continuously increasing δ98mo values in neoarchean black shales and iron formations from the hamersley basin abstract we present mo c and o isotope data from black shales carbonate and oxide facies iron formations from the hamersley group western australia that range in age from 2 6 to 2 5 billion years the data show a continuous increase from near crustal 98 mo values of around 0 50 for the oldest marra mamba and wittenoom formations towards higher values of up to 1 51 for the youngest sample of the brockman iron formation thereby the trend in increasing 98 mo values is portrayed by both carbonate facies iron formations and black shales considering the positive correlation between mo concentration and total organic carbon we argue that this uniformity is best explained by molybdate adsorption onto organic matter in carbonate iron formations and scavenging of thiomolybdate onto sulfurized organic matter in black shales a temporal increase in the seawater 98 mo over the period 2 6 2 5 ga is observed assuming an overall low mo isotope fractionation during both mo removal processes oxide facies iron formations show lowest mo concentrations lowest total organic carbon and slightly lower 98 mo compared to nearly contemporaneous black shales this may indicate that in iron formation settings with very low organic matter burial rates the preferential adsorption of light mo isotopes onto fe oxyhydr oxides becomes more relevant a similar mo isotope pattern was previously found in contemporaneous black shales and carbonates of the griqualand west basin south africa the consistent and concomitant increase in 98 mo after 2 54 billion years ago suggests a more homogenous distribution of seawater molybdate with uniform isotopic composition in various depositional settings within the hamersley basin and the griqualand west basin the modeling of the oceanic mo inventory in relation to the mo in and outflux suggests that the long term build up of an isotopically heavy seawater mo reservoir requires a sedimentary sink for isotopically light mo the search for this sink i e adsorption onto mn oxides in well oxygenated surface oceans and or subaerial environments or incomplete thiomolybdate formation in weakly sulfidic settings remains debated but its relevance becomes more important closer to the great oxidation event and is probably related to already weakly oxidizing conditions even prior to the 2 5 ga whiff of oxygen

burial history and diagenetic reaction kinetics relations among burial histories deduced from stratigraphy thermal regimes imposed by tectonics and sedimentation and rates of diagenetic reactions extrapolated from experimental data have been explored by using sedimentation diagrams plots of burial with respect to the sediment water or sediment air interface as a function of time elapsed since deposition one can estimate paleogeotherms from such plots using assumptions on heat flow thermal conductivities compaction and cementation effects on porosity and convecting formation water systems from these one can deduce the temperature history of a sediment as a function of postdepositional time that history can be converted to the integrated value of temperature as a function of time this value which takes into ccount the total contribution of time and temperature is the relevant parameter for reaction kinetics a plot of the time temperature integral versus postdepositional time for a given formation can then be compared with the time temperature integral for specific yields of a diagenetic reaction to deduce at what stage in its history a sediment would have accumulated sufficient time and thermal energy to accomplish a given extent of reaction these deductions can be compared with petrologic information on the time of origin of diagenetic phases

depositional environment and mechanism of preservation of microfossils upper proterozoic bitter springs formation australia microfossils preserved in black chert nodules from the upper proterozoic bitter springs formation of australia represent the preserved remnants of cyanobacterial communities that inhabited a series of nonmarine saline lakes and ponds groundwaters of halite salinity pickled the microorganisms and thus inhibited their early bacterial degradation these same groundwaters were saturated in silica so that upon evaporation the silica and halite precipitated and formed a series of chert concretions nodules and bipyramidal quartz crystals where the chert nodules formed in areas of buried cyanobacterial mats further degradation of the buried sheaths and cells was arrested and the microorganisms were preserved

biostratigraphic and chemostratigraphic correlation of neoproterozoic sedimentary successions upper tindir group northwestern canada as a test case recent advances in proterozoic micropaleontology and sedimentary isotope geochemistry suggest that improved interbasinal correlation of neoproterozoic 1000 540 ma successions is possible because widely varying interpretations of its age have been suggested and no reliable radiometric dates or paleomagnetic data are available the upper tindir group of northwestern canada provides an opportunity to test this hypothesis the age of these strata is of paleontological importance because silicified carbonates near the top of the group contain disc shaped scale microfossils that may provide insights into the early evolution of biomineralization a reinterpretation of upper tindir microfossil assemblages suggests a late riphean age although diagenesis and contact metamorphism have altered the isotopic compositions of some carbonates least altered samples indicate that delta 13c of contemporaneous seawater was at least 4 7 typical of neoproterozoic but not cambrian carbonates strontium isotopic compositions of the least altered samples yield values of approximately 0 7065 which can be uniquely correlated with late riphean seawater together micropaleontology and the isotopic tracers of c and sr constrain the upper tindir carbonates and their unique fossils to be late riphean likely between 620 and 780 ma

formation of magnesium smectite during lacustrine carbonates early diagenesis study case of the volcanic crater lake dziani dzaha mayotte indian ocean the volcanic crater lake of dziani dzaha in mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of mg phyllosilicates associated with carbonate rocks the dziani dzaha is characterized by intense primary productivity volcanic gases bubbling in three locations and a volcanic catchment of phonolitic alkaline composition the lake water has an alkalinity of ca 0 2 mol l 1 and ph values of ca 9 3 cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon hydrogen nitrogen elemental analyzer x ray fluorescence spectrometry and x ray powder diffraction in surface sediments the content of total organic carbon reaches up to 20 weight the mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment below 30 cm depth x ray diffraction analyses of the 2 m clay fraction indicate the presence of a saponite like mineral a mg rich smectite the saponite like mineral accumulates at depth to reach up to ca 30 weight concurrent with a decrease of the contents of hydromagnesite and organic matter thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions the formation of the saponite like mineral instead of al free mg silicates resulted from high aluminum availability which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting supersaturation of the lake water relative to saponite is especially due to high ph values themselves derived from high primary productivity this suggests that a genetic link may exist between saponite and the development of organic rich carbonate rocks which may be fuelled by the input of co2 rich volcanic gases this provides novel insights into the composition and formation of saponite rich deposits under a specific geodynamic context such as the cretaceous south atlantic carbonate reservoirs

siliceous sponge expansion and phosphogenesis in a shallow water environment in the malyi karatau range kazakhstan during the precambrian cambrian transition abstract ocean oxygenation during the precambrian cambrian transition is thought to be one of the key factors for the rapid diversification of animals known as the cambrian explosion siliceous sponges inhabiting continental slope settings were capable of shifting the respiratory oxygen demand in the water column to depth resulting in an increase in oxygen levels of shallow water to assess the global importance of the impact of ecosystem engineering by sponges we evaluated the palaeo environmental conditions that confine sponge expansion using multiple geochemical proxies including silicon and carbon stable isotopes as well as major and trace elements in cherts and siliceous sedimentary rocks that were deposited in shallow water around the precambrian cambrian transition we investigated three lithosections in the malyi karatau range in southern kazakhstan the lowermost chert samples of these sections are typically characterized by comparably low 30si values and exhibit molds of siliceous sponge spicules upsection the occurrence of cross bedding structures and granular phosphorites document the shallowing of the depositional environment where overall increasing 30si values and increasing al si indicate less settlement of sponges and higher suspended particle loads corresponding carbon isotope records of the carbonate fraction decrease from near zero to negative carbon isotope values averaging 5 9 the 13ccarb values overall coincide with y ho the redox indicator ce ce and mass fractions of carbonate fluorapatite suggesting that these ratios are predominantly controlled by the mineralization of organic matter in phosphatic sediment the studied sections document the ecological requirements for sponge settlement and proliferation such as the presence of a hard substrate and sufficiently low suspended particle concentrations and or low wave energy this work suggests that the widespread appearance of karsted surfaces around the precambrian cambrian transition that were subsequently submerged during the cambrian transgression would have created permissive habitats for sponges and large phosphate deposits the capability of sponges to oxygenate shallow seawater and extensive phosphorous burial in epicontinental basins are thus feasible mechanisms for the seawater oxygenation around the precambrian cambrian transition

