precambrian gneiss and late proterozoic sedimentation in north central british columbia u pb analyses of zircons from gneissic granite in the sifton and deserters ranges flanking the northern rocky mountain trench in north central british columbia indicate crystallization ages of 1 85 b y and 728 m y respectively augen gneiss in the sifton ranges appears to be basement rock associated with a sequence of amphibolite quartzite schist and carbonate thrust southerly over upper proterozoic strata of the ingenika group gneissic granite in the deserters range is nonconformably overlain by pebble conglomerate and pure quartzite in turn overlain by a thick sequence of amphibolite schist phyllite gritty clastic rocks carbonate quartzite and diamictite assigned to the upper proterozoic misinchinka group the deserters data establish a probable maximum age for rocks of the windermere supergroup in the northern rocky mountains the age and structural setting of these gneisses are similar to other gneisses of the omineca crystalline belt of the canadian cordillera

dolostones from grand cayman british west indies abstract pervasively dolomitized tertiary strata on grand cayman are dominated by heterogeneous dolostones that are composed of both low ca calcium dolomite lcd 55 mol caco3 homogeneous dolostones formed of either lcd or hcd are rare some dolostones contain dolomite crystals that are characterized by oscillatory zones 1 10 m thick of lcd and hcd whereas others have rims of lcd surrounding cores of hcd in some dolostones bioclasts formed of lcd are embedded in a matrix of hcd an 1 18o difference between the lcd and hcd can be attributed to the natural covariance that exists between the 18o and mol caco3 in dolomite there are no significant differences in the 87sr 86sr ratios of the lcd and hcd there is no consistent vertical or lateral pattern to the distribution of lcd and hcd in the cayman dolostones even in closely spaced sampling sites such variability is more consistent with growth of individual dolomite crystals being controlled by processes that operated around the surfaces of individual crystals rather than by large scale changes in the dolomitizing fluids this interpretation is consistent with the development of oscillatory zoning which is attributed to precipitation of different phases or minerals from the same parent fluid because of competition between interface controlled and diffusion controlled growth kinetics dolomitization of the tertiary strata on grand cayman was probably mediated by seawater or modified seawater nevertheless small scale intrinsic factors controlled the incorporation of ca in the dolomite lattice and accordingly the geochemical parameters that are intimately linked to the mol caco3 these small scale intrinsic controls must be factored into any model to explain the formation of tertiary island dolostones

temperature limits for preservation of primary calcite clumped isotope paleotemperatures abstract solid state reordering of c o bonds in the calcite lattice can alter the clumped isotope composition of paleotemperature archives such as fossil brachiopod shells without inducing significant changes in shell microstructure and trace element concentrations metrics commonly used to gauge preservation quality to correctly interpret the paleoenvironmental significance of clumped isotope derived paleotemperatures it is necessary to understand the temperature time domain in which solid state c o bond reordering is important we address this question using a combination of laboratory and natural geological experiments on paleozoic brachiopod shells the laboratory experiments involve heating fossil brachiopod calcite at different temperatures and times to directly observe rates of 13 c 18 o bond reordering the resulting arrhenius parameters are indistinguishable from values previously determined for an optical calcite with similar trace element compositions we develop an alternative kinetic model for reordering that accounts for non first order reaction progress observed during the initial several hundred minutes of laboratory heating experiments and show that the simplified first order approximation model passey and henkes 2012 predicts reaction progress equally well for temperatures and timescales relevant to sedimentary basins we evaluate our laboratory based rate predictions by studying brachiopod calcite from several sedimentary basins with independently constrained burial temperature histories specifically we use the laboratory derived arrhenius parameters to predict the evolution of brachiopod calcite clumped isotope compositions during successive one million year time steps reflecting the burial and exhumation temperature paths of each basin while this exercise is limited by the relatively large uncertainties in the temperature histories of these basins we find general correspondence within error between predicted and observed clumped isotope values we present simplified temperature time diagrams for calcite showing domains where primary clumped isotope compositions will be preserved partially reordered and fully reordered in conclusion calcite samples dwelling at 100 c or lower for 10 6 10 8 year timescales should not be affected by solid state c o bond reordering

evaluating formation fluid models and calibrations using clumped isotope paleothermometry on bahamian dolomites abstract the use of stable oxygen isotopes to understand the mechanisms of dolomite formation has been hampered by the inability to precipitate well ordered dolomite under normal earth surface conditions several studies have attempted to address this problem either by precipitating high temperature ordered dolomites and extrapolating the data to low temperatures or by using more disordered very high mg calcites as a proxy for low temperature dolomites the result is eight equations that disagree significantly from each other by as much as 3 6 in the 18o value of the precipitating fluid at 25 c and produce differences which can greatly affect the interpretation of the formation mechanisms for low temperature dolomites however by combining the recently developed clumped isotope paleothermometer an independent isotopic measurement 47 that directly relates to the temperature of formation to late miocene to pleistocene aged dolomites from the bahamas with a well constrained thermal and fluid history we have attempted to narrow down the viable equations used to interpret the 18o values of dolomites the clumped isotope temperatures measured on the bahamian dolomites 16 37 c agrees with the range of temperatures expected in the bahamas pairing these temperatures with geological and mineralogical arguments we favor the equation suggested by matthews and katz 1977 as it is the only one that produces realistic 18o fluid values across the range of clumped isotope temperatures both the clumped isotope temperatures and 18o values of the precipitating fluid show a strong positive covariance that we have interpreted as reflecting the mixing of surface brines that have undergone varying amounts of evaporation and normal seawater the different mechanisms driving these fluids included formation by normal marine seawater driven by the compensatory flow of the mixing zone bank wide kohout convection and evaporative brine reflux

2 2 ga age of zircons in three occurrences of upper proterozoic clastic rocks of the northern cassiar terrane yukon and british columbia analyses of detrital zircons from three occurrences of upper proterozoic clastic rocks of the northern cassiar terrane in yukon and northern british columbia yield a u pb age of 2224 22 ma the zircons apparently belong to a single population similar in age to zircons in stratigraphically equivalent rocks of the southern cassiar terrane and to zircons in rocks in the yukon tanana terrane of alaska a source terrane or area of the required age and extent has not yet been identified

time capsule concretions unlocking burial diagenetic processes in the mancos shale using carbonate clumped isotopes abstract septarian carbonate concretions contain carbonate precipitated during progressive growth of the concretion and subsequent fracture filling as such they have been used to track variations in 13c and 18o of pore waters during diagenesis and to define diagenetic zones in clastic rocks however the 18o value of the carbonate is dependent on precipitation temperature and the 18o value of the pore fluid from which it precipitated interpretations must assume one of these parameters both of which are highly variable through time in diagenetic settings carbonate clumped isotopes of the cement can provide independent estimates of temperature of precipitation allowing the pore water 18o to be back calculated here we use this technique on carbonate concretions and fracture fills of the upper cretaceous prairie canyon member mancos shale colorado we sampled concretions from two permeable horizons separated by a 5 m shale layer with one permeable horizon containing concretions with septarian fractures we show cores precipitated at cooler temperatures 31 c 660 m burial depth than the rims 68 c 1980 m burial depth and relate that to the 13ccarbonate values to suggest the concretion core precipitated in the methanogenic zone with increasing input from thermogenically produced co2 the two concretion bearing horizons have different back calculated 18oporewater values mean 2 65 and 1 13 vsmow for cements formed at the same temperature and similar 13c values suggesting the shale layer present between the two horizons acted as a barrier to fluid mixing additionally the 18ocarbonate of the septarian fractures 13 8 vpbd are due to precipitation at high temperatures 102 to 115 c from a fluid with a mean 18oporewater of 0 32 vsmow therefore we can tie in the cementation history of the formation to temporal and spatial variations in 18oporewater

magnetic structural and geochronologic evidence bearing on volcanic sources and oligocene deformation of ash flow tuffs northeast nevada magnetic properties of mid tertiary volcanic rocks west of jiggs in northeast nevada were investigated for the purposes of interpreting igneous structural and tectonic processes in this part of the basin and range province anisotropy of magnetic susceptibility ams patterns reflect flow fabrics and suggest previously unknown sources for these ash flow tuffs paleomagnetic and structural evidence suggest counterclockwise relative rotation of the southern part of the area with respect to the north new stratigraphic chemical and k ar isotopic data support these interpretations dacite to rhyolite ash flow tuffs of the indian well formation were subdivided into two main units the lower and predominant tuff of jiggs 35 8 37 0 ma and the unconformably overlying but areally restricted tuff of hackwood 30 8 ma the jiggs unit has two polarities whereas the hackwood has only a reversed polarity remanence together these units with tilt correction define a pole 92 e 87 n dp 6 dm 8 for approximately 30 37 ma this pole is concordant with coeval north american reference poles indicating that this zone of approximately 30 km ns extent has not undergone significant vertical axis rotation relative to the north american reference andesite lavas of normal magnetic polarity and of 32 5 ma age characterize the diamond hills immediately to the south we interpret this region from both structural evidence and discordant paleomagnetic direction to have rotated approximately 25 counterclockwise relative to the indian well volcanic units to the north the apparent rotation of the diamond hills is possibly the result of drag on the left lateral garcia fault which limits the diamond hills on the southwest analysis of ams data suggests by patterns of the k1 axes two distinct sources for the jiggs unit a northern buried source and a central partially buried source lithologic evidence consistent with proximal vent facies is found near the latter source an unusual pattern in the ams k3 axes suggested as resulting from the tilt correction applied to initial dip is consistent with the interpreted flow directions tight age constraints can be placed on the deformation of the main ash flows of the jiggs unit in the 35 8 to 30 8 ma interval because the diamond hills unit is slightly younger and appears to have been affected by the same deformative episode as the jiggs the regional deformation affecting both units can be more narrowly confined to the 32 5 30 8 ma interval this deformation is inferred to coincide with the major early tertiary episode of crustal extension in this region of the basin and range

geochemical and isotopic constraints on the diagenetic history of a massive stratal late cambrian royer dolomite lower arbuckle group slick hills sw oklahoma usa abstract the diagenetic history of the late cambrian massive royer dolomite from the lower arbuckle group slick hills sw oklahoma usa has been evaluated through comprehensive geochemical analyses the dolomite probably formed during early diagenesis from contemporaneous late cambrian seawater evidenced by similar 87 sr 86 sr ratios from 0 70891 to 0 70913 of most 70 dolomite samples to coeval late cambrian seawater from 0 7089 to 0 7092 very light 18o values 10 2 to 12 9 pdb however indicate that the dolomite was significantly modified during burial correlation of various geochemical parameters sr fe 18o and 87 sr 86 sr demonstrates that dolomite samples with increased modification are characterized by lower sr and higher fe concentrations lighter 18o values and less radiogenic 87 sr 86 sr ratios this correlation coupled with the light 18o values of the dolomite demonstrates that meteoric water recharged into overlying carbonates was responsible for burial modification of the dolomite geochemical comparison of the dolomite with associated limestone and quantitative modelling of 18o sr and 87 sr 86 sr variation during water rock interaction suggest that the dolomite underwent long lasting modification the timing of meteoric modification was coincident with the development of regional overlying unconformities corresponding to emergence of the carbonate platform similar diagenetic modification has probably been responsible for 18o depletion and trace element modification of ancient dolomites in many other areas although many of these dolomites have been interpreted to have formed either in mixing zone environments or in subsurface environments

