microstructure of high temperature 73 c siliceous sinter deposited around hot springs and geysers yellowstone national park the role of biological and abiological processes in sedimentation slightly alkaline hot springs and geysers in yellowstone national park exhibit distinctive assemblages of high temperature 73 c siliceous sinter reflecting local hydrodynamic conditions the main depositional zones include subaqueous pool and channel bottoms and intermittently wetted subaerial splash surge and overflow areas subaqueous deposits include particulate siliceous sediment and dendritic and microbial silica framework silica framework forms thin porous microbe rich films coating subaqueous surces spicules with intervening narrow crevices dominate in splash zones surge and overflow deposits include pool and channel rims columns and knobs in thin section subaerial sinter is composed of i dark brown nearly opaque laminated sinter deposited on surfaces that evaporate to dryness ii clear translucent silica deposited subaqueously through precipitation driven by supersaturation iii heterogeneous silica representing silica encrusted microbial filaments and detritus and iv sinter debris brownish laminations form the framework of most sinter deposited in surge and overflow zones pits and cavities are common architectural features of subaerial sinter and show concave upward pseudo cross laminations and micro unconformities developed through migration marked birefringence of silica deposited on surfaces that evaporate to dryness is probably a strain effect repeated wetting and evaporation often to dryness and capillary effects control the deposition morphology and microstructure of most high temperature sinter outside of the fully subaqueous zone microbial filaments are abundant on and within high temperature sinter but do not provide the main controls on morphology or structuring except in biofilms developed on subaqueous surfaces millimetre scale lamination cyclicity in much high temperature sinter represents annual layering and regular seasonal fluctuations in silica sedimentation

microstructural changes accompanying the opal a to opal ct transition new evidence from the siliceous sinters of geysir haukadalur iceland opaline silica opal a has formed in marine lacustrine and geothermal environments throughout geological time during diagenesis opal a normally changes to opal ct then opal c and finally to quartz such changes commonly destroy the original fabrics and any fossils that opal a contained the physical changes that accompany the opal a to opal ct transition however are known poorly x ray diffraction analyses electron microprobe analyses and high resolution high magnification scanning electron microscope imagery of siliceous sinters from the geysir geothermal area in iceland show that opal a is formed of heterometric arrays of randomly packed microspheres up to 5 m diameter with neighbouring spheres commonly being joined by small connection pads in contrast enlarged spheres lepispheres inverse opal two types and spindle frameworks with hexagonal motifs characterize opal ct the textures in opal ct which vary on a microscale reflect the complex interplay between dissolution e g inverse opal and precipitation e g enlarged spheres spindle frameworks that probably was mediated by groundwater in a near surface environment the processes deciphered from these young rocks should however be applicable to sedimentary opal a and opal ct of all ages irrespective of their origin

high archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3 5 ga swaziland supergroup south africa new and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3 5 3 2 ga swaziland supergroup barberton greenstone belt south africa the low 1 8 o values of these cherts indicate extremely high ocean temperatures of 55 85 c previously the large depletion in 1 8 o shown by all barberton cherts relative to their phanerozoic counterparts has been attributed to low 1 8 o in archean oceans chert formation during late diagenesis wholesale loss of 1 8 o during alteration and or regional silicification of sediments around hot springs these alternative explanations are not compatible with the new results cherts in the onverwacht group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying fig tree and moodies groups demonstrating that the chert 8 0 o values were fixed prior to archean uplift and erosion which started at 3 26 ga the maximum 1 8 o value in barberton cherts 22 is lower than the minimum values 23 in phanerozoic bedded cherts precluding late diagenesis as the explanation of the overall low 1 8 o values regional metamorphic hydrothermal or long term resetting of original 1 8 o values is also precluded by preservation of 1 8 o values across different metamorphic grades and by systematic 1 8 o differences among interbedded chert types stratigraphic units and conglomerate clasts the 7 1 8 o variation of these archean cherts is similar to that of phanerozoic deep sea cherts formed when opal converted to microquartz during burial but the actual archean values are 10 lower marine opal was apparently converted to microquartz during burial to depths of 1 km cherts with 1 8 o 15 reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and or hydrothermal activity cherts with higher 1 8 o values formed during early diagenesis and indicate an extremely hot archean ocean and surface environment