highly alkaline high temperature hydrothermal fluids in the early archean ocean abstract based on the petrology of hydrothermally altered archean basaltic greenstones thermodynamic calculations of phase equilibria were conducted to estimate the composition of a high temperature 350 c hydrothermal fluid in an archean subseafloor basalt hosted hydrothermal system the results indicate that the hydrothermal fluid was highly alkaline attributed to the presence of calcite in the alteration minerals under a high co2 condition and predict a generation of sio2 rich fe poor hydrothermal fluids in the archean subseafloor hydrothermal system the chemically reactive mixing zones between alkaline hydrothermal fluids and slightly acidic neutral seawater are characterized by inverse ph and chemical polarity to modern hydrothermal systems leading to extensive precipitation of silica and iron oxyhydroxides on under the seafloor such processes can be responsible for the abiotic formation of voluminous chert and subseafloor silica dike the mechanism of silicification and the ph controlled generation of banded iron formation that has been arising mainly from the redox chemistry in the archean ocean such high temperature alkaline fluids could have had a significant role not only in the early ocean geochemical processes but also in the early evolution of life

isotopic sm nd signatures of precambrian banded iron formation from the fennoscandian shield east european platform and bundelkhand craton india abstract in this communication we report geochemical and sm nd isotope data of banded iron formation bif from kola karelia fennoscandian shield east european platform and bundelkhand craton n e india the bifs from kola karelia algoma type and bundelkhand craton algoma type north central india and east european platform superior type characterized by alternate domains of quartz rich and iron rich layers except for magnetite and hematite orthopyroxene amphibole and garnet are the other dominant minerals present the bif s the shale normalized ree spectra of iron rich layers of bundelkhand kola karelia and east european bifs are similar the end t for silica rich layers is higher compared to the iron rich layers for all studied samples the higher end t values around 5 0 for silica rich layers from kola karelia bundelkhand bif and east european platform suggest a juvenile source for silica the lower end t values 5 to 2 for iron rich layers from the same samples implies continental source for the iron the present study suggests that the sedimentation mechanisms of silica and iron for algoma as well as superior types bifs are similar despite spatial diversity

elastic strength of the slave craton at 1 9 gyr and implications for the thermal evolution of the continents the secular cooling of the earth since its formation together with the law of radioactive decay imply that the earth s internal heat generation and heat loss must have been two to three times higher in archaean and proterozoic times than today but this inference is difficult to test directly here we present a flexural analysis of the proterozoic kilohigok basin in the archaean slave craton of canada which shows that 1 9 gyr ago the craton had an effective elastic thickness of only 12 4 km because the present effective elastic thickness of the region is 100 25 km these results imply a two to fourfold decrease in the thermal gradient beneath the slave craton since 1 9 gyr and indicate that at 1 9 gyr 700 myr after the accretion of the continental lithosphere forming the craton the thick continental root beneath this part of the canadian shield had not yet formed

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

micrometer scale chemical and isotopic criteria o and si on the origin and history of precambrian cherts implications for paleo temperature reconstructions abstract oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperature during the precambrian these reconstructions have been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation existing criteria used to assess the pristine sedimentary origin of a chert are based on petrography criterion 1 chert is composed mostly of microquartz on the bulk oxygen isotopic composition criterion 2 bulk 18 o has to be close enough to the maximum 18 o value previously measured in other cherts of the same age and on the presence of a large 18 o range at the micrometer scale criterion 3 18 o range of 10 at 2 m however these criteria remain incomplete in determining precisely the origin and degree of preservation of ancient cherts we report in situ si and o isotope compositions and trace element concentrations in seven chert samples ranging from 1 88 to 3 5 ga in age correlations between 30 si and al 2 o 3 and k 2 o tio 2 reveal that microquartz is of three different origins i e diagenetic hydrothermal or silicification moreover chert samples composed mostly of diagenetic microquartz show a large range of 30 si at the micrometer scale 1 7 4 5 consistent with the large range of 18 o previously found in the gunflint diagenetic cherts we propose two further quantitative criteria to assess the origin state of preservation and diagenetic history of cherts criterion 4 uses trace element concentrations coupled with 30 si to ascribe the origin of cherts among three possible end members diagenetic hydrothermal and silicified criterion 5 is the presence of a large range of 30 si in pure diagenetic microquartz in the seven samples analyzed in this study only one from the gunflint iron formation at 1 88 ga passes all the criteria assessed here and can be used for paleo temperature reconstructions the criteria outlined here will help guide to the identification of pristine diagenetic cherts and or to better constrain seawater paleo temperature reconstructions by taking into account the effect of diagenesis

the origin of carbonaceous matter in pre 3 0 ga greenstone terrains a review and new evidence from the 3 42 ga buck reef chert abstract the geological record of carbonaceous matter from at least 3 5 ga to the end of the precambrian is fundamentally continuous in terms of carbonaceous matter structure composition environments of deposition preservation and abundance in host rocks no abiotic processes are currently known to be capable of producing continuity in all four of these properties although this broad view of the geological record does not prove that life had arisen by 3 5 ga the end of the early archean it suggests a working hypothesis most if not all carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms this hypothesis must be tested by studies of specific early geological units designed to explore the form distribution and origin of enclosed carbonaceous matter the carbonaceous environmentally diverse 3416 ma buck reef chert brc of the barberton greenstone belt south africa provides an opportunity for such a study upward facies progressions in the brc reflect deposition in environments ranging from shallow marine evaporitic brine ponds to a storm and wave active shelf to a deep low energy basinal setting below storm wave base abundances and ratios of al2o3 zr tio2 and cr track inputs of various types of volcaniclastic and terrigenous clastic materials in particular zr al2o3 and zr serve as proxies for concentration of windblown dust and indirectly as proxies for sedimentation rate cu zn ni and feo were concentrated in the most slowly deposited transitional and basinal sediments inconsistent with a hydrothermal setting but consistent with a normal marine setting the distribution of microfacies defined by associations and layering of clastic ferruginous and carbonaceous grains correlates with facies transitions fine carbonaceous laminations which occur only in shallow platform settings represent photosynthetic microbial mats these were ripped up and the debris widely redistributed in shallow and deep water by waves and storms the isotopic composition of carbonaceous matter ranges from 35 to 30 in shallow water settings and to 20 in deep water units the heavier 13c in deep water carbonaceous matter is thought to reflect microbial processing possibly by fermentation and methanogenesis of organic matter originally produced in shallow water hydrothermal origins for brc carbonaceous matter are clearly excluded by the inferred depositional setting of the rocks as a whole an inference supported by field petrographic and geochemical analysis we suggest that the biological model proposed here for brc carbonaceous matter is the best currently available the hypothesis that at least some carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms should be regarded as likely until extraordinary contradictory evidence is presented

evaluation of siderite and magnetite formation in bifs by pressure temperature experiments of fe iii minerals and microbial biomass abstract anoxygenic phototrophic fe ii oxidizing bacteria potentially contributed to the deposition of archean banded iron formations bifs before the evolution of cyanobacterially generated molecular oxygen o 2 by using sunlight to oxidize aqueous fe ii and precipitate fe iii oxyhydr oxides once deposited at the seafloor diagenetic reduction of the fe iii oxyhydr oxides by heterotrophic bacteria produced secondary fe ii bearing minerals such as siderite feco 3 and magnetite fe 3 o 4 via the oxidation of microbial organic carbon i e cellular biomass during deeper burial at temperatures above the threshold for life thermochemical fe iii reduction has the potential to form bif like minerals however the role of thermochemical fe iii reduction of primary bif minerals during metamorphism and its impact on mineralogy and geochemical signatures in bifs is poorly understood consequently we simulated the metamorphism of the precursor and diagenetic iron rich minerals ferrihydrite goethite hematite at low grade metamorphic conditions 170 c 1 2 kbar for 14 days by using 1 mixtures of abiotically synthesized fe iii minerals and either microbial biomass or glucose as a proxy for biomass and 2 using biogenic minerals formed by phototrophic fe ii oxidizing bacteria mossbauer spectroscopy and xrd showed that thermochemical magnetite formation was limited to samples containing ferrihydrite and glucose or goethite and glucose no magnetite was formed from fe iii minerals when microbial biomass was present as the carbon and electron sources for thermochemical fe iii reduction this could be due to biomass derived organic molecules binding to the mineral surfaces and preventing solid state conversion to magnetite mossbauer spectroscopy revealed siderite contents of up to 17 after only 14 days of incubation at elevated temperature and pressure for all samples with synthetic fe iii minerals and biomass whereas 6 of the initial fe iii was reduced to sideritic fe ii in biogenic fe iii minerals during incubation based on our data magnetite in bif is unlikely to be formed by thermochemical fe iii reduction in sediments of biogenic ferrihydrite hematite or goethite dominated sediments with complex microbial biomass present instead our results suggest that diagenetic magnetite in bif was either formed by microbial fe iii reduction during early diagenesis i e below 120 c or by thermochemical fe iii reduction with simple organic compounds at higher temperatures whereas siderite was formed by both microbial diagenetic fe iii reduction and thermogenic fe iii reduction with complex biomass thermochemical fe iii reduction coupled to biomass oxidation during metamorphism provides another origin for bif siderites and could have led to a significant increase in fe ii content in bif after deposition over geological timescales

