application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

early dolomitisation of late triassic platform carbonates in the transdanubian range hungary abstract in the late triassic a large carbonate platform system came into existence in the transdanubian range tr producing a 1 5 to 2 km thick carbonate succession the lower part of the platform carbonate series is made up of cyclic peritidal dolomite main dolomite formation while the upper one consists of cyclic peritidal limestone dachstein limestone formation however there is a transitional member of remarkable thickness between them consisting of partially dolomitised limestone and calcitic dolomite this study focuses on the transitional member where the process of early dolomitisation was halted at various stages in the investigated sections the early dolomitisation was controlled by the climate and high frequency sea level oscillation which led to periodic subaerial exposure of large areas and palaeo hydrological conditions increasing humidity and gradual extension of the permanently inundated outer platform led to cessation of early dolomitisation in the middle to late norian

methane dominated thermochemical sulphate reduction in the triassic feixianguan formation east sichuan basin china towards prediction of fatal h2s concentrations abstract new sour pools have recently found in the lower triassic feixianguan fm carbonate reservoirs in the east sichuan basin in china with h 2 s up to 17 4 by volume a recent blowout from a well drilled into this formation killed hundreds of people as a result of the percentage concentrations of h 2 s in order to assess the origin of fatal h 2 s as well as the cause of petroleum alteration h 2 s concentrations and the isotopes 34 s and 13 c have been collected and measured in gas samples from reservoirs anhydrite pyrite and elemental sulphur 34 s values have been measured for comparison the high concentrations of h 2 s gas are found to occur at depths 3000 m temperature now at 100 c in evaporated platform facies oolitic dolomite or limestone that contains anhydrite nodule occurrence within the reservoirs where h 2 s concentrations are greater than 10 its 34 s values lie between 12 0 and 13 2 cdt this is within the range of anhydrite 34 s values found within the feixianguan fm 11 0 to 21 7 average 15 5 3 5 cdt thus h 2 s must have been generated by thermochemical sulphate reduction tsr locally within the reservoirs burial history analysis and fluid inclusion data reveal that the temperature at which tsr occurred was greater than about 130 140 c suggesting that the present depth temperature minimum is an artifact of post tsr uplift both methane and ethane were actively involved in tsr since the petroleum became almost totally dry no alkanes except methane and methane 13 c values become significantly heavier as tsr proceeded methane 13 c difference thus reflects the extent of tsr while it is tempting to use a present day depth control 3000 m to predict the distribution of h 2 s in the feixianguan fm this is an invalid approach since tsr occurred when the formation was buried some 1000 2000 m deeper than it is at present the likelihood of differential uplift across the basin means that it is important to develop a basinal understanding of the thermal history of the feixianguan fm so that it is possible to determine which parts of the basin have been hotter than 130 140 c

dolomites and dolomites in mesozoic platform carbonates of the southern apennines geometric distribution petrography and geochemistry abstract a multidisciplinary study of the dolomitized bodies present among the late triassic to late cretaceous carbonate deposits of the monti lattari belt southern apennines of italy compares the different dolomitization styles present within this succession of these mesozoic platform carbonates and furnishes an overview of their formative genetic models the revealing traces of the diverse modes of diagenesis in this region include both surface and subsurface mechanisms utilizing a geometric reconstruction of the dolomitized bodies an integrated facies and petrographic analysis of the components and a geochemical characterization of the sediments including detailed mineralogy and associated stable isotope values the data presented demonstrate that some intervals are characterized by surface related dolomitization processes middle late triassic and cretaceous while others latest triassic to lower middle jurassic and are mainly dominated by later large scale subsurface dolomitization processes all of the middle late triassic norian successions are pervasively dolomitized exhibit fabric preserving textures and have a distinctly heavy stable isotope signature their origin has been related to diagenesis due to particular climatic and palaeoceanographic conditions leading to the formation of extended blankets of shallow warm saline seawater this setting was terminated by climatic and palaeoceanographic changes during the latest triassic which dramatically reduced the significance of surface related dolomitization on a regional scale the overlying latest triassic rhaetian outcrops consist of a widespread coarse fabric destructive massive dolomite with a lighter isotopic signature these dolomites seem genetically related to the ones of the lower middle jurassic which share the same geometrical petrographic and geochemical characteristics caused by large scale fluid circulation this diagenetic pattern apparently started in the early jurassic during burial driven by thermal convection due to the different temperatures between the oceanic waters and the warmer carbonate platform the middle upper jurassic units feature local episodes of early dolomitization that partially replaced the muddy carbonate facies due to tidal pumping and reflux mechanisms dolomitization is not a pervasive feature during this phase of deposition in the cretaceous portion of the succession dolomites have a totally different and distinct character and are generally related to bedding and show variability in textures and mg ca ratios local association with evaporites geometry of dolomitized bodies and stable isotope geochemistry suggest processes comprising evaporative pumping and reflux all related to the surface environment this evidence points to a renewal of a climatic context more favorable for early dolomitization thus this study permits recognition of the presence of three quite different dolomitization styles within the carbonate platform succession of the southern apennines fold thrust belt one ascribed to the norian one to the rhaetian lower jurassic and one to the cretaceous

