the origin and evolution of canadian shield brines evaporation or freezing of seawater new lithium isotope and geochemical evidence from the slave craton abstract new chemical and isotopic data for deep seated calcium chloride brine from the miramar con gold mine yellowknife n w t strongly suggest that the brine salinity is of marine origin diagnostic marine properties include uniformly elevated br cl ratios typical of seawater concentrated beyond halite saturation and li br ratios 0 0254 0 0325 and 6 li compositions 32 1 to 36 3 similar to seawater 32 3 the mean 6 li for all mine water samples of 35 1 may reflect minor uptake of li by secondary minerals this interpretation is supported by analyses of altered metabasalt from fault zones which is enriched in li but depleted in 6 li 14 7 to 15 6 relative to the unaltered metabasalt 5 4 the mechanism responsible for concentrating the hypersaline brine end member is not unequivocal as evidence exists to support both evaporative and cryogenic processes on the one hand the devonian sedimentary record in the great slave lake region in conjunction with yellowknife brine isotopic compositions 2 h and 34 s so 4 that are similar to various devonian fluids support an evaporative origin on the other hand the na cl br cl relationship in the brine strongly suggests a cryogenic mechanism regardless of the concentrative mechanism the chemical data indicate that the yellowknife parent brine was concentrated 28 to 30 fold relative to seawater the extreme depletion of mg and enrichment of ca in the brines accompanied by sr ca ratios similar to that of seawater are accounted for by dolomitization of an aragonite rich marine sediment by the brine before infiltration into the crystalline basement rocks subsequent alteration of silicate minerals in the shield added additional ca and sr to the brine as indicated by their radiogenic 87 sr 86 sr ratios up to 0 7147 based on mineral balance calculations the major mineral products of the cryogenic and evaporitic concentration and evolution paths are significantly different the cryogenic evolution results in some 15 mirabilite 60 hydrohalite and 18 dolomite whereas the major minerals formed from the evaporitic evolutionary sequence are 36 halite 8 gypsum 17 dolomite and 30 albite the great similarity between the calcium chloride brine from yellowknife and other such canadian shield brines indicates that they may share a common marine origin

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

strontium isotopic compositions of cambrian upper miaolingian furongian series dolomites from south eastern sichuan basin china significance of sources of dolomitizing fluids and timing of dolomitization abstract tracing the nature of dolomitizing fluids and constraining the dolomitization time are of great significance for understanding dolomite genesis and reservoir formation especially for ancient dolomites strontium isotopes can be used as very useful tools to study this aspect thick dolomite bodies developed in the loushanguan group of the upper miaolingian furongian series cambrian in the south eastern margin of the sichuan basin china these dolomites are probably good hydrocarbon reservoirs little research has been carried out on the sr isotopes in these ancient dolomites three different textures of dolomites have been identified in the study area dolomicrite fabric retentive oolitic dolomite and fabric obliterative dolomite the dolomicrite displays a higher sr isotopic value avg 0 71052 compared with the contemporaneous seawater as well as a high sr and low mn content these chemical characteristics indicate that the dolomitizing fluids of the dolomicrite were mainly derived from modified seawater mixed with a little continental input the 87 sr 86 sr ratios avg 0 70936 of the fabric retentive oolitic dolomites are close to those of normal seawater indicating that the dolomitizing fluids were derived from the contemporaneous seawater and that they possibly formed soon after sedimentation as a result of an early replacement during shallow burial the low sr isotopic values avg 0 70854 of the fabric obliterative dolomites indicate that these dolomitizing fluids are related to late cambrian seawater and that the dolomites are the result of recrystallization of the fabric retentive dolomites in a relatively closed diagenetic system during burial with combined characteristics of relatively high sr and low mn contents the recrystallization began approximately 490 ma and was completed not earlier than 485 ma this study is not only beneficial to understand the characteristics and multistage process of the dolomitization of ancient cambrian dolomites but provides a reference for sourcing dolomitizing fluids and constraining the dolomitization time using sr isotopes

fluid compartmentalization and dolomitization in the cambrian and ordovician successions of the huron domain michigan basin abstract strata bound dolomite occurs in deep seated ordovician and cambrian sediments within the huron domain of southern ontario canada core samples were analyzed for petrographic stable and sr isotopic composition and fluid inclusion microthermometry to characterize dolomitization and evaluate diagenetic fluid composition the samples represented a range of host rocks from dolomitized limestones dolostones sandy dolostones and sandstones within ordovician black river group and underlying cambrian formation the petrographic and geochemical attributes have provided a basis to gain insight on the source fluids that modified these rocks as well as the possible timing of formation evidence indicates that the formations were subjected to high temperatures average 115 c that cannot be explained by burial history alone this suggests the occurrence and migration of hydrothermal fluids within the low permeability dolomite horizons possibly during paleozoic orogenesis dolomite and calcite fracture infill isotopic and fluid inclusion data point to two possibly individual diagenetic fluid systems i an earlier cambrian system that is characterized by a pronounced negative shifts in oxygen and carbon isotopic composition 18o average 9 0 for dolomite and 13 9 for calcite respectively and 13c 3 06 for dolomite and 4 82 for calcite respectively a more radiogenic sr87 sr86 ratios range from 0 70977 to 0 71100 warm th values range from 84 to 156 c for dolomite average 113 6 c and 87 141 c for calcite average 101 5 c and saline signature salinity range from 23 2 to 27 2 wt nacl eq for dolomite average 24 3 for dolomite and 23 6 wt nacl eq for calcite and ii a later ordovician system that is characterized by less negative shifts in both oxygen and carbon isotopes 18o average 8 7 for dolomite and 7 5 for calcite and 13c average 0 37 for dolomite and 0 36 for calcite respectively hypersaline salinity range from 22 4 to 30 1 wt nacl eq average 27 0 for dolomite and 27 5 to 29 7 for calcite average 29 2 comparable homogenization temperature th ranges from 85 to 132 c for dolomite average 109 6 c and 66 153 c for calcite average 107 2 c and a less radiogenic sr87 sr86 ratios range from 0 70818 to 0 71 000 fluid system the observation of highly discrete strata bound dolomites combined with only trace quantities of saddle dolomite and its associated geochemical signature suggest that diagenesis as a result of hydrothermal fluids was neither pervasive in volume or extent within the north western huron domain

rare earth element geochemistry of dolomites in the middle devonian presqu ile barrier western canada sedimentary basin implications for fluid rock ratios during dolomitization rare earth elements ree were determined in fine medium and coarse crystalline replacement dolomites and for saddle dolomite cements from the middle devonian presqu ile barrier from pine point and the subsurface of the northwest territories and north eastern british columbia ree patterns of the fine crystalline dolomite are similar to those of middle devonian limestones from the presqu ile barrier fine crystalline dolomite occurs in the back barrier facies and may represent penecontemporaneous dolomitization at or just below the sea floor medium crystalline dolomite is widespread in the lower southern and lower central barrier medium crystalline dolomite is slightly depleted in heavy ree compared with devonian marine limestones and fine crystalline dolomite and has negative ce and eu anomalies medium crystalline dolomites replaced pre existing limestones or were recrystallized from earlier fine crystalline dolomites during these processes the ree patterns of their precursors were modified late stage coarse crystalline replacement dolomite and saddle dolomite cements occur together in the upper barrier and have similar geochemical signatures coarse crystalline dolomites have negative eu anomalies and those from the pine point area also have positive la anomalies saddle dolomites are enriched in light ree and have positive la anomalies the ree patterns of coarse crystalline dolomite and saddle dolomite differ from those of marine limestones and fine and medium crystalline dolomites suggesting that different diagenetic fluids were responsible for these later dolomites although massive dolomitization requires relatively large volumes of fluids in order to provide the necessary amounts of mg2 dolomitization and subsequent recrystallization may not necessarily modify the ree signatures of the precursor limestones because of the low concentrations of ree in most natural fluids thus relative fluid rock ratios during diagenesis may be estimated from ree patterns in the diagenetic and precursor minerals fine crystalline dolomites retain the ree patterns of their limestone precursors in the medium and coarse crystalline dolomites the precursor ree patterns were apparently altered by large volumes of fluids involved during dolomitization this study suggests that ree compositions of dolomites and their limestone precursors may provide important information about the relative amounts of fluids involved during diagenetic processes such as dolomitization

correlating paleomagnetic geochemical and petrographic evidence to date diagenetic and fluid flow events in the mississippian turner valley formation moose field alberta canada abstract petrographic geochemical and paleomagnetic analyses of the mississippian turner valley formation provide constraints on diagenesis and fluid flow events in the western canada sedimentary basin paleomagnetic plugs and companion geochemical samples were taken from two drillcores with fullbore microimage log orientations dolomite from both wells yielded two magnetization directions the low temperature low coercivity direction is a drilling induced remanence rather than a viscous remanent magnetization the high temperature high coercivity remanence direction is cretaceous and there is no sign of a primary mississippian direction geochemical analyses of matrix dolomite yield 18o values ranging from 0 65 to 3 34 vpdb standard and 13c values ranging from 1 77 to 3 05 vpdb the least depleted samples have stable isotope values consistent with or only slightly depleted from postulated mississippian dolomite values the remaining sample values exhibit a negative covariant trend consistent with either mixing with another diagenetic fluid or recrystallization during burial petrographic analysis reveals the presence of a recrystallization event that caused zoning and a gradual increase size of the dolomite crystals this event is thought to have caused both the cretaceous paleomagnetic remanence and the altered geochemical values the minor enrichment in sr radiogenic isotopes relative to coeval seawater values suggests that both an extrabasinal source for any fluid and large scale fluid flow are unlikely the results also indicate that magnetic remanences are very sensitive to visually minor changes in carbonate recrystallization from heat or pressure so that great care must be taken in correlating paleomagnetic and geochemical data

a strontium oxygen and hydrogen isotopic composition of brines michigan and appalachian basins ontario and michigan abstract 87 sr 86 sr ratios of brine from samples from the michigan and appalachian basins in ontario and michigan covering the stratigraphic interval from the cambrian to mississippian vary from 0 708 to 0 711 with the exception of the salt units of the salina formation silurian most values are greater than seawater for the time in question indicating water rock interaction the sources of the radiogenic sr has not been identified all samples plot below the gmwl in 18 o 2 h space with the cambrian and ordovician samples closest to the line mixing of brines meteoric and glacial pleistocene water is indicated in some cases the more concentrated brines from each stratigraphic unit show a very narrow spread in values all the ordovician brines show a narrow range over a 200 km area for sr o and h isotopes indicating extensive lateral migration of the fluids strontium in the brine has not equilibrated isotopically with its host rock in some cases the late stage minerals saddle dolomite calcite and anhydrite have the same 87 sr 86 sr ratios as the brine indicating that they precipitated from the brine in isotopic equilibrium

