structurally controlled hydrothermal dolomites in albian carbonates of the ason valley basque cantabrian basin northern spain abstract shallow marine carbonate sedimentation dominated during the albian in the western part of the basque cantabrian basin in northern spain forming the large ramales platform this platform originated on a less subsiding tectonic block facing deeper and more subsiding areas to the south and east which were created by tectonic activity in the basin fracture related hydrothermal dolomites hosted in these albian carbonates are well exposed in the ason valley area mapping in the studied area revealed several dolomite bodies related to main faults that cut the stratification almost at right angles the bodies show a vertical development along fault strike up to 900 m thick from which kilometre scale branches expand following the stratification dolomitization is pervasive and independent of the limestone facies main dolomite facies are fine replacive sucrosic and saddle petrography c o and sr isotopes and fluid inclusion analysis support a polyphase hydrothermal dolomitization at fluid temperatures between 75 c and 240 c and highly variable salinity of up to 22 wt nacl fine dolomite replaced limestone first and then sucrosic and saddle dolomites replaced part of the first dolomite and cemented newly created fracture porosity together with different calcite cements zebra dolomites and hydroclastic breccias are products of this later stage burial analysis of the host rock supports maximum burial temperatures of 80 c and intense tectonic activity from the albian to turonian with a latest albian peak subsidence albian stretching of the crust and subsequent ascent of the isotherms in the area is suggested to have produced sufficient heat to the dolomitizing fluids the structural analysis indicates a strong transtensional tectonic activity in the studied area during albian to turonian time with the creation of an overstep between w e trending and n s trending faults fluids moved from subsiding areas to fractured uplifted parts of the ramales platform enhanced by diapiric activity

reactive transport modelling of reflux dolomitization in the arab d reservoir ghawar field saudi arabia it has long been recognized that the arab d reservoir in ghawar field has been significantly dolomitized and that the distribution of dolomites is highly heterogeneous across this reservoir previous studies indicated that dolomite occurs with either a stratigraphic or non stratigraphic distribution when mapped dolomite tends to form several parallel linear trends across the field although stratigraphic dolomite was suggested to be formed early from highly evaporated pore fluids sourced from overlying evaporite deposits non stratigraphic dolomite was thought to be generated primarily from hydrothermal fluids sourced from below this study focuses primarily on these non stratigraphic dolomites and proposes that i these dolomites initially formed via seepage reflux but were reinforced by late stage hydrothermal dolomitization and ii reflux is also responsible for the formation of parallel linear trends of dolomite the reflux model hypothesizes that an evaporative lagoon which is the source of dolomitizing fluids formed during the falling stage systems tract of a depositional sequence and that with continuing sea level fall this lagoon migrated progressively towards deeper parts of an intrashelf basin adjacent to the ghawar field leaving behind lines of dolomite bodies along a series of temporary coastlines two dimensional reactive transport models have been built to test this hypothesis and have resulted in a predicted pattern of dolomite bodies that agrees with both the observed vertical distribution of non stratigraphic dolomite as well as the mapped lateral distribution of the dolomite trends in addition the major ion compositions of late jurassic seawater are calculated based on fluid inclusion data in the literature using jurassic seawater in current models leads to the absence of anhydrite cements and less potential of over dolomitization than using modern seawater

geochemical implications of sulfate in sedimentary carbonates abstract sulfate content reaches maximum partitioning in inorganically deposited mn calcite from aqueous solution when the carbonate contains 4 wt mno similar partitioning was also found for sr which rises to a maximum when calcite sediments contain 0 5 wt mno the behavior of the sulfate is explicable in terms of substitution of co 2 3 by so 2 4 this argument for the distribution of so 2 4 in inorganically formed carbonates is also consistent for sulfate in marine carbonates ir spectra of recent biogenic carbonates revealed no organic but only inorganic sulfate indicating that the sulfate occupies lattice sites in the carbonates total sulfate content of recent calcitic marine carbonates and pleistocene limestones increases with increasing mg content up to a maximum near 2 3 wt mg and decreases at larger mg contents this decrease in the sulfate content of these marine carbonates from that incorporated originally into the carbonate lattices is attributed to gradual exsolution during diagenesis in subaerial fresh water or on the sea floor in low so 2 4 waters the sulfate in coralline algae apparently increases the mg content above that predicted from observed shifts in the x ray diffraction peaks based upon so 2 4 free calcites

geochemistry of the fluids related to epigenetic carbonate hosted zn pb deposits in the maestrat basin eastern spain fluid inclusion and isotope cl c o s sr evidence epigenetic zn pb deposits in the maestrat basin eastern spain are hosted by dolostones that replace aptian limestones the deposits have morphologies ranging from mineralized breccias to stratiform massive replacements sphalerite and galena cementing breccias precipitated as well formed crystals whereas the stratiform replacements are characterized by textures such as colloform and skeletal or dendritic growths two mineralized areas are found valdelinares and mas de la mina cedraman the highest ore grades up to 11 zn and largest deposits ca 30 000 t of sulfides occur as stratiform deposits in the valdelinares area resurreccion and restauracion mines in the valdelinares area microthermometry of fluid inclusions reveals that ore stage minerals precipitated by the mixing of two distinct fluids fluid a a high salinity brine with salinity greater than 26 wt percent nacl equiv and temperatures above120oc and fluid b a low salinity fluid with less than 15 wt nacl equiv and temperatures around 40oc fluid a dominated brines are found in mineralized breccias located in the vicinity of major fracture zones whereas more dilute fluids are typical of massive ores in the mas de la mina area the presence of two fluids is less evident although the ranges in both temperature and salinity also point to mixing na k li cl br systematics of the inclusion fluids indicates that fluids a and b were evaporated seawater that did not significantly interact with k and li rich sediments the most diluted mixtures fluid b are characterized by relatively heavy cl 37 cl 1 11 and 1 19 compared to fluid a dominated brines which have 37 cl values close to seawater 0 the sulfur isotope compositions range from 10 5 to 13 4 per mil for sulfides and between 16 and 17 8 per mil for barite such values are consistent with mixing of a metal rich brine with small amounts of 32 s enriched sulfur and a sulfur rich fluid with a 34 s close to the marine sulfate of late cretaceous to early paleocene age the c and o isotope compositions of ore stage calcite 13 c from 0 6 to 7 8 18 o from 17 2 to 20 9 are also explained by the precipitation from different mixtures of fluids a and b from the geochemical data it is inferred that fluid mixing played a key role in the formation of the zn pb deposits in the maestrat basin ore precipitation occurred in places where brines ascending through faults interacted with a dilute sulfur rich fluid deposits with the highest ore grade and size are related to areas where the most extensive mixing occurred

