isolation of diagenetic overgrowths on quartz sand grains for oxygen isotopic analysis abstract diagenetic overgrowths on detrital quartz sand grains in sandstones can be isolated for direct measurement of their oxygen isotope ratios interfacial materials between detrital cores and diagenetic overgrowths are dissolved by etching with dilute hf the weakened interface breaks when grains are agitated ultrasonically subsequent particle size separation is an effective means of concentrating the diagenetic component

submillimeter scale zonation of δ18o in quartz and feldspar isle of skye scotland abstract laser probe analysis of oxygen isotope ratios in submillimeter domains of quartz and feldspar in hydrothermally altered granite reveals complex intragranular zonation that can be contoured within individual grains two samples collected within 10 m of each other in the maol na gainmhich epigranite mge isle of skye scotland are similar petrographically yet display very different 18o zonation patterns sk1 has isotopically zoned quartz 18 o 1 8 8 2 with intragranular gradients of 18o as great as 5 mm and individually homogeneous grains of k feldspar 0 2 which also display a large range of values 18 o 1 7 to 7 3 oxygen isotope gradients of more than 11 mm were measured in this sample in contrast sk4 has homogeneous quartz 18o 5 7 0 3 n 14 and a zoned k feldspar phenocryst 18 o 3 3 to 6 9 isotopic heterogeneity in quartz correlates with the degree of chlorite alteration in adjacent hornblende grains the isotopic heterogeneity revealed by the laser probe in single grains of quartz and feldspar is greater than the entire range of values displayed by bulk mineral analysis of samples collected throughout the mge pluton although the average values of 18o obtained by laser probe are identical to the average bulk values values in k feldspar suggest exchange at 400 c with infiltrating meteoric water 18 o 10 the marked differences in zonation profiles between samples on the outcrop scale suggest that permeability and isotope exchange during hydrothermal alteration was highly heterogeneous these microanalytical data for quartz and feldspar provide the first oxygen isotope evidence from pervasively altered granites that paleopermeability is caused at least in part by intracrystalline microfractures

diagenesis of a mixed siliciclastic evaporitic sequence of the middle muschelkalk middle triassic the catalan coastal range ne spain the middle muschelkalk middle triassic of the catalan coastal range north east spain comprises sandstone mudstone anhydrite and minor carbonate layers interbedded sandstones and mudstones which are dominant in the north eastern parts of the basin are terminal alluvial fan deposits south westward in the basin the rocks become dominated by interbedded evaporites and mudstones deposited in sabkha mudflat environments the diagenetic and pore water evolution patterns of the middle muschelkalk suggest a strong facies control during eodiagenesis formation of microdolomite anhydrite baryte magnesite k feldspar and mixed layer chlorite smectite was favoured within and adjacent to the sabkha mudflat facies whereas calcite haematite mixed layer illite smectite and quartz formed mainly in the alluvial facies low 18osmow values for microdolomite 23 7 to 28 4 and k feldspar overgrowths 17 3 to 17 7 suggest either low temperature isotopic disequilibrium or precipitation from low 18o porewaters low 18o waters might have developed at least in part during low temperature alteration of volcanic rock fragments during mesodiagenesis precipitation of quartz overgrowths and coarse dolomite occurred in the alluvial sandstones whereas recrystallization of microdolomite was dominant in the sabkha mudflat facies the isotopic compositions of these mesogenetic phases reflect increasing temperature during burial upon uplift and erosion telogenetic calcite and trace haematite precipitated in fractures and replaced dolomite the isotopic composition of the calcite 18osmow 21 5 to 25 6 o 13c 7 7 to 5 6 o and presence of haematite indicate infiltration of meteoric waters

