the seabee inigok and ikpikpuk wells of the alaskan north slope have been analyzed by electron paramagnetic resonance epr spectroscopy the sample suite includes representatives of the torok kingak lisburne sadlerochit and endicott formations power saturation studies show a definite correlation with the downhole maturation the extent of aromaticity of the sample as defined by 13 c nmr and with the beginning of the oil window for the kingak formation strong correlations within formations across the depositional basin are seen abrupt changes in epr parameters indicate a change in the nature of the organic matter present and may reflect a facies change peak widths and spin densities correlate with maturity as indicated by vitrinite reflectance and sample aromaticity as measured in solid state 13 c nmr spectra

iron silicate microgranules as precursor sediments to 2 5 billion year old banded iron formations banded iron formations bifs are chemical sedimentary rocks comprising alternating layers of iron rich and silica rich minerals that have been used to infer the composition of the early precambrian ocean and ancient microbial processes however the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues petrographic examination of the ca 2 5 ga dales gorge member of the brockman iron formation hamersley group western australia reveals the presence of abundant silt sized microgranules composed of stilpnomelane the microgranules are most common in the least altered bif where they define sedimentary laminations implying a depositional origin we suggest that the precursor mineral was an iron rich silicate that formed either in the water column or on the seafloor the microgranular texture may have developed due to clumping of amorphous mud forming silt sized floccules the microgranules were resedimented by dilute density currents and deposited in lamina sets comprising a basal microgranular rich lamina overlain by amorphous mud with dispersed microgranules the lamina sets collectively define plane lamination structure probably of the lower flow regime the microgranular textures are preserved only where early diagenetic silica prevented the compaction of lamina sets episodic resedimentation of iron silicates alternating with periods of nondeposition and seafloor silicification provides an explanation for some of the characteristic banding in bif we propose that for most of the early precambrian the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

making magnetite late again evidence for widespread magnetite growth by thermal decomposition of siderite in hamersley banded iron formations abstract banded iron formations bifs are marine chemical sedimentary rocks that were widely deposited before the start of the great oxidation event 2 45 2 32 ga they represent an important archive into the chemistry of the precambrian oceans and atmosphere and the evolution of the marine biosphere it is believed that bifs were derived from ferric oxides hydroxides that were deposited by iron oxidizing bacteria or by reaction with oxygen released by cyanobacteria hematite is traditionally interpreted to represent the dehydrated product of the ferric oxide hydroxide precipitate whereas magnetite is thought to have formed from ferric oxides hydroxides via bacterial iron reduction and oxidation of microbial biomass however studies are equivocal about when and how magnetite formed in bifs we present the results of a detailed petrographic study of magnetite in bifs of the hamersley group western australia we find no evidence that magnetite was precipitated from seawater or was a product of bacterial iron reduction instead most of the magnetite formed by replacement of siderite and other ferrous rich minerals after burial our observations suggest that magnetite growth was linked to the breakdown of siderite during metamorphism published experimental data indicate that in the presence of water and under the metamorphic conditions experienced in the southern pilbara craton 200 350 c the thermal decomposition of siderite could account for much of the magnetite in the hamersley bifs 3feco3 h2o fe3o4 3co2 h2 the relationship between magnetite and the primary precipitates which are preserved as greenalite nanoparticles and microgranules in chert bands suggests that much of the magnetite formed in siderite rich iron silicate bands that developed by compaction of non silicified iron silicate muds these bands acted as reactive pathways for fluid flow and the formation of magnetite if most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron our results are consistent with models for the deposition of bifs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial

esr assessment of kerogen maturation and its relation with petroleum genesis abstract petroleum hydrocarbons are formed by breakdown of kerogen preserved in source rocks throughout the process of catagenesis this process is accompanied by free radical generation in kerogen the availability of measuring free radicals in kerogen using esr to deduce a maturation estimation of potential source rocks has been hindered due to the presence of the solvent extractable organic molecules seom trapped within the kerogen matrix spin concentration ns of the kerogen treated with pyridine k py represents the paramagnetic centers of the kerogen matrix itself and provides a potential parameter to evaluate kerogen maturation