ediacaran sulfur cycle insights from sulfur isotope measurements δ33s and δ34s on paired sulfate pyrite in the huqf supergroup of oman abstract the ediacaran period 635 541 ma was a time of dramatic environmental and ecological change sulfur isotopic records preserved in marine strata provide insights into these changes previous work using 34 s in pyrite and carbonate associated sulfate in the ediacaran aged huqf supergroup sultanate of oman suggested increases in marine sulfate concentrations changing microbial pathways and redox conditions and a long term increase in 34 s of sulfate delivery to the ocean however 34 s based analyses on their own were insufficient to uniquely reconstruct these environmental and ecological changes here we present new paired 34 s and 33 s data from carbonate associated sulfate and pyrite in these strata the results allow us to provide three new constraints on ediacaran sulfur cycling first a notable increase 34 s cas py previously attributed to possible disproportionation in upper buah strata is associated with 33 s evidence for an increase in sulfide reoxidation indicating a transition to more oxidizing conditions possibly associated with the onset of bioturbation secondly 33 s data indicate that a large positive shift in both sulfate and pyrite 34 s in ara group strata is mostly likely due to an increase in delivery of 34 s enriched sulfate to the basin finally paired 34 s 33 s data constrain possible changes in seawater sulfate concentrations suggesting an increase in sulfate supply related to enhanced evaporite weathering consistent with geological and geochemical observations that imply high so 4 17 mm during deposition of the ara group

fault controlled dolomitization at swan hills simonette oil field devonian deep basin west central alberta canada the partly dolomitized swan hills formation middle upper devonian in the simonette oil field of west central alberta underwent a complex diagenetic history which occurred in environments ranging from near surface to deep 2500 m burial five petrographically and geochemically distinct dolomites that include both cementing and replacive varieties post date stylolites in limestones depths 500 m these include early planar varieties and later saddle dolomites fluid inclusion data from saddle dolomite cements th 137 190 c suggest that some precipitated at burial temperatures higher than the temperatures indicated by reflectance data tpeak 160 c thus at least some dolomitizing fluids were hydrothermal fluorescence microscopy identified three populations of primary hydrocarbon bearing fluid inclusions and confirms that saddle dolomitization overlapped with upper cretaceous oil migration the source of early dolomitizing fluids probably was devonian or mississippian seawater that was mixed with a more 87sr rich fluid fabric destructive and fabric preserving dolostones are over 35 m thick in the swan hills buildup and basal platform adjacent to faults thinning to less than 10 cm thick in the buildup between 5 and 8 km away from the faults this plume like geometry suggests that early and late dolomitization events were fault controlled late diagenetic fluids were in part derived from the crystalline basement or palaeozoic siliciclastic aquifers based on 87sr 86sr values up to 0 7370 from saddle dolomite calcite and sphalerite cements and 206pb 204pb of 22 86 from galena samples flow of dolomitizing and mineralizing fluids occurred during burial greater than 500 m both vertically along reactivated faults and laterally in the buildup along units that retained primary and or secondary porosity

fault controlled and stratabound dolostones in the late aptian earliest albian benassal formation maestrat basin e spain petrology and geochemistry constrains this study was developed under the exxonmobil fc2 alliance fundamental controls on flow in carbonates the authors wish to thank exxonmobil production company and exxonmobil upstream research company for providing funding the views in this article by sherry l stafford are her own and not necessarily those of exxonmobil this research was supported by the sedimentary geology research group of the generalitat de catalunya 2014sgr251 we would like to thank andrea ceriani and paola ronchi for their critical and valuable reviews and associated editor piero gianolla for the editorial work

diurnal sea breezes force near inertial waves along rottnest continental shelf southwestern australia abstractobservations of upper ocean dynamics close to the critical latitude ratio of the local inertial to diurnal frequency is 0 94 from a range of platforms surface currents using hf radar moored instruments and satellite remote sensing data off southwest australia indicated the presence of energetic near inertial waves generated through the diurnal inertial resonance during the austral summer when southerly winds and land sea breeze lsb system dominated the wind regime strong counterclockwise diurnal motions amplitudes surpassing 0 3 m s 1 penetrated to 300 m depth with diurnal vertical isotherm fluctuations up to 60 m the upward phase propagation speed of 140 m day 1 deep penetration of diurnal currents below the mixed layer and the 180 phase difference between the upper and lower water column suggested that the local lsb system caused the resonant diurnal motions relative vorticity fluctuations along two cross shore transects indicated changes to the local effective coriolis freq