seafloor hydrothermal fluids ben nevis area abitibi greenstone belt implications for archean 2 7ga seawater properties abstract a central part of the 2 7 ga abitibi greenstone belt between the destor porcupine and cadillac larder lake structures has been subjected to sub greenschist facies metamorphism t the majority of observed fluid inclusions are two phase and liquid dominated with a vapor bubble volume of 10 microthermometry on 58 samples 1100 fluid inclusions and eutectic temperatures between 45 and 30 c indicate the presence of at least one significant divalent cation in the fluids hence salinity is calculated as wt cacl2 equiv the peak of the salinity distribution of inclusions from vesicles and interstitial and drainage cavity quartz falls between 4 and 10 wt cacl2 equiv a small subset of inclusions yielded salinities between 18 and 22 wt cacl2 equiv most homogenization temperatures fall in an interval between 110 and 160 c a geothermal gradient estimated from the data of 10 15 c km is reasonable and consistent with the interpretation of the area as a regional seafloor recharge zone primary porosity in the modern ocean floor does not show the rapid precipitation of quartz that is indicated for the archean however the global occurrence of silica rich banded iron formations and the lack of silica consuming marine organisms in the precambrian indicates that marine geochemistry and sio2 precipitation mechanisms may have been different during the precambrian the total salinity of a fluid is dependent primarily on its total chlorinity hence water rock interactions with ion exchange cannot be the only reason for the higher salinities encountered in this study indications for boiling or high temperature phase separation are not evident suggesting that these options are not a reason for the observed high salinities our data are in agreement with six other data sets from the 3 75 ga isua greenstone belt w greenland the 3 24 ga pilbara region w australia the 3 2 ga barberton greenstone belt south africa 2 the 2 7 ga matagami area abitibi greenstone belt canada and the proterozoic 2 2 ga ongeluk formation south africa the consistency of fluid inclusion data suggests that archean and early proterozoic seawater may have been more saline and potentially stratified compared to modern seawater

sedimentology and geochemistry of archean silica granules the production of biogenic silica has dominated the marine silica cycle since early paleozoic time drawing down the concentration of dissolved silica in modern seawater to a few parts per million ppm prior to the biological innovation of the first silica biomineralizing organisms in late proterozoic time inputs of silica into precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes although the mechanics of this abiotic marine silica cycle remain poorly understood cherty sedimentary rocks are abundant in archean sequences and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in archean seawater the recent discovery that many pure chert layers in early archean rocks formed as sedimentary beds of sand sized subspherical silica granules has provided direct evidence for primary silica deposition here we provide further sedimentological and geochemical analyses of early archean silica granules in order to gain a better understanding of the mechanisms of granule formation silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths the abundance and widespread distribution of silica granules in archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column the regular occurrence of silica granules as centimeter scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis looking to modern siliceous sinters for insight into silica precipitation we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres consistent with this hypothesis archean ocean chemistry would have favored particle aggregation over gelling granule formation would have been most favorable under conditions promoting rapid silica polymerization including high salinity and or high concentrations of dissolved silica our observations suggest that granule sedimentation was often episodic suggesting that granule formation may have also been episodic perhaps linked to variations in these key parameters

silica replaced oolites bedded shelf cherts and paleozoic changes in the silica cycle abstract oolites that were replaced by early diagenetic silica and were not reworked from their original depositional setting can serve as a useful tool to evaluate a previously suggested retreat of biosiliceous facies from peritidal environments to deeper water settings in the early paleozoic the silica replaced oolites suggest that the retreat was largely over by the middle ordovician which is consistent with emerging compilations of the spatial and temporal distribution ordovician cherts whether the retreat was a function of declining concentration of dissolved silica in shallow marine waters or of radiation of faunas from nearshore into offshore settings or some combination of both is an issue that may be clarified with improved resolution of the timing of the retreat a small upper carboniferous peak in silica replaced oolite abundance coincides with a minor rebound in shelf chert accumulation and the common presence of coastal flint deposits in upper carboniferous cyclothems suggesting that cherty facies returned to shallow water environments at this time

silicon isotopes in 3 8 ga west greenland rocks as clues to the eoarchaean supracrustal si cycle we report mc icp ms si isotopic measurements 29si recalculated as 30si on micro subsamples at 500 m scale from several major rock types of the 3 8 ga old isua greenstone belt igb southern west greenland and surrounding eoarchaean terrains with a large overall range of variations 2 80 30si 0 68 they demonstrate strong involvement of surface fluids enriched with dissolved si the resistance of si isotopes to metamorphic resettings and metasomatic overprints is also established metabasaltic pillows and metasediments display similar 30si enriched signatures suggesting that emergent surfaces of the eoarchaean protocrust were composed of slightly weathered hydrothermally altered mafic ultramafic bodies isua magnetite quartz banded iron formation bif is strongly depleted in 30si relative to all coeval rocks this depletion supports rayleigh controlled precipitation from seafloor vented hydrothermal fluids in contrast banded quartz pyroxene rocks from akilia island some 150 km southwest of the igb which some authors have identified as bif related yield quartz with si isotopic composition 30si 0 36 similar to metamorphic derived quartz 30si 0 50 this supports their derivation from tectonic reworking of ultramafic protoliths penetrated by metamorphic silica and is at variance with their proposed role as harboring earliest biogenic tracers

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

precambrian si isotope mass balance weathering and the significance of the authigenic clay silica sink abstract the geological cycles of carbon and silicon are linked via the silicate weathering feedback but growing understanding of reverse weathering the formation of authigenic clay minerals via back reaction of silica with cations indicates that the nature of the connection between these two elemental cycles may be more complex than previously assumed recent carbon cycle modeling has suggested that enhanced reverse weathering during precambrian time could have played an important role in regulating climate the topology of the precambrian silica cycle including the size of the authigenic clay sink and its influence on the carbon cycle is challenging to reconstruct due to inherent preservation biases in the rock record here we used an alternative approach using si isotope values to invert for the magnitudes of precambrian silica fluxes by assuming that the silica cycle must be in mass balance with the bulk silicate earth over geological time scales with respect to si isotopes to estimate uncertainties associated with these flux reconstructions we used monte carlo simulations based on the natural variation exhibited in si isotope ratio data from the precambrian sedimentary record n 2118 spanning 3 8 ga to 525 ma and modern authigenic clays n 123 the perspective of si isotope mass balance shows that the previously observed increase in chert 30si values across precambrian time is a signal of an evolving silica cycle rather than a secular change in seawater temperature results yielded a secular increase in the relative contribution of the authigenic clay sink across most of precambrian time this view is consistent with previous interpretations of a concomitant decrease in the concentration of dissolved silica in seawater this result is consistent among a variety of model versions incorporating different assumptions about the abundance of iron formations and possible relationships between authigenic clay formation and concepts that invoke slow growth in the absolute amount of the continental crust this approach also highlights key areas where improving si isotope records could further enhance our constraints on the precambrian silica cycle and its relationship with the carbon cycle and climate stability