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

morpho structural evolution of the cordon caulle geothermal region southern volcanic zone chile insights from gravity and 40ar 39ar dating the cordon caulle geothermal region 40 5 s is a 13 km 6 km nw se elongate volcano tectonic depression hosting numerous monogenetic volcanic centers fumaroles and hot springs most of which occur spatially associated with the edges of the depression volcanism started at approximately 0 3 ma with dominantly mafic effusions and expressed ultimately in post glacial and historic dacite rhyolite eruptions 1921 1922 and 1960 three ne trending gravimetric profiles were carried out across the depression of cordon caulle yielding a residual negative anomaly with the greatest amplitude 20 mgal at the center of the depression a 2d gravity model given by a 490 m thick shallow body with a density contrast 650 kg m3 interpreted as a shallow package of dacite rhyolite lavas and tephra was found to match the residual anomaly a dense rock equivalent volume of felsic material on the order of 28 km3 25 of the total output of cordon caulle is derived from this thickness previous workers explain the origin of felsic rocks at cordon caulle in terms of the orientation of the borders of cordon caulle relative to the maximum stress direction 1 i e ne trending prevailing during the quaternary according to which nw trending domains were compressional favoring stagnation storage and differentiation of magmas the dominantly monogenetic style of volcanism the plateau like morphology and the negative bouguer anomaly on the contrary are interpreted as an indication of extensional tectonics intra arc extension is proposed to be related to coseismic or postseismic stress relaxation accompanying major thrust events such as the great 1960 chilean earthquake which is consistent at cordon caulle with the direction of propagation of dikes of the 1960 eruption i e coseismic extension and present day subsidence inferred from insar data i e postseismic extension prior to the quaternary 8 2 1 6 ma nearly east west compression prevailed at the intra arc zone a 5 44 0 04 ma old granodiorite intrusion bracketed by two sub parallel branches of the regional margin parallel liquine ofqui fault zone lofz in a prominent pop up structure 15 km to the east of cordon caulle accounts for reverse reactivation of the lofz and rapid fault related exhumation of basement rocks 1 mm year we argue that under compression reverse reactivation of the lofz was facilitated by the combined effect of cyclic increase in fluid pressure induced by magmatic hydrothermal input and removal of lithostatic load induced by glacial erosion 1 5 mm year in the vicinity of cordon caulle

contrasting nature of deformation along an intra arc shear zone the liquine ofqui fault zone southern chilean andes abstract the liquine ofquifault zone lofz cuts the patagonian batholith and the modern volcanic arc of southern chile for ca 1000 km the rock fabric along three transects of the lofz combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike in the liquine transect 39 s a 1 km wide northeast striking subvertical mylonitic zone shows north plunging stretching lineations this mylonitic zone has been juxtaposed by high angle reverse faulting against a nearly undeformed miocene granitoid metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral reverse displacement deformation pre dates a 100 2 ma undeformed porphyritic dike hornblende 40ar 39ar mean age in the reloncavi transect 41 42 s deformation in cretaceous and miocene plutons is predominantly brittle kinematic analysis of two fault populations yields compressional and dextral strike slip stress regimes interpreted as late miocene in age in the hornopiren transect 42 43 s a 4 km wide mylonitic zone developed in plutons and wallrocks shows subhorizontal stretching lineations and dextral displacement a single fault population overprinting the mylonites supports a dextral strike slip stress regime available u pb k ar and 40ar 39ar dates on deformed cenozoic plutons and wallrock range from 9 to 13 ma on hornblende and from 6 to 3 ma on biotite microstructures and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies similar to the biotite ar closure temperature subparallel magmatic and solid state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons dextral oblique subduction of the nazca plate beneath western south america has driven long term intra arc deformation at the southern andes plate boundary zone ridge collision in turn has favored dextral displacement at the leading edge of the continent since the pliocene

dolomitization of the middle devonian winnipegosis carbonates in south central saskatchewan canada the middle devonian winnipegosis carbonate unit in south central saskatchewan is partially to completely dolomitized two major types of replacive dolomite are distinguished microcrystalline to finely crystalline dolomite type 1 displays nonplanar a to planar s textures mimetically replaces the precursor limestone accounts for about four fifths of dolomite phases volumetrically and mainly occurs in the winnipegosis mounds and the lower winnipegosis member directly underlying the mounds medium crystalline dolomite type 2 shows planar s to planar e textures commonly occurs in the lower winnipegosis and brightholme members and decreases upward in abundance the 87 sr 86 sr ratios of type 1 dolomite 0ae70795 to 0ae70807 fall within the estimated sr isotopic range for middle devonian marine carbonates stratigraphic petrographic and geochemical data constrain the formation of type 1 dolomite to hypersaline sea water in a near surface environment after marine cementation and sub aerial diagenesis and prior to precipitation of the middle devonian leofnard salts movement of dolomitizing fluids could be driven by density differences and elevation head the shift to lower d 18 o values of type 1 dolomite 7ae 4t o 5ae1 vienna pee dee belemnite vpdb is interpreted as the result of recrystallization at elevated temperatures during burial type 2 dolomite has higher 87 sr 86 sr ratios 0ae70809 0ae70928 suggesting that the dolomite probably formed from basinal fluids with an increased richness in the radiogenic sr isotope in type 2 dolomite sr 2 concentrations are lower and fe 2 and mn 2 concentrations are higher compared with the associated limestone and type 1 dolomite type 2 dolomite is interpreted as having been formed from upward migrating basinal fluids during latest devonian and carboniferous period

gangue mineral 87 sr 86 sr ratios and the origin of mississippi valley type mineralization the 87 sr 86 sr ratio measurements in mississippi valley type gangue minerals and host marine carbonates ranging in age from upper cambrian to mississippian show that in northwestern ohio where the mineralization is sparse the minerals precipitated from fluids in local isotopic equilibrium with the host carbonates in which the minerals were emplaced in contrast most gangue minerals from districts of major mineralization in illinois kentucky virginia and tennessee contain strontium with higher 87 sr 86 sr ratios than the carbonate hosts and appear to have crystallized from fluids containing strontium from silicate minerals in the sedimentary succession no evidence has been found for a magmatie component in the ore fluid strontium data from elmwood tennessee show that the early ore fluids like those of northwestern ohio were in isotopic equilibrium with the host carbonates but that they were mixed with and displaced by introduced fluids containing more radiogenic strontium these findings are consistent with the two fluid mixing model for the mississippi valley mineralization where a fluid indigenous to the host carbonates and containing reduced sulfur was mixed with introduced fluids analogous to metal bearing and low sulfide oil field brines with an extensive history of chemical exchange with clays and other silicate minerals in the sedimentary succession

87 sr 86 sr applied to age discrimination of the palaeozoic carbonates of the ossa morena zone sw iberia variscides four distinct palaeozoic episodes of marine carbonate sedimentation are reported in the ossa morena zone sw iberian variscides among them the cambrian and the devonian episodes are those that are well represented in the portuguese domains of the ossa morena zone in this zone the strong variscan deformation and metamorphic recrystallization obliterate the biostratigraphic contents of some carbonate local successions and their ages have been based on lithostratigraphic correlations the 87sr 86sr values for the ossa morena zone carbonate rocks allow to make an isotopic lithostratigraphic correlation based on the comparison between strontium ratios of marine carbonate rocks without chronological data and carbonate rocks with a good age control moreover the established 87sr 86sr fingerprints of the marine carbonate episodes from ossa morena zone should be simultaneously correlated with the worldwide seawater 87sr 86sr curve despite the interaction of the marine carbonate sedimentation with late secondary dolomitization processes or with high temperature metamorphic metasomatic fluids which increases the primary 87sr 86sr ratios this methodology allows to define two main distinct clusters of 87sr 86sr for the ossa morena zone marine carbonates a devonian cluster with strontium ratios lower than 0 708000 and a cambrian cluster ranging from 0 708777 to 0 708299 0 708575 0 000161 that can reach 0 709227 0 708866 0 709087 0 000118 if incipient late secondary dolomitization or high temperature metamorphism metasomatism operated the obtained 87sr 86sr isotopic ratios corroborate the lithostratigraphic correlations between the carbonate rocks of abrantes assumar estremoz viana alvito ficalho and escoural successions with the ovetian marianian limestones of alter do chao elvas succession moreover this methodology allows to establish the isotopic discrimination between the upper silurian devonian limestones of ferrarias barrancos and bencatel successions and the marbles of estremoz succession

a basement interacted fluid in the n81 deposit pine point pb zn district canada sr isotopic analyses of single dolomite crystals the mississippi valley type pb zn deposits of the pine point district northwest territories canada are located close to the eastern edge of the present day western canadian sedimentary basin the deposits are thought to have formed as the result of basin wide fluid flow in the presqu ile barrier the host to the ore deposits a laser ablation multi collector inductively coupled plasma mass spectrometric study of 87sr 86sr ratios of ore related dolomites from the n81 deposit at pine point indicates that at least two sources of sr were present in the mineralizing system one fluid has a range in sr isotopic values from 0 07073 to 0 71200 and is interpreted to be derived from middle devonian seawater that interacted with clastic units in the basin the second fluid has higher sr isotopic values up to 0 71520 similar to those found in some canadian shield brines and is interpreted to represent an evaporated seawater derived brine which has interacted with crystalline basement rocks reactivation of old structures in the basement may have provided a pathway for cross formation fluid flow to the site of mineralization the data suggest that the stratigraphic location of the pine point district near the interface between the western canadian sedimentary basin and its basement may have exerted a fundamental control on the formation of these deposits