rare earth elements in solnhofen biogenic apatite geochemical clues to the palaeoenvironment rare earth element ree concentrations in biogenic apatite samples coprolite bone and soft tissue were used to investigate the environment of deposition of the celebrated solnhofen fossil lagerstatten the measured ree patterns are similar between different localities lithologies flinz faule and levels in the upper solnhofen plattenkalk suggestive of a stable ree supply during deposition the behaviour of cerium in the solnhofen samples implies that bottom water conditions were not anoxic and variations in the cerium anomaly can be explained by differences in burial rate these results provide further geochemical support for current depositional models barthel k w 1978 solnhofen ein blick in die erdgeschichte ott verlag thun barthel k w swinburne n h m conway morris s 1990 solnhofen a study in mesozoic palaeontology cambridge univ press cambridge that propose that extra basinal processes are responsible for the interbedded nature of the solnhofen deposits rather than intra basinal processes such as water turnover events

cerium a chemical tracer for paleo oceanic redox conditions abstract the ce anomalies found in marine carbonates provide a sensitive indicator for the study of paleo oceanic redox conditions the dependency of cce the observed ce concentration and cepo04 the activity of the cepo4 complex to the po2 ph and pco2 of seawater can be used for deciphering some aspects of the paleo ocean chemistry and is log c ce log cepo 0 4 13 2 0 25 log p o 2 3 0 ph 1 95 0 25 log p o 2 1 50 log p co 2 1 50 log ca the ph which is related to pco2 and po2 are major factors for controlling the ce concentration in seawater therefore ce anomalies ceas ce observed ce interpolated between la and pr or nd serve as indicators for ph or pco2 or po2 changes nearly uniform ce abundances found in pacific 5000 m water columns piepgras and jacobsen 1988 and the ceas in ichthyoliths can be explained using this model a general cea shift in sediments from d s d p holes 525a 530a b 516 516f suggest that the entire south atlantic water redox condition changed from reducing to oxidizing at 56 my in the north atlantic the galicia margin samples show a redox change between 132 148 my indicating active surface currents between the atlantic ocean and the shallow sea off the iberian peninsula the carbonate samples from d s d p hole 316 in the central pacific yielded an average cea of 0 15 0 01 which is 2 3 the ceas observed in deep pacific seawater by piepgras and jacobsen 1988 the relative enrichment of ce in these samples is ascribed to coprecipitation of ce oh 4 onto fe mn o coatings the striking similarities in the trace element profiles of ree sc hf th and ceas and the low u abundances 0 08 ppm in many marine carbonates indicate insignificant ce redox and trace elemental concentration changes for the pacific ocean over the past 95 my no significant ce redox change occurred in the pacific ocean within k t boundary one 240 my china limestone deposited under oxic sedimentary conditions yielded a cea of 0 53 calculations indicate a pco2 of 1 9 present value prevailed over the 240 my tethyan ocean u th ratios and low u concentrations are diagnostic indicators for obtaining reliable ceas in marine carbonates and perhaps for biogenic apatites and conodonts as well

diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies an isotopic and ree study of cambrian phosphorites seawater rare earth element ree distributions show heavy ree enrichment and cerium depletion but these characteristics are only rarely found in ancient marine authigenic minerals such as carbonate fluorapatite and calcite this enigma may be due to changing ree distributions in seawater through time or to post depositional redistribution of ree differentiating between these two factors is however far from straightforward as their relative importance is likely to vary and evidence is often ambiguous in this article we present ree as well as strontium and sulphur isotope data for basal cambrian phosphorites of south china which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm and tend to undergo therefore less diagenetic alteration of ree spectra than skeletal apatites and other phosphate types this study reveals stratigraphic trends in ce anomalies ce ce that could be taken to imply the preservation of a palaeoenvironmental signal however further investigation of our data shows that such trends in phosphorites can also be caused by post depositional alteration which tends to 1 increase ree contents 2 reverse ce depletion 3 alter eu anomalies and 4 concentrate preferentially the middle ree mree further alteration which may be due to weathering has led to hree depletion and positive la and y anomalies tetrad effect this is the first study where ceanom and phosphate bound sulphate s isotope values could be compared for one particular stratigraphic section which provides us with an additional tool for the interpretation of diagenetic redox conditions least altered phosphorites from meishucun that yield a strongly negative ceanom constrain early cambrian seawater 34s to about 33 we conclude that phosphorite ceanom primarily records variations in the local conditions of diagenesis while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom water redox conditions

a terrane interpretation of the archaean limpopo belt the limpopo belt is a zone of thickened archaean crust whose origin is currently explained by a late archaean continent continent collision between the kaapvaal and zimbabwe cratons this review shows that the two cratons have fundamentally different geological histories and that the zimbabwe craton was unlikely to have behaved as a stable cratonic block at the time of the limpopo belt collision the geological histories of the zimbabwe craton the north marginal central and south marginal zones of the limpopo belt and the kaapvaal craton are shown to be sufficiently different from one another to warrant their consideration as discrete terranes the boundaries between the five units outlined above are all major shear zones further supporting a terrane model for the limpopo belt the five units were all intruded by late to syn tectonic granites c 2 6 ga constraining the accretion event to c 2 6 ga

the behaviour of trace and rare earth elements ree during hydrothermal alteration in the rangan area central iran the rhyolitic dome in the rangan area has been subjected to hydrothermal alterations by two different systems 1 a fossil magmatic hydrothermal system with a powerful thermal engine of a deep monzodioritic magma 2 an active hydrothermal system dominated by meteoric water based on mineralogical and geochemical studies three different alteration facies have been identified phyllic advanced argillic and silicic with notable differences in ree and other trace elements behaviour in the phyllic alteration zone with assemblage minerals such as sericite pyrite quartz kaolinite lree are relatively depleted whereas hree are enriched the advanced argillic zone is identified by the presence of alunite jarosite and pyrophyllite as well as immobility of lree and depletion in hree in the silicic zone most of lree are depleted but hree patterns are unchanged compared to their fresh rock equivalents all the ree fractionation ratios la yb cn la sm cn tb yb cn ce ce cn and eu eu cn are low in the phyllic altered facies eu eu cn in both advanced and silicic facies is low too in all alteration zones high field strength elements hfse e g ti zr nb are depleted whereas transition elements e g v cr co ni fe are enriched geochemically speaking trace and rare earth elements behave highly selective in different facies

the incorporation of rare earth elements in modern coral we report measurements of rare earth elements rees which show that these trace elements are being incorporated in modern coral in proportion to their seawater concentrations four bermuda north rock coral species diploria strigosa diploria labyrinthiformis montastrea annularis and porites astreoides and two tarawa atoll samples of the species hydnophora microconos were analyzed by ti idms following cleaning techniques to isolate the lattice bound rees based on the replicate analyses of the same piece of diploria strigosa excellent reproducibility was achieved the ree ca ratios 0 1 3 nmol mol of the bermuda and tarawa corals are similar to those of cd ca the trace metal with the lowest seawater concentration used in coral studies with the exception of ce the distribution coefficients e g dnd nd coral nd seawater ca seawater ca coral between bermuda coral lattice and sargasso seawater have fairly flat patterns across the ree series the values of d range from 1 to 3 like those reported for other trace elements in corals this suggests but does not prove that rees are incorporated in the aragonite lattice of these corals the shale normalized ree patterns of the tarawa corals also have seawater like distributions however no local seawater data are available to calculate values of d two bermuda species d labyrinthiformis and p astreoides have values of d for ce that are high with respect to the d values of la and nd implying that there is preferential uptake of ce into the lattice this may be related to the fact that ce is the only ree with an active redox chemistry in seawater there is considerable interest in using the chemical and isotopic composition of coral as indicators of climatic variations the combination of ree ca ratios and neodymium isotopic composition of coral has the potential to help understand important natural processes a primary application could be directed toward a tracer for river water discharge in tropical regions and by inference a proxy for rainfall climate variation and weathering history this application would exploit the fractionated composition of rees in river water as a terrigenous signature in coastal corals