iron isotopes constrain biologic and abiologic processes in banded iron formation genesis abstract the voluminous 2 5 ga banded iron formations bifs from the hamersley basin australia and transvaal craton south africa record an extensive period of fe redox cycling the major fe bearing minerals in the hamersley transvaal bifs magnetite and siderite did not form in fe isotope equilibrium but instead reflect distinct formation pathways the near zero average 56fe values for magnetite record a strong inheritance from fe3 oxide hydroxide precursors that formed in the upper water column through complete or near complete oxidation transformation of the fe3 oxide hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of 56fe values 1 addition of marine hydrothermal fe 2 aq 2 complete reduction by bacterial dissimilatory iron reduction dir and 3 interaction with excess fe 2 aq that had low 56fe values and was produced by dir most siderite has slightly negative 56fe values of 0 5 that indicate equilibrium with late archean seawater although some very negative 56fe values may record dir support for an important role of dir in siderite formation in bifs comes from previously published c isotope data on siderite which may be explained as a mixture of c from bacterial and seawater sources several factors likely contributed to the important role that dir played in bif formation including high rates of ferric oxide hydroxide formation in the upper water column delivery of organic carbon produced by photosynthesis and low clastic input we infer that dir driven fe redox cycling was much more important at this time than in modern marine systems the low pyrite contents of magnetite and siderite facies bifs suggests that bacterial sulfate reduction was minor at least in the environments of bif formation and the absence of sulfide was important in preserving magnetite and siderite in the bifs minerals that are poorly preserved in the modern marine record the paucity of negative 56fe values in older early archean and younger early proterozoic bifs suggests that the extensive 2 5 ga hamersley transvaal bifs may record a period of maximum expansion of dir in earth s history

diagenetic replacement controlled by force of crystallization many diagenetic replacements in sedimentary rocks occur along thin solution films between the authigenic and host phases rather than conventionally invoking bulk pore water undersaturation for host phase dissolution we propose that many diagenetic replacements occur by a force of crystallization controlled replacement mechanism whereby nonhydrostatic stresses resulting from authigenic crystal growth are principally responsible for host phase dissolution this model explains diagenetic features such as the restriction of host phase dissolution to authigenic host crystal contacts and euhedral authigenic crystal faces in planar contact with unreplaced host phases as well as the relative replacement tendencies of some common authigenic minerals

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

a petrographic and geochemical study of carbonate and silica phases from the ediacaran doushantuo formation in the three gorges area of south china implications for diagenetic conditions abstract the carbonate of the ediacaran doushantuo formation contains various silica phases in the three gorges area of south china to better understand the diagenetic conditions under which these silica phases formed we carried out a petrographic and geochemical study of the silica phases and the coadjacent carbonate phases at the xiaofenghe section xfh in the three gorges area of south china five types of silica phases can be distinguished based on petrographic characteristics s1 and s2 are composed of microcrystalline quartz in chert nodules and disseminated in the host dolostone respectively s3 is composed of the megaquartz while s4 and s5 are microcrystalline quartz and fibrous silica respectively both replacing calcite cement six distinct types of carbonate can be recognized including the host matrix carbonate c1 sparry calcite nodules c2 calcite around chert nodules c3 dispersed calcite cement within chert nodules c4 fine micropeloidal calcite cements c5 and calcite veins c6 sedimentary structures suggest that formation of the s1 through s5 silica phases and c2 through c5 carbonate phases occurred during early burial diagenesis and therefore their formation recorded early diagenetic conditions assuming that ediacaran surface seawater temperature ranged from 28 to 39 c surface seawater 18o values can be calculated based on the oxygen isotopic composition of the host dolostone matrix c1 which range from 6 to 0 4 to 0 and 2 to 0 for the d2 d3 and d4 subunits of the doushantuo formation respectively based on the 18o values of chert nodules and beds mainly phase s1 diagenetic temperatures can be constrained to between 27 and 60 c under such low diagenetic temperatures the primary geochemical signatures of the host dolostone such as ree y patterns 13ccarb and 34spy values are likely to be preserved those silica phases without evidence of contamination of continental silicate detritus show characteristic ree y patterns with minor light ree depletions positive la eu and gd anomalies negative ce anomalies and slightly super chondritic y ho ratios implying that the source of silica may be derived from seawater negative ce anomalies in the silica phases in d2 through d4 subunits indicate that the bottom seawater or early diagenetic porewater was oxic at the time of silica precipitation

a paleoproterozoic drowned carbonate platform on the southeastern margin of the wyoming craton a record of the kenorland breakup abstract the nash fork formation in the upper part of the early paleoproterozoic snowy pass supergroup medicine bow mountains of wyoming was deposited on a mature passive margin along the southern flank of the wyoming craton and straddles the end of the ca 2 2 2 1 ga carbon isotope excursion two drowning events marked by black shales subdivide the carbonate platform into three parts the lower nash fork formation consists of outer shelf to supratidal deposits represented by massive and stromatolitic dolomites heterolithic siliciclastics carbonates large silicified domal digitate stromatolites nodular dolomites and stromatolitic dolomites molds after evaporite crystals are pervasive in the heterolithic siliciclastics carbonates large silicified domal digitate stromatolites formed biostromes and bioherms following flooding events the middle nash fork formation comprises two intervals of black shale separated by inner shelf heterolithic siliciclastics nodular carbonates black shales are organic and pyrite rich contain turbidites and developed in response to drowning of the platform overlying massive dolomite of the upper nash fork formation was deposited in an unprotected intertidal setting and displays an upward shallowing trend terminated by a prominent karstic surface in the middle of the unit the nash fork formation is open marine with no evidence for restricted circulation on the carbonate platform the two drowning events on the carbonate platform are likely related to dissection of the mature passive margin associated with the breakup of kenorland the younger drowning event is associated with the end of the carbon isotope excursion the main building elements of the lower and middle nash fork carbonate platform are dolomitic mudstones and stromatolites macroscopic seafloor precipitates are volumetrically negligible with the exception of tufa deposits and domes in the massive and stromatolitic dolomites and possibly digitate stromatolites within domal digitate stromatolites the upper nash fork formation comprises dolomitic mudstones relatively rare stromatolites and inorganic precipitates that are more common than in the underlying carbonates styles of carbonate deposition on this early paleoproterozoic platform differ from those documented on late archean carbonate platforms there are fewer macroscopic seafloor precipitates and more dolomitic mudstones this pattern is considered to be related to a rise of the atmospheric oxygen level that led to a decrease in bicarbonate saturation in the ocean

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

germanium silica ratios in diagenetic chert nodules from the ediacaran doushantuo formation south china abstract germanium silica ge si ratios of dolostone and mudstone hosted chert nodules from the ediacaran 635 542 ma doushantuo formation in the yangtze gorges area south china are reported these chert nodules typically have a calcite rim a pyrite rim and a silica core the latter sometimes containing disseminated pyrite the silica core was precipitated by early diagenetic replacement of carbonate and silty muddy sediments two types of chert nodules are identified based on their mineralogy and geochemistry type 1 chert nodules are poor in disseminated pyrite in the silica core they also have low al and show a strong positive correlation between al contents and ge si with a near zero or negative intercept in contrast type 2 chert nodules contain abundant disseminated pyrite in the silica core and show a weakly positive correlation between ge si ratios and al contents with a large positive intercept on the ge si axis the ge si of doushantuo nodules are greater than those of cretaceous deep sea cherts suggesting that the ge si ratio of ediacaran seawater porewater was greater than the cretaceous due to the more effective discrimination against ge by inorganic opal precipitation relative to biogenic opal precipitation the positive correlation between ge si and al can be interpreted using a mixing model with a pure chert characterized by a low ge si ratio and an al rich endmember characterized by a high ge si ratio the latter is most likely represented by a clay component but the model based estimate of the ge si ratio inferred for the al rich clay endmember is much higher than that of phanerozoic clay minerals these high ge si ratios for the clay endmember may be related to the generally high ge si ratio of ediacaran seawater but could also be related to clay organic matter interactions organic matter absorbed to clays could provide an additional source of ge because certain organic molecules are known to have a high affinity for ge due to their strong metal ion chelating properties the high ge si ratio of the al rich endmember in type 1 chert nodules suggests that ge in porewaters from which these cherts precipitated may have been dominated by ge organic complexes the low ge si ratio inferred for the al rich endmember in type 2 chert nodules is therefore taken to indicate that ge was released from organic matter perhaps due to anaerobic degradation of organic matter accompanied by the formation of pyrite and was redistributed between clay organic endmembers and pure chert resulting in a non zero intercept in the ge si vs al 2 o 3 plots these observations suggest that a strong terrestrial influence in a restricted sedimentary basin or a high content of dissolved organic carbon in ediacaran seawater and porewater may have contributed to the dominance of ge organic complexes in the doushantuo basin in the yangtze gorges area