thermo chronological approach to reservoir diagenesis in the offshore angola basin a fluid inclusion 40 ar 39 ar and k ar investigation 1 microthermometric analyses of fluid inclusions and geochronological surveys of authigenic silicates in neocomian sandstones and albian dolomitic reservoirs from the offshore angola basin have allowed determination of the age and the physical chemical conditions for fluid migration in a late diagenetic stage the 40 ar 39 ar laser probe dating of adularia overgrowths from the albian reservoirs shows that the trapping of primary aqueous and oil inclusions occurred during the early miocene 25 6 or 3 2 ma the k ar age of authigenic illites from the neocomian sandstones is slightly younger 19 or 1 ma and the crystallization of regular illite smectite mixed layers is related to a previous eocene to paleocene diagenetic stage 51 58 ma there appears to have bee both a regional and vertical variation in the salinity of aqueous fluids during the miocene the fluid inclusion trapping temperatures were estimated by correcting the homogenization temperatures of aqueous fluids assuming that pressures during the miocene were close to the present reservoir pressures and by intersecting the isochores of aqueous and oil inclusions assuming they were trapped at strictly similar pressure and temperature conditions the fluid inclusion temperatures are consistent with those derived from the tetrahedral aluminum content aliv of authigenic chlorites 168 206 degrees c in the albian reservoirs at depths between 2350 and 3580 m and 175 237 degrees c in the neocomian sandstones at depths between 2770 and 4470 m in both reservoir formations miocene tempe atures were higher than the present ones and related either to deeper burial or higher geothermal gradient

separation kar dating and 18o16o ratio measurements of diagenetic k feldspar overgrowths an example from the lower cretaceous arkoses of the angola margin abstract k feldspar overgrowths adularia on detrital k feldspar were removed from the lower cretaceous 130 115 ma arkoses of angola and concentrated for oxygen isotopic analysis and k ar dating the concentration technique consisted of etching with dilute h2sif6 followed by ultrasonic agitation and particle size separation the diagenetic k feldspar content of each of fourteen subfractions as estimated by cathodoluminescence was between 45 and 92 percent the rest being mostly detrital microcline and in some cases small amounts of quartz and or albite extrapolation of 40 ar 40 k and 18 o 16 o ratios indicated a k ar age of the adularia of 98 16 ma and a 18o of 20 2 0 8 per mil smow the age is consistent with petrographic observations which suggest that precipitation of adularia occurred relatively early in the diagenetic history the k ar age together with the 18o value burial history and fluid inclusion data indicate that adularia formed at relatively shallow depth

an integrated thermal and isotopic study of the diagenesis of the brent group alwyn south u k north sea abstract the petrography fluid inclusion thermometry and isotope geochemistry of diagenetic cements are used to reconstruct the pore fluid history of the middle jurassic brent group reservoir sandstones in the alwyn south area of the u k north sea the study focuses on a relatively limited area of three adjacent reservoir compartments at successively higher structural levels the cement assemblage of kaolinite quartz and illite has resulted in severe deterioration of otherwise good reservoir quality early precipitation of vermiform and late blocky kaolinite was succeeded by a period of relatively intense illite precipitation temperature estimates for kaolinite precipitation of 80 c and 18o of 15 3 suggest co existing fluids of 18o 3 quartz cementation overlapped both kaolinite and illite formation fluid inclusion data indicate that quartz cementation took place at temperatures of 109 7 c pore fluid salinities were 4 wt nacl with a h2o o isotopic composition of 1 o 0 5 smow the fluids which precipitated coexisting illite were compositionally homogeneous with equilibrium 18o water compositions of 0 5 smow illite sd values range from 33 to 50 smow these fluid inclusion and isotopic data suggest that both quartz and illite were precipitated from pore waters with a uniform marine signature this is consistent with the predominantly marine to paralic depositional context of the brent group in alywn south illite precipitation was followed by hydrocarbon emplacement between the middle eocene and lower oligocene

