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

indian punch rifts tibet the strikes of late cenozoic normal faults in tibet fan systematically from a northwesterly to northeasterly direction from west to east they exhibit an axis of symmetry that bisects the himalayan arc and trends subparallel to india asia motion the two dimensional stress field inferred from the spatial pattern of normal fault orientations can be explained solely by collisional stresses localized along a southern part of the himalayan arc continued insertion of indian crust into tibet can explain the persistence of extension and may be increasing plateau crustal thickness and elevation coeval with rifting

a systematic rare earth elements and yttrium study of archean cherts at the mount goldsworthy greenstone belt in the pilbara craton implications for the origin of microfossil bearing black cherts abstract a systematic ree y study of archean ca 3 0 ga cherts from the mount goldsworthy greenstone belt in the northeastern pilbara craton western australia was performed in order to understand their origin and depositional environment analyzed samples include microfossil bearing black cherts from the farrel quartzite and a black vein chert from the underlying warrawoona group and laminated to banded chert including carbonaceous chert jaspilite and banded iron formation from the overlying deepening upward cleaverville formation laminated to banded cherts from the cleaverville formation show a clear stratigraphic trend upsection of increasing y ho and positive eu anomalies with hree enrichment and positive la anomalies the data comprise a mixing array between two end member components on a newly proposed y ho eu anomaly diagram one end member is interpreted to be archean seawater with a super chondritic y ho ratio 100 and a weak positive eu anomaly 3 the other end member with a chondritic y ho ratio and a negligible eu anomaly is assumed to be non marine water such as continental run off ground water or geothermal water black cherts containing microfossils in the farrel quartzite are characterized by a positive la anomaly hree enrichment negligible to a slight positive eu anomaly and a chondritic to slightly super chondritic y ho ratio they are distinct from vein cherts with a distinct mree enrichment and contemporaneous hydrothermal cherts with pronounced eu anomaly and plot close to the supposed non marine end member component on the y ho eu anomaly diagram the black cherts were thus precipitated from a water mass influenced significantly by for example continental run off ground water and or geothermal water but not from high t hydrothermal solution increasing the credibility of microfossils contained in them

interpreting benthic oxygen levels in mudrocks a new approach quantified paleoecology and gamma ray spectrometry have been applied in the analysis of the kimmeridge clay a highly organic rich british jurassic mudrock decreasing benthic oxygen trends are reflected in decreasing species richness and dominance diversity values similarly the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen the calculation of authigenic uranium values from data gathered by gamma ray spectrometry shows enrichment in more oxygen deficient environments the good correlation between the independently derived paleoecological and authigenic u data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks

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

the behavior of rare earth elements in seawater precise determination of variations in the north pacific water column abstract we present new highly precise and accurate data for ten rare earth elements rees in two water column profiles and in bottom waters from three additional localities in the north pacific our results presented here and in an earlier publication indicate that the water column in the western north pacific is stratified with respect to the isotopic composition of nd as well as nd la sm nd and er la both the nd isotopic and the ree data show that ree transport is strongly influenced by horizontal advection even in the sluggish circulation of the pacific in general rees increase in concentration with depth at mid latitude they reach a maximum concentration at about 3000 m and then decrease to the bottom at high latitude heavy rees reach a maximum concentration near 3000 m whereas light rees continue to increase in concentration with depth the deep water maximum and bottom water depletion is interpreted as an advective feature resulting from the flow of a lower concentration of rees in antarctic bottom water under a higher concentration of rees in the north pacific concentrations of rees do not show a linear relationship with those of si indicating that the ree cycle is not directly coupled to that of the si cycle ree element ratios i e nd la correlate with hydrographie properties which suggests that major features of the ree patterns may be conserved during lateral transport comparison of our ree data to those in previously published reports on pacific seawater shows that the identification of ree distributions affected by advection is only made possible by the high precision analysis we developed for this study the ce concentration we observe is relatively constant over 200 4500 m depth at both sites where we have complete profiles this unique constancy of ce is in contrast to the contiguous rees la and nd which show substantial concentration variations with depth

u rich archaean sea floor sediments from greenland indications of 3700 ma oxygenic photosynthesis abstract 3700 ma metamorphosed pelagic shale from west greenland contains up to 0 4 wt reduced carbon with 13c values down to 25 6 pdb peedee belemnite the isotopic signature and mode of occurrence suggest that the carbon derived from planktonic organisms the pb isotopic composition shows that the shale had high primary u th this indicates that organic debris produced a local reducing environment which precipitated u transported to the site of sedimentation by oxidized ocean water the existence of highly productive plankton that fractionated c isotopes strongly and set up oxidation contrast in the environment suggests that oxygenic photosynthesis evolved before 3700 ma

