aragonite relic preservation in jurassic calcite replaced bivalves shells of the aragonite bivalve neomiodon great estuarine group jurassic scotland replaced by coarse neomorphic calcite contain oriented relics of the original aragonite ultrastructure the presence of these relics in such old altered shells as well as the high sr content of the replacement calcite indicate that the process of calcite replacement of aragonite is not a cumulative slow process involving repeated alteration events but rather a rapid one step process aragonite relics once encased in neomorphic spar will survive as unequivocal evidence of original aragonite mineralogy barring total remobilization of the enclosing stable calcite a generally unlikely event the retention of this residual aragonite and high sr calcite supports recent isotopic studies which suggest that the multiple phases of alteration recrystallization invoked in earlier literature are unlikely events in the diagenesis of most undolomitized limestones retention of aragonite relics appears to be independent of whether alteration occurs in shallow meteoric or as in the case of our neomiodon material deeper burial environments pseudopleochroism of the replaced neomiodon shells appears to be due to organic largely graphitic relics not to the aragonite relics

chlorine cycling during subduction of altered oceanic crust eclogitic rocks that have experienced devolatilization with little or no interaction with external fluid sources can be viewed as analogues of crustal material which may be transferred back into the mantle during subduction thus they can be used to evaluate the extent of the recycling of volatile elements such as chlorine we report new oxygen isotope ratios of omphacite and fluid inclusion data determined from eclogitic metagabbros of the rocciavre massif italian alps the data are compared with those obtained for the monviso cyclades and the franciscan complex high pressure rocks in all localities relics of early dehydration fluids are preserved as primary fluid inclusions in the cores of omphacite megacrysts rocciavre monviso and franciscan complex or garnet cyclades salinity estimates of the inclusion fluids range from 32 to 45 wt nacl in rocciavre 17 to 21 wt nacl in monviso and are similar to seawater in other areas omphacite and bulk rock 18o values of rocciavre 5 1 6 8 and monviso 3 0 5 3 metagabbros are markedly lower than those of cyclades 6 8 14 3 and the franciscan 6 7 13 1 metabasites the fluid salinity 18o systematics of eclogitic rocks is similar to that documented along a typical section of the altered oceanic crust and unmetamorphosed ophiolites this suggests that high pressure metamorphism and associated processes did not modify significantly the variability in chlorine concentrations and oxygen isotope ratios inherited from a stage of sea floor hydrothermal alteration under low basaltic layer and high temperature gabbroic layer conditions respectively extrapolating the estimated h2o and cl contents of eclogitic rocks to a representative section of the subducted oceanic crust indicates that a minimum of 100 to 200 ppm cl could be recycled into the mantle during subduction this yields a cl h2o ratio of 3 6 to 7 5 10 2 for the subducted oceanic crust which is similar to e morb on the basis of available cl isotopic data we infer that a large proportion 70 of the cl stored in the altered crust should be recycled to the mantle to generate an isotopic composition of the subducted crust equivalent to the source of unaltered mid ocean ridge basalt 37cl 4 7

meteoric diagenesis below a submerged platform implications for δ13c compositions prior to pre vascular plant evolution middle ordovician alabama u s a abstract the middle ordovician chickamauga formation at red mountain expressway rme and tidwell hollow twh sections in alabama u s a has a complex early diagenetic history early diagenetic history of the buildup facies consisted of two phases of marine cementation separated by two generations of meteoric equant calcite with moldic dissolution separating equant calcite generations a third generation of meteoric equant drusy calcite is present after secondary marine diagenesis equant drusy calcite is interpreted as meteoric because its stable isotopic composition 13 c 0 1 to 1 6 18 o 4 8 to 7 3 is too variable to be accounted for by marine or mixing zone precipitation equant drusy calcite from buildup facies is mostly non luminescent and non ferroan this is unlike early ferroan equant calcite dull to bright luminescent from below the unconformity that overlies the chickamauga formation at rme meteoric diagenesis below the unconformity is unrelated to early diagenesis in the underlying buildup facies equant drusy calcite from the buildup facies at rme has a covariation of 13c and 18o values that forms a truncated inverted j and has elevated mg values these geochemical signatures indicate precipitation at a site that was distal from the recharge zone meteoric diagenesis at rme occurred during rising relative sea level and flow of meteoric fluids under a submerged platform is the only mechanism able to reconcile available stratigraphic petrologic and geochemical data ordovician meteoric 13c values from twh and other formations are slightly depleted 2 4 relative to coeval marine carbonate especially when compared with post silurian meteoric carbonate minor depletion of ordovician carbonate is the result of limited generation of light soil gas co2 before the development of vascular plants

diagenetic stabilization of aragonite and low mg calcite i trace elements in rudists abstract cretaceous rudists with thick multilayered bimineralic aragonite and low mg calcite shells have been studied for the effects of meteoric diagenesis on their textural and chemical attributes the original fine compact prismatic or cellular prismatic microstructures of low mg calcitic layers have been preserved frequently despite the vagaries of their subsequent diagenetic histories these layers usually suffered only partial recrystallization which caused fusion of crystal units rounding of prism comers and partial cementation confined to intercrystalline and intraparticle pore spaces in contrast the originally aragonitic parts of the shell usually suffered either complete dissolution or at best retained some vestiges of their crossed lamellar or complex crossed lamella microstructures synsedimentary submarine diagenetic events such as borings by endolithic fauna internal micritic and or peloidal sedimentation and submarine aragonite and or high mg calcite cements within the primary inter and intraparticle pore spaces were coeval with rudist accretion the subsequent meteoric cementation has been characterized by the formation of bladed and equant ferroan calcites which occluded the residual primary and secondary porosities diagenetic stabilization of aragonite to diagnetic low mg calcite and low mg calcite to diagenetic low mg calcite is believed to have been a two stage process initially sr and na have been partially depleted during an early meteoric diagenetic phase calcitization of metastable phases with expulsion of sr at a slower rate than that of na this stage is followed by incorporation of more mn fe and mg due to precipitation of late ferroan calcite stabilization of aragonite causes more pronounced trace element repartitioning than the stabilization of low mg calcite however the degree of repartitioning depends not only on mineralogy but also on the structural buildup of a given shell layer suggesting that factors such as water rock ratio surface kinetics are of considerable sig ificance for the process of diagenetic stabilization rudist skeletal components with preserved original mineralogy and texture have trace element chemical signatures analogous to holocene marine bivalves this similarity argues for comparable chemical composition of the cretaceous and holocene seawater as well as for similar modes of incorporation of trace elements into mollusc shells

geochemistry of coal measure source rocks and natural gases in deep formations in songliao basin ne china abstract the natural gases developed in deep volcanic rock reservoirs of the songliao basin ne china are characterized by enriched 13 c value for methane and frequently reversal carbon isotopic distribution pattern although many researchers consider such gas type as an abiogenic origin we believe the natural gases have a biogenic origin mainly except little inorganic gases and the reversal carbon isotopic distribution pattern of gases is caused by mixing of different origin gases methane carbon isotopic values for majority samples fall in the range from 24 to 32 which is heavier than typical coal type gases in other chinese basins there are several reasons caused heavy carbon isotope of methane 1 carbon isotopic values of source kerogen are 3 5 heavier than these from other basins 2 source rocks are at extremely high maturity stage with vitrinite reflectance mostly above 3 0 3 portion of gas is derived from basement mudrock or slate with higher maturity the observation on the organic from deep formation reveals that there is a relatively high content for liptinite which reaches approximately 8 to 10 the macerals component of source rock shows that the source rocks have some ability to generate oil small portion of oil was generated from high hydrogen content macerals in coals and shales as proof by oil found in microcrack and in micropore of coal and oil bearing fluid inclusions grown in volcanic reservoir the occurrence of pyrobitumen in volcanic reservoir indicates preexisted oil had been cracked into wet gas and this kind of gas had also been found in gas pools heavy isotopic methane is derived from coal at extremely high maturity stage there may be little inorganic alkane gases in deep layers for their geochemistry and special geological setting of songliao basin artificial mixing experiments of different origins gases confirm that inorganic gas such as gas from well fs1 mixed with other end members can turn out full reversal isotopic distribution pattern based on simple calculations of 13 c 1 values it can be concluded that the deep natural gases in songliao basin are mainly derived from coal

evaluating asbestos fibre concentration in metaophiolites a case study from the voltri massif and sestri voltaggio zone liguria nw italy this work is part of the project study for a road tunnel bypassing the town of genova and was aimed at evaluating the amount of asbestos fibres in the metaophiolites belonging to the voltri group and the sestri voltaggio zone liguria northern italy the 85 studied rock samples mainly mafic and ultramafic rocks derive from exposed outcrops and prospecting boreholes the study of field relations and petrographic microtextural investigations under the optical microscope allowed for the identification and characterisation of asbestos bearing settings and lithotypes mineralogy and concentration of asbestos fibres in powdered specimens were determined by means of a scanning electron microscope equipped with an energy dispersive x ray spectroscopy device these investigations were combined with petrography on thin section x ray diffraction analysis and phase contrast optical microscopy on rock powders mafic and ultramafic rocks commonly contain asbestos in concentrations below 1 000 mg kg considered as the contamination threshold under italian law however the fibre concentration rises abruptly within localised zones where the metaophiolite sequences were involved into late ductile to brittle tectono metamorphic events two groups of asbestos bearing settings have been so far identified in the area a fracture networks within serpentinites dominated by fibrous chrysotile and b boudins of chlorite tremolite schists likely deriving from dynamic recrystallisation of mafic rocks under greenschist facies conditions dominated by fibrous amphibole even considering the low volumetric incidence of these settings metres to few tens of metres their high asbestos content locally controls the total fibre amount in the excavation products thus requiring special prevention measures during excavation management and final storage of the contaminated debris

evolution of synmetamorphic veins and their wallrocks through a western alps transect no evidence for large scale fluid flow stable isotope major and trace element systematics quartz rich synfolial veins and wallrocks from different areas of a western alps cross section are examined in an attempt to constrain the scale and mechanisms of fluid flow through metamorphic terrains different portions of this cross section underwent distinct p t evolutions as reflected by the mineralogy of veins a in the greenschist external dauphinois domain veins are characterized by quartz and calcite together with an aluminosilicate pyrophyllite na k bearing phyllosilicates and a femg bearing silicate chlorite b in narrow domains of the brianconnais and piemontais zones with a non eclogitic hp lt evolution followed by rapid cooling veins are characterized by quartz calcite ca and fe mg bearing silicates lawsonite and fe mg carpholite respectively and c in the major part of the brianconnais andpiemontais zones where low grade blueschistfacies metamorphism is followed by a greenschist facies evolution veins are characterized by the replacement of earlier lawsonite and carpholite by na and k bearing micas in order to constrain the provenance of fluids involved in vein formation we determined stable isotope c 0 h and major and trace element compositions in minerals and whole rocks for a large set of veins and wallrocks these rocks are compared with some non metamorphic shales from the northern paris basin considered as protoliths of the alpine schists stable isotope compositions of calcites are regionally distinct 6r80 28 0 smow in the northern paris basin 26 0 in the brianconnais zone 22 0 in the piemontais zone and 18 0 in the dauphinois zone within each studied region calcite g o values are quite homogeneous regardless of rock type vein or wallrock this feature is explained as an isotopic homogenization with the most abundant lithology in each zone rock buffered system where the overall ratio of calcite of marine origin to detrital silicate minerals determines the final isotopic composition of the whole rock w r after equilibration during alpine metamorphism g o gd values of analysed hydrated minerals and whole rocks si80 15 to t 25 0 and 6d 60 0 smow lie within the field of alpine cover rocks and of fluids in equilibrium with these rocks interaction with an external o or d depleted fluid such as hydrothermal waters meteoric water or basement fluid can be ruled out major and trace element systematics indicate an enrichment in si ca p sr y ta and locally eu in the veins with respect to the wallrock related to massive crystallization of quartz and carbonates together with phosphate and oxide potential fluid sources chemical composition of the fluid phase and the mechanisms of fluid flow during vein formation are discussed fluid flow takes place within tectonic units at both hectometric and decimetric scale fluid may also have migrated along and across the major tectonic contacts but this process is poorly documented