is quartz cathodoluminescence color a reliable provenance tool a quantitative examination abstract we examined cathodoluminescence cl colors of quartz by using red 590 780 nm green 515 590 nm and blue 380 515 nm optical filters interfaced with a cathodoluminescence cl detector attached to a scanning electron microscope sem sem cl images taken through these filters were captured digitally and transferred to a computer luminescence intensities luminosities of the images were measured by using available commercial software measured luminosities of these cl images are directly related to relative intensities of red green and blue cl emissions luminosity data were then used to construct plots that display relative luminosities of the cl images acquired through the red green and blue filters an unfiltered cl image of each quartz grain generated by photons with wavelengths ranging from 200 700 nm was also acquired by subtracting the numerical luminosity values of the images acquired through the red green and blue filters from the luminosity value of the unfiltered image the contribution to total luminosity provided by cl emission in the near ultraviolet uv was calculated the cl colors of quartz from a variety of volcanic plutonic and metamorphic rocks and hydrothermal deposits were examined volcanic quartz phenocrysts have the most restricted cl color range with strongest emission intensity in the blue wavelength band cl colors of plutonic quartz overlap those of volcanic phenocrysts but extend over a broader range to include quartz that displays higher intensity of red emission cl emission in hydrothermal vein quartz is similar to that in plutonic quartz although some hydrothermal quartz exhibits stronger green cl emission than does plutonic quartz the cl colors of metamorphic quartz exhibit the widest variation overlapping the color fields of both volcanic and plutonic quartz and extending further into the red cl emission in the near uv makes a significant contribution 5 85 percent to the total luminosity of sem cl images particularly images of plutonic quartz because of overlap in the cl color ranges of volcanic plutonic metamorphic and hydrothermal quartz unambiguous identification of quartz provenance on the basis of cl color alone is problematic it is difficult to distinguish between volcanic and some plutonic quartz and between some plutonic and hydrothermal quartz or to distinguish magmatic quartz from metamorphic quartz that exhibits blue cl color only metamorphic quartz that exhibits moderately strong red emission appears distinguishable on the basis of color from quartz of other origins our work thus suggests that cl color is not a reliable indicator of quartz provenance

silica diagenesis ii general mechanisms abstract amorphous silica phases opal a precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions opal a dissolves and yields solutions of still relatively high silica content in pore waters containing abundant cations open framework polymers form which flocculate to yield opal ct opal ct becomes increasingly ordered primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase opal ct dissolves to yield pore waters of low silica concentration which allows slow growth of quartz crystals from monomeric solution the quartz crystals then slowly increase in size and crystallinity carbonates appear to enhance opal ct formation possibly due to the activity of positively charged hydroxyl complexes hence polymerization in relatively pure systems is involved in opal a formation and polymerization in impure systems is involved in opal ct formation and slow growth from monomeric low silica concentration solutions is involved in precipitation of quartz in sedimentary realms

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

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

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

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

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

stratigraphic changes of ge si ree y and silicon isotopes as insights into the deposition of a mesoarchaean banded iron formation in order to determine the origin of silicon si in banded iron formation bif we have undertaken a multi tracer study combining ree y data ge si ratios and si isotopes d 30 si on stratigraphically resolved layers from a 2 95 ga bif from the pongola supergroup south africa si in both si rich and fe rich layers has a common origin represented by a seawater reservoir strongly influenced by continent derived freshwaters 10 and very limited o 0 1 high t hydrothermal fluids as indicated by eu anomalies and y ho ratios the coevolution of d 30 si signatures of si and fe rich

another continental pool in the terrestrial silicon cycle silicon is the second most abundant element on earth and is involved in important terrestrial and marine biogeochemical processes however its terrestrial cycle is not well quantified and there is a missing reservoir in the budget of silicon isotopes water in seas and rivers is enriched in silicon 30 compared with igneous rocks suggesting that there is an unknown continental reservoir that is depleted in silicon 30 now with the help of a new generation mass spectrometer basile doelsch et al have obtained in situ measurements of silicon isotopes in quartz from sandstones in southern france the results showing greater silicon 30 depletion than in any previously reported terrestrial samples suggest that silicified rocks formed from quartz precipitates in soils and groundwaters can account for the missing reservoir silicon is the second most abundant element on earth it is an important nutrient for phytoplankton1 and is readily absorbed by terrestrial vegetation2 it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates3 but the continental cycle of silicon is not well known and only a few studies have attempted to use silicon stable isotopes 28si 29si and 30si 4 5 6 7 8 9 10 11 12 13 to quantify the continental silicon reservoirs dissolved silicon in sea and river waters forms a reservoir of mean isotopic value 1 1 refs 7 10 it is enriched in 30si with respect to the igneous rocks reservoir which has a mean isotopic value of 0 3 refs 4 9 this enrichment can only be produced by a major fractionation during weathering and should result in the formation of a continental 30si depleted reservoir such a reservoir however has not been identified to date here we analyse silicon isotopes of in situ quartz from a sandstone series in france using a new generation secondary ion mass spectrometry apparatus we show that quartz that precipitates as siliceous cements forms a strongly 30si depleted reservoir with isotopic values down to 5 7 a more negative value than any previously published for terrestrial samples our findings suggest that quartz re precipitation plays an important role in the biogeochemical cycle of silicon