geochemical characteristics of archean cherts and other sedimentary rocks in the pilbara block western australia evidence for archean seawater enriched in hydrothermally derived iron and silica abstract geochemical and mineralogical characteristics of archean sedimentary rocks such as cherts banded iron formations bifs shales and sandstones collected from the pilbara block western australia were studied lithologic features of various types of chert occurring within the pilbara supergroup are closely related to their chemical compositions red and brown bands of banded cherts generally show a higher fe 2 o 3 total iron as fe 2 o 3 concentration than do other cherts whereas grey greenish grey and pale yellowish grey cherts are characteristic of relatively higher concentrations of al and ti white cherts are generally composed almost exclusively of sio 2 geochemical characteristics of the cherts suggest that they were precipitated from hydrothermal solutions evidence for the hydrothermal origin includes 1 low mno fe 2 o 3 values 2 low concentrations of heavy metals 3 positive euanomalies 4 low co zn and ni zn values and 5 high concentrations of mn and fe in carbonates positive correlations between fe and heavy metals such as ni and zn imply the intermittent precipitation of fe oxides hydroxides along with these heavy metals in the ancient ocean cherts except for grey greenish grey and pale yellowish grey cherts are highly depleted in detrital materials this may have been caused by rapid precipitation of hydrothermally derived fe and si the archean pilbara ocean is believed to have been enriched in fe and si derived from hydrothermal activity

a multiple sulfur and organic carbon isotope record from non conglomeratic sedimentary rocks of the mesoarchean witwatersrand supergroup south africa abstract multiple sulfur isotope ratios 36 s 34 s 33 s 32 s and organic carbon isotope ratios 13 c 12 c were measured from 198 non conglomeratic sedimentary samples that were collected from five deep diamond drill cores that cover the majority of the ca 2 96 2 82 ga witwatersrand supergroup 13 c org 34 s py and 33 s py values of the sample set range from 44 3 to 21 9 3 7 to 16 5 and 4 0 to 1 9 respectively these geochemical data vary relative to depositional facies proximal marine distal marine and fluvial dominated and tectonic setting trailing margin and foreland basin in the trailing margin setting of the 2 96 ga hospital hill subgroup the proximal marine depofacies is characterized by relatively high organic carbon contents up to 0 9 wt and 13 c org values around 28 in contrast to the distal marine depofacies that yields low organic carbon contents 0 01 wt and high 13 c org values up to 22 both depofacies yield low sulfur contents 0 02 wt a narrow range of 34 s values 3 2 and positive 33 s values up to 1 9 this data is consistent with photoautotrophic carbon fixation in shallow marine environments and limited organic carbon production preservation in deeper water settings due to longer transport distances and effective biological degradation e g fe reducing microbes positive 33 s values imply that sulfur was largely derived from a photochemical elemental sulfur reservoir in the foreland basin tectonic setting of the 2 94 ga government and 2 92 ga jeppestown subgroups shelf deposits associated with fluvial braidplain depofacies are characterized by 13 c depleted organic carbon 44 to 38 relatively high sulfur contents 0 2 1 3 wt variable 34 s values 3 7 to 16 5 and small negative 33 s values 0 4 these data suggest that the microbial community in the fluvial dominated depofacies may have consisted of photoautotrophs methanogens anaerobic methanotrophs and sulfate reducers sulfate was derived from a variety of sources that include photochemical crustal and marine sulfur reservoirs the occurrence of three regionally persistent diamictite deposits in the government subgroup coincides with the onset of compressional tectonics and development of pyritic shales with highly 13 c depleted organic carbon suggesting that an increase in continental sulfur flux and methane oxidation may have triggered the mesoarchean glaciations drawdown of h 2 and ch 4 however the link between large 33 s anomalies 4 0 to 1 2 and diamictite suggests low levels of atmospheric oxygen and minimal dilution of photochemical signatures

the geochemistry of late archaean microbial carbonate implications for ocean chemistry and continental erosion history trace element concentrations and combined sr and nd isotope compositions were determined on stromatolitic carbonates microbialites from the 2 52 ga campbellrand carbonate platform south africa shale normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive la and y anomalies and in being depleted in light rare earth elements in contrast to modern seawater and microbialite proxies the 2 52 ga samples lack a negative ce anomaly but possess a positive eu anomaly these latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where unlike today hydrothermal fe input was not oxidised and scavenged and rare earth elements were not coprecipitated with fe oxyhydroxides the persistence of a positive eu anomaly in relatively shallow campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated archaean to continental erosion dominated phanerozoic rare earth element flux ratio the dominant hydrothermal input is also expressed in the initial sr and nd isotope ratios there is collinear variation in sr nd systematics which range from primitive values 87sr 86sr of 0 702386 and end of 2 1 to more evolved crustal ratios mixing calculations show that the range in trace element ratios e g y ho and initial isotope ratios is not a result of contamination by trapped sediment but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed calculated nd flux ratios yield a hydrothermal input into the 2 52 ga oceans one order of magnitude larger than continental input such a change in flux ratio most likely required substantially reduced continental inputs which could in turn reflect a plate tectonic causation e g reduced topography or expansion of epicontinental seas