paleoceanographic and paleoclimatic controls on ooid mineralogy of the smackover formation mississippi salt basin implications for late jurassic seawater composition abstract the late jurassic smackover formation in the mississippi salt basin consists of two 150 m thick shoaling upward cycles each capped by ooid grainstones during deposition of the lower cycle originally calcite ooids preserved radial fabric 18o 3 8 pdb 13c 4 5 pdb sr2 315 ppm formed on the seaward side of the basin and former aragonite ooids calcitized textures 18o 3 0 pdb 13c 5 5 pdb sr2 1790 ppm were precipitated on the landward side in the upper cycle originally calcite ooids preserved radial fabric were precipitated on both the seaward 18o 2 5 pdb 13c 4 2 pdb sr2 300 ppm and the landward 18o 2 5 pdb 13c 3 2 pdb sr2 250 ppm sides of the basin because kinetic variables mg2 ca2 rate of co32 supply po43 so42 are incapable of totally preventing aragonite formation we suggest that smackover calcite ooids were precipitated from seawater with low carbonate saturation state possibly undersaturated relative to aragonite the shift from seaward calcite to landward aragonite ooids in the lower cycle was controlled by a shoreward increase in seawater salinity the net effect of the salinity gradient was a landward increase in the carbonate saturation state in response to decreasing dissolved co2 and increasing co32 ca2 and temperature in seawater supersaturated with respect to both arago ite and calcite kinetic variables favored dominance of aragonite over calcite the landward increase in seawater salinity reflects extensive evaporation in an arid climate resulting in antiestuarine circulation the monomineralogic calcite nature of ooids of the upper cycle suggests that the salinity gradient across the basin was not sufficient to alter the seawater saturation state this is attributed to a less arid climate and or a less restricted circulation

naturally occurring asbestos in an alpine ophiolitic complex northern corsica france this paper provides a field description and an analytical characterization of the fibrous minerals associated with ultrabasic and basic rocks from the corsican ophiolitic complex on the island of corsica in order to examine their asbestos potential thirty five fibrous samples taken from serpentinites magnesium rich meta gabbros and meta basalts were studied using combined epma ramanand fesem methods the results highlight that naturally occurring asbestos noa are abundant in serpentinites and regularly occur in magnesium rich meta gabbros and meta basalts in northern corsica the spatial distribution abundance and mineralogical types of these noa strongly depend on the petrographic nature of the hosting rocks and their structural pattern noa in serpentinites correspond to chrysotile vein networks in the internal parts of the thickest rocky masses and to tremolite veins and shear planes carrying tremolite fibers both in the external parts of these masses and in highly sheared serpentinites within or close to tectonic contacts noa in highly deformed magnesium rich meta gabbros are associated with the opening and filling of albite tremolite veins associated with the syntectonic boudinage of the most competent meta gabbros in the meta basalts noa are associated with late metamorphic actinolite bearing polymineralic veins cross cutting the foliation planes fragments and pebbles of serpentinites meta gabbros and meta basalts containing noa are also present in colluvium scree and alluvium resulting from erosion processes special attention should be paid to serpentinites and or magnesium rich meta gabbros bearing colluvium in which fibrous occurrences of tremolite regularly evolved into whitish clusters consisting of very long easily separable flexible and entangled fibers with a higher asbestos potential the characterization of noa in the coc serpentinites meta gabbros and meta basalts leads us to consider them as hazardous materials as these lithologies are very abundant within the whole structural edifice they may be regularly impacted by development or construction work and thus require suitable monitoring

the slip fiber chrysotile asbestos deposit in the zidani area northern greece abstract the chrysotile asbestos deposit of zidani in northern greece is an example of a slip fiber deposit associated with alpine type peridotites the deposit occurs in the northern part of an antigorite serpentinite and peridotite body tectonically emplaced on the pelagonian basement of the area a continuous retrograde alteration event is recognizable along the thrust fault between a metaclastic and carbonate sequence and the serpentinite body a phase of ductile deformation that formed antigorite mylonites was followed by a brittle one which formed schistose serpentinite the sense of shear of the overthrusting metaclastic mylonite coincides with that of the immediate underlying schistose and mylonite serpentinites thus retrogression of both groups of rocks is assumed to be the result of the thrust event the numerous altered amphibolite dikes crosscutting the ore created planar anisotropies during the thrust related deformation these planar anisotropies localized the deformation and consequently the formation of the schistose serpentinite and the asbestos ore in the northern part of the serpentinite body slip fiber chrysotile asbestos was formed along anastomosing non penetrating shear surfaces in the schistose serpentinite the chrysotile fibers replaced antigorite in situ in shear planes most likely at a transitional stage of deformation exhibiting both ductile and brittle fabrics however syntectonic filling of shear veins at the brittle stage is the major cause of the origin of the asbestos deposit

sediment hosted geothermal systems review and first global mapping abstract sediment hosted geothermal systems shgss are hybrid geological systems where volcanic and sedimentary domains interact leading to mixtures of inorganic and organic gases typically characterized by geothermal thermometamorphic or mantle derived co2 and biotic microbial or thermogenic ch4 shgss occur in sedimentary basins crossed by magmatic intrusions or involved in volcanic plumbing systems these systems can be of considerable interest for petroleum exploration and natural greenhouse gas emission studies but systematic studies for their characterization and worldwide distribution are missing here we provide a review of shgss identified so far and propose methodological criteria for their definition and identification based on integrated geological and gas geochemical parameters we find that shgss are typically characterized by a fluids dominated by mantle or decarbonation methamorphic co2 50 vol b considerable amounts of ch4 and heavier hydrocarbons at least 1 5 vol generally up to 30 40 vol produced by microbial or thermogenic degradation of organic matter hosted in sedimentary rocks c tectonically active sedimentary basins back arc rift zones and foredeep generally hosting petroleum fields and within 300 km from recent or ancient volcanic centers this analysis resulted in a global map including a first set of 33 shgss located in north america central and eastern europe far east eastern oceania and northern new zealand and a second set of potential shgs prone areas occurring also in south america north africa middle east and kamchatka the present shgs map can evolve on the basis of more detailed geological analysis and new gas geochemical data

volcanic rocks as prolific gas reservoir a case study from the qingshen gas field in the songliao basin ne china abstract the qingshen gas field with over 100 billion cubic meters bcms of proven gas reserves was discovered recently in the upper jurassic lower cretaceous strata of the songliao basin northeastern china the stratigraphic relationships and gas chemical compositions indicate mixed gas sources dominantly from the upper jurassic lower cretaceous coals and associated dark mudstones underlying the gas reservoirs with minor contribution from deep inorganic sources the gas reservoirs consist mainly of volcanic rocks rhyolite and tuff with some production also from the conglomerate gas charge into these largely volcanic rock reservoirs appears to coincide with the peak gas generation in the deep upper jurassic lower cretaceous coal measures the presence of several thick lacustrine mudstone beds in the upper cretaceous strata as regional caprocks and relatively mild structural deformation since the late cretaceous are among the favorable conditions for the preservation of large volume of natural gases in these relatively deep gas reservoirs the gas yields are the highest in the paleomorphological highs where fractures and secondary porosity are well developed in the gas reservoirs

channelized fluid flow and mass transfer along a rift related detachment fault eastern alps southeast switzerland remnants of a rift related low angle detachment system are spectacularly exposed in the continental err nappe and occur discontinuously in the underlying oceanic platta nappe in southeast switzerland rocks along the detachment fault exhibit systematic changes in mineralogy fabrics and structures that are related to deformation and hydration of granite to form cataclasite and subsequent gouge structural mineralogical and geochemical data indicate a continuous evolution of fluid flow and mass transfer along the detachment fault mass balance calculations together with stable isotope data suggest 1 an altered seawater source and local interaction with mantle rocks 2 limited fluid rock interaction and mass transfer during initial cataclasis at deeper crustal levels and temperatures of 150 to 200 c 3 increased fluid flux and mass transfer at slightly lower temperatures 100 150 c during gouge formation at shallower crustal levels and 4 strongly localized fluid rock interaction at low temperatures 65 80 c near the sea floor at a final stage of detachment faulting we propose a model in which seawater dominated fluids penetrated and locally interacted with mantle rocks before they were caught and channelized along large scale detachment faults flow direction is upward along the detachment from higher to lower temperatures suggesting that the driving force of fluid flow was thermal fluid convection associated with mantle exhumation our study suggests that this large scale detachment fault is a good analog for extensional structures along present day nonvolcanic rifted margins such structures along ocean continent transitions are important in controlling fluid pathways and mass transfer between mantle and continental crust

an ion microprobe study of diagenetic carbonates in the devonian nisku formation of alberta canada abstract diagenetic calcites and dolomites of the upper devonian nisku formation in central alberta canada were analyzed for trace elements by ion microprobe the ion microprobe permits analyses on sample spots comparable in size 20 m to those of conventional electron probes detection limits on the ion microprobe are 1 ppm for many trace elements of interest trace element data obtained by ion microprobe are comparable to those obtained by icp trace element variations in and between upper devonian diagenetic calcites from the nisku formation aid in the interpretation of the fluid composition during precipitation and or recrystallization as well as in the interpretation of primary carbonate mineralogy e g high mg calcite vs aragonite using the ion probe microenvironments as well as temporal variations in diagenetic fluid composition can be resolved the presence of significant trace element variations in nisku carbonates on a scale of 100 m indicates that fluid rock interaction during burial of up to 4000 m and during more than 300 m y was very limited although several of the measured trends probably reflect incomplet retention of primary compositional variations trace element variations in some paleozoic limestones and dolostones reflect variations in fluid composition during primary precipitation as opposed to fluid compositions during recrystallization these findings encourage the use of trace element compositions for diagenetic interpretations even in very old and deeply buried carbonates

naturally occurring asbestos in eastern australia a review of geological occurrence disturbance and mesothelioma risk potential asbestos bearing rocks account for about 0 2 of the land area of eastern australia the main mode of occurrence is as narrow cross fibre and slip fibre veins of chrysotile asbestos in serpentinised ophiolite complexes along the boundaries of major tectonic domains smaller deposits of chrysotile and amphibole asbestos occur in metamorphosed mafic and ultramafic rocks associated with the macquarie volcanic arc in new south wales amphibole asbestos is also known from proterozoic and palaeozoic amphibolite and from devonian basalt natural asbestos bearing materials in eastern australia have been disturbed by mining road construction agriculture and forestry urban development and through natural weathering processes persons most at risk of potential exposure to asbestos from natural sources include farmers who work or live in areas where asbestos bearing materials may be routinely disturbed by agricultural activities construction workers involved in large scale earthwork projects in areas underlain by asbestos bearing rocks and quarry workers who unwittingly disturb asbestos bearing materials government authorities and private enterprise need to take geological factors into account to reduce the likelihood of unplanned disturbance of natural asbestos bearing materials

transformation of aragonite dominated lime muds to microcrystalline limestones scanning electron microscopy of textures in several well lithified pleistocene micrites mixed micrite and microspar from south florida and the bahamas shows significant diagenetic alteration of the lime mud precursors the dominance of aragonite in those precursor muds is supported by the surprising presence of abundant aragonite relics in the neomorphic calcite crystals confirmed by feigl stain x ray and electron diffraction polished very lightly etched surfaces of neomorphic calcite crystals even in the microspar show aragonite relics and pits both absent in coexisting void fill cements similarities between micrite and microspar crystals indicate their common origin through a one step neomorphic process of calcitization that is the mud did not first calcitize to micrite and then alter to microspar by aggrading neomorphism this conclusion has more general implications for the interpretation of the origins of other older microsparitic carbonates both early calcitization and concurrent early meteoric cementation were significant in producing the florida bahamas micrites studied which have low porosities similar to those found in most ancient fine grained limestones these low porosities despite the absence of significant overburden indicate that major porosity reduction without compaction can occur in calcitized aragonite dominated muds