hump shaped 40ar 39ar age spectra in k feldspar and evidence for cretaceous authigenesis in the fountain formation near eldorado springs colorado the fountain formation near eldorado springs co usa shows evidence of alteration by hydrothermal fluids that precipitated authigenic potassium feldspar adularia as rims on detrital feldspars and as interstitial cement deposits detailed 40ar 39ar step heating experiments of samples from the fountain formation reveal age spectra having a characteristic hump at laboratory temperatures between 900 and 1080 c the humps appear to be related to the presence of adularia laser analyses of small grains of adularia indicate that the hump can be physically dissected unlike age gradients found in igneous feldspars such behavior is consistent with the mixing of gas from two or more generations of k feldspar each having unique diffusion properties the feldspars studied here indicate that two pulses of 150 c hydrothermal fluids migrated through the fountain formation at 135 and 94 ma prior to the main phase of laramide tectonic activity in the region the limited occurrence of authigenic cements suggests that reactivation of the underlying precambrian idaho springs ralston creek shear zone was significant enough to heat and mobilize large quantities of meteoric fluids in addition these data also suggest that cretaceous movements along the transcontinental arch correlated with stratigraphic events began approximately 40 million years earlier than previously thought

oxygen isotope analyses of fine silica grains using laser extraction technique comparison with oxygen isotope data obtained from ion microprobe analyses and application to quartzite and silcrete cement investigation the laser fluorination technique reported here for analyzing the oxygen isotope composition d 18 o of fine quartz size fractions 50 20 20 10 10 5 5 2 2 1 and 1 lm has been validated by comparison with the ion microprobe technique it yields accurate d 18 o data with an external precision better than 0 15 this is a significant methodological improvement for isotopic studies dealing with materials such as soil or biogenic oxides and silicates particles are often too small and recovered in insufficient amount to be easily handled for ion microprobe analysis both techniques were used to investigate d 18 o composition of a cretaceous quartzite and silcrete sequence from the southeast of france quartzite cements average 31 04 1 93 they formed from mid cretaceous seawater higher in the series silcretes cements average 26 66 1 36 they formed from upper or post upper cretaceous soil water and groundwater oxygen isotope data show that the silicification steps from one mineralogical phase to another and from one layer to another including from an upper pedogenic silcrete to a lower groundwater silcrete occurred in a closed or weakly evaporating hydrological system

oxygen isotope studies of clastic diagenesis in the lower cretaceous viking formation alberta implications for the role of meteoric water

investigation on support pattern of a coal mine roadway within soft rocks a case study abstract supporting coal mine roadway within soft rocks is a typical challenge in underground mining practices as the most widely used support structure rock bolt system has been successfully used to support coal mine roadways in various complex geological and geotechnical conditions including roadway in extremely weak rock masses roadways 1000 m below ground open off cut roadways with large section and roadways along the edges of mined out areas with thin pillars or even no pillars however the effectiveness and applicability of rock bolt system for the reinforcement of soft rock masses has not yet been established in this paper we present a case study of rock bolt system as used for ground reinforcement of longwall entries within soft rock masses the study site was the tailgate of the longwall panel 5 2s at the hongmiao coal mine in the pingzhuang coal mining district in china a typical soft rock coal mine of the region since traditional rock bolt system failed to maintain the stability of the tailgate the effectiveness of rock bolt systems for this application remained doubtful the reasons for the failure of the rock bolt system were first examined a discrete element method simulation was then performed to better understand the mechanism of rock bolts in supporting soft rocks an improved rock bolt system was finally proposed that was used to support the tailgate the field monitor showed that the improved rock bolt system successfully suppressed cracking and dilation of the tailgate this case study is useful in enhancing engineering applications of rock bolts to support longwall entries excavated in soft rock masses