hydrothermal dolomitization in platform and basin carbonate successions during thrusting a hydrocarbon reservoir analogue mesozoic of venetian southern alps italy abstract the early jurassic dolomitized carbonates in the venetian alp represent a surface analogue of the hydrocarbon exploration targets in adriatic offshore and po plain italy dolomitization affected the carbonate platform of monte zugna formation lower jurassic and the neptunian dikes breccia in the pelagic maiolica formation uppermost jurassic lower cretaceous improving the poro perm characteristics petrography stable isotope strontium isotope ratio trace element and fluid inclusion analyses were carried out on samples from the monte grappa anticline which is the direct analogue for subsurface the petrographic analyses showed a first pervasive replacement dolomitization phase d1 followed by volumetrically less important dolomite cement precipitation phases d2 d3 ds the same quite wide range of oxygen isotope 9 to 2 v pdb is observed in all dolomite types the 13 c range is in the positive field of marine derived carbonate from 0 5 to 3 2 pdb the trace element analysis showed a slight enrichment in fe and mn contents in the monte zugna dolostones with respect the original limestone the same dolomite precipitation temperature up to 105 c th was observed in the replacement and cement dolomites suggesting a unique dolomitization event this temperature largely higher than the maximum burial temperature about 50 c supports a hydrothermal origin of the dolomitizing fluids which had a seawater to brackish composition the data collected suggest a hydrothermal dolomitization occurring during to the south alpine thrusting according to the squeegee model the interpretation is consistent with the dolomitization model proposed for similar jurassic successions in the central southern alps this study indicates that the deformed foreland and thrust fold belts carbonates in po plain and adriatic offshore are suitable to be dolomitized and therefore reflect an efficient hydrocarbon exploration play

seawater normalized ree patterns of dolomites in geshan and panlongdong sections china implications for tracing dolomitization and diagenetic fluids abstract in conventional studies of tracing dolomitization and diagenetic fluids rees of dolomites were widely used as been normalized by paas nasc or chondrite however most dolomites are formed in seawater or seawater derived fluids thus we conduct a new attempt to normalize the rees of dolomite using seawater standard based on case studies on 36 triassic limestone dolomite samples from the geshan section of southeast china and 26 permian triassic dolomite samples from the panlongdong section of northeastern sichuan basin southwest china the geshan seawater normalized sn ree patterns are characterized by notable positive ce sn average ce sn ce 6 823 sd 0 192 and negative pr sn anomalies average pr sn pr 0 310 sd 0 010 and slightly negative gd sn anomalis average gd sn gd 0 864 sd 0 053 with no obvious eu sn anomaly average eu sn eu 1 036 sd 0 094 the signatures of ree patterns barely changed during the dolomitization process for the ree compositions of the panlongdong dolomite it can be found that 1 the recrystallization process can result in varied total ree concentrations between 7 16 ppm and 37 87 ppm but do not alter the ree patterns including consistent positive ce sn anomalies average 4 074 sd 0 27 and lree enrichment average nd sn yb sn 3 164 sd 0 787 2 meteoric incursion can reverse ce anomaly from the strong positive ce anomalies ce sn ce 5 059 to slightly positive ce sn ce 2 459 or even negative ce anomalies and 3 hydrothermal fluid altered ree pattern is complicated by fluctuated distribution curve negative ce anomaly and positive eu anomaly eu sn eu 1 862 these results suggest that the seawater normalized ree patterns of dolomite can serve as an index to study the source of the dolomitization fluids and distinguish complex diagenetic processes providing a complement to previous works