morphological adaptations of 3 22 ga old tufted microbial mats to archean coastal habitats moodies group barberton greenstone belt south africa microbial life was well established and widespread by the paleoarchean however the degree of evolutionary advancement such as microbial motility intra and inter species interactions phototropism or oxygenic photosynthesis by that time remains highly debated the 3 22 ga moodies group in the barberton greenstone belt bgb south africa are earth s oldest well preserved siliciclastic tidal deposits they exhibit a unique assemblage of microbial mats providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings the fossil mats are preserved as kerogenous laminations 0 5 1 mm thick that can be traced laterally for 15 km in a 1000 m thick succession of fine to coarse grained tidal sandstones and conglomerates we here present a detailed stratigraphic and depositional facies analysis documenting the association of the three principal mat morphotypes with specific environmental settings 1 planar type in coastal floodplain 2 wavy type in intertidal and 3 tufted type in upper inter to supratidal facies all mat types indicate a flourishing phototrophic biota moreover the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments

photochemistry of methane in the earth s early atmosphere abstract a detailed model is presented of methane photochemistry in the primitive terrestrial atmosphere along with speculation about its interpretation steady state ch 4 mixing ratios of 10 6 10 4 could have been maintained by a methane source of about 10 11 cm 2 s 1 which is comparable to the modern biogenic methane production rate in the absence of a source methane would have disappeared in 4 years being either oxidized or polymerized into more complex hydrocarbons the source strength needed to maintain a steady ch 4 mixing ratio and the degree to which methane could have polymerized to form higher hydrocarbons depend upon the amount of co 2 present in the early atmosphere the dependence on h 2 is much weaker infrared absorption by methane and especially by other hydrocarbon species may have supplemented the greenhouse warming due to carbon dioxide a radiative model is needed to establish this effect quantitatively the destruction of the methane greenhouse early in the proterozoic may have triggered the huronian glaciation these calculations also suggest that atmospheres rich in both co 2 and ch 4 may be photochemically unstable with respect to conversion to co

the geochemistry of uranium and thorium in coastal marine sediments and sediment pore waters pore water profiles of uranium and thorium isotopes in the muddy sediments of buzzards bay ma permit an assessment of the effect of diagenetic redox reactions on the geochemical behavior of these elements uranium shows a pronounced minimum pore water concentration 1 2 dpm kg near the sediment water interface 0 3 cm which coincides with the pore water fe maximum u concentrations increase with depth to a broad maximum which is greater than the overlying seawater value then decrease laboratory sediment tank experiments maintained without macrofauna also show the near interface minimum and increase and sediment incubation experiments show removal of u from the pore water to a constant value by about 40 days the 234u238u activity ratio is equal to the seawater value in all samples oxidation state separations of pore water u in hcl solution using anion exchange demonstrate that u is present in the pore water as u vi the coincidence of the pore water uranium and fe maximum is consistent with the hypothesis of reduction of u vi to u iv and removal from solution at about the same depth or pe as fe is reduced increases in pore water u with depth appear to be related to release of authigenic seawater u vi to solution possibly as alkalinity and ph increase and organic matter with which the u is associated is oxidized lower concentrations at depth in the sediment column may be linked to increases of effective u reducing agents like hydrogen sulfide in the pore water or to the activities of macrofauna in contrast pore water profiles of the long lived th isotopes 232th and 230th show low activities 02 dpm kg and relatively little change with depth although values are greater than those in the overlying water solid phase activities are similarly constant with depth and the distribution may be controlled by a sorption equilibrium between pore water and the solid phase the short lived isotope 234th shows greatest pore water activities in the upper 5 cm and is present at virtually zero activities elsewhere in the sediment column the upper few centimeters of the sediment generally contain excess 234th scavenged from the overlying water column and distributed by bioturbation as well as pore water fe and mn profiles which show maxima indicating reduction and dissolution of fe and mn oxyhydroxides the pore water 234th profile can be explained by release of 234th from the solid phase as femn oxide surface coatings are reduced and dissolved

geologic evidence for archean atmospheric and climatic evolution fluctuating levels of co2 ch4 and o2 with an overriding tectonic control the 3 5 3 2 ga sedimentary record shows evidence of surface temperatures of 70 15 c nahcolite nahco 3 as a primary evaporitic mineral and an aggressive weathering regime even in the absence of land vegetation these features are best explained by a mixed ch 4 and co 2 atmospheric greenhouse in which ch 4 co 2 ratios were 1 and pco 2 was at least 100 1000 times the present value and perhaps as high as several bars the formation of large areas of continental crust at 3 2 3 0 ga including the kaapvaal and pilbara cratons resulted in the gradual depletion of atmospheric co 2 through weathering by 2 9 2 7 ga declining pco 2 was associated with climatic cooling and siderite free soils transitory ch 4 co 2 ratios of 1 may have resulted in the sporadic formation of organic haze from atmospheric ch 4 reflected in one or more isotopic excursions involving global deposition of abnormally 1 3 c depleted organic c surface temperatures of 60 c after 2 9 ga may have also increased the distribution and productivity of oxygenic photosynthetic microbes eventual lowering of new continental blocks by erosion reduced loss of atmospheric co 2 due to weathering and continued long term tectonic recycling of co 2 resulted in rising pco 2 and decreasing ch 4 co 2 ratios in the later archean and eventual reestablishment of a mainly co 2 greenhouse similar events may have been repeated in the latest archean and earliest proterozoic but gradually rising production of o 2 effectively kept ch 4 co 2 ratios to 1 at this time

