a foray into false positive results in mineral dissolution and precipitation studies abstract this study presents and discusses several case studies of false positive results in mineral rock dissolution and precipitation studies and the lessons that can be learned from them

a model for fibrous illite nucleation and growth in sandstones we have developed a model for the formation of fibrous illite in sandstones where kaolinite is a primary reactant and potassium is derived from in situ k feldspar grain dissolution or imported into the model reference frame illite fiber nucleation and growth are modeled using arrhenius expressions that consider saturation state in addition to temperature and time nucleation occurs on pore walls and muscovite and detrital illite may be defined as energetically favorable substrates the model is integrated with other touchstone models to account for the influence of other diagenetic processes on surface area and reactant volumes and to provide input for permeability simulations we evaluated the illite model performance on two data sets 1 jurassic quartzose samples from offshore mid norway with maximum temperatures ranging from 108 to 173c 226 to 343f and 2 miocene lithic samples from offshore southeast asia that have maximum temperatures ranging from 157 to 182c 315 to 360f the model matches measured abundances of illite kaolinite and k feldspar in both data sets using identical kinetic parameters predicted k ar ages are consistent with available data given uncertainties associated with detrital contaminants although no illite particle size data are available from the analyzed samples modeled crystallite thicknesses from the norway data set are comparable to published measurements of 0 004 to 0 012 m from north sea samples with similar temperature histories

stable isotope geochemistry and phase equilibria of coesite bearing whiteschists dora maira massif western alps peak metamorphic temperatures for the coesite pyrope bearing whiteschists from the dora maira massif western alps were determined with oxygen isotope thermometry the 18o smow values of the quartz after coesite 18o 8 1 to 8 6 n 6 phengite 6 2 to 6 4 n 3 kyanite 6 1 n 2 garnet 5 5 to 5 8 n 9 ellenbergerite 6 3 n 1 and rutile 3 3 to 3 6 n 3 reflect isotopic equilibrium temperature estimates based on quartz garnet rutile fractionation are 700 750 c minimum pressures are 31 32 kb based on the pressure sensitive reaction pyrope coesite kyanite enstatite in order to stabilize pyrope and coesite by the temperature sensitive dehydration reaction talc kyanite pyrope coesite h2o the a h2o must be reduced to 0 4 0 75 at 700 750 c the reduced a h2o cannot be due to dilution by co2 as pyrope is not stable at x co2 0 02 t 750 c p 30 kb in the absence of a more exotic fluid diluent e g ch4 or n2 a melt phase is required granite solidus temperatures are 680 c 30 kb at a h2o 1 0 and are calculated to be 70 c higher at a h2o 0 7 consistent with this hypothesis kyanite jadeite quartz bands may represent a relict melt phase peak p t f h2o estimates for the whiteschist are 34 2 kb 700 750 c and 0 4 0 75 the oxygen isotope fractionation between quartz 18o 11 6 and garnet 18o 8 7 in the surrounding orthognesiss is identical to that in the coesitebearing unit suggesting that the two units shared a common final metamorphic history hydrogen isotope measurements were made on primary talc and phengite d smow 27 to 32 on secondary talc and chlorite rite after pyrope d 39 to 44 and on the surrounding biotite d 64 and phengite d 44 gneiss all phases appear to be in nearequilibrium the very high d values for the primary hydrous phases is consistent with an initial oceanicderived connate fluid source the fluid source for the retrograde talc chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization the similar d but dissimilar 18o values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium but not oxygen isotope equilibrium the unusual hydrogen and oxygen isotope compositions of the coesite bearing unit can be explained as the result of metasomatism from slab derived fluids at depth