natural emissions of methane from geothermal and volcanic sources in europe it has recently been demonstrated that methane emission from lithosphere degassing is an important component of the natural greenhouse gas atmospheric budget globally the geological sources are mainly due to seepage from hydrocarbon prone sedimentary basins and subordinately from geothermal volcanic fluxes this work provides a first estimate of methane emission from the geothermal volcanic component at european level in europe 28 countries have geothermal systems and at least 10 countries host surface geothermal manifestations hot springs mofettes gas vents even if direct methane flux measurements are available only for a few small areas in italy a fair number of data on co2 ch4 and steam composition and flux from geothermal manifestations are today available for 6 countries czech republic germany greece iceland italy spain following the emission factor and area based approach the available data have been analyzed and have led to an early and conservative estimate of methane emission into the atmosphere around 10 000 ton yr 4000 16 000 ton yr basically from an area smaller than 4000 km2 with a speculative upper limit in the order of 105 ton yr only 4 18 of the conservative estimate about 720 ton yr is due to 12 european volcanoes where methane concentration in volcanic gases is generally in the order of a few tens of ppmv volcanoes are thus not a significant methane source while the largest emission is due to geothermal areas which may be situated next to volcanoes or independent here inorganic synthesis thermometamorphism and thermal breakdown of organic matter are substantial methane flux can reach hundreds of ton yr from small individual vents geothermal methane is mainly released in three countries located in the main high heat flow regions italy greece and iceland turkey is likely a fourth important contributor but the absolute lack of data prevents any emission estimate therefore the actual european geothermal volcanic methane emission could be easily projected to the 105 ton yr levels reaching the magnitude of some other natural sources such as forest fires or wild animals

ordovician meteoric carbon and oxygen isotopic values implications for the latitudinal variations of ancient stable isotopic values abstract columnar and clear blocky calcite cement from a middle ordovician carbonate succession in east tennessee is interpreted as meteoric in origin columnar and clear blocky calcite from this succession does not show extremely large 13c depletions reported from meteoric phases of younger rocks meteoric fluid 18o values calculated from clear blocky calcite are 2 to 3 more negative than approximately coeval sea water a relationship typical of modern low latitude coastal meteoric water comparison with meteoric 18o values from ordovician units elsewhere suggests that the geographic distribution of these values may be broadly similar to that observed today therefore we tentatively suggest that geographic distribution of meteoric 18o values during both icehouse and greenhouse eras are similar

origins of some early carboniferous calcrete fabrics revealed by cathodoluminescence implications for interpreting the sites of calcrete formation abstract calcretes from the lower carboniferous marginal marine limestones of south wales germany and belgium exhibit complex calcite microspar fabrics with rhombic irregular and spherical crystals in variable mosaics while many of these fabrics resemble classic recrystallization microspars having irregular crystal forms and non planar crystal boundaries cathodoluminescence microscopy reveals that they result from multiple phases of precipitation and dissolution no evidence for recrystallization by crystal boundary migration or replacement during the transformation of less stable calcite minerals aragonite or high magnesian calcite is seen identical microspar fabrics have been widely described in quaternary calcareous soils which may also be primary fabrics and not due to recrystallization or mineral transformation cathodoluminescence provides a useful tool for interpreting calcrete fabrics and in the case of these carboniferous forms indicates multiple phases of carbonate saturation and leaching reflecting the prominent wetting and drying phases evidenced by associated vertic soils horizons the complex cycles of precipitation and dissolution are characteristic of pedogenic calcretes and the recognition of such features using cathodoluminescence may provide a means of differentiating pedogenic from groundwater calcretes

the utility of methane clumped isotopes to constrain the origins of methane in natural gas accumulations methane clumped isotope compositions provide a new approach to understanding the formational conditions of methane from both biogenic and thermogenic sources under some conditions these compositions can be used to reconstruct the formational temperatures of the gas and this capability can be applied to common subsets of both biogenic and thermogenic systems additionally there are examples in which clumped isotope compositions do not reflect gas formation temperatures but instead mixing effects and kinetic phenomena such kinetic effects also occur in common and recognizable subtypes of biogenic and thermogenic gases here we review the use of methane clumped isotope measurements for understanding the origin of methane in the subsurface we review methane clumped isotope measurements from numerous biogenic and thermogenic natural gas reservoirs we then place these measurements in the context of common frameworks for identifying the formational conditions of methane including the use of methane 13 c and d values and c 1 c 2 3 ratios finally we propose a framework for how methane clumped isotopes can be used to identify the origin of methane accumulations

structurally controlled growth of fibrous amphibole in tectonized metagabbro constraints on asbestos concentrations in non serpentinized rocks the nucleation and concentration of asbestos in rocks are mostly associated with mechanisms of fibre formation combined with the water dependent mineralogical alteration produced during the serpentinization of ultramafic masses very little is known about the structural settings and tectonic histories that influence and control the occurrence of asbestos in non serpentinized rocks diffusely embedded within tectonized ophiolitic suites focusing on a case history provided by a tectonized metagabbro from the ligurian alps northern italy a multiscale structural petrographic approach is used to investigate the relationships between rock fabric and fibrous amphibole growth within the metagabbro meso to microstructural observations are used to document the role of structurally controlled fluid rock interactions in localizing the growth of fibrous amphibole during ductile to brittle shearing mylonitic foliation to shear veins a qualitative structural scenario is provided to illustrate the growth of asbestos amphiboles in shear veins during the progression of shear deformation towards semi brittle rheological conditions the mechanisms of the structurally controlled growth of fibrous amphibole in non serpentinized rocks imply an examination of the tectonic boundary conditions that are at the origin of the concentration of asbestos in ophiolitic rocks involved in orogenic belt construction

asbestos fibre identification vs evaluation of asbestos hazard in ophiolitic rock melanges a case study from the ligurian alps italy in recent years the high incidence of harmful health effects through inhalation of airborne asbestos from amphibole bearing rock melanges has been thoroughly documented here we present a field based multi scale geological approach aimed at illustrating the occurrence of amphibole fibrous mineralisation in an ophiolitic suite from the ligurian alps italy and discussing the implication on in situ determination of the asbestos hazard the rock melange is composed of plurimetre sized blocks of different lithotypes metagabbro serpentinite chloritoschist juxtaposed by the meaning of tectonic structures the geological structural survey revealed that the fibrous mineralisation is localised in specific structural sites of the rock volume including veins and schistosity both micro chemical and crystal structure analyses on selected fibrous samples revealed that actinolite fibres grow in veins within the metagabbro and in chloritoschists while fibrous tremolite occurs in serpentinite schistosity the morphological features of these amphibole fibres have been analysed in tem images and used for classifying them as asbestiform or non asbestiform the results show that the asbestos hazard determination is not unequivocally identified when different procedures for asbestos fibre identification and classification are applied this may have impact on normatives and regulations in defining environmental hazards due to asbestos occurrence

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

tiber delta co2 ch4 degassing a possible hybrid tectonically active sediment hosted geothermal system near rome fiumicino town in the tiber river delta near rome international airport italy is historically affected by large amounts of carbon dioxide co2 in the ground and gas eruptions triggered by shallow drilling while it is known that co2 originates from carbonate thermometamorphism and or mantle degassing the origin of methane ch4 associated with co2 is uncertain and the outgassing spatial distribution is unknown combining isotope gas geochemistry soil gas and structural stratigraphic analyses we provide evidence for a hybrid fluid source system classifiable as sediment hosted geothermal system shgs where biotic ch4 from sedimentary rocks is carried by deep geothermic co2 through active segments of a half graben molecular and isotopic composition of ch4 and concentration of heavier alkanes ethane and propane obtained from gas vents and soil gas throughout the delta area reveal that thermogenic ch4 up to 3 7 vol in soil gas 13cch4 37 to 40 vpdb vienna peedee belemnite and 2hch4 162 to 203 vsmow vienna standard mean ocean water in gas vents prevails over possible microbial and abiotic components the hydrocarbons likely result from known meso cenozoic petroleum systems of the latium tyrrhenian coast overmaturation of source rocks or molecular fractionation induced by gas migration are likely responsible for increased c1 c2 ratios co2 and ch4 soil gas anomalies are scattered along nw se and w e alignments which based on borehole geomorphologic and structural stratigraphic analyses coincide with active faults of a half graben that seems to have controlled the recent evolution of the tiber delta this shgs can be a source of considerable greenhouse gas emissions to the atmosphere and hazards for humans and buildings

geochemical and stable isotope resetting in shear zones from taschalp constraints on fluid flow during exhumation in the western alps fluid flow at greenschist facies conditions during exhumation of the western alps occurred in several penecontemporaneous systems including shear zones at lithological contacts deformed contacts between serpentinite bodies and metabasalts albite veins within metabasalts and calcite quartz veins within calcareous schists fluid flow in shear zones that juxtapose metasediments and ophiolitic rocks within the piemonte unit reset o and h isotope ratios 18o values are buffered by the wall rocks however calculated fluid 2h values are similar within all the shear zones suggesting that they formed an interconnected network the similarity of 2h values of the sheared rocks and those of unsheared calcareous schists suggests that the fluids were derived from or had equilibrated with the schists that envelop the ophiolite rocks time integrated fluid fluxes at the sheared contacts estimated from changes in si in metabasalts were up to 105 m3 m 2 with the fluid flowing up temperature driven either by topography or seismic pumping individual shear zones were active for c 2 3 myr implying average fluid fluxes of up to 10 9 m3 m 2 s 1 rocks in shear zones within the ophiolite away from contacts with the metasediments show much less marked isotopic and geochemical changes implying that fluid volumes decreased into the ophiolite unit consistent with the source of fluids being the metasediments fluids were generated by dehydration reactions that were intersected during exhumation and while many rocks show the affects of fluid rock interaction large scale fluid flow between major units was not common

structures in natural serpentinite gouges abstract serpentinite gouges from cataclastic fault zones in the voltri massif nw italy are dominated by few discrete boundary parallel y shears and oblique synthetic riedel r 1 shears separating domains of massive gouge unlike natural clay rich fault gouges the cohesive microbreccia domains only rarely show foliations these structures dominated by discrete shears indicate a significant degree of localization of the deformation at the scale of the gouge the dominant gouge forming mechanisms were fracturing of olivine and pyroxene grains along serpentinized grain boundaries and initiation of transgranular fractures in pyroxene grains in domains of lattice bending and grain indentation syn tectonic serpentinization added to the weakening of fragments textural similarities with experimentally formed gouges suggest that the voltri fault zones were dominated by stick slip sliding hence that they may represent fossil seismogenic faults a notable aspect of the gouge texture in the voltri fault zones is a sigmoidal shape of the riedel shears which bend into parallelism with the y shears it is argued that this geometry was largely controlled by local perturbations of the stress field near heterogeneities on y or p shears the average spacing between the r 1 shears increases linearly with 0 47 times the distance between bounding y shears this proportionality may reflect a balance between the cohesive strength of the gouge material and the shear stresses acting on the bounding slip planes

amphibole in vermiculite mined in argentina morphology quantitative and chemical studies on the different phases of production and their environmental impact vermiculites with impurities of amphibole a dry fine residue and calcined materials from three deposits in the province of cordoba argentina were studied from a commercial perspective amphiboles are considered as asbestos a group of silicate minerals with strong and flexible fibres that are heat resistant and chemically inert and thus well suited for heat insulation these fibrous particles have provoked controversies about the toxicity impact on human health and the development of diseases such as mesothelioma asbestosis or lung cancer their commercialization and exploitation are currently prohibited the sampled minerals were identified with a petrographic microscope chemical analyses sem and xrd it was concluded that amphibole minerals are present in all the deposits studied and in every phase of plant production but not all of them have asbestiform characteristics

