chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

the los alamos sferic array a research tool for lightning investigations 1 since 1998 the los alamos sferic array lasa has recorded electric field change signals from lightning in support of radio frequency rf and optical observations by the fast on orbit recording of transient events forte satellite by sferic a colloquial abbreviation for atmospheric we refer to a remote measurement of the transient electric field produced by a lightning flash lasa consisted of five stations in new mexico in 1998 and was expanded to 11 stations in new mexico texas florida and nebraska in 1999 during the 2 years of operation described in this paper the remote stations acquired triggered 8 or 16 ms duration 12 bit waveforms and gps based sferic time tags 24 hours per day year round source locations were determined daily using differential time of arrival techniques and the waveforms from all geolocated events were transferred to los alamos national laboratory lanl where they have been archived for further analysis including event classification and characterization we evaluated lasa location accuracy by comparing temporally coincident occurring within 100 s lasa and national lightning detection network nldn event locations approximately one half of the locations agreed to within 2 km with better agreement for events that occurred within the confines of lasa subarrays in new mexico and florida of the 900 000 events located by the sferic array in 1998 and 1999 nearly 13 000 produced distinctive narrow bipolar field change pulses resembling those previously identified as intracloud discharges

dolomitization a critical view of some current views the principal models in vogue today for dolomitization are the mixing zone and the sabkha models despite the wide acceptance of these models there has been little critical assessment of their validity such an assessment is the objective of the present paper a close look at the dorag mixing zone model badiozamani 1973 reveals several serious weaknesses 1 the range of freshwater seawater mixtures that meet the dorag requirement for dolomitization shrinks to a small window if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based 2 in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed 3 in a number of dolomites interpreted to be of mixing zone origin dolomite has precipitated without dissolution of the calcite substrate evidence that negates the fundamental premise of the dorag model in addition to these weaknesses isotope and trace element data used to identify mixed water dolomite are inadmissible because 1 isotopic fractionation factors for dolomites remain unresolved 2 isotope values uncorrected for temperature for a host of dolomites interpreted to be of different origins overlap 3 nonisomorphous trace elements such as na in dolomite cannot on theoretical grounds be relied on to identify dolomitizing fluids similar overall objections can be brought against folk and land9s 1975 schizohaline version of mixing zone dolomitization in summary mixing zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization contemporaneous dolomite formation in modern sabkhas is well documented but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved in the well studied sabkha at abu dhabi brine chemistry changes have been used as evidence of a replacement origin for the dolomite but it is shown here that this evidence is far from unequivocal an alternative origin of direct precipitation of dolomite is offered an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes and in line with our laboratory experience with dolomite synthesis moreover it is suggested here that in general contemporaneous dolomite will form at low temperatures only by direct precipitation a mechanism that requires special conditions of highly super saturated waters of high mg ca ratio and elevated co 3 hco 3 concentrations this may explain why modern dolomite is mainly restricted to evaporitic environments a bias not shared by ancient dolomites in contrast replacement dolomite appears to require at low temperatures long reaction times or 10 4 yr a requirement that is mainly to be met in large regional groundwater flow systems both marine and nonmarine a third dolomitization model considered here is that of baker and kastner 1981 based on the experimental finding that sulfate ions inhibit or retard dolomitization a number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations 2 to 70 times that of seawater thus the baker kastner model should be held in abeyance until these serious contradictions are resolved the current emphasis on mixing zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem four of these avenues each of which deemphasizes the special water approach are briefly addressed and are as follows 1 influence of temperature and time 2 mass transfer processes 3 burial diagenesis of epigenetic dolomites 4 fluid inclusion studies at elevated temperatures the dolomite problem essentially disappears ordered dolomite can be made in the laboratory in days at 100 degrees c what is more at temperatures above 60 degrees c ca rich waters become dolomitizing fluids which makes most natural subsurface waters capable of dolomitization at low temperatures time may be the key element so that seawater will become a major dolomitizing fluid only where stable circulation systems such as kohout convention can drive seawater through carbonate platforms for many thousands to millions of years this paper has tried to show that currently favored models of dolomitization carry serious uncertainties enough to warn us to look more critically at the validity of these models and also it is hoped enough to spur new efforts to find new solutions to the problems of dolomitization