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

ophiolitic thrust nappes in western nevada implications for the cordilleran orogen new field and geophysical evidence from jurassic mafic and ultramafic rocks in western nevada indicate the presence of two major ophiolite complexes tectonically overlying the rocks of the north american continental margin coupled with the existence of jurassic and older island arc marginal basin complexes and overthrust relations in the adjacent sierran klamath region these ophiolitic thrust nappes in nevada support collision models for the middle jurassic deformation the geometry of the observed structures and their relative age relationships suggest that this deformation might have resulted from the collision of the north american continent with a west dipping subduction zone which resulted in ophiolite emplacement and continental underthrusting continued westward motion of north america beneath the allochthonous oceanic terranes during the late mesozoic developed extensive thrust and possible strike slip faulting in the western us cordillera and caused migration of deformation towards the east this eastward propagation of deformation is interpreted to have resulted in the development of the sevier foreland thrust belt of latest jurassic to latest cretaceous age these collisional processes and the geometry of the proposed collision orogen are reminiscent of the alpine type collision belts

laboratory calibration of the calcium carbonate clumped isotope thermometer in the 25 250 c temperature range abstract many fields of earth sciences benefit from the knowledge of mineral formation temperatures for example carbonates are extensively used for reconstruction of the earth s past climatic variations by determining ocean lake and soil paleotemperatures furthermore diagenetic minerals and their formation or alteration temperature may provide information about the burial history of important geological units and can have practical applications for instance for reconstructing the geochemical and thermal histories of hydrocarbon reservoirs carbonate clumped isotope thermometry is a relatively new technique that can provide the formation temperature of carbonate minerals without requiring a priori knowledge of the isotopic composition of the initial solution it is based on the temperature dependent abundance of the rare 13c 18o bonds in carbonate minerals specified as a 47 value the clumped isotope thermometer has been calibrated experimentally from 1 c to 70 c however higher temperatures that are relevant to geological processes have so far not been directly calibrated in the laboratory in order to close this calibration gap and to provide a robust basis for the application of clumped isotopes to high temperature geological processes we precipitated caco3 mainly calcite in the laboratory between 23 and 250 c we used two different precipitation techniques first minerals were precipitated from a caco3 supersaturated solution at atmospheric pressure 23 91 c and second from a solution resulting from the mixing of cacl2 and nahco3 in a pressurized reaction vessel at a pressure of up to 80 bar 25 250 c the calibration lines of both experimental approaches overlap and agree in the slopes with theoretical estimates and with other calibration experiments in which carbonates were reacted with phosphoric acid at temperatures above 70 c our study suggests a universal 47 t calibration t in k 47 in 47 0 98 0 01 3 407 10 9 t 4 2 365 10 7 t 3 2 607 10 3 t 2 5 880 t 0 293 0 004 this new 47 t calibration given in the absolute reference frame that extends the experimentally calibrated temperature range for clumped isotopes to 250 c can be applied to carbonates that grew at intermediate temperatures 20 250 c

growth demise and dolomitization of miocene carbonate platforms on the marion plateau offshore ne australia abstract strontium isotope stratigraphy has been used to examine the timing of depositional events and dolomitization in two miocene carbonate platforms cored by ocean drilling program odp leg 194 just seaward of the great barrier reef the results provide firm constraints for correlating surfaces and depositional stages between the two platforms and thereby relating seismic sequences previously defined in the off platform sediments to the lithostratigraphic units described from cores in the seismically transparent platform top sites oyster bearing beds at the base of both platform successions yield early oligocene ages 29 31 ma thus dating initial transgression of the marion plateau s volcanic basement there followed a period of slow accumulation of shallow water grainstones rich in quartz and phosphate grains in late oligocene time 29 23 ma seismic megasequence a the main growth of the carbonate platforms took place in early to late miocene time 23 7 ma comprising five depositional sequences the first four of these seismic megasequence b are common to both platforms and terminated with a possible karst surface at 10 7 ma different sedimentologic expression of this megasequence in the two platforms reflects contrasting progradational versus aggradational geometries in the locations studied the final growth stage seismic megasequence c occurred only in the southern platform and terminated at 6 9 ma both platform demise events 10 7 and 6 9 ma approximately coincide with falls in global sea level combined with longer term trends of decreasing water temperature sr isotope ages of dolostones increase with depositional age and older dolostones in the southern platform have more coarsely crystalline and fabric destructive textures than overlying younger dolostones these relationships are consistent with dolomitization by normal seawater shortly after deposition and overprinting of multiple times of dolomite recrystallization and cementation in the deeper strata

geochronology of augen gneiss and related rocks yukon tanana terrane east central alaska using several isotopic techniques we have determined the ages of selected metamorphic rocks in the yukon tanana terrane ytt of east central alaska u pb zircon data from an augen gneiss body in the big delta quadrangle indicate that the granitoid protolith of the gneiss was intruded 341 3 m y ago lower intercept age an upper intercept age of 2 136 31 m y indicates an inherited early proterozoic component in these zircons this inheritance age is substantiated by a sm nd whole rock model age of 2 09 0 08 b y from the big delta augen gneiss body detrital zircons from quartzitic wall rocks to this body were also derived from an early proterozoic 2 1 to 2 3 b y old crustal source s zircons from three other augen gneisses occurring in an east west belt which extends into the southern yukon territory canada have similar mississippian and early proterozoic intercept ages a rb sr whole rock isochron from widely separated bodies of augen gneiss has an age of 333 26 m y and an initial 87 sr 86 sr ratio of 0 728 0 002 confirming the mississippian intrusive age for the protolith the high initial 87 sr 86 sr ratio further indicates an old crustal component in these rocks a rb sr mineral isochron 115 4 m y k ar data from hornblende and micas 128 to 107 m y and u pb data from sphene 134 m y from augen gneiss and related rocks are similar to many k ar ages in this region and confirm the occurrence of an early cretaceous thermal event u pb ages of zircons from three metavolcanic units in the ytt suggest that extrusion of the protoliths of these rocks occurred 360 380 m y ago scatter in the data is caused by ubiquitous inheritance and multiple lead loss events significant devonian mississippian igneous activity 380 340 m y took place in the ytt followed by one or possibly two metamorphic episodes mississippian and cretaceous as yet no early proterozoic source rocks for the paleozoic magmas have been identified in the ytt but rocks of similar early proterozoic ages occur to the east and south in the northwest territories and british columbia canada

oxygen isotope exchange in a two layer model of oceanic crust abstract a numerical model is presented for the transport and exchange of oxygen isotopes in an idealized two layer oceanic crust basalt carbonate that is saturated with a non advecting pore fluid open to the overlying ocean the model allows for time varying sedimentation rate porosity chemical diffusivity and temperature dependent 18o fractionation factors reaction is modeled as solution precipitation of calcite in the carbonate sediment and as modified solution precipitation basalt dissolves smectite precipitates in the basalt layer generic models using input parameters that are typical of those measured in dsdp odp cores and constant sedimentation rates illustrate the behavior of the system sufficiently well to allow first order effects to be discerned that have important implications for paleotemperature and ocean floor weathering studies the solution precipitation reactions allow oxygen to be exchanged between the low 18o basalt layer and the high 18o carbonate sediment through the medium of the pore fluids this results in 18o increasing upwards in the basalt and decreasing downwards in the sediment pore fluid 18o values decrease with depth from zero to values of 1 to 3 at the basalt sediment interface and to values of 7 to 15 within the basalt because reaction rates are slow relative to diffusion in the pore fluid high frequency 18o variations in the solid are preserved indefinitely but the amplitude of the variations decreases as e rt where r is the reaction rate fraction reacted per unit time the results suggest that the diagenetic shifts of the 18o values of benthic and high latitude planktonic foraminifera samples younger than oligocene age are typically negligible for samples older than 50 ma diagenetic effects are typically significant and generally cause estimates of bottom and high latitude surface ocean temperatures in the cretaceous and early tertiary to be too high on the other hand the model suggests that diagenesis will shift the 18o values of low latitude planktonic foraminifera to higher values and that paleotemperature estimates of low latitude surface waters particularly in the eocene and oligocene may be too low using a synthetic 18o record for dsdp site 167 we demonstrate how numerical models can be used to correct specific measured records for diagenesis this approach could provide a means of extracting paleotemperature information from heavily altered late cretaceous and older samples and for quantifying the subtle effects of diagenesis on the 18o record at all sites

early dolomitization and partial burial recrystallization a case study of middle triassic peritidal dolomites in the villany hills sw hungary using petrography carbon oxygen strontium and clumped isotope data

burial estimates constrained by clumped isotope thermometry example of the lower cretaceous qishn formation haushi huqf high oman

p t evolution of the teslin suture zone and cassiar tectonites yukon canada evidence for a and b type subduction mineral composition and quantitative thermobarometric studies indicate that the teslin taylor mountain and nisutlin terranes within the teslin suture zone tsz yukon record widespread high p t metamorphic conditions consistent with subduction zone dynamothermal metamorphism the highest p t conditions 575 750 c and 9 17 kbar are preserved in tectonites formed during normal dip slip ductile shear dextral strike slip tectonites record lower p t conditions 400 550 c and 5 8 kbar and tectonites which show reverse shear have peak temperatures of c 420 c and a minimum peak pressure of 3 kbar dynamothermal metamorphism took place in a west dipping b type subduction zone outboard of western north america in permo triassic time tsz tectonites were underplated against the hangingwall plate of the subduction zone following subduction of the ocean basin which separated north america from the hangingwall plate tsz tectonites were overthrust eastward as a coherent structural package as a result of a type subduction of cassiar strata in early jurassic time par autochthonous cassiar tectonites which comprised the leading edge of the western north american margin record prograde moderate p high t metamorphism 550 750 c and 7 13 kbar synchronous with top to the east ductile shear metamorphism occurred as a result of subduction of the north american margin into the tsz subduction zone in early jurassic time following metamorphism cassiar tectonites cooled slowly from 500 to 300 c during the period middle jurassic to middle cretaceous tsz and cassiar tectonites were deformed during changing p t conditions data from each of these tectonite packages indicate that grain scale strain partitioning may have allowed local recrystallization of individual minerals by the addition of mechanical energy the composition of the new grains reflects the p t conditions under which that particular grain was deformed