diagenetic alteration of early marine cements of upper silurian stromatactis stromatactis is a spar network whose elements in cross section have flat to undulose lower surfaces and digitate upper surfaces the network is composed principally of isopachous crusts of centripetal cement and commonly occurs embedded in finely crystalline limestone it is the cement filling of interconnected cavities stromatactis of upper silurian red stromatactis limestone from gaspe peninsula quebec appalachians exhibits two types of cements 1 an isopachous cement that lined the walls of the conduits and is interpreted as early marine and 2 a later blocky cement that occupies the centres of cavities the first cement is composed exclusively of non ferroan calcite whereas the second cement is mixed non ferroan and ferroan calcite the early isopachous cement is white on polished slabs and has a turbid aspect under transmitted light in a few samples the relative homogeneity of this early cement is broken by the presence of distinctive grey clear calcite under cathodoluminscence the grey clear calcite is non luminescent and exhibits well defined bladed crystal shapes whereas the white turbid cement has a dull orange luminescence and indistinct crystal shapes the relationships between the two cements indicate that the dull luminescent cement is a secondary form of the non luminescent cement and it is concluded that the dull cement is the product of alteration of the non luminescent cement by burial or meteoric fluids the later blocky cement has the same dull luminescence as the white turbid cement and is thought to have been precipitated from the same fluids as those responsible for the alteration of the early marine cements oxygen isotopic values of the dull cement of the early isopachous crusts mean 18o 6 8 o are intermediate between those of the non luminescent early marine cement mean 18o 5 3 o and the dull luminescent blocky cement mean 18o 11 8 o while carbon isotopic values do not differ significantly 13c 2 9 2 4 and 2 6 o respectively the alteration also has affected the distribution of some trace elements particularly mg both unaltered and altered cements contain less than 1 microdolomite inclusions but the mg content of the background calcite of unaltered cement is three times that of altered cement 14171 vs 5502 ppm precursor early marine cement is thought to have been low mg calcite the mean 18o value 5 3 o of unaltered early marine cement is higher than values for the oxygen isotopic signature of silurian oceans provided by brachiopod shells

calcite cementation during bacterial manganese iron and sulphate reduction in jurassic shallow marine carbonates faunally restricted argillaceous wackestones from the middle jurassic of eastern england contain evidence of early diagenetic skeletal aragonite dissolution and stabilization of the carbonate matrix closely followed by precipitation of zoned calcite cements and precipitation of pyrite distinctive cathodoluminescence and trace element trends through the authigenic calcites their negative 13c compositions and the location of pyrite in the paragenetic sequence indicate that calcite precipitation took place during sequential bacterial mn fe and sulphate reduction calcite 18o values are compatible with cementation from essentially marine pore fluids although compositions vary owing to minor contamination with 18o depleted late cements mg and sr concentrations in the calcites are lower than those in recent marine calcite cements this may be a result of kinetic factors associated with the shallow burial cementation microenvironments bicarbonate for sustained precipitation of the authigenic calcites was derived largely from aragonite remobilization augmented by that produced through anaerobic organic matter oxidation in the metal and sulphate reduction environments aragonite dissolution is thought to have been induced by acidity generated during aerobic bacterial oxidation of organic matter distinction of post oxic metal reduction and anoxic sulphate reduction diagenetic environments in modern carbonate sediments is uncommon outside pelagic settings and early bacterially mediated diagenesis in modern platform carbonates is associated with extensive carbonate dissolution high detrital fe contents of the jurassic sediments and their restricted depositional environment were probably the critical factors promoting early cementation these precipitates constitute a unique example of calcite authigenesis in shallow water limestones during bacterial mn and fe reduction

petrography trace elements and oxygen and carbon isotopes of gordon group carbonates ordovician florentine valley tasmania australia gordon group carbonates are a thick 2 km stratigraphically continuous ordovician sequence that ranges in age from the arenig to the ashgill they were deposited in subtidal intertidal supratidal and channel depositional environments at a paleolatitude of about 10 n the biota is similar to modern tropical chlorozoan assemblages with abundant corals oncolites calcareous algae and stromatolites non skeletal grains are abundant and are predominantly intraclasts and pellets with some oolites and aggregates micrite is abundant with some sparry calcite cement dolomite which commonly replaces micrite extensive burrows and mud cracks formed mostly during early diagenesis by marine to mixed marine waters the gordon group limestones are characterized by high sr na ratios 2 6 4 6 similar to modern aragonitic carbonates the stratigraphic variations of sr na and mn are related to depositional environments diagenesis and dolomitization the low mn concentrations indicate oxidizing peritidal depositional environments with high rates of carbonate deposition na varies with salinity and the sr varies with facies the largest sr concentrations are in corals and micrites the sr na and mn concentrations have been affected by two stage meteoric diagenetic stabilization the first stage involves marked depletion of sr and na with no appreciable gain of mn due to inversion of aragonite to low mg calcite the second stage involves no significant change in sr and na with appreciable gain of mn which is due to recrystallization of low mg calcites and addition of spar cement in a semi closed diagenetic system sr concentrations decrease whereas mn and fe concentrations increase with increasing mg concentrations due to dolomitization in predominantly marine waters with some influx of meteoric waters the 18o and 13c isotopic fields of calcites and dolomites partly overlap and the dolomites are enriched relative to co existing calcites by 2 to 3 in 18o and by about 1 in 13c similar to modem marine and mixed marine dolomites low mn and fe concentrations indicative of least meteoric diagenetic change correspond to 5 0 0 5 18o and 1 0 05 13c suggesting that these values were the original late ordovician marine calcite values in tasmania these isotopic values are similar to the least altered late ordovician brachiopods and marine calcite cements from other areas the middle ordovician 18o and 13c values of marine cements are 7 and 0 respectively and these values match the marine cement values of the same age from other areas this shift towards heavier isotopic values from middle to late ordovician is probably due to both glaciation in the higher latitudes of gondwana and a slight drop in temperature 4 c depletion in 13c 18o sr and na and enrichment in mn in the lower and upper most part of the late ordovician tasmanian limestones indicative of pronounced meteoric diagenesis coincide with a major australia wide marine regressions

low magnesian calcite marine cement in middle ordovician hardgrounds from kirkfield ontario abstract middle ordovician trentonian biosparites from the bobcaygeon and verulam formations near lake simcoe ontario exhibit numerous features demonstrating lithification near the sediment water interface these include planar erosion surfaces which truncate shells and intraclasts undercut hummocks on upper surfaces vertical borings which cross cut fossil debris and early cement and a diverse fauna of encrusting organisms which grew attached to lithified surfaces a marine origin for early cement is indicated by the development of hardgrounds within a transgressire regionally onlapping sequence the occurrence of dozens of such units within normal marine sediments and a lack of features recording early oxidation and or partial dissolution during exposure to meteoric water marine cement from four of these units examined in thin section by sem and by cathodoluminescence consists of clear zoned syntaxial overgrowths on echinoderm debris and equant spar on trilobite bryozoan and brachiopod fragments as such this cement 1 is morphologically identical to meteoric phreatic low magnesian calcite cement from calcitized pleistocene sequences 2 is interpreted as having been precipitated as low magnesian calcite 3 is anomalous in habit and inferred composition in comparison with holocene marine cement and 4 demonstrates that generalizations which relate cement morphology and composition to environmental water chemistry in modern diagenetic systems may not be valid when applied to more ancient carbonate sequences

the flow of surface derived fluids through alice springs age middle crustal ductile shear zones reynolds range central australia the south east reynolds range central australia is cut by steep north west trending alice springs age c 334 ma shear zones that are up to hundreds of metres wide and several kilometres long with reverse senses of movement amphibolite facies 550 600 c 500 600 mpa shear zones cut metapelites while greenschist facies shear zones 420 535 c 400 650 mpa cut metagranites the sheared rocks commonly underwent metasomatism implying that the shear zones were the pathways of significant fluid flow altered granites within greenschist facies shear zones have gained si and k but lost ca and na relative to their unsheared counterparts suggesting that the fluid flowed down temperature and hence probably upward through the shear zones time integrated fluid fluxes calculated from silica addition are up to 2 1 1010 mol m 2 c 4 2 105 m3 m 2 similar time integrated fluid fluxes are also estimated from changes in k and na the sheared granitic rocks locally have 18o values as low as 0 which is much lower than the 18o values of the adjacent unsheared granites 7 to 9 implying that the fluid which flowed through these shear zones was derived from the surface for the estimated time integrated fluid fluxes the fluids would be able to retain their isotopic signature for many tens to hundreds of kilometres the flow of surface derived fluids into the ductile middle crust with subsequent expulsion upwards through the shear zones may have been driven by seismic activity accompanying the alice springs deformation

composition and origin of coalbed gases in the lower silesian basin southwest poland abstract coalbed gases in the lower silesian coal basin lscb of poland are highly variable in both their molecular and stable isotope compositions geochemical indices and stable isotope ratios vary within the following ranges hydrocarbon chc index chc ch4 c2h6 c3h8 from 1 1 to 5825 wet gas c2 index c2 c2h6 c3h8 c4h10 c5h12 ch4 c2h6 c3h8 c4h10 c5h12 100 from 0 0 to 48 3 co2 ch4 cdmi index cdmi co2 co2 ch4 100 from 0 1 to 99 9 13c ch4 from 66 1 to 24 6 d ch4 from 266 to 117 13c c2h6 from 27 8 to 22 8 and 13c co2 from 26 6 to 16 8 isotopic studies reveal the presence of 3 genetic types of natural gases thermogenic ch4 higher gaseous hydrocarbons and co2 endogenic co2 and microbial ch4 and co2 thermogenic gases resulted from coalification processes which were probably completed by late carboniferous and early permian time endogenic co2 migrated along the deep seated faults from upper mantle and or magma chambers minor volumes of microbial ch4 and co2 occur at shallow depths close to the abandoned mine workings late stage microbial processes have commenced in the upper cretaceous and are probably active at present however depth related isotopic fractionation which has resulted from physical and physicochemical e g diffusion and adsorption desorption processes during gas migration cannot be neglected the strongest rock and gas outbursts occur only in those parts of coal deposits of the lscb which are dominated by large amounts of endogenic co2

secular variation in abiotic marine carbonates constraints on phanerozoic atmospheric carbon dioxide contents and oceanic mg ca ratios abiotic marine carbonates have been precipitated as ooids and synsedimentary cement within shallow marine carbonate sequences throughout the phanerozoic these components exhibit variable abundances and compositions that temporally correlate with estimates of global eustasy with rising sea level aragonite and acicular magnesium calcite precipitation gave way to the formation of equant calcite and possibly dolomite similarly the abundance of ooids was limited by positions of global sea level and associated changes in ocean chemistry modern abiotic marine carbonates also exhibit variable abundances and compositions as a function of changing fluid chemistry with latitude and depth chemical data on ambient marine waters suggest that carbonate ion concentrations and rates of precipitation presently control the nature of marine cement and ooids using data from modern oceans to calculate the requisite atmospheric pco 2 levels necessary to form this range of phases in ancient seas suggests that atmos

origin of carbon in fumarolic gas from island arcs abstract we have measured 3 he 4 he ratios 13c values and co 2 3 he ratios of fumaroles and hot spring gases in three volcanic areas in japan the 3 he 4 he ratios 13c values and co 2 3 he ratios vary from 02 05 ratm 9 7 to 2 7 and from 7 74 109 to 1 18 1011 respectively based on these data and those in the literature we present the first quantitative estimates of the upper mantle sediment and limestone including slab components in co2 of volcanic and hydrothermal fluids from subduction zones taking compiled c isotopic data of limestone and organic sediment assuming that the co 2 3 he ratio and 13c value of mid ocean ridge basalt morb are representative of the upper mantle component and that later isotopic and elemental fractionations are minimal it is possible to deconvole each fraction of c on the basis of simple mixing equations up to 20 of the carbon in high temperature volcanic gases in subduction zones is derived from a morb type source while the major fraction of the gases is attributable to co2 produced by decarbonation of subducted marine limestone and slab carbonate

diagenetic modification of dolomite in middle ordovician carbonates taiyuan city area china abstract patchy dolomite occurs in the middle ordovician limestones of the taiyuan city area china the dolomite is finely crystalline subhedral to euhedral and is considered to have formed initially by syndepositional subtidal dolomitization in evaporative or modified marine water as indicated by the close association of supratidal gypsum deposits with subtidal dolomite by the widespread distribution of dolomite and by the close association of dolomite with burrows the much depleted 18o and 13c values of both dolomite and coexisting limestone relative to middle ordovician marine carbonates suggest intensive diagenetic modifications the average 18o and 13c values of present day dolomite are 6 0 and 1 2 enriched by about 3 and 1 respectively when compared with the coexisting limestone 8 8 18o and 2 4 13c on average this is consistent with the known fractionation between dolomite and calcite and may indicate that both the dolomite and coexisting limestone were finally modified or stabilized in the same diagenetic fluid the trace element sr fe and mn concentration difference between the dolomite and coexisting limestone also supports a simultaneous modification or stabilization although the diagenetic modification of both dolomite and coexisting limestone by seawater modified seawater or basin derived water is possible the simultaneous modification by meteoric water seems most likely