speed and current for lightning stepped leaders near ground as determined from electric field records the speed and current during the final stages of the lightning stepped leader were determined from features of the electric field waveform the average speed of the leader tip was found below the height where de dt was zero for close lightning this height was in the range 0 6 2 0 km for flashes at the distances considered here 3 6 5 7 km for 10 stepped leaders observed near tampa florida the mean final one dimensional speed was 4 4 105 m s the standard deviation was 3 8 105 m s and the range was 1 0 14 105 m s these speeds are consistent with other stepped leader speeds reported in the literature that were found using optical techniques the current was estimated when the leader was near ground from de dt during the last hundred microseconds before the return stroke for 62 stepped leaders observed at distances of 3 6 14 km the mean final leader current was 1 3 ka the standard deviation was 1 0 ka the geometric mean was 1 0 ka and the range was 110 a to 5 0 ka errors resulting from uncertainties in distance charge center height calibration ground channel inclination and channel branching were found to be about 40 50 rms for both speed and current

pore water evolution in oilfield sandstones constraints from oxygen isotope microanalyses of quartz cement abstract oxygen isotope microanalyses of authigenic quartz in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the brae formation in the miller and kingfisher fields north sea oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a cameca ims 4f ion microprobe in conjunction with quartz cement paragenesis in the reservoirs constrained from textural and cathodoluminescence cl microscopy studies pore water evolution was reconstructed from the time of deposition of the sandstones in the upper jurassic until the present cl photomicrographs of quartz overgrowths in the brae formation sandstones show three cement zones a b and c which can be related to different oxygen isotope compositions 1 the earliest and thinnest zone a homogeneous cl pattern with probable 18 o values between 23 and 26 direct measurements were not possible precipitated in the sandstones at temperatures 18 o values between 15 and 18 precipitated in the sandstones most likely between 70 and 90 c 3 the third zone c homogeneous cl pattern and directly measured 18 o values between 16 and 22 precipitated in the sandstones most likely at temperatures 90 c calculated oxygen isotope compositions of pore waters show that zone a quartz cements and enclosing concretionary calcite precipitated from a meteoric type fluid 7 during shallow burial 18 o 7 to a more 18 o enriched fluid 18 o 4 as burial continued to 3 0 km data from zone c quartz cements are consistent with further fluid evolution from 18 o 4 to basinal type fluids with 18 o similar to the present day formation water oxygen isotope composition 0 6 at 4 0 km burial a similar pore water evolution can be derived for all three reservoirs studied indicating that hydrogeologic evolution was similar across sandstones of the whole brae formation the quartz cement zones observed in the brae formation sandstones and the pore water history derived for the area studied is analogous to published petrographic and pore water evolution data from the nearby brent group reservoirs and from reservoirs located in the haltenbanken area on the atlantic margin offshore norway considering quartz cement is a major porosity occluding phase in many reservoir sandstones and because pore waters both dissolve quartz and carry the dissolved silica to cementation sites the data presented are valuable for improving the understanding and prediction of reservoir quality development in sandstones globally

oxygen isotope microanalysis by secondary ion mass spectrometry suggests continuous 300 million year history of calcite cementation and dolomitization in the devonian bakken formation calcite cementation and dolomitization are key diagenetic processes in many sedimentary systems unravelling detailed histories and timescales of cementation and replacement is however often compromised by the limited spatial resolution of many analytical techniques in some cases multiple grains are co analyzed so that diagenetic histories are blurred and reaction periods are difficult to establish in this study we have used 10 micrometer resolution in situ secondary ion mass spectrometry to determine the oxygen isotope composition of 197 individual 10 50 micrometer size crystals of dolomite and calcite from six samples in a single core of upper devonian middle bakken member siltstones and sandstones the major tight oil formation of the williston basin usa this amount of data places important constraints on the range of temperatures and times that carbonate cementation and replacement occurred petrographic data show that microcrystalline calcite cement is an early phase and combined with mineralogical data suggest that much of the dolomite replaces calcite over spatial scales of less than a centimeter analyses of individual calcite crystals have a range of 5 for 18o in the group of crystals and for the group of individual dolomite crystals 10 these sub centimeter ranges are as great as those observed in previous studies of carbonate cements sampled over many meters and remind us that previous low resolution studies may have inadvertently analyzed mixed phases there is no relationship between dolomite texture and isotopic composition at this spatial scale microscale backscattered electron imagery and scanning electron microscopy cathodoluminescence zoning is seen but cannot be resolved with a 10 micrometer spot size assuming since it is an early cement that calcite precipitated from seawater 18o 1 5 it formed at ca 15 40 c mainly at the lower temperatures present day formation waters in devonian rocks in this region have oxygen isotope compositions of 7 8 vsmow using these values as a likely dolomitizing fluid we suggest that dolomitization occurred continuously between 40 and 140 c over 150 200 million years most likely in a fluid with a high mg ca ratio resulting from gypsum formation in local evaporites we suggest that this exceptionally low rate of dolomitization was controlled by the low rate of supply of mg in a very sluggish flow regime dolomitization is incomplete because of a limited supply of mg

methodological improvements for investigating silcrete formation petrography ft ir and oxygen isotope ratio of silcrete quartz cement lake eyre basin australia the oxygen isotopic composition 18o of the cement of a silcrete sample from the cordillo silcrete late eocene early oligocene of lake eyre basin australia was investigated the massive structure of the silcrete outcrop and the absence of pedogenic features suggest that this silcrete formed by precipitation from groundwater in order to avoid errors inherent in estimates of the cement 18o value from isotopic analyses of bulk material and mass balance considerations methodological improvements are described the silcrete cement is physically separated from the detrital phases its purity is checked using cathodoluminescence cl spectroscopy optical microscopy x ray diffraction xrd analyses and fourier transform infrared ft ir spectroscopy show the cement is made of three phases i overgrowth ii crypto crystalline quartz and iii micro crystalline quartz thus its 18o value must be interpreted as that of a mixture although the mixture consists mainly of crypto crystalline quartz as the amount of si oh exchangeable oxygen in the cement would bias the results by an amount less than the analytical error 0 09 it is unnecessary to go through an equilibration procedure prior to isotopic analysis the 18o value of the cement sample is between 24 8 and 25 8 relative to v smow assuming an eocene oligocene temperature of 15 20 c in agreement with published regional temperature estimates the inferred 18o value of the quartz forming solution was between 6 9 and 12 2 this is lower than the minimum 18o value of 6 70 estimated for the modern groundwater near the sample site a plausible paleoenvironmental scenario is that silcrete formed in a climate colder and wetter than the modern one from shallow groundwater frequently recharged in a setting of slow tectonic uplift

conditions and timing of quartz cementation in brent reservoirs hild field north sea constraints from fluid inclusions and sims oxygen isotope microanalysis abstract the conditions and timing of quartz cementation in the brent sandstones in the hild field norwegian north sea were constrained on the basis of oxygen isotope microanalysis and fluid inclusion microthermometry combined with optical and cathodoluminescence cl microscopy studies samples of the tarbert and the ness formations were investigated in three different wells presently lying at 3 8 4 3 km subsea and at 140 155 c quartz cement is present in all wells abundance varies from 3 to 26 and tends to increase with depth of burial two distinct separate episodes of quartz cementation referred to as q1 and q2 are readily distinguished by cl microscopy the two episodes are also recognized in the bimodal distribution of fluid inclusion temperatures and oxygen isotope microanalyses the data indicate that q1 quartz formed in the early tertiary 65 35 ma at 95 110 c and 2 3 3 0 km burial depth and that q2 mainly precipitated in the oligocene miocene 40 10 ma at 125 135 c and 3 1 3 7 km q1 quartz precipitation follows the onset of overpressuring in the reservoir while precipitation of q2 seems to slightly precede and accompany the main phase of oil filling 35 10 ma secondary ion mass spectrometry sims oxygen isotope microanalyses yield 18o values ranging from 17 to 25 averaging 23 2 for q1 and 19 5 1 5 for q2 calculated 18o values of quartz forming waters are similar for q1 and q2 ranging from 1 to 3 and are similar to the 18o value 2 of present day formation water in the reservoir quartz cementation in the brent reservoir of the hild field occurred during deep burial at high temperature from evolved 18o rich basinal water most likely derived from the viking graben two episodes of enhanced quartz precipitation are recorded around 100 c and 130 c no evidence was found for any significant contribution of 18o depleted jurassic meteoric water and or low temperature precipitation of quartz cement

illitization and paleothermal regimes in the middle ordovician st peter sandstone central michigan basin k ar oxygen isotope and fluid inclusion data hydrocarbon reservoirs occur in the middle ordovician st peter sandstone in the central michigan basin at depths of 1 5 3 5 km and are diagenetically altered latest diagenetic cements include saddle dolomite pervasive microcrystalline illite and chlorite and quartz a k ar and 18o 16o study of the fine grained authigenic illite in 25 samples from 16 wells covering a large area within the basin yields k ar ages ranging from 367 to 322 ma and dgr18o values between 12 7 and 16 9 pmil smow the dgr18o values of diagenetic quartz overgrowths range from 15 2 to 18 9 pmil fluid inclusion temperatures in the quartz cement range from 70 to 170 c reflecting multiple generations of diagenetic quartz and or precipitation over m st of the diagenetic history reequilibrated fluid inclusions in the saddle dolomite cement yield temperatures ranging from 90 to 150 c a regionally significant episode of illitization occurred during the late devonian mississippian temperatures of illite formation are indirectly estimated to be in the range of 125 170 c and most paleodepths of illitization are between 2 8 and 3 2 km these results imply that 1 illite formed from 18o rich fluids and 2 elevated geothermal gradients i e greater than 34 c km existed in the michigan basin in the late paleozoic the k ar ages and the dgr18o values are not correlated to present depths of the samples or paleodepths of illitization illites with young ages and low dgr18o values tend to be geographically distributed along the north south branch of the buried precambrian rift the dgr18o values of the diagenetic quartz follow a similar trend the spread of illite k ar ages and dgr18o values and their geographic distribution are best explained as reflecting abnormally high thermal regimes in the part of the basin located above the presumably highly fractured basement along the rift

diagenesis of quartz in the upper proterozoic kaimur sandstones son valley central india abstract the upper proterozoic kaimur sandstones in central india are quartz sublithic and lithic arenites cemented by quartz illite and hematite diagenetic quartz occurs in five modes syntaxial overgrowths fracture healings aggregates of small euhedral crystals quartz resulting from the alteration of detrital silicates and from the recrystallization of quartz intergranular pressure dissolution is suggested as the main source of silica with smaller contribution from other sources such as silica dissolved in meteoric waters stylolitization clay mineral diagenesis and the alteration of detrital silicates studies on the fluid inclusions and oxygen isotopes of diagenetic quartz suggest that meteoric water modified by diagenetic reactions has mediated the quartz cementation

quartz cementation history of sandstones revealed by high resolution sims oxygen isotope analysis abstract evaluating the timing and origin of quartz cement is central to understanding how porosity is lost in sandstones during burial kinetic models of quartz cementation have been calibrated using large scale datasets but have never been tested at the microscopic level at which cement forms here we use high precision in situ oxygen isotope analyses on sandstone from the jurassic ness formation from the north sea to reveal the growth history of single quartz overgrowths to a resolution of 2 m measured 18o cement range from 28 to 20 v smow in early to late cement and are consistent with quartz cementation models that propose the bulk of quartz precipitates as a continuous process beginning at 60 70 c quantitative x ray diffraction analyses and clay mineralogical analysis of interbedded shales are inconsistent with a silica source from shale implying that silica for the cement is sourced internally to the sand these isotope data are broadly consistent with predictive conceptual quartz cementation models and provide a critical link from micron scale measurements to basin scale predictions and observations

influence of early diagenesis on reservoir quality of rotliegende sandstones northern netherlands reservoir quality of rotliegende sandstones lower permian in the northern netherlands was significantly affected by early diagenetic processes that are related to sedimentary facies in marginal desert playa lake environments in the past burial related products and processes have received more attention than early diagenetic processes neglecting the effects that early diagenesis can have on reservoir quality and well productivity of rotliegende sandstones shallow groundwater related diagenesis is a major factor affecting rotliegende reservoir sandstones the precipitation of the main porosity occluding cements dolomite anhydrite and quartz occurred early in the diagenetic history oxygen isotopes of dolomite d18o 18 4 to 23 4 smow suggest that meteoric water has played a major role throughout the diagenetic history and that precipitation of these cements occurred at low temperatures 80 c the strontium isotopes of dolomite cements 87sr 86sr 0 70935 0 71387 and the sulfur isotopes of anhydrite cements d34s 6 7 to 9 7 cdt rule out permian or younger seawater as a source for these cements and are consistent with formation from meteoric water within a continental basin an early diagenetic origin of the cements is also suggested by the fact that the majority of sandstone samples have lost more porosity due to cementation than through compaction most samples with high 20 amounts of porosity loss via cementation were affected by early dolomite and anhydrite the relative abundance of dolomite and anhydrite cements can be correlated with depositional environments wet depositional environments e g interdune and fluvial sandstones with water tables close to the surface show the highest amount of dolomite and anhydrite cements whereas dry environments with relatively deep water tables e g dune sandstones are characterized by low amounts of cements the precipitation of early diagenetic cements has strongly influenced the present day porosity patterns with depositional environments interpreted as drier showing less cements and significantly altering the porosity patterns formed during early diagenesis knowledge of the influence of sedimentology and paleogeography on the diagenetic patterns is a key element for improved understanding and prediction of reservoir quality in the rotliegende sandstones of the northern netherlands

ion microprobe analyses of δ18o in early quartz cements from 1 9ga granular iron formations gifs a pilot study abstract the low 18 o values of precambrian cherts have been widely used to infer that temperatures were higher and or seawater 18 o was lower compared to today s oceans however the precambrian cherts presented as evidence for these temperatures are neomorphosed from amorphous precursors that originally precipitated from ocean water suggesting diagenetic alteration is an important cause of the widespread low 18 o values many pores between sand size clasts in granular iron formations gifs are filled with coarsely crystalline quartz cements that are texturally primary in unmetamorphosed gif samples from the 1 9 ga gunflint and sokoman iron formations synsedimentary clasts of gif and high minus cement porosities the volume of pore space occupied by recognizable cement inversely proportional to the degree of compaction indicate such cements precipitated directly from pore water near the depositional interface growth bands identified in quartz cement crystals using cathodoluminescence cl imaging further confirm their void filling and unrecrystallized nature in order to test the effect of neomorphism on the 18 o of silica in these rocks unrecrystallized primary quartz cements and neomorphic quartz in adjacent clasts were both analyzed by ion microprobe values of 18 o range from 23 5 to 26 4 vsmow in cements and 21 3 26 8 in clasts in samples from the gunflint iron formation samples from the sokomon iron formation record deeper more evolved pore waters and in some cases contain lower 18 o cements in individual pores of gunflint samples cement values vary only slightly more than analytical uncertainty from edge to center and no consistent increasing or decreasing trends were observed overall the 18 o of the cements are similar to the most elevated 18 o reported from cherts of similar age but span a substantially smaller range this suggests most of the lower values reported previously reflect late diagenetic conditions at greater burial depths the shallow possibly restricted seas above the gunflint and sokoman gifs appear to have been warmer and may have been lower in 18 o than present day seawater but they may not have been representative of global seawater