fabric development in proterozoic bedded chert penganga group adilabad india sedimentologic implications laterally persistent horizons of interstratified chert limestone manganese ores within a siliceous micritic limestone are interpreted to have been deposited on a distally steepened deep carbonate ramp in the proterozoic penganga group of the pranhita godavari valley south india the association of intraformational limeclast conglomerate turbidites and pelagic micritic limestone indicates deposition of the chert bearing horizons at the toe of a distally steepened ramp slope environment the chert is characterized by a wide variety of fabrics including cryptocrystalline and microcrystalline quartz equant megaquartz and chalcedony cryptocrystalline and microcrystalline quartz are most common they occur mainly as lenses of mosaic quartz and define laminae megaquartz in contrast occurs as irregular patches as fenestroids within cryptocrystalline or microcrystalline fabrics or in complex aggregates and disrupted mosaics the origin of the cryptocrystalline and microcrystalline quartz by maturation parallels the morphological evolution of many deep sea cherts that form by maturation of biogenic opal a to quartz chert through an opal ct stage within the quartz stage the microcrystalline fabric formed by pervasive space centered grain growth of cryptocrystalline crystals and porphyroid growth resulted in porphyrotopic fabric the megaquartz formed directly from pore water in continuity with maturation of opal ct the disrupted and curdled mosaics attest to high fluid activity within semilithified sediments the variation in fabric types at the microscale suggests fabric evolution at low temperature the presence of chert clasts in debris flow conglomerates and development of a plethora of water escape structures suggest maturation of silica at shallow burial depth maturation at shallow burial was favored by very low detrital content of these cherts and a mg rich pelagic carbonate depositional geochemical milieu estimated sedimentation rates from 2 to 10 mm ky super 1 closely match the average sedimentation rate from modern pelagic siliceous deposits

a paleoweathering profile from griqualand west south africa evidence for a dramatic rise in atmospheric oxygen between 2 2 and 1 9 bybp a core drilled near wolhaarkop in griqualand west south africa intersected highly oxidized kuruman iron formation below red beds of the gamagara formation the lateral equivalents of the kuruman iron formation in this drill hole consist largely of siderite ankerite magnetite greenalite and quartz the oxidation of the kuruman iron formation in wol 2 occurred almost certainly during weathering prior to the deposition of the gamagara formation the date of this weathering episode is bracketed between about 2 2 and 1 9 bybp by the age of the ongeluk lavas in the transvaal sequence below the unconformity and by the age of the hartley lavas in the olifantshoek group above the unconformity the ratio of iron to sio2 in the several facies of the weathered kuruman iron formation in wol 2 is nearly the same as that in their unweathered equivalents since sio2 loss during weathering was almost certainly minor the similarity of the fe sio2 ratio in the weathered and unweathered bif indicates that nearly all the feo in the kuruman iron formation was oxidized and retained as feo3 during weathering such a high degree of iron retention is best explained by an o2 content of the atmosphere or 0 03 atm at the time of weathering such an o2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt or 2 2 bybp but is consistent with the highly oxidized nature of the 1 85 by flin flon paleosol the new data suggest that po2 rose dramatically from about 1 percent pal present atmospheric level to or 15 percent pal between 2 2 and 1 9 bybp

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

fossil preservation through phosphatization and silicification in the ediacaran doushantuo formation south china a comparative synthesis phosphatized and silicified microfossils preserved through replication of organic templates by authigenic calcium phosphate and silica respectively form through mechanistically similar mineralization processes but under notably different conditions in order to characterize the environments and taphonomic processes associated with these preservational pathways we compare phosphatized and silicified fossil assemblages in the ediacaran doushantuo formation south china in terms of stratigraphic and paleogeographic distribution preservation and biodiversity several differences are recognized 1 phosphatization and silicification both occurred in sub tidal doushantuo sediments but phosphatized fossils generally occur in shallower sub tidal facies than silicified fossils 2 although both phosphatized and silicified fossils are preserved three dimensionally the latter are typically more compressed and 3 whereas phosphatized assemblages are numerically dominated by multicellular acritarchs putative metazoan eggs and embryos silicified assemblages are dominated by prokaryotes these differences can be explained by environmental and taphonomic distinctions phosphatized fossils occur in intraclastic phosphorites and phosphatic carbonates and were likely fossilized near the suboxic anoxic boundary in sediments under oxic bottom waters with redox and biogeochemical gradients conducive to p remineralization phosphate burial via iron pumping and calcium phosphate precipitation reworking and winnowing of phosphatized fossils preferentially concentrated certain taxa prior to final burial in contrast silicified fossils typically occur in chert nodules which formed deeper in the sediment column in ferruginous pore waters and perhaps under ferruginous bottom waters where bacterial sulfate reduction and pyrite formation created low ph conditions conducive to calcite dissolution and silica precipitation once formed chert nodules mostly remained in situ but some were reworked with turbidites and olistostromes developed in slope facies despite these distinctions phosphatized and silicified fossil assemblages in the doushantuo formation generally overlap in time and space and representatives of all major groups and many common genera of doushantuo organisms are present in both phosphorites and cherts consequently although both preservational pathways have paleoenvironmental and taphonomic biases their relative biases on taxonomic presence absence data are minimal given sufficient sampling efforts and paleobiologic and biostratigraphic information based on presence absence but not abundance data from the doushantuo formation appears to be robust

a field guide to finding fossils on mars the martian surface is cold dry exposed to biologically harmful radiation and apparently barren today nevertheless there is clear geological evidence for warmer wetter intervals in the past that could have supported life at or near the surface this evidence has motivated national aeronautics and space administration and european space agency to prioritize the search for any remains or traces of organisms from early mars in forthcoming missions informed by 1 stratigraphic mineralogical and geochemical data collected by previous and current missions 2 earth s fossil record and 3 experimental studies of organic decay and preservation we here consider whether how and where fossils and isotopic biosignatures could have been preserved in the depositional environments and mineralizing media thought to have been present in habitable settings on early mars we conclude that noachian hesperian fe bearing clay rich fluvio lacustrine siliciclastic deposits especially where enriched in silica currently represent the most promising and best understood astropaleontological targets siliceous sinters would also be an excellent target but their presence on mars awaits confirmation more work is needed to improve our understanding of fossil preservation in the context of other environments specific to mars particularly within evaporative salts and pore fracture filling subsurface minerals

source of silica and silicification of the lowermost cambrian yanjiahe formation in the three gorges area south china abstract the rise of siliceous organisms is generally considered to have driven the abrupt decrease of silica in the ocean during the ediacaran cambrian e c interval even if the exact timing and biological triggers of this event are still debated although ediacaran cambrian chert depositions are widely distributed multiple evidences converge to support their hydrothermal origin in this study we provide petrographic study supplemented by elemental concentration sem and raman spectral data on the siliceous rocks from the yanjiahe formation the three gorges area south china in order to ascertain the origin of included silica and discuss the involved processes of silica deposition contrary to most previous studies our analyses suggest that studied cherts mainly resulted from biogenic silica and subsidiary from abiotic precipitation influenced by surrounding hydrothermal activity and from detrital input the deposition model of silica includes direct deposition of siliceous bioclasts secondary replacement of carbonate by dissolved biosilica and microbial induced silicification from seawater the sink of silica recorded in the yanjiahe formation was characterized of a transition from typical precambrian to phanerozoic process in addition micropalaeontological and taphonomical analyses of the yanjiahe formation revealed two pulses of microfossil abundance and diversity in the fortunian and stage 2 respectively they can be correlated with a regression and an enrichment in dissolved phosphate that probably derived from enhanced detrital flux

decoupled sources of the 2 3 2 2ga yuanjiacun banded iron formation implications for the nd cycle in earth s early oceans abstract 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 2 ga 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 al 2 o 3 e nd t 3 5 derived from interaction with a depleted mantle source and associated with high fe fluxes the second is ambient surface seawater e nd 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 3 ga 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