electromagnetic induction studies recent developments in electromagnetic induction studies of the lithosphere and the asthenosphere are reviewed attention is given to geoelectrical studies of active tectonic areas in terms of the major zones of crustal extension the basin and range province along western regions of north america and the rio grande rift studies have also been performed of tectonic activity around iceland the salton trough and cerro prieto and the subduction zones of the cascade mountains volcanic belt where magnetotelluric and geomagnetic variation studies have been done geomagnetic variations experiments have been reported in the central appalachians and submarine electromagnetic studies along the juan de fuca ridge controlled source electromagnetic and dc resistivity investigations have been carried out in nevada hawaii and in the adirondacks mountains laboratory examinations on the conductivity of representative materials over a broad range of temperature pressure and chemistry are described

overview of electromagnetic methods applied in active volcanic areas of western united states abstract a better understanding of active volcanic areas in the united states through electromagnetic geophysical studies received foundation from the many surveys done for geothermal exploration in the 1970 s investigations by governmental industrial and academic agencies include but are not limited to mapping of the cascades long valley mono area the jemez volcanic field yellowstone park and an area in colorado for one example mt konocti in the mayacamas mountains california gravity magnetic and seismic as well as electromagnetic methods have all been used in an attempt to gain a better understanding of the subsurface structure in each of these volcanic regions anomalous zones were mapped when conductive these anomalies were interpreted to be correlated with hydrothermal activity and not to represent a magma chamber electrical and electromagnetic geophysical methods can offer valuable information in the understanding of volcanoes by being the method which is most sensitive to change in temperature and therefore can best map heat budget and hydrological character to aid in prediction of eruptions

error analysis for a long range lightning monitoring network of ground based receivers in europe 1 an experimental long range lightning detection system consisting of a network zeus of six ground based radio receivers has been deployed in europe and has been operated since june 2001 the receivers detect and measure the electromagnetic signal emitted by a lightning source in the very low frequency band between 5 and 15 khz sferics which propagates over thousands of kilometers in the earth ionosphere waveguide in this study lightning location retrievals from the zeus network are compared against more definitive sources in order to investigate issues on long range lightning retrieval accuracy and detection efficiency the validation study is carried out over three regions the u s east coast northwestern atlantic the african continent and within the network spain data originate from the u s national lightning detection network the lighting imaging sensor aboard the tropical rainfall measurement mission satellite and the spanish national lightning network we investigate the nature of the errors involved in the lightning location retrieval and propose a new approach for moderating some of these effects location errors are shown to vary between 40 and 400 km mode at 220 km for very long ranges 5000 km within the network the distance error does not exceed 40 km mode at 20 km random simulation results are shown to be in good agreement with validation data error reduction of 12 or 20 km at long range can be achieved by accommodating for the varying propagation velocities in the sferics wave the long range detection efficiency is shown to have a mode at approximately 20 and a relatively small spread with no obvious deviation from day to night

an enhancement of the ionospheric sporadic e layer in response to negative polarity cloud to ground lightning 1 lightning data collected using a boltek storm tracker system installed at chilton uk were used to investigate the mean response of the ionospheric sporadic e layer to lightning strokes in a superposed epoch study the lightning detector can discriminate between positive and negative lightning strokes and between cloud to ground cg and inter cloud ic lightning superposed epoch studies carried out separately using these subsets of lightning strokes as trigger events have revealed that the dominant cause of the observed ionospheric enhancement in the es layer is negative cloud to ground lightning