the joffre banded iron formation hamersley group western australia assessing the palaeoenvironment through detailed petrology and chemostratigraphy abstract the joffre member of the brockman iron formation is by volume the largest single known banded iron formation bif in the world here we present detailed petrology and chemostratigraphy through the entire 355 m core section of this 2 45 billion year old unit oxide bif and silicate carbonate oxide bif dominate the lithology with minor amounts of interbedded stilpnomelane mudrock stilpnomelane rich tuffaceous mudrock and calcareous mudrock besides chert and magnetite the prominent mineralogy is riebeckite ankerite hematite stilpnomelane and crocidolite the bif is characterized by an average of 50 wt sio 2 and 44 5 wt fe 2 o 3 and an overall low abundance of al 2 o 3 2 sn of 0 24 the rey patterns also show a positive la sn anomaly no ce sn anomaly and a weakly developed positive y sn anomaly iron isotopes 56 fe with positive 56 fe values of 0 04 to 1 21 suggest that a large part of the hydrothermal iron was partly oxidized in the upper water column and subsequently precipitated as ferric oxyhydroxides no epiclastic grains have been found rather submarine hydrothermal fluids and fine grained volcanogenic detritus controlled bif chemistry the former source is reflected through a constant positive eu sn anomaly throughout the core average eu sn anomaly of 1 6 with a peak of 2 1 between 100 and 155 m depth while the latter source is best reflected through the stilpnomelane rich tuffaceous mudrock consisting of volcanic ash fall tuff with relict shards set in a stilpnomelane matrix the mudrock is overlain by well preserved wavy laminae and laminae sets of stilpnomelane microgranules that likely originated from re worked volcanic ash formed either on the seafloor or in the water column prior to deposition an enriched hree to lree pattern a high iron content 30 wt and a 56 fe value of 0 59 collectively imply that the mudrock facies interacted with the fe rich seawater prior to deposition the tio 2 zr ratio of the bif and the associated mudrocks suggest a felsic only source related to the same style of volcanics as the slightly younger woongarra rhyolites given the observation that the dominant control on the seawater chemistry was associated with felsic volcanics we speculate that the fine grained pelagic ash particles may have sourced bio available nutrients to the surface water this would have facilitated enhanced biological productivity including bacterial fe ii oxidation which is now recorded as the positively fractionated 56 fe iron oxide minerals in the joffre bif alongside submarine hydrothermal input to the basin the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways thereby making the joffre bif poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses

neoarchaean banded iron formation within a 2620 ma turbidite dominated deep water basin slave craton nw canada abstract banded iron formations bifs are iron and silica rich chemical metasedimentary rocks that span much of the archaean and early palaeoproterozoic rock record given that bifs precipitated from ambient seawater they provide an important archive of earth s ancient marine environments within the slave craton occurs one of the largest and well preserved achaean turbidite basins in the world interstratified within the greywacke mudstone turbidites is bif that is commonly associated with ferruginous sediments and locally with a felsic tuff bed field investigations were conducted at four bif locations from west to east across the slave craton these bifs are composed of micro and mesobands of quartz and magnetite with various amounts of metamorphic grunerite ferro actinolite ferro hornblende as well as greenalite and stilpnomelane the latter two are interpreted as retrograde metamorphic phases as the bif was deposited in a deep water turbidite dominated basin s the chemical component was occasionally diluted by clastic detritus as illustrated by highly variable al 2 o 3 content and al 2 o 3 tio 2 and cr ni that are similar to the clastic sediments on a layer by layer basis the iron content in the bif ranges from as low as 4 wt and up to 75 wt fe 2 o 3 whereas the sio 2 content ranges from 20 wt to as high as 94 wt this together with a hyperbolic germanium silicon distribution suggests that continentally sourced silica comprised the background precipitation which was periodically interrupted by pulses of hydrothermally sourced iron high temperature hydrothermal fluids contributed with an average shale normalized positive eu anomaly of 1 92 which indicates input of reduced eu 2 to the ambient seawater the bif shows characteristic aqueous fluid derived rare earths yttrium fractionation pr yb 3 to ce 4 the age and trace element signature of the felsic tuff is similar to arc type volcanic rocks potentially linking the bif and turbidite basin s to the defeat suite plutons ca 2630 2620 ma the slow chemical rainout behind bif deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump generated debris flows sourced and triggered by the tectonically active arc terrain episodically delivered clastic sediment