evaluating the relationship between the carbon and sulfur cycles in the later cambrian ocean an example from the port au port group western newfoundland canada abstract we present a high resolution 34 s sulfate and pyrite and 13 c carbonate record from the middle upper cambrian port au port group a mixed carbonate siliciclastic succession exposed in western newfoundland canada the results illustrate systematic 34 s sulfate shifts of 15 over relatively short stratigraphic intervals 10 m likely 34 s sulfate pyrite ca 23 and a generally positive coupling between changes in 13 c carbonate and 34 s sulfate together these results indicate that middle to late cambrian sulfate concentrations were low and that the sulfate reservoir was more sensitive to change than it was in either terminal neoproterozoic or cenozoic oceans however a simple carbon c and sulfur s isotope box model of the late cambrian ocean illustrates that low sulfate concentrations alone fail to account for the 15 34 s sulfate shifts recognized in port au port strata such large shifts can be generated only if fluctuating oceanic redox is invoked marine anoxia forces reduced c s burial and elevated 34 s driving larger 34 s changes per mole of organic carbon buried the conclusion that later cambrian oceans featured both low sulfate levels and widespread subsurface anoxia supports hypotheses that link fluctuating marine redox conditions in the delayed recovery of skeletal animals and metazoan reefs from late early cambrian extinction

controls on reflux dolomitisation of epeiric scale ramps insights from reactive transport simulations of the mississippian madison formation montana and wyoming abstract prediction of the geometry and petrophysical properties of dolomite geobodies depends on understanding both the hydrological system supplying reactive fluids and the chemistry of these fluids however patterns are complicated by the non linear response of the diagenetic system to depositional texture which controls both fluid flux via permeability architecture and reaction rate via effective surface area this study explores interactions between extrinsic controls spatial distribution of brine composition and temperature and intrinsic controls permeability and reactivity using local and regional scale reactive transport models of sequential episodes of brine reflux that resulted in partial dolomitisation of the mississippian madison ramp inter well scale models show preferential early dolomitisation of fine grained more reactive beds pervasive dolomitisation can occur most readily beneath the brine pool where flow is perpendicular to bedding and is most rapid at high brine fluxes down dip of the brine pool bedding parallel flow is focused in relatively permeable coarse grained beds providing reactants for strongly preferential alteration of intervening more reactive fine grained beds in contrast thicker sequences of fine grained beds dolomitise more slowly limited by the rate of supply of magnesium regional scale models with injection of brines of increasing salinity towards the ramp interior reproduce the observed pattern of dolomitisation however more realistic simulations in which reflux is driven by lateral density contrasts generate flow rates orders of magnitude too low for significant dolomitisation simulations suggest pervasive dolomitisation of epeiric scale ramps by a platform wide reflux circulation as often invoked is not feasible rather dolomitisation of such extensive systems critically requires local scale flow systems such as may result from topographically controlled variations in restriction of platform top seawater circulation

compilation and time series analysis of a marine carbonate δ18o δ13c 87sr 86sr and δ34s database through earth history abstract the sr s o and c isotope database of marine carbonates contains over 55 000 published isotope values of low mg calcite from diagenetically little altered phanerozoic fossil shells as well as samples of whole rocks and calcite cements of ordovician to archean age carbon and oxygen isotope data for the shell material are divided into habitat subsets high mid low latitude and deep sea and whole rock data are separated by mineralogy into calcite dolomite subsets trend correlation wavelet and spectral analyses on gaussian filtered isotope records were applied to detect and quantify similarities and patterns in temporal records with the following results 1 oxygen isotope trends from the high latitude and deep sea habitats are almost indistinguishable through the last 115 ma consistent with the existence of the oceanic conveyor belt throughout this interval 2 all oxygen isotope habitat records show a strong coherent 30 45 ma 38 ma cyclicity throughout the cretaceous and the cenozoic 3 up to 70 of the multi million year variability in the 18o record of the last 115 ma can be simulated by the following equation 18 o 0 64 sin 2 t 120 ma 0 9 x sin 2 t 38 3 ma 1 1 with x ranging from 0 4 0 6 for the low high latitude and deep sea habitats to 0 8 for the mid latitude realm 1 a 120 20 ma cycle occurs in the paleozoic and neoproterozoic 18o record consistent with paleoclimate variability as interpreted from sedimentological and faunal records 2 the offset of 13c values between deep water and high latitude vs surficial habitats at lower latitudes is consistent with the operation of a biological pump in the oceans since at least the cretaceous 3 sr and s isotope records exhibit a 60 70 ma cyclicity throughout the phanerozoic

th pb and u pb dating of ore stage calcite and paleozoic fluid flow thorium 232 lead 208 and uranium 238 lead 206 radiometric ages for ore stage calcite show that mississippi valley type mvt ore deposits can form in distinct tectonic settings an age of 251 11 million years for the jumbo mine in kansas united states is in agreement with other ages for mvt deposits in the midcontinent of north america the similarity of ages of these deposits supports the concept that they formed in response to fluid flow during the late paleozoic alleghenian ouachita orogeny an age of 351 15 million years for twelve mile bore and bloodwood kapok deposits in australia indicates that these mvt ores were deposited in a rifting environment

a simple analytical model of low frequency wind driven upwelling on a continental slope abstract a simple closed form analytical solution of a classical coastal upwelling is found in terms of two dimensional baroclinic waves trapped in a two layer ocean on a steep continental slope and forced by the wind driven reversals of currents on an adjoining wide and vertically well mixed continental shelf bottom friction on the continental shelf plays a dominant role this solution agrees qualitatively with the sparse available records of low frequency oscillations of currents and temperature on the upper continental slope of the great barrier reef