paleomagnetism of the borden dykes of baffin island and its bearing on the grenville loop northern baffin island is intersected by spectacular swarms of predominantly northwesterly trending diabase dykes cross cutting relationships k ar ages and paleomagnetic contact tests indicate that these dykes represent at least two major episodes of hadrynian igneous activity the borden dykes were emplaced during the earlier of these episodes they are about 950 ma old and have a paleomagnetic pole position of 153 3 e 26 7 s m 8 6e p 6 1 the later episode saw the emplacement of the previously defined franklin dykes which are about 750 ma old the newly defined borden pole lies at the southern extremity of the grenville loop and has an age consistent with poles of similar position that were derived from rocks of the grenville province all of the dykes probably reflect tension acting in an east northeast west southwest direction the directions of thick northwest trending dykes which form a considerable angle to that expected from east northeast tension are thought to have been controlled

mesoproterozoic carbonate systems in the borden basin nunavut existing stratigraphic nomenclature lithologic descriptions and geological interpretations for an economically important mesoproterozoic dolostone in the milne inlet graben borden basin nunavut do not adequately portray its unusual facies or their spatio temporal configuration four new stratigraphic units are introduced to replace this dolostone formerly known as the society cliffs formation in the southeastern half of the graben the iqqittuq formation represents a distally steepened ramp that grades northwestward into deep water mudstone that is indistinguishable from that of the underlying arctic bay formation the overlying angmaat formation represents a rimmed restricted peritidal platform that grades northwestward across a tepee cortoid shoal barrier into the unusual deep water dolo laminite of the nanisivik formation deep water carbonate mounds up to hundreds of metres thick and kilometres in areal dimensions belong to the ikpiarjuk formation these mounds are geometrically equivalent

occurrence and formation of dolomite in organic rich continental margin sediments dolomite is presently forming throughout a large part of the oceans in continental margin marine sediments accumulating at rates less than 500 m m y and having organic carbon contents greater than 0 5 wt at higher sedimentation rates the dolomite content of the sediments is greatly diluted at lower organic carbon content the porewater chemistry precludes dolomite formation the reactants for dolomite formation have the following sources magnesium supplied from overlying seawater calcium supplied by the dissolution of calcium carbonate and carbon supplied from carbonate dissolution and organic carbon oxidation the rate of dolomite formation appears to be limited by the rate of calcium carbonate dissolution this rate can be quantitatively calculated from the magnesium diffusional and advective fluxes conversely in ancient sediments the depth and timing of dolomite formation can be calculated from the amount of dolomite and the sedimentation rate in dolomite rich sediments these calculations demonstrate that most of the dolomite formed within a few tens of meters of the sea floor within the zone of microbial sulfate reduction such dolomites have ne ative values of dgr13c dolomite in dolomite poor sediments may form at greater burial depth below the zone of sulfate reduction resulting in positive values of dgr13c the distribution coefficient of strontium in dolomite has been determined to have an average value of 0 06 using this value predicted strontium contents for submarine dolomites range from 150 ppm in dolomite rich sections to 1 290 ppm in dolomite poor sections

regional magnetotelluric surveys in hydrocarbon exploration parana basin brazil the magnetotelluric geophysical method has been used effectively as a hydrocarbon exploration tool in the intracratonic parana basin of south america the parana basin has an area of about 1 200 000 km2 463 000 mi2 extending over portions of brazil paraguay uruguay argentina and bolivia the paleozoic marine sedimentary rocks in the parana basin are covered by the world s most extensive flood basalt complex making geophysical exploration extremely difficult although modern seismic techniques recently applied are achieving good results in some areas the 1 2 km 3 300 6 600 ft thick surface basalts and buried diabase sills pose no problem for the magnetotelluric method because the natural electromagnetic fields used as the energy source pass easily through the basalt data for the regional study were taken on six profiles with soundings spaced 8 to 15 km 5 to 9 mi apart the magnetotelluric sounding data outline a linear uplift known as the ponta grossa arch this major structural feature cuts across the northeast trending intracratonic basin almost perpendicularly and is injected with numerous diabase dikes although its character is reasonably well known in the shallow eastern parts of the basin it is poorly delineated in the deeper parts of the basin where promising natural gas zones have been tested in several wells in the survey area mt interpretations show that basalts have aggregate thickness of as much as 2 km 6 600 ft and basement may be as much as 6 km 20 000 ft below the surface over most of the basin the basalts are covered by upper cretaceous to holocene continental sediments of a few hundred meters thickness and are underlain by 2 to 4 km 6 600 to 13 100 ft thick paleozoic sediments with possible hydrocarbon potential the ponta grossa arch is interpreted to be the failed arm of a triple rift system that formed during the separation of the african and south american continents shales of the devonian ponta grossa formation are important parana basin source rocks for hydrocarbons as are several permian units significant electrical contrasts occur between the permian sediments and older units so that magnetotelluric measurements can give an indication of the regional thickness of the permian and younger sediments to aid in interpreting hydrocarbon migration patterns and possible trap areas