δ18o values 87sr86sr and sr mg ratios of late devonian abiotic marine calcite implications for the composition of ancient seawater abstract late devonian frasnian abiotic marine calcite has been microsampled and analyzed for 87 sr 86 sr ratios 18o and 13c values and minor element concentrations portions of marine cement crystals from the alberta and canning basins have escaped diagenetic alteration and preserve original marine 18o values 4 8 0 5 pdb 13c values 2 0 to 3 0 pdb 87 sr 86 sr ratios 0 70805 3 and sr mg weight ratios 0 04 to 0 05 marine 87 sr 86 sr ratios are globally consistent and can be correlated within the alberta basin and among the alberta canning and williston basins correlation of isotopic and chemical data strengthen the conclusion that marine cements from the leduc formation preserve original marine 18o values which are 3 to 4 lower than those of modern marine cements these low 18o values are best explained by precipitation from 18o depleted seawater and not by elevated seawater temperature or diagenetic alteration for comparison with devonian data analogous data were collected from holocene mg calcite and aragonite marine cements from enewetak atoll marshall islands mg calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater and mg calcite cements are homogeneous with respect to sr and mg contents empirically derived homogeneous distribution coefficients for mg and sr in modern abiotic mg calcite from enewetak atoll are 0 034 and 0 15 respectively an equation describing the dependence of dsr on mg content was based on a compilation of sr and mg data from holocene abiotic marine calcite dsr 3 52 10 6 ppm mg 6 20 10 3 unlike that derived from experimental data this sr mg relation is consistent over a range of 4 to 20 mol mgco3 and may represent precipitation phenomena which are minimally controlled by kinetic effects comparison of sr and mg contents of analogous devonian and holocene marine cements suggests that the mg ca ratio of late devonian seawater was significantly lower and that the sr ca ratio was significantly higher than that of modern seawater

zebra dolomitization as a result of focused fluid flow in the rocky mountains fold and thrust belt canada discordant zebra dolomite bodies occur locally in the middle cambrian cathedral and eldon formations of the main ranges of the canadian rocky mountains fold and thrust belt they are characterized by alternating dark grey a and white b bands forming an abba diagenetic cyclicity these bands developed parallel to both bedding and cleavage dark grey a bands consist of fine 300 m non planar crystalline impure dolomite the white b bands are composed of coarse up to several millimetres milky white pure saddle dolomites b1 which are often covered by pore lining zoned dolomite b2 the b phases often possess a saddle shaped morphology in contrast to the replacement origin of the a dolomite the zoned b2 dolomite rims are interpreted as a cement formed in open cavities the b1 dolomite is interpreted as the result of recrystallization with diagenetic leaching of non carbonate components all the zebra dolomites studied are nearly stoichiometric and are characterized by enriched na and depleted sr concentrations fe and mn concentrations in these dolomites differ depending on the sample locality fluid inclusion data indicate that the dolomites formed from relatively hot th 130 200 c saline 20 23 wt cacl2 eq fluids a diagenetic high temperature origin is also supported by depleted 18o values 20 to 14 vpdb a contribution of 87sr enriched fluids is reflected in the 87sr 86sr values 0 7091 0 7123 zebra dolomite development is explained by focused fluid flow which exploited areas of structural weaknesses e g basin platform rim areas faults etc expulsion of hot basinal brines in a tectonically active regime generated overpressures which explains the development of secondary porosity during zebra dolomitization as well as the intra zebra fracturing at decimetre to micrometre scale

emplacement of volcanic vents and geodynamics of central anatolia turkey abstract observations on synthetic aperture radar sar scenes of the european remote sensing ers satellite and digital elevation models dems complemented by field structural analysis permit a new understanding of relationships between tectonics and volcanism since the late miocene 10 ma in central anatolia volcanic edifices form elongate stratovolcanoes linear clusters and volcanic ridges they indicate emplacement on tension fractures and tail crack or horsetail features for instance the kara dag volcano is rooted on a tail crack which accommodates a horizontal left lateral throw component at a fault termination caldera complexes of cappadocia are associated with horsetail fault patterns the emplacement of volcanoes also benefits from larger scale tectonic structures the erciyes dag volcano is localized by the sultan saz releasing bend which opens along the sinistral strike slip ecemis fault deformation has been analysed from tension fractures which are perpendicular to the direction of extension and from field structural analysis on a regional scale the tectonic regime responsible for the distribution of volcanic vents in this area of convergence and lateral extrusion is not compression but extension the central taurus range is the thermally uplifted shoulder of the adana cilicia basin which is related to lithosphere thinning westward movements in the northwestern part of the studied area are influenced by the active back arc aegean extension situated to the west farther to the south the direction of motion turns southwest and south under the influence of the opening of the adana cilicia basin we interpreted that extension in the central anatolian plateau is related to crustal blocks moving above sub horizontal detachment surfaces located in the lower crust this is based on several facts the tuz golu fault zone is a within crust detachment the tuz golu basin does not affect the whole lithosphere because otherwise it would have been bordered by thermally uplifted shoulders movements change trend within a small 50 km region

manetoe facies a gas bearing megacrystalline devonian dolomite yukon and northwest territories canada the manetoe facies is a broadly stratiform and regionally developed 38 000 km2 or 15 000 mi2 body of white sparry dolomite hosted in lower and middle devonian strata and is the northwestward continuation of the similar but better known presqu ile facies dolomite several large gas fields have been discovered within anomalously thick developments of the manetoe dolomite close to the northwest limit of the devonian elk point basin of alberta the external geometry of the manetoe facies indicates that it may have been a cavern or karst system excavated in the late middle devonian by a large vertically confined coastal aquifer the local regions of anomalously thick manetoe facies may have developed where the aquifer stoped upward into strata above the confining aquiclude of the headless formation shale dolomite cementation may have occurred during any of three possible episodes of large scale fluid movement fluid inclusion and strontium isotope data favor the hypothesis that manetoe dolomites precipitated from heated 150 210 c or 300 410 f hypersaline evaporitic brines that have interacted with the underlying precambrian crust probably in the late devonian lead isotope data support the general concept of pre mesozoic dolomite cementation and largely preclude a late early tertiary deep burial setting spectacular zones of detrital dolomite cement within the manetoe are most readily accommodated by a near surface very early middle devonian aquifer mixing model origin but the presence of these zones does not preclude the other possible origins carbon and xygen isotope data from the manetoe facies support the concept that the presqu ile and manetoe facies are physically continuous and had a common origin

pine point orebodies and their relationship to the stratigraphy structure dolomitization and karstification of the middle devonian barrier complex mississippi valley type ore field on the south side of great slave lake pine point has produced 58 2 million tons of 3 0 percent pb and 6 7 percent zn with present reserves of 25 7 million tons of 2 7 percent pb and 6 3 percent zn most pine point orebodies were deposited in interconnected paleokarst networks that developed at the lower limits of coarse dolomite alteration of upper barrier limestones karstic structures which exhibit the largest cavity development and consequently the greatest amounts of infilling internal sediments host the best grades of mineralization the following conclusions are suggested the barrier developed due to subtle epeirogenic crustal movements and or eustatic sea level changes affecting normal sedimentary processes deformation of the barrier is possibly due to differential sediment compaction or warping of underlying evaporites coarse dolomitization and paleokarstification metals introduced by chloride rich brines brines expelled by overburden pressures migrated eastward along the barrier and precipitated metals in paleokarst structures upon encountering reduced sulfur species modified journal abstract

the liquine ofqui fault zone a long lived intra arc fault system in southern chile abstract plate reconstructions for the cenozoic document relatively steady right oblique subduction of the farallon nazca plate beneath the chilean continental margin the kinematic signature recorded along the intra arc liquine ofqui fault zone lofz a major feature of the southern andes may help to understand the way in which the nazca south america slip vector has been partitioned into strain and displacement along and across the continental margin the lofz consists of two nne trending right stepping straight lineaments a strike slip duplex at the right step and curved features which splay off the straight lineaments toward the northwest the lofz runs mostly through heterogeneously deformed cenozoic plutonic rock of the north patagonian batholith and patchy metamorphic wall rock early cenozoic volcano sedimentary rocks and dyke swarms found in close spatial association with the strike slip duplex are believed to have developed in strike slip related basins quaternary volcanoes are aligned parallel to the lofz both ductile and brittle kinematic indicators within centimeter to meter wide high strain zones document late cenozoic dextral shear deformation contrasting left lateral deformation recorded on older and wider mylonitic zones suggests that the lofz may be a long lived shear zone that accommodated continental scale deformation arising from the farallon nazca south america plate convergence a block rotation pattern as indicated by paleomagnetic data is consistent with the geometry and cenozoic kinematics of the lofz