δ30si systematics in a granitic saprolite puerto rico granite weathering and clay mineral formation impart distinct and interpretable stable si isotope 30 si signatures to their solid and aqueous products within a saprolite clay minerals have 30 si values 2 0 more negative than their parent mineral and the 30 si signature of the bulk solid is determined by the ratio of primary to secondary minerals mineral specific weathering reactions predominate at different depths driving changes in differing 30 si pore water values at the bedrock saprolite interface dissolution of plagioclase and hornblende creates 30 si pore water signatures more positive than granite by up to 1 2 these reactions are the main contributor of si to stream water and determine its 30 si value throughout the saprolite biotite weathering releases si to pore waters but kaolinite overgrowth formation modulates its contribution to pore water si the influence of biotite on 30 si pore water is greatest near the bedrock where biotite derived si mixes with bulk pore water prior to kaolinite formation higher in the saprolite biotite grains have become more isolated by kaolinite overgrowth which consumes biotite derived si that would otherwise influence 30 si pore water because of this isolation which shifts the dominant source of pore water si from biotite to quartz 30 si pore water values are more negative than granite by up to 1 3 near the top of the saprolite

an archean impact layer from the pilbara and kaapvaal cratons the barberton greenstone belt of south africa and the eastern pilbara block of western australia provide information about earth9s surface environments between 3 2 and 3 5 billion years ago including evidence for four large bolide impacts that likely created large craters deformed the target rocks and altered the environment we have obtained identical single zircon uranium lead ages of 3470 2 million years ago for the oldest impact events from each craton these deposits represent a single global fallout layer that is associated with sedimentation by an impact generated tsunami and in western australia is represented by a major erosional unconformity

early silica cementation and subsequent diagenesis in arenites from four early proterozoic iron formations of north america abstract in four unmetamorphosed north american iron formations that display arenitic granular textures much of the chert and megaquartz occupies former intergranular pore space and exhibits textures typical of cements iron bearing phases are very rare in these cements excluding replacive crystals cracks and more irregular cavities that formed in these arenites soon after deposition probably via the shrinkage of gelatinous silica are filled with similar chert and megaquartz cements silica cemented intraclastic pebbles of arenite local occurrences of cross stratified cherty internal sediments resting on the cements and numerous zones where the abundance of cement equals the porosity of comparable modern sands indicate the cementation commenced near the sediment water interface opa ine silica fibrous silica polymorphs and megaquartz druses are inferred to have been the most voluminous initial precipitates and they are very inhomogeneously distributed silica concentrations in the pore waters of newly deposited iron formation sands must therefore have been high but variable the lack of evidence for subaerial desiccation or normal saline deposits renders an evaporative origin for either the cements or iron formations as a whole highly unlikely steeper geothermal gradients are suggested as a more likely cause of rapid silica cementation and silica cements from geothermal areas show very similar textures most of the cements in iron formation arenites were affected by neomorphism replacement mechanical compaction and or pressure solution regardless of their origin the cements are noteworthy for the following reasons they indicate that modern iron formations are enriched in silica relative to their precursor sediments they provide new evidence that iron formations are not replaced limestones or dolomites and the iron minerals they are associated with are probably the closest approaches to primary ferruginous phases in iron formation arenites early silica cements also appear to be widespread in lutitic or banded iron formations in the form of chert pods

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

stable silicon isotopes of groundwater feldspars and clay coatings in the navajo sandstone aquifer black mesa arizona usa abstract we present some of the first analyses of the stable isotopic composition of dissolved silicon si in groundwater the groundwater samples were from the navajo sandstone aquifer at black mesa arizona usa and the si isotope composition of detrital feldspars and secondary clay coatings in the aquifer were also analyzed silicon isotope compositions were measured using high resolution multi collector inductively coupled mass spectrometry hr mc icp ms nu1700 nuplasma hr the quartz dominated bulk rock and feldspar separates have similar 30 si of 0 09 0 04 and 0 15 0 04 95 sem respectively and clay separates are isotopically lighter by up to 0 4 compared to the feldspars from isotopic mass balance considerations co existing aqueous fluids should have 30 si values heavier than the primary silicates positive 30 si values were only found in the shallow aquifer where si isotopes are most likely fractionated during the dissolution of feldspars and subsequent formation of clay minerals however 30 si decreases along the flow path from 0 56 to 1 42 representing the most negative dissolved si isotope composition so far found for natural waters we speculate that the enrichment in 28 si is due to dissolution of partly secondary clay minerals and low temperature silcretes in the navajo sandstone the discovery of the large range and systematic shifts of 30 si values along a groundwater flow path illustrates the potential utility of stable si isotopes for deciphering the si cycling in sedimentary basins tracing fluid flow and evaluating global si cycle