hydrothermal nontronite formation at eolo seamount aeolian volcanic arc tyrrhenian sea abstract a sediment core containing a yellowish green clay bed was recovered from an area of extensive hydrothermal deposition at the se slope of the eolo seamount tyrrhenian sea the clay bed is composed of pure nontronite described for the first time in the tyrrhenian sea which appears to be the most aluminous nontronite ever found among the seafloor hydrothermal deposits the high al content suggests precipitation from al containing hydrothermal solutions the ree distribution of the eolo nontronite has a v shape pattern the heavy ree enrichment is in part due to their preferential partitioning in the nontronite structure this enrichment was possibly further enhanced by the hree preferential sorption on bacterial cell walls the light ree enrichment is the result of scavenging uptake by one of the nontronite precursors i e poorly ordered fe oxyhydroxides from the hydrothermal fluids oxygen isotopic composition of the nontronite yields a formation temperature of 30 c consistent with a low temperature hydrothermal origin the relatively radiogenic nd isotopic signature of the nontronite compared to the present day mediterranean seawater indicates that approximately half of nd and presumably the rest of the lree are derived from local volcanic sources on the other hand 87 sr 86 sr is dominated by present day seawater sr scanning electron microscopy investigation revealed that the nontronite is composed of aggregates of lepispheres and tube like filaments which are indicative of bacteria assisted precipitation bacteria inhabiting this hydrothermal site likely acted as reactive geochemical surfaces on which poorly ordered hydrothermal fe oxyhydroxides and silica precipitated upon aging the interactions of these primary hydrothermal precipitates coating bacterial filaments and cell walls likely led to the formation of nontronite finally the well balanced interlayer and layer charges of the crystal lattice of seafloor hydrothermal nontronite decrease its sorption capacity to zero thus the ubiquitous nontronite precipitation along the active plate boundaries and around the hot spots has no significant impact on oceanic trace element chemistry

radical concentration of kerogen as a maturation parameter a good correlation was found between spin concentration and burial depth for kerogen extracted with benzene methanol followed by pyridine or dioxane differences in slopes of the regression lines of spin concentration vs burial depth are dependent upon the geothermal gradient of the studied areas free radical concentration might be a good maturation parameter for kerogens of the same type

free radical transformations in the irati oil shale due to diabase intrusion abstract electron spin resonance esr spectroscopy was used to evaluate the effect of a diabase intrusion on the production and decomposition of free radicals in the irati oil shale parana basin brazil free radical concentration measurements on the stratigraphic column ceri 2 indicated that the ratio between the extent of thermal alteration on sedimentary organic matter and the intrusive thickness was 1 14 kinetics of free radical transformation obtained from laboratory pyrolysis of irati oil shale can be described by first order production and decomposition reactions of distinct species although these species present different kinetic parameters they are not resolved and lie inside the envelope of the single line esr spectrum the activation energies for parallel first order production reactions of two main species of free radicals in the temperature range of 360 420 c were found to be 54 2 kcal mol 1 and 36 9 kcal mol 1