paleointensity during periods of rapid reversal a case study from the middle jurassic shamrock batholith western nevada paleointensity is one of the least deter mined parameters in geomagnetism yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the earth s core and lower mantle we test two important hy potheses that affect our understanding of how paleointensity has changed in earth s past 1 the mesozoic dipole low mdl hypoth esis and 2 the inverse relationship between geomagnetic polarity reversal rate and paleo intensity we report paleointensity results determined using both the coe modified thellier method and the izzi protocol on the medium grained hornblende biotite granite of the middle jurassic shamrock batholith in the southern singatse range western nevada usa previous zircon u pb dating gives an age of batholith emplacement at ca 165 8 0 4 ma this age estimate is coincident with a period of unusually high reversal frequency 10 reversals ma in the middle jurassic remanence in the shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase the labora tory un blocking temperature range for the well de fined thermal remanent magnetization is nar row between 540 and 575 c and median destructive fields are between 40 and 50 mt thus we infer that the age of rema nence ac quisition is approximated by the u pb age estimate for pluton emplacement two sets of quality criteria were used to assess the paleo intensity results the first set yielded an 87 success rate while the second set using more stringent criteria resulted in only a 20 suc cess rate mean paleointensity values for the shamrock batholith using the loose and strict quality criteria are 34 0 6 4 t n 13 and 33 6 9 6 t n 3 respectively with errors reported at one standard deviation after correction for both remanence anisot ropy and cooling rate the mean values that pass loose and strict criteria are respec tively 17 9 2 7 t and 17 5 2 6 t the uncorrected and corrected paleointensity values for the strict estimates yield respec tive mean virtual dipole moments vdms of 62 7 18 zam 2 z 10 21 and 32 7 4 9 zam 2 the uncorrected value is within error of the vdms reported for plagioclase grains isolated from ca 160 ma basalts from ocean drilling program site 801 however these values have an absolute difference ranging from 6 to 20 zam 2 while the corrected vdm value is more consistent with the esti mated average jurassic field strength of 29 zam 2 using a recent estimate for the long term stable field strength of 42 zam 2 our corrected values add further credence to both the mdl and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses

sr mg ratios of modern marine calcite empirical indicators of ocean chemistry and precipitation rate holocene biotic and abiotic marine calcite have a similar range of mg contents 0 to 22 and 4 to 21 mol mgco3 respectively yet biotic calcite has sr2 concentrations that are consistently 1250 ppm higher than those of abiotic calcite as in laboratory experiments a positive linear relation is observed between dsr and calcite mg content this produces two distinct linear trends on a plot of sr2 vs mg2 concentrations principal axes of variation for both trends have similar slopes yet distinctly different sr2 concentration intercepts biotic y 0 024x 1298 r2 0 70 abiotic y 0 027x 47 r2 0 77 the similar slopes of these trends reflect the constancy of mgca and srca ratios of modern seawater equations describing the dependence of dsr on calcite mg content are derived from both trends biotic dsr 3 16 x 10t 6 ppm mg 0 169 abiotic dsr 3 52 x 10 6 ppm mg 0 0062 characterization of sr mg trends for holocene materials allows comparison with analogous trends from ancient samples to estimate relative changes in seawater mgca and srca ratios the relatively high sr contents of biotic calcite result from rapid precipitation rates associated with shell accretion in marine organisms calcites precipitated from seawater in laboratory experiments have dsr values that are similar to those of biotic marine calcite suggesting that both precipitate at approximately the same rate our estimates of surface area normalized precipitation rates in planktonic and benthonic foraminifera are comparable to those of seeded ph stat experiments we conclude that the dsr values for biotic and experimental marine calcite are kinetically controlled whereas the lower precipitation rates of abiotic marine calcite yield dsr values that approximate equilibrium conditions experimentally derived equations describing the relation between dsr and calcite precipitation rate indicate that the offset in sr content between biotic and abiotic calcite is the result of abiotic precipitation rates that are two to five orders of magnitude lower than those of biotic precipitates however observations of naturally occurring marine cements suggest that the five order of magnitude offset best represents natural system processes

clumped isotope fractionation during phosphoric acid digestion of carbonates at 70 c the application of clumped isotopes 47 in carbonate minerals as a sensitive temperature proxy in paleo environments depends on a well constrained clumped isotope fractionation for the necessary step of phosphoric acid digestion of the carbonate mineral to produce co2 published estimates for clumped isotope fractionations vary and the effect of different carbonate mineralogies is still under debate differences in the sample preparation design and sample digestion temperatures are potential sources for varying acid fractionations and could be a source for discrepant 47 temperature calibrations observed in different laboratories to evaluate the clumped isotope acid fractionation at 70 c and simultaneously account for a potential cation effect we analyzed a set of eight carbonate minerals calcite aragonite dolomite and magnesite that were driven to a stochastic isotope distribution by heating them to temperatures of 1000 c our study reveals significant cation and mineral specific differences for the 47 acid fractionation of carbonate minerals digested at 70 c or 100 c the 47 acid fractionation at 70 c for calcite is 0 197 0 002 for aragonite 0 172 0 003 whereas dolomite has a significantly larger acid fractionation of 0 226 0 002 for magnesite digested at 100 c we observed a 47 acid fractionation of 0 218 0 020 projected to an acid digestion at 25 c our acid fractionation for calcite of 0 260 is statistically indistinguishable from existing studies we further show that the 47 of the calcite standards eth 1 and eth 2 of 0 265 and 0 267 respectively are in the range of the determined acid fractionation projected to 25 c suggesting that they have an identical and near stochastic isotope distribution the observed differences in the 47 acid fractionation between calcite and aragonite 47 0 025 and between calcite and dolomite 47 0 029 does not correlate with the phosphoric acid fractionation of oxygen isotopes but rather depends on the radius of the cation as well as on the mineral structure our results reveal that the acid fractionation of dolomite at 70 c is significantly distinct from the one of calcite but at 90 c the two are within error of each other due to the different acid fractionation temperature dependence of calcite and dolomite thus it is necessary to use a mineral specific 47 acid fractionation factor for dolomite to avoid differences in the final 47 signal from dolomites digested at 90 c and dolomites digested at lower temperatures similar effects may apply also to other carbonates such as magnesite and siderite however their mineral specific 47 acid fractionation at digestion temperatures around 90 c might be also similar to the one of calcite so that potential differences could be within the range of the analytical error

u pb geochronology of detrital zircons from a continental margin assemblage in the northern coast mountains southeastern alaska metamorphic rocks within and west of the northern coast mountains in southeastern alaska consist of an upper proterozoic to upper paleozoic continental margin assemblage that we interpret to belong to the yukon tanana terrane u pb geochronologic analyses of single detrital zircon grains from four samples of quartzite suggest that the zircons were shed from source regions containing rocks of 495 ma 750 ma 1 05 1 40 ga 1 75 2 00 ga 2 3 ga 2 5 2 7 ga and 3 0 ga multigrain fractions from two samples yield upper intercepts between 2 0 and 2 3 ga but the scarcity of single grains of similar age suggests that these fractions comprise a mixture of 2 3 ga grains zircons in these rocks generally overlap in age with i detrital zircons in metasedimentary rocks of the yukon tanana terrane in eastern alaska and yukon ii detrital zircons in strata of the cordilleran miogeocline and iii plutonic and gneissic rocks that intrude or are overlain by miogeoclinal strata in addition the p

general equations for modeling fluid rock interaction using trace elements and isotopes abstract general equations for modeling open and closed system fluid rock interaction using trace elements and isotopes are presented taylor s 1977 open system equation for stable isotopes is extended to account for a change in the value between rock and fluid during interaction it is shown that hydrothermal systems in which fractionation factors vary during fluid rock interaction will significantly alter the isotopic ratios of rocks compared to equivalent systems with invariant fractionation factors the equations for modeling trace element exchange are analogous to the stable isotope equations and permit the calculation of fluid rock ratios using changes in trace element concentrations in the rock similarly to stable isotope exchange the change of the trace element concentration in a rock during infiltration is a function of the trace element concentration in the fluid the fluid rock partition coefficient and the fluid rock ratio rather than simply the concentration of the trace element in the fluid

carbonate clumped isotope bond reordering and geospeedometry carbonate clumped isotope thermometry is based on the preference of 13c and 18o to form bonds with each other at elevated temperatures such bond ordering is susceptible to resetting by diffusion of c and o through the solid mineral lattice this type of bond reordering has the potential to obscure primary paleoclimate information but could also provide a basis for reconstructing shallow crustal temperatures and cooling rates we determined arrhenius parameters for solid state reordering of c o bonds in two different calcites through a series of laboratory heating experiments we find that the calcites have different susceptibilities to solid state reordering reaction progress follows a first order rate law in both calcites but only after an initial period of non first order reaction that we suggest relates to annealing of nonequilibrium defects when the calcites are first heated to experimental temperature we show that the apparent equilibrium temperature equations or closure temperature equations for carbonate clumped isotope reordering are analogous dodson s equations for first order loss of daughter isotopes for each calcite the sensitivity of apparent equilibrium temperature to cooling rate is sufficiently high for inference of cooling rates within a factor of 5 or better for cooling rates ranging from tens of degrees per day to a few degrees per million years however because the calcites have different susceptibilities to reordering each calcite defines its own cooling rate apparent equilibrium temperature relationship the cooling rates of carrara marble inferred from carbonate clumped isotope geospeedometry are 10 6 10 3 degrees per annum and are in broad agreement with rates inferred from thermochronometric methods cooling rates for 13c depleted calcites from the late neoproterozoic doushantou cap carbonates in south china are on the order of 102 104 degrees per annum consistent with rapid cooling following formation of these calcites by a short lived hydrothermal event bristow et al 2011 nature v 474 p 68 71 most of the uncertainty in these estimates relates to uncertainty in arrhenius parameters for different calcites thus while the carbonate clumped isotope geospeedometer shows promise for recording cooling rates in settings and lithologies where other geospeedometers may not be applicable the uncertainty in cooling rate will be large without independent knowledge of the reordering kinetics of each study material thus the full potential of the method will only be realized if reordering kinetics can be accurately determined for each study material or predicted on the basis of mineral composition texture or other observable parameters

shallow burial alteration of dolomite and limestone clumped isotope geochemistry carbonate clumped isotope paleothermometry is a promising method for paleoclimate reconstructions and understanding diagenetic processes although clumped isotope compositions measured as 47 values can be altered by recrystallization and thermal resetting several studies have investigated the effects of deep burial and exhumation on 47 values we present 47 values of samples from an 4500 m long drill core from andros island bahamas which represent the spectrum from shallow to deep burial conditions these limestones and dolomites originally formed under near surface temperature conditions which are reflected by 27 c 47 temperatures in samples buried to 1 3 km depth below this depth calculated 47 temperatures increase by 10 c indicating a shift from preserved near surface temperatures to diagenetically modified values importantly this shift is not accompanied by a change in 18o and 13c values or cathodoluminescence and non uniform micrometer scale crystallization is observed in both 47 altered and unaltered rocks similar 47 temperatures recorded by each carbonate phase suggest that mineral specific differences are minor fine grained dolomites and calcites are viable materials for recording surface temperature conditions and both are susceptible to alteration of their primary clumped isotope abundances during burial diagenesis our results demonstrate that original 47 values of fine grained carbonates can be altered under mild diagenetic conditions