microfossils of sulphur metabolizing cells in 3 4 billion year old rocks of western australia sulphur isotope data from early archaean rocks suggest that microbes with metabolisms based on sulphur existed almost 3 5 billion years ago leading to suggestions that the earliest microbial ecosystems were sulphur based 1 5 however morphological evidence for these sulphur metabolizing bacteria has been elusive here we report the presence of microstructures from the 3 4 billion year old strelley pool formation in western australia that are associated with micrometre sized pyrite crystals the microstructures we identify exhibit indicators of biological affinity including hollow cell lumens carbonaceous cell walls enriched in nitrogen taphonomic degradation organization into chains and clusters and 13 c values of 33

the indigenous origin of witwatersrand carbon in the witwatersrand approximately 40 of the gold is intimately associated with so called carbon in carbon seam reefs which occur in over a dozen paleoplacers many of them concentrated at two stratigraphic levels in the 7000 m thick succession of archean siliciclastic sedimentary rocks this is reduced carbon present as kerogen admixed in various proportions with derivative now solid bitumen s oil generation and migration were active geological processes during early earth history numerous possible source rocks for oil generation including the carbon seams themselves occur within the witwatersrand basin in the witwatersrand ore oil bearing fluid inclusions are also present derived like the bitumen by thermal maturation of the kerogen the presence of kerogen and bitumen in the witwatersrand sedimentary rocks together with a wealth of observations on the spatial distribution of the carbon seams confirm that the carbon originated in situ from living organisms in microbial mat cover as opposed to flowing in from elsewhere as liquid hydrocarbons as some researchers have suggested paleochannels which truncated auriferous carbon seams early in the depositional history are of widespread occurrence and micro synsedimentary faults offset carbon seams the carbon seams are thus indigenous biogenic markers that grew contemporaneously with placer development the various features highlighting the nature and spatial distribution of witwatersrand carbon seams provide a classic case where field evidence trumps laboratory data in the reconstruction of geological processes

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

miocene shallow marine cold seep carbonate in fold and thrust western foothills sw taiwan a mound shaped authigenic carbonate buildup 50 m wide and 5 m high occurred in the middle part of the late miocene shallow marine succession the hunghuatzu formation exposed along the nantzuhsien river sw taiwan the carbonate concretions are classified into four types based on morphology which appeared to develop in upward sequence with vague boundaries 1 flat pipe shaped nodules highly dolomitic small pipes or nodules subparallel to the host strata 2 bulb shaped nodules discrete or combined small mesoclots with subspherical to irregular shapes 3 mushroom shaped concretions large mushroom like or irregular shaped blocks with chimney or vent structures and 4 carbonated layers with sedimentary structures these concretions are mainly composed of dolomite micrites quartz feldspar muscovite and clay minerals ubiquitously interweaved with blackish brown colored low magnesium calcite lmc veins low delta c 13 vpdb values 51 8 parts per thousand to 29 8 parts per thousand of all types of the carbonates reveal that the carbonates were mainly derived from anaerobic oxidation of hydrocarbons many samples are also o 18 depleted delta o 18 vpdb 11 5 parts per thousand to 1 00 parts per thousand due to the mixture with isotopically light lmc veins likely precipitated from meteoric waters there are numerous chemosymbiotic bivalve fossils genus monitilora diameter 3 4 cm in growth position just above the carbonate buildup according to the geological and isotopic signatures the hunghuatzu carbonates are related to a cold seep pseudobioherm which formed in a very shallow marine environment like tidal flat in a passive margin setting exposures of the hunghuatzu pseudobioherm may provide clues for tracking and comparing the evolution history of gas reservoir systems and hints of active tectonics of the sw taiwan region c 2012 elsevier ltd all rights reserved