strontium isotopic composition of devonian dolomites western canada sedimentary basin significance of sources of dolomitizing fluids abstract dolomites from the middle and upper devonian of five regions in the western canada sedimentary basin wwere studied middle devonian winnipegosis formation outcrop keg river formation reefs in the rainbow field and presqu ile barrier and the upper devonian miette buildup and the wabamun group of northern alberta data from the upper middle devonian swan hills formation in the rosevear field and nisku formation reefs were also included in these areas of western canada there are at least three dolomitizing events 1 early dolomites in winnipegosis reefs and the subsurface part of the presqu ile barrier have 87sr 86sr values near 0 7080 wabamun sabkha dolomites have 87sr 86sr values near 0 7083 and all fall on their respective parts of the seawater curve thus the sources of dolomitizing fluids were devonian seawaters 2 secondary matrix dolomites from the presqu ile barrier rainbow rosevear miette nisku and wabamun all have slightly higher 87sr 86sr values 0 7081 0 7094 than the correspnding devonian seawater dolomitizing fluids were probably modified from devonian seawater with small amounts of radiogenic 87sr being added during early compaction from adjacent or underlying clastics or older carbonates 3 late stage saddle dolomites from presqu ile rainbow rosevear miette nisku and wabamum are all highly radiogenic 0 7100 up to 0 7151 except for most pine point saddle dolomites indicating significant input of radiogenic 87sr with likely sources from the underlying cambrian and precambrian clastics and or the precambrian basement at pine point some presqu ile saddle dolomites have sr isotopic values similar to matrix dolomites suggesting a different fluid source possibly from subsurface brines that may have evolved from pressure solution of devonian strata alternatively formation waters derived from the deeper part of the basin may have been progressively diluted with low 87sr 86sr shallow formation waters as they moved updip along the presu ile barrier the 87sr 86sr values support and refine earlier conclusions based on textual evidence c and o isotopes geochemistry and fluid inclusions they considerably limit the sources of the dolomitizing fluids in open diagenetic systems however the timing of the widespread secondary matrix dolomites can only be broadly constrained to sometime during the late paleozoic the similarity of the o and sr isotopes in the replacement dolomites from widely separated locations geographically and stratigraphically tentatively suggests that these dolomites may have precipitated from similar formation waters on a basin wide scale the late stage saddle dolomites and probably the coarsely crystalline dolomites could only have formed near maximum burial conditions in the late cretaceous to early tertiary from warm hydrothermal brines that moved updip from more deeply buried portions of the basin later coarse crystalline calcites precipitated from highly saline brines and are generally more radiogenic than saddle dolomites they apparently formed from more radiogenic formation waters than have curretly been reported from the basin

strontium isotope trends during diagenesis in ore bearing carbonate basins strontium isotope studies 120 analyses were carried out on ore and gangue minerals from five ore districts in different carbonate basins most of the samples were separated from consecutive diagenetic generations even for diverse stratigraphic positions and mineralogies very consistent trends are recognized in all cases87sr 86sr ratios increase with advancing diagenetic stage however each district shows a different behaviour in the magnitude of increase over the first diagenetic generation which ideally is close to contemporaneous seawater the increase of the strontium ratios in the subsequent diagenetic generations is explained by equilibration during crystallization with fluids evolving towards more radiogenic compositions this is interpreted as a consequence of the reaction of the fluids with rb bearing phases mainly detrital clay minerals variation ranges of87sr 86sr ratios can constitute a sensitive tool for determining the stage of basin evolution in a similar way as vitrinite reflectance or illite crystallinity this method has the disadvantage that the increase of radiogenic strontium depends not only on the evolution stage i e temperature pressure and time but also on the amount of rb bearing minerals present in the basin on the other hand this method has the important advantage that within a given basin this last parameter can be modelled each evolution stage is recorded in the cements or crystallization generations by its strontium isotope signatures which tend towards increased radiogenic values with advancing diagenesis

tectonically driven dolomitization of cambrian to lower ordovician carbonates of the quruqtagh area north eastern flank of tarim basin north west china dolomites occur extensively in cambrian to lower ordovician carbonates in the tienshan orogen of the quruqtagh area north east tarim basin where thick up to 1 km dark grey lenticular limestones of semi pelagic to pelagic origin are prominent the dolomites generally occur as beige anastomosed geobodies that cross cut well stratified limestones based on detailed field investigations and petrographic examination three types of matrix dolomite are identified fine crystalline planar e md1 fine to coarse crystalline planar s e md2 fine to coarse crystalline non planar a md3 dolomites one type of cement dolomite the non planar saddle dolomite cd is also common the preferential occurrence of md1 along low amplitude stylolites points to a causal link to pressure dissolution by which minor mg ions were probably released for replacive dolomitization during shallow burial compaction type md2 md3 and cd dolomites commonly co occurring within the fractured zones have large overlaps in isotopic composition with that of host limestone implying that dolomitizing fluids inherited their composition from remnant pore fluids or were buffered by the formation water of host limestones through water rock interaction however the lower 18o and higher 87sr 86sr ratios of these dolomites also suggest more intense fluid rock interaction at elevated temperature and inputs of mg and radiogenic sr from the host limestones with more argillaceous matter and possibly underlying neoproterozoic siliciclastic strata secondary tensional faults and fractures within a compressional tectonic regime were probably important conduits through which higher temperature mg rich fluids that had been expelled from depth were driven by enhanced tectonic compression and heating during block overthrusting forming irregular networks of dolomitized bodies enclosed within the host limestones this scenario probably took place during the late hercynian orogeny as the tarim block collided with tienshan island arc system to the north and north east subsequent downward recharges of meteoric fluids into the dolomitizing aquifer probably terminated dolomitization as a result of final closure of the south tienshan ocean or palaeo asian ocean and significant tectonic uplift of the tienshan orogen this study demonstrates the constructive role of notably tensional or transtentional faulting fracturing in channelling fluids upward as a result of intense tectonic compression and heating along overthrust planes on the convergent plate margin however a relatively short lived low fluid flux may have limited the dolomitization exclusively within the fractured faulted limestones in the overthrust sheets this article is protected by copyright all rights reserved

seismological aspects of the 1988 1989 lonquimay chile volcanic eruption abstract more than 1600 earthquakes were recorded in a four month period on portable seismographic stations installed in the lonquimay area immediately after the initiation of the volcanic eruption of december 25 1988 hypocentral parameters were calculated on a subset of 150 events showing clear p and s arrivals seismic activity was restricted to a north south elongated region that included the main crater of the lonquimay volcano the new navidad crater lies along the eastern margin of the area of seismicity most of the hypocenters are located at depths shallower than 6 km and none have depths exceeding 10 km the largest earthquakes of the sequence took place on dec 28 m 4 7 jan 12 13 m 4 8 and 5 0 and feb 24 m 5 3 the number of events per day varied it peaked on dec 27 with more than 260 events and more than 200 events were recorded on jan 13 both of which corresponded to the days of large earthquakes the relation of large to small events given by the b value of the magnitude frequency distribution log n a bm is typical of volcanic sequences b 1 7 and much larger than the equivalent for tectonic environments the rate of decay of the number of aftershocks of each large earthquake is proportional to t p with p increasing from 0 63 to 1 08 as the eruption progressed in time initial low values of p indicate a slow stress release in the region larger absolute values of p are comparable with tectonic environments and might be indicators of the last stages of the eruption process the total seismic energy release is 9 6 10 19 ergs about five orders of magnitude less than the estimated thermal energy the spatial distribution of earthquakes suggests that the sources of stress change associated with the eruption are located under and possibly to the south of the main crater of the lonquimay volcano and that the evacuation channels of magma occupied a zone of weakness in the region of the new crater

geochemical and isotopic evidence for a genetic link between canadian shield brines dolomitization in the western canada sedimentary basin and devonian calcium chloridic seawater hypersaline brines of marine origin deep within fractured crystalline rocks of the canadian shield are characterized by calcium chloride compositions with relatively low concentrations of magnesi

systematic decrease of high δ13c values with burial in late archaean 2 8 ga diagenetic dolomite evidence for methanogenesis from the crixas greenstone belt brazil abstract remnants of fe poor 2700 4400 ppm dolomite crystals crystal aggregates and microconcretions are cryptically distributed within stratiform bodies tens of metres across of metamorphic fe rich 3 6 wt fe dolomite that occur in graphitic schists of the 2 8 ga ribeira o das antas formation in the crixas greenstone belt brazil fe poor dolomite is a diagenetic carbonate formed during burial of organic bearing muddy sediments using cathodoluminescence microscopy a consistent sequence of dolomite growth stages is well defined within individual crystals crystal aggregates and microconcretions from comparison with similar textures in phanerozoic carbonates we suggest that the early dolomite formed during progressive burial aggregates having developed from mergent growth of individual crystals during mechanical and then chemical compaction the 13cdolomite values of crystals and concretion cores are very high 12 14 pdb and a negative shift of about 7 occurs over millimetres in subsequent dolomite generations early diagenetic fe poor dolomite and late stage fe rich dolomite show overlapping ranges of initial 87sr86sr ratios between 0 7080 and 0 7164 and 18o values 14 to 16 pdb these similarities may indicate isotopic exchange between the early dolomite and subsequent metamorphic fluids yet the well preserved growth zonation of the fe poor dolomite suggests that such exchange did not follow a process of recrystallization the decrease in the 13c of dolomite is correlated with crystal and concretion growth during burial this behaviour is interpreted to indicate a change in the 13c of pore water bicarbonate resulting from bacterially mediated organic diagenesis within the lower part of the zone of methanogenesis the crixas dolomite represents direct evidence that methanogenic bacteria were important contributors in shallow subsurface geochemical systems in the early precambrian the pattern of 13c values found for the crixas dolomite is also documented in other early to middle precambrian dolomites and limestones previous interpretations have considered positive 13c values to be the result of primary oceanographic controls whereas variability in 13c is commonly linked to metamorphic alteration we suggest that the role played by organic diagenesis in controlling the 13c values of diagenetic carbonates has been underestimated