silica alteration zones in the barberton greenstone belt a window into subseafloor processes 3 5 3 3 ga ago abstract silicification of volcanic rocks ranging from komatiitic to dacitic in composition is a common phenomenon of the 3 5 to 3 2 ga old volcano sedimentary succession of the barberton greenstone belt silica enrichment occurs in zones several tens of metres thick at the top of volcanic sequences that are capped by sedimentary chert horizons sio2 contents increase upsection from the original igneous value up to 92 silicification is associated with a depletion of most elements commonly mobile during water rock interaction while k2o rb and ba are enriched in these zones some transition metals sc v and cr were immobile whereas ni and co and some other metals cu zn were mobile and are depleted the 18o values of volcanic rocks show a positive linear relationship with silica content and range from 9 0 to 17 3 the element depletion enrichment patterns and oxygen isotope data indicate low temperature c 100 150 c hydrothermal processes for the origin of the alteration zones the loss of most elements coupled with the addition of sio2 as well as ree systematics indicate high water rock ratios and a fluid ree composition typical of archaean seawater that had a 18o value of 0 hydrothermal activity is attributed to heat derived from cooling of volcanic rocks and a high regional heat flow in an oceanic plateau like setting and resulted in the establishment of shallow subseafloor convection cells diffuse venting of hydrothermal fluids over broad areas of the ancient ocean floor provided an ideal environment for both seafloor and subsurface biota 3 5 ga ago

sulfate reducing bacteria and silica solubility a possible mechanism for evaporite diagenesis and silica precipitation in banded iron formations selective replacement of sulfate evaporite minerals by silica and the precipitation of silica in association with sulfide mineral phases in banded iron formations may be mediated by the metabolic activities of sulfate reducing bacteria hydrogen sulfide is known to be a product of this metabolism and is often called upon as a source of sulfur for metallic sulfides in sedimentary rocks we report here on the influence that chemical changes induced by bacterial sulfate reduction have on silica solubility controlled in vitro growth experiments with desulfovibrio desulfuricans and silica show 1 this organism can grow in silica concentrations as great as 400 ppm with no inhibition and 2 growth in the presence of silica yields a decrease in dissolved silica growth experiments with 80 ppm silica produced a lowering in dissolved silica from 80 ppm to 60 ppm a 25 decrease in just 30 h control experiments in the absence of cells resulted in no effective decrease in dissolved silica the ability of sulfate r

rare earth elements in hamersley bif minerals abstract minerals from the hamersley banded iron formation western australia were analyzed for y and rare earth elements yrees by laser ablation icp ms to investigate diagenetic pathways from precursor phases to bif minerals one group of apatites carries the seawater ree signature giving evidence that p and rees thoroughly scavenged from the water column by si ferrihydrite particles were released upon microbial fe 3 reductive dissolution of si ferrihydrite in pore water and finally sequestered mainly in authigenic apatite the absence of fractionation between apatite and seawater suggests that ree were first incorporated into an amorphous calcium phosphate as fully hydrated cations i e as outer sphere complexes the iron oxides and carbonates carry only a small fraction of the whole rock ree budget their ree patterns are distinctly enriched in yb and show some m type tetrad effect consistent with experimental k d ree between solid and saline solution with low carbonate ion concentrations it is deduced that hematite formed at an incipient stage of fe 2 catalyzed dissolution of si ferrihydrite via a dissolution reprecipitation pathway the ree pattern of greenalite found as sub micron particles in quartz in a chert siderite sample is consistent with its authigenic origin by precipitation in pore water after dissolution of a small amount of si ferrihydrite magnetite carries very low yrees ppb level has an homogeneous pattern distinctly enriched in the mid rees compared to hematite and includes a late population depleted in light rees ba and as magnetite forming aggregates and massive laminae is tentatively interpreted as reflecting some fluid aided hematite magnetite re equilibration or transformation at low grade metamorphic temperatures

alternating si and fe deposition caused by temperature fluctuations in precambrian oceans precambrian banded iron formations contain thin alternating layers of iron and silica rich minerals microbial culture experiments show that these alternations could be caused by changes in the relative dominance of microbial iron oxidation and abiotic silica precipitation driven by fluctuating ocean temperature

diagenetic fe carbonates in paleoarchean felsic sedimentary rocks hooggenoeg formation barberton greenstone belt south africa implications for co2 sequestration and the chemical budget of seawater abstract in order to evaluate the potential of felsic sediments as a co2 sink in the archean we studied felsic volcaniclastic epiclastic sedimentary rocks of the 3 45 ga hooggenoeg formation barberton greenstone belt which were affected by metasomatic processes during seafloor alteration and diagenesis water rock interactions leading to k si and co2 metasomatism were quantified the precursor rock a k metasomatized dacite was leached of ca mg fe na sr and ba and enriched in k rb and si prior to erosion and deposition the formation of k mica and quartz and minor k feldspar in the dacites suggests low ph metasomatic conditions likely produced by hydrothermally induced circulation of hot acidic and reduced archean seawater in equilibrium with a high p c o 2 atmosphere erosion and transport of k metasomatized dacitic detritus away from the felsic volcanic centers resulted in the deposition of conglomerate sandstone and shale by mass flow processes early diagenetic silicification affected mainly the fine grained sediments with higher silica sorption capacity forming impermeable layers while sand rich sediments were partly silicified and remained permeable trapped fluids precipitated two generations of fe rich dolomites and finally calcite up to 30 vol of siliciclastic coarse grained sediment was replaced by carbonates in a shallow burial high heat flow diagenetic regime depth 750 m temperature 80 160 c and likely throughout deposition of overlying volcano sedimentary units the carbon isotopic composition of fe rich dolomites 13cpdb 1 9 to 2 4 and the strong fe ca mg leaching of the paleoarchean volcanic formations support the influence of seawater derived fluids throughout co2 metasomatism for each gram of eroded dacite the overall chemical exchange involved by k si co2 metasomatism was characterized by a mass transfer of fe 1 2 mmol g na 2 1 mmol g and o2 2 0 mmol g to seawater in contrast seawater was depleted in si 10 mmol g ca 0 51 mmol g mg 0 43 mmol g k 1 5 mmol g and h 0 93 mmol g during incorporation of these elements in the volcanic and sedimentary rocks the average co2 uptake by the sedimentary rocks studied here is estimated to be 1 8 mmol g in the same order of magnitude as previous estimates for the paleoarchean basaltic crust although mafic ultramafic rocks are the most abundant rocks in paleoarchean greenstones belts and represent the most important atmospheric co2 sink upon seafloor alteration in the paleoarchean coarse felsic sedimentary rocks provide a non negligible contribution to the build up of the continental co2 reservoir

microbial and animal evolution in relation to redox fluctuations in a deep water setting of south china during the ediacaran cambrian transition ca 551 523 ma abstract the ediacaran cambrian e c transition witnessed significant ecosystem perturbations of the oceanic environment e g oxygen concentrations of seawater and biological evolution however in spite of abundant ediacara type megafossils trace fossils small shelly fossils ssfs macroscopic algae and metazoans recovered from shallow water e c transition strata throughout south china the manifestation of geological events in deep water settings remains elusive here we present new paleontological and geochemical evidence from the 38 m thick deep water liuchapo formation ca 551 523 ma of the yangtou section northeastern guizhou province china which consists mainly of chert through the e c transition the liuchapo formation can be divided into two members based on biostratigraphy and redox proxies the lower member 0 32 m is characterized by low mo al 0 15 1 82 ppm and u al 0 47 2 51 ppm values that suggest deposition under suboxic conditions these deposits are dominated by horodyskia minor paleopascichnus jiumenensis spherical cyanobacteria myxococcoides and random acritarchs the upper member 32 38 m records a transition to anoxic non sulfidic conditions as suggested by marked increases in mo al 5 47 ppm and u al 8 810 ppm the upper member contains ssfs sponge spicules filamentous cyanobacteria multicellular algae and sheet like fossils the survival of diverse organisms under strongly reducing conditions reflected in the upper member suggests that marine redox fluctuations may limit the growth of late ediacaran organisms and provide the living space and elevated nutrients for early cambrian biota

a lagrangian perspective on parameterizing deep convection abstractthe parameterization of deep moist convection as a subgrid scale process in numerical models of the atmosphere is required at resolutions that extend well into the convective gray zone t