relative control of paleoceanography climate and eustasy over heterozoan carbonates a perspective from slope sediments of the marion plateau odp leg 194 in this paper we explore the relative control of paleocean ography eustasy and water temperature over the evolution of a car bonate slope system deposited on the marion plateau northeastern australia growth of several carbonate platforms started in the early miocene on this plateau and although they occurred in low latitude subtropical waters they are composed mainly of heterozoan organisms we investigated an upper to distal slope transect drilled during odp leg 194 and located close to the northern marion platform we re constructed mass accumulation rates of carbonate as well as the evo lution in the ratios of carbon and oxygen stable isotopes power spec trum analysis of the carbon isotope record revealed the existence of cycles with main frequencies centered around 409 kyr and 1800 kyr we interpret the 409 kyr cycle as being paced by changes in the ec centricity of the earth orbit and we suggest that the 1800 kyr cycle could be linked to long term eustatic changes finally on the basis of the timing of changes in mass accumulation rates of carbonate we infer that the strength and direction of oceanic currents affected sedimen tation on the marion plateau by shifting depocenters of slope sedi mentation a process probably further modulated by sea level changes we argue that the evolution and demise of the heterozoan carbonate systems present on the marion plateau were controlled mainly by the evolution of strong benthic currents and that eustasy and water tem perature alone did not account for the drowning of the platforms in the mid miocene

modeling the effects of diagenesis on carbonate clumped isotope values in deep and shallow water settings the measurement of multiply isotopically substituted clumped isotope carbonate groups provides a way to reconstruct past mineral formation temperatures however dissolution reprecipitation i e recrystallization reactions which commonly occur during sedimentary burial can alter a sample s clumped isotope composition such that it partially or wholly reflects deeper burial temperatures here we derive a quantitative model of diagenesis to explore how diagenesis alters carbonate clumped isotope values we apply the model to a new dataset from deep sea sediments taken from ocean drilling project site 807 in the equatorial pacific this dataset is used to ground truth the model we demonstrate that the use of the model with accompanying carbonate clumped isotope and carbonate 18 o values provides new constraints on both the diagenetic history of deep sea settings as well as past equatorial sea surface temperatures specifically the combination of the diagenetic model and data support previous work that indicates equatorial sea surface temperatures were warmer in the paleogene as compared to today we then explore whether the model is applicable to shallow water settings commonly preserved in the rock record using a previously published dataset from the bahamas we demonstrate that the model captures the main trends of the data as a function of burial depth and thus appears applicable to a range of depositional settings

dolomitization of holocene mg calcite supratidal deposits ambergris cay belize dolomitized crusts and associated sediments 845 2925 yr old on ambergris cay belize central america compose a significant portion of the holocene sediment section on many supratidal flats the dolomitic sections are as thick as 0 7 m and contain an average of 70 calcic dolomite dolomite occurs as a replacement of high mg calcite micrite matrix and allochems and as passively precipitated cements isotopic analyses suggest that dolomitization appears to be promoted primarily by reactions of the magnesian calcite supratidal sediments with essentially near normal marine waters the effects of seasonal influxes of meteoric and hybrid fluids however are also clearly evident in the crusts as indicated by isotopes dolomite crystal etching the selective leaching of aragonite and the local loss of mg 2 from the host micrite and included skeletal fragments the amounts rapid rates of dolomitization and initial mg calcite mineralogy of the sediments make the ambergris cay supratidal flats exceptional in the holocene and this area may be a model for the genesis of some ancient peritidal dolomites

on the evolution of dolomite stoichiometry and cation order during high temperature synthesis experiments an alternative model for the geochemical evolution of natural dolomites abstract data collected from a series of high temperature dolomitization experiments in which dolomite replaces calcite in mg ca cl solutions indicate that dolomite composition mol mgco 3 and cation order evolve independently as a function of reaction progress despite a wide range of initial solution mg ca 0 43 1 50 the first product to form in all experiments is disordered dolomite both the rate of replacement and the composition of these dolomite products are strongly dependent on the initial mg ca in solution experimental solutions with lower mg ca yield less stoichiometric more ca rich dolomite and have reactions that progress at a slower rate conversely solutions with higher mg ca yield more stoichiometric dolomite and are characterized by faster overall reaction rates in all experiments dolomite composition remains constant during most of the reaction despite a solution chemistry that continually evolves to lower mg ca as magnesium is captured from solution by growing dolomite and calcium is liberated from calcite into solution despite a dolomite composition that remains relatively constant the degree of cation order increases with the percentage of dolomite shortly before complete consumption of calcite reactants when 95 of the calcite reactants have been replaced by dolomite products dolomite composition rapidly becomes more stoichiometric after all calcite reactants have been consumed dolomite is completely stoichiometric regardless of the mg ca in solution at the onset of experimental conditions however the degree of cation order in the stoichiometric dolomite products continues to increase with time in dolomitizing solutions the observation that dolomite composition remains constant throughout most of the dolomitization reaction is different than data from some previous studies this suggests that the mechanism by which dolomite replaces caco 3 is not clear and is more complex than previously recognized the data presented here suggest that the initial dolomite phase provides a compositional template for future dolomite growth thus new material being added to the crystal surface is forced to follow the established composition the data presented here support an alternative model for the commonly observed field relationship between the percent dolomite in a carbonate rock and the stoichiometry of that dolomite we propose that dolomite does not evolve toward more stoichiometric compositions simply because the percent dolomite in the rock increases i e indicating further reaction progress rather carbonate rocks with a higher dolomite to limestone ratio where the dolomite is more stoichiometric can be better explained as having been dolomitized in solutions with higher mg ca which cause both faster rates of dolomitization and more stoichiometric compositions this model explains field observations from evaporative reflux settings that show spatial variations in dolomite composition and crystal texture as the result of chemical evolution of interstitial pore fluids along a flow path the new model may also provide a new framework for investigating the chemical evolution of dolomite in reactive transport models which numerically track chemical reactions and constituents

a comparison of nanometer scale growth and dissolution features on natural and synthetic dolomite crystals implications for the origin of dolomite abstract this paper describes experiments in which mechanisms of crystal growth are inferred from the surface nanotopography of natural and synthetic dolomite using atomic force and scanning electron microscopy synthetic dolomite was formed by replacement of calcite at 218 c in mg ca cl solutions the first products to form during dolomite synthesis experiments as detected with x ray diffraction xrd are invariably poorly ordered nonideal dolomite as the reaction proceeds the degree of cation order increases yet the products remain relatively disordered upon examination the surfaces of nonideal dolomite are covered with mounds 10 200 nm wide and 1 20 nm high following depletion of the calcite reactant dolomite becomes relatively well ordered and stoichiometric ideal the surfaces of ideal dolomite are covered with broad flat layers separated by steps that measure tenths to tens of nanometers high comparison of natural and synthetic dolomite surfaces after etching in 0 5 h2so4 for 10 20 seconds indicates that high temperature synthetic and low temperature natural nonideal dolomite surfaces are covered with mounds identical to the growth features found on unetched nonideal synthetic dolomite in contrast synthetic and low temperature ideal dolomite surfaces when etched are characterized by flat layers with deep euhedral etch pits these results suggest that despite the wide range in formation conditions natural and synthetic dolomites form by the same growth mechanisms furthermore the two different surface nanotopographies described here are consistent with a model in which nonideal dolomite forms by precipitation of amorphous mounds that quickly crystallize to nonideal dolomite while ideal dolomite later replaces nonideal dolomite and grows by spiral growth

high late miocene pliocene elevation of the zhada basin southwestern tibetan plateau from carbonate clumped isotope thermometry the timing and pattern of tibetan pla teau rise provide a critical test of possible mechanisms for the development and sup port of high topography yet views range widely on the history of surface uplift to modern elevations of 4 5 km to address this issue we present clumped isotope ther mometry data from two well studied ba sins in central and southwestern tibet for which previous carbonate 18 o data have been used to reconstruct high paleoeleva tions from late oligocene to pliocene time clumped isotope thermometry uses mea surements of the 13 c 18 o bond ordering in carbonates to constrain the temperature t 47 and 18 o value of the water from which the carbonate grew these data can be used to infer paleoelevation by exploiting the systematic decrease of surface tempera ture and the 18 o based paleoaltimetry and with paleontological and isotopic data indicating the presence of cold adapted mammals living in a cold high ele vation climate we suggest that late neogene elevation loss across the zhada basin catch ment probably related to local expression of east west extension across much of the south ern tibetan plateau at this time

origin of island dolostones a case study from the cayman formation miocene cayman brac british west indies abstract cayman brac the easternmost of the cayman islands is 19 km long 1 5 to 3 km wide and rises 40 m above sea level at its eastern end geographically isolated by the deep oceanic waters of the caribbean sea this island with its thick succession of pervasively dolomitized tertiary rocks is a natural laboratory for assessing the origin of island dolostones dolostones in the miocene cayman formation 100 m thick with most crystals 55 mol caco3 geochemically they are characterised by 13c values of 1 6 to 3 5 18o values of 2 3 4 0 80 279 ppm sr 52 340 ppm fe and 9 82 ppm mn the inverse correlation between 18o and mol caco3 and positive correlation between sr and mol caco3 largely reflect kinetic effects on oxygen isotope fractionation and the sr partition behaviour between dolomite and water dolomite stoichiometry indicates formation from waters with a high mg ca ratio and 18o values corrected for biases associated with dolomite stoichiometry and phosphoric acid fractionation indicate formation from normal seawater like fluids under near surface conditions the 87sr 86sr ratios point to two phases of dolomitization phase i in the late miocene 6 8 ma caused partial dolomitization of the basal part of the cayman formation phase ii during the pliocene to early pleistocene 1 5 ma completed dolomitization of the cayman formation both phases of dolomitization were mediated by similar fluids although available information suggests that dolomitization was probably linked to sea level fluctuations the timing of dolomitization relative to the transgressive regressive cycle remains open to debate

mineralogy nucleation and growth of dolomite in the laboratory and sedimentary environment a review dolomite camg co3 2 forms in numerous geological settings usually as a diagenetic replacement of limestone and is an important component of petroleum reservoir rocks rocks hosting base metal deposits and fresh water aquifers dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of ca2 and mg2 cations interspersed with co32 anion layers normal to the c axis dolomite has r3 symmetry lower than the caco3 r3 c symmetry of calcite primarily due to ca mg ordering high magnesium calcite also has r3 c symmetry and differs from dolomite in that ca2 and mg2 ions are not ordered high magnesium calcite with near dolomite stoichiometry 50 mol mgco3 has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high magnesium calcite many dolomites display some degree of cation disorder ca2 on mg2 sites and vice versa which is detectable using transmission electron microscopy and x ray diffractometry laboratory syntheses at high temperature and pressure as well as studies of natural dolomites show that factors affecting dolomite ordering stoichiometry nucleation and growth include temperature alkalinity ph concentration of mg and ca mg to ca ratio fluid to rock ratio mineralogy of the carbonate being replaced and surface area available for nucleation in spite of numerous attempts dolomite has not been synthesized in the laboratory under near surface conditions examination of published x ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near ambient conditions by microbial mediation are unsubstantiated these laboratory products show no evidence of cation ordering and appear to be very high magnesium calcite elevated temperature and elevated pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high magnesium calcite formation it remains to be demonstrated whether microbial mediated growth of very high magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high temperature experiments