melting of a subducting oceanic crust from u th disequilibria in austral andean lavas understanding crustal genesis at convergent plate boundaries is important for determining mass transfer between different geochemical reservoirs in the earth s mantle and for deciphering the long term growth of the continental crust most arc magmas are thought to be generated from fluid induced melting of the mantle wedge above slabs of subducting oceanic crust1 such magmas frequently display 238u enrichments or radioactive equilibrium2 3 between 238u and its radiogenic product 230th but where a young and hot oceanic crust is being subducted it may itself partially melt and produce calc alkaline andesites and dacites termed adakites4 here we report a uniform excess of 230th over 238u but variable th isotope ratios in young adakites from the andean austral volcanic zone south of the triple junction where the chile ridge subducts beneath south america we show that these results are compatible with the adakites having been formed by approximately 20 equilibrium melting due to amphibole decomposition in a heterogeneous5 oceanic crust moreover both the degree of melting of the oceanic crust and its thermal structure appear to be uniform under most of the andean austral volcanic zone such partial melting of subducted oceanic slabs may have occurred throughout the earth s history where young oceanic plates were subducted

neotectonics in the foothills of the southernmost central andes 37 38 s evidence of strike slip displacement along the antinir copahue fault zone 1 the antinir copahue fault zone acfz is the eastern orogenic front of the andes between 38 and 37 s it is formed by an east vergent fan of high angle dextral transpressive and transtensive faults which invert a paleogene intra arc rift system in an out of sequence order with respect to the cretaceous to miocene fold and thrust belt 3 1 1 7 ma volcanic rocks are folded and fractured through this belt and recent indicators of fault activity in unconsolidated deposits suggest an ongoing deformation in spite of the absence of substantial shallow seismicity associated with the orogenic front neotectonic studies show the existence of active faults in the present mountain front the low shallow seismicity could be linked to the high volumes of retroarc derived volcanic rocks erupted through this fault system during pliocene and quaternary times this thermally weakened basement accommodates the strain of the antinir copahue fault zone absorbing the present convergence between the south america and nazca plates

87 sr 86 sr ratio measurements on sulfides carbonates and fluid inclusions from pine point northwest territories canada an 87 sr 86 sr ratio increase accompanying the mineralizing process crystallization of fine grained early diagenetic dolomite and later coarse dolomite has occurred in sr isotope equilibrium with devonian sea water 87 sr 86 sr 0 7082 extraction of radiogenic sr from silicates in the metal source sediments by mineralizing brines modified journal abstract

abu dhabi sabkha persian gulf revisited application of strontium isotopes to test an early dolomitization model a study of brines and diagenetically altered minerals from coastal sabkha of abu dhabi persian gulf was undertaken to test the flood recharge evaporative pumping model for early dolomitization using strontium isotopes as a tracer the sup 87 sr sup 86 sr ratio of the sabkha brines defines a pattern of both horizontal and vertical mixing of coastal seawater and continental ground water consistent with a previously proposed model the sup 87 sr sup 86 sr ratio of the diagenetic dolomites indicates early dolomitization by marine recharge waters the sup 87 sr sup 86 sr ratio of gypsum from the dolomitization area 1 4 to 4 4 km from high tide mark and seaward reflects precipitation from marine brines but more landward 6 to 9 km gypsum and anhydrite record a continental signal in the minerals suggests a paragenetic evaporative sequence dolomite yields gypsum yields anhydrite the dolomitizing fluid is evaporated seawater that recharges the sabkha aquifer during supratidal flooding and is afterward pumped upward through the saturated zone by evaporative processes

geochemical constraints on the origin of the kicking horse and monarch mississippi valley type lead zinc ore deposits southeast british columbia canada two mississippi valley type mvt ore deposits kicking horse and monarch have been studied with the aim of comparing the ores at the two localities and to characterize the origin of the mineralizing fluids and the ore formation process es both deposits are hosted by the middle cambrian cathedral formation carbonate host rocks kicking horse on the north and monarch on the south flank of the kicking horse valley near field se british columbia the ore bodies are situated at the transition of western basinal to eastern shallow water strata of the paleo pacific passive margin succession in the cordilleran foreland province of the western canada sedimentary basin both deposits are related spatially to normal faults in both localities the ore minerals are dominated by pyrite sphalerite and galena dolomite minor quartz and calcite are also present in close association with the ores the salinity 21 30 wt nacl eq and homogenization temperatures 63 182 c measured in fluid inclusions in carbonate quartz and sphalerite lie within the typical range of mvt fluid conditions the good stoichiometry 50 53 mol caco3 low 18o values 21 to 14 vienna peedee belemnite and relatively high homogenization temperatures 95 c of the dolomite suggest the dolomites were formed under burial diagenesis the ore forming fluids probably interacted with siliciclastic units based on elevated li contents and 87sr 86sr ratios which are highest in the dolomite type after the main ore stage we propose that the ores formed from the mixing of a downward infiltrating sulfur bearing halite dissolution fluid with an upward migrating metal rich evaporated seawater fluid which had already undergone minor mixing with a dilute fluid

brines in the carboniferous sydney coalfield atlantic canada abstract formation waters within upper carboniferous sandstones in the sub sea prince and phalen coal mines nova scotia originated as residual evaporative fluids probably during the precipitation of windsor group lower carboniferous salts which underlie the coal measures salinity varies from 7800 to 176 000 mg l and the waters are na ca cl brines enriched in ca sr and br and depleted in na k mg and so4 relative to the seawater evaporation curve br cl and na cl ratios suggest that the brine composition corresponds to an evaporation ratio of as much as 30 the brines lie close to the meteoric line on h o isotopic plots but with a compositional range of 18o from 4 18 to 6 99 and of d from 42 4 to 23 5 distant from modern meteoric or ocean water mine water composition contrasts with that of nearby salt spring brines which are inferred to have originated through dissolution of windsor group evaporites by modern meteoric waters however a contribution to the mine waters from halite dissolution and from br in organic matter cannot be ruled out present concentrations of several elements in the brines can be explained by water rock interaction the original windsor brines probably moved up into the overlying coal measure sandstones along faults prior to the late triassic the high salinity and irregular salinity distribution in the phalen sandstones suggests that the brines have undergone only modest dilution and are virtually immobile in contrast prince waters show a progressive increase in salinity with depth and are inferred to have mixed with surface waters basinal brines from which these modern formation fluids were derived may have been important agents in base metal and ba mineralisation from the mid carboniferous onwards as saline fluid inclusions are common in zn pb sulphide deposits in the region

saddle dolomite as a by product of chemical compaction and thermochemical sulfate reduction petrographic stable and radiogenic isotope trace element and fluid inclusion data from the devonian nisku reef trend in the subsurface of alberta canada suggest that saddle dolomite was a by product of chemical compaction and thermochemical sulfate reduction in subsiding dolomitized rocks furthermore thermochemical sulfate reduction may be representative of other processes that increase the amount of saddle dolomite via an increase in carbonate alkalinity the data also confirm that saddle dolomite is most likely to form at elevated temperatures from hypersaline brines

the strontium isotopic composition of ordovician and silurian brachiopods and conodonts relationships to geological events and implications for coeval seawater utilized for strontium isotope measurements the results document a gradual decrease in 87 sr 86 sr from 0 7091 to 0 7087 from tremadoc to llandeilo a sharp decline to 0 7078 during the late llandeilo early caradoc little change during caradoc and the ashgil and a steady rise to 0 7087 through the silurian these long term 10 7 yr variations with magnitudes in the range of 10 23 are interpreted to be controlled primarily by continental collisional tectonics and its associated erosion and weathering the gradual decrease in 87 sr 86 sr ratio during the early ordovician may record the reduction in uplift and weathering rates due to waning of the pan african orogenies the rapid decline near the llandeilo caradoc boundary suggests a strong hydrothermal flux likely due to increased sea floor spreading and a possible superplume event the latter may have caused the prominent transgressive phase the largest in the phanerozoic which would have muted continental flux input although the caradoc was the main interval for the taconic orogeny its impact on the sr continental flux may have been delayed until the early llandovery this effect complemented by reworking of glacial deposits near the ordovician silurian boundary and enhanced by phases of the silurian salinic orogeny may have combined to give the progressive increase in the strontium isotope ratio through the silurian the scale and directionality of these changes makes the strontium isotope curve valuable for dating and correlation purposes copyright 1998 elsevier science ltd

fluid inclusion and isotopic evidence on dolomitization devonian of western canada the presqu ile and manetoe facies are diagenetic features developed in lower and middle devonian formations of the elk point basin presqu ile and the mackenzie shelf manetoe both facies contain coarsely crystalline dolomite and white sparry dolomite cement less extensive diagenetic phases in order of paragenesis include fluorite anhydrite barite calcite quartz sphalerite and galena conditions of dolomitization are outlined from core and outcrop examination thin section and cathodoluminescent petrography fluid inclusion analyses and c and o isotopic data fluid inclusion and stable isotope analyses from dolomite are combined to determine the isotopic composition of the dolomitizing fluids chemical analyses of fluid inclusion waters are compared with formation water analyses to derive a proposed origin for the diagenetic fluids dolomite formed from hot high salinity fluids early in the diagenetic history of the basin limestone was dolomitized as a result of the same process that created white sparry dolomite cement in the manetoe facies remnants of the dolomitizing fluids are present as formation waters in some devonian formations in alberta subsequent invasion of meteoric waters produced more dilute diagenetic fluids which resulted in the precipitation of calcite and quartz cement in the manetoe facies these fluids are present in fluid inclusionsmore and are responsible for the low sup 18 o content of the calcite cement the temperatures of calcite and quartz formation differ as a function of burial depth within the manetoe facies during the mesozoic and cenozoic while the temperature of formation for dolomite does not less