ion microprobe analysis of 18o 16o in authigenic and detrital quartz in the st peter sandstone michigan basin and wisconsin arch usa contrasting diagenetic histories abstract the oxygen isotopic compositions of authigenic quartz cements in sandstones provide a monitor of the temperatures compositions and origins of pore occluding fluids during diagenesis but quartz overgrowths are too fine grained to be amenable to conventional isotopic analysis we have used a cameca ims 4f ion microprobe to determine oxygen isotopic variations in authigenic and detrital quartz in four samples of the ordovician st peter sandstone from the michigan basin and wisconsin arch midwestern usa ion microprobe isotopic analyses have been successfully accomplished with an internal precision of 1 1 and a spatial resolution of 20 30 m at low mass resolution using a high voltage offset technique repeated analyses of the quartz standard demonstrate a reproducibility of close to 1 1 sd in good agreement with that expected from counting statistics the four ion probe samples were chosen as representative of thirteen samples that were mechanically and chemically disaggregated sieved and analysed as a function of sieve size by conventional bulk mineral techniques conventional and ion microprobe analyses are mutually consistent supporting the accuracy of the ion microprobe analyses within sample isotopic variations of up to 13 and micro scale isotopic variations of at least 4 over a distance of 100 m have been measured within quartz overgrowths in a sandstone from the wisconsin arch overgrowths are uniformly higher in 18o than detrital grains and gradients of up to 25 exist across a few microns 18o enriched quartz overgrowths in sandstones from the wisconsin arch show complex cl zonation and reflect one of two possible processes 1 low temperature quartz precipitation during mixing of meteoric waters with upwelling basinal fluids 2 higher temperature quartz precipitation during episodic gravity driven upwelling of warm basinal fluids of comparable isotopic composition to michigan basin fluids from the illinois basin related to evolution of mississippi valley type pb zn ore forming fluids quartz overgrowths in michigan basin sandstones which derived their silica locally by pressure solution are thought to have precipitated continuously over a range of temperatures from hot basinal fluids of restricted isotopic composition and circulation detrital quartz shows significant intea grain and intergrain isotopic variation 5 13 but no significant inter sample variation consistent with a uniform source of granitoid and metamorphic quartz in space and time

imported quartz cement in aeolian sandstone grew from water of uniform composition but has complex zonation the origins and volumes of waters which mass transport silica in sedimentary basins remain obscure many previous analytical studies have illustrated cases where quartz cements in sandstones originate from complex and variable fluids by contrast we show by using a combination of separation and analytical techniques that in lower permian sandstones of the southern north sea the cementing fluid was isotopically uniform during growth of quartz cement with a 1b o of 19 6 1 0 v smow in this relatively uniform fluid quartz cements grew and developed complex cathodoluminescence cl zonations petrographic data show that 8 10 quartz cement locally 30 was imported into this 380 km 2 180 m thick aeolian sandstone and cement distribution controlled by depositional permeabilities we infer a large scale high volume flux of evolved meteoric fluid during 2 km deep burial and show that complex cl zonation may arise from relatively subtle changes in water composition

timing and conditions of permian rotliegende sandstone diagenesis southern north sea k ar and oxygen isotopic data illitic clay ranging from pure illite to highly illitic illite smectite i s is the most abundant diagenetic phase in the eolian and sabkha facies of the rotliegende formation of the southern north sea and northeastern netherlands most k ar ages of diagenetic illite and i s in the eolian sandstones are between 100 and 175 ma illite formation is related to two major phases of tectonic activity the jurassic kimmerian orogenic movements and late cretaceous early tertiary inversion movements the burial depth of a sample at the time of diagenetic illite formation can be derived from the k ar age and the appropriate burial history curve illite formed in different localities at different times and at different depths generally diagenetic illite is more abundant in samples more deeply buried at the time of illite formation the correlation between illite formation and tectonic disturbance and the correlation between illite abundance and burial depth may be useful for predicting reservoir properties specifically we would expect reservoir permeability of the rotliegende in the study area to be least impaired by illite growth in those samples less deeply buried from the late jurassic through the late cretaceous calculated dgr18o values of most illite forming fluids lie between 0 and 4 pmil smow indicative of meteoric or marine waters modified by interaction with rock however 18o depleted fluids as low as 5 pmil indicative of a major meteoric component are inferred to have been involved in the formation of illite in the groningen gas field at a time when nearby sections of the rotliegende were eroded and exposed at the surface