reactive transport modeling of brine reflux dolomitization anhydrite precipitation and porosity evolution abstract both geothermal convection and brine reflux drive circulation of sea water derived fluids through carbonate platforms during early burial but dynamic interactions between heat and solute transport and resulting diagenesis are at present poorly understood this paper describes high resolution reactive transport model rtm simulations that suggest that reflux of 85 ppt brines rapidly restricts geothermal convection to the platform margin with flow focused in the more permeable shallow carbonates in a baseline simulation involving an elongate 25 km wide grain dominated packstone platform brine reflux resulted in complete dolomitization beneath the 5 km wide brine pool in 335 ky the dolomite body then extends downward at c 22 m 100 ky into an underlying broad area of partial dolomitization this process enhances porosity at shallow depth but beneath the dolomite body precipitation of anhydrite occludes porosity and limits the depth of reflux in contrast geothermal convection at the platform margin forms a smaller partially dolomitized body over a longer time 60 dolomite after 1 my with very minor associated anhydrite cementation reflux diagenesis is sensitive to platform geometry with higher rates of fluid flow increasing the depth of alteration beneath the brine pool for a circular platform compared to the linear baseline fluid flow across thermal gradients enhances reaction rates and ignoring heat transport by 85 ppt brine reflux underestimates the extent of reflux dolomite by 25 and associated anhydrite by 90 the depth and rate of anhydritization is sensitive to the geothermal heat flux whereas platform top temperatures affect dolomitization rate reflux diagenesis is also sensitive to brine density which affects both fluid flow and reaction rates sediment permeability and reactive surface area rsa are key intrinsic controls on diagenesis where the permeability structure permits sufficient fluid flow diagenesis preferentially affects more reactive fine grained sediments however as flow rates decline reactions become focused in the more permeable but less reactive sediments simulations thus shed light on why in some settings reflux preferentially dolomitizes muddy sediments but elsewhere favors grainstones once active reflux ceases brines continue to flow in the subsurface but this latent reflux causes only minor dolomitization due to prior mg2 consumption at shallow depth

early development and late preservation of porosity linked to presence of hydrocarbons in precambrian microbialite gas reservoirs within the sichuan basin southern china abstract the ancient ediacaran dengying formation consists of microbialites with unusually high porosity that represent a large proportion of the weiyuan anyue gas reservoirs in southwestern china s sichuan basin however a clear model for the development and preservation of these microbialite reservoirs has yet not been determined we used samples from outcrops and borehole cores to characterize these microbialites and propose a model for early development and subsequent reservation of their porosity primary porosity in the microbialite framework was mainly represented by fenestrate vugs and elongated cavities between microbial layers that were formed during initial sedimentation as well as secondary karst vugs and caves formed by meteoric karst resulting from subsequent tectonic uplift these features represent the early development of the microbialite reservoirs subsequent filling of vugs by early stage radial dolomite rd and late stage saddle dolomite sd occurred during the deep burial stage with 18o and homogenization temperature patterns suggesting continuous cementation however most vugs were not cemented due to combination of 1 a rigid dolomitized framework 2 a closed burial system restricting the flow of mineralized fluid 3 the presence of petroleum or pyrobitumen and 4 acidic fluid conditions related to high concentrations of h2s and co2 the third and fourth factors were linked to presence of hydrocarbons and had made big contribution to porosity retention during deep burial diagenetic stage

carbonate porosity creation by mesogenetic dissolution reality or illusion many authors have proposed that significant volumes of porosity are created by deep burial dissolution in carbonate reservoirs we argue however that this model is unsupported by empirical data and violates important chemical constraints on mass transport because of the ubiquitous presence and rapid kinetics of dissolution of carbonate minerals the mesogenetic pore waters in sedimentary basins can be expected to be always saturated and buffered by carbonates providing little opportunity for the preservation of significantly undersaturated water chemistry during upward flow even if the initial generation of such undersaturated pore water could occur a review of the literature where this model has been advanced reveals a consistent lack of quantitative treatment in consequence the presumption of mesogenetic dissolution producing a net increase in secondary porosity should not be used in the prediction of carbonate reservoir quality

petrography and geochemistry of fault controlled hydrothermal dolomites in the riopar area prebetic zone se spain abstract the present paper reports the first detailed petrographical and geochemical studies of hydrothermal dolomites related to mvt zn fe pb deposits in the riopar area mesozoic prebetic basin se spain constraining the nature origin and evolution of dolomitizing and ore forming fluids mapping and stratigraphic studies revealed two stratabound dolostone geobodies connected by other patchy bodies which replace carbonate units of upper jurassic to lower cretaceous ages these dolostones are associated to the w e trending san jorge fault indicating a main tectonic control for fluid flow seven different dolomite types were identified i matrix replacive planar s red i ii matrix replacive planar e red ii iii planar e sucrosic cement sud iv non planar grey saddle dolomite cement sad i pre dating zn fe pb sulfides v non planar milky to pinkish saddle dolomite cement sad ii post dating zn fe pb ores vi ore replacive planar e porphyrotopic pod and vii planar s cloudy cement ced meteoric calcite types were also recognized the different dolomite types are isotopically characterized by i depleted 18 o from 25 1 to 27 6 v smow and 13 c from 2 3 to 0 9 v pdb values compared to upper jurassic to lower cretaceous limestone signature 18 o 27 6 to 30 9 v smow 13 c 0 5 to 3 2 v pdb and ii 87 sr 86 sr ratios for the main dolomitization phases red and sud 0 70736 0 70773 close to the jurassic and cretaceous carbonate values 0 70723 0 70731 whereas more radiogenic values 0 70741 0 70830 for saddle dolomites sad related to the zn fe pb sulfide mineralization prevailed after fluid interaction with rb bearing minerals microthermometrical studies on two phase liquid and vapor fluid inclusion populations in planar and non planar dolomites and sphalerite show homogenization temperatures between 150 and 250 c these data indicate that both planar and non planar dolomite textures formed at high temperatures under hydrothermal conditions in deep burial diagenetic environments the main dolomitizing phase red i red ii and sad i shows low to moderate fluid inclusions salinity 5 14 wt eq nacl whereas the dolomitization related to ore precipitation sphalerite and sad ii spreads to higher salinity values 5 25 wt eq nacl these data may respond to a mixing between a low salinity fluid fluid a less than 5 wt eq nacl and a more saline brine fluid b more than 25 wt eq nacl at different fluid proportions