dolomitization of the devonian swan hills formation rosevear field alberta canada the swan hills formation middle upper devonian of the western canada basin is host to several nw se trending gas fields developed in massive replacement dolostone one of these the rosevear field contains two major dolostone trends along opposing margins of a marine channel that penetrates into a platform reef complex dolostones consist predominantly of branching and bulbous strdmatoporoid floatstones and rudstones with well developed moldic and vuggy porosity replacement dolomite is coarsely crystalline 100 600 m inclusion rich composed of euhedral through anhedral crystals and has a blotchy to homogeneous red cathodoluminescence geochemically replacement dolomite is characterized by i nearly stoichiometric composition 50 1 51 1 mol caco3 ii negative 18o values mean 7 5 pdb and iii variable 87sr 86sr ratios ranging from values similar to late devonian early mississippian seawater 0 7082 to radiogenic compositions comparable to saddle dolomite cements 0 7100 dolomitization began after widespread precipitation of early equant calcite spar and after the onset of pressure solution implying that replacement dolomite formed in a burial environment oxygen isotope data suggest that dolomite formed at 35 75 c temperatures reached during burial in late devonian through jurassic time at minimum depths of 450 m the linear nw se orientation of most dolomite fields in the swan hills formation is suggestive of fault control on fluid circulation two models are proposed for fault controlled circulation of dolomitizing fluids at the rosevear field in the first compaction driven updip fluid migration occurred in response to basin tilting commencing in the late palaeozoic deep basinal fluids migrating updip were focused into channel margin sediments along fault conduits the second model calls upon fault controlled convective circulation of i warm devonian mississippian seawater or ii middle devonian residual evaporitic brines the overlap in 87sr 86sr and 18o compositions and similar cathodoluminescence properties between replacement and saddle dolomites provide evidence for neomorphism of some replacement dolomite quantitative modelling of sr and o isotopes and sr abundances suggests partial equilibration of some replacement dolomite with hot radiogenic brines derived during deep burial of the swan hills formation in the late cretaceous palaeocene interaction of replacement dolomite with deep brines led to enrichment in 87sr while leaving 18o similar to pre neomorphism values

formation of saddle dolomites in upper cambrian carbonates western tarim basin northwest china implications for fault related fluid flow abstract the saddle dolomites occur more intensely in cores closely to fault than that in cores far away from the fault in upper cambrian carbonate of western tarim basin suggesting that formation of the saddle dolomites is likely related to fault controlled fluid flow they partially fill in fractures and vugs of replacement dolomite the saddle dolomites exhibit complex internal textures commonly consisting of core and cortex in comparison with the matrix dolomites the saddle dolomites show lower sr content and 87 sr 86 sr ratios higher fe and mn content and more negative 18 o values combined with high th 100 130 c of primary fluid inclusions it is suggested that the saddle dolomites precipitated from hydrothermal fluid derived from the deep evaporite bearing middle cambrian strata and the magnesium source may be due to dissolution of host dolomite during hydrothermal fluid migration fault activity resulted in petrographic and geochemical difference of the core and cortex of the saddle dolomites the cores precipitated from the formation water mixed by deep brines at the early stage of fault activity and the cortexes precipitated from the deep fluid with higher temperatures through the middle cambrian later in summary the formation of the saddle dolomites implies a hydrothermal fluid event related to fault activity which also resulted in high porosity in upper cambrian carbonate in western tarim basin

brine mixing in the middle devonian of western canada and its possible significance to regional dolomitization abstract the desiccation that most large evaporite basins experienced in their history created large elevation heads these heads must have induced flows of ground or formation waters into the evaporite basins the consequences of such groundwater flows are believed to be 1 provision of the necessary hydrodynamic drive to allow dolomitization of carbonates beneath and adjacent to the evaporite basin and 2 the formation of anomalously distributed evaporites in the basin adjacent to sites of groundwater inflow sites where the groundwaters mixed with marine derived brines such brine mixing may also have been responsible for depleting the marine derived brines of their contained sulfate so explaining later development of sulfate impoverished potash deposits anomalous developments of anhydrite surrounding older carbonate buildups in the distal parts of the middle devonian elk point basin of western canada may represent such products of brine mixing and provide evidence of dolomitization of underlying lower palaeozoic and devonian carbonates coevally with the filling of the evaporite basin regional dolomitization in the williston basin is stratigraphically confined beneath the middle prairie evaporite suggesting a genetic link between the dolomitization and evaporite deposition dolomitization by refluxing brines generated in the evaporite basin appears unlikely because of the presence of an aquitard at the base of the evaporite succession developments of laminar and pisolitic carbonates adjacent to the carbonate buildups are believed to indicate the former presence of subaerial springs that carried calcium rich groundwaters into the evaporite basin anhydrite envelopes around the buildups are interpreted to be reaction products that formed when calcium bearing spring waters mixed with sulfate rich but calcium depleted marine brines the calcium in the spring waters is believed to have entered the groundwaters as a by product of the dolomitization of carbonates underlying the evaporite basin precipitation of gypsum now anhydrite as a consequence of brine mixing must have depleted the sulfate content of the marine derived brines it is possible that the sulfate depleted potash deposits in the prairie evaporite originate by this mechanism on the other hand the stratigraphic position of these deposits lends little support for this hypothesis

detailed record of the neogene sr isotopic evolution of seawater from dsdp site 590b a detailed study of strontium isotope variations in neogene marine carbonate sediments from deep sea drilling project site 590b using techniques that allow the 87sr 86sr ratio to be determined to better than 0 00001 gives a high resolution record of the sr isotopic evolution of seawater the data show that the rate of change of the marine 87sr 86sr ratio has varied significantly even on time scales as short as 1 m y periods of particularly rapid growth appear to follow major marine regressions and probably reflect an increase in the delivery of radiogenic sr from the continents coupled with a decreased submarine carbonate dissolution rate greater carbonate compensation depth periods of relatively slowly changing 87sr 86sr follow major marine transgressions on the basis of correlations with the marine oxygen isotope record and the times of major continental glacier growth it is inferred that the effects of sea level variations are modified by climatic factors that affect the intensity of continental weathering and runoff the effects of sea floor generation rate variations are not discernible for the neogene the maximum attainable stratigraphic resolution using sr isotopes is between 0 1 and 2 m y for this time period

structural controls of volcanism in transversal chains resheared faults and neotectonics in the cordon caulle puyehue area 40 5 s southern andes abstract the nature of the relationship between tectonics and volcanism in a specific area is the result of the interaction between the crustal structures of the basement and ongoing regional stress field in the southern andes different kinematic environments represented by two nearly orthogonal active fissure systems show apparently incompatible arrays of volcanic chains moreover a local quaternary compressive regime is inferred from basement faults however these mutually orthogonal chains which are also oblique to the volcanic front have contrasting magma evolution and eruptive styles that can be related with their tectonic setting whereas northeast trending monogenetic cones carran los venados can be interpreted as resulting from magma upwelling along regional tension cracks in a quaternary dextral transpressive compressive domain northwest striking fissure system cordon caulle volcanic complex can be the result of extensional fault reactivation and long magma residence in the upper crust different kinematic environments cause contrasting magma evolution and eruptive styles a two way coupling mechanism is suggested for to explain local depart of the regional stress field geometric array of extensional features and final magmatic evolution in these volcanic domains of the southern andes

burial cementation in the swan hills formation devonian rosevear field alberta canada abstract saddle dolomite anhydrite and coarsely crystalline late mosaic calcite cements in the devonian swan hills formation at the rosevear field in alberta canada are interpreted from petrographic and geochemical evidence as the products of burial diagenesis these inferred burial cements are commonly found as a partial to complete filling of moldic and vuggy porosity in massive replacement dolostones fluid inclusions th 125 159 c tm 18 to 22 c 87sr 86sr ratios 0 7101 0 7115 enrichment in ba and zn and light 18o values 6 to 11 2 pdb suggest that saddle dolomite precipitated from hot radiogenic brine with salinities six times greater than seawater ions for saddle dolomite precipitation may have been sourced from off reef shales the precambrian basement middle devonian halite mesozoic clastics and swan hills replacement dolomite undergoing pressure solution in the presence of a normal paleogeothermal gradient 30 c km saddle dolomite would have formed in the late cretaceous paleocene at depths of 3 5 km this time corresponds to the emplacement of gas at the rosevear field dolomite hosted anhydrite cement with nonradiogenic 87sr 86sr 0 7083 and low 34s compositions 15 to 23 cdt has been quantitatively modeled using a two component mixing equation to constrain fluid and ion sources the results indicate that ions for cementation were derived from fluids that passed through and dissolved middle devonian anhydrite this produced a sr rich brine with nonradiogenic 87sr86sr and low 34s these fluids mixed with radiogenic formation water already in the reservoir to precipitate anhydrite cement with the observed isotopic composition fluid inclusions th 90 to 170 c tm 6 to 17 c and elevated 87sr 86sr compositions 0 7105 0 7173 in coarsely crystalline late mosaic calcite support precipitation during the hydrological transformation of the western canada basin into a topographically controlled flow regime the reduced precipitation temperatures of some fluid inclusions in late mosaic calcite compared to saddle dolomite may be attributed to the decrease in western basin paleogeothermal gradients following the laramide orogeny the presence of calcitized anhydrite coarsely crystalline calcite with light 13c values 5 4 to 13 2 pdb s enriched pyrite and 8 h2s in reservoir gases provide evidence for thermochemical sulfate reduction during burial diagenesis at rosevear