high resolution tephra and u pb chronology of the 3 33 3 26 ga mendon formation barberton greenstone belt south africa abstract the mendon formation in the barberton greenstone belt of south africa marks the boundary between the onverwacht and fig tree groups these groups are characterized by mafic to ultramafic volcanism and felsic volcanism with related epiclastic sedimentation respectively this transition marks the end of komatiitic volcanism in the barberton greenstone belt and is accompanied by numerous impact related spherule layers this study characterizes the upper mendon formation texturally and geochemically over a wide areal extent and across structure and facies changes in an attempt to better understand the evolution of tectonic processes at this boundary a suite of whole rock and handheld x ray fluorescence analyses are presented in conjunction with textural information stratigraphic relationships and u pb ages to create a temporal and chemostratigraphic framework for the mendon formation local and regional stratigraphic variations including absence of distinctive layers and variation in layer thickness seen across the mendon preclude ascription of a single stratigraphy that accurately describes the 1 2 km of section present in this formation these variations indicate diachronous deposition of the mendon formation over a wide areal extent and into multiple basins or sub basins by more than one magmatic source 204 pb corrected 206 pb 238 u and 207 pb 235 u concordia model ages of 3279 9 1 ma and 3287 3 2 9 ma for two samples from upper portions of the mendon formation provide temporal context for deposition two samples from the basal 10 m of the fig tree group above the s2 spherule bed that marks the boundary between the onverwacht and fig tree groups give model ages of 3267 8 6 9 ma and 3261 18 ma these ages provide added constraints for the onverwacht fig tree boundary and confirm that the weltevreden formation is roughly age correlative with the uppermost mendon formation while the mendon and weltevreden formations are in part age correlative and have similar lithologies they do not appear to be genetically related the dominance of ultramafic volcanic rocks and the paucity of felsic volcanic and terrigenous sedimentary rocks within the mendon and weltevreden formations indicate that the primary mode of crustal formation was likely plume related magmatic accretion and not subduction the relatively sharp transition within the barberton greenstone belt from ultramafic volcanic sequences to more felsic volcanic and epiclastic sedimentary sequences is everywhere marked by impact related spherule layers which suggest that major impacts may have played a role in the evolution of early tectonics to more modern subduction related styles

early archean silicate spherules of probable impact origin south africa and western australia layers of sand sized silicate spherules in 3 2 to 3 5 ga archean greenstone belts in south africa and western australia formed by the accumulation of quenched liquid silicate droplets having primary compositions resembling those of immediately underlying rock sequences the layers and particles appear to be unrelated to volcanic activity and we suggest that both occurrences may represent melt droplets formed during meteorite or comet impacts if so these spherules are the oldest known terrestrial impact products if these objects are representative of a larger population of early archean impact features it may be possible to directly estimate the meteorite flux on the early archean earth

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

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

formation of silica oncoids around geysers and hot springs at el tatio northern chile siliceous oncoids up to 4 cm in diameter are common on the laterally extensive sinter aprons that surround the spectacular geysers and hot springs at el tatio in northern chile many of these complex oncoids developed close to geyser and spring vents that discharge boiling water internally the oncoids which are composed of precipitated amorphous silica are formed of complex arrays of spicules and concentric laminae as well as detrital volcanic grains spicular growth is dominant in most examples the formation and growth of the spicules and concentric laminae were mediated by a microbial community which included filamentous microbes mucus and possibly bacteria the microbes and mucus were silicified by replacement and encrustation in some laminae the filamentous microbes lay parallel to the growth surface in other laminae most filaments forming the thin mats were suberect amorphous silica precipitated between the filaments occluded porosity and commonly disguised the microbial fabric the oncoids grew on the proximal sinter aprons around the geyser vents and hot spring pools most growth took place subaerially with the silica delivered to the precipitation sites by splashing water from the geysers and or periodic shallow flooding of the discharge aprons unlike silica oncoids at other geothermal sites vertical growth of oncoids that formed in some rimstone pools was not limited by water depth

photosynthetic microbial mats in the 3 416 myr old ocean recent re evaluations of the geological record of the earliest life on earth have led to the suggestion that some of the oldest putative microfossils1 and carbonaceous matter were formed through abiotic hydrothermal processes2 3 similarly many early archaean more than 3 400 myr old cherts have been reinterpreted as hydrothermal deposits rather than products of normal marine sedimentary processes2 4 5 here we present the results of a field petrographic and geochemical study testing these hypotheses for the 3 416 myr old buck reef chert south africa from sedimentary structures and distributions of sand and mud we infer that deposition occurred in normal open shallow to deep marine environments the siderite enrichment that we observe in deep water sediments is consistent with a stratified early ocean6 7 we show that most carbonaceous matter was formed by photosynthetic mats within the euphotic zone and distributed as detrital matter by waves and currents to surrounding environments we find no evidence that hydrothermal processes had any direct role in the deposition of either the carbonaceous matter or the enclosing sediments instead we conclude that photosynthetic organisms had evolved and were living in a stratified ocean supersaturated in dissolved silica8 9 3 416 myr ago