sedimentary and tectonic history of the holowilena ironstone a neoproterozoic iron formation in south australia abstract the holowilena ironstone is a neoproterozoic iron formation in south australia associated with glacial deposits of the sturtian glaciation through a comprehensive field study coupled with optical and scanning electron microscopy x ray fluorescence and x ray diffraction a detailed description of the stratigraphy sedimentology mineralogy and structure of the holowilena ironstone was obtained the holowilena ironstone comprises ferruginous shales siltstones diamictites and is largely made up of hematite and jasper early diagenetic replacement minerals of precursor iron oxyhydroxides and silica these chemical precipitates are variably influenced by turbidites and debris flows contributing clastic detritus to the depositional system structural and stratigraphic evidence suggests deposition within a synsedimentary half graben a model for the holowilena ironstone is proposed in which dense oxic fluids expelled during sea ice formation in the cryogenian pool in the depression of the half graben allowing for long lived mixing with the ferruginous seawater and the deposition of iron oxides this combination of glacial dynamics tectonism and ocean chemistry may explain the return of iron formations in the neoproterozoic

siliceous sediments of the guaymas basin the effect of high thermal gradients on diagenesis recent siliceous sediments of the guaymas basin a rapid spreading center in the gulf of california are intruded by basaltic sills which induced significant diagenetic and metamorphic changes in the sediments the transformation from opal a to opal ct opal ct to quartz and opal a directly to quartz in these sediments cored on dsdp leg 64 can be used to infer the temperature history and order of emplacement of intrusives at or near contacts with sills of 30 m or greater average thickness opal a inverts directly to authigenic quartz but there is less quartz than would be expected from the amount of opal a dissolution opal ct forms in sediments sandwiched between adjacent sills based on high rates of quartz nucleation and growth at high temperatures 150 c and on considerations of convective solution transfer opal ct is thought to form only where temperatures were lower or at positions between sills an environment which prevents rapid convective dissipation of silica in solution where temper

nodular chert formation in carbonate rocks mapping of carbonate dissolution in nodular chert bearing strata indicates that nodular chert formation in a diverse suite of phanerozoic limestones occurred in bulk pore waters that were at calcite saturation opal ct and quartz supersaturation was achieved in most chert bearing strata by the intraformational dissolution of amorphous silica skeletal material chertification occurred by a combination of force of crystallization controlled replacement of the host carbonate whereby non hydrostatic stresses resulting from opal ct and quartz crystal growth caused calcite dissolution and the isomineralic heterogenous nucleation of new opal ct and quartz crystal at nodule peripheries the restriction of the solution phase at silica carbonate contacts to thin films permitted the preservation of ghosts of micron sized features in the chert heterogeneities in sediment organic matter content porosity and biogenic silica concentrations promote the nucleation of chert nodules in some formations

depositional gaps in abitibi greenstone belt stratigraphy a key to exploration for syngenetic mineralization models of greenstone belt development are crucial for exploration allochthonous models predict belts to be a collage of unrelated fragments whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary timiskaming style assemblages and field and geochronological data indicate that the abitibi greenstone belt developed autochthonously we describe major revisions to stratigraphy of the abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization the abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous u pb zircon ages of pre 2750 2750 to 2735 2734 to 2724 2723 to 2720 2719 to 2711 2710 to 2704 and 2704 to 2695 ma we present revised lithotectonic and or stratigraphic nomenclature using these time intervals including 1 isotopic inheritance in younger episodes which indicates that the older episodes 2750 2735 and 2734 2724 ma had greater extent than is presently seen 2 dikes feeding younger volcanic episodes 2706 ma cutting older volcanic units 2734 2724 ma and 3 2710 to 2704 ma mafic to ultramafic sills intruding the 2719 to 2711 ma episode changes to the nomenclature include the identification of pre 2750 ma volcanic episode supracrustal fragments in the northern and southern abitibi greenstone belt and subdivision of the 2719 to 2711 ma 2710 to 2704 ma and 2704 to 2695 ma episodes into lower and upper parts we present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and or lithotectonic map of the abitibi greenstone belt many of the volcanic episodes are intercalated with and capped by a relatively thin sedimentary interface zone dominated by chemical sedimentary rocks stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m y between volcanic episodes the zones consist of up to 200 m of iron formation chert breccia heterolithic debris flows of volcanic provenance sandstone and or argillite and conglomerate modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years whereas the time interval between basalt rhyolite cycles is 106 years the sedimentary interface zones are therefore interpreted as condensed sections zones with very low rates of sedimentation in a basinal setting or zones with negligible rates of sedimentation marked by silicification of existing rock types the sedimentary interface zones are therefore considered submarine correlative conformities disconformities or unconformities separating the equivalent of group level stratigraphic and lithotectonic units the unconformity bounded stratigraphic model provides a new regional to deposit scale interpretive model for use in exploration for syngenetic mineralization

weathering before the advent of land plants evidence from unaltered detrital k feldspars in cambrian ordovician arenites potassium chelation by land plants and the consequent depletion of k ion activity in soil waters make k feldspars unstable in present day soils other factors remaining constant the absence of land plants in pre silurian time would make k feldspars stable dissolution of plagioclase under the action of soil water would be the same in both pre and post silurian time the presence of abundant fresh k feldspars and the absence of plagioclase in the cambrian ordovician arenites of ohio wisconsin minnesota missouri texas wyoming and montana are probably due to the absence of potassium chelating agents in the soils of the time

silicification in the belt supergroup mesoproterozoic glacier national park montana usa nodular cherts can provide a window on the original sediment composition diagenetic history and biota of their host rock because of their low susceptibility to further diagenetic alteration the majority of phanerozoic cherts formed by the intraformational redistribution of biogenic silica particularly siliceous sponge spicules radiolarian tests and diatom frustules in the absence of a biogenic silica source precambrian cherts necessarily had to have had a different origin than phanerozoic cherts the mesoproterozoic belt supergroup in glacier national park contains a variety of chert types including silicified oolites and stromatolites which have similar microtextures and paragenesis to phanerozoic cherts despite their different origins much of the silicification in the belt supergroup occurred after the onset of intergranular compaction but before the main episode of dolomitization the belt supergroup cherts probably had an opal ct precursor in the same manner as many phanerozoic cherts although it is likely that precambrian seas had higher silica concentrations than at present because of the absence of silica secreting organisms no evidence was observed that would suggest that high dissolved silica concentrations in the belt sea had a significant widespread effect on silicification the rarity of microfossils in belt supergroup cherts indicates that early silicification if it occurred was exceptional and restricted to localized environments the similarity of microtextures in cherts of different ages is evidence that the silicification process is largely controlled by host carbonate composition and dissolved silica concentration during diagenesis regardless of the source of silica

biomineralization and global biogeochemical cycles biological activity is a dominant force shaping the chemical structure and evolution of the earth surface environment the presence of an oxygenated atmosphere hydrosphere surrounding an otherwise highly reducing solid earth is the most striking consequence of the rise of life on earth biological evolution and the functioning of ecosystems in turn are to a large degree conditioned by geophysical and geological processes understanding the interactions between organisms and their abiotic environment and the resulting coupled evolution of the biosphere and geosphere is a central theme of research in biogeology biogeochemists contribute to this understanding by studying the transformations and transport of chemical substrates and products of biological activity in the environment biogeochemical cycles provide a general framework in which geochemists organize their knowledge and interpret their data the cycle of a given element or substance maps out the rates of transformation in and transport fluxes between adjoining environmental reservoirs the temporal and spatial scales of interest dictate the selection of reservoirs and processes included in the cycle typically the need for a detailed representation of biological process rates and ecosystem structure decreases as the spatial and temporal time scales considered increase much progress has been made in the development of global scale models of biogeochemical cycles although these models are based on fairly simple representations of the biosphere and hydrosphere they account for the large scale changes in the composition redox state and biological productivity of the earth surface environment that have occurred over geological time since the cambrian explosion mineralized body parts have been secreted in large quantities by biota because calcium carbonate silica and calcium phosphate are the main mineral phases constituting these hard parts biomineralization plays an important role in the global biogeochemical cycles of carbon calcium silicon and phosphorus the chapter starts with introducing the basic concepts of