stable isotope geochemistry of alpine ophiolites a window to ocean floor hydrothermal alteration and constraints on fluid rock interaction during high pressure metamorphism the subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle however despite its potential importance fluid rock interaction during high pressure metamorphism is relatively poorly understood the stable isotope and major element geochemistry of pennine ophiolite rocks from italy and switzerland that were metamorphosed at high pressures are similar to that of unmetamorphosed ophiolites suggesting that they interacted with little pervasive fluid during high pressure metamorphism cover sediments also have oxygen isotope ratios within the expected range of their protoliths in the rocks that escaped late greenschist facies retrogression different styles of sub ocean floor alteration may be identified using oxygen isotopes petrology and major or trace element geochemistry within the basalts zones that have undergone high and low temperature sub ocean floor alteration as well as relatively unaltered rocks can be distinguished serpentinites have 18o and 2h values that suggest that they were formed by hydration on or below the ocean floor the development of high pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub ocean floor alteration and which contained hydrated fine grained reactive assemblages given that the transformation of blueschist facies metabasic rocks to eclogite facies assemblages involves the breakdown of hydrous minerals e g lawsonite zoisite and glaucophane and will thus liberate considerable volumes of fluids metamorphic fluid flow must have been strongly channelled high pressure quartz calcite omphacite glaucophane titanoclinohumite veins that cut the ophiolite rocks represent one possible channel however stable isotope and major element data suggest that they were not formed from large volumes of exotic fluids fluids were more likely channelled along faults and shear zones that were active during high pressure metamorphism such strong fluid channelling may cause fluids to migrate toward the accretionary wedge especially along the slab mantle interface which is probably a major shear zone this may preclude all but a small fraction of the fluids entering the mantle wedge to flux melting additionally because fluids probably interact with relatively small volumes of rock in the channels they cannot scavenge elements from the subducting slab efficiently

two phase diagenetic alteration of carbonate matrix middle ordovician limestones taiyuan city area china both pseudospar and micrite are present in carbonate matrix of middle ordovician wackestones in the taiyuan city area china under the scanning electron microscope pseudospar crystals display unpitted surfaces and an absence of aragonite relics in polished and slightly etched sections micritic crystals show a similar feature with slightly pitted surfaces and absence of aragonite relics this possibly indicates that both pseudospar and micrite have a similar original mineralogy i e as calcites this is further supported by their low sr contents 417 ppm the geochemistry of pseudospar and micrite suggests that they were the products of two distinct episodes of diagenesis affecting the matrix micrite was possibly formed in near surface meteoric environments but pseudospar which is characterized by a population of more depleted 18o values 9 3 to 10 2 pdb than that of micrite 7 4 to 8 7 pdb is explained to have formed during shallow burial diagenesis because its 18o composition is too light to be accounted for by near surface meteoric origin the lower sr contents 417 ppm of both pseudospar and micrite in conjunction with their more depleted 18o values 7 4 to 10 2 pdb are interpreted to be the results of meteoric diagenesis when compared with the geochemistry of marine diagenetic calcites the more negative and wider range of 13c values of pseudospar and micrite 0 3 to 4 7 in comparison with those of other coeval meteoric calcites was attributed to the influence of the decay of organic matter in carbonate matrix as well as to meteoric diagenesis although the contents of fe and mn between pseudospar and micrine overlap widely an inverse relationship exists between the concentrations of fe and mn and the 13c values in both pseudospar and micrite this can be accounted for by variable redox by variable redox conditions during diagenesis the 18o values of void fill sparry calcite cements in micrite 8 2 to 8 6 pdb fall into the category of near surface meteoric diagenesis possibly indicating that the majority of micrites were cemented earlier and consequently were resistant to any further alterations so that only minor amount of pseudospars were formed during shallow burial diagenesis by recrystallization

asbestiform and non asbestiform morphologies in a talc and vermiculite mine from the province of cordoba argentina a case study in this work a talc and vermiculite mine from the province of cordoba argentina was investigated with special emphasis on the occurrence of asbestiform and non asbestiform phases the meta ultramafic rock was studied by a multimethodological approach complementing field studies with petrographic mineralogical compositional and morphological analyses samples were examined by stereomicroscopy polarizing light microscopy sem eds xrd dsc tga and ftir complementary compositional and textural analyses were performed with fe sem eds and epma talc rich veins with a laminar and fibrous appearance were at first recognized however the fibrous morphology observed both in the field and by microscopy is due to an apparent habit because of the sample orientation to avoid erroneous interpretations studies by secondary electron images sem are fundamental to carrying out this type of analysis tremolite was identified in different zones of the outcrop however only 40 of the crystals located in the vermiculite zone have dimensions to be considered as asbestiform fibres in the range of respirable particles in these types of complex deposits affected by superimposed metamorphic igneous and deformational events multimethodological approaches are necessary to develop models of occurrence of asbestiform morphologies that may be applicable to other with similar characteristics

horizontal contraction or horizontal extension heterogeneous late eocene and early oligocene general shearing during blueschist and greenschist facies metamorphism at the pennine austroalpine boundary zone in the western alps mylonitic structures related to two orogenic events are described from the upper and lower contacts of the combin zone and the immediately overlying upper austroalpine dent blanche nappe mont mary klippe and the directly underlying lower austroalpine etirol levaz slice the first event late eocene in age commenced during blueschist facies p t conditions but pre dated the peak of subsequent greenschist facies overprint the second event early oligocene in age took place during retrograde greenschist facies conditions most sense of shear indicators associated with the retrograde mylonites indicate top se shearing but subordinate top nw displacing shear sense indicators have also been mapped mylonitic top se shearing appears to be restricted to the combin zone and its upper and lower contacts within the dent blanche nappe and mont mary klippe and at the base of the etirol levaz slice structures were observed which developed during blueschist greenschist facies conditions and are in conjunction with the p t t history of these rocks inferred to be older associated kinematic data indicate a top nw shear sense comparable blueschist greenschist facies shear sense indicators have not been observed in the combin zone nonetheless the foliation in the combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions this indicates that the combin zone and the immediately over and underlying austroalpine units shared a common tectono metamorphic evolution since the late eocene finite strain data reveal oblate strain fabrics which are thought to result from a true flattening strain geometry flow path modelling reveals a general non coaxial deformation regime and corroborates significant departures from a simple shear deformation in the study area mylonitic top se shearing in the combin zone is attributed to early oligocene backfolding and backthrusting of the mischabel phase temperature time curves suggest slight reheating in the monte rosa nappe underneath and cooling in the dent blanche nappe above the combin zone hence confirming a thrust interpretation for this event the top nw displacing structures are thought to result from late eocene emplacement of the dent blanche nappe and the combin zone onto the middle pennine barrhorn series along the combin fault as related structures initiated during mildly blueschist facies conditions in the dent blanche nappe and the underlying combin zone and both were emplaced together onto the greenschist facial barrhorn series it is concluded that the structures developed as the nappes moved upward relative to the earth s surface thus the combin fault is regarded as a thrust the geometry of this structure indicates that the combin fault is an out of sequence thrust that locally cut down section hence top nw out of sequence thrusting caused local thinning of the metamorphic structural section in association with horizontal shortening out of sequence thrusts cutting down section and back thrusts offer the possibility of explaining the pronounced break in the grade of metamorphism across the combin fault i e the contact between the eclogite facial zermatt saas zone and the overlying lower grade combin zone by contractional deformation

the gabbro eclogite transformation an oxygen isotope and petrographic study of west alpine ophiolites both magmatic and eclogitic parageneses are preserved in the gabbros of western alpine ophiolites samples with relic magmatic mineralogies display partial transformation to eclogitic assemblages along cracks and grain boundaries gabbros with eclogitic mineralogies contain zoned pseudomorphs after olivine comprising talc rich cores with kyanite mg chloritoid and omphacite in outer cores and garnet rims the compositional zonation of these olivine pseudomorphs closely parallels that shown by olivines in hydrothermally altered ocean floor gabbros the eclogitic gabbros are hydrous containing paragonite zoisite and other water bearing minerals and it has been suggested that water was introduced during high pressure metamorphism however the similarity of olivine alteration patterns to those of ocean floor gabbros suggests that hydration and local metasomatism leading to the stability of aluminous minerals in olivine sites occurred during hydrothermal alteration prior to subduction oxygen isotope systematics are consistent with this proposal alpine gabbros with magmatic relics have a mean 18o value of 5 7 0 7 similar to that of unaltered oceanic crust whereas eclogitic gabbros have a mean 18o value of 4 8 0 9 this statistically significant difference is consistent with the eclogitic samples having undergone high temperature ocean floor alteration the preservation of magmatic and hydrothermal 18o values in ocean floor gabbros that have been metamorphosed at 2 2 5 gpa 60 75 km implies that the deeper levels of ocean crust have not experienced pervasive fluid flow during subduction or subsequent exhumation magmatic assemblages were preserved despite an overstep of eclogitization reactions by at least 0 6 1 1 gpa implying that equilibrium was not attained in undeformed parts of the system because of slow diffusion in water deficient rock volumes

abiotic methane on earth 1 over the last 30 years geochemical research has demonstrated that abiotic methane ch4 formed by chemical reactions which do not directly involve organic matter occurs on earth in several specific geologic environments it can be produced by either high temperature magmatic processes in volcanic and geothermal areas or via low temperature 100 c gas water rock reactions in continental settings even at shallow depths the isotopic composition of c and h is a first step in distinguishing abiotic from biotic including either microbial or thermogenic ch4 herein we demonstrate that integrated geochemical diagnostic techniques based on molecular composition of associated gases noble gas isotopes mixing models and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic ch4 in natural gases which are frequently mixtures of multiple sources although it has been traditionally assumed that abiotic ch4 is mainly related to mantle derived or magmatic processes a new generation of data is showing that low temperature synthesis related to gas water rock reactions is more common than previously thought this paper reviews the major sources of abiotic ch4 and the primary approaches for differentiating abiotic from biotic ch4 including novel potential tools such as clumped isotope geochemistry a diagnostic approach for differentiation is proposed

development of schistosity by dissolution crystallization in a californian serpentinite gouge to understand the behaviour and deformation mechanisms of serpentinites in the seismogenic zone we study the deformation macro and microstructures of serpentinites along the santa ynez fault in the san andreas system at the outcrop scale deformation is localized in a gouge zone that shows three different structures 1 micrometric undeformed fragments clasts of the previously serpentinized peridotite 2 localized shear planes y and r and 3 a penetrative schistosity s observations under sem and tem reveal that the schistosity corresponds to serpentine fibres parallel to each other and whose orientation varies as they wrap around clasts tem micro textures indicate that these long fibres result from continuous syntectonic growth rather than from reorientation of pre existing fibres implying a slow transfer process that occurs at short distances we propose a dissolution diffusion crystallization process for the formation of the schistosity that corresponds to a low strain rate creeping process of deformation that can be effective in aseismic fault segments

holocene dolomitization of calcium carbonate sediments from the coastal sabkhas of abu dhabi u a e a stable isotope study secondary dolomitization of holocene calcium carbonate sediments occurs beneath the coastal sabkhas of abu dhabi u a e a stable isotope study of the sabhka sediments and brines indicates that the