relationship between spring and geyser activity and the deposition and morphology of high temperature 73 c siliceous sinter yellowstone national park wyoming u s a abstract six classes of alkaline hot springs and geysers are here distinguished in yellowstone national park based on activity and eruptive style 1 non surging non boiling springs 2 gently surging non boiling springs 3 non surging to gently surging boiling springs 4 strongly surging springs 5 fountain geysers and 6 cone geysers each displays morphologically distinctive siliceous sinter deposits in the high temperature 73 c parts of the vent and outflow systems non surging and gently surging non boiling springs that do not overflow their rims develop thin digitate to lilypad like sinter rims that grow horizontally over the pool surfaces similar springs with rim overflow have rims that accrete vertically boiling springs often have nearly circular steep sided pools and rims dominated by splash and rim overflow and composed mainly of spicular and columnar sinter fountain geysers characterized by broad fountain like eruption bursts have a central vent surrounded by a zone of spicular sinter beyond which is a broad flat bottomed moat but lack steep sinter cones cone geysers have narrow vents jet like discharges that cool rapidly and fall close to the vent and steep sided spicule dominated sinter cones sinter deposition takes place within three main zones 1 fully subaqueous sides and bottoms of vent pools outflow basins and outflow channels 2 periodically wetted splash zones surrounding hot springs and geysers and 3 largely subaerial parts of the rim and outflow areas wetted mainly by overflow and surging and that dry quickly fully subaqueous surfaces are covered by fine earthy particulate siliceous sediment pss composed largely of amorphous silica spherules and spherule aggregates that appear to have precipitated around heterogeneous nuclei including microbes splash zones of hot springs and geysers are characterized by spicular sinter largely subaerial zones wetted mainly by rim overflow and surging with little or no splash show complex varieties of terraced columnar and knobby sinter the highest silica deposition rates occur where surfaces are wetted frequently and evaporate to dryness such as the tops of spicules columns and knobs associated lower areas and crevices that remain wet or moist show slow accumulation of pss most continuously wet surfaces in high temperature 73 c hot spring and geyser vent and outflow systems are probably covered by microbial biofilms although microbes may locally influence rates of silica deposition and sinter microstructure the results of this study suggest that the depositional sites morphology and macrostructuring of high temperature sinter are controlled largely by the processes of surface wetting evaporation and drying

size shape and orientation of grains in sands and sandstones image analysis applied to rock thin sections abstract a computer based image analysis system is used to determine the grain size distribution grain shape and grain orientation from rock thin sections a monte carlo simulation allows correction of the grain size distribution for the corpuscle effect grains cut marginally in the thin section plane have smaller apparent diameters thin section derived grain size distributions of modern and ancient samples have been compared with distributions obtained from sieving of the same samples different grain shape parameters have been compared with the wadell formula the grain orientation can be easily and reliably measured in thin sections from different cuts of a sample grain orientation data can help to determine paleocurrent directions

silicon in the earth s core the earth and moon have iron isotopic abundances that are slightly heavy compared with mars the asteroid vesta and primitive meteorites the reason for this has been unclear one suggestion is that there was a large scale isotopic equilibration during the giant impact the collision between the early earth and a mars sized body that created the moon but this has seemed unlikely given that most other elements do not show the effect an analysis of 44 meteorite and terrestrial samples now shows that the silicon isotopic compositions of basaltic rocks from the earth and moon are also distinctly heavy the finding is consistent with the isotopic equilibration model and it suggests that silicon was already partitioned into the earth s core as a light element before the moon formed the silicon isotopic compositions of basaltic rocks from the earth and moon are distinctly heavy the similar isotopic composition of the bulk silicate earth and the moon are consistent with large scale isotopic equilibration during the giant impact indicating that si was already a light element in the earth s core before the moon formed small isotopic differences between the silicate minerals in planets may have developed as a result of processes associated with core formation or from evaporative losses during accretion as the planets were built up basalts from the earth and the moon do indeed appear to have iron isotopic compositions that are slightly heavy relative to those from mars vesta and primitive undifferentiated meteorites1 2 3 4 chondrites explanations for these differences have included evaporation during the giant impact that created the moon when a mars sized body collided with the young earth however lithium5 and magnesium6 lighter elements with comparable volatility7 8 9 reveal no such differences rendering evaporation unlikely as an explanation here we show that the silicon isotopic compositions of basaltic rocks from the earth and the moon are also distinctly heavy a likely cause is that silicon is one of the light elements in the earth s core we show that both the direction and magnitude of the silicon isotopic effect are in accord with current theory10 based on the stiffness of bonding in metal and silicate the similar isotopic composition of the bulk silicate earth and the moon is consistent with the recent proposal11 that there was large scale isotopic equilibration during the giant impact we conclude that si was already incorporated as a light element in the earth s core before the moon formed

petrography and provenance of silicified early archaean volcaniclastic sandstones eastern pilbara block western australia intense post depositional alteration has profoundly affected sandstones in the volcanic portions of early archaean 3 5 3 3 ga greenstone belts the mineralogy and bulk compositions of most grains have been completely destroyed by pervasive metasomatism but grain textures are commonly well preserved consequently microtextural information coupled with present alteration compositions as determined petrographically can be used to estimate original framework modes silicified early archaean volcaniclastic sandstones assigned to the panorama formation and duffer formation warrawoona group eastern pilbara block western australia were originally composed of volcanic vrf and sedimentary srf rock fragments volcanic quartz feldspar traces of ferromagnesian minerals and pumice only volcanic megaquartz remained stable during alteration all other primary components were replaced by granular microcrystalline quartz gmc and sericite in most areas the sandstones were composed of dacitic to rhyolitic vrfs now totally replaced by sericite poor gmc and recognized by preserved microporphyritic textures in a few areas quartz poor dacitic to andesitic vrfs dominated the detrital assemblage minor srfs and mafic vrfs now replaced by gmc are recognized on the basis of colour internal structures and internal textures including skeletal possible spinifex textures detrital feldspar is represented by blocky sericite rich grain pseudomorphs a semi quantitative point count scheme developed for the analysis of heavily altered sandstones indicates the following primary detrital mode ranges for panorama arenites quartz 0 28 feldspar 0 28 vrfs 58 86 and srfs 0 25 in about half the point counted samples feldspar could not be distinguished from rock fragments in such cases both were counted as one grain type lv which makes up from 84 to 100 of the framework modes of these rocks these sands were derived from a terrane composed largely of fresh felsic volcanic rocks and sediments but locally including minor mafic ultramafic and sedimentary rocks much but not all of the felsic volcaniclastic sand represents reworked pyroclastic debris there is no evidence for contributions from plutonic or metamorphic sources the panorama modal assemblage represents a provenance that is lithologically more restricted than that of archaean greywackes and other siliciclastic units common in the sedimentary portions of these same early archaean greenstone belts and younger greenstone belts 3 0 2 7 ga