rare earth element contents of jurassic fish and reptile teeth and their potential relation to seawater composition anglo paris basin france and england the rare earth element ree chemistry of jurassic shelf seawater from western europe anglo paris basin was investigated by analyzing the fish and reptile teeth deposited in shallow to deeper water 200 m environments ree patterns in apatites are controlled by the host sediments vertebrate teeth sampled from the siliciclastic sediments calcareous sandstones and shales show flat shale normalized ree patterns that reflect the dominant influence of the continental source from which the ree were derived carbonate deposits protected from the clastic sources contain fish and reptile teeth whose ree patterns reflect more accurately the ree composition of the overlying water column the ree patterns are similar through the bajocian to the oxfordian and are characterized by a depletion in heavy rare earth element dy ybn 1 8 5 0 compared to modern seawater compositions dy ybn 0 8 0 9 the fractionations among hree in the jurassic seawater are more efficient than in modern oceans and increases with depth instead of decreasing these hree removal processes operated as early as the paleozoic and disappeared during the late cretaceous the fractionation of hree relative to lree is correlated with the increasing depth of the basin as indicated by two independent criteria that are the sedimentary structures and the oxygen isotope composition of coexisting brachiopods the dy ybn ratio of the marine biogenic phosphates can be used as a proxy of paleo water depths at the scale of a water mass ce anomalies ce in biogenic apatite bearing limestones are variable and systematically negative relative to modern surface seawater and bathonian values mean ce 0 63 0 10 weak ce anomalies during the callovian mean ce 0 21 0 09 suggest the onset of more reducing conditions this redox change coincided with an increase of seawater temperatures as suggested by the oxygen isotope compositions measured on the same teeth we speculated that the callovian low ce could result from a decrease in the po2 due to the warming of seawater

dissolved rare earth elements in the southern ocean cerium oxidation and the influence of hydrography abstract analysis of dissolved ree distributions in the southeastern atlantic ocean has revealed that progressive oxidative removal of dissolved ce from deepwater 1 km occurs over the mixing time of the oceans ce anomaly ce ce values measured in deep waters from the southeastern atlantic are similar to but slightly higher than those from deep ocean stations throughout the indian and pacific oceans typically falling in the range 0 08ce ce 0 12 only deepwater samples from the north atlantic exhibit values greater than ce ce 0 12 with the exception of southeastern atlantic bottomwaters which are influenced by antarctic bottomwater covariation of dissolved ree concentrations with salinity and or dissolved si concentrations at this site has allowed dissolved ree compositions for various southern ocean water masses to be predicted end member concentrations for antarctic bottom water aabw derived in this way indicate high dissolved ree concentrations similar to average pacific deep water but with significantly enhanced dissolved ce concentrations and less pronounced hree enrichment this result is consistent with the proximity of our study site to active aabw deepwater formation and evolution of that water mass during transport to the pacific ocean

dolomite characteristics and diagenetic model of the calcari grigi group asiago plateau southern alps italy an example of multiphase dolomitization a multidisciplinary study conducted over the carbonate platform deposits of the liassic calcari grigi group southern alps highlighted how the use of outcrop analogues can contribute to better define the distribution of dolomitic bodies related to fault networks to characterize the petrophysical properties of the dolomitic sequence and unravel a complex diagenetic history this study was carried out in the asiago plateau southernmost part of the eastern southern alps northern italy which provides excellent outcrops of the jurassic calcari grigi group the dolomitization of the jurassic sequence is variable in terms of stratigraphic extension and geographic distribution in the studied localities the dolomitization is generally limited to the mount zugna formation and is characterized by an undulatory front with sub vertical dolomitic chimneys along the major faults within this unit and often associated with faults stacked high porosity and permeability bed parallel dolomitic bodies are developed that show excellent petrophysical properties the dolomitic intervals are characterized by pervasive unimodal and patchy polymodal dolomite crystals thin section cathodoluminescence isotopic and fluid inclusion analyses were used to constrain the paragenetic evolution of the sequence which is similar in all the studied localities the first dolomitization stage is marked by zoned dolomite crystals with a dull luminescent core the porosity is thought to have increased after this stage with dark blue luminescent dolomite accompanied by the corrosion of older crystals the appearance of saddle dolomite marks the onset of the porosity reduction stage ending with the infilling of vugs and the remaining open pores with calcite cement the diagenetic evolution locally stopped at the saddle dolomite stage with the complete occlusion of the remaining pores paragenetic and fluid inclusion data suggest an evolutionary trend of increasing temperatures and decreasing salinity toward brackish fluids responsible for dolomite and calcite precipitation the integration of the available data seem to indicate that the diagenetic evolution of the study area is related to i the interplay between evolving fluids from marine to brackish ii the burial of the sequence increasing temperature and iii the evolution of the hydrogeological system fault and fracture network fluid mixing this complex paragenetic evolution is strongly linked to the evolution of the porosity framework that evolved from a good widespread network in the early stages of the burial history to a confined system in the later stages due to reduction of porosity by the deposition of late calcite and dolomite cements