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

spatial patterns of diagenesis during geothermal circulation in carbonate platforms geothermal convection of seawater deep in carbonate platforms could provide the necessary supply of magnesium for dolomitization at temperatures high enough to overcome kinetic limitations we used reactive transport simulations to predict the rates and spatial patterns of dolomitization during geothermal convection in a platform that was 40 km across and 2 km thick in the simulations porosity and permeability decrease with depth to account for sediment compaction dolomitization of a platform consisting of medium grained 0 0 5 mm sediments occurred in a broad band ranging from 2 5 km depth near the margin to 1 5 km depth near the platform center the area of dolomitization is deep enough that temperatures exceed 50 c but not so deep that low permeabilities restrict mass transport complete dolomitization in the center of this zone is estimated to require at least 60 my incorporation of permeability contrasts permeable beds and reactive beds focused dolomitization strongly and reduced the estimated time required for dolomitization by as much as 50 percent dolomitization created magnesium depleted calcium rich fluids in less than 10 ky and results support a link between dolomitization and anhydrite precipitation where adequate sulfate is available

δ34s and δ33s records of paleozoic seawater sulfate based on the analysis of carbonate associated sulfate abstract this study updates the 34s and 33s temporal records of paleozoic and early mesozoic seawater sulfate using analysis of carbonate associated sulfate of biogenic and whole rock carbonate the time resolution of carbonate samples studied here is on the order of millions of years which is longer than the timescale for homogenization of seawater sulfate sulfur one tenth of a million years but is comparable to the timescale for isotope evolution of sulfate in the oceans this 34s record confirms the long term decreasing trend that is discernible over the 250 million year timescale of sampling and the 33s record of seawater sulfate for the paleozoic is consistent with an average value of 0 002 0 004 2 540 251 ma that is distinct from the positive 33s observed today and inferred for the rest of the cenozoic both 34s and 33s records of seawater sulfate suggest the presence of shorter timescale variations that occur on timescales of tens of millions of years arguably driven by changes in intensity of sulfide oxidation in cycling of sulfur and or by rapid changes in sulfur influx to the oceans and its associated sulfur isotopes the permian triassic boundary marks a transition in co trajectories of 34s and 33s from in phase to anti phase the biogeochemical forcing that causes this is unclear this newly calibrated record remains consistent with an earlier assertion wu et al 2010 that the sulfur isotope fractionation 34ssw py between oceanic sulfate 34ssw and coeval sedimentary pyrite 34spy was smaller during the paleozoic than in the cenozoic and reached lowest values during the carboniferous the 34sin and 33sin estimates of influx sulfur to the oceans afforded by the isotope mass balance model for each geologic period throughout the paleozoic reveal a change from higher positive values of 34sin more negative values of 33sin approximately 0 030 to lower positive values of 34sin slightly negative values of 33sin approximately 0 003 this change is interpreted to reflect the evolution of the earth surface sulfur pool and the rapid recycling of newly formed evaporite sulfate minerals as arguably supported by our estimates of integrated erosion rates over geologic time for pyrite and evaporite 6 1 6 4 10 9 a 1 and 7 7 7 8 10 9 a 1 respectively

the sulphur isotopic composition of trace sulphates in carboniferous brachiopods implications for coeval seawater correlation with other geochemical cycles and isotope stratigraphy the carboniferous 34s record based on the analysis of structurally substituted sulphate sss in brachiopod shells is characterized by a significant decline from values around 21 1 in the tournaisian to 12 1 in the gzelian superimposed on this broad evolution is a higher order structure with a relative minimum in the late visean followed by a maximum in the bashkirian on the 107 year time scale 34s is positively correlated with 13ccarb in contrast to the long term negative linkage between sulphur and carbon observed for the entire phanerozoic this indicates that mechanisms other than the redox balance control the carbon and sulphur cycles on this time scale the 87sr 86sr and 34s records display parallel trends during the tournaisian to moscovian this positive correlation suggests that tectonics may be of importance for the evolution of 34s on time scales of 107 years a possible scenario to explain the co evolution of the isotope systems and their linkage to tectonism and palaeoclimate invokes an increased nutrient supply to the oceans during periods of enhanced uplift and continental weathering this nutrient flux may have resulted in increased burial of organic matter and enhanced bacterial sulphate reduction