new u pb age constraints on latest cretaceous magmatism and associated mineralization in the fawnie range nechako plateau central british columbia new u pb ages and k ar dates primarily for rocks proximal to mineral occurrences in the fawnie range of central british columbia document latest cretaceous ca 74 66 ma continental arc igneous activity and date associated base and precious metal mineralization u pb ages of ca 73 69 ma for the capoose pluton and hypabyssal to extrusive garnet rhyolites at the capoose prospect demonstrate a latest cretaceous age for mineralization and a likely plutonic source for mineralizing fluids a u pb age of ca 67 ma for a late mineralized felsic dyke and two k ar dates ca 70 and 68 ma for hornfelsed jurassic volcanic rocks at the blackwater davidson prospect constrain a latest cretaceous age for mineralization a u pb age of ca 74 ma for a fine grained diorite sill that cuts a significant epithermal gold vein at the tsacha prospect places a minimum age on mineralization at this probable jura cretaceous deposit and documents latest cretaceous magmatism latest cretaceous k ar dates are reported for an andes

anatomy of siliceous hot springs examples from yellowstone national park wyoming usa abstract numerous siliceous hot spring systems in the norris and lower geyser basins of yellowstone national park wyoming provide insights into spring geometries depositional facies and lithofacies associated with modern hot springs analyses of active cistern spring octopus spring deerbone spring and spindle geyser and inactive pork chop geyser siliceous hot springs have facilitated the construction of a facies model for siliceous hot spring deposits at yellowstone yellowstone s siliceous springs tend to group into four broad morphological categories siliceous spires and cones domal mounds terraced mounds and ponds siliceous spires cones are subconical accumulations up to 5 7 m high and about 2 m in diameter and are common deposits in yellowstone lake domal mounds are characterized by siliceous precipitates with a broad lens or shield geometry 2 3 m in vertical relief discharge channels and an areal accumulation of approximately 150 m2 in contrast terraced mounds have a stair step morphology a substantial pool 8 10 m in diameter shrubby precipitates and occupy areas of 2000 m2 siliceous ponds are variable in size have little outflow and exhibit low amounts of silica precipitation of these morphological varieties domal mounds and terraced mounds are thought to have the best long term preservation potential the four spring morphotypes are composed of up to eight cumulative hot spring depositional facies 1 vent 95 c 2 proximal vent precipitation of siliceous sinter is the result of abiotic and biotic processes abiotic precipitational processes are dominant in the vent area whereas biotic influences on the precipitate fabric become progressively more important downflow

geochemistry and sedimentology of a facies transition from microbanded to granular iron formation in the early proterozoic transvaal supergroup south africa abstract a transition from microbanded kuruman to granular griquatown iron formation is described in terms of sedimentological petrographic and geochemical characteristics major minor trace and rare earth elements as well as whole rock carbon and oxygen isotopic compositions transitions between microbanded and granular iron formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron formations the kuruman griquatown transition zone differs from other granular iron formation sequences in being non oolitic i e peloidal and magnetite rich instead of hematite rich it is concluded that the coarsening upward microbanded granular iron formation units in the kuruman griquatown sequence represent upward shallowing storm dominated deposits that is primary microbanded relatively deep water fair weather deposits having been intermittently reworked by storm wave action form wavy sharp based graded and poorly sorted irregular interbeds of granular iron formation this is in contrast to typical hematitic oolitic iron formation units which represent true shoal deposits geochemical results strongly support the storm reworking hypothesis microbanded and granular iron formation beds have similar ree patterns with negative ce anomalies and depletion in light ree reflecting deposits in a marine setting below the zone of shallow surface water positive eu anomalies in the ree patterns suggest admixture of a hydrothermal component of the ocean water carbonates in the iron formation are depleted in 13c indicating involvement of organic matter during diagenetic alteration of the iron formations there are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13c than in shallower water deposition of the iron formations was sporadically interrupted by influx of tuffaceous material now represented by centimeter thick shard and lapilli bearing stilpnomelane lutite beds these beds differ chemically from the iron formations in being enriched in elements such as al sc y zr nb hf ta th u and the lanthanides and displaying ree patterns with strong negative eu and slight positive ce anomalies the composition of the parent magma of the volcanics remains uncertain because of probable post depositional alteration of the volcanic glass however it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds typical of plinian type deposits chemical variations in the minor trace and ree compositions of the iron formations are closely correlated with the amounts of admixed volcanic ash material most of the iron formations appear chemically very uncontaminated being highly depleted in all minor trace and ree and organic carbon contents the iron formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository

texture specific si isotope variations in barberton greenstone belt cherts record low temperature fractionations in early archean seawater sedimentary cherts are unusually abundant in early archean pre 3 0 ga sequences suggesting a silica cycle that was profoundly different than the modern system previously applied for the purpose of paleothermometry si isotopes in ancient cherts can offer broader insight into mass fluxes and mechanisms associated with silica concentration precipitation diagenesis and metamorphism early archean cherts contain a rich suite of sedimentological and petrographic textures that document a history of silica deposition cementation silicification and recrystallization to add a new layer of insight into the chemistry of early cherts we have used wavelength dispersive spectroscopy and then secondary ion mass spectrometry sims to produce elemental and si and o isotope ratio data from banded black and white cherts from the onverwacht group of the barberton greenstone belt south africa this geochemical data is then interpreted in the framework of depositional and diagenetic timing of silica precipitation provided by geological observations sims allows the comparison of si and o isotope ratios of distinct silica phases including black carbonaceous chert beds and bands many including well defined sedimentary grains white relatively pure chert bands including primary silica granules early cavity filling cements and later quartz filled veins including all chert types and textures analyzed the 30 si dataset spans a range from 4 78 to 3 74 with overall mean 0 20 median 0 51 and standard deviation 1 30 n 1087 most samples have broadly similar 30 si distributions but systematic texture specific 30 si differences are observed between white chert bands mean 0 60 n 750 which contain textures that represent primary and earliest diagenetic silica phases and later cavity filling cements mean 1 41 n 198 we observed variations at a 100 m scale indicating a lack of si isotope homogenization at this scale during diagenesis and metamorphism although fractionations during diagenetic phase transformations may have affected certain textures we interpret these systematic variations to reflect fractionation during silica precipitation as well as isotopically distinct fluids from which later phases originated sims 18 o values fall in a range from 16 39 to 23 39 n 381 similar to previously published data from bulk gas source mass spectrometry of onverwacht cherts we observed only limited examples of texture related variation in 18 o and did not observe correlation of 18 o with 30 si trends this is consistent with hypotheses that si isotope ratios are more resistant to alteration under conditions of rock buffered diagenesis marin carbonne et al 2011 our results indicate that low temperature processes fractionated silicon isotopes in early archean marine basins a behavior that probably precludes the application of chert 30 si as a robust paleothermometer the values we observe for facies that sedimentological and petrographic observations indicate formed as primary and earliest diagenetic silica precipitates from seawater are more 30 si rich than that expected for bulk silicate earth this is consistent with the hypothesis that the silicon isotope budget is balanced by the coeval deposition of 30 si enriched cherts and 30 si depleted iron formation lithologies precipitation of authigenic clay minerals in both terrestrial and marine settings may have also comprised a large 30 si depleted sink with the corollary of an important non carbonate alkalinity sink consuming cations released by silicate weathering

geyser eggs from te whakarewarewatangaoteopetauaawahiao north island new zealand waikorohihi and mahanga two geysers located on geyser flat at te whakarewarewatangaoteopetauaawahiao new zealand discharge chloride waters with a ph of 8 5 9 25 at 100 c many large up to 10 8 5 cm coated grains composed of amorphous silica opal a that are informally termed geyser eggs surround the vent of waikorohihi whereas smaller 3 cm geyser eggs are present in the proximal outflow channel of mahanga the geyser eggs have nuclei of detrital sinter or rhyolitic tuff their cortices are formed of thin concentric laminae and or spicular geyserite most cortical laminae contain a low diversity biota that includes filamentous microbes three types and scattered spherical microbes silicified grains of pinus radiata pollen are common in some laminae multiple phases of silica encrustation around the filamentous microbes disguised most features that are needed to confidently identify them most filaments are probably cyanobacteria variations in the microporosity and or microbe concentrations define individual cortical laminae growth of the geyser eggs in the pool around waikorohihi vent was controlled by the eruptive behavior of the geyser the eggs are submerged by boiling water during eruptions during dormant phases the geyser eggs are subaerially exposed while exposed their surfaces may remain moist from steam or rainwater or they may desiccate the low microbial diversity in the geyser eggs is probably related to their periodic inundation by boiling water and the highly variable conditions the abundant microbial laminae in the geyser eggs confirm that microbial templates are found in siliceous coated grains from almost all environmental settings in siliceous thermal spring systems

silicon isotopes and the tracing of desilication in volcanic soil weathering sequences guadeloupe abstract silicon si stable isotopes have the potential to become a useful weathering proxy given that light si isotopes are preferentially incorporated into secondary clay minerals here we investigate how si depletion in soils and associated clay mineralogy influence the si isotope fractionation associated with clay mineral formation we report 30 si compositions in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from guadeloupe that were formed under contrasting climatic conditions strongly desilicated soils containing kaolinite that formed in wet areas high precipitation are compared with less desilicated soils containing smectite formed in drier conditions low precipitation clay fractions are isotopically lighter than the parent andesite 30 si 0 23 and increasingly lighter with si depletion in soils which supports the view that the si isotope composition in secondary clay fractions is controlled by the degree of soil desilication it is shown that the si isotope fractionation factor between the parent silicate material and the secondary clay minerals is smaller for si rich secondary clay minerals such as smectite and larger for si poor secondary clay minerals such as kaolinite this study provides new insights to better define si isotopes as a proxy for environmental conditions for clay neoformation