hydrogeochemical characterization of oilfield waters from southeast maracaibo basin venezuela diagenetic effects on chemical and isotopic composition free water and water in emulsion with oil from hydrocarbon wells onshore offshore lake maracaibo were extracted and analyzed the use of classical and new chemical diagrams evidenced that na hco3 and na cl are the main chemical facies whereas na so4 facies was rare and revealed only in faulted areas in which shallow groundwater was involved in deep circulation cretaceous ca cl waters were not revealed but their imprint on some samples was traced and evidenced by langelier ludwig brine differentiation and caexcess vs nadeficit diagrams these square plots allowing a discrimination between the water contributions from the different source rocks should be more exploited as hydrogeochemical tools in hydrocarbon exploration original data on monocarboxylic anions showed acetate concentration higher than ten times that of formate at sampling conditions but speciation under deep conditions indicated a reversed activity dominance calculation of saturation indexes and kinetic modeling showed an active precipitation of chalcedony kaolinite and clays especially where the temperature was lower t 100 c formation waters and lake water showed quite similar chloride concentration and hydrogen isotope composition however thermodynamic calculations at the oilfield p t conditions revealed chlorite and illite produced during previous diagenetic stages of misoa sandstones recrystallization to smectite and vermiculite in present day stage the diagenetic water produced in these reactions could have enriched the 18o isotopic composition of approximately 2 this isotope diagenetic effect was more evident in na hco3 waters which also showed a decrease in chloride at constant boron concentration as a consequence of clay dewatering in this regard a new boron geothermometer useful for formation waters is also proposed t c 5 1364 ln 1 b mg 1 5 728 0 180 0 164 0 012

evidence for deep anaerobic biodegradation associated with rapid sedimentation and burial in the beaufort mackenzie basin canada abstract formation waters of the 14 km thick late cretaceous cenozoic beaufort mackenzie basin were examined as part of a larger project to better understand the petroleum potential of the region where early exploration defined petroleum reserves of 744 109 bbls recoverable crude oil and 11 74 tcf gas historical water analyses 2583 samples from 250 wells drilled up to 5 km depth were compiled and culled to remove incomplete and poor quality samples the resultant database shows a broad range of salinity and water chemistry that has no systematic relationship with depth three main water types are defined paleo seawater and freshwaters related to a miocene age gravity driven flow system and low tds high alkalinity waters high alkalinity waters are isolated in overpressured fault blocks that were rapidly buried by post miocene iperk shale deposition the high alkalinities up to 9000 mg l are interpreted to be related to in situ co2 generation through anaerobic methanogenesis in response to freshwater invasion the dominant control on biogenic gas generation appears to be maximum burial temperature rather than the modern temperature distribution this is consistent with the paleopasteurization model that suggests once critical burial temperatures are reached sterilized rocks are inhibited from further biodegradation even when temperatures subsequently drop back into the habitable zone

mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon reservoirs abstract secondary porosity created by the post depositional leaching of detrital grains and cements can be of great economic importance the term secondary porosity is only descriptive and does not give any information on whether the porosity of the rock has been enhanced in reality there are two end member volumetric outcomes to the leaching process firstly the mineral can be completely dissolved and its components transported out of the system in such an instance the porosity of the rock will be enhanced secondly it is possible that a mineral will decompose and by product minerals will be precipitated which will occupy approximately the same volume as the original mineral the latter case is more likely where the pore waters are close to equilibrium with the majority of the minerals of the rock this will occur at elevated temperatures and or where the pore waters have been in contact with the rock for a long time these conditions are commonly met during intermediate to deep burial many metastable minerals such as high temperature feldspars probably decompose in this way a consequence of the decomposition of such metastable minerals is that primary porosity will be swapped for secondary and that older and more deeply buried rocks will contain a higher proportion of secondary porosity the mass transport equation may be used to examine the physico chemical environment where enhanced porosities most likely occur during leaching by using a simple non equilibrium mass transport model based on the silica system it is possible to demonstrate the relative importance of flow rate reaction rates specific surfaces and temperature in controlling the areal extent of leaching wide leaching zones are related to high flow rates which in turn promote a high throughput of undersaturated fluid and dispersion leaching over a wide area is also favoured by low reaction rates which will occur at low temperatures although it is true to say that less of a given silicate mineral can be dissolved by an undersaturated pore water at low temperature than at high temperature it is probable that pore fluids from deeply buried sediments are more likely to be close to equilibrium with a given mineral than for instance meteoric water thus the most likely environment for the production of widespread enhanced porosity is in the shallow subsurface the lower flow rates and higher temperatures that are encountered during deep burial diagenesis will result in far narrower more intense leaching zones as a result providing sufficient reactive mineral is present any undersaturated pore water will approach equilibrium in a short distance in order to transport undersaturated pore waters appreciable distances in the deep subsurface it is necessary to minimize their interaction with the host rock this could occur if the pore fluid were to move through a fracture system where it would interact with a far smaller surface area of reactive mineral than if it moved through the pore network

simulating the impact of glaciations on continental groundwater flow systems 1 relevant processes and model formulation 1 in the recent literature it has been shown that pleistocene glaciations had a large impact on north american regional groundwater flow systems because of the myriad of complex processes and large spatial scales involved during periods of glaciation numerical models have become powerful tools to examine how ice sheets control subsurface flow systems in this paper the key processes that must be represented in a continental scale 3 d numerical model of groundwater flow during a glaciation are reviewed including subglacial infiltration density dependent i e high salinity groundwater flow permafrost evolution isostasy sea level changes and ice sheet loading one dimensional hydromechanical coupling associated with ice loading and brine generation were included in the numerical model hydrogeosphere and tested against newly developed exact analytical solutions to verify their implementation other processes such as subglacial infiltration permafrost evolution and isostasy were explicitly added to hydrogeosphere a specified flux constrained by the ice sheet thickness was found to be the most appropriate boundary condition in the subglacial environment for the permafrost frozen and unfrozen elements can be selected at every time step with specified hydraulic conductivities for the isostatic adjustment the elevations of all the grid nodes in each vertical grid column below the ice sheet are adjusted uniformly to account for the earth s crust depression and rebound in a companion paper the model is applied to the wisconsinian glaciation over the canadian landscape in order to illustrate the concepts developed in this paper and to better understand the impact of glaciation on 3 d continental groundwater flow systems

fault reactivation and structural inversion backarc and intraplate compressive deformations example of the eastern sunda shelf indonesia the tectonic evolution of the eastern sunda platform in indonesia is revealed by the study of seismic profiles and comprises three general tectonic periods during the cenozoic extension with graben infill from the paleogene to the early miocene a tectonically quiet period with only broad subsidence and a middle miocene recent period of folding inversion and thrusting these structures reveal changes in the intraplate stress field which was controlled by the changes in the boundary conditions of the plates collisions on the northern and eastern sides and interplate coupling southward along the java subduction zone inversions often occurred without reactivation of pre existing normal fault planes at the upper levels of these faults the factors controlling these deformations are discussed with special interest placed on the conditions for the potential reactivation of cohesionless normal faults inversion and reactivations were mostly controlled by the orientation of the pre existing fault planes in relation with the regional stress field and by the tectonic stress regime in a strike slip regime effective principal stresses 1 and 3 horizontal with 1 2 3 the optimum conditions for reactivation in isotropic rocks are pre existing faults that strike close to the potential two vertical conjugate failure planes the same mechanical properties as for pre existing fault prevail in such conditions strike slip movements and reverse strike slip movements and associated en echelon folds were generally observed in a compressive regime 1 and 2 horizontal with 1 2 3 typical steeply dipping normal faults are difficult to reactivate by slip movements we suggest that reverse slip movements occurred in the deeper levels of faults and were accommodated by flexure above the pre existing fault planes and the inversion of grabens the strongly inverted grabens are those whose orientation is perpendicular to the regional compression nevertheless with strong regional shortening large uplifts and thrust faults were observed with reactivation in the form of reverse or reverse strike slip movements along pre existing steeply dipping normal fault planes

chemistry of oil field brines in relation to diagenesis of reservoirs 1 use of mineral stability fields to reconstruct in situ water composition example of the mahakam basin abstract during mineral diagenesis the chemical composition of pore water can be strongly influenced by mineral reactions especially under geological conditions where water rock ratios are low to model and eventually to predict diagenesis it is important to have an understanding of formation water composition however available water analyses in many basins are not complete enough for this purpose in most cases reliable values for ph dissolved silica and aluminium are lacking such data can be reconstructed using various relations developed herein these data are constrained by petrographic observations because of their well preserved or authigenic character some alumino silicates are considered as relatively stable and can be thought of as effectively saturated with respect to them the saturation state of the other minerals of the matrix can be discussed according to these constraints the sub arkosic series of the mahakam basin resevoirs mio pliocene are used as a reference example to perfect a method capable of reconstructing consistent compositions of diagenetic waters suitable for input into diagenetic modelling