abiotic methane flux from the chimaera seep and tekirova ophiolites turkey understanding gas exhalation from low temperature serpentinization and implications for mars abstract the emission of abiotic methane ch 4 into the atmosphere from low temperature serpentinization in ophiolitic rocks is documented to date only in four countries the philippines oman new zealand and turkey serpentinization produces large amounts of hydrogen h 2 which in theory may react with co 2 or co to form hydrocarbons fischer tropsch type synthesis ftt similar mechanisms have been invoked to explain the ch 4 detected on mars so that understanding flux and exhalation modality of ophiolitic gas on earth may contribute to decipher the potential degassing on mars this work reports the first direct measurements of gas ch 4 co 2 flux ever done on onshore ophiolites with present day serpentinization we investigated the tekirova ophiolites at cirali in turkey hosting the chimaera seep a system of gas vents issuing from fractures in a 5000 m 2 wide ophiolite outcrop at this site at least 150 190 t of ch 4 is annually released into the atmosphere the molecular and isotopic compositions of c 1 c 5 alkanes co 2 and n 2 combined with source rock maturity data and thermogenic gas formation modelling suggested a dominant abiotic component 80 90 mixed with thermogenic gas abiotic h 2 rich gas is likely formed at temperatures below 50 c suggested by the low deuterium hydrogen isotopic ratio of h 2 d h2 720 consistent with the low geothermal gradient of the area abiotic gas synthesis must be very fast and effective in continuously producing an amount of gas equivalent to the long lasting 2 millennia emission of 100 t ch 4 yr 1 otherwise pressurised gas accumulation must exist over the same ophiolitic formation 3 km away from chimaera we detected an invisible microseepage of abiotic ch 4 with fluxes from 0 07 to 1 g m 2 d 1 on mars similar fluxes could be able to sustain the ch 4 plume apparently recognised in the northern summer 2003 10 4 or 10 5 t yr 1 over the wide olivine bedrock and outcrops of hydrated silicates in the syrtis major and nili fossae just one seep like chimaera or more realistically a weak spatially sporadic microseepage would be sufficient to maintain the atmospheric ch 4 level on mars

geochemistry of shale gases from around the world composition origins isotope reversals and rollovers and implications for the exploration of shale plays abstract we studied a large dataset of 2600 shale gas samples from 76 geological formations in 38 sedimentary basins located in eleven countries shale gases contain mostly hydrocarbons dominated by methane shale gases can have primary microbial secondary microbial and thermogenic origin however gases produced from most commercially successful shale plays e g the marcellus haynesville eagle ford and barnett in the usa the vaca muerta in argentina and the wufeng longmaxi in china are thermogenic it appears that formations with greater gas endowment such as the marcellus and the haynesville contain late mature thermogenic gas shale plays with early mature thermogenic and secondary microbial gas such as the antrim usa and the new albany usa formations have relatively low endowments of recoverable gas shale plays with primary microbial gas are not significant from commercial exploration perspective isotope reversals 13c of methane 13c of ethane are observed in shale plays with mature organic matter vitrinite reflectance 2 that experienced significant uplift 2 km it appears that isotope fractionation during desorption from depressurized late mature shales leads to isotope reversal in the residual gas produced from shale formations e g the wufeng longmaxi significant contribution of adsorbed gas enriched in 13c rich c2 hydrocarbons relative to the co occurring free gas in the production from some plays e g the fayetteville usa may result in isotope rollovers

petrographic and geochemical evidence for original aragonite mineralogy of upper jurassic carbonates mozduran formation sarakhs area iran abstract diverse skeletal and non skeletal grains with abundant red and blue green calcareous algae evaporites and early diagenetic dolomites in the mozduran carbonates are similar to those of modern warm water shallow marine carbonates isopachous and fibrous intragranular sparry calcite cements resemble modern aragonite morphologies concentric and radial ooids are like modern aragonitic forms which are formed in turbulent and in low energy environments respectively deformed ooids shattered micritic envelopes and spalled ooids indicate aragonite dissolution during meteoric diagenesis meteoric cements are equant mosaic to drusy calcites burial cements only occur as vein fillings sr and na covariance with mn supports evidence of inversion of aragonite to calcite with subsequent open phreatic diagenesis fe concentrations are a magnitude higher than mn due to a reducing phreatic environment high sr na ratios suggest original aragonite mineralogy the 18 o and 13 c field of mozduran limestone lies at the edge of the modern shallow marine field indicating a range from marine to meteoric cements differences in 18 o values between marine calcites and meteoric derived calcites is similar to that in the modern tropics the surface marine diagenetic trend line that passes through shallow marine modern whole carbonates cuts the mozduran limestone isotope field due to their formation in warm shallow marine waters given that atmospheric co 2 levels were similar to those at present increasing mn and fe values decreasing na and uniform sr values with increasingly lighter values of both 18 o and 13 c are exlained by extensive meteoric diagenesis the least altered marine calcite 18 o value of 2 6 pdb corresponds to 29 or 24 c depending on whether 0 of present seawater and 1 2 of ancient seawater with no ice caps are taken as a base an original calcite mineralogy postulated for some jurassic limestones is related to a non tropical origin

geochemistry of natural gases in deep strata of the songliao basin ne china chemical composition and stable carbon isotopic studies were undertaken for 27 gas samples from deep strata of the xujiaweizi depression in the songliao basin to investigate their origin gas molecular and carbon isotopic compositions show great variety methane is the main component for all studied samples and its content ranges from 57 4 to 98 2 with an average of 90 1 gas wetness ranges from 0 8 to 16 7 with an average of 2 7 the main non hydrocarbon gases are carbon dioxide and nitrogen with an average of 4 0 and 3 2 respectively carbon isotope data suggest that these deep strata gases are mainly coal type gases mixed with minor amounts of associated oil type gases coal type gases are characterized by heavier carbon isotopic values and drier chemical compositions these gases were generated from the lower cretaceous shahezi formation coals interbedded shales with type iii kerogen during the postmature stage of hydrocarbon generation oil type gases are characterized by lighter carbon isotope and higher wetness which were generated from the lower cretaceous shales with type ii kerogen in the shallow strata during the early mature stage of hydrocarbon generation mixing of two different gases causes unusual carbon isotopic distribution patterns with lighter isotopic values in higher numbered carbons in most gases the discovery of coal type gases in the songliao basin provides new prospects for the exploration in this region

enhanced hydrothermal processes at the new born lusi eruptive system indonesia abstract little is known about the processes taking place in new born hydrothermal systems we investigate the eruptive activity of lusi a sediment hosted hydrothermal system active since 2006 in east java indonesia we show that superimposed on the regular geysering activity lusi features periods of enhanced hydrothermal processes during which boiling mud breccia is violently discharged from the eruptive vents this results in sudden flooding lasting from several minutes to several hours and featuring almost instantaneous temperature peaks i e from 35 c to more than 65 c in less than 2 min such activity here named hydrothermal waves is marked by high frequency seismic radiations and is distinguished by sharp peaks of ch4 and co2 concentrations in the erupted fluids this suggests fluids upwelling from at least tents of meters below the near surface the injection into the shallow plumbing system of ch4 and co2 rich batches of fluids may promote instabilities in the mud breccia column leading to the hydrothermal waves our findings provide key insights into the deep charging dynamics of a new born hydrothermal system and highlight the effects imposed by subtle variations in fluid chemistry changes at depth

clumped isotope geochemistry the study of naturally occurring multiply substituted isotopologues clumped isotope geochemistry is concerned with the state of ordering of rare isotopes in natural materials that is it examines the extent to which rare isotopes d 13 c 15 n 18 o etc bond with or near each other rather than with the sea of light isotopes in which they swim abundances of isotopic clumps in natural materials are influenced by a wide variety of factors in most cases their concentrations approach within ca 1 relative the amount expected for a random distribution of isotopes deviations from this stochastic distribution result from enhanced thermodynamic stability of heavy isotope clumps slower kinetics of reactions requiring the breakage of bonds between heavy isotopes the mass dependence of diffusive and thermo gravitational fractionations mixing between components that differ from one another in bulk isotopic composition biochemical and photochemical fractionations that may reflect combinations of these simpler physical mechanisms and in some cases other processes we do not yet understand although clumped isotope geochemistry is a young field several seemingly promising applications have already emerged most importantly it appears that proportions of 13 c 18 o bonds in carbonate minerals are sensitive to their growth temperatures independent of bulk isotopic composition thus clumped isotope analysis of ancient carbonates can be used as a quantitative paleothermometer that requires no assumptions about the 18 o of waters from which carbonates grew this approach has been used to reconstruct marine temperatures across the phanerozoic reaching back to the silurian terrestrial ground temperatures across the cenozoic thermal histories of aqueously altered meteorites among other applications clumped isotope geochemistry is also placing new constraints on the atmospheric budget and stratospheric photochemistry of co 2 and should be capable of placing analogous new constraints on the budgets of other atmospheric gases finally this field could be extended to encompass sulfates volatile hydrocarbons organic moieties and other materials

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

early ordovician cool creek dolomite middle arbuckle group slick hills sw oklahoma u s a origin and modification abstract dolomite in the early ordovician cool creek formation of the arbuckle group slick hills sw oklahoma occurs as massive bodies in sabkha like sequences petrographically the dolomite displays euhedral to anhedral and xenotopic fine to coarse crystalline textures chemically it is characterized by nearly stoichiometric composition ca1 01mg0 99 co3 2 and low 18o values 7 6 to 9 9 pdb and has a large range of 87sr 86sr ratios 0 70843 to 0 70894 relative to associated limestone 0 70879 to 0 70897 and coeval early ordovician seawater the 87sr 86sr ratios of most 65 dolomite samples fall in the 87sr 86sr range of coeval seawater the distribution of cool creek dolomite in sabkha like sequences coupled with dominant 87sr 86sr ratios similar to coeval seawater suggests that the dolomite probably formed initially in a marine environment during early diagenesis the low 18o values of the dolomite however indicate considerable burial modification correlations between texture and geochemistry as well as among various parameters sr fe 18o and 87sr 86sr demonstrate that dolomite samples with increased modification are characterized by coarser textures lower sr and higher fe concentrations and lower 18o values and 87sr 86sr ratios these correlations along with geochemical comparison with associated limestone not only suggest that the dolomite was modified by fluids depleted in 18o but also indicate that the lower 87sr 86sr ratios of many 35 dolomite samples relative to early ordovician seawater resulted from diagenetic modification on this basis we conclude that 1 modification of the dolomite by basin derived brines can be ruled out 2 modification by younger seawater seawater meteoric mixtures and fluids derived from early compaction of younger shales is possible but 3 the dolomite seems most likely to have undergone long lasting modification by meteoric water which gained low sr 86sr ratios by infiltrating younger carbonates probably during emergence of the carbonate platform coincident with regional paleozoic unconformities

predictive assessment of the asbestos content in the western italian alps an essential tool for an effective approach to risk analysis and management in tunneling operations and muck reuse the modern approach to the design and management of tunnel excavation and muck reuse can be influenced to a great extent by the possible presence of rock formations containing asbestos minerals this situation creates problems concerning the protection of the workers health and the expectable environmental criticalities while a drastic re consideration of the muck destination is necessary since in the case of carcinogens corrective action following exposure or dispersion is not acceptable detailed knowledge of the characteristics of the material to be excavated is all the more essential only on this basis it is possible to design the tunneling operations in a prevention through design approach so that the tunnel driving techniques fittings and technologies and special equipment and modus operandi can grant where necessary minimized risk conditions and make a correct decision on whether how and when these measures must be fully activated since such an approach involves a remarkable impact on costs and work organization in the western alps a detailed identification and quantification of the asbestos content in rocks is a difficult task due to the fact that the asbestos in the host rocks mainly serpentinite shows a highly variable distribution typical of ophiolitic belts throughout the world as it is mostly associated with joints and shear zones the possibilities of a predictive assessment of the asbestos content in the formations to be excavated are here discussed with special reference to the reliability of the achievable results