calibration of dolomite clumped isotope thermometry abstract use of the clumped isotope thermometer to decipher environments of dolomite formation has been inhibited by a lack of empirical data from dolomites formed at known temperatures calibrations for aragonite calcite siderite and apatite indicate that a universal 47 temperature relationship may exist across all carbonate bearing mineral phases however theoretical modeling and proposed acid fractionation differences have suggested that dolomites may have different 47 values relative to calcites precipitated at the same temperature to resolve this question we analyzed five synthetic and four natural dolomites formed at known temperatures these included synthetic dolomites grown in mg ca cl solutions at temperatures of 200 250 c and natural samples constrained by fluid inclusion analyses 70 c climate 28 and 27 c and deep ocean borehole temperature 21 c when using calcite 47 acid fractionation values these data result in a calibration line that is statistically indistinguishable from other clumped isotope calibrations at least with current precision we find no evidence for a consistent dolomite 47 offset these results further support a universal calibration for carbonate clumped isotope thermometry and enable new investigations into conditions of dolomite formation

trace element and isotope geochemistry of zoned calcite cements lake valley formation mississippian new mexico insights from water rock interaction modelling abstract regionally extensive pre pennsylvanian meteoric zoned calcite cements from the mississippian age lake valley formation new mexico comprise three major zones zones 1 2 and 3 each with a distinct isotope and trace element geochemistry zone 1 has the highest mg 18o and 13c contents zone 2 intermediate values and greatest mn and fe and zone 3 has the lowest trace element 18o and 13c values covariation of c and o isotopes for the three zones define an inverted l shaped trend and covariations of mg mn and mg fe define l shaped trends all suggestive of rock evolution curves resulting from water rock interaction these covariant trends were the impetus for quantitative mass balance modelling of water rock interaction iterative mass balance modelling of covariations of stable isotopes c and o and trace elements mg mn fe of these zoned cements implies that water rock interaction progressively decreased water rock ratios increased during precipitation cementing waters for the oldest cement zone zone 1 resulted from water rock ratios in the range of tens to a few hundred whereas the youngest cement zone 3 resulted from water rock ratios in the 100 s to 1000 s the modelling also implies that differences in geochemistry of the cement zones cannot be explained solely by changes in water rock ratios but require greater 18o and greater ca and mg concentrations in input waters that evolved to zone 1 precipitational waters than those for zones 2 and 3 zones 2 and 3 can reasonably be modelled using input waters with the same isotopic compositions the same ca concentration and for some zone 2 cement the same mg concentrations the minimum mg ca ratios in input waters is constrained by the lower mg plateau in mg mn space and is about the same for all three zones about 1 25 the maximum mg ca ratios about 1 20 occurred in zone 2 input waters mn fe covariations also show an l shaped trend a trend whch fits neither a simple water rock interaction model nor a simple eh model a model is proposed in which eh and limited mn sources determine the amount of fe and mn available in solution to partake in water rock interaction water rock interaction modelling provides one useful approach to quantitatively interpreting isotopic and trace element data from diagenetic carbonates to gain a clearer understanding of evolution of diagenetic fluids

powder x ray diffraction analysis of homogeneous and heterogeneous sedimentary dolostones abstract the ca molar ca ca mg 100 in sedimentary dolomite which ranges from 48 ca to 62 5 ca can be determined by atomic absorption spectrometry electron microprobe analysis or powder x ray diffraction analysis the latter procedure which relies on the fact that the position of the d104 reflection depends linearly on the ca content offers an easy and relatively inexpensive method of determining ca in dolomite to date such analyses have assumed that the dolostone is homogeneous with respect to the ca content of its constituent dolomite crystals herein the xrd technique is extended to compositionally heterogeneous dolostones a considerable amount of important information can be derived from x ray diffractograms of dolostones if they are analyzed with peak fitting techniques pf xrd specifically this technique allows 1 determination of the ca with an accuracy of 0 5 ca 2 identification of heterogeneous dolostone samples formed of more than one population of dolomite crystals as defined by their ca content and 3 derivation of the proportion of each population of dolomite crystals with an accuracy of 10 the pf xrd technique is a powerful analytical tool that provides valuable information on the major element composition of low fe dolomite compared to other techniques the pf xrd technique is accurate rapid relatively inexpensive and easy to use

rb sr and u pb dating of bentonites a 6 in 15 cm bentonite in the z coal cretaceous tertiary boundary in eastern montana was sampled at four different places and biotite sanidine and zircon were separated from the clay u pb analyses of purified zircons yielded small systematic variations from concordant u pb dates plotting the data on a concordia diagram a short linear discordia line intersects the concordia at with an mswd of 1 16 the systematic variation of the four sets of zircon u pb data on the concordia plot may be an artifact of the sampling and purification procedure or could result from natural sample variation from minor contamination biotite fractions of varying specific gravity were obtained for each of the four z coal bentonite samples and together with the matching purified sanidine fraction were analysed for rb sr dating excluding those lighter biotite fractions found to have lost 30 or more of their original rb an isochron was obtained giving an age of 63 7 0 3 ma with an initial 87sr 86sr ratio of 0 706

the role of bacterial sulfate reduction during dolomite precipitation implications from upper jurassic platform carbonates the early diagenetic formation of dolomite in modern aquatic environments is limited mostly to evaporitic and marine anoxic organic rich sediments dominated by bacterial sulfate reduction bsr in such environments bacterial activity lowers the energy barriers for the nucleation and growth of dolomite and thus promotes the formation of non stoichiometric highly disordered and metastable proto dolomite although the boundary conditions for the formation of modern proto dolomites are considered to be generally understood the role of bsr during limestone dolomitization in ancient marine environments remains questionable herein we present a study about the physicochemical conditions and processes which led to the formation of partly dolomitized limestone and dolostone in the presence of bsr on a stable carbonate platform during the upper jurassic at oker northern german basin the dolomite textures the spatial trace elemental patterns of the dolomite and of the surrounding limestone and the results of 18o and 13c isotope analyses reveal that the oker dolomite has been formed by the early diagenetic replacement of magnesian calcite precursors at temperatures between 26 c and 37 c we interpret the mineralizing fluids responsible for dolomitization as pristine marine to slightly evaporitic and reducing seawater being modified during shallow seepage reflux and or evaporitic tidal pumping the elevated 34scas values 17 9 to 19 7 v cdt of the oker dolomite compared to ambient upper jurassic seawater indicate that bsr facilitated dolomite formation for the first time we show that a linear anti correlation exists between decreasing carbonate associated sulfate cas contents in dolomite and increasing ordering ratio of the dolomite lattice structure with the degree of cation order in dolomite to be given by degree of cation order dol 0 018 cas dol 68 3 r2 0 98 this correlation implies that the cas content of sedimentary dolomite can be used as a measure for dolomite maturity the relationships between the ambient paleo environmental controls the resultant dolomitization pathways and subsequently the structure and the composition of the precipitating dolomite are presented and discussed in relation to the stability of modern and ancient proto dolomites throughout burial diagenesis

absolute magnitude of the second order middle to late miocene sea level fall marion plateau northeast australia the miocene carbonate platform of the marion plateau in northeast australia preserves an excellent record of sea level fluctuations the warm water platform consists of an early to middle miocene second order highstand complex whereas the late miocene platform is a second order lowstand complex analysis of the sea level fall that led to this relation shows that the amplitude of this event was at least 180 m furthermore we suggest a methodology for determining the amplitude of sea level fluctuations based on measurement of sea level falls in structurally simple parts of passive margins or in distal parts of foreland basins

direct physical evidence of dolomite recrystallization the possibility of recrystallization is a long standing barrier to deciphering the genetic origin of dolomites there is often uncertainty regarding whether or not characteristics of ancient dolomites are primary or the consequence of later recrystallization unrelated to the original dolomitization event results from 65 new high temperature dolomite synthesis experiments 1 m 1 0 mg ca ratio solutions at 218 c demonstrate dolomite recrystallization affecting stoichiometry cation ordering and nanometre scale surface texture the data support a model of dolomitization that proceeds by a series of four unique phases of replacement and recrystallization which occur by various dissolution precipitation reactions during the first phase induction period no dolomite forms despite favourable conditions the second phase replacement period occurs when ca rich dolomite products with a low degree of cation ordering rapidly replace calcite reactants during the replacement period dolomite stoichiometry and the degree of cation ordering remain constant and all dolomite crystal surfaces are covered by nanometre scale growth mounds the third phase primary recrystallization period which occurs in the experiments between 97 and 100 dolomite is characterized by a reduced replacement rate but concurrent increases in dolomite stoichiometry and cation ordering the end of the primary recrystallization period is marked by dolomite crystal growth surfaces that are covered by flat laterally extensive layers the fourth phase of the reaction secondary recrystallization period occurs when all calcite is consumed and is characterized by stoichiometric dolomite with layers as well as a continued increase in the degree of cation ordering with time inferences of recrystallization in natural dolomite based on cation order or stoichiometry of dolomite usually depend on assumptions about the precursor dolomite subjected to recrystallization if it is assumed that the experimental evidence presented here is applicable to natural low temperature dolomites then the presence of mounds is direct evidence of a lack of recrystallization and the presence of layers is direct evidence of recrystallization

stratigraphic patterns of deep water dolomite northeast australia abstract in odp hole 815a located along the edge of the townsville trough and marginal to the marion plateau of northeast australia dolomite forms about 5 20 wt of carbonate within a distal part of a muddy contourite succession of pliocene 2 6 4 0 ma age integration of seismic lithic and biostratigraphic data identifies a history of current controlled episodic deposition subsequently shut down during rapid subsidence by 2 6 ma overlying pelagic periplatform sediment contains no or trace amounts of dolomite high order rhythmic 1 5 m thick variation in dolomite quartz and carbonate content within the contourite succession defines a past high order stratigraphic variation in diagenetic potential for dolomite d13c values of dolomite reflect incorporation of at least two sources of bicarbonate mixed with seawater 1 in more carbonate rich sediment d13c values 1 to 2 0 indicate likely contribution from dissolution of metastable carbonate 2 in more siliciclastic rich sediment with locally elevated total organic content d13c values 4 to 6 reflect contribution from organic matter diagenesis the majority of d18o values of dolomite over the entire contourite succession vary between 3 2 and 2 and are interpreted to reflect near surface to shallow tens of meters subsurface marine derived diagenesis a few dolomites with lower 2 to 0 values are restricted to two stratigraphic intervals and reflect one or some combination of the following processes a second stage of dolomitization at greater depths near surface diagenesis with much warmer bottom waters or burial diagenesis with mixture of meteoric and marine derived pore fluids magnesian calcite is absent today in the contourite succession however the depositional and diagenetic frameworks of the contourite succession suggest that metastability of aragonite and magnesian calcite variable current activity and seawater diffusion across the sediment water interface all promoted dolomitization compilation of dolomite stratigraphy along slopes off northeast australia illustrates that dolomite distribution in hole 815a is one of four intervals d1 d4 in strata younger than 4 ma d1 0 3 0 8 ma d2 1 1 3 ma d3 d3a 1 8 and d3b 2 0 2 4 ma and d4 2 6 4 0 ma the stratigraphy is linked to temporal variation in supply of shelf and pelagic derived metastable carbonate regulated by changes in paleoceanography eustasy and tectonics d1 d3b and d4 are in sediments with ages similar to previously documented dolomitization stages in shallow water platforms elsewhere in the world and suggest a coincidence between elevated diagenetic potential for deep water dolomitization in platform margins and substantive changes in ocean c volume transport linked to eustatic change in sea level