electrical properties of granite with implications for the lower crust the electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure structurally bound water oxygen fugacity and other parameters the effect of sulfur fugacity may be important but is experimentally unconfirmed in addition to changing the magnitude of electrical properties the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties with increasing temperature changes in water content retain large but lessened effects on electrical properties near room temperature a monolayer of water will decrease the electrical resistivity by an order of magnitude several weight percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5 at elevated temperatures just below granitic melting a few weight percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2 above the melting temperature 650 to 1100 c depending upon water pressure a few weight percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy remarkably the few weight percent water must be present as free water experiments with hydrated hornblende schist with structural water indicate an electrical resistivity very similar to that for dry granite the implications of these results together with the findings of deep magnetic sounding and magnetotelluric surveys suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle

structure along the northwest edge of the snake river plain interpreted from seismic refraction the results of a seismic refraction survey at the idaho national engineering laboratory along the northwest boundary of the snake river plain show that velocities of the volcanic rocks beneath the plain increase from 1 5 km s at the surface to 5 2 5 5 km s at depths between approximately 2200 and 2500 m an exploration well in the area inel 1 encountered cenozoic volcanic rock with some interbedded sediments to a depth of 2460 m underlain by 700 m of a rhyodacite porphyry the refraction data demonstrate a nearly vertical faultlike discontinuity of about 1000 m displacement beneath the plain about 1 8 km from the southeast flank of the arco hills lost river range whether this discontinuity represents the northwest flank of the snake river plain graben the east flank of the lost river range fault or a caldera wall is not known the results further indicate that paleozoic rocks may extend beneath the plain as far as 5 6 km southeastward from the arco hills

the control of cathodoluminescence in dolomite by iron and manganese variations in the cathodoluminescent properties of carbonates are usually attributed to differing proportions of manganese mn2 as the most important activator and iron fe2 as the main inhibitor of luminescence interactions between manganese and iron concentrations and the luminescent properties of dolomite are demonstrated by petrographic and chemical analyses of 86 samples of dolomite representing a range of depositional environments and ages cambrian to cretaceous and a wide geographical distribution north america and europe iron and manganese are positively correlated in the dolomites with the former showing a greater range of variation very small amounts of manganese are sufficient to activate the luminescence and as little as 100 ppm mn2 is present in highly luminescing samples the intensity of luminescence is not proportional to the manganese concentration iron begins to quench luminescence as its concentration reaches 10 000 ppm above that level luminescence is rapidly lost and total extinction occurs among samples containing more than 15 000 ppm fe2 regardless of the manganese concentration

ground truth confirmation and theoretical limits of an experimental vlf arrival time difference lightning flash locating system during trials of a vlf arrival time difference atd technique for long range lightning flash location an apparently well organized and isolated flash sequence was located internal atd evidence suggests flashes were located at a thousand kilometre range to within 5 km ground truth based on synoptic mesoscale considerations observer reports and lightning damage claims confirms locations to 10 15 km in addition elimination of systematic effects from internal atd evidence demonstrates relative flash location errors of 0 9 1 8 km and a tight cluster of atd fixes features scatter in location which is not inconsistent with these values the nature of vlf lightning flash observation is examined and shown to be consistent with errors even smaller than this