recognition of original mineralogy in micrites abstract detailed sem study of selected micrites 4 m and microspars 4 to 12 m from all phanerozoic systems and various geographic localities suggest that textural properties of micrites and microspars are mineralogically controlled those micrites and microspars with apparent aragonite dominated lime mud precursors adp have neomorphic calcite crystals which show pitted surfaces or relic aragonite inclusions in polished etched sections the presence of relics in all crystal sizes in adp micrites and microspars indicates an absence of secondary dissolution precipitation or aggrading neomorphism that is formation of all neomorphic crystal sizes occurred in a single diagenetic event micritic limestones with apparent calcite dominated precursors cdp howeve are characterized by finely crystalline 4 m textures lack any inclusions and have unpitted crystal surfaces strontium content of micrites and microspars studied are bimodally distributed a similar distribution was recognized by veizer in 1977 and veizer and demovic in 1973 and 1974 who suggested it was the result of original mineralogy preliminary results on the micrites and microspars studied show sr distribution generally well correlated with textural properties adp and cdp micrites and microspars possess sr values which fall with few exceptions within the high sr and low sr groups respectively of veizer and others several adp samples which fall within the low sr group are molluscan rich thus likely aragonite contribution to precursor muds was more probably low sr low sr adp could also be the result of open system diagenetic alteration in such cases low sr adp micrites and m crospars are associated with high mn content we have found several low sr adp samples that are high in mn high sr adp micrites and microspars are therefore interpreted as being originally composed of high sr aragonite mineralogy the sr content of cdp end page 499 micrites studied thus far is low and suggestive of high mg calcite mud precursors for those micrites oxygen isotopic composition of both cdp and adp samples are rather broad ranges suggesting varying contributions of original mineralogies for both groups there is however an overall negative trend toward lighter dgr18o isotopic values with increasing age indicative of either progressively higher temperatures with age or lower 18o16o ratios in ocean water end of article last page 500

extensional faulting in an intraoceanic subduction complex working hypothesis for the palaeogene of the alps apennine system abstract within the ophiolite complexes exposed in the ligurian alps and apennines nw italy several extensional detachment faults are identified which overprint late cretaceous high pressure metamorphic fabrics developed in an intraoceanic subduction system one of these detachment faults is the sestri voltaggio line which marks an 8 kbar pressure gap between blueschist facies ophiolites and calcareous sediments of the sestri voltaggio zone and underlying eclogite fades ophiolites and metasediments of the voltri massif radiometrie and stratigraphie constraints suggest that extension has occurred during the late paleocene to possibly early eocene from the middle eocene onward compressional tectonics prevailed again resulting in westerly directed imbrications in the ligurian alps and easterly directed folding and thrusting in the ligurian apennines a strong correlation is identified between the africa europe convergence rate and the stability of the accretionary wedge i e the paleocene extension coincides with a period of strongly reduced convergence velocities whilst the middle eocene transition to compressional tectonics concurs with an acceleration of the convergence rate this inferred relation between convergence rate and style of internal deformation matches very closely the results from mechanical models addressing the stability of orogenic wedges with a strain rate dependent rheology and provides the basis for a working hypothesis on the palaeogene evolution of the alps apennine orogenic system

geochemistry of brachiopods oxygen and carbon isotopic records of paleozoic oceans abstract combined trace element and isotope studies of 319 brachiopods covering the ordovician to permian time span show that 13c and 18o in well preserved specimens varied during the paleozoic the overall 13c secular trend is in accord with the previously published observations but its details are obscured by vital isotopic fractionation effects at generic level nonetheless the results suggest that the negative correlation between marine 13ccarbonate and 34ssulphate deteriorates at time scales of 106 years due to the long residence time and thus slow response of so42 in the ocean for oxygen isotopes all devonian and older specimens have 18o of 4 while the well preserved permian samples have near present day 18o of about 1 pdb this isotopic dichotomy is probably not due to post depositional phenomena salinity or biogenic fractionation effects this leaves open the perennial arguments for a change in 18 o 16 o of sea water versus warmer ancient oceans the present data are difficult to explain solely by the temperature alternative the coincidence of the proposed shift in 18o with the large late paleozoic changes in marine 87 sr 86 sr 13 c 12 c 34 s 32 s and sea level stands argues for a tectonic cause and for a change in 18 o 16 o of sea water although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites whatever the precise cause or combination of causes the implications for tectonism and or paleoclimatology are of first order significance

abnormal carbon and hydrogen isotopes of alkane gases from the qingshen gas field songliao basin china suggesting abiogenic alkanes abstract it is great debate that the alkane gases of abiogenic origin would constitute a major portion of the commercial accumulation of the qingshen gas field songliao basin china in this study abiogenic gases characterized by heavy 13c1 values reversal of the usual carbon isotopic trend of c1 c5 alkanes very narrow variation in 2hc1 values and low ch4 3he ratios associated with high r ra values 1 were identified the hydrocarbon gas in the qingshen gas field is a mixture of thermogenic alkanes derived from cretaceous mudstone type i kerogen or jurassic coal type iii kerogen and abiogenic alkanes mainly ch4 from mantle degassing a quantitative estimation of abiogenic alkanes contribution to the qingshen gas field is made based on a 13c1 vs 13c2 plot about 30 40 of alkane gases in the qingshen gas field along with its helium are estimated to be derived from the mantle via magmatic activity particularly the abiogenic formation of ch4 generated from the reduction of co2 by hydrothermal activity may contribute our study suggests that abiogenic alkane gases in certain geological settings could be more widespread than previously thought and may accumulate into economic reservoirs

the δ18o record of phanerozoic abiotic marine calcite cements monomineralic abiotic marine cements formed in low latitude phanerozoic reefs provide the direction and amplitude of secular variation of delta sup 13 c and delta sup 18 o in marine calcite and defines two end member compositions 580 to 360 my minus 7 to minus 5 per thousand delta sup 18 o sub pdb and 360 to present minus 3 to 0 per thousand delta sup 18 o sub pdb sampling of the devono carboniferous transition 375 320 my at several global sites reveals a rapid change in carbonate isotopic compositions bracketed within fammenian to early visean aged strata a 7 to 15 my time interval this shift corresponds to a 2 offset in mean delta sup 13 c and 3 4 offset in delta sup 18 o the abruptness of such change and its overall correlation with variations in sup 87 sr sup 86 sr delta sup 34 s delta sup 13 c and li al ratios in marine sediments suggests a primary offset in marine water composition

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

focused fluid flow during subduction oxygen isotope data from high pressure ophiolites of the western alps abstract during subduction hydrothermally altered oceanic crust undergoes extensive dehydration the water released migrates upwards and may escape along the slab mantle interface or continue rising to hydrate the mantle overlying the subducting slab oxygen isotope ratios from eclogitic ophiolites of the western alps correlate closely with lithology samples with little altered basalt as a protolith have 18 o values of 5 1 6 0 material representing low temperature alteration zone rocks have 18 o values of 5 9 6 6 and rocks orginating in high temperature alteration zones have 18 o values of 4 6 5 3 these isotopic values closely approach those of the ocean crust and unmetamorphosed ophiolites these data suggest that the alpine rocks have not experienced significant fluid flow during metamorphism implying that the escape of fluid from subducted ocean crust is highly focused probably along structurally controlled pathways to the top of the slab the fluid may then continue largely along the slab mantle interface although a portion of it rises further to hydrate the overlying mantle

methane clumped isotopes in the songliao basin china new insights into abiotic vs biotic hydrocarbon formation abiotic hydrocarbon gas typically generated in serpentinized ultramafic rocks and crystalline shields has important implications for the deep biosphere petroleum systems the carbon cycle and astrobiology distinguishing abiotic gas produced by chemical reactions like sabatier synthesis from biotic gas produced from degradation of organic matter or microbial activity is sometimes challenging because their isotopic and molecular composition may overlap abiotic gas has been recognized in numerous locations on the earth although there are no confirmed instances where it is the dominant source of commercially valuable quantities in reservoir rocks the deep hydrocarbon reservoirs of the xujiaweizi depression in the songliao basin china have been considered to host significant amounts of abiotic methane here we report methane clumped isotope values 18 and the isotopic composition of c 1 c 3 alkanes co 2 and helium of five gas samples collected from those xujiaweizi deep reservoirs some geochemical features of these samples resemble previously suggested identifiers of abiotic gas 13 c enriched ch 4 decrease in 13 c 12 c ratio with increasing carbon number for the c1 c4 alkanes abundant apparently non biogenic co 2 and mantle derived helium however combining these constraints with new measurements of the clumped isotope composition of methane and careful consideration of the geological context suggests that the xujiaweizi depression gas is dominantly if not exclusively thermogenic and derived from over mature source rocks i e from catagenesis of buried organic matter at high temperatures methane formation temperatures suggested by clumped isotopes 167 213 c are lower than magmatic gas generation processes and consistent with the maturity of local source rocks also there are no geological conditions e g serpentinized ultramafic rocks that may lead to high production of h 2 and thus abiotic production of ch 4 via co 2 reduction we propose that the songliao gas is representative of an atypical type of thermogenic gas that can be mistaken for abiotic gas such gases may be encountered more frequently in future exploration of deep or over mature petroleum systems

geochemical characteristics and genesis of natural gas in the yan an gas field ordos basin china abstract the yan an gas field in the ordos basin china was discovered in 2014 and contains reserves of up to 100 billion m3 its occurrence in the basin contradicts the distribution model of gas in the north oil in the south upper paleozoic natural gas in the yan an area is dominated by methane with an average dryness coefficient c1 c1 4 of 99 5 13c1 30 8 to 27 5 with minor amounts of co2 3 8 n2 0 5 and ethane 13c2 37 2 to 30 5 and traces of higher hydrocarbons the gas is predominantly coal derived originating from carboniferous permian humic hydrocarbon source rocks and minor oil type gas from sapropelic source rocks the carbon isotope series includes reversals with 13c2

holocene dolomitization of supratidal sediments by active tidal pumping sugarloaf key florida abstract sugarloaf key is an area where holocene dolomitization is occurring calcium rich dolomite is found in a 0 25 to 10 cm thick surface crust which transgresses a thin layer of carbonate mud overlying the karsted pleistocene miami oolite radiocarbon ages of the crust range from 160 years b p to 1420 years b p with a corresponding increase in dolomite content from 0 to 80 the relatively high permeability of the underlying pleistocene oolite and low permeability of the holocene carbonate mud results in a tidal lag between surface water and the partly confined aquifer consequently seawater is pumped upward and downward through the holocene sediment during spring tides the highest concentration of dolomite is found in areas where the sediment layer is thinnest and tidal pumping is most effective limited analyses of surface and subsurface water taken at intervals throughout the pumping cycle suggest that the dolomitizing fluid is essentially florida bay water the earliest diagenesis is caused by precipitation of dolomite cement which occurs as 0 1 to 0 3 micron subrounded crystallites that show no distinct crystal form during further cementation and somewhat later during replacement of preexisting crystallites poorly ordered dolomite forms as 1 to 5 micron euhedral rhombs x ray diffraction data indicate that the crystallized dolomite is better ordered and less calcium rich than the dolomite composed of crystallites

genetic characterization of natural gases natural gases can be characterized genetically using four properties c2 concentration carbon and hydrogen isotope variations in methane and carbon isotope variation in ethane three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters in these diagrams compositional fields have been defined for primary gases biogenic thermogenic associated and thermogenic nonassociated and for gases which result from mixing of these gases these fields are strictly empirical and comprise compositional variations found in about 500 natural gases the isotopic and compositional variations in natural gases can be described in terms of 1 processes during formation of the gases such as bacterial fermentation or maturation of organic matter and 2 processes during secondary migration mixing of primary gases is an important and common process migration of gases predominantly affects the c2 concentration whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged allowing an assessment of the origin of migrated gases and properties of their source rocks the formation of gas from humic organic matter and coals is not yet clear from published data the diagrams use data from various basins and areas interstitial gases from the gulf of california are entirely of bacterial origin traces of thermogenic gases are formed only in the vicinity of dolerite sills gases in the south german molasse basin and in the vienna basin are of bacterial mixed and thermogenic origins data from the north italian po basin provide examples for genetic characterization of migrated gases