secular variations in seawater chemistry controlling dolomitisation in shallow reflux systems insights from reactive transport modelling dolomitization often plays a critical role in the pore network development of platform carbonates with implications for reservoir quality distribution understanding both the hydrological system driving dolomitization and the chemistry of the fluids involved is fundamental to constrain predictions of the geometry and the petrophysical properties of dolomite bodies here the role of secular variations in seawater mg ca as a control on dolomitization and early porosity modification was evaluated using one dimensional reactive transport models and fluids based on modern aragonite sea mississippian and aptian calcite sea seawaters the sensitivity of dolomitization to a range of extrinsic controls brine salinity temperature fluid flow rate and pco2 and to intrinsic reactivity of the sediments effective reactive surface area was also explored simulations suggest faster calcite replacement by dolomite for seawaters with higher mg ca indicating that dolomitization potential is determined more by mg ca rather than saturation index increasing evaporative concentration enhances reaction rate independent of the effect of enhanced density driven fluid flux in addition to brine composition effective surface area of precursor sediments and temperature exert a critical control on replacement rate while secular variations of ph and carbonate alkalinity associated with changes in pco2 are only secondary controls above flow rates of 0 01 m yr 1 replacive dolomitization is reaction limited rather than flux limited favouring alteration of fine grained carbonates and suggesting that preferential alteration of grainstone units is rare unless head gradients are low post replacement dolomite cementation is flux dependent and thus favoured in areas of high head gradient and high permeability sediments and contrary to replacement supersaturation is a more important driver than mg ca while uncertainties remain regarding low temperature dolomitization kinetics the capability of numerical simulations to decouple individual controls provides new insights which can be used in conjunction with traditional comparative sedimentology to generate more rigorous conceptual models for individual reservoir settings

3d reactive transport modeling of porosity evolution in a carbonate reservoir through dolomitization abstract dolomitization of carbonate reservoirs is considered a major process for porosity enhancement and is therefore of great relevance in hydrocarbon exploration despite the ubiquitous occurrence of massive dolostones the question of whether reasonable amounts of dolomitizing fluid and mg supply are capable of substantial porosity generation remains in this work a model of hydrothermal dolomitization of a carbonate reservoir is assessed by means of 3d reactive transport simulations implemented in icp an interface between comsol multiphysics and phreeqc the model is based on a currently producing carbonate reservoir with large regions of high porosity values deduced through seismic inversion the model geometry and parameterization are based on the present day configuration of the basin using an embedded model approach the geochemical reactions in the reservoir have been coupled to the thermo hydrogeological behavior of the entire basin the results indicate that after 100 kyr dolomitization of limestone by fluids with high mg ca ratio is restricted to areas around faults predicted volumes affected by dolomitization are consistent with dolomite bodies observed in nature however the intensity of dolomitization up to 20 of limestone replacement and the corresponding porosity enhancement up to 3 is not in agreement with the observed porosity values in the studied area even considering uncertainties in the fluid chemistry and in the hydrological conditions numerical predictions suggest that times much longer than 100 kyr of recurrent water rock interaction are needed to account for the generation of massive dolostones formed from limestone replacement however longer periods of time are not consistent with the tectonic evolution of the basin therefore additional or alternative porosity generation mechanisms must be involved such as compaction and late diagenetic corrosion

application of numerical modelling to a case of compaction driven dolomitization a jurassic palaeohigh in the po plain italy dolomitization of a carbonate platform can occur at different times and in different diagenetic environments from synsedimentary to deep burial settings numerical simulations are valuable tools to test and select the model that among different hypotheses compatible with field and geochemical data best honour mass balance kinetic and thermodynamic constraints moreover the simulation can predict the distribution of the dolomitized bodies in the subsurface and evaluate porosity changes valuable information for the oil industry this study is the first attempt to reproduce and investigate the compaction dolomitization model the diagenetic study of the jurassic carbonate basin and palaeohigh system of the po plain indicates that the carbonates of the palaeohighs were dolomitized by basin compaction fluids the main goal of the simulations is to evaluate the origin and evolution of the dolomitizing fluids and to provide insights regarding the distribution of the potential reservoir dolomitized bodies in the po plain the modelling process is subdivided into two steps basin modelling and reactive transport modelling the sebe3 basin simulator eni proprietary was used to create a three dimensional model of the compacting system the results include compaction fluid flow rate from the basin to the palaeohigh compaction duration and a determination of the total amount of fluid introduced into the palaeohigh these data are then used to perform reactive transport modelling with the toughreact code sensitivities on dolomite kinetic parameters suggest that dolomitization was an efficient process even at low temperatures with differences mainly related to the dynamics of the process fluid composition is one of the main constraints the sea water derived compaction fluid is proven to be efficient for dolomitization due to its relatively high mg content simulations also confirmed that permeability is the most important factor influencing fluid flow and consequently the dolomite distribution in the formation permeable fractured zones have a strong influence diverting the dolomitizing fluids from their normal path towards overlying or lateral zones moreover the simulations showed that after dolomite replacement is complete the dolomitizing fluids can precipitate dolomite cement causing over dolomitization with related localized plugging effects in the zone of influx mass balance calculations indicate that in the dolomitization compaction model the amount of compaction water fluxed from the basin to the carbonate is the main constraint on dolomitization efficiency this observation implies that the ratio between the volume of the basin undergoing compaction and the volume of the palaeohigh is a limiting factor on the final size of the dolomitized bodies an isolated palaeohigh could be an ideal site for pervasive replacement dolomitization due to the large volume of compaction fluids available compared with the carbonate rock volume in the case of large platforms the more permeable margin lithofacies are the most likely sites for compaction model dolomitization the combined use of a basin simulator and reactive transport modelling has proved to be a successful method to verify model reliability and it provides insights into the volumetric distribution of diagenetic products