dedolomitization in the taum sauk limestone upper cambrian southeast missouri abstract coarse dolomite crystals in the taum sauk limestone upper cambrian southeast missouri contain both calcite and iron oxides in rhombohedral zones and along fractures and cleavage traces the dolomite crystals are demonstrably nonstoichiometric averaging 57 mole percent caco3 and are compositionally zoned ranging from 1 40 to 4 56 mole percent feco3 cathodoluminescene study and microprobe analyses of the dolomite coupled with scanning electron microscope sem examination of etched fracture surfaces reveal that 1 scattered dolomite within the calcite iron oxide zones is the most iron rich 2 calcite within dolomite rhombs is brightly luminescent nonferroan and is intergrown with iron oxides 3 the iron oxides display two crystal habits thin red hematite circular plates typically 0 7 to 1 0 microns across grouped in rosettes and prismatic bundles and prismatic radiating aggregates of brown crystals goethite typically 1 to 3 microns in length 4 narrow veinlets of dolomite associated with calcite and iron oxides along cleavage traces form a three dimensional network this feature which aided in interpreting the iagenetic history of the dolomite was not observed during petrographic examination but was readily observed with the sem the occurrence of nonferroan calcite and ferric oxides in rhombohedral zones in taum sauk dolomite suggests that dedolomitization occurred by the oxidation and alteration of ferroan dolomite zones and probably reflects alteration related to recent weathering

an experimental study of the remote location of lightning flashes using a vlf arrival time difference technique the operational cathode ray direction finding crdf network for the location of lightning flashes at ranges up to thousands of kilometres can be replaced by a system using the arrival time difference atd technique the atd technique has theoretical advantages over crdf which could lead to substantially improved flash location but the extent to which it suffers from certain propagation effects in practice must be quantified in an experiment using the atd technique equipments were deployed in the u k and gibraltar these captured the vertical electric field of vlf electromagnetic radiation from individual lightning flashes synchronized to crdf selected flashes and recorded against an accurate timebase by analysing the data using techniques analogous to those of hyperbolic navigation the flash location was inferred together with a measure of the data consistency atd location of independent flashes observed in time proximity frequently gave the tight grouping expected from an isolated active thunderstorm cell groups of diameter 2 10 km were observed at megametre ranges atd flash locations were compared with crdf and other meteorological data positioning agreed within the limited accuracy of current techniques but the crdf system s potential for poor accuracy was highlighted an analysis method was developed relating the internal consistency of the atd flash data to geographical location accuracy and atd accuracy of less than 5 s was measured experimentally on this basis an atd system can locate flashes over a synoptic scale area with errors comparable with a storm cell diameter

exploration beneath volcanics snake river plain idaho coincident gravity magnetotelluric mt and seismic reflection refraction surveys were conducted along a line traversing the boundary between the volcanic covered eastern snake river plain idaho and the sediment covered western overthrust belt one dimensional mt and seismic refraction analysis away from the immediate boundary between sediments and volcanics successfully maps the depths to units of igneous and sedimentary or metasedimentary rock substantially different models of the volcanic covered terrain and of the sediment covered terrain are linked by a gravity anomaly and by an elongation and rotation of mt anisotropy ellipses which indicate a fault fault dip depth extent and motion are constrained by marked changes in the first arrival apparent velocities and by an electrical marker layer the analysis shows that metasediments extend laterally at least 16 km beneath the volcanic cover the two terrains are connected by a boundary zone which is a major normal fault

experimental evidence of resistivity frequency dispersion in magnetotellurics in the newberry oregon snake river plain idaho and campi flegrei italy volcano geothermal areas abstract we show that magnetotelluric measurements carried out in volcano geothermal areas can be significantly affected by the resistivity frequency dispersion phenomenon the polarization effects are recognized by comparing magnetotelluric and dc geoelectrical deep soundings performed in the same station indeed the conventional separate analysis of the two data sets sometimes gives unconformable results especially as it regards the resistivity and thickness of the conductive layers ascribable to the geothermal fluid filled rocks these apparent discrepancies can be overcome by admitting the occurrence of intense induced polarization effects inside the rocks having extensive alterations due to hydrothermal paragenesis the basic theory of the induced polarization effects over magnetotelluric measurements is at first briefly recalled then three impressive field examples taken from previous experiments performed in the newberry volcano snake river plain and campi flegrei geothermal areas are presented and discussed