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

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

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

coupled silicon oxygen isotope fractionation traces archaean silicification silica alteration zones and cherts are a conspicuous feature of archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early earth in order to understand the process es of silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the theespruit hooggenoeg and kromberg formations of the barberton greenstone belt south africa the 30si and 18o values of bulk rock increase with increasing amount of silicification from unsilicified basalts 0 64 30si 0 01 and 8 6 18o 11 9 to silicified basalts 30si and 18o values as high as 0 81 and 15 6 respectively cherts generally have positive isotope ratios 0 21 30si 1 05 and 10 9 18o 17 1 except two cherts which have negative 30si values but high 18o up to 19 5 the pronounced positive correlations between 30si 18o and sio2 imply that the isotope variation is driven by the silicification process which coevally introduced both 18o and 30si into the basalts the oxygen isotope variation in the basalts from about 8 6 to 15 6 is likely to represent temperature dependent isotope fractionation during alteration our proposed model for the observed silicon isotope variation relies on a temperature controlled basalt dissolution vs silica deposition process

the petrogenesis of volcaniclastic komatiites in the barberton greenstone belt south africa a textural and geochemical study the onverwacht group of the 3 5 3 2 ga barberton greenstone belt south africa contains multiple stratigraphic units that include laterally extensive beds of komatiitic ash accretionary lapilli and lapilli these units have been affected by pervasive silicification serpentinization or less commonly carbonate metasomatism silicification resulted in sio2 k2o485 wt and depletion of most other major and trace elements most of these tuffs have prominent high hf hf and zr zr 0 5 12 which cannot result from normal magmatic processes but are due to the typically immobile rare earth elements migrating during post silicification fluid rock interaction similarly their low ce ce values do not reflect archean surface redox conditions but the circulation of later oxidizing fluids despite this intense alteration ratios of al2o3 and tio2 remain uniform and coherent within single volcanic units these ratios indicate that most silicified tuffs are not petrogenetically related to the underlying or overlying komatiitic flow rocks and that each originated from either separate mantle sources or different partial melting conditions serpentinized tuffs retain komatiitic element abundances but al2o3 fails to define a tight linear array with the demonstrably immobile elements ti and zr we speculate that this is due to post depositional mixing of al depleted and al undepleted tuff layers by aqueous currents excellent textural preservation of the silicified tuffs shows they are characterized by a dearth of phenocrysts low particle vesicularity and abundance of fine vitric ash suggesting the eruption and rapid quenching of superheated or near liquidus anhydrous magmas minor assimilation of hydrated basaltic or ultramafic crust within the dry magma may have enhanced the surface phreatomagmatic explosivity while still allowing the magma to rise close to an adiabatic ascent path however textural and geochemical evidence for such a process is scarce temporal and compositional constraints show that the diversity in the types of komatiites throughout the onverwacht group can be accounted for by variations in plume mantle dynamics and that komatiitic tuffs were deposited during intervals of volcanism characterized by low effusive eruptive volumes and or low emplacement rates

geochemical processes evidence and thermodynamic behavior of dissolved and precipitated carbonate minerals in a modern seawater freshwater mixing zone of a small tropical island abstract the geochemical processes and thermodynamic behavior of dissolved and precipitated carbonate minerals controlling the hydrochemistry of an aquifer in the seawater freshwater mixing zone of a small island are identified field and laboratory analyses geochemical modeling phreeqc and multivariate statistical analysis msa provide a quantitative interpretation for the geochemistry of the carbonate dominated aquifer geochemical analyses and modeling results show that dissolution and re precipitation of caco3 are the prevalent processes governing geochemical reactions in the mixing zone furthermore this was confirmed by coherent statistical output that incorporates principle component analysis pca and k means cluster analysis k ca generally the composition of the lowland sandy soil was rather homogeneous and was primarily composed of quartz aragonite calcite and mg calcite thermodynamic model calculations indicate that the carbonate minerals calcite aragonite and dolomite are supersaturated in the mixing zone nevertheless powder x ray diffraction pxrd and scanning electron microscope sem examination verified the occurrence of low mg calcite lmc and the absence of dolomite attributed to thermodynamic kinetic hindrance cation disorder and the presence of dolomite crystal growth rate inhibitors such as so4 the results suggest that dissolution of aragonite and precipitation of lmc drives the solid phase geochemistry in the small tropical island aquifer

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