reactive transport modeling an essential tool and a new research approach for the earth sciences abstract reactive transport modeling is an essential tool for the analysis of coupled physical chemical and biological processes in earth systems and has additional potential to better integrate the results from focused fundamental research on earth materials appropriately designed models can describe the interactions of competing processes at a range of spatial and time scales and hence are critical for connecting the advancing capabilities for materials characterization at the atomic scale with the macroscopic behavior of complex earth systems reactive transport modeling has had a significant impact on the treatment of contaminant retardation in the subsurface the description of elemental and nutrient fluxes between major earth reservoirs and in the treatment of deep earth processes such as metamorphism and magma transport active topics of research include the development of pore scale and hybrid or multiple continua models to capture the scale dependence of coupled reactive transport processes frontier research questions that are only now being addressed include the effects of chemical microenvironments coupled thermal mechanical chemical processes controls on mineral fluid reaction rates in natural media and scaling of reactive transport processes from the microscopic to pore to field scale

a simple analytical model of periodic coastal upwelling abstractan existing reduced gravity model that reproduces the response of the coastal sea to alongshore wind forcing at subinertial frequencies is extended by allowing for cross shore wind forcing and by considering superinertial frequencies the obtained explicit solution shows that the wind driven currents are predominantly controlled by friction and the coriolis force at subinertial frequencies and by friction and local acceleration at superinertial frequencies the effect of the coast is manifested by coastal trapped variability at subinertial frequencies and baroclinic inertia gravity waves propagating away from the coast at superinertial frequencies the pycnocline oscillates at the coast not only at subinertial but also at superinertial frequencies with the alongshore wind contributing more to the former and the cross shore wind influencing more the latter the oscillations are most pronounced when the periodic wind forcing is resonantly coupled to the local inertial oscillations but only if the

pre ore burial dolomitization adjacent to the carbonate hosted gays river zn pb deposit nova scotia the gays river formation of the lower windsor group in the vicinity of the gays river zn pb deposit was completely dolomitized via a volume for volume transformation prior to mineralization premineralization porosity was the precursor limestone porosity comprising fenestrae dissolution molds and intercrystalline intraskeletal and growth pores which remained essentially unchanged through dolomitization regionally the dolomites one replacement and one cement overlapped with sediment compaction and dissolution of aragonite the dolomites are characterized by low 18opdb values 14 to 4 average 8 slightly radiogenic 87sr 86sr ratios 0 70778 0 70900 and locally high percentages of feco3 and mnco3 up to 13 5 and 9 5 mol respectively the presented petrographic and geochemical data compiled with existing fluid inclusion microthermometry data suggest dolomitization by hot brines in a burial setting high fe and mn concentrations south of the studied area with progressive depletion toward

the evaporation path of seawater and the coprecipitation of br and k with halite brines and salt were sampled at the morton bahamas solar salt production facility on great inagua island in the bahamas the brines were analyzed by ion chromatography to define more precisely than heretofore the evaporation path of seawater to the end of the halite facies at inagua calcium carbonate begins to precipitate at a brine concentration factor of 1 8 times that of seawater gypsum begins to precipitate at a brine concentration of 3 8 times seawater and halite at a concentration factor of 10 6 three of the most concentrated brines from inagua 40 times seawater were evaporated further in the laboratory magnesium sulfate first precipitated at brine concentrations about 70 times those of seawater and potassium bearing phases began to precipitate for these brines at concentrations greater than 90 times those of seawater the distribution of coefficients of br and k between brines and halite were determined by combining analytical data for the inagua brines with measurements of the br and k content of halites from inagua and of halite which had precipitated from inagua brines during storage the observed average value of dbr is 0 032 in good agreement with some of the previous measurements the measured values of dk are highly variable 0 001 to 0 021 dk for halite precipitated early in the halite facies is in the vicinity of 0 015

fault controlled dolomitization in a rift basin there are numerous examples of fault controlled so called hydrothermal dolomite htd many of which host economic mineral deposits or hydrocarbons but there remains a lack of consensus as to how they form in particular multiple phases of diagenetic overprinting can obscure geochemical fingerprints study of a cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds the hammam faraun fault hff block within the suez rift egypt hosts both massive and stratabound dolostone bodies non fabric selective massive dolostone is limited to the damage zone of the fault while fabric selective stratabound dolostone bodies penetrate nearly 2 km into the footwall oligo miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the hff escape of fluids from the incipient hff into the lower thebes formation led to differential stratabound dolomitization once the hff breached the surface fluid circulation focused along the fault plane to form younger massive dolostone bodies this study provides a snapshot of dolomitization during the earliest phases of extension unobscured by subsequent recrystallization and geochemical modification contrary to many models stratabound dolomitization preceded non stratabound dolomitization fluids were hydrothermal but with little evidence for rapid cooling and brecciation common to many htd bodies these results suggest that many of the features used to interpret and predict the geometry of htd in the subsurface form during later phases of structural deformation perhaps overprinting less structurally complex dolomite bodies

massive hydrothermal dolomites in the southwestern cantabrian zone spain and their relation to the late variscan evolution abstract large scale massive burial dolomitization affected upper carboniferous carbonates of the southwestern variscan cantabrian zone spain replacive and void filling dolomites formed and were post dated by calcite cementation the process of dolomitization was due to the circulation of hypersaline and hydrothermal marine derived brines controlled by rock anisotropies the dolomites probably formed in early permian time during post thrusting orocline formation in an extensional setting lithospheric delamination induced increased heat flow and allowed thermal convection of the fluids the dolomites from the cantabrian zone are excellently exposed and represent an interesting outcrop analogue for hydrocarbon reservoirs elsewhere in comparable geotectonic settings of the european variscan orogen large masses of similar dolomites could have formed if mg rich brines were available