pervasive subsurface dolomitization of the nisku formation in central alberta abstract the nisku formation in the subsurface of central alberta canada contains pervasive dolomites that occur mainly as a replacement of reefal limestones the degree of pervasive dolomitization increases downdip along a structural homocline from about 25 to 100 vol and associated replacive anhydrites increase from about 5 to 20 vol pervasive dolomites occur as two types one type consists of almost non porous and impermeable mosaics of cloudy crystals that have diameters of 5 300 microns the other type has significant intercrystalline porosity and permeability and consists of cloudy and clear crystals ranging in size from 100 500 microns cathodoluminescence zonation is rare but modulated structures observed with tem are common both types of dolomite have strong xrd ordering refl ctions and are nearly stoichiometric sr concentrations 25 125 ppm increase and mn concentrations 20 400 ppm decrease upwards in several cores the boron contents of both dolomite types 20 50 ppm are enriched relative to other diagenetic phases 18 ppm the sr isotope ratios vary from 0 7089 0 7107 and are enriched relative to devonian and younger seawater oxygen and carbon isotope ratios cluster around 5 8 permil pdb and 2 0 permil pdb respectively and are similar to those of remnant calcites from the combination of stratigraphic petrographic petrophysical and geochemical data it is inferred that both types of matrix dolomite formed contemporaneously from the same fluids in a flow regime with predominantly upward fluid movement at temperatures between about 40 50 c and at burial depths of about 300 1 000 m most anhydrites formed originally as gypsum during the advanced stages and after matrix dolomitization and later recrystallized to the anhydrites now present based on all accumulated data and on mass balance calculations at least two types of subsurface flow could have caused pervasive dolomitization and gypsification anhydritization 1 expulsion of burial compaction water i e chemically modified seawater with or without additional contributions of m bearing formation waters from deeper parts of the basin e g middle devonian brines and 2 thermal convection of formation fluids originally underlying and or overlying the nisku

multistage dolomitization in the society cliffs formation northern baffin island northwest territories canada dolostones of the upper proterozoic society cliffs formation on northern baffin island are host to the nanisivik lead zinc sulfide deposits two distinct stages of dolomitization have occurred distinguished by their petrographic cathodoluminescence and isotopic characteristics 1 massive dolomitization of precursor carbonates and 2 late stage cementation sabkha facies of the lower member of the formation and laminated algal stromatolitic to massive dolostones of the upper member are isotopically similar 18o ranges from 1 5 to 6 5 13c from 1 7 to 3 1 pdb and 87sr 86sr averages 0 70761 petrographic and geochemical evidence indicates that these dolostones formed chiefly by seawater thermal convection beneath shale of the overlying victor bay formation although mixing of meteoric and marine water also could have occurred later fracture filling dolomitic cements have consistently lower 18o but 13c is similar to that in the host dolostones microtraverses of single fractures filled wit

magnetotelluric soundings on the idaho national engineering laboratory facility idaho the magnetotelluric mt method was used as one of several geophysical tools to study part of the idaho engineering laboratory inel facility the purpose of the geophysical study on inel was to investigate the facility for a possible site to drill a geothermal exploration well the initial interpretation of the mt sounding data was done with one dimensional models consisting of four or five layers the minimum number required to fit the data after the test well inel 1 was completed the electric log was used to guide an improved one dimensional id interpretation of the mt sounding data profile models derived from the well log provided good agreement with velocity models derived from refraction seismic data a resolution study using generalized inverse techniques shows that the resolution of resistive layers in the lower part of the mt models is poor as is the definition of a shallow altered basalt unit the only major structure observed on the mt data was the faulted contact between the snrp and basin and range structures on the west modeling of the data near this structure with a two dimensional computer program showed that the mt data near the fault require a model similar to the seismic refractionmore models and that structure on a deep crustal conductor is also required less