exceptional preservation of soft bodied ediacara biota promoted by silica rich oceans reply the ediacara biota earth s earliest fossilized ecosystem of complex macroscopic multicellular organisms occurs in terminal ediacaran strata worldwide yet how the fossils are preserved remains controversial ediacara assemblages consist of exceptionally preserved soft bodied forms of enigmatic morphology and phylogenetic affinity many of these fossil assemblages are anactualistically preserved as casts and molds in sandstones ediacara style preservation here we present evidence from the ediacara member of south australia that ediacara style preservation was due to rapid early stage precipitation of silica cements facilitated by the high silica saturation state of the oceans prior to the appearance of prolific silica biomineralizers an early silicification model provides a coherent mechanistic and empirically supported explanation for the widespread preservation of soft bodied organisms of ediacaran early paleozoic age as sandstone casts and molds the prevalence of early silicification confirms that ediacara style fossil assemblages can provide an accurate window into life on the ediacaran seafloor that can be used to reconstruct critical steps in the development and diversification of early animal ecosystems

heterogeneous seawater phosphorus concentrations during the sturtian glaciation evidence from p fe ratios of fulu formation ironstone in south china abstract phosphorus p is one of the key bio limiting elements in the modern ocean and controls marine primary productivity over the geological time scales it is proposed that the ediacaran atmosphere oxygenation and animal evolution might have been triggered by high seawater p concentration in cryogenian oceans seawater p concentration can be reconstructed by the ratio of phosphorus to iron expressed as p fe 100 in ironstones here we measured the p fe ratios of the banded iron formation bif from the cryogenian fulu formation in south china the p fe 100 ratios of the fulu bif vary between 0 08 and 1 81 mean 0 45 n 22 considerably lower than the values of coeval bif deposited in other continents mean 1 24 n 74 variation of p fe ratio of ironstones may suggest a large seawater p concentration gradient during the sturtian glaciation which is comparable to or even higher than that in the modern ocean 1 2 m in north atlantic and 2 6 m in north pacific though being explained in different ways the heterogeneous seawater p concentration requires active organic matter production in the surface ocean and or decomposition in the ocean interior as well as possible onset of thermohaline circulation during the sturtian glaciation finally our study suggests that precipitation of bif in the completely ice covered ocean was unlikely

objective analysis of marine winds with the benefit of the radarsat 1 synthetic aperture radar a nonlinear regression framework 1 surface wind analyses are constructed from spaceborne synthetic aperture radar sar measurements along coastal regions of eastern and western north america and collocated operational marine wind forecasts each analysis minimizes the error sum of squares of the wind forecast the sar backscatter and wind direction that is inferred from the sar backscatter gradient the relative importance of the sar wind information is defined by its expected error covariances parameters that define these covariances are postulated for half the analyses by comparison with an independent set of buoy observations the remaining analyses are found to compare better with buoy observations than conventional analysis approaches it is suggested that sar wind information generally be weighted strongly in an analysis and that an error covariance length scale of o 10 km is appropriate

role of biomineralization as an ultraviolet shield implications for archean life cyanobacteria isolated from the krisuvik hot spring iceland were mineralized in an iron silica solution and irradiated with high levels of ultraviolet light analysis of the rates of photosynthesis chlorophyll a content and phycocyanin autofluorescence revealed that these mineralized bacteria have a marked resistance to uv compared to nonmineralized bacteria naturally occurring sinters composed of iron silica biominerals collected from

evolution of early proterozoic passive margin carbonate platform rocknest formation wopmay orogen northwest territories canada abstract the superbly exposed rocknest formation wopmay orogen northwest territories canada is an early proterozoic 1 89 ga carbonate platform that formed along the eastern passive margin of slave craton archean the palinspastically restored shelf sequence is an eastward thinning prism 0 1 100 m thick extending for over 220 km parallel to strike and over 200 km perpendicular to strike it contains from west to east slope outer shelf shoal complex and inner shelf facies slope facies are carbonate rhythmites megabreccias and platy rhythmite breccias outer shelf facies include stromatolitic reefal boundstone and backreef ooid intraclast grainstore shoal complex facies are shallowing upward cycles of tufa cryptalgalaminite laminated dolosiltite lutite overlain by tepees b eccias and pisolite these formed a paleotopographic high restricting an eastern lagoon from western ocean inner shelf facies are dominantly asymmetric shallowing upward cycles of lagoonal mixed carbonates and siliciclastics that grade into intertidal stromatolitic dolomites cyclic facies pass eastward into noncyclic siliciclastics of central and eastern lagoon the rocknest lagoon spanned the width of slave craton and was bounded by a western carbonate shoreline shoal complex and by a tectonically active eastern siliciclastic shoreline the rocknest carbonate shelf developed on an underlying siliciclastic shelf an initial ramp rapidly evolved into a progradational accretionary rim and shoal complex followed by vertical aggradation of the rim concurrently the shoal complex underwent numerous west to east progradational events to produce many shallowing upward cycles in the shelf interior cyclic sedimentation was interrupted during three events of incipient shelf drowning associated with local backstepping of the rim and noncyclic sedimentation over most of the shelf probably caused by fourth order eustatic sea level oscillations the shelf was terminally drowned and overlain by deep water synorogenic siliciclastics during attempted subduction of the margin carbonates capping the shelf are stromatolitic patch re fs which overlie a possible unconformity suggesting emergence of the shelf prior to drowning

microbiological processes in banded iron formation deposition banded iron formations have been studied for decades particularly regarding their potential as archives of the precambrian environment in spite of this effort the mechanism of their deposition and specifically the role that microbes played in the precipitation of banded iron formation minerals remains unresolved evidence of an anoxic earth with only localized oxic areas until the great oxidation event ca 2 45 to 2 32 ga makes the investigation of o2 independent mechanisms for banded iron formation deposition relevant recent studies have explored the long standing proposition that archean banded iron formations may have been formed and diagenetically modified by anaerobic microbial metabolisms these efforts encompass a wide array of approaches including isotope ecophysiological and phylogeny studies molecular and mineral marker analysis and sedimentological reconstructions herein the current theories of microbial processes in banded iron formation mineral deposition with particular regard to the mechanisms of chemical sedimentation and post depositional alteration are described the main findings of recent years are summarized and compared here and suggestions are made regarding cross disciplinary information still required to constrain the role of the biosphere in banded iron formation deposition

a key role for green rust in the precambrian oceans and the genesis of iron formations iron formations deposited in marine settings during the precambrian represent large sinks of iron and silica and have been used to reconstruct environmental conditions at the time of their formation however the observed mineralogy in iron formations which consists of iron oxides silicates carbonates and sulfides is generally thought to have arisen from diagenesis of one or more mineral precursors ferric iron hydroxides and ferrous carbonates and silicates have been identified as prime candidates here we investigate the potential role of green rust a ferrous ferric hydroxy salt in the genesis of iron formations our laboratory experiments show that green rust readily forms in early seawater analogue solutions as predicted by thermodynamic calculations and that it ages into minerals observed in iron formations dynamic models of the iron cycle further indicate that green rust would have precipitated near the iron redoxcline and it is expected that when the green rust sank it transformed into stable phases within the water column and sediments we suggest therefore that the precipitation and transformation of green rust was a key process in the iron cycle and that the interaction of green rust with various elements should be included in any consideration of precambrian biogeochemical cycles large iron deposits formed episodically in the archaean oceans experimental data and geochemical modelling suggest that green rust was an important contributor to the formation of these deposits and the archaean iron cycle in general

micro scale silicon isotope heterogeneity observed in hydrothermal quartz precipitates from the 3 7 ga isua greenstone belt sw greenland pillow basalt and chert form integral lithologies comprising many archean greenstone belt packages to investigate details of these lithologies in the 3 7 ga isua greenstone belt sw greenland we measured silicon isotope compositions of quartz crystals by secondary ion mass spectrometry from a quartz cemented quartz amygdaloidal basaltic pillow breccia recrystallized chert and chert clasts thought to represent silica precipitation under hydrothermal conditions the recrystallized chert chert clasts and quartz cement have overlapping 30si values while the 30si values of the quartz amygdules span nearly the entire range of previously published values for quartz precipitates of any age despite amphibolite facies metamorphism we suggest that the heterogeneity is derived from kinetic isotope fractionation during quartz precipitation under disequilibrium conditions in a hydrothermal setting consistent with the pillow breccia origin on the basis of the present data we conclude that the geological context of each sample must be carefully evaluated when interpreting 30si values of quartz