oxygen and hydrogen isotope evidence for a temperate climate 3 42 billion years ago the widely held view that the archaean climate around 3 5 billion years ago was remarkably warm with ocean temperatures perhaps as high as 80 c has been questioned on the grounds that the established method of estimating ancient ocean temperature from the oxygen isotope ratios of sedimentary deposits is subject to significant uncertainty michael hren et al have adopted a different approach to estimating ocean temperature based on an analysis of both oxygen and hydrogen isotopes of 3 4 billion year old buck reef chert sediments from south africa the isotopes sampled are consistent with formation in waters no warmer than about 40 c suggesting that earth s early oceans may have been far cooler than previously thought the study of stable oxygen isotope ratios 18o of precambrian cherts suggests that ocean temperatures during the archaean era about 3 5 billion years ago were between 55 c and 85 c but uncertainty about the 18o of the primitive ocean has led to considerable debate regarding this conclusion here a combined analysis of oxygen and hydrogen istopes sampled from 3 42 billion year old buck reef chert rocks in south africa indicates that the ancient ocean was much cooler than previously thought stable oxygen isotope ratios 18o of precambrian cherts have been used to establish much of our understanding of the early climate history of earth1 2 3 and suggest that ocean temperatures during the archaean era 3 5 billion years ago were between 55 c and 85 c ref 2 but because of uncertainty in the 18o of the primitive ocean there is considerable debate regarding this conclusion examination of modern and ancient cherts indicates that another approach using a combined analysis of 18o and hydrogen isotopes d rather than 18o alone can provide a firmer constraint on formational temperatures without independent knowledge of the isotopic composition of ambient waters4 5 here we show that 18o and d sampled from 3 42 billion year old buck reef chert rocks in south africa are consistent with formation from waters at varied low temperatures the most 18o enriched buck reef chert rocks record the lowest diagenetic temperatures and were formed in equilibrium with waters below 40 c geochemical and sedimentary evidence suggests that the buck reef chert was formed in shallow to deep marine conditions so our results indicate that the palaeoarchaean ocean was isotopically depleted relative to the modern ocean and far cooler 40 c than previously thought2

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

carbonaceous cherts of the barberton greenstone belt south africa isotopic chemical and structural characteristics of individual microstructures abstract carbonaceous matter occurring in chert deposits of the 3 4 3 2 ga old barberton greenstone belt bgb south africa has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration here a detailed study is presented of in situ analysis of carbonaceous particles by lrs laser raman spectroscopy and sims secondary ion mass spectrometry reporting degree of structural disorder carbon isotope ratio and nitrogen to carbon ratio this combination of in situ analytical tools is used to interpret the 13 c values of only the best preserved carbonaceous remains enabling the rejection of non indigenous unmetamorphosed material as well as the exclusion of strongly hydrothermally altered carbonaceous particles raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub to lower greenschist facies metamorphic event although this identifies these organics as indigenous to the cherts it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re deposition of organics this suggest that morphological interpretation of these carbonaceous particles and in general of putative microfossils or microlaminae in hydrothermally altered early archean cherts should be made with caution a chert in the hooggenoeg formation which is older than and has been hydrothermally altered by a volcanic event 3445 ma ago contains strongly altered carbonaceous particles with a uniform n c ratio of 0 001 and a range of 13 c that is shifted from its original value cherts of the kromberg formation post date this volcanic event and contain carbonaceous particles with a n c ratio between 0 002 and 0 006 both the buck reef chert and the footbridge chert of the kromberg formation have retained fairly well preserved 13 c values with ranges from 34 to 24 and 40 to 32 respectively abiologic reactions associated with hydrothermal serpentinization of ultramafic crust such as fischer tropsch synthesis were an unlikely source for carbonaceous material in these cherts the carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material

uranium isotope fractionation suggests oxidative uranium mobilization at 2 50ga abstract geochemical data from late archean sedimentary rocks point to photosynthetic o 2 production and at least intermittent occurrences of locally mild oxidative weathering and surface ocean oxygenation oxygen oases prior to the early paleoproterozoic great oxidation event for example distinctive authigenic enrichments of mo and re in euxinic anoxic and sulfidic black shales are best explained by the oxidative mobilization of these metals from crustal sulfide minerals and their accumulation as oxyanions in seawater in contrast it is not clear if low u enrichments in the same shales reflect negligible oxidation of u from the upper crust or a very small oceanic u inventory that was derived from oxidative u mobilization here we report u isotope data for the 2 50 ga mt mcrae shale hamersley basin western australia which provides a more sensitive test for the presence or absence of authigenic u compared to u concentrations and enrichment factors normalized to average shale or upper crustal compositions that may not be representative of the local detrital composition we find instances where the u isotope composition in the upper mt mcrae shale 238 u 0 2 to 0 0 relative to standard srm950a is isotopically heavier than average upper crust 238 u 0 31 0 14 2sd based on granitoids and basalts the high 238 u values point to u isotope fractionation in the late archean marine environment and hence indicate the presence of a small amount of dissolved u in seawater and authigenic u in the mt mcrae shale volume dependent equilibrium isotope fractionation during the reduction of dissolved u vi to insoluble u iv like that observed in the modern black sea may explain the high 238 u signatures if u removal from bottom waters was not quantitative alternatively quantitative u removal would require that late archean seawater had high 238 u which could have arisen from the preferential sequestration of 235 u to fe oxyhydr oxide minerals elsewhere in the hamersley basin the supracrustal 238 u signatures are associated with some of the highest mo and re enrichments in the mt mcrae shale as well as distinctive mo s and n isotope signatures that are indicative of mild environmental oxygenation hence our findings suggest that small amounts of u were oxidatively mobilized from the upper crust at 2 50 ga unlike mo and re however u oxidation may not have occurred on land instead we hypothesize that oxidative u mobilization occurred by submarine weathering in an oxygen oasis