epigenetic dolomitization and the origin of xenotopic dolomite texture abstract xenotopic texture which is commonly observed in pre cenozoic rocks is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and usually undulatory extinction xenotopic dolomite texture is similar in appearance to neomorphic limestone textures idiotopic dolomite texture euhedral to subhedral crystals with straight intercrystalline boundaries contrasts with xenotopic texture and is commonly observed in both cenozoic and more ancient dolomites texture may be controlled by the temperature at which crystals grow crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites this results in faceted crystals and euhedral to subhedral crystal mosaics above a critical roughening temperature crt a rough surface is energetically favored surface nucleation does not require dislocations and atoms are randomly added to the crystal surface resulting in nonfaceted growth and an anhedral crystal mosaic it is hypothesized that a critical roughening temperature exists for dolomite above 50 c xenotopic dolomites are produced by dolomitization of limestone and or neomorphic recrystallization of dolomite at elevated temperature above the crt after burial idiotopic dolomites are produced below the crt by near surface processes and are occasionally produced above crt if growth inhibiting effects of the solution or impurities stabilize crystal faces calcite has a crt below 25 c and therefore develops anhedral crystal mosaics characteristic of neomorphic texture both at near surface and at elevated temperatures synthetic xenotopic dolomite was produced in the laboratory at 250 c and 300 c by dolomitization of aragonite and calcite skeletal fragments and by recrystallization neomorphism of nonstoichiometric cenozoic dolomites xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic hueco dolomite permian texas near the marble canyon intrusion at temperatures between 350 c and 600 c hydrothermal dolomitization of periclase calcite marble near the intrusion also resulted in a xenotopic texture xenotopic dolomite in the galena group ordovician wisconsin was produced by neomorphism of a preexisting dolomite during the emplacement of lead zinc sulfides at temperatures between 80 c and 227 c in the trenton limestone ordovician michig n xenotopic dolomite replaced limestone during the migration of hot 50 c fluids along fracture systems xenotopic dolomite was not observed in cenozoic dolomites that were subjected to temperatures near surface values only

strontium isotopic evolution of oil field waters from carbonate reservoir rocks in bindley field central kansas u s a abstract oil field waters produced from mississippian carbonate reservoir rocks in bindley field kansas had an average salinity of about 42 8 mg l they were enriched in ca sr na k rb and li and depleted in mg relative to sea water at the same level of either cl or br concentration the average abundances of different elements are as follows na 13 460 mg l k 325 mg 1 rb 0 9 mg l li 12 mg l ca 1 515 mg l sr 42 mg l mg 430 mg l cl 23 000 mg l so4 2 630 mg l br 32 mg l the 87 sr 86 sr values of the waters ranged between 0 7221 and 0 7230 whereas the values of the host carbonate rocks were between 0 7090 and 0 7093 the very high 87 sr 86 sr values of the waters probably occurred as a result of their reaction with alkali feldspar minerals in buried precambrian crystalline rocks to the east of the bindley field the study demonstrates that despite their residence in carbonate reservoir rocks oil field waters can retain sufficiently distinct isotopic memory that may provide important dues about mineral water interactions in the chemical evolutionary history of the waters the study further indicates that very late diagenetic reactions in many carbonate rocks are of minor extent difference in the sr isotopic data among the oil field waters suggests that the sr isotopic data can be used as a reliable guide in recognising existence of separate pools in an oil field

the location of lightning affecting the ionospheric sporadic e layer as evidence for multiple enhancement mechanisms we present a study of the geographic location of lightning affecting the ionospheric sporadic e es layer over the ionospheric monitoring station at chilton uk data from the uk met office s arrival time difference atd lightning detection system were used to locate lightning strokes in the vicinity of the ionospheric monitoring station a superposed epoch study of this data has previously revealed an enhancement in the es layer caused by lightning within 200km of chilton in the current paper we use the same data to investigate the location of the lightning strokes which have the largest effect on the es layer above chilton we find that there are several locations where the effect of lightning on the ionosphere is most significant statistically each producing different ionospheric responses we interpret this as evidence that there is more than one mechanism combining to produce the previously observed enhancement in the ionosphere