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

the effect of fluid and deformation on zoning and inclusion patterns in poly metamorphic garnets within the bergen arcs of w norway caledonian eclogite facies assemblages t 650 c p 15 kbar have formed from grenvillian granulites t 800 900 c p 10 kbar along shear zones and fluid pathways garnets in the granulites grti pyr56 40 alm45 25gro19 14 are unzoned or display a weak ca 1 wt feo over 1000 m zoning the eclogite facies rocks contain garnets inherited from their granulite facies protoliths these relict garnets have certain areas with compositions identical to the garnets in their nearby granulite but can be crosscut by bands of a more almrich composition grtii pyr31 41alm40 47gro17 21 formed during the eclogite facies event these bands orientated preferentially parallel or perpendicular to the eclogite foliation may contain mineral filled veins or trails of eclogite facies minerals omphacite amphibole white mica kyanite quartz and dolomite steep compositional gradients up to 9 wt feo over 40 m separate the two generations of garnets indicating limited volume diffusion the bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature controlled volume diffusion at mineral interfaces in the granulites the re equilibration of granulite facies garnets during the eclogite facies event must therefore be a function of fracture density deformation and fluid availability the results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re equilibration of garnets

regional trends in water chemistry smackover formation southwest arkansas geochemical and physical controls we present results on regional geochemical analyses of formation waters from jurassic carbonate strata of the upper smackover formation from the southwest arkansas shelf gulf coast basin these formation waters have been noted in earlier studies for their exceptional salinities and abnormally br rich composition our detailed and regionally extensive database shows that chemical heterogeneities occur on a relatively small scale and have spatial relations to faults which may act as pathways for fluid migration from deeper and hotter portions of the stratigraphic section one of the most pronounced heterogeneities in water chemistry is an h 2 s rich anomaly in the center of the shelf enrichments in h 2 s are accompanied by large increases in alkali elements li k and b and b which could be produced by higher temperature diagenesis of clay minerals and feldspars in deeper seated siliciclastic strata bromide enrichments in all smackover formation waters are apparent from relations between br and cl contents as well as br and divalent cation carpenter function values as suggested by earlier studies of these waters evaporatively concentrated seawater expelled from associated thick monomineralic evaporite sections may be an important source of br cl and na whereas ca mg and sr concentrations have been modified by carbonate mineral recrystallization and dolomitization mineral equilibria exert significant controls on concentrations of ca mg sr and so 4 in these brines br enrichment likely was produced by halite recrystallization in fluids already enriched in br from seawater evaporation elevated cl br ratios occur in regional association with lower salinity water near t e south arkansas fault system consistent with halite dissolution during meteoric water recharge thus despite relatively uniform reservoir lithology and simple structural setting processes on small spatial scales have resulted in significant heterogeneity in water chemistry within a small area of the gulf coast sedimentary basin although movement of fluids along these fault systems is not demonstrated in our study the regional trends in geochemistry are strongly suggestive of their important role in controlling chemical heterogeneities in smackover formation waters

sandstone diagenesis in the pattani basin gulf of thailand history of water rock interaction and comparison with the gulf of mexico abstract in the pattani basin a failed rift basin extensive water rock interaction has occurred between subquartzose alluvial sandstones of miocene age and their pore fluids diagenetic rates and pathways have been strongly influenced by high geothermal gradients high co 2 fugacities and low pore water salinities depositional pore water was fresh to brackish depending on the depositional environment of the sediments chloride concentrations in modern formation water are believed primarily to reflect the proportions of river and sea water in the depositional environment however the concentration of other important solutes and the isotopic composition of the formation waters can not be explained by roportional mixing of these two end member waters dissolution of detrital plagioclase an 3 and k feldspar are reactions of major significance that are reflected chemically in the na cl and k cl ratios of the formation water despite the high temperature of the sandstones 120 200 c diagenetic albite does not occur geochemical calculations indicate the formation water is undersaturated with respect to both orthoclase and albite this style of feldspar diagenesis differs significantly from that of sandstones of similar composition in other basins and has probably influenced other aspects of silicate diagenesis important authigenic minerals are 1 locally abundant calcite cement 13 c 12 8 18 o 17 3 pdb an early diagenetic phase that formed at about 60 c 2 pore filling kaolinite 18 o 9 9 d 83 5smow that was closely associated with feldspar dissolution and formed over a range of temperatures and 3 fibrous pore lining and pore bridging illite 18 o 9 8 d 86 7 smow the last significant cement formed at temperatures of 120 to 150 c potassium argon dates on illite indicate that sandstone diagenesis took place during a period of rapid sedimentation in the first two thirds of the burial history comparison of pattani basin diagenesis with diagenesis of sandstones of similar age in other sedimentary basins demonstrates that chemical diagenesis relative to mechanical compaction has been especially rapid in the pattani basin this reflects the effect of high temperatures on reaction rates the net effect is a high average rate of porosity loss with burial 11 km