origin and mechanisms of k si metasomatism of ca 3 4 3 3ga volcaniclastic deposits and implications for archean seawater evolution examples from cherts of kittys gap pilbara craton australia and msauli barberton greenstone belt south africa abstract the effects of k si metasomatism during the formation of early archean replacement cherts have been quantified in this study by the investigation of two well known stratigraphic sections the msauli chert mc barberton greenstone belt south africa and the kittys gap chert kgc pilbara craton western australia the kgcs have a dacitic precursor similar to duffer formation dacites pilbara craton while the mcs are derived from al depleted komatiites similar to those from the weltevreden formation barberton greenstone belt mass balance calculations reveal that the volcaniclastic deposits had initial porosities of up to 85 vol for the kgc and of 65 vol for the mc secondary porosities 27 vol mc 8 vol kgc produced during k metasomatism are proportional to the dissolution of fe ca mg rich glass and precursor minerals komatiites have a higher chemical exchange potential than dacites each gram releasing 1 2 mmol fe2 2 8 mmol mg2 1 4 mmol ca2 and 1 1 mmol na to seawater together with 4 4 mmol o2 k metasomatism of 1 g of komatiite further implies an uptake of 0 67 mmol of k and 2 7 mmol of h the highest silica uptake is achieved for the kgc 82 mmol g of precursor this silica enrichment most likely operated in the water column and at the sediment water interface by sorption mechanisms on the surface of detrital particles and particulate organic matter as a result of seawater silica saturation acidic conditions ph 5 5 6 5 and hot temperatures 70 c favored the formation of k rich phyllosilicates by interaction with seawater during the early diagenetic alteration of the volcaniclastic particles the widespread occurrence of k si metasomatism in volcanic and sedimentary rocks can be regarded as a general alteration process of the early archean seafloor with a major influence on seawater composition the highly k selective metasomatism confirms previous studies suggesting that the archean ocean was acidic and probably in equilibrium with a co2 rich atmosphere

construction of a composite pressure temperature path revealing the synorogenic burial and exhumation history of the sevier hinterland usa metamorphic pressure temperature p t paths derived from 16 growth zoned garnets nine from this study and seven from a previous study have been combined to construct a detailed composite path for an area in the hinterland of the cretaceous to early tertiary sevier orogenic belt in southern idaho and north west utah samples are from two proterozoic units in the footwall of the basin elba thrust the schist of mahogany peaks in the central albion mountains idaho and the schist of stevens spring in the basin creek area of the grouse creek mountains utah 40 km to the south the simulated portions of the garnets analysed in this study grew from reactions involving the breakdown of chlorite in the upper greenschist to lower amphibolite facies multiple garnets were analysed from three samples overlapping segments of p t paths from different garnets in the same sample correlate with respect to slope and garnet mn concentration the composite p t path documents three episodes of sharply increasing pressures separated by two episodes of pressure decrease all during progressively increasing temperatures the path is interpreted to represent alternating episodes of synconvergent thrusting and extensional exhumation in the hinterland of the sevier orogen burial was probably caused by the basin elba fault the only major thrust exposed in the region extensional exhumation may have occurred along the mahogany peaks or emigrant spring faults or by extensional reactivation of the basin elba fault this method of correlating partial p t paths to reveal a more complete composite path provides a powerful tool in unveiling orogenic histories in metamorphic terranes where evidence of major structures responsible for burial and exhumation is commonly obscured by later events

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

tectonic controls on atmospheric climatic and biological evolution 3 5 2 4ga abstract this paper presents a speculative scenario for the evolution of the earth s surface environment and biological community 3 5 2 4 ga based on the geologic record and its interpretation available geologic evidence suggests that the earth s climate before 3 2 ga was hot probably 60 73 c as a result of a co 2 and ch 4 greenhouse the early biological community was probably dominated by anoxygenic photosynthetic thermophiles cyanobacteria if they evolved before 3 1 ga would have struggled to survive at such high temperatures as a result the atmosphere contained extremely low levels of o 2 and 33 s shows wide departures from 0 indicating mass independent fractionation of s isotopes in the upper atmosphere this hot early climate collapsed 3 1 3 0 ga due to growth of large new blocks of continental crust cratons the weathering of which resulted in the depletion of atmospheric co 2 and eventual drawdown of ch 4 due to formation and rainout of methane aerosols cooling may have culminated in glaciation about 3 0 2 9 ga this climatic catastrophe and attendant changes in atmospheric composition drove a major biological revolution 3 0 2 7 ga characterized by the emergence of new low temperature taxa including cyanobacteria and their spread throughout surface environments at the expense of extreme thermophiles including methanogens during a period of general tectonic stability 3 0 2 7 ga the earth s surface was characterized by overall clement conditions with temperatures 2 after 2 75 ga and by 2 7 ga surface temperatures had returned to 60 c cyanobacteria were again suppressed o 2 production waned and atmospheric mass independent fractionation is again indicated by extreme variations in 33 s these conditions persisted perhaps intermittently until about 2 5 2 4 ga when weathering and erosion of vast new blocks of continental crust formed about 2 65 2 6 ga caused the second and probably last collapse of a 60 c surface climate broad tectonic climatic and biological events 3 5 2 9 ga are remarkably parallel to those of the late archean and paleoproterozoic 2 75 2 2 ga and neoproterozoic 1 0 0 50 ga these 500 myr long cycles of greenstone volcanism and crustal generation climatic and atmospheric instability and biological innovation reflect the long term interaction of the tectonic atmospheric climatic and biological components of the earth s surface system they suggest that the evolution of the earth s interior expressed through its control on the formation of large blocks of continental crust has influenced atmospheric composition and climate which in turn have provided a fundamental control on the timing and directions of biological evolution throughout earth history