early diagenetic recrystallization of holocene 3000 years old peritidal dolomites ambergris cay belize three peritidal carbonate crusts and associated intercrust sediments total thickness of 30cm aged 3000 years bp on ambergris cay belize contain 32 100 calcian dolomite x 72 5 dolomite ranging in composition from 40 to 46 mol mgco3 x 43 3 dolomite replaced high mg calcite foraminiferal muds penecontemporaneously with sedimentation forming partially dolomitized sediments and lithified crusts dolomitization probably occurred in normal to moderately evaporated seawater and is apparently continuing at the present detailed scanning electron microscope analysis shows a linear increase in mean dolomite crystal size with depth 0 4 m near the top of the section to 1 0 m near the base of the dolomitized section this size increase is not accompanied by any significant decrease in porosity crystal size distributions appear to be log normal and become increasingly broad and flat with depth rietveld x ray pattern fitting structure refinements indicate increasing ca and mg concentrations on their respective sites cation ordering as a function of increasing depth most of the ordering occurs within the first 15 cm of the surface stoichiometry does not increase with depth indicating no relationship between the ca mg ratio and cation ordering strong geochemical trends were observed down section in the dolomite including 1 increasing mn content 44 to 274 ppm and 2 decreasing 13c values 0 9 to 5 5 pdb oxygen isotope values range from 18o 1 3 pdb in the upper part of the section to 2 6 pdb in the lower part of the section and are interpreted to represent two distinct groups of values rather than a continuous trend down section dolomite crystal size increase and shapes of crystal size distributions are consistent with recrystallization via a surface energy driven dissolution reprecipitation process ostwald ripening the observed trends in carbon isotopes and mn content probably result from geochemical re equilibration during recrystallization and reflect reducing conditions and an isotopically light organically derived carbon source oxygen isotope compositions probably reflect relict original dolomite values and are a result of decreasing evaporation due to rising sea level

precambrian crust beneath the mesozoic northern canadian cordillera discovered by lithoprobe seismic reflection profiling 1 the cordillera in northern canada is underlain by westward tapering layers that can be followed from outcrops of proterozoic strata in the foreland belt to the lowermost crust of the orogenic interior a distance of as much as 500 km across strike they are interpreted as stratified proterozoic rocks including 1 8 0 7 ga supracrustal rocks and their basement the layering was discovered on two new deep seismic reflection profiles in the yukon line 3 650 km and northern british columbia line 2 1245 km in two segments that were acquired as part of the lithoprobe slave northern cordillera lithospheric evolution snorcle transect in the mackenzie mountains of the eastern yukon the layering in line 3 is visible between 5 0 and 12 0 s 15 to 36 km depth it is followed southwestward for nearly 650 km 500 km across strike and thins to less than 1 0 s 3 0 3 5 km thickness near the moho at the yukon alaska international boundary in the northern rocky mountains of british columbia the upper part of the layering on line 2 correlates with outcrops of proterozoic 1 76 1 0 ga strata in the muskwa anticlinorium at this location the layering is at least 15 km thick and is followed westward then southward into the middle and lower crust for 700 km 300 km across strike it disappears as a thin taper at the base of the crust 150 km east of the coast of the alaskan panhandle the only significant disruption in the layering occurs at the tintina fault zone a late to postorogenic strike slip fault with up to 800 km of displacement which appears as a vertical zone of little reflectivity that disrupts the continuity of the deep layering on both profiles 300 km apart the base of the layered reflection zone coincides with the moho which exhibits variable character and undulates in a series of broad arches with widths of 150 km in general the mantle appears to have few reflections however at the southwest end of line 3 near the alaska british columbia border a reflection dips eastward from 14 0 s to 21 0 s 45 to 73 km depth beneath exposed eocene magmatic rocks it is interpreted as a relict subduction surface of the kula plate our interpretation of proterozoic layered rocks beneath most of the northern cordillera suggests a much different crustal structure than previously considered 1 ancient north american crust comprising up to 25 km of metamorphosed proterozoic to paleozoic sediments plus 5 10 km of pre 1 8 ga crystalline basement projects westward beneath most of the northern canadian cordillera 2 the lateral 500 km by at least 1000 km and vertical up to 25 km extent of the proterozoic layers and their internal deformation are consistent with a long lived margin for northwestern north america with alternating episodes of extension and contraction 3 the detachments that carry deformed rocks of the mackenzie mountains and northern rocky mountains are largely confined to the upper crustal region above the layering 4 accreted terranes include thin klippen that were thrust over north american pericratonic strata e g yukon tanana and terranes such as nisling and stikinia that thicken westward as the underlying proterozoic layers taper and disappear 5 the ages of exposed rocks are not necessarily indicative of the ages of underlying crust a frequent observation in lithoprobe interpretations so that estimates of crustal growth based on surface geology may not be representative

a paleomagnetic study of the structural deformation in the yerington district nevada 2 mesozoic basement units and their total and pre oligocene tectonism

timing and magnitude of miocene eustasy derived from the mixed siliciclastic carbonate stratigraphic record of the northeastern australian margin article eustasy is a key parameter to understand sedimentary sequences on continental margins and to reconstruct continental ice volume in the cenozoic but timing and magnitude of global sea level changes remain controversial especially for the miocene epoch we analyzed sediment cores recovered from the marion plateau offshore northeastern australia during ocean drilling program odp leg 194 to define the mechanisms and timing of sequence formation on mixed carbonate siliciclastic margins and to estimate the amplitude of miocene eustatic adjustments we identified sequence boundaries on seismic reflection lines significantly revised the existing biostratigraphic age models and investigated the sedimentary response to sea level changes across the marion plateau we subdivided the miocene sediments into three sequence sets comprising a set of prograding clinoforms a muddy prograding carbonate ramp evolving into an aggrading platform and a lowstand ramp evolving into a backstepping ramp we recognized eight individual sequences dated at 18 0 ma 17 2 ma 16 5 ma 15 4 ma 14 7 ma 13 9 ma 13 0 ma and 11 9 ma we demonstrate that sequences on the marion plateau are controlled by glacio eustasy since sequence boundaries are marked by increases in 18 o deep sea miocene isotope events mi1b mbi 3 mi2 mi2a mi3a mi3 mi4 and mi5 respectively which reflects increased ice volume primarily on antarctica our backstripping estimates suggest that sea level fell by 26 28 m at 16 5 ma 26 29 m at 15 4 ma 29 38 m at 14 7 ma and 53 81 m at 13 9 ma combining backstripping with 18 o estimates yields sea level fall amplitudes of 27 1 m at 16 5 ma 27 1 m at 15 4 ma 33 3 m at 14 7 ma and 59 6 m at 13 9 ma we use a similar approach to estimate eustatic rises of 19 1 m between 16 5 and 15 4 ma 23 3 m between 15 4 and 14 7 ma and 33 3 m between 14 7 and 13 9 ma these estimates can be combined into a eustatic curve that suggests that sea level fell by 53 69 m between 16 5 and 13 9 ma this implies that at least 90 of the east antarctic icesheet was formed during the middle miocene the new independent amplitude estimates are crucial as the miocene is the geologic epoch for which the new jersey margin sea level record is poorly constrained

the origin of carbonate cements in the hildasay reservoir cambo field faroe shetland basin clumped isotopic analysis and implications for reservoir performance abstract the early eocene paralic sandstones of the hildasay member of the flett formation form the major oil bearing reservoir in the cambo field located in the faroe shetland basin the sandstones locally contain calcite cemented intervals that vary in thickness from decimetre to over 1 m a calcite cemented interval from well 204 10a 5 has been analysed petrographically and using clumped isotopes to determine its mode of formation and potential lateral extent petrographic analysis shows the cemented interval to consist of ferroan calcite with a consistent dull red cathodoluminescence suggesting a single phase of precipitation the centre of the cemented interval comprises a finer grained unit with detrital kerogen and early sphaerosiderite while the rest comprises homogeneous porosity occluding ferroan calcite the early sphaerosiderite in the centre is replaced by ferroan calcite with a high 13 carbon isotopic signature 13 cvpdb 7 53 suggesting that it formed during anaerobic methanogenesis or fermentation samples from the intermediate zone have a lower 13 carbon isotopic composition 13 cvpdb 0 72 to 3 68 in comparison the outer margin of the cemented unit has an even lower 13 carbon isotopic signature 13 cvpdb 15 5 to 15 9 more typical of a strong aerobic oxidation source the oxygen isotopic signature of the cements is similar 18 ovpdb 10 9 to 12 2 analysis of the clumped isotopes suggest that the ferroan calcite formed at 40 50 c from pore waters that were predominantly meteoric in origin 18 ovsmow 4 8 to 5 7 burial history modelling would suggest that the cements formed at a depth of 500 1000 m meteoric water was likely to have been introduced during the formation of the mid lutetian unconformity 45mya approximately 10 mya after the sandstones were deposited the model proposed is that ferroan calcite precipitation was initiated in the fine lag deposits that contained the kerogen and sphaerosiderite and then grew outwards if the model is correct the cemented units should be restricted to this facies and consequently of limited lateral extent consequently it is likely that the cemented intervals will have a limited impact upon the reservoir performance and are unlikely to act as major barriers to fluid flow

reconstructing past sea surface temperatures correcting for diagenesis of bulk marine carbonate abstract a numerical model which describes oxygen isotope exchange during burial and recrystallization of deep sea carbonate is used to obtain information on how sea surface temperatures have varied in the past by correcting measured 18o values of bulk carbonate for diagenetic overprinting comparison of bulk carbonate and planktonic foraminiferal 18o records from odp site 677a indicates that the oxygen isotopic composition of bulk carbonate does reflect changes in sea surface temperature and 18o at odp site 690 we calculate that diagenetic effects are small and that both bulk carbonate and planktonic foraminiferal 18o records accurately reflect paleogene warming of high latitude surface oceans biased from diagenesis by no more than 1 c the same is likely to be true for other high latitude sites where sedimentation rates are low at dsdp sites 516 and 525 the effects of diagenesis are more significant measured 18o values of eocene bulk carbonates are more than 2 lower at deeply buried site 516 than at site 525 consistent with the model prediction that the effects of diagenesis should be proportional to sedimentation rate model corrections reconcile the differences in the data between the two sites the resulting paleotemperature reconstruction indicates a 4 c cooling of mid latitude surface oceans since the eocene at low latitudes the contrast in temperature between the ocean surface and bottom makes the carbonate 180 values particularly sensitive to diagenetic effects most of the observed variations in measured 18o values are accounted for by diagenetic effects rather than by sea surface temperature variations we show that the data are consistent with constant equatorial sea surface temperatures through most of the cenozoic with the possible exception of the early eocene when slightly higher temperatures are indicated we suggest that the lower equatorial sea surface temperatures for the eocene and oligocene reported in other oxygen isotope studies are artifacts of diagenetic recrystallization and that it is impossible to reconstruct accurately equatorial sea surface temperatures without explicitly accounting for diagenetic overprinting