diagenetic calcite from the chazyan group vermont an example of aragonite alteration in a greenhouse ocean abstract marine diagenetic calcite with both a calcitic low to intermediate mg and aragonitic origin was examined from the middle ordovician buildups of the chazyan group in vermont all marine phases have elevated sr up to 1800 ppm compared with that observed from marine precipitates in other middle ordovician units stromatoporoids labechiids which were originally aragonitic have higher sr values than phases with an original calcite mineralogy trilobites marine cement additional evidence supporting precursor mineralogy interpretations includes elevated mg values up to 3 6 mole mgco 3 and the presence of microdolomite in interpreted calcitic phases originally aragonitic precipitates have lower mg values and most significantly lack microdolomite this study demonstrates the presence of elevated sr values in marine precipitates that formed during a period when calcite not aragonite was the dominant physiochemically precipitated calcium carbonate mineralogy that formed from sea water elevated sr is attributable to at least a partially open system diagenetic stabilization of biogenic aragonite

recrystallization of dolomite evidence from the monterey formation miocene california dolomites from the upper calcareous siliceous member of the miocene monterey formation exposed west of santa barbara california were analysed for geochemical isotopic and crystallographic variation the data clearly document the progressive recrystallization of dolomite during burial diagenesis in marine pore fluids recrystallization is recognized by the following compositional and crystallographic variations dolomites have decreasing 18o and 13c compositions decreasing sr contents and increasing mg contents with increasing burial depths and temperatures from east to west in the study area 18o values vary from 5 3 in the east to 5 5 pdb in the west and are interpreted to reflect the greater extent and higher temperature of dolomite recrystallization in the west 13c values correlate with 18o and decrease from 13 6 in the east to 8 7 pdb in the west sr concentrations correlate positively with 18o values and decrease from a mean of 750 ppm in the east to a mean of 250 ppm in the west mol mgco3 values inversely correlate with 18o values and increase from a minimum of 41 0 in the east to a maximum of 51 4 in the west rietveld refinements of powder x ray diffraction data indicate that the more recrystallized dolomites have more contracted unit cells and increased cation ordering the fraction of the ca sites in the dolomites that are occupied by ca atoms increases slightly with the approach to stoichiometry the fraction of the mg sites occupied by mg atoms strongly correlates with mol mgco3 even in early diagenetic non stoichiometric dolomites there is little substitution of mg in ca sites during recrystallization the amount of mg substituting for ca in ca sites decreases even further most of the disorder in the least recrystallized non stoichiometric dolomites is related to substitution of excess ca on mg sites

alteration of early formed dolomite during shallow to deep burial mississippian mission canyon formation central to southwestern montana dolomite in the mississippian mission canyon formation osagean to middle meramecian of central and southwestern montana precipitated during multiple episodes of dolomitization and or recrystallization the earliest generation of dolomite most likely precipitated from evaporated sea water in near surface diagenetic environments when the mission canyon platform intermittently aggraded to sea level this dolomite has relatively positive delta sup 18 o values minus 1 5 to 7 5 and typically is nonstoichiometric and trace element enriched a second generation of dolomite has very negative delta sup 18 o values minus 11 2 to minus 1 5 and always is stoichiometric and trace element depleted dolomite with delta sup 18 o values minus 6 8 is in the upper 100 180 m of the mission canyon formation beneath a regional post mission canyon unconformity and karst surface that extends across the entire study area dolomite depleted in sup 18 o may have 1 precipitated initially in a regional mixing zone during post mission canyon subaerial exposure and karstification 2 precipitated at elevated temperatures during later diagenesis or 3 recrystallized from an earlier generation of dolomite during later shallow to deep burial diagenesis very negative delta sup 18 o values and low trace element contents for the second generation of dolomite suggest that diagenetic solutions were dominated by meteoric water whereas delta sup 13 c values indicate that diageneticmore fluids were buffered by osagean carbonate rocks 54 refs 10 figs less

tectonic controls of phanerozoic sedimentary rock cycling the cyclic nature of the phanerozoic sedimentary rock distribution carbon sulphur coupling and material transfer among sedimentary reservoirs appears to be controlled by tectonic factors the distribution of preserved sedimentary mass in terms of rock mass remaining u geologic age shows a minimum c 300 350 ma ago which separates two subcycles of erosion and deposition of sedimentary rocks the older subcycle was interrupted because of the major continental collisions of the devonian and late carboniferous these collisions resulted in a reduction of outcrop areas of rocks of the older cycle relative to their masses leading to a decline in the probability of destruction and an increase in half life of these older sediments a strong correlation exists between the long term cyclicity in the phanerozoic global sea level curve and the distribution of carbon and sulphur among their major exogenic reservoirs this correlation is related to two principal tectonic modes of the phanerozoic oscillatory and submergent it is postulated that the submergent mode of active plate convergence obduction and subduction of sediments large ridge volume and high sea level gave rise to low erosion and sedimentation rates less restricted environments of carbonate deposition and relatively high atmospheric co 2 levels high temperatures resulting from an increased rate of production of co 2 from diagenetic and metamorphic reactions at subduction zones as sea level rose carbon was transferred from the sedimentary reservoir of reduced organic carbon to that of oxidized inorganic carbon in limestones whereas sulphur moved from the oxidized sulphate reservoir to the reduced sulphide reservoir as sea level fell reservoir transfers were opposite to those above culminating in the oscillatory mode of generally elevated continental interiors these reservoir transfers are consistent with secular changes observed in the distribution of 13 c and 34 s in sedimentary materials during the phanerozoic petrographic examination of phanerozoic oolite formations shows that ooids with preserved calcitic relict textures are characteristic of pre carboniferous carbonate rocks whereas ooids with relict textures indicative of initial aragonite mineralogy are dominant in rocks of younger age these changes in ooid mineralogy may be interpreted as reflecting changes in co 2 levels of the ocean atmosphere system consistent with the above tectonic considerations atmospheric co 2 levels were higher prior to carboniferous time favouring formation of calcitic ooids and skeletal parts after the carboniferous co 2 levels fell and aragonite and mg calcites of greater than 8 mol mg increased in abundance as precipitates

deep fluid circulation in alpine shear zones pyrenees france field and oxygen isotope studies a combined field stable isotope and whole rock chemical study was made on late cretaceous to tertiary metasomatic shear zones cutting hercynian gneisses in the aston massif pyrenees france mylonitisation occurred during the early stages of alpine compression under retrograde conditions at 400 450 c and about 10 km depth whole rock 18o values of 11 to 12 in the gneisses was lowered to 5 to 9 in the shear zones with the quartz muscovite 18o 16o fractionations of about 2 to 4 essentially unchanged these 18o 16o systematics together with d muscovite 40 to 50 indicate that large volumes of formation waters or d rich meteoric waters passed through the shear zones during deformation the same fluids also redistributed major elements as shown by the correlation of 18o shift with muscovitisation and albitisation reactions in granitic wall rocks however even though 18o was universally lowered within the shear zones the 18o 16o ratios were not homogenised nor do they correlate in detail with the presence or absence of muscovitisation suggesting that fluid flow was probably fracture controlled and episodic field mapping shows that along the length of a particular shear zone muscovitisation of granite gneiss dies out 150m above the contact with underlying sillimanite gneiss thus muscovitisation and albitisation of granite gneiss in shear zones and their wall rocks probably occurred during re equilibration of acidic chloride rich aqueous fluids that had previously moved upward within the shear zones through underlying sillimanite gneiss extremely high material balance fluid rock ratios 103 are required to explain the extent of muscovitisation along this shear zone implying integrated fluid mass fluxes of about 108 kg m2 this is probably close to the maximum value for other shear zones in the network similar volumes of a more chemically evolved fluid must have passed through the unmuscovitised mylonites showing that the absence of alteration cannot necessarily be used to infer low values of fluid flux for reasonable pressure gradients and time scales of fluid movement effective permeabilities of 10 15 to 10 17 m2 are required such values can be accounted for by short lived widely spaced cracks produced during seismic activity a model is presented in which formation waters were seismically pumped down an underlying shallow southward dipping decollement and then upward through the steeply dipping shear zone network

retrogressive fabric development during exhumation of the voltri massif ligurian alps italy arguments for an extensional origin and implications for the alps apennines linkage geological mapping coupled with structural investigations carried out in the voltri massif eastern ligurian alps italy provide new data for the interpretation of the tectonic context controlling main fabric development during exhumation of its high pressure core the voltri massif is here interpreted as a c 30 km long eclogite bearing asymmetric dome formed by the progressive verticalisation of the regional second phase mylonitic foliation developed during retrogressive greenschist metamorphic conditions in this light the exhumation history is driven by a ductile to brittle extensional process operating through low angle top to the w multiple detachment systems a late eocene early oligocene age for this extensional episode is proposed on the basis of structural correlations stratigraphic and radiometric constraints in this scenario the voltri massif is interpreted as an extensional domain developed to accommodate the late eocene early oligocene arching of the western alps northern apennines orogenic system

estimation of natural asbestos content in rocks by fracture network modeling and petrographic characterization abstract asbestos may constitute a severe health risk when meta ophiolites are excavated for large infrastructural projects for public acceptance a reliable estimation of the content of naturally occurring asbestos noa is necessary for the design of construction sites workers safety and spoil management in the framework of a research project supporting the final design of a highway tunnel system in nw italy sem eds scanning electron microscopy energy dispersive spectrometry quantitative analyses were performed to provide a direct noa content estimation by counting and weighing the asbestos fibers in the rocks after a chemical and geometrical characterization the direct noa content estimation was compared with an indirect estimation obtained through a fracture network modeling based on a structural survey on a selected outcrop and statistical analysis of a relative digital image the fracture intensity inferred from the fracture network model was multiplied by coefficients deriving from the semi quantitative estimation of the geological relations between asbestos mineral occurrence and fracture size thickness and distribution a good agreement between the indirect noa estimation and the average result of the sem eds analysis was obtained thus the statistical analysis of the fracture network may represent a valuable support to the sem eds quantitative analysis based on mineral fibers counting however the quality of the indirect noa estimation depends on the postulates for inferring the coefficients describing the distribution and occurrence of the asbestos minerals within the fractures this note discusses the above mentioned issues as well as those concerning the procedure for a representative sampling of noa bearing rocks and fractures

isotopic reversals with respect to maturity trends due to mixing of primary and secondary products in source rocks abstract carbon isotopic composition 13 c reversals with respect to maturity trends have been reported for ethane and propane in barnett shale gas in the fort worth basin u s a this trend also exists in some conventional gas reservoirs such as in the ordos basin china and appalachian basin u s a fractionation during mass transport is unlikely to be a significant mechanism because its effect is minor under geological conditions it would have caused more obvious reversal for methane than for ethane and propane and mass transport can not account for the occurrence of the trend both inside and outside source rocks this paper demonstrates that the reversal may result from mixing of indigenous primary gas generated directly from kerogen and secondary gas generated from oil and condensate within source rocks and presents deconvolution results using end members derived from a kinetic isotope fractionation model the results including the wetness and mixing ratio of primary and secondary gases are consistent with geological conditions condensates may be the major precursor of the secondary gas in shale when the reversal occurs a complete trend of 13 c variations in methane and ethane with maturity is proposed that is based on the indigenous mixing mechanism

neomorphism and cementation in ancient deep water limestones cow head group cambro ordovician western newfoundland canada abstract lithification of thinly bedded lime mudstones and grainstones in the cow head group resulted mainly from precipitation of calcite cement and neomorphic spar at shallow burial depths below the sea floor these crystals are predominantly non ferroan although the iron content and the size of neomorphic spar commonly increase near the margins of nodular and continuous beds of lime mudstone displacive fringes of ferroan fibrous calcite represent the last stage of carbonate precipitation in some limestones most calcite 13c values range between 0 and 3 pdb but 13c values as negative as 15 indicate that bacterial degradation of orgnaic matter variably influenced pore water composition serial analyses from the centres to the margins of nodular and continuously bedded lime mudstones suggest that many of the limestones in this sequence were lithified prior to the precipitation of calcite having the prominent negative 13c values typical of concretionary carbonate calcite 18o values range from 5 to 8 pdb indicating precipitation from 18o depleted pore waters of uncertian origin cathodoluminescence shows that aggrading neomorphism in the lime mudstones did not take place by porphyroid or coalescive consumption of a more finely crystalline mosaic to yield a more coarsely crystalline mosaic instead scattered crystals