a new geochemical approach for constraining a marine redox condition of early archean abstract this paper presents a new approach for determining the paleo redox conditions of the early archean deep sea using u th systematics of seafloor hydrothermally altered basalts in the modern ocean system fresh unaltered basaltic glasses from the seafloor exhibit a tight u th correlation which reflects magmatic processes however basalts that have been hydrothermally altered are noticeably enriched in u due to adsorption and precipitation of dissolved u from oxygenated deep sea water onto the altered rocks irrespective of alteration temperature they exhibit much higher u th up to 34 than the primary igneous ratio of 0 35 in marked contrast hydrothermally altered basalts from the early archean 3 46 ga seafloor have maintained igneous u th 0 25 perfectly in spite of having been subjected to intense alteration this can be explained by a lack of dissolved u in early archean deep sea water due to the prevalence of anoxic deep sea conditions

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

hydrogen based carbon fixation in the earliest known photosynthetic organisms thin carbonaceous laminations preserved in shallow water facies of the 3416 ma buck reef chert south africa have been interpreted to represent some of the oldest known mats constructed by photosynthetic microbes preservation of these mats within a unit containing facies deposited at water depths ranging from 0 m to 200 m provides an opportunity to explore the electron donors employed in early microbial photosynthesis the presence of siderite feco3 as a primary sediment lack of hematite fe2o3 and lack of cerium anomalies throughout the buck reef chert imply that the entire water column was anoxic despite the presence of photosynthetic organisms authigenic uranium ua u th 3 correlates inversely with siderite abundance suggesting that variations in carbonate rather than oxygen activity controlled uranium mobility the inferred lack of oxygen and ferric minerals and the presence of dissolved fe2 in the water column imply that h2o fe2 and h2s could not have served as primary electron donors for carbon fixation it is most likely that buck reef chert bacteria utilized h2 as the primary reductant for photosynthesis

tectonic and magmatic development of the salinian coast ridge belt california 1 we present new field structural petrographic and geochronologic data on a rare midcrustal 25 km exposure of a cordilleran arc the coast ridge belt located in the santa lucia mountains of central california the study area is composed primarily of a deformed suite of upper amphibolite to granulite facies rocks the sur series which is dominated by metaigneous tonalites diorites and gabbros with subordinate metasedimentary quartzite and marble inherited zircons in magmatic rocks suggest that the provenance of framework rocks is drawn heavily from miogeoclinal formations and that sedimentation occurred in the late paleozoic or later minor magmatism in the coast ridge belt began in the early or middle cretaceous but magmatic activity was most intense during a short period time from 93 to 81 ma based on u pb zircon ages of a felsic gneiss and two less deformed diorites the time period 93 81 ma also brackets a period of extensive thickening and high temperature ductile deformation while a thrusting cause for ductile deformation cannot be ruled out we favor the hypothesis that the exposed rocks correspond to a zone of return flow of supracrustal rocks locally displaced by granitoid plutons in the shallower crust magmatism ended throughout salinia between81and76ma coincidentwiththeattainmentof peak pressure and temperature conditions of 0 75 gpa and 800 c exhumation followed immediately bringing the coast ridge belt to the surface within 8 my at a rate of at least 2 3 mm yr exhumation was coincident with an episode of extensional collapse that has been documented elsewhere in the southern california arc during the early laramide orogeny and that may be related to underthrusting of the forearc at that time index terms 8102 tectonophysics continental contractional orogenic belts 8035 structural geology pluton emplacement 3640 mineralogy and petrology igneous petrology 8030 structural geology microstructures 8124 tectonophysics earth s interior composition and state 1212