preliminary paleomagnetic data from the jurassic humboldt lopolith west central nevada evidence for thrust belt rotation in the fencemaker allochthon layered and massive gabbros of the humboldt lopolith and extrusive equivalents in west central nevada yield well grouped paleomagnetic data on a site basis with corrections for attitudes of compositional layering in gabbros and flow contacts in lavas kcorr kin situ 5 5 the mean of 15 site means is d 307 i 21 95 9 k 17 while k ar data imply a late jurassic age we prefer an older probably early jurassic age for the lopolith and its magnetization based upon magnetization data and relations of these rocks with host strata compared with early jurassic expected directions the data reveal a discordancy in declination of 37 14 22 which implies a significant counterclockwise rotation of the lopolith local rotation of fence maker allochthon units most likely occurred when southeasterly directed thrust sheets obliquely contacted a generally north south trending shelf margin mesozoic deformation will be more accurately estimated with better age control and measurement of possible cenozoic crustal rotation

recrystallization versus neomorphism and the concept of significant recrystallization in dolomite research abstract the term neomorphism originally introduced in 1965 as a substitute for the term recrystallization in limestones is now in use also for dolostones however only very few dolomite researches use the term neomorphism even though it is well and strictly defined many do not know what this term means and using the term commonly leads to confusion among carbonate researchers as well as between them and other geologists partially or largely for these reasons most dolomite researchers use the term recrystallization rather than neomorphism in addition the term neomorphism does not encompass all the important characteristics of recrystallization known today it is recommended therefore to abandon the term neomorphism in dolomite research and apply the term recrystallization with an extended definition that includes those characteristics that commonly are involved in recrystallization and that can be measured in dolomites and dolostones textural changes size shape structural changes ordering strain compositional changes stoichiometry isotopes trace elements including zoning fluid inclusions and changes in the paleomagnetic properties for practical applications the absence presence and or degree of recrystallization is often important for a genetic interpretation of dolostones if changes via recrystallization in texture structure composition and or paleomagnetic properties results in data ranges that are larger than the original ones which must be known for reference a dolomite dolostone is said to be significantly recrystallized and the reset properties are no longer representative of the fluid and environment of dolomitization but characterize the last event of recrystallization however not all measurable properties must be reset during recrystallization in this case inherited properties represent the event of dolomitization whereas reset properties represent the fluids and process of recrystallization in recrystallization resets only one of 10 properties e g 87 sr 86 sr ratios beyond the range of the pristine samples within the analytical errors this dolomite is significantly recrystallized with respect to sr isotopes yet insignificantly recrystallized with respect to the other nine properties in either case values of those properties that are identical to the pristine reference values can be used for genetic interpretations of dolomitization the concept of significant recrystallization is of great use in genetic interpretations of dolomites dolostones in particular a reliable interpretation of the chemistry of the dolomitizing fluids no longer depends on an absolute recognition of recrystallization rather it is sufficient to recognize that a dolomite is insignificantly recrystallized application of this principle also shows that many ancient dolomites and dolostones are insignificantly recrystallized finally the concept of significant recrystallization is independent of mineralogy hence it can and should be applied also to limestones

a 600 million year carbonate clumped isotope record from the sultanate of oman carbonate clumped isotope thermometry is a promising technique that has the potential to help decode the significance of the variability of both physical and geochemical compositions of ancient carbonate rocks this study utilizes a 600 million year record of marine carbonate rocks from the subsurface and surface of the sultanate of oman to explore how burial and exhumation affected the carbonate clumped isotope thermometer samples span 6 km of burial depth and include calcite and dolomite mineralogies and a range of carbonate rock textures we find evidence for two broad patterns in the physical and geochemical behavior of carbonate rocks during burial the first group of carbonates yield water 18 o vsmow compositions slightly enriched or equal to an expected ice free seawater composition of 1 2 and display good to fair textural preservation suggesting that cementation and lithification occurred within tens of meters of the sediment water interface temperatures from the second group sit on the present day geotherm yield highly enriched water 18 o vsmow compositions and display fair to poor textural preservation we find no evidence for solid state reordering in paired analyses of calcites and dolomites our results contribute to a growing body of work that indicates that the seawater 18 o vsmow composition has not changed significantly over 600 myr and was not 6 in the ediacaran

shallow burial dolomite cement a major component of many ancient sucrosic dolomites dolomite cement is a significant and widespread component of phanerozoic sucrosic dolomites cements in dolomites that were never deeply buried are limpid have planar faces non saddle forms often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores five samples of sucrosic dolomites interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers provide insights into the abundance of planar cements in sucrosic dolomites such cement comprises 11 to 45 32 mean of peritidal to sub tidal dolomites on an outcrop in the edwards aquifer early cretaceous of central texas 19 to 33 25 mean of ramp dolomites in the hawthorn group oligomiocene and 50 to 70 in shelf dolomites of the avon park formation eocene in the upper floridan aquifer of sub surface peninsular florida 18 to 45 32 mean of sub tidal shelf dolomites in quarry sections of the burlington keokuk formation early mississippian in south eastern iowa and 18 to 76 50 mean in shallow cores and outcrops of outer shelf dolomites from the gambier limestone oligo miocene of south australia backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening pore space reduction induration and general maturation of these dolomites most pre holocene dolomites are multiphase crystalline rocks composed of i seed crystals or cores ii crystal cortices that concentrically enlarged the cores and iii free space syntaxial precipitates of limpid cement around the crystals remaining caco3 grains and micrite can be replaced by dolomite but typically they are dissolved between stages ii and iii creating systems of intercrystal and mouldic pores typical of sucrosic dolomites networks of cement overgrowths aided by water filled pore systems under hydrostatic to lithostatic pressure are judged to slow or prevent compaction in sucrosic dolomites it can be argued that cortex growth involves both replacement of caco3 particles and microcementation of their interparticle pores this interpretation and the abundance of cements in so many dolomites would obviate the controversy over the volumetrics of replacement dolomitization limpid planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters at low temperatures 30 to 35 c and shallow burial depths 100 m in water saturated networks of dolomite silt and sand cements in many dolomites in island and continental aquifer systems appear to result from event driven processes related to sea level highstands cementation events can follow replacement

style and age of late oligocene early miocene deformation in the southern stillwater range west central nevada paleomagnetism geochronology and field relations paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late oligocene to early miocene episode of large magnitude extension in the southern stillwater range and adjacent regions of west central nevada the southern stillwater range was the site of an oligocene to early miocene volcanic center comprising 1 28 3 to 24 3 ma intracaldera ash flow luffs lava flows and subjacent plutons associated with three calderas 2 24 8 to 20 7 ma postcaldera silicic dikes and domes and 3 unconformably overlying 15 3 to 13 ma dacite to basalt lava flows plugs and dikes the caldera related luffs lava flows and plutons were tilted 60 70 either west or east during the initial period of cenozoic deformation that accommodated over 100 extension directions of remanent magnetization obtained from these extrusive and intrusive caldera related rocks are strongly deflected from an expected miocene direction in senses appropriate for their tilt a mean direction for these rocks after tilt correction however suggests that they were also affected by a moderate 33 4 11 8 component of counterclockwise vertical axis rotation paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24 8 to 20 7 ma postcaldera dikes and domes in detail an apparent decrease in rotation with decreasing age of individual isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24 4 and 24 2 ma the onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked deformation was not driven by magma buoyancy however because tilting equally affected the caldera systems of different ages including their plutonic roots it is more likely that regional extension was focused in the southern stillwater range due to magmatic warming and reduction of tensile strength of the brittle crust faults that accommodated deformation in the southern stillwater range initially dipped steeply and cut deeply to expose more than 9 km of crustal section the exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early miocene brittle ductile transition

fluid inclusions in sedimentary and diagenetic systems abstract some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals important fluids in the sedimentary realm include atmospheric gases fresh water of meteoric origin lake water seawater mixed water evaporated water formation waters deep in basins oil and natural gas preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global change research is essential for improving understanding of diagenetic systems and provides useful information in petroleum geology applications of fluid inclusions to sedimentary systems are not without their complexities some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration which results in stretching or leakage and refilling of some fluid inclusions similarly overheating in the laboratory can also cause reequilibration of fluid inclusions so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment fluid inclusion assemblages fia consistency in homogenization temperatures within a fluid inclusion assemblage consisting of variably sized and shaped inclusions is the hallmark of a data set that has not been altered through thermal reequilibration in contrast fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration if a fluid inclusion assemblage has not been altered by thermal reequilibration its fluid inclusions may be useful as geothermometers for low and high temperature systems or useful as geobarometers applicable throughout the sedimentary realm if a fluid inclusion assemblage has been altered partially by thermal reequilibration techniques for distinguishing between altered and unaltered fluid inclusions may be applied in studies of global change fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments fluid inclusions also preserve microsamples of ancient seawater and atmosphere the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans in petroleum geology fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons they can delineate the evolution of the chemistry of hydrocarbons and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems in studies of diagenesis fluid inclusions can be the most definitive record most diagenetic systems are closely linked to temperature and salinity of the fluid thus fluid inclusions are sensitive indicators of diagenetic environments

diagenesis and sr isotopic evolution of seawater using data from dsdp 590b and 575 new sr isotopic and concentration analyses of bulk carbonate and porewaters from a carbonate rich miocene section sampled at dsdp site 575 are reported and used along with our earlier data from a section of similar age at dsdp site 590b in a numerical model to determine for each site the rate of sr exchange during diagenesis and the degree to which this amount of exchange has shifted the87sr86sr of the bulk carbonate from the ratio each increment of sediment had when first deposited despite the fact that the two sites have very different water depth 4536 m at 575 vs 1299 m at 590b different sedimentation rate 10 m my at 575 vs 40 m my at 590b and significantly different porewater sr2 gradients we find that the rate of sr exchange as a function of sediment age is almost indistinguishable between the two sites the rate of sr exchange determined at the two sites is such that the resulting shift in bulk carbonate87sr86sr due to diagenesis is small compared to the total range of87sr86sr values measured but large compared to the analytical uncertainty of the individual isotopic ratio measurements by taking this shift into account we reconstruct the original87sr86sr of each increment of carbonate sampled which when plotted as a function of age becomes our best estimate of the sr isotopic evolution of seawater because sr is very well mixed in the ocean at any given time there is a single worldwide value of seawater87sr86sr therefore if we are quantitatively accounting for the effect sr exchange we should find the same seawater evolution curve regardless of what dsdp site is used when we compare the observed bulk carbonate87sr86sr vs age at the two sites they are seen to differ by amounts that are sometimes large compared to the analytical uncertainties of the measurements however when these data are corrected for the post depositional sr isotopic shifts predicted by our diagenetic model we find almost perfect agreement this agreement suggests that we have made a realistic determination of the rate of sr exchange and its consequences in terms of shifting the87sr86sr of the bulk carbonate and more importantly that sr isotopes can be used to correlate marine sediments with an accuracy comparable to the very small analytical uncertainties of modern isotopic measurements