diagenesis and paleofluid flow in the devonian southesk cairn carbonate complex in alberta canada abstract one of the largest geological features in the deep part of the alberta basin in western canada is the middle to upper devonian southesk cairn carbonate complex sccc the sccc lies at depths of about 5000 m in the vicinity of the disturbed belt of the rocky mountains and is rising to depths of 2000 m in a northeasterly direction over a distance of 150 km the complex contains several natural gas pools which contain up to about 30 h2s the major focus of this study is to construct a detailed diagenetic history and concurrent pore fluid flow over time the southesk cairn complex reveals a complicated diagenetic history that consists of five phases of pore water evolution 1 synsedimentary to shallow burial 2 intermediate burial 3 deep burial 4 maximum burial and 5 present depth each of these phases influenced the character and the quality of the reservoir rocks phase 4 is of special interest because the strontium isotope data of late diagenetic calcite cements suggest the influence of basin external fluids the 87sr 86sr isotope ratios of these calcite cements are high up to 0 7320 directly adjacent to the disturbed belt and they decrease with increasing distance towards the foreland basin 0 7100 the radiogenic strontium signal can be traced for about 100 km into the foreland basin circumstantial evidence suggests that the radiogenic strontium was carried by fluids that were expelled during the laramide orogeny from the rocky mountain main ranges and or the underlying basement

thermochemical sulphate reduction in the upper devonian cairn formation of the fairholme carbonate complex south west alberta canadian rockies evidence from fluid inclusions and isotopic data the fairholme carbonate complex is part of the extensively dolomitized upper devonian carbonate reefs in west central alberta the studied formations contain moulds up to 10 cm in diameter which are filled partially with saddle dolomite quartz and calcite cements these cements precipitated from a mixture of brines that acquired high salinity by dissolution of halite and brines derived from evaporated sea water the fluids were warm homogenization temperature of primary fluid inclusions of 76 to 200 c and saline 20 to 25 wt nacl equivalent and testify to thermochemical sulphate reduction processes the latter is deduced from s in solid inclusions co2 and h2s in volatile rich aqueous inclusions and depleted d 13 c values down to 26 vienna pee dee belemnite high 87 sr 86 sr values 0ae7094 to 0ae7110 of the cements also indicate interaction of the fluids with siliciclastic sequences the thermochemical sulphate reduction related cements probably formed during early laramide burial another younger calcite phase characterized by depleted d 18 o values 23ae9 to 13ae9 vienna pee dee belemnite low na 27 to 37 p p m and sr 39 to 150 p p m concentrations and non saline 0 wt nacl equivalent fluid inclusions is attributed to post laramide meteoric water

fabric retentive non luminescent brachiopods as indicators of original delta 13 c and delta 18 o composition a test abstract fabric retentive non luminescent helderbergian brachiopods from open marine deposits analogous to brachiopods used in other studies to determine paleo oceanic stable isotopic composition exhibit depleted 13c values immediately below the erosional contact and more enriched values of 13c at greater stratigraphic distances below this surface in addition 13c values of these brachiopods display a marked shift across the erosional surface and values above it are essentially the same as those 30 m or more beneath the surface complementing these trends whole rock analyses metastable high agnesian calcite constituents echinoderms as well as early equant calcite cement also exhibit an increased depletion trend in 13c values as the helderberg oriskany contact is approached from below fabric retentive luminescent as well as obviously recrystallized brachiopods display stable isotopic values similar to those of fabric retentive non luminescent brachiopods thus best preserved fabric retentive non luminescent brachiopods exhibit values similar to obviously recrystallized brachiopods diagenetically altered metastable constituents echinoderms stable early meteoric cements and whole rock samples helderberg strata of new york state were affected by early meteoric diagenesis during a siegenian exposure event prior to deposition of the overlying marine oriskany sandstone meteoric diagenesis during this exposure gave rise to a characteristic carbon isotopic depletion trend approaching the helderberg oriskany contact all of the skeletal constituents underwent isotopic exchange during early diagenetic exposure to meteoric waters consequently the practice of using fabric retentive non luminescent brachiopods as indicators of their original chemistry and by extension paleo oceanic chemistry is questioned

strontium and its isotopes in canadian rivers fluxes and global implications systematic sampling of the 39 largest canadian rivers shows that the weighted average 87sr86sr ratio in the dissolved load is 0 7111 similar to previous measurements on such large rivers as the amazon and mississippi consequently we believe that the above estimate is likely representative of the global average this imposes a limit of 6 5 1011g yr 1 on the amount of sr exchanged with basalts in hydrothermal cells on mid oceanic ridges evaluation of geological information suggests that sr from silicate sources is of considerable importance for all but the largest canadian rivers the latter have chemical and isotopic composition consistent with 4 1 carbonate vs silicate derivation of sr but such interpretation is not unique in terms of their water discharge the 39 canadian rivers studied account for 4 2 of the world total and their weighted average concentrations for other dissolved solutes are tds 176 ppm ca 18 ppm cl 6 8 ppm and sr 84 ppb

focused fluid flow along faults in the monterey formation coastal california fluid flow in fractured siliceous mudstone of the miocene monterey formation of california is inferred to be highly focused toward map scale faults that locally contain extensive amounts of carbonate and minor silica cement the distance of cross stratigraphic flow as inferred based on the strontium isotopic composition of carbonate fault cement is close to the thickness of the monterey formation of 700 m in one of two study locations jalama beach and less than the formation thickness at another location arroyo burro beach fluid is thus derived from within the monterey formation rather than from underlying older units based on mass balance estimates of the fluid volume required for fault cementation at jalama beach the minimum distance of formation parallel flow into the fault zone is 4 km and possibly 12 km the inferred distance of flow parallel to the formation into this fault thus exceeds the distance of cross formational upward flow along the fault by at least a factor of six the mass balance estimate requires that fluid flow along the fault is channeled into a pipe shaped conduit rather than distributed along fault strike fluid flow from the surrounding formation into fault pipes is inferred to follow a radial rather than uni or bilateral flow symmetry using bedding confined sets of extension fractures and stratabound breccia bodies radial fluid flow toward fault pipes requires fairly isotropic fracture permeability for flow along bedding and a low permeability across bedding the inferred flow geometry illustrates the combined effect of fault permeability structure permeability anisotropy of the surrounding formation and hydraulic head distribution in controlling basinal fluid flow in faulted sequences

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

petrogenesis of 1 9 ga limestones and dolostones and their record of paleoproterozoic environments abstract fibrous calcite cements from limestones of the paleoproterozoic pethei group in northwest canada have relatively constant 13 c values near 1 that indicate the c exogenic cycle of ca 1 9 ga ago had similar balances between oxidized and reduced reservoirs of carbon as has existed throughout most of the phanerozoic 87 sr 86 sr ratios of these cements are near 0 7051 and support a progressively increasing influx of radiogenic sr from continental weathering into the paleoproterozoic oceans 18 o values of the cements vary from 14 to 8 5 although the original values of the cements were probably higher because the cements have been subseqently altered as evidenced by their variable trace element contents proposed higher temperatures or substantially lower 18 o values of ancient seawater based on such values may be in error fabric retentive dolomites generally regarded as syndepositional also have variable 18 o values the highest of which are ca 30 greater than the highest values of the fibrous calcite cements the dolomites have more radiogenic sr isotopic compositions indicating that they formed from fluids that interacted with rocks such as those of the clastic sediments which underlie the limestones these fluids may have partially altered the 18 o values and chemical compositions of the fibrous calcite cements late stage calcitic and dolomitic cements that formed from late diagenetic fluids have similar 18 o values and radiogenic sr isotopic ratios as the fabric retentive dolomites if these potentially syndepositional dolomites have had their primary isotopic compositions reset through interaction with diagenetic fluids this would have significant implications to other studies using early dolomite to constrain ancient environmental conditions

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

dolomites in the tazekka pb zn district eastern morocco polyphase origin from hydrothermal fluids many different types of water and processes have been proposed for the formation of dolomites the three phases of hydrothermal dolomites in the middle atlas causse were investigated to elucidate their formation processes the first two of these are associated with sphalerite and galena in stratiform and open space filling deposits these formed early in the history of the deposition of the pb zn mineralization and commonly reveal a paragenetic overlap a later phase post dating pb zn mineralization is reflected in saddle dolomite all three phases show a decrease in 18o and 13c values passing from sterile unmineralized to mineralized rocks and isotopic signatures are independent of the carrier facies however early formed dolomites can be separated into two distinct groups on the basis of 18o values type 1 dolomites host stratiform ore deposits whereas type 2 dolomites host an open space filling ore body later saddle dolomites are more depleted in 18o than either of these the early hydrothermal and saddle dolomites precipitated from similar fluids during three distinct events but formed by two mechanisms replacement hydrothermal dolomite and cement precipitation saddle dolomite they show different isotopic signatures and apparently formed at different temperatures field data petrographic and stable isotope results suggest a continuum of replacement during the carixian for the early hydrothermal dolomite 1 and during the toarcian for early hydrothermal dolomite 2 followed by a cement precipitation phase for saddle dolomite