microbial life and biogeochemical cycling on land 3 220 million years ago the colonization of emergent continental landmass by microbial life was an evolutionary step of paramount importance in earth history here we report direct fossil evidence for life on land 3 220 million years ago ma in the form of terrestrial microbial mats draping fluvial conglomerates and gravelly sandstones of the moodies group south africa combined field petrographic carbon isotope and raman spectroscopic analyses confirm the synsedimentary origin and biogenicity of these unique fossil mats as well as their fluvial habitat the carbon isotope compositions of organic matter 13corg from these mats define a narrow range centred on 21 in contrast to fossil mats of marine origin from nearby tidal deposits that show 13corg values as low as 34 bulk nitrogen isotope compositions 2 15n 5 are also significantly different from their marine counterparts 0 15n 3 which we interpret as reflecting denitrification in the terrestrial habitat possibly of an atmospheric source of nitrate our results support the antiquity of a thriving terrestrial biosphere during the palaeoarchaean and suggest that a complex and microbially driven redox landscape existed during the deposition of the moodies group with distinct biogeochemical cycling occurring on land by 3 220 ma microbial life colonized the land surface by 3 2 billion years ago forming complex communities distinct from those in nearby marine environments according to analyses of fossilized microbial mats in the moodies group south africa

subduction and exhumation of the pelona orocopia rand schists southern california the pelona orocopia and rand schists of southern california and southwestern arizona are thought to have formed in either the same east dipping subduction zone as the franciscan complex or in a southwest dipping subduction zone related to collision of an outboard continental fragment with north america the principal justification for the collision model has been the observation that continental rocks overlying the schists locally show transport to the northeast field and petrographic studies now confirm that the northeast movement occurred during exhumation of the schist not during subduction combined with regional geologic relations this evidence makes the collision model unlikely

earliest life on earth evidence from the barberton greenstone belt south africa abstract studies investigating the structure and diversity of earth s record of life older than 3 2 ga are restricted to two locations worldwide in which sedimentary rocks have escaped regional high grade metamorphism and penetrative deformation the pilbara craton of western australia and the barberton greenstone belt in the kaapvaal craton of south africa this paper provides a south african perspective on the evidence of paleoarchean life a record that is often overlooked in the literature it aims to summarize and critically review previously reported claims of early life in the bgb gives an overview of the latest findings and provides an outlook on potential future discoveries the 15 km thick volcanic sedimentary succession making up the barberton supergroup was deposited between 3 55 to ca 3 20 ga and can be subdivided in three stratigraphic units that provide a unique window into a diverse and widespread paleoarchean microbial ecosystem landscape putative biosignatures occur almost throughout the entire bgb stratigraphy and range from carbonaceous cherts containing filamentous spheroidal and lenticular microstructures traces of hydrothermal biofilms photosynthetic microbial mats remnants of pseudocolumnar stromatolites and large organic walled spheroidal microfossils of currently unknown affinity the bgb also contains one of the world s oldest known record of tufted microbial mats which extensively colonized tidally influenced siliciclastic shorelines and were most likely formed by filamentous photosynthesizers other mat associated biosignatures include silicified gas bubbles domes and lenses that likely formed due to metabolic activity or the decay of buried organic matter some of these subsurface voids beneath the cohesive mats were inhabited by the earliest known forms of cavity dwelling microbial communities that were probably dominated by chemotrophic or photosynthetic microbes recently discovered terrestrial microbial mats once thriving in a fluvial dominated setting represent the oldest macroscopically visible fossil traces of life on land which is also supported by the occurrence of nearby paleosols that carry signals of biogenic sulfur fractionation the wealth of preserved microbial biosignatures from marine fluvial hydrothermal and possibly planktonic settings combined with the high spatial and temporal resolution of the barberton greenstone belt deposits is truly exceptional consequently the bgb deserves an equal level of attention and protection for future generations like its australian counterpart

thermal evolution of a portion of the sevier hinterland the northern ruby mountains east humboldt range and wood hills northeastern nevada conventional thermobarometry and gibbs method modeling have been used to obtain new information pertaining to the mesozoic tertiary thermal evolution of the ruby mountains east humboldt range and wood hills metamorphic complexes northeastern nevada kyanite grade assemblages from clover hill in the extreme northeastern part of the east humboldt range were produced at temperatures of 780 810 k and depths in excess of 35 km previously published geochronologic data suggest an early cretaceous or late jurassic age for this event the clover hill area subsequently experienced a major unroofing interval prior to late cretaceous time during which it was brought roughly 20 km closer to the surface the lack of stratigraphic evidence for substantial cretaceous erosion in the region and the topology of the pressure temperature paths for the clover hill samples suggest that the unroofing mechanism may have been tectonic denudation related to the relaxation of large topographic gradients produced by crustal thickening in the hinterland of the sevier orogen metamorphic rocks at clover hill and within the wood hills core complex equilibrated at 820 900 k and 500 640 mpa 18 24 km in cretaceous time prior to 115 ma other parts of the ruby mountains east humboldt range metamorphic complex presently structurally separated from the clover hill sequence were intruded extensively by late cretaceous granitic magmas and attained substantially higher temperatures than the clover hill wood hills block sillimanite grade assemblages from these areas apparently underwent roughly 10 km of unroofing and 130 k of cooling prior to final equilibration at 820 920 k and 360 430 mpa 13 16 km during oligocene extension

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