carbonate equilibria and ph buffering by organic acids response to changes in pco2 abstract organic acid anions such as acetate a natural product of the thermal breakdown of kerogen may buffer the ph of natural formation waters the extent of ph buffering by acetate should determine whether an increase in the partial pressure of carbon dioxide p co 2 will lead to calcite dissolution porosity enhancement or calcite precipitation porosity destruction successful prediction of porosity therefore requires a quantitative understanding of this process this paper presents computer simulations of the response of the acetic acid carbonate system to changes in p co 2 for typical reservoir fluid compositions and temperatures the effects of variations in temperature ionic strength initial ca 2 concentration and the magnitude of the change in p co 2 were investigated natural formation waters from cenozoic reservoirs in the gulf coast basin and california show maximum organic acid concentrations at 100 20 c at 100 c the simulations show that for concentrations of acetic acid up to 0 06 m 3600 mg l 1 increases in the p co 2 of initially calcite saturated solutions will promote calcite undersaturation at higher temperatures equivalent buffering responses occur at lower acetic acid concentrations but acetic acid concentrations in natural formation waters also decrease rapidly with increasing temperature since present day concentrations of acetic acid in formation waters from cenozoic reservoirs rarely exceed 0 06 m increases in p co 2 will generally promote calcite dissolution by these waters in the gulf coast basin variation in organic acid concentrations as a function of reservoir age suggests that reductions in dissolved organic acid concentrations over geologic time have been relatively small it can be concluded therefore that at any point in the histories of these reservoirs an increase in p co 2 would have promoted calcite dissolution despite the effects of ph buffering by acetate since increases in p co 2 will in most cenozoic basins promote calcite dissolution knowledge of the location and timing of co 2 production in the subsurface can more easily be applied to the prediction of calcite cementation and dissolution

appraisal of the use of experimental and analogue studies in the assessment of the role of organic acid anions in diagenesis abstract to fully understand and predict the processes which act to produce the diagenetic phenomena of porosity enhancement or reduction observed within sandstone reservoir rocks it is necessary to understand the role of formation waters in mineral dissolution and precipitation reactions of particular interest are the mechanisms of feldspar and clay mineral dissolution precipitation which involve the transport of al and si in solution it has been suggested that organic acid anions known to be present within formation waters may enhance aluminosilicate mineral solubility by complexing with al and si reducing the volumes of water required for transport to take place however the possible significance of organic acid anion complexing still cannot be fully assessed because of limited experimental evidence and poor knowledge of the stability of dissolved organic species under appropriate pressure temperature conditions a series of experiments was undertaken to investigate the possible role of acetate oxalate and citrate representing potential complexing agents present within formation waters in diagenetic processes involving feldspar and quartz dissolution and clay precipitation and to determine solute yields from organic rich mudrocks at 150 c 50 mpa albite solubility is enhanced to the greatest extent in the presence of citrate whereas acetate enhances the rate of albite dissolution however in experiments with pb rich alkali feldspar the rapid decay of acetate reduces its complexing potential and hence its availability for the transport of metals artificial maturation of organic rich mudrocks under conditions appropriate for diagenesis yields a mixture of organic acid anions including acetate oxalate and citrate but there is no evidence for solute yields significantly in excess of those predicted for mineralogical buffers of fluid composition in the absence of organic species domestic waste disposal sites are considered as potential analogues to diagenetic systems the associated chemical processes show close parallels between the chemical maturation of landfill leachates and the development of sediment pore waters

physical and chemical controls on the composition of waters in sedimentary basins abstract the composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion the south west louisiana gulf coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section the simultaneous systematic variation in dissolved na k mg ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate carbonate mineral assemblages is a first order control on fluid compositions the chemical potential of chloride or alternatively total anionic charge appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region this variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper mudstone dominated sediments where the composition of the fluid is largely rock buffered the ultimate origin of the fluid and its pathway of chemical evolution may be obscured at least in terms of major solute composition non buffered components such as cl and br or isotopic compositions are more likely to retain information on original end member fluid compositions and reaction pathways

correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium the mol co2 of gas reservoired in sandstones tends to increase with increasing temperature along both the us gulf coast and the norwegian continental shelf each of these provinces displays identical linear correlations between temperature and the log of the partial pressure of co2 i e pco2 in the reservoir fluid that is the product of the mol co2 the reservoir pressure this correlation is the same both in gulf coast reservoirs younger than 17 ma and in norwegian reservoirs of jurassic to triassic age 144 245 ma indicating that it is not dependent on reaction kinetics norwegian shelf data indicate that the same correlation also applies for gas condensate and oil reservoirs we suggest that the log pco2versus temperature correlation is the result of inorganic chemical equilibria between feldspar clay and carbonate minerals a trend of decreasing ph with increasing temperature is fixed by equilibria between aluminosilicate minerals feldspar kaolinite at lower temperatures followed by various reactions between kaolinite illite chlorite and feldspar as temperatures exceed 110 140 c responding to the ph set by the aluminosilicate system co2 activity in the formation water and thereby co2 partial pressure in any gas phase present is controlled by equilibrium with carbonate minerals this model predicts that co2 released by organic maturation will in general result in precipitation of carbonate minerals below 100 120 c and that there should be widespread carbonate dissolution at higher temperatures

garnet omphacite phengite thermobarometry of eclogites from the coesite bearing unit of the southern dora maira massif western alps abstract using relevant geothermobarometric methods p t data were collected for the reconstruction of the metamorphic evolution of 34 eclogite samples taken from small lenses and boudins within the ultrahigh pressure uhp metamorphic coesite bearing brossasco isasca unit biu of the dora maira massif the mineral phases used clinopyroxene garnet phengite or growth zones thereof were identified as being coexistent for different stages of metamorphism on the basis of careful petrographic studies of several published geothermobarometers the garnet clinopyroxene thermometer of powell powell r 1985 regression diagnostics and robust regression in geothermometer geobarometer calibration the garnet clinopyroxene geothermometer revisited j metamorph geol 3 pp 231 243 combined with the garnet clinopyroxene phengite barometer after waters and martin waters d martin h n 1993 the garnet clinopyroxene phengite barometer terra abstr 5 pp 410 411 was chosen here because it provided the most reliable results nevertheless the scatter of p t data points for the prograde stage i peak metamorphic stage ii and retrograde stage iii development of the eclogites is still considerable among the many possible reasons for this inconsistency discussed a partial lack of equilibration of some of the eclogites during their metamorphic history should be taken into account despite the data scatter an average p t path could be estimated which includes the following coordinates for stage i 15 kbar 500 c 25 kbar 570 c 32 kbar 650 c for stage ii 36 kbar 720 c and for stage iii 24 kbar 680 c and 14 kbar 620 c this is in fair agreement with p t paths derived earlier for other rock types of the biu on the basis of other geothermobarometers

formation water geochemistry of the sverdrup basin implications for hydrocarbon development in the high arctic abstract the sverdrup basin holds significant reserves of discovered and undiscovered petroleum that have not been produced to date given its remote arctic setting loss of perennial sea ice increases the prospects for producing reserves from this region potential renewed drilling and production requires knowledge of formation water geochemistry to aid both exploration as well as regulatory development historic data shows formation waters typically have a seawater origin although there is high variability in formation water geochemistry salt diapirs lead to localized brine generation with salinities over an order of magnitude higher than average for the basin evidence also shows fresh water influx occurred along the basin margins that initiated biogenic gas generation