the effects of diagenesis and dolomitization on ca and mg isotopes in marine platform carbonates implications for the geochemical cycles of ca and mg abstract the ca mg o and c isotopic and trace elemental compositions of marine limestones and dolostones from odp site 1196a which range in depth 58 to 627 mbsf and in depositional age 5 and 23 ma are presented the objectives of the study are to explore the potential for non traditional isotope systems to fingerprint diagenesis to quantify the extent to which geochemical proxies are altered during diagenesis and to investigate the importance of diagenesis within the global ca and mg geochemical cycles the data suggest that ca which has a relatively high solid to fluid mass ratio can be isotopically altered during diagenesis in addition the alteration of ca correlates with the alteration of mg in such a way that both can serve as useful tools for deciphering diagenesis in ancient rocks bulk carbonate 44 ca values vary between 0 60 and 1 31 srm 915a scale the average limestone 44 ca is 0 97 0 24 1sd identical within error to the average dolostone 1 03 0 15 1sd magnesium isotopic compositions 26 mg dsm 3 scale range between 2 59 and 3 91 and limestones 3 60 0 25 and dolostones 2 68 0 07 are isotopically distinct carbon isotopic compositions 13 c pdb scale vary between 0 86 and 2 47 with average limestone 1 96 0 31 marginally offset relative to average dolostone 1 68 0 57 the oxygen isotopic compositions 18 o pdb scale of limestones 1 22 0 94 are substantially lower than the dolostones measured 2 72 1 07 the isotopic data from 1196a suggest distinct and coherent trends in isotopic and elemental compositions that are interpreted in terms of diagenetic trajectories numerical modeling supports the contention that such trends can be interpreted as diagenetic and suggests that the appropriate distribution coefficient k mg associated with limestone diagenesis is 1 to 5 10 3 distinctly lower than those values 0 015 reported in laboratory studies with respect to mg isotopes the modeling also suggest that diagenetic fractionation factors of 0 9955 4 5 and 0 9980 2 are appropriate for limestone diagenesis and dolomitization respectively

testing clumped isotopes as a reservoir characterization tool a comparison with fluid inclusions in a dolomitized sedimentary carbonate reservoir buried to 2 4 km constraining basin thermal history is a key part of reservoir characterisation in carbonate rocks conventional palaeothermometric approaches cannot always be used fluid inclusions may be reset or not present while 18o palaeothermometry requires an assumption on the parent fluid composition the clumped isotope palaeothermometer however is a promising technique for constraining the thermal history of basins in this study we test if clumped isotopes record temperatures of recrystallisation in deeply buried dolomitic reservoirs through comparison with fluid inclusion data the studied reservoir is the cretaceous pinda formation offshore angola a deeply buried dolomitised sedimentary carbonate hydrocarbon reservoir it provides an ideal test case as samples from industry wells are available over a relatively wide burial depth range of 2000 4000 metres below sea floor mbsf and the constituent dolomites are relatively homogeneous across this depth range fluid inclusion homogenisation temperatures for the pinda formation record a range of temperatures from 110 170 c increasing with depth these closely match present day ambient well temperatures indicating recent resetting of the fluid inclusions clumped isotopes however record temperatures significantly 20 60 c below fluid inclusion and well temperatures for the seven samples analysed the deepest five samples 2800 3700 mbsf record clumped isotope temperatures around 100 120 c interpreted to represent a deep burial recrystallisation event responsible for a massive re dolomitisation of the reservoir the lower clumped isotope temperatures 65 and 82 c of the shallower 2055 and 2740 mbsf samples are interpreted to represent physical mixing of two dolomite generations due to incomplete burial recrystallisation of an early shallow dolomite determination of temperature through clumped isotopes allows calculation of the parent fluid 18o values in the five deepest samples the fluid 18o values of 3 7 6 5 cluster around the modern day porewater composition 5 suggesting burial dolomitisation occurred in the presence of evolved brine mineral 18o values of 4 5 to 7 are lower than pristine cretaceous marine dolomite and are in accordance with burial recrystallisation clumped isotopes are therefore interpreted to record temperatures corresponding to open system burial recrystallisation events this study shows that clumped isotopes are a valuable tool in characterising the thermal history of deeply buried 2000 m carbonate hydrocarbon reservoirs

petrology and tectonic significance of augen gneiss from a belt of mississippian granitoids in the yukon tanana terrane east central alaska an approximately east west trending belt of porphyritic peraluminous granitoids metamorphosed and deformed to augen gneiss is exposed for 400 km across the yukon tanana terrane this belt consists of compositionally texturally and isotopically similar concordant intrusions of augen gneiss in the big delta eagle and tanacross quadrangles of east central alaska in the fiftymile batholith of west central yukon territory and in two batholiths in the offset part of the yukon tanana terrane northeast of the tintina fault in southeastern yukon territory chemical analyses of augen gneiss from widely separated localities within the big delta intrusion suggest that the various samples are related by crystal fractionation comparison of major and trace element data from concordant layers of augen poor gneiss and aplitic gneiss with similar data from adjacent augen gneiss suggests that the layers represent co magmatic sills of more highly differentiated filter pressed melt similarities between elemental abundances particularly those of rare earth elements in augen gneiss from the large bodies in the big delta eagle and tanacross quadrangles alaska and southeastern yukon territory suggest a common origin for these bodies the peraluminous composition of the gneiss its high initial 87 sr 86 sr ratio and high th concentration indicate that the augen gneiss protolith contains a large component of crustal material geochronologic studies of augen gneiss in east central alaska and southeastern yukon territory indicate a mississippian intrusive age and an early proterozoic age for the crustal component the augen gneiss bodies were intruded late in a middle paleozoic volcanic plutonic episode and they may represent a deeply eroded continental magmatic arc regional amphibolite facies metamorphism and mylonitization may have occurred late during the tectonic episode that resulted in intrusion of the porphyritic granitoid protoliths

oxygen and hydrogen isotope characteristics of hydrothermal alteration at the ann mason porphyry copper deposit yerington nevada late cenozoic normal faulting has tilted a 3 by 5 km block containing the mid jurassic ann mason porphyry copper deposit 90o w and exposed the hydrothermal alteration pattern in cross section from 1 to 6 km palcodepth oxygen and hydrogen isotopes have been used to deduce sources of hydrothermal fluids fresh samples of the host yerington batholith pluton have typical calc alkaline magmatic isotopic compositions 18 o whole rock 6 7 7 0 d biotite 85 to 88 the various types of hydrothermally altered rocks have similar d and 18 o values only slightly isotopically shifted from original magmatic values except along an axial zone within and above the orebody in this zone altered rocks are enriched in 18 o by 1 to 3 5 per mil and 18 o values increase with decreasing palcodepth the 18 o values of the whole rock and or feldspar and the d values of biotite chlorite and or actinolite in the various alteration zones are as follows propylitic alteration is widespread and is isotopically similar to fresh rock 18 o 6 2 to 7 2 per mil d 82 to 92 per mil potassic alteration characterized by 100 percent alteration of hornblende to biotite and cu mineralization occurs at 3 5 km palcodepth along the axis of the mineralizing granite porphyry dike swarm 18 o 6 5 to 8 4 per mil d 68 to 96 per mil sodic calcic alteration characterized by hydrothermal oligoclase actinolite sphene occurs laterally alongside and below the ore zone at 3 to 6 km palcodepth 18 o 5 7 to 8 4 per mil d 67 to 90 per mil late stage sodic albite chlorite alteration occurs at 1 to 4 km palcodepth in the vicinity of the axial zone 18 o 6 7 to 9 6 per mil d 79 to 85 per mil the youngest late stage sericitic alteration occurs at the shallowest levels primarily at 1 to 2 km palcodepth and displays a distinct 18 o and d enrichment 18 o 9 8 to 10 4 per mil d 61 per mil integration of stable isotope data geologic field relations and phase equilibria constraints indicates that the high salinity fluids responsible for potassic alteration and cu mineralization were of internal magmatic origin but that synchronous deep convecting saline sodic calcic fluids were largely derived from external nonmagmatic sources a third type of fluid also external but lower in temperature and much more dilute was prevalent at shallow levels and caused latestage sericitic alteration this was either an isotopically heavy coastal meteoric water or a seawater

paleomagnetic assessment of oroflexural deformation in west central nevada and significance for emplacement of allochthonous assemblages abrupt westerly deflections of north south trending facies boundaries the isopleth of 87sr 86sr 0 076 and arcuate structural trends of paleozoic mesozoic and cenozoic age in the west central basin and range province have been explained by 1 post early jurassic oroflexural folding and shear 2 the existence of a pre late jurassic resistant block along the western continental margin possibly representing late precambrian breakup and 3 regional flexure mina deflection prior to intrusion of late cretaceous plutons which crosscut flexure related structural trends paleomagnetic data gathered from late cretaceous plutons and remagnetized metasedimentary rocks along and north of the northern margin of the zone of east west structural trends are not in support of significant oroflexural folding and attending major clockwise rotation since late cretaceous time the data water canyon pluton d 1 i 57 95 4 k 9 n 132 gunmetal pluton d 354 i 60 95 6 k 21 n 29 east garfield hills pluton d 349 i 74 95 11 k 14 n 12 la panta pluton d 22 i 66 95 10 k 16 n 13 gillis canyon pluton d 354 i 50 95 4 k 20 n 50 and folded units of the excelsior mountains d 309 i 79 95 7 k 12 n 37 are dispersed from expected cretaceous directions for the mina deflection region but they do not conform to a model of major systematic regional clockwise rotation the generally small deflections of unit means from expected directions could be explained by irregular components of late cenozoic tilting and or rotation the paleomagnetic data do not deny the possibility of pre late cretaceous oroclinal bending nevertheless regional structural analysis complimented by the paleomagnetic results indicate that the mina deflection is in all likelihood a primary feature whose geometry reflects the late precambrian fragmentation of western north america structures related to the accretion of allochthonous assemblages between middle devonian and cretaceous time in this part of the cordillera also reflect the primary nature of this feature

rock buffered recrystallization of marion plateau dolomites at low temperature evidenced by clumped isotope thermometry and x ray diffraction analysis abstract much debate exists on the extent to which early dolomites recrystallize and preserve the signature of their primary diagenetic setting here we combine clumped isotopes thermometry with x ray diffraction and thin section petrography to study dolomite recrystallization under shallow burial