geometrically coherent continuous deformation in the volume surrounding a seismically imaged normal fault array abstract we calculated an apparent dip attribute which was used to ascertain the spatial distribution of fault related continuous deformation the vertical component of displacement calculated from the continuous deformation acts to fill in missing displacement in the fault throw profile this result shows that apparently complex 3d patterns of continuous strain in the volumes surrounding the fault array developed as part of a single geometrically coherent fault array however if this component of continuous deformation was not added to the throw profile the fault array could have been misinterpreted as a series of isolated fault segments with coincidental overlaps this technique permits the analysis of continuous deformation structures which are up to an order of magnitude smaller than previously described in the study area these structures are interpreted as small fault propagation folds forming in a shale dominated cover sequence the fault propagation folds above the upper tip line of the mapped fault array bifurcate upwards from the fault surface into three coherent lobes and resemble secondary fault segments the near constant along strike length of the region of continuous deformation throughout the syn rift sequence implies that the length of the fault array was established at an early stage in its growth prior to the establishment of a seismically visible fault surface

chemical study of tectonically controlled hydrothermal dolomitization an example from the lessini mountains italy in the central southern alps feltre verona italy a 750 m thick interval of jurassic and cretaceous pelagic limestones shows post depositional partial brecciation and dolomitization the overlying 500 m thick tertiary sedimentary sequence is unaffected through the paleogene mostly submarine mafic volcanism has been documented in the area small scale extensional features were observed in the limestone near the contact with the dolomitized breccias their orientations measured in the field correspond to the tectonic framework of the area and give evidence for the contemporaneousness of the volcanic activity and the brecciation the distribution and petrologic characteristics of the basalts combined with the orientation of the extensional features observed in the field allow a paleotectonic reconstruction this tectonic scenario can be viewed as a back arc extension an eastward prolongation of the spreading that divided southern france from the sardinia corsica calabria block generating the ligurian balearic basin in the late oligocene

reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection reactive transport models rtms permit quantitative investigation of diagenesis and its effects on reservoir quality the rtm toughreact is used to investigate diagenesis in an isolated platform driven by geothermal kohout convection of seawater which has been invoked to explain dolomitization during early burial previous short 0 1 m y rtm simulations suggested that convection can drive dolomitization mostly at greater than 50c and anhydritization but complete dolomitization requires greater than 30 60 m y our more extended rtm simulations 30 m y indicate significant nonlinearities in the system consistent with high temperature experiments with parts of the platform completely dolomitized within 10 15 m y as dolomitization proceeds the process becomes predominantly flux controlled with development of a wedge shaped dolomite body which thins from the margin to the interior at considerably shallower depth and cooler temperatures 20 30c than suggested by short simulations dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms sediment permeability and reactive surface area commonly inversely related are key controls dolomitization is limited to the margin of low permeability muddy platforms despite a high reactive surface area dolomitization of more permeable grainy platforms is limited by a lower reactive surface area occurring only in the platform core due to widespread cooling sedimentary layering produces a complex diagenetic stratigraphy dolomitization favoring more reactive beds at shallow depth where permeability is not limiting but switching to more permeable beds at depth bank marginal fracturing limits dolomitization of the platform interior whether the fractures are baffles or conduits for flow

behavior of carbonate associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy abstract although carbonate associated sulfate cas is used widely as a proxy for the sulfur isotope composition of ancient seawater little is known about the effects of diagenesis on retention of primary 34 s signals our case study of the key largo limestone pleistocene florida is the first systematic assessment of the impact of meteoric diagenesis on cas properties geochemical and petrographic data show that meteoric diagenesis has affected the exposed coralline facies to varying degrees yielding differences now expressed as sharp reaction fronts between primary and secondary carbonate minerals within individual coral heads specifically analyses across high resolution transects in the key largo limestone show that concentrations of strontium and sodium decrease across the recrystallization front from original aragonite to meteoric low magnesium calcite by factors of roughly 5 and 10 respectively predictably 18 o values decrease across these same fronts the 13 c relationships are more complex with the most depleted values observed in the latest formed calcite such trends likely reflect carbon isotope buffering capacity that decreased as reaction progressed as well as protracted development of soil profiles and the associated terrestrial biomass and thus depleted 13 c during sea level lowstand conversely 34 s values of cas vary within a narrow buffered range from 20 6 to 22 6 compared to 20 8 22 0 of coeval pleistocene seawater across the same mineralogical transition despite sulfate concentrations that drop in the diagenetic calcite by an average factor of 12 collectively these data point to robust preservation of primary 34 s for carbonates that have experienced intense meteoric diagenesis which is encouraging news for those using the isotopic composition of cas as a paleoceanographic proxy at the same time the vulnerability of cas concentrations to diagenetic resetting is clear

mass dependent fractionation of quadruple stable sulfur isotope system as a new tracer of sulfur biogeochemical cycles abstract sulfur isotope studies of post archean terrestrial materials have focused on the ratio 34s 32s because additional isotopes 33s and 36s were thought to carry little information beyond the well known mass dependent relationship among multiple isotope ratios we report high precision analyses of 33s and 36s values defined as deviations of 33s and 36s from ideal mass dependent relationships for international reference materials and sedimentary sulfides of phanerozoic age by using a fluorination technique with a dual inlet isotope ratio mass spectrometer measured variations in 33s and 36s are explained as resulting from processes involve branching reactions two or more reservoirs formed or mixing irreversible processes in closed systems rayleigh distillation amplify the isotope effect we outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation the isotope systematics discussed here cannot explain the much larger variation of 33s and 36s observed in archean rock records furthermore phanerozoic samples we have studied show a characteristic 33s and 36s relationship that differs from those measured in archean rocks and laboratory photolysis experiments thus high precision analysis of 33s and 36s can be used to distinguish small non zero 33s and 36s produced by mass dependent processes from those produced by mass independent processes in archean rocks and extraterrestrial materials