strontium isotopes and rare earth element geochemistry of hydrothermal carbonate deposits from lake tanganyika east africa at cape banza north tanganyika lake fluids and aragonite chimneys have been collected many times since the discovery of this sublacustrine field in 1987 this sampling has been investigated here for the sr isotopic compositions and the rare earth element features of the carbonates and a few fluid samples the 87sr 86sr ratios of the chimneys indicate that they have precipitated from a mixture of lake water more than 95 and hydrothermal fluids no zoning in the chimneys was detected with our sr data for the rare earth elements the situation is more complex the external walls of the chimneys are rare earth element poor la 500 ppb yb 200 ppb la yb 2 to 3 4 their shale normalised rare earth element patterns suggest that they are in equilibrium with the inferred carbonate depositing fluids the rare earth element concentrations of the internal walls of the chimneys are significantly light rare earth elements lree enriched with la contents sometimes up to 5 ppm we suggest that they contain more vent fluid rare earth elements than the external wall samples possibly adsorbed on the surface of growing crystals or simply hosted by impurities it was not possible to constrain the nature of these phases but the variations of the compositions of the internal wall materials of the active chimneys with time as well as data obtained on an inactive chimney indicate that this rare earth element excess is mobile partition coefficients were calculated between the external wall aragonite and carbonate depositing fluid the results are strikingly similar to the values obtained by sholkovitz and shen 1995 on coral aragonite and suggest that there is no significant biologic effect on the incorporation of rare earth elements into coral aragonite and that the various carbonate complexes involved me co3 complexes are the main lree carriers in seawater cantrell and byrne 1987 instead of me co3 2 in banza fluids have the same behaviour during aragonite precipitation

new distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites late tertiary dolomites form the bahamas exhibit a strong inverse correlation between sr and mol mgco sub 3 it is proposed that this relation is a result of a distribution coefficient dependent on the concentration of ca in dolomite because these dolomites precipitated from water with a sr sup 2 ca sup 2 ratio similar to that of seawater a distribution coefficient d sub sr sup dolomite of 0 0118 is calculated for sr in stoichiometric dolomite this coefficient increases by about 0 0039 for every additional mol caco sub 3 in nonstoichiometric dolomite these data have three important implication first the new d sub sr sup dolomite is substantially lower than previously suggested second a relation has been established among the sr sup 2 ca sup 2 ratio of dolomite forming waters the major element composition of dolomites and the sr content of dolomites allowing the determination of the sr sup 2 ca sup 2 ratio of fluids involved in the formation of ancient dolomites third some published data sets on dolomites show a similar relation between sr content and major element composition suggesting that these dolomites may also have formed from seawater other data sets deviate from the correlation defined by little bahama bank dolomites indicating either dolomitization in fluids other than seawater or changes ofmore the sr sup 2 ca sup 2 ratio of seawater through time less

dissolution and its impacts on reservoir formation in moderately to deeply buried strata of mixed siliciclastic carbonate sediments northwestern qaidam basin northwest china abstract petroleum exploration is increasingly extending from shallowly to moderately to deeply buried strata with reservoir quality being a key the formation mechanism of conventional clastic and carbonate reservoirs under such conditions has been widely investigated however the mechanism of mixed siliciclastic carbonate reservoirs has not been well studied in this paper we address this issue using a case study in the northwestern qaidam basin northwest china n1 e1 2 age 2500 4500 m dissolution pores and fractures dominate the reservoirs however only the fractures were the focus of previous studies thus here we investigate dissolution and its impacts on reservoir formation providing a complementary understanding of the reservoir formation in the basin using core observations and examining thin section it was discovered that dissolution was both random and followed bedding the dissolved components primarily included calcareous and gypsum minerals with fingerprints that are characteristic of burial dissolution further electron probe analysis on authigenic minerals revealed that the dissolution fluid might have originated from acidic formation fluids associated with hydrocarbon generation the fluids passed through faults and fractures dissolution pores were an important component of the reservoir providing approximately 60 of the porosity in addition segments of high porosity generally above 5 are associated with dissolution based on these observations a schematic model was established to explain the impacts of dissolution on reservoir formation specifically organic acidic formation fluids enter tectonic fractures resulting in dissolution as they pass through them this dissolution enhances the size of the pore space and the reservoir properties of the rocks eventually developing a fracture dissolution reservoir