petrology of very high pressure eclogitic rocks from the brossasco isasca complex dora maira massif italian western alps petrographical and mineral chemical data are given for the eclogites which occur in the garnet kyanite micaschists of the penninic dora maira massif between brossasco isasca and martiniana italian western alps and for a sodic whiteschist associated with the pyrope coesite whiteschists of martiniana the brossasco isasca bi eclogites are fine grained foliated and often mica rich rocks with a strong preferred orientation of omphacite crystals and white micas porphyroblasts of hornblende are common in some varieties whilst zoisite and kyanite occur occasionally in pale green varieties associated with leucocratic layers with quartz jadeite and garnet these features differentiate the bi eclogites from the eclogites that occur in other continental units of the western alps which all belong to type c garnet sodic pyroxene and glaucophane are the major minerals in the sodic whiteschist sodic pyroxene in the eclogites is an omphacite often close to jd50di50 with very little acmite and virtually no aliv and impure jadeite in the leucocratic layers and in the sodic whiteschist garnet is almandine with 20 30 mol for each of the pyrope and grossular components in the eclogites and a pyrope rich variety in the sodic whiteschist white mica is a variably substituted phengite and paragonite apparently only occurs as a replacement product of kyanite amphibole is hornblende in the eclogites but the most magnesian glaucophane yet described in the sodic whiteschist quartz pseudomorphs of coesite were found occasionally in a few pyroxenes and garnets the p t conditions during the vhp event are constrained in the eclogites by reactions which define a field ranging from 27 28 kbar to 35 kbar and from 680 to 750 c these temperatures are consistent with the results of garnet pyroxene and garnet phengite geothermometry which suggest that the eclogites may have equilibrated at around 700 c in the sodic whiteschist pressures ranging from 29 to 35 kbar can be deduced from the stability of the jadeite pyrope garnet glaucophane compatibility as in the eclogites water activity must have been low such conditions are close to the p t values estimated for the early alpine recrystallization of the pyrope coesite rock and like petrographical and mineralogical features set aside the bi eclogites from the other eclogites of the western alps instead indicating a close similarity to some of the eclogite bodies occurring in the adula nappe of the central alps an important corollary is that glaucophane stability at least in na and mg rich compositions and under very high pressures may extend up to 700 c in agreement with the ht stability limit suggested by experimental studies

femg partitioning between coexisting garnet and phengite at high pressure and comments on a garnet phengite geothermometer abstract the fe mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite mg 67 in the presence of quartz and water at pressures of 20 35 kb and temperatures of 800 1000 c compositions of coexisting garnet and mica indicate a linear relation between both the inkd fe mg garnet fe mg phengite and temperature and inkd and pressure in the above p t range this fe mg exchange reaction between garnet and phengite is shown to be dependent on the ca content of the garnet and on the mg number of the bulk composition these two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46 and by using a synthetic basaltic composition the overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems respectively however because of non ideality in the exchange reaction this geothermometer should not be used in any practical application outside the composition ranges studied also careful consideration of the presence of fe3 in phengite must be made if the fe3 content of the natural phengite is unknown then the temperatures obtained will be maximum temperatures only

an isochemical model for formation of authigenic kaolinite k feldspar and illite in sediments abstract although thermodynamic calculations indicate a narrow range of conditions for coprecipitation of kaolinite and k feldspar coexistence of authigenic kaolinite and k feldspar is common in jurassic sandstones of the north sea this paper shows that the coexistence of authigenic k feldspar and kaolinite may indicate that this narrow range of conditions did prevail and that the system behaved as a closed system these conditions are presence of muscovite in the sediment pore waters supersaturated with respect to quartz and temperatures lower than 50 c at higher temperatures the authigenic kaolinite and authigenic k feldspar may react and form authigenic illite muscovite and quartz cement the direction of the isochemical reaction is thermodynamically determined by the degree of supersaturation with respect to quartz in the formation water published data on the degree of supersaturation with respect to quartz in formation waters suggest that the illitization of kaolinite may take place at temperatures as low as 50 c much lower than previously believed

thermobarometry of phengite bearing eclogites in the dabie mountains of central china pressure temperature conditions for formation of the peak metamorphic mineral assemblages in phengite bearing eclogites from dabieshan have been assessed through a consideration of fe2 mg2 partitioning between garnet omphacite and garnet phengite pairs and of the reaction equilibrium celadonite pyrope grossular muscovite diopside which incorporates an evaluation of the extent of the strongly pressure dependent inverse tschermak s molecule substitution in the phengites for the latter equilibrium the calibration and recommended activity composition models indicated by waters martin 1993 have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others confirmation that in some phengite eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite and in some cases even diamond rather negates previous suggestions that coesite may have been stabilized in only very localized possibly just intracrystalline domains inherent difficulties in the evaluation of peak metamorphic temperatures from fe2 mg2 partitioning between mineral phases due to uncertainties over fe3 fe2 ratios in the minerals especially omphacites and to re equilibration during extensive retrograde overprinting in some samples are also assessed and discussed our results indicate the existence in south central dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes thus our data do not support earlier contentions that south central dabieshan comprises a structurally coherent continental crust terrane with a regional p t gradient signalling previous deepest level subduction in the north instead we recognize the central dabie ultra high pressure coesite eclogite bearing terrane to be structurally overlain by a southern dabie high pressure quartz eclogite bearing terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section