epr study of kerogens from middle valley northern juan de fuca ridge odp leg 139 abstract isolated kerogens from four sites in the middle valley hydrothermal region of the juan de fuca ridge ocean drilling project odp leg 139 were analysed by electron paramagnetic resonance epr spectroscopy measurements of peak width spin density and power saturation for site 857 kerogens which increased regularly in maturity downhole show correlation with vitrinite reflectance values from 0 61 to 2 5 indicating the start of the oil window at depths from 200 to 400 m spin density increases to 1 56 10 17 spins per gram and peak width decreases to 3 45 g gauss with increasing depth the tendency to power saturate also decreases with increasing maturity and increasing vitrinite reflectance within the oil window these trends are consistent with a model in which exchange processes are occurring and cause changes in the epr behavior of samples from this site sediments from other middle valley sites 855 856 and 858 contain large quantities of pyrite with mn 2 impurities which interact with the carbon radical to distort the epr measurements

modeling of gas generation from the barnett shale fort worth basin texas the generative gas potential of the mississippian barnett shale in the fort worth basin texas was quantitatively evaluated by sealed gold tube pyrolysis kinetic parameters for gas generation and vitrinite reflectance ro changes were calculated from pyrolysis data and the results used to estimate the amount of gas generated from the barnett shale at geologic heating rates using derived kinetics for ro evolution and gas generation quantities of hydrocarbon gas generated at ro 1 1 are about 230 l t 7 4 scf t and increase to more that 5800 l t 186 scf t at ro 2 0 for a sample with an initial total organic carbon content of 5 5 and ro 0 44 the volume of shale gas generated will depend on the organic richness thickness and thermal maturity of the shale and also the amount of petroleum that is retained in the shale during migration gas that is reservoired in shales appears to be generated from the cracking of kerogen and petroleum that is retained in shales and that cracking of the retained petroleum starts by ro 1 1 this result suggests that the cracking of petroleum retained in source rocks occurs at rates that are faster than what is predicted for conventional siliciclastic and carbonate reservoirs and that contact of retained petroleum with kerogen and shale mineralogy may be a critical factor in shale gas generation shale gas systems together with overburden can be considered complete petroleum systems although the processes of petroleum migration accumulation and trap formation are different from what is defined for conventional petroleum systems

ge si in hamersley bif as tracer of hydrothermal si and ge inputs to the paleoproterozoic ocean abstract ge si and ge fe ratios have been measured by laser ablation icp ms in selected sections of the hamersley banded iron formation bif including chert siderite bands i e carbonate and silicate facies bif the major ge carriers are magnetite and hematite in the oxide facies and the fe rich silicates greenalite minnesotaite and stilpnomelane in chert siderite bands while quartz has the lowest ge si ratios 0 15 0 7 mol mol in accord with discrimination against ge in quartz the homogeneous si fe ratio of 1 25 for dales gorge oxide facies bif is consistent with si ferrihydrite as precursor phase this si concentration is sufficiently high for formation of iron free silica clusters at the surface of ferrihydrite so that only a minor fractionation of the ge si ratio should result from preferential ge adsorption relative to si hematite chert bands that have been minimally altered by metamorphic reactions and or fluid metasomatism are the preferred material to determine the sources of silica in hamersley bif variations of fe si and ge si ratios in chert mesobands are interpreted as the result of diagenetic mineral segregation a consistent ge si 3 mol mol is then derived by extrapolation of measured ge si ratios to the average fe si composition of dales gorge bif simple mass balance calculations indicate hydrothermal inputs to the ocean of the order of 25 for si and 90 for ge assuming ge si 10 mol mol for high temperature hydrothermal fluids the high si concentration in siderite chert bands si fe 4 demands only a small amount of si ferrihydrite precursor and some excess silica likely precipitated in pore water oversaturated with regard to amorphous silica the average ge si 0 80 0 1 mol mol for bulk chert siderite samples including some stilpnomelane rich cherts is within the compositional range of modern riverine and aeolian inputs pointing to a continental si source during their deposition