origin of dolomite in the late cretaceous paleocene limestone turbidites eastern pontides turkey abstract the late cretaceous paleocene tonya formation of the hacimehmet region in trabzon eastern pontides turkey is composed of limestone turbidites these limestone turbidites were deposited at water depths of around 500 to 600 m and remained buried until uplifted in late paleocene time the lower part of this formation which is made up of clast supported calcirudites and pebbly calcarenites is partially to completely dolomitized two morphological types of dolomite are identified 1 fine to medium crystalline dolomite rd1 2 coarse to very coarse crystalline dolomite rd2 each one of the dolomite types is non stoichiometric ca rich ca 57 61 mg 43 39 and has similar geochemical features and uniform dull red brown luminescence these dolomites have 18 o pdb values from 2 6 to 3 3 average 3 0 13 c pdb values from 2 3 to 3 1 average 2 7 and 87 sr 86 sr ratios from 0 707719 to 0 707772 fluid inclusion data of rd2 dolomites yield homogenization temperatures t h of 48 58 c and salinities of 20 4 to 21 7 wt nacl equivalent the sr concentration in dolomites varying from below a detection limit of 20 to 360 ppm is not homogeneous within each dolomite crystal the na concentrations of dolomites are similar varying from 600 to 760 ppm average 680 ppm and are higher than those for dolomites formed in normal marine environments the fe and mn concentrations range from 2500 to 7000 ppm average 3800 ppm and from 50 to 850 ppm average 330 ppm respectively considering the petrography paragenesis trace element and stable isotope data the dolomites in the tonya formation are interpreted to have formed in the subsurface during mechanical compaction at shallow to intermediate burial depths dolomitization in the tonya formation was most probably driven by seawater that was slightly modified through rock water interaction and in part at elevated temperatures

extension and basin formation in the southern andes caused by increased convergence rate a mid cenozoic trigger for the andes the southern andes between 33 and 45 s latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin which itself was characterized by volcanic activity the basins are filled with thick up to 3000 m accumulations of interbedded sedimentary and volcanic strata of late oligocene early miocene age we interpret that these basins developed during a phase of moderate extension within the plate margin system triggered by an increased rate of convergence of the farallon nazca and south american plates between 28 and 26 ma this history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling although extensional basins of this age are only well described in the southern andes the convergence history and volcanic chronology are similar farther north in the central andes 18 33 s leading to the speculation that extension may have characterized the late oligocene early miocene interval regionally we hypothesize that this extension was a necessary condition to subsequent building of the modern andes mountains

diagenesis of the devonian southesk cairn carbonate complex alberta canada marine cementation burial dolomitization thermochemical sulfate reduction anhydritization and squeegee fluid flow abstract one of the largest geological features in the deep part of the alberta basin in western canada is the middle to upper devonian southesk cairn carbonate complex sccc the sccc is located at depths of about 5000 m adjacent to the disturbed belt of the rocky mountains rising to depths of 3000 m in a northeastward direction over a distance of 150 km the sccc produces oil and gas with up to about 30 h2s from several stratigraphic levels the diagenesis of the sccc was investigated by means of petrography and geochemistry stable isotope compositions were determined for fibrous calcite cements matrix dolomite saddle dolomite and sparry calcite fibrous calcites have stable isotope values of 18o 7 5 to 4 5 pdb and 13c 2 0 1 0 pdb matrix dolomite samples have stable isotope values of 18o 8 1 to 4 9 pdb and 13c 0 to 1 0 pdb saddle dolomite samples range from 18o 9 8 to 7 1 pdb and 13c 1 0 to 0 pdb sparry calcite samples have 18o 13 7 to 7 2 pdb and 13c 24 0 to 0 pdb the sr isotope values of all analyzed samples span a broad range from 0 7080 to 0 7320 showing considerable scatter but also one distinctive pattern i e values higher than 0 7220 occur only at depths in excess of about 4000 m whereas lower values scatter throughout the entire depth range furthermore the sr isotope values of sparry calcites decrease northeastward similarly the fluid inclusion homogenization temperatures of these calcites display a general decrease updip whereas the salinities show no general spatial trends the uncorrected homogenization temperatures range from about 85 to 180 c and the nacl equivalent salinities range from about 17 to 24 weight the d values of brines obtained from seven wells range from 79 to 20 smow and their 18o values range from 1 5 to 8 7 smow the sulfur isotope values of the dissolved sulfate ranges from 34s 10 1 to 24 5 cdt the sr isotope values of the brine samples range from 0 7109 to 0 7255 elemental sulfur isotope values cluster tightly around 34s 18 0 to 18 4 cdt the values of secondary anhydrites range from 34s 18 4 to 26 7 cdt values for solid bitumen vary from 34s 19 5 to 20 2 cdt integration of all data reveals a complex diagenetic evolution from the middle devonian to the recent the paragenetic sequence consists of 24 distinct diagenetic phases which have been grouped into 5 diagenetic stages stage i density driven convection in near surface settings likely superimposed on and or aided by tidal pumping facilitating marine cementation stage ii cellular convection facilitating pervasive matrix dolomitization aided by compaction stage iii hydrocarbon migration stage iv thermochemical sulfate reduction forming sour gas and regional squeegee flow forming sparry calcite cements and localized flow through faults stage v fluid flow driven by erosional rebound in the overlying mesozoic strata and remnant squeegee flow this study yielded several important results 1 the prolific repertoire of diagenetic textures found in the southesk cairn carbonate complex serves as a means of comparison for petroliferous carbonate complexes elsewhere especially in sour gas provinces close to orogenic fronts 2 the diagenetic evolution contains long periods of relative tranquility that were punctuated by several episodes of intense diagenetic activity 3 both thermochemical sulfate reduction and squeegee type fluid flow are recorded in late diagenetic sparry calcite cements 4 the effect of the various burial diagenetic episodes on the hydrocarbon reservoir properties throughout the entire reef complex was highly variable specifically matrix dolomitization thermochemical sulfate reduction and the emplacement of secondary and tertiary anhydrites affected the hydrocarbon reservoir properties throughout the entire reef complex on the other hand squeegee fluid flow was insignificant in terms of affecting the reservoir properties

petrographic rare earth elements and isotope constraints on the dolomite origin of ordovician majiagou formation jizhong depression north china abstract dolomites from the ordovician majiagou formation are considered as major hydrocarbon reservoir rocks in the yangshuiwu area jizhong depression however the origin of the dolomites remains unclear this study utilizes a composite analysis of seawater normalized rare earth elements rees petrography and isotopes c o and sr to trace dolomitizing fluids and processes three types of dolomite are identified planar s to nonplanar microcrystalline type d1 dolomite

lithospheric radial anisotropy beneath the gulf of mexico the lithosphere asthenosphere boundary lab where a layer of low viscosity asthenosphere decouples with the upper plate motion plays an essential role in plate tectonics most dynamic modeling assumes that the shear velocity can be used as a surrogate for viscosity which provides key information about mantle flow here we derive a shear velocity model for the lab structure beneath the gulf of mexico allowing a detailed comparison with that beneath the pacific pac and atlantic atl our study takes advantage of the usarray data from the march 25th 2013 guatemala earthquake at a depth of 200 km such data is unique in that we can observe a direct upward traveling lid arrival which remains the first arrival ahead of the triplications beyond 18 this extra feature in conjunction with upper mantle triplication sampling allows good depth control of the lab and a new upper mantle seismic model atm a modification of atl to be developed atm has a prominent low velocity zone similar to the structure beneath the western atlantic the model contains strong radial anisotropy in the lid where v sh is about 6 faster than v sv this anisotropic feature ends at the bottom of the lithosphere at about the depth of 175 km in contrast to the pacific where it extends to over 300 km another important feature of atm is the weaker velocity gradient from the depth of 175 to 350 km compared to pacific models which may be related to differences in mantle flow

petrography and geochemistry of early stage fine and medium crystalline dolomites in the middle devonian presqu ile barrier at pine point canada the petrography and geochemistry of fine and medium crystalline dolomites of the middle devonian presqu ile barrier at pine point western canada sedimentary basin are different from those of previously published coarse crystalline and saddle dolomites that are associated with late stage hydrothermal fluids fine crystalline dolomite consists of subhedral to euhedral crystals ranging from 5 to 25 m mean 8 m the dolomite interbedded with evaporitic anhydrites that occur in the back barrier facies in the elk point basin fine crystalline dolomite has 18 values between 1 6 to 3 8 pdb and 87sr 86sr ratios from 0 7079 0 7081 consistent with derivation from middle devonian seawater its sr concentrations 55 225 p p m mean 105 p p m follow a similar trend to modern little bahama seawater dolomites its rare earth element ree patterns are similar to those of the limestone precursors these data suggest that this fine crystalline dolomite formed from middle devonian seawater at or just below the sea floor medium crystalline dolomite in the presqu ile barrier is composed of anhedral to subhedral crystals 150 250 m mean 200 m some of which have clear rims toward the pore centres this dolomite occurs mostly in the southern lower part of the barrier medium crystalline dolomite has 18o values between 3 7 to 9 4 pdb mean 5 9 pdb and 87sr 86sr ratios from 0 7081 0 7087 mean 0 7084 sr concentrations from 30 to 79 p p m mean 50 p p m and mn content from 50 to 253 p p m mean 161 p p m and negative ce anomalies compared with those of marine limestones the medium crystalline dolomite may have formed either 1 during shallow burial at slightly elevated temperatures 35 40 c from fluids derived from burial compaction or more likely 2 soon after deposition of the precursor sediments by middle devonian seawater derived from the elk point basin these results indicate that dolomitization in the middle devonian presqu ile barrier occurred in at least two stages during evolution of the western canada sedimentary basin the geochemistry of earlier formed dolomites may have been modified if the earlier formed dolomites were porous and permeable and water rock ratios were large during neomorphism