geochemical constraints from formation water analyses from the north sea and the gulf coast basins on quartz feldspar and illite precipitation in reservoir rocks

chemistry of oil field brines in relation to diagenesis of reservoirs 2 reconstruction of palaeo water composition for modelling illite diagenesis in the greater alwyn area north sea abstract input data for water composition are required in order to numerically simulate diagenetic episodes in rock dominated closed system modelling the composition of water initially present in the pores has at most only a minor influence on the stability of the system and is controlled by the mineral composition in open system modelling the composition of infiltrating water can play an important role brent water salinities of the greater alwyn area exhibit a regional trend from the north to the south the k cl ratio of these waters varies along two trends it is higher in the north than in the south alkalinity and published pco2 values are used to calculate ph under subsurface conditions very few silica measurements are available aluminium analyses of pore water are not available some reservoirs of the greater alwyn area experienced a major reduction in permeability due to illite and quartz precipitation coeval with kaolinite and k feldspar dissolution saturation of present day brent waters with respect to these minerals is examined the method proposed in part 1 is applied to reconstruct a range of possible compositions for ancient waters in brent reservoirs between several extreme conditions one of these extremes is provided by buffering water with a set of minerals chosen from petrographic considerations the other extremes correspond to reasonable states of over saturation with respect to particular minerals e g quartz and concern the values of dissolved si and al which are the least well characterised concentrations from the routine analyses of formation waters

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

pb sr nd isotopic behavior of deeply subducted crustal rocks from the dora maira massif western alps italy ii what is the age of the ultrahigh pressure metamorphism pb nd and sr isotope data are reported from two localities on mineral separates from mg rich metapelites and associated rocks that have been subducted to depths of at least 100 km for which metamorphic conditions are estimated at 28 33 kilobars pressure and 700 800 c and then returned to the surface initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites the initial ratios indicate predominantly recycled aged granitic crustal materials for the sources of all of the samples five zircon samples 4 from pyrope megacrysts and 1 from fine grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304 10 and 38 0 1 4 ma in a concordia diagram zircon from the country rock also plots along the chord the zircon data together with initial nd and sr data indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic hercynian rocks ellenbergerite from pyrope megacrysts and monazite from the fine grained ground mass yield slightly younger ages of 30 34 ma apparently reflecting lower blocking temperatures than that of zircon sm nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 ma in agreement with the zircon date a major question concerns the timing of the ultrahigh pressure metamorphism experimental data suggest that pyrope and quartz coesite as well as ellenbergerite formed by various metamorphic reactions if as generally assumed the ultrahigh pressure metamorphism occurred ca 100 ma ago our data require that the zircon did not experience measurable lead loss at that time but lost major amounts of lead 38 ma ago during late alpine metamorphism estimates of diffusion rates for nd in pyrope further suggest that the apparent sm nd age of 38 ma for the megacryst is not consistent with that model those problems are resolved if the ultrahigh pressure metamorphism occurred 38 40 ma ago but problems remain from ar ar dates of 100 ma on phengite an inferred 120 ma age for zircon lead loss from another study and possibly by the very rapid uplift required if the metamorphism is that young

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

coesite in the straumen kyanite eclogite pod norway the occurrence of coesite in the straumen kyaniteeclogite pod in the western gneiss region wgr norway is described although the microtextures of coesite and of its partial or complete transformation into quartz are similar to those described at the first coesite occurrence in norway the grytting dolomite eclogite pod there are some significant differences at this second locality these include the host mineral of the sio2 inclusions being garnet instead of clinopyroxene and also occasionally kyanite an initial coesite eclogite sub facies paragenesis was followed by a quartz eclogite sub facies paragenesis which in addition to rutile and dolomite or calcite contains the highest number 7 of silicate phases known in any norwegian eclogite garnet clinopyroxene kyanite quartz phengite zoisite and clinoamphibole this second occurrence of definite coesite i consolidates the very high pressury estimates about 30 kbar minimum previously deduced for certain eclogite pods occurring in the wgr and ii justifies the recognition of the norwegian coesite eclogite province constituted of a nappe terrane of about 30 kbar parageneses now residing in the wgr