origin of the breno and esino dolomites in the western southern alps italy implications for a volcanic influence the esino limestone of the western southern alps represents a differentiated ladinian lower carnian carbonate platform comprised of margin slope and peritidal inner platform facies up to 1000 m thick a major regional subaerial exposure event lead to coverage by another peritidal lower carnian carbonate platform breno formation multiphase dolomitization affected the carbonate sediments petrographic examinations identified at least three main generations of dolomites d1 d2 and d3 that occur as both replacement and fracture filling cements these phases have crystal size ranges of 3 35 m dolomicrite d1 40 600 m eu to subhedral crystals d2 and 200 m to 5 mm cavity and fracture filling anhedral to subhedral saddle dolomite d3 respectively the fabric retentive near micritic grain size coupled with low mean sr concentration 76 37 ppm and estimated 18o of the parent dolomitizing fluids of d1 suggest formation in shallow burial setting at temperature 45 50 c with possible contributions from volcanic related fluids basinal fluids circulated in volcaniclastics or related to volcanic activity which is consistent with its abnormally high fe 4438 4393 ppm and mn 1219 1418 ppm contents the larger crystal sizes homogenization temperatures d2 108 9 c d3 111 14 c of primary two phase fluid inclusions and calculated salinity estimates d2 23 2 eq wt nacl d3 20 4 eq wt nacl of d2 and d3 suggest that they formed at later stages under mid to deeper burial settings at higher temperatures from dolomitizing fluids of higher salinity which is supported by higher estimated 18o values of their parent dolomitizing fluids this is also consistent with their high fe 4462 4888 ppm and 1091 1183 ppm respectively and mn 556 289 ppm and 1091 1183 ppm contents and low sr concentrations 53 31 ppm and 57 24 ppm respectively the similarity in shale normalized sn ree patterns and ce ce ce sn and la pr pr sn anomalies of the investigated carbonates support the genetic relationship between the dolomite generations and their calcite precursor positive eu anomalies coupled with fluid inclusion gas ratios n2 ar co2 ch4 ar he high f concentration high f cl and high cl br molar ratios suggest an origin from diagenetic fluids circulated through volcanic rocks which is consistent with the co occurrence of volcaniclastic lenses in the investigated sequence

longwavelength gravity anomalies and the deep thermal structure of the baikal rift the analysis of the gravity field over the baikal rift area has been carried out in order 1 to detect the amount of the deep hot material and 2 to constrain the flexural rigidity of the lithosphere we removed a few first harmonics of the global field and the gravity effects due to the crust from the observed field and found a residual anomaly which is aligned with the rift this residual which we attribute to the mantle shows a minimum of about 15 mgal in amplitude and 900 km in width which is superimposed over a wider minimum with smaller amplitude a model involving a simple stretching of the lithosphere with diffusion of heat predicts right order of magnitude for both the amplitude and the wavelength of the 900 km anomaly our results confirm that the stretching factor is of the order of 1 2 to 1 5 interpretation of the coherence function computed between gravity and topography shows that the lithosphere in the area has a significant equivalent elastic thickness of about 30 km i e flexural rigidity about 2 3 1023n m

pervasive dolomitization with subsequent hydrothermal alteration in the clarke lake gas field middle devonian slave point formation british columbia canada the clarke lake gas field in british columbia canada is hosted in pervasively dolomitized middle devonian carbonates of the slave point formation the clarke lake field consists mostly of pervasive matrix dolomite and some saddle dolomite the latter varying in volume from about zero in limestones to normally 20 40 locally up to 80 in dolostones over any given 10 m 33 ft core interval some of the saddle dolomite is replacive some is cement and both varieties are associated with dissolution porosity and recrystallized matrix dolomite the major objective of this study is to identify the causes and timing of matrix and saddle dolomite formation specifically whether these dolomites are hydrothermal a comprehensive petrographic and geochemical examination indicates that pervasive matrix dolomitization was accomplished by long distance migration of halite saturated brines during the late devonian to mississippian fluid inclusion homogenization temperatures suggest about 150 uncorrected to 190c corrected at the time of matrix dolomitization these temperatures differ markedly from most published work on the dolomitized devonian reefs in the alberta basin south of the peace river arch where pervasive matrix dolomitization was accomplished by advection of slightly modified seawater at temperatures of about 60 80c and where no hydrothermal influence was ever present the saddle dolomites at clarke lake are not cogenetic with matrix dolomite and are not the product of hydrothermal dolomitization sensu stricto instead they formed through the hydrothermal alteration of matrix dolomite by way of invasion of a gypsum saturated brine during periods of extremely high heat flow and regional plate margin tectonics in the late devonian to mississippian fluid inclusion homogenization temperatures suggest that hydrothermal alteration occurred between 230 uncorrected and 267c corrected which is significantly higher than the maximum temperature of about 190c attained by the slave point formation during burial the sources of the halite and gypsum saturated brines are middle devonian evaporite depositional environments roughly 200 km 124 mi south and or east of clarke lake near the peace river arch

dolomitization and isotope stratigraphy of the vazante formation sao francisco basin brazil the vazante formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the vazante carbonates dolomites can be classified based on petrography and geochemistry into four generations ranging in crystal size between 4 m almost micritic and 3 mm and occurring as both replacements and cements the sr ca molar ratios calculated for the dolomitizing fluid 0 0006 0 0138 suggest a contribution from a non marine possibly meteoric water component the 18 oa nd 13 c values vary from 0 1 to 14 3 pdb and 0 2 to 2 3 pdb respectively fluid inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 c a mixing zone model of dolomitization for dolomites i iii satisfies the constraints from elemental chemistry 18 o 87 sr 86 sr and fluid inclusions the petrographic and chemical criteria of the latest generation dolomite iv reflect conditions of deep burial environment at temperatures above 120 130 c the 13 c values show only small variations at the base of the formation followed by a major negative plunge 4 at the top this plunge correlated with the sturtian glacial phase in contrast to c isotopes only few samples may have retained their near primary 87 sr 86 sr values of 0 70614 0 70734 the lowest 87 sr 86 sr value 0 70614 from fibrous cement in the upper part of the formation correlates with the negative 13 c shift and is also consistent with the sr isotope signature proposed for the glacial sturtian seawater the 34 s values obtained from sulfates trapped in carbonates range between 10 8 and 16 9 with a jump to 21 3 in the overlying formation all isotope signals are within the range suggested for the early neoproterozoic seawater but the precambrian baseline is poorly known and additional work is required to confirm this tentative assignment 2001 elsevier science b v all rights reserved

sources of basinal and mississippi valley type mineralizing brines mixing of evaporated seawater and halite dissolution brine abstract origins of basinal brines and mississippi valley type mvt mineralizing fluids have been separately attributed to evaporation of seawater or dissolution of halite although brines originating from the two processes are not mutually exclusive in basins this study shows that the na br cl br diagram cannot distinguish between evaporated seawater and halite dissolution fluid using the na deficit ca excess diagram which was previously proposed to characterize fluid rock interactions of basinal brines it is shown that most basinal brines including wt mineralizing fluids of the viburnum trend deposits were probably initially a mixture of halite dissolution fluid and evaporated seawater using the same diagram we suggest that the mineralizing brines of the gays river mvt deposit were derived from an aquifer of clastics underlying a thick succession of evaporites where halite dissolution fluid and evaporated seawater could have mixed

imaging of subducted lithosphere beneath south america tomographic images are produced for the deep structure of the andean subduction zone beneath western south america the data used in the imaging are the delay times of p pp and pwp phases from relocated teleseismic earthquakes in the region regionally structural features larger than about 150 km are resolved by the data presentations of layer anomaly maps and cross sections reveal 1 the nazca slab is probably continuous laterally and at depth over most regions 2 the offset between the north and south deep earthquake zones containing the 1994 deep bolivia main shock and its aftershocks can be modelled by a northwest striking and steeply northeast dipping slab structure and 3 the nazca slab clearly penetrates the lower mantle beneath central south america but is partly deflected in the southern deep zone

evidence for small scale mantle convection in the upper mantle beneath the baikal rift zone 1 inversion of teleseismic p wave travel time residuals collected along a 1280 km long profile traversing the baikal rift zone brz reveals the existence of an upwarped lithosphere asthenosphere interface which causes a travel time delay of about 1 s at the rift axis central high an area with early arrivals relative to the stable siberian platform of up to 0 5 s is observed on each side of the rift about 200 km from the rift axis flank lows while the location of the central high is approximately fixed in the vicinity of the rift axis those of the flank lows vary as much as 200 km with the azimuth of the arriving rays we use three techniques to invert the travel time residuals for velocity anomalies beneath the profile two of the techniques assume an isotropic velocity structure and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry we use independent geophysical observations such as gravity active source seismic exploration and crustal thickness measurements to compare the applicability of the models other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2 5 lateral velocity reduction and a velocity increase due to transverse isotropy with a vertical axis of symmetry we consider the anisotropy to be the result of the vertical component of a lithosphere asthenosphere small scale mantle convection system that is associated with the rifting

velocity structure around the baikal rift zone from teleseismic and local earthquake traveltimes and geodynamic implications abstract we present new results on the velocity structure of the baikal rift zone asia deduced from a comparative teleseismic and local tomography analysis the aim of this paper is to better identify the role of deep mantle processes versus that of far field tectonic effects on the occurrence of extensional tectonics within a continental plate we use 36000 traveltimes of p refracted waves from the isc catalogues and pg and pn traveltimes of 578 earthquakes recorded by the russian regional network to determine a velocity model by the use of local and teleseismic inversion procedures the models show that some velocity patterns are continuous from the surface down to at least 400 km among them a narrow negative anomaly goes through mongolia and follows the southern and eastern margins of the siberian craton this structure is interpreted as a thin mantle plume rising beneath the rift axis however our results do not evidence any wide asthenospheric upwarp at this place other velocity anomalies observed near the surface are not deeply rooted in particular a negative anomaly is observed at shallow levels 48 km beneath the northern third of lake baikal which is disconnected from deeper structures it may be explained by the existence of underplated magmatic material at the bottom of the crust by comparing the geometry of deep rooted anomalies to the present day stress field patterns we conclude that the sub lithospheric mantle dynamics is not the main factor controlling extensional processes in the baikal rift however it does contribute to a thermal weakening of the lithosphere along a mechanical discontinuity bounding the siberian shield we finally conclude that three favourable conditions are gathered in the baikal area to generate extension far field extensional stress field mechanical inherited lithospheric weakness and heat supply further studies should help to precise the genetic link between these three factors

dolomitization of the lower ordovician aguathuna formation carbonates port au port peninsula western newfoundland canada implications for a hydrocarbon reservoir the lower ordovician aguathuna formation 100 m thick is formed of shallow marine carbonates which constitute the uppermost part of the st george group of western newfoundland sedimentation wa

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

evolution of pyrite trace element compositions from porphyry style and epithermal conditions at the lihir gold deposit implications for ore genesis and mineral processing the lihir also known as ladolam au deposit in papua new guinea is a telescoped ore deposit in which volcanic sector collapse s led to superimposition of shallow level au rich epithermal mineralization upon preexisting but genetically related porphyry style alteration this superimposition created a giant 56 moz au resource but also created complications for ore processing specifically with regard to the difficulties in processing the refractory au rich pyritic ore we have analyzed trace element zonation and composition of pyrite grains using laser ablation inductively coupled plasma mass spectrometry imaging coupled with naocl etching from a subset of spatially and paragenetically constrained pyrite bearing samples from the lienetz orebody pyrite grains belong to either porphyry or epithermal stages or are composite pyrite grains with a multistage history trace element zonation and metal contents of pyrite are unique for each paragenetic event providing insights into the nature of the mineralizing fluids early generations of coarse grained pyrites that formed under higher temperature porphyry style conditions have low trace element contents compared with epithermal stage pyrites except for co ni and se later generations of oscillatory zoned pyrites that formed under lower temperature epithermal conditions are enriched in trace elements such as as mo ag sb au and tl the composite pyrites are relatively coarse grained and display textural and geochemical evidence of modification i e dissolution and reprecipitation they are interpreted to be porphyry stage pyrite grains that have been overgrown by rims of delicate banded epithermal style pyrite enriched in au as and other trace elements the composite pyrite grains are volumetrically dominant in the deep seated anhydrite zone at lienetz because au is concentrated only along the rims of these pyrite grains the pyrite can be subjected to a shorter period of oxidation and leaching to liberate most of the au this is in contrast to areas dominated by high grade epithermal stage mineralization where pyrite grains are as and au rich throughout and thus require longer oxidation and processing time understanding au deportment in telescoped deposits is therefore essential for optimizing mineral processing and can significantly impact the economics of mining complex hybrid ore deposits

fluid inclusion gas compositions from an active magmatic hydrothermal system a case study of the geysers geothermal field usa abstract hydrothermal alteration and the active vapor dominated geothermal system at the geysers ca are related to a composite hypabyssal granitic pluton emplaced beneath the field 1 1 to 1 2 million years ago deep drill holes provide a complete transect across the thermal system and samples of the modern day steam the hydrothermal system was liquid dominated prior to formation of the modern vapor dominated regime at 0 25 to 0 28 ma maximum temperatures and salinities ranged from 440 c and 44 wt nacl equivalent in the biotite hornfels adjacent to the pluton to 305 c and 5 wt nacl equivalent at distances of 1730 m from the intrusive contact the major minor and noble gas compositions of fluid inclusions in the hydrothermally altered rocks were integrated with microthermometric and mineralogic data to determine their sources and the effects of mixing and boiling major and minor gaseous species were released from the inclusions by crushing or thermal decrepitation noble gases were released by crushing the samples were analyzed by mass spectrometry the analyses document the presence of magmatic crustal and meteoric components in the trapped fluids hydrothermal fluids present during the liquid dominated phase of the system contained gaseous species derived mainly from crustal and magmatic sources at the geysers n 2 ar ratios greater than 525 and 3 he 4 he ratios of 6 10 7 ra are diagnostic of a magmatic component crustal gas has co 2 ch 4 ratios less than 4 n 2 ar ratios between 45 and 525 and low 3 he 4 he ratios 0 5 ra meteoric fluids have co 2 ch 4 ratios greater than 4 and n 2 ar ratios between 38 air saturated water and 84 air however n 2 ar ratios between 15 and 110 can result from boiling ratios less than 15 reflect the incorporation of n 2 into nh 3 bearing clay minerals in the central geysers the incursion of meteoric fluids occurred during the transition from the liquid to vapor dominated regime variations in the relative ch 4 co 2 and h 2 contents of the gas analyses demonstrate that boiling took place under open system conditions the gas data indicate that the inclusions have remained closed to the diffusion of he and h 2 since their formation

origin of lower ordovician dolomites in eastern laurentia controls on porosity and implications from geochemistry the catoche formation of the st george group in eastern laurentia western newfoundland consists of early ordovician arenigian shallow marine platform carbonates 160 m thick which were extensively dolomitized during the course of their diagenetic history the dolomites occur as both replacement and pore filling cements and are a major control on porosity distribution in the formation the origin and diagenetic history of the catoche dolomites at daniel s harbour western newfoundland were analysed in comparison to equivalent successions at port au choix pac and port au port peninsula pap to assay the reservoir potential of these dolomites in eastern laurentia petrographic examination identified at least three generations of dolomites in the catoche formation which are 1 an early replacement sub to eu hedral micritic dolomite 4 m 30 m d1 2 eu to sub hedral dolomite 70 m 1 mm often with cloudy cores and clear rims d2 and 3 subhedral to anhedral saddle dolomite cement 200 m 3 mm d3 the micritic dolomite d1 exhibits a dull cathodoluminescence cl under cathodoluminoscope whereas dolomite d2 exhibits consistent concentric cl zonation some subhedral crystals of d3 appear zoned both in plane polarized light and cathodoluminoscope otherwise d3 exhibits a dull cl stoichiometric dolomite occurs in all three generations with d2 as the dominant dolomite by abundance the low strontium 47 25 ppm content coupled with depleted 18o value of dolomitizing fluids 10 to 11 2 vsmow and near micritic grain size suggests an early precipitation of dolomite d1 at low temperatures of near surface conditions from solutions likely formed by mixing of early ordovician sea and meteoric waters in contrast microthermometric measurements of primary two phase fluid inclusions in dolomite d2 homogenization temperatures of 102 168 c with a salinity range of 19 8 25 eq wt nacl and dolomite d3 homogenization temperatures of 158 190 c with a salinity range of 20 2 22 2 eq wt nacl suggest that both dolomite generations were generated in mid to deep burial settings from high salinity low temperature 200 c hydrothermal fluids likely under suboxic conditions this is consistent with the low sr concentrations for d2 36 4 8 ppm and d3 38 7 9 ppm 18o values of dolomitizing fluids for d2 2 1 to 8 1 vsmow and d3 6 to 8 1 vsmow coupled with fe contents of d2 1684 1096 ppm and d3 1783 7 618 ppm as well as the respective mn d2 131 2 50 and d3 197 5 55 ppm concentrations ree and shale normalized reesn values of catoche carbonates indicate enrichment in rare earth element ree composition of the earliest calcite c1 relative to those of arenig seawater whereas the reesn profiles of the dolomite generations mimic that of calcite c1 the ce ce ce sn and la la la pr pr sn anomalies of the catoche dolomites are consistent with precipitation in equilibrium with source fluids in slightly oxic to suboxic conditions whereas eu eu eu cn anomalies suggest similar source fluids for d2 and d3 results of fluid inclusion gas analysis are consistent with petrographic features and geochemical compositions and support the exclusion of magmatic fluids during dolomitization visual estimates of porosity from thin sections indicate four porous 4 12 horizons vugs and intercrystalline pores are two types of porosity associated with the dolomites with the latter being the dominant type and associated mainly with dolomite d2

global azimuthal anisotropy in the transition zone surface wave dispersion measurements for love wave overtones carry evidence of azimuthal anisotropy in the transition zone of earth s mantle 400 to 660 kilometers deep a backus gilbert inversion of anisotropic phase velocity maps with resolution kernels mainly sensitive to the transition zone shows a robust long wavelength azimuthally anisotropic velocity structure this observation puts new constraints on the mineralogy and dynamics of the transition zone because this anisotropy may result from aligned minerals tilted laminated structures or even organized pockets of fluid inclusions

burial dolomitization and dissolution of upper jurassic abenaki platform carbonates deep panuke reservoir nova scotia canada a large gas reservoir was discovered in the previously unproductive jurassic aged abenaki carbonate margin in 1998 most of the reservoir porosity is developed in dolostones these dolostones replaced preexisting wackestones packstones and grainstones associated with reefal and adjacent depositional environments many dolomites were subsequently recrystallized or dissolved accounting for much of the preserved secondary porosity subsequent fracturing helped enhance reservoir permeabilities enhanced petrographic techniques established that dissolution of previously dolomitized fabrics generated much of the secondary porosity in these dolostones diffused plane polarized light revealed relict grains and textures invisible with standard microscopic observations petrographic and geochemical observations also confirmed that dissolution occurred under deep burial conditions after incipient pressure solution dissolution was not confined to the centers of dolomitized grains as is commonly seen when remnant calcitic grains dissolve out during the advanced stages of replacement dolomitization instead dissolution was random within relict grains as isolated dolomite crystals were also variably dissolved the geochemistry of these dolomites and associated late stage calcites implied precipitation from basinal hot fluids as well as hydrothermal fluids later diagenetic fluids either acidic or calcium rich or perhaps both at different times based on associated mineralization seemingly promoted dolomite dissolution the presence of tectonic fractures and stylolites helium gas and faults observed in seismic data implied that dolomitization and subsequent dissolution along the abenaki platform margin were controlled by reactivated wrench faults tied to basement on a finer scale diagenetic fluids moved through fractures and pressure solution seams the data collected to date support our contention that the dolomitization and dissolution process which has created most of the porosity in the abenaki reservoir was poststylotization and deeper burial in origin given the timing of tectonic activity in the area and its inferred connection to diagenesis it is probable that at least a part of the diagenetic fluids were hydrothermal in nature

high resolution isotope stratigraphy of the lower ordovician st george group of western newfoundland canada implications for global correlation the lower ordovician st george group of western newfoundland consists mainly of shallow marine platform carbonates 500 m thick it is formed from bottom to top of the watts bight boat harbour catoche and aguathuna formations the top boundary of the group is marked by the regional st george unconformity outcrops and a few cores from western newfoundland were sampled at high resolution and the extracted micritic materials were investigated for their petrographic and geochemical criteria to evaluate their degree of preservation the d13c and d18o values of wellpreserved micrite microsamples range from 4 2 to 0 vpdb and from 11 3 to 2 9 vpdb respectively the d13ccarb profile of the st george group carbonates reveals several negative shifts which vary between 2 and 3 and are generally associated with unconformities disconformities or thin shale interbeds thus reflecting the effect of or link with significant sea level changes the st george unconformity is associated with a negative d13ccarb shift 2 on the profile and correlated with major lowstand around the end of arenig on the local sea level reconstruction and also on those from the baltic region and central australia thus suggesting that the st george group unconformity might have likely had an eustatic component that contributed to the development enhancement of the paleomargin other similar d13ccarb shifts have been recorded on the st george profile but it is hard to evaluate their global extension due to the low resolution of the documented global lower ordovician tremadoc middle arenig d13ccarb profile

diagenesis and origin of calcite cement in the flemish pass basin sandstone reservoir upper jurassic implications for porosity development abstract the flemish pass basin is a deep water basin located offshore on the continental passive margin of the grand banks eastern newfoundland which is currently a hydrocarbon exploration target the current study investigates the petrographic characteristics and origin of carbonate cements in the ti 3 member a primary clastic reservoir interval of the bodhran formation upper jurassic in the flemish pass basin the ti 3 sandstones with average q 86 0 f 3 1 r 10 9 contain various diagenetic minerals including calcite pyrite quartz overgrowth dolomite and siderite based on the volume of calcite cement the investigated sandstones can be classified into 1 calcite cemented intervals 20 calcite and 2 poorly calcite cemented intervals porous petrographic analysis shows that the dominant cement is intergranular poikilotopic 300 500 m calcite which stared to form extensively at early diagenesis the precipitation of calcite occured after feldspar leaching and was followed by corrosion of quartz grains intergranular calcite cement hosts all liquid inclusions mainly in the crystal core but rare primary two phase liquid and vapor fluid inclusions in the rims with mean homogenization temperature t h of 70 2 4 9 c and salinity estimates of 8 8 1 2 eq wt nacl the mean 18 o and 13 c isotopic compositions of the intergranular calcite are 8 3 1 2 vpdb and 3 0 1 3 vpdb respectively whereas fracture filling calcite has more depleted 18 o but similar 13 c values the shale normalized rare earth element ree sn patterns of calcite are generally parallel and exhibit slightly negative ce anomalies and positive eu anomalies fluid inclusion gas ratios co 2 ch 4 and n 2 ar of calcite cement further confirms that diagenetic fluids originated from modified seawater combined evidence from petrographic microthermometric and geochemical analyses suggest that 1 the intergranular calcite cement precipitated from diagenetic fluids of mixed marine and meteoric riverine waters in suboxic conditions 2 the cement was sourced from the oxidation of organic matters and the dissolution of biogenic marine carbonates within sandstone beds or adjacent silty mudstones and 3 the late phases of the intergranular and fracture filling calcite cements were deposited from hot circulated basinal fluids calcite cementation acts as a main controlling factor on the reservoir quality in the flemish pass reservoir sandstones over 75 of initial porosity was lost due to the early calcite cementation the development of secondary porosity mostly enlarged moldic pores and throats by later calcite dissolution due to maturation of organic matters e g hydrocarbon and coals was the key process in improving the reservoir quality

whole earth tomography from delay times of p pcp and pkp phases lateral heterogeneities in the outer core or radial anisotropy in the mantle we perform tomographic inversions of compressional wave travel time measurements in order to image the laterally varying radial anisotropy of p wave velocity in the entire mantle in order to achieve an adequate sensitivity to both horizontal and vertical p velocity throughout the mantle we include in our data set travel time measurements corresponding to ray paths of very different geometries specifically we use data associated with the phases p pcp core reflected and pkpbc pkp df core refracted all extracted from the bulletins of the international seismological centre 1964 1993 pcp pkpbc and pkpdf data are also sensitive to the topography of the core mantle boundary cmb and pkp only to the velocity structure of the earth s core these circumstances require that we also allow for cmb topographic anomalies as free parameters in our inversions and investigate the possibility of the existence of lateral heterogeneities in the outer core e g ritzwoller et al 1986 stevenson 1987 wahr and de vries 1989 vasco and johnson 1998 the solution models that we propose include alternatively a laterally heterogeneous outer core a radially anisotropic mantle or both so that trade offs between those features can also be evaluated we conclude that only minor and very localized radially anisotropic anomalies are present in the middle and lower mantle the most prominent being located in the deepest 400 km under the western pacific we find a discrepancy particularly evident by a comparison between different models of the cmb topography between the travel time measurements that are sensitive to the earth s core and those that are not we show that it is not possible to explain this discrepancy only in terms of radial anisotropy of the mantle and or lateral heterogeneity of the outer core

geochemical characteristics and origins of the diagenetic fluids of the permian changxing formation calcites in the southeastern sichuan basin evidence from petrography inclusions and sr c and o isotopes abstract the upper permian changxing formation in the southeastern sichuan basin exhibited complex fluid activities and the petrographic features fluid inclusions and stable isotopes c o sr isotopes of calcites and host rocks from this formation were determined the aim of the study is to determine the crystallization sequence fluid trapping time and origins of diagenetic fluids of calcites in the changxing formation the results show that four generations of calcite cements were formed in the changxing formation and the corresponding four groups of primary inclusions were captured by these calcites group inclusions are characterized by high salinity 16 15 23 18 wt average 19 5 wt nacl equivalent and low temperature 60 97 c and they were trapped in fine grained calcite during from the early triassic to the early middle triassic t1 t2 inclusions of the other three groups which were captured by medium grained to ultra coarse grained calcites are characterized by moderate salinity average

n2 ar he compositions in fluid inclusions indicators of fluid source abstract a quadrupole mass spectrometer was used to measure bulk samples of conservative gas species n 2 ar and he in fluid inclusions from a variety of hydrothermal systems analyses of these tracer elements help determine 1 1 if gases extracted by bulk inclusion analyses can provide accurate measurement of n 2 ar he in active and fossil geothermal systems 2 2 if hydrothermal fluids associated with paleogeothermal systems in a continental setting follow n 2 ar he systematics similar to th the western pacific rim active geothermal systems specifically new zealand and 3 3 whether different deposit types systematically vary with regard to n 2 ar he the n 2 ar he ratios of fluid inclusion volatiles released from recently deposited minerals from the valles system are similar to those of present day valles thermal waters those inclusion samples from deep within the valles system below a regional aquitard increase in n 2 compositions for inclusions from the questa and copper flat porphyry deposits are n 2 rich similar to those of arc related volcanic gases whereas those from taylor creek sn deposit appear to be mixtures of magmatic and crustal components n 2 ar he ratios of the precambrian tribag deposit suggest a basalt source but significant levels of self generated he from u and th in the inclusion fluids are also possible inclusions from two epithermal deposits with low salinity inclusions have n 2 ar he ratios trending towards air saturated meteoric waters asw and those inclusions with higher salinities indicate minor to no asw component the n 2 ar he ratios in fresnillo and cochiti inclusions which have magmatic helium isotopic ratios indicate additions of magmatic gases to meteoric fluids inclusions from sediment hosted deposits that contain hydrocarbon bearing brines are he rich as are meteoric waters with a long residence time in the crust at relevant pressure temperature composition conditions henry s law constants of n 2 ar and he are similar and thus the relative amounts of these species trapped under boiling conditions will not vary appreciably from solubility controlled amounts in coexisting liquid furthermore there is no evidence of he loss or gain in inclusions by diffusion process data from the various systems that were examined indicate that fluid inclusion n 2 ar he compositions may be related to magmatic fluid sedimentary brine deep circulating meteoric fluid and shallow circulating meteoric fluid sources which we modified from those proposed by giggenbach 1986 relationships between the n 2 ar he tracer gases and other measurable quantities in fluid inclusions are indicated to be helpful in understanding fluid mixing processes in paleogeothermal systems

terrestrial heat flow of continental rifts abstract the results of heat flow studies in the east african rhine and baikal rift systems show the the value of terrestrial heat flow depends on the type of continental rift rifting stage intrarift structures and the age of recent tectonothermal activity in the region they are closely associated with endogenic processes and the position of asthenospheric diapirs

sea level and paleotemperature records from a mid holocene reef on the north coast of java indonesia mid holocene age fossil fringing reefs occur along the tectonically stable north coast of java indonesia presenting an opportunity for sea level and paleoclimate reconstruction the fossil reef at point teluk awur near jepara central java contains two directly superposed horizons of porites lobata microatolls corals in the lower horizon 80 cm above modern sea level yielded uranium series dates of 7090 90 year bp while corals in the upper horizon at 1 5 m grew at 6960 60 year bp these dates match the transgressive phase of regional sea level curves but suggest a mid holocene highstand somewhat older than that recorded on mid pacific islands paleotemperature was calibrated using sr ca and 18o values of a modern p lobata coral and the locally measured sea surface temperature sst yielding sst sr ca and sst 18o calibration equations tsr ca 91 03 7 35 sr ca and t delta 18 text o 3 77 to 5 52 18o the application of the local equations to sr ca and 18o measurements on these corals yielded a range of temperatures of 28 8 1 7 c comparable to that of the modern java sea 28 4 0 7 c a paleo salinometer 18o 18o t t delta 18 text o tsr ca re calculated using the local parameters also suggests java sea mid holocene paleosalinity similar to modern values

chemostratigraphic correlation of neoproterozoic successions in south america abstract the neoproterozoic successions of south america largely accumulated during extensional events associated with the breakup of rodinia these units are preserved as thick carbonate and siliciclastic strata in epicontinental seaways and shallow marine environments on passive cratonal margins which have been intensely deformed in mobile belts surrounding the craton volcaniclastic and siliciclastic materials are associated with some of the marginal fold belts three mega sequences are represented in the cratonal area glaciogenic carbonate and molasse at least two transgressive regressive sea level cycles occurred during the evolution of the carbonate mega sequence which lies above iron cemented glacio marine diamictite of probable sturtian age we report high resolution carbon oxygen strontium and sulfur isotope trends from analyses of well preserved sample sets and use principally strontium and carbon isotopes from the best preserved samples in concert with lithostratigraphic and biostratigraphic observations to provide detailed correlations of the south american neoproterozoic successions lead zinc deposits some of which with important metal reserves seem to be restricted to the sturtian carbonate succession and associated with a sequence boundary at the end of the first transgressive regressive cycle phosphate deposits are found in both sturtian and varangerian marinoan carbonate successions above glaciogenic sequences

model of the transient temperature field of the baikal rift lithosphere abstract in order to coordinate available geophysical data on the deep seated structure with observed heat flow it has to be borne in mind that the thermal field of the baikal rift zone is not in a steady state a two dimensional model of the transient temperature field of the lithosphere was constructed by a numerical method for the solution of the energy conservation equation the authors suggest that asthenospheric upwelling 250 km wide was formed on account of roof block detachment such a mechanical replacement of the lithosphere by the asthenosphere was simulated by a positive compulsory upward movement of the 1200 c isotherm with constant velocity the duration of this movement was taken as long as 30 m y which corresponds to the duration of the first slow rifting stage as soon as the asthenosphere had reached the mohorovicic discontinuity its ascending movement on a wide scale ceased because the density of the crust was less than that of the asthenosphere temperature calculations were carried out in the lithosphere above the 1200 c isotherm this model permits us to reproduce theoretically the present value of the regional heat flow and the curie depth when the time passed from the cessation of the upwelling top movement became equal to 3 5 m y the last figure corresponds to the duration of the second fast stage of rifting on the regional scale the crust of the rift zone sayan baikal domal uplift continues to be heated cooling of the crust can only occur at the segments corresponding to the local heat flow anomalies which are suggested as being produced by dikes the intrusion time of the dike beneath the southern baikal is estimated to be 3 m y

terrestrial heat flow in the south of east siberia abstract we summarize the results of heat flow studies in the paleozoic southern parts of the siberian platform mesozoic zabaikalye and cenozoic baikal rift activity areas it is established that regional heat flow values are greatly influenced by the latest tectonic activity rather than by the age of crust formation

seismic modelling of subduction zones with inhomogeneity and anisotropy i teleseismic p wavefront tracking summary traveltimes and amplitudes for p waves from a 500 km deep source in the kuril subduction zone have been synthesized by ray tracing in smooth 3 d models that allow general anisotropy and inhomogeneity the aim is to compare the effects of proposed anisotropy in or near slabs with those of lateral heterogeneity alone to concentrate on these effects the source position slab thickness 90 km dip 63 and velocity anomaly 5 per cent are held constant results are presented for isotropic models with slab penetration to 670km and 1000 km anisotropic models with 670 km deep slabs have anisotropy within the slab anderson 1987 and in a 10 wedge above the slab ribe 1989a mckenzie 1979 the resulting wavefront topology is never as simple as that in a laterally homogeneous reference earth and there is strong model dependence of shadow zones caustics and areas of multipathing rays are traced through slab models defined by 3 d cubic spline interpolation of up to 21 elastic constants outside the slab region 1 d ray tracing through prem and spherical trigonometry are used to complete the ray path the results illustrate the importance of using both traveltime and amplitude information when interpreting slab structure from teleseismic data some anisotropic slab models have been found which produce large 2 s traveltime residuals that are similar in many parts of the world to those for the deep isotropic model but the amplitude patterns are substantially different the model with a deep isotropic slab produces a narrow band of large traveltime residuals 3 s and high amplitudes in a region across northern canada this feature is due to the focusing of rays that have travelled down the high velocity core of the deep slab regions where ray theory fails i e caustics are obvious through multipathing and amplitude singularities hilbert transforms and airy type decay caustics should be observed in many places if the models presented are good representations of the earth multipathing in along strike regions is a pervasive feature of the models considered and the degree of such multipathing is highly dependent on the nature of the slab boundary velocity gradients the model with anisotropy above the slab produces multipathing traveltime triplication in the down dip region i e a narrow region through europe identifying such non linear or catastrophic features in teleseismic data is potentially more diagnostic than linearized interpretations automated inversion overall the results show that a range of conservative models representing a range of structural theories can encompass a wide range of wavefront consequences drs m weber and v cerven are thanked for helpful reviews of the manuscript advice and reprints from dr k fujita are appreciated and dr d l anderson is thanked for suggesting the anisotropic slab model with no isotropic velocity anomaly the authors acknowledge the support of nserc canada through operating grant number a1465 j m k is supported by an amoco postgraduate scholarship and an ontario graduate scholarship

teleseismic imaging of subaxial flow at mid ocean ridges traveltime effects of anisotropic mineral texture in the mantle summary deformation of peridotite caused by mantle flow beneath an oceanic spreading centre can result in the development of seismic anisotropy traveltime anomalies and shearwave splitting will develop as seismic energy propagates through such an anisotropic region thus providing a signature of the deformation field at depth in this study we investigate the nature of deformation associated with mantle upwelling for two models of flow in the upper 100 km of the mantle the finite strain fields of the passive upwelling model versus the buoyancy enhanced upwelling model are quite different this suggests that mineral aggregates deform differently in the two models thus developing seismic signatures that are distinguishable numerical estimates of the corresponding mineral textures are made using polycrystal theory for olivine with four operative slip systems the activation of a slip system is determined for each grain on the basis of the local critical resolved shear stress the computed grain deformation reflects a balance between stress equilibrium for the aggregate as a whole and strain continuity between neighbouring grains within the aggregate this approach enables a direct link to be made between the model flow fields and the resulting texture development given these mineral orientation distributions elastic parameters are calculated and wavefronts are propagated through the anisotropic structure traveltimes for teleseismic body waves are computed using ray theory and amplitudes are estimated for an across axis profile extending 100 km from the ridge axis relative p wave residuals of up to 1 s are predicted for the buoyant model with on axis arrivals being earliest since near vertical velocities are fastest beneath the axis on axis p wave arrivals for the passive model are half a second earlier than arrivals 60 km off axis and relative delays continue to increase slowly as distance from the ridge increases s wave splitting of almost a second is predicted for the buoyant model whereas less than a half second of splitting is determined for the passive model

seismic anisotropy and mantle flow beneath the baikal rift zone seismic studies have shown that continental rifts such as lake baikal and the great rift valley of east africa are like mid ocean rifts in that they lie above broad regions of asthenospheric upvvarp of much greater extent than the surface expression of rifting1 4 the direction of mantle flow in such regions can be investigated using the seismic anisotropy created by flow induced orientation of mantle olivine crystals5 8 seismic studies of the mid atlantic ridge have revealed upwelling mantle flow beneath the ridge and flow normal to the ridge axis on either side8 10 here we present results from an array of seismic stations across the baikal rift zone in southern siberia the splitting in arrival times of sks seismic waves indicates that the upper mantle beneath the rift zone is anisotropic with the fast direction which reflects the direction of mantle flow being horizontal and normal to the rift axis this suggests that the broad upwarp associated with this continental rift is caused by similar mantle flow to that at mid ocean rifts this may help to elucidate the processes involved in continental rifting

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

isotope stratigraphy of the lapa formation sao francisco basin brazil implications for late neoproterozoic glacial events in south america the lapa formation is a thick carbonate sequence 900 m that constitutes the upper part of the vazante group on the s francisco craton brazil it conformably overlies a previously unrecognized glacial diamictite unit of poorly constrained age the sequence above the glacial unconformity consists predominantly of organic rich shale subtidal rhythmic dolomicrites and microbialaminites and intertidal stromatolites four boreholes spanning different depositional settings were sampled at highresolution and investigated for their petrographic and chemical criteria to evaluate their degree of preservation the 13 c and 18 o values of well preserved lapa carbonate microsamples range from 8 2 to 3 3 vpdb and from 13 6 to 0 9 vpdb respectively each of the 13 c profiles of the investigated cores reveals two strong negative excursions of up to 8 an event in post glacial dolomicrites immediately above the glaciogenic unit and a 10 m interval of organic rich shale and a second near the top of the sequence associated with a shale interval based on the observation of dropstones and sedimentary iron formation in the underlying diamictite as well as the distinguishable carbon isotope trends the lapa formation is considered as a cap carbonate lithofacies the age of the lapa formation is presently unknown but the least radiogenic 87 sr 86 sr value 0 7068 associated with a negative carbon isotope excursion matches that from the rasthof formation in namiba on the congo craton which is radiometrically constrained to be younger than ca 750 ma

anisotropic convection with implications for the upper mantle on the basis of polycrystalline theory describing the plasticity in olivine and enstatite the flow in a convection cell has been simulated using a finite element formulation the spatial variations in anisotropic properties are computed from the textures that evolve with the flow a kinematically constrained equilibrium based assumption is used to partition the macroscopic deformation among crystals within an aggregate we model the convection for one specific cell geometry and two sets of boundary conditions a complete map of textures throughout the cell is obtained the textured convection cell is structurally very heterogeneous and textures along streamlines do not correlate with the finite strain the results of the simulations indicate that during up welling a strong texture develops rapidly it convects during spreading and is attenuated during subduction results are compared with features of the upper mantle in our predictions the pattern of preferred orientation during spreading is inclined to the flow coordinates due to deformation by simple shear this is contrary to hess 1964 intuition that 001 slip planes of olivine orient themselves parallel to the flow planes yet the pattern is consistent with natural fabric data significant differences are observed as a function of depth within the cell the variations in the p wave velocities in this textured model mantle are analyzed and correspond well with observed seismic data

source of diagenetic fluids from fluid inclusion gas ratios abstract alteration of marine carbonates such as those from the mesoproterozoic and lower ordovician during burial history is carried out by diagenetic fluids that could potentially have different sources during precipitation diagenetic minerals may trap tiny amounts of those solutions in fluid inclusions thus providing fossil archives and aliquots that offer valuable record about the origin of fluids the analysis of various fluid inclusion gases e g ch 4 co 2 n 2 he and ar may provide reliable information which allows the discrimination among different fluid sources generally speaking the meteoric cements should have n 2 ar ratio around 38 the value for air saturated water however the gases in inclusions of the current study trapped by diagenetic carbonates cements from independent depositional basins of different ages mesoproterozoic early ordovician and modern and locations canada usa brazil and java sea have n 2 ar ratios 92 which match non magmatic modified marine or meteoric fluids also the ar he ratios 3 in the studied fluid inclusions are significantly lower than those of calc alkaline magmatic fluids 20 thus excluding magmatic fluids as a potential source on the other hand some n 2 ar and ar he ratios of the studied samples approach values reported for basinal fluids from the hansonburg mvt deposit in new mexico 68 and 1 respectively suggesting that both meteoric mixed marine meteoric evolved meteoric and basinal fluids may be involved during carbonate diagenesis

global correlation of the vazante group sao francisco basin brazil re os and u pb radiometric age constraints abstract the vazante group consists of precambrian carbonate dominated platform deposits that extend along more than 300 km in the external zone of the brasilia fold belt of the sao francisco basin in east central brazil the sequence is about 4 8 km thick and contains a preserved glaciomarine diamictite unit containing dropstone at the top and a lower diamictite unit at the bottom previous c and sr isotope profiles suggested the correlation of the upper diamictite unit with the sturtian glacial event ca 750 643 ma however new re os isotope data from the shales associated with the upper diamictites yield radiometric age estimates between 993 46 and 1100 77 ma u pb measurements on a suite of clear euhedral zircon crystals that were separated from the same shales associated with the upper diamictite and from the arkosic sandstone above the lower diamictite yield ages as young as 988 15 and 1000 25 ma respectively based on the re os and u pb ages the best age estimate of the vazante group is constrained to be 1000 1100 ma and thus the two diamictite units are not correlative with the sturtian glaciation s but most likely are records of glacial events that occurred during the late mesoproterozoic

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

upwarp of anomalous asthenosphere beneath the rio grande rift continental rifts are possible analogues of mid ocean ridges although major plate tectonic features are less clearly observed1 current thermal models of mid ocean ridges2 4 consist of solid lithospheric plates overlying the hotter less viscous asthenosphere with plate thickness increasing away from the ridge axis the lithospheric lower boundary lies at or near the melting point isotherm so that at greater depths higher temperatures account for lower viscosity lower seismic velocities and possibly partial melting upwarp of this boundary at the ridge axis concentrates heat there thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium at slow spreading ridges a major central graben forms owing to the mechanics of magma injection into the crust5 topography heat flow gravity and seismic studies support these models on the continents a low velocity channel has been observed although it is poorly developed beneath ancient cratons6 9 plate tectonic models have been applied to continental basins and margins10 12 but further similarities to the oceanic models remain elusive topographic uplift is often ascribed to airy type isostatic compensation caused by crustal thickening rather than thermal compensation in the asthenosphere here we discuss the rio grande rift in southwestern united states teleseismic p wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening

dolomitization of the lower ordovician watts bight formation of the st george group western newfoundland evidence of hydrothermal fluid alterationgeological survey of canada contribution 20080544 the watts bight formation in western newfoundland consists of a lower ordovician succession of shallow water carbonates and has been extensively dolomitized these dolomites occur as both replaceme

biogenic and abiogenic low mg calcite blmc and almc evaluation of seawater ree composition water masses and carbonate diagenesis the ree and shale normalized paas reesn values of modern brachiopods biogenic low mg calcite blmc represented by several species from high to low latitudes from shallow to deep waters and from warm and cold water environments define three distinct average seawater trends the warm and cold water brachiopods define two indistinguishable p 0 050 groups that mimic open ocean seawater ree chemistry exhibiting the typical lree enrichment with a slightly positive to negative ce anomaly followed by an otherwise invariant series other recent brachiopods from an essentially siliciclastic seabed environment are distinct in both ree and reesn trends from the previous two populations showing a slight enrichment in the mrees and an increasing trend in the hrees other groups of modern brachiopods are characterized by elevated reesn trends relative to the normal groups as well as by complexity of the series trends the most characteristic feature is the decrease in the hrees in these brachiopods from areas of unusual productivity i e such as upwelling currents fluvial input and aerosol dust deposition preserved brachiopods from the eocene and silurian exhibit reesn trends and ce anomalies similar to that of the open ocean modern brachiopods although their enriched ree concentrations suggest precipitation of blmc influenced by extrinsic environmental conditions preservation of the blmc was tested by comparing the ree and reesn trends of preserved eocene brachiopods to those of oligocene brachiopods that were altered in an open diagenetic system in the presence of phreatic meteoric water the altered blmc is enriched by approximately one order of magnitude in both ree and reesn trends relative to that in blmc of their preserved counterparts similarly the ree and reesn of preserved silurian brachiopod blmc were compared to those of their enclosing altered lime mudstone which exhibits features of partly closed system phreatic meteoric water diagenesis despite these differences in the diagenetic alteration systems and processes the rees and reesn trends of the blmc of altered brachiopods and of originally mixed mineralogy lime mudstones now diagenetic low mg calcite are enriched by about one order of magnitude relative to those observed in the coeval and preserved blmc in contrast to the changes in ree and reesn of carbonates exposed to phreatic meteoric water diagenesis are the ree compositions of late burial calcite cements precipitated in diagenetically open systems from burial fluids the ree and reesn trends of the burial cements mimic those of their host lime mudstone with all showing slight lree enrichment and slight hree depletion exhibiting a chevron pattern of the reesn trends the overall enrichment or depletion of the cement reesn trends relative to that of their respective host rock material reflects not only the openness of the diagenetic system but also strong differences in the elemental and ree compositions of the burial fluids evaluation of the ce ce sn and la pr pr sn anomalies suggests precipitation of the burial calcite cements essentially in equilibrium with their source fluids

provenance and burial history of cement in sandstones of the northbrook formation carboniferous western newfoundland canada a geochemical investigation abstract the carboniferous north brook formation western newfoundland canada consisting of fluvial lacustrine arkosic litharenites has undergone two cementation cycles the first cycle comprises chlorite followed by minor quartz and calcite mono phase aqueous fluid inclusions hosted in first cycle calcite reflect precipitation in a near surface meteoric diagenetic environment chlorite linings and cements generally inhibit precipitation of quartz overgrowths thus maintaining open pore space until a subsequent cycle when temperatures are within the oil window the second cycle comprises chlorite followed by calcite which hosts two phase fluid inclusions with homogenization temperatures ranging from 91 7 to 120 7 c and salinity from 6 3 to 8 4 eq wt nacl these results suggest conditions in line with a deep burial environment and calculated fluid inclusion isochors stipulate a trapping temperature of around 132 c and pressure of 500 bar 5 km hydrostatic pressure for these cements quantitative fluid inclusion gas analysis further confirms that fluids were sourced from evolved meteoric fluids with fluctuating co2 ch4 n2 ratios manganese analysis confirms an oxidizing environment for the first cycle calcite cement but a reducing one for the infilling second cycle cement both cements have similar ree signatures are slightly lree depleted and during migration the fluids were influenced by the sedimentary host rock although there is a prominent negative ce anomaly there is no eu anomaly in the cements the 13c and 18o values are mostly in the negative in the calcite cement hosted in the coarse grained clastics which were influenced by migrating brines and or hydrocarbons in the diagenetic history of this sedimentary basin the first cementation event is characterized by chlorite quartz and calcite emplaced under shallow oxidizing meteoric water conditions and appropriate temperatures a diagenetic hiatus ensued until the units were more deeply buried at this stage a second generation of cement characterized by minor chlorite and major calcite infilled vugs and voids under higher temperatures equivalent to the upper limit of the oil window and 5 km burial depth the lack of porosity precludes the north brook formation from being a petroleum reservoir although limited petroleum may have been sourced during the geologic history

baikal rift zone structure and geodynamics abstract the cenozoic baikal rift zone is superimposed on the caledonian baikal fold belt representing the suture between the precambrian siberian craton and several microcontinents the baikal rift zone consists of a system of disconnected fault bounded basins and extensional and wrench faults that straddles a major arch having a topographic relief of 2 3 km rifting activity commenced during the oligocene and is still active as evident from high seismicity of the baikal rift zone across the baikal rift zone upper crustal extension is considerably smaller than would be expected from its crustal thickness which decreases by 8 11 km across the baikal basins as compared to the adjacent unextended areas the discrepancy between upper and lower crustal attenuation could be attributed to intrusion of a major diapir indeed geophysical data indicate that the sayan baikal dome coincides with such a diapir the top of which is located at the crust mantle boundary this intrusion is held responsible for oligocene and quaternary doming of the rift zone development of the intra continental baikal rift system is thought to be related to asthenospheric diapirism active rifting however intra plate stresses in conjunction with the himalayan collision may have played some role

a deep geophysical study in the baikal region experimental data show that in east siberia resistivity curves irrespective of their trends are affected by galvanic local distortions the preliminary step of the magnetotelluric data processing is to obtain a steady shape of resistivity curves reflecting a true deep section for this purpose statistical averaging and different criteria of impedance rejecting were used the available mts curves were normalized by level to the global magnetovariation curves two dimensional modelling was performed from several sublatitudinal profiles crossing the baikal rift zone three dimensional models based on two dimensional modelling and on induction vector distribution have been computed via programs of m n yudin following other researchers two conductive layers are distinguished i the mid and low crustal and ii the mantle one with the layer surface uplifted from 100 110 km depth in the southern baikal rift zone to 60 70 km northeastwards along the eastern baikal coast the top of this layer seems to correspond to the asthenospheric roof the asthenosphere deepening in southern brz is likely to be related to a decrease in the asthenospheric bulge width and an increase in the rate of lithospheric thickening with mantle degasing the origin and evolution of the baikal rift is considered proceeding from the model of passive rifting with regard to a long existing lithospheric inhomogeneity between the siberian platform and the sayan baikal folded area

thickness of the lithosphere beneath the baikal rift zone and adjacent regions abstract methods of separation of gravity anomalies related to mantle density inhomogeneity are developed based on the inversion of these anomalies with regard to some constraints obtained from seismic data the map of lithospheric thickness beneath the baikal rift zone and adjacent regions was made the lithospheric thickness beneath the baikal rift zone is estimated to be 40 50 km i e the lithosphere is thinned here to a crustal thickness beneath the siberian platform the lithospheric thickness increases to 200 km and underneath the trans baikal region of moderate cenozoic tectonic activity it ranges from 75 to 160 175 km therefore a wide asthenospheric upwelling was revealed beneath the rift zone the data available on the configuration of the baikal depression and that of the crust and the lithosphere as a whole made it possible to estimate the magnitudes of extension at different lithospheric levels the significant increase in extension with depth implies that rifting in the baikal zone was caused by asthenospheric diapirism

massive dolomitization of a pinnacle reef in the lower devonian west point formation gaspe peninsula quebec an extreme case of hydrothermal dolomitization through fault focused circulation of magmatic fluids devonian pinnacle reefs of the west point formation in gaspe peninsula eastern canada were built on paleotectonic highs in a foreland basin of the nine pinnacles known in outcrop one is dolomitized and occurs at the junction of two acadian faults the petrography of the dolomitized facies has revealed the presence of three dolomite phases and one late calcite cement a first dolomite phase of small crystals is volumetrically minor the following dolomite phase dominates and consists of centimeter size replacive saddle dolomite crystals that contain fluid inclusions with homogenization temperatures ranging between 301 and 382c the third dolomite consists of millimeter to centimeter size saddle dolomite crystals that fill late fractures this phase is characterized by lower temperature fluid inclusions 159 171c a lower temperature calcite phase 107 123c fills some voids fluid inclusions are saline 8 7 to 13 3 wt naclequiv the dolomite and calcite phases are characterized by very negative 18ovpdb vienna peedee belemnite values between 19 and 14 and negative 13cvpdb values between 8 and 1 the replacive saddle dolomite phase originated from a fluid with very positive 18ovsmow vienna standard mean ocean water values 8 whereas the following dolomite and calcite precipitated from fluids with lower 18ovsmow values 3 4 and 4 5 we propose that fault focused circulation of magmatic fluids is responsible for the very high temperature massive dolomite replacement of the calcite host and high temperature burial fluids later used fractures to circulate in the dolomitized host to precipitate late dolomite and calcite regionally hydrocarbon migration is recognized at the time of late calcite cementation

a global isostatic gravity model of the earth the long wavelength features of the external gravity field of the earth contain the gravitational signal from deep seated lateral mass and density inhomogeneities sustained by dynamic earth mantle processes to interpret the observed gravity field with respect to mantle dynamics and structures it is essential first to remove the lithosphere induced anomalous gravitational potential which is generated by the topographic surface load and its isostatically compensating masses based upon the most recent global compilation of crustal thickness and density data and the age distribution of cooling oceanic lithosphere residual topography and gravity are calculated by subtracting the known crustal and oceanic lithosphere compensating masses and gravitational effects from the surface fields empirical admittances between residual topography and gravity are then computed to estimate the effective depths of the remaining compensating masses which are not explained by the initial data and model assumptions this additional compensation is eventually placed by adjusting the density in the uppermost mantle between the moho and on average 70 km depth with a maximum of 118 km under tibet the lithospheric mass distribution is used in a subsequent forward computation to create a global model of the lithosphere induced gravitational potential the resulting isostatic model is considered to be valid for spatial wavelengths longer than 500 km the isostatic lithosphere model field expressed in terms of both gravity and geoid heights is subtracted from the observed free air gravity field to yield a global set of 1 1 isostatic gravity disturbances and from a satellite derived long wavelength geoid to yield the isostatic residual geoid the comparison of residual mantle gravity residual topography and isostatic corrected gravity allows us to identify the main characteristics of the underlying mantle for example dynamic support by mantle flow of the north atlantic topographic high applying the isostatic correction the overall pattern of the geoid becomes smoother and the most pronounced features which are separated in the observed geoid tend to get connected to larger structures these results stress the importance of separation of the lithospheric gravitational impact for a correct interpretation of the external gravity field even in its very long wavelength constituents also the isostatic corrected geoid spectrum reveals a stronger decrease in power from degree 3 to degree 4 and degree 5 to degree 6 which is in accordance with seismological models of deep mantle structures

volcanism of the baikal rift zone abstract the volcanism of the baikal rift zone is a specific case of intraplate volcanism where the tensional stress field in the lithosphere under the rift zone does not greatly exceed local tension in other volcanic regions of central asia in which rifting was not morphologically expressed the main phase of volcanism miocene pliocene preceded active extension and rift formation migration of volcanism from the periphery to the axial part of the rift as well as subsequent variation of the composition of volcanics from alkaline to tholeiitic are not specific to the baikal rift zone the volcanism of this zone and other interior regions of asia is considered to be a consequence not only of deep regional processes under volcanic areas connected to the formation and rise of a highly anomalous mantle but also of global processes caused by the indo eurasian collision which was responsible for the large scale deformations within the asiatic continent including tensional stress fields

faulting within the eastern baikal rift as characterized by earthquake studies earthquake relocations focal mechanism determinations waveform modeling and stress analyses using focal mechanism information are used to examine the faulting processes within the western baikal rift 95 to 108 5 results of the study show that high angle dip 70 faulting is most likely to occur in the more youthful parts of the rift and is most likely to be associated with m 4 5 events like the eastern rift nearly all earthquakes occurred on faults dipping 30 suggesting that in no part of the rift is significant deformation occurring on low angle structures focal mechanism information does not indicate there is a sharp boundary to the western rift as observed in the eastern rift less normal faulting rakes 60 to 120 occurs in the western rift than the eastern rift waveform modeling of the two largest earthquakes 1950 mondy and 1959 srednebaikal in the western rift gives focal depths of 10 to 15 km the 1959 event appears to have occurred along the eastern boundary fault on lake baikal just to the north of proval bay a feature created a m 7 1 earthquake in 1862 the 1950 event may have occurred along the southern boundary fault of the tunka depression stress orientation information indicates a near horizontal nw se to wnw ese striking 3 in the eastern part of the western rift

dolomitization of the lower ordovician catoche formation implications for hydrocarbon exploration in western newfoundland the lower ordovician st george group in western newfoundland consists of a sequence of subtidal and peritidal carbonates which are extensively dolomitized the current study investigates the diagenetic evolution of the catoche formation from the port aux choix and port au port peninsulas in order to study the controls on reservoir quality in western newfoundland the catoche formation dolomites are classified into three main generations early and pervasive replacement dolomite d1 indicates that dolomitization began during early stages of diagenesis stable isotope and trace element data indicate significant variations between d1 dolomite on the port aux choix and port au port peninsulas the depleted 18o signature of d1 dolomite fluids 8 7 1 3 vpbd on the port aux choix peninsula is consistent with partial dolomitization associated with mixing of seawater and meteoric waters on the flanks of structural highs in contrast 18o values 6 1 0 7 vpbd and trace element data from the port au port peninsula indicate that pervasive d1 was associated with mixing of possibly post evaporitic brines with meteoric waters later stage replacement dolomites d2 are associated with enhancement in porosity through the development of intercrystalline pores while latest stage saddle dolomite d3 significantly occluded the pores in some horizons d2 dolomite formed due to the influx of warm 100 c saline 15 eq wt nacl fluids intercrystalline porosity in d2 formed due to the dolomitization of precursor calcite due to the lower molar volume of dolomite compared to calcite therefore porosity development is lower on the port au port peninsula with no significant volume change during the recrystallization of the pervasive early d1 dolomicrite similarly extensive porous horizons on the port aux choix peninsula are related to the limited extent of d1 dolomitization this suggests that the quality of a potential dolomite reservoir is strongly controlled by tectonic and diagenetic history of host carbonates

teleseismic arrivals at a mid ocean ridge effects of mantle melt and anisotropy recent observational evidence of upwelling mantle anisotropy at a slow spreading center has motivated the modeling of teleseismic arrivals at mid ocean ridges the models consider a variety of types of anisotropy and heterogeneity where the emphasis is to ascertain whether or not travel times can be used to discriminate between the existence of partial melt and anisotropy two mechanisms for anisotropy in the upwelling asthenosphere are considered one due to the preferential alignment of the fast axes of olivine crystals in the direction of mantle flow and the other due to the preferential alignment of cracks that feed melt towards the spreading axis the results indicate that p waves are most sensitive to even modest amounts of flowinduced asthenospheric anisotropy while s waves are most sensitive to the presence of mantle melt multiple s wave arrivals are predicted for many models most notably the ones with anisotropy due to crack alignment where very large s wave separations develop the model which best fits existing data requires a higher degree of crystal alignment anisotropy in the upwelling asthenosphere than in the lithosphere this effect has been predicted in studies of the evolution of crystal alignment anisotropy in polycrystalline aggregates

faults and tectonic stresses of the baikal rift zone abstract the cenozoic baikal rift system consists of a central extensional segment which links a southwestern and a northeastern sinistral shear segment statistical studies on fault length amount and sense of slip depth of crustal penetration and spacing has led to the establishment of empirical relationships which appear to apply also for non extensional regimes results are graphically displayed in figs 4 7 tectonic stress regimes were determined by means of fault and joint analyses and on the basis of seismological data results of both approaches are compatible and explain the evolution of the baikal rift zone in terms of tensional reactivation of pre existing crustal discontinuities

on the structure and mechanical behaviour of the extending lithosphere in the baikal rift from gravity modelling abstract the crustal structure and lithospheric flexure of the baikal rift zone siberia are examined by means of gravity modelling we model the bouguer anomaly ba along five 1200 km long gravity profiles we first evidence that continuous elastic plate flexure due to surface loading cannot explain the observed ba then we introduce plate discontinuities coupled with a realistic brittle elasto ductile plate rheology which allow us to model of external tectonic forces acting on the plates and determination of the moho geometry we show that the clearest expression of extensional processes occurs in the central part of the rift which exhibits the highest crustal thinning it evolves southwards to a rapidly increasing compression resulting in an overthickening of the southern plate s crust and in the long wavelength flexure of the siberian plate north of the central rift crustal thinning which is always less than 7 km gives way to a more diffuse zone of deformation inside the sayan baikal folded belt based on plate flexure models we propose that the rift shoulders surrounding the central and north baikal basins are not supported by upward bending plate but have a deep crustal root caused by a downward flexure the other parts of the rift depict two adjacent plates with antithetic flexures we also infer that the axial mantle material upwarping is not related to a large scale asthenospheric upwelling since the lithosphere rheological interfaces are not significantly disturbed our results favour the role of horizontal forces and motions resulting from the india asia collision combined with the effect of inherited tectonic structures and especially the paleozoic suture bounding the siberian craton for explaining the crustal structure and plate flexure modeled

asymmetric upwarp of the asthenosphere beneath the baikal rift zone siberia in the summer of 1991 we installed 27 seismic stations about lake baikal siberia aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the baikal rift zone and adjacent regions sixty six teleseismic events with high signal to noise ratio were recorded travel time and q analysis of teleseisms characterize an upwarp of the lithosphere asthenosphere boundary under baikal theoretical arrival times were calculated by using the international association of seismology and physics of the earth s interior 1991 earth model and travel time residuals were found by subtracting computed arrival times from observed ones a three dimensional downward projection inversion method is used to invert the p wave velocity structure with constraints from deep seismic sounding data our results suggest that 1 the lithosphere asthenosphere transition upwarps beneath the rift zone 2 the upwarp has an asymmetric shape 3 the velocity contrast is 4 9 in the asthenosphere 4 the density contrast is 0 6 and 5 the p wave attenuation contrast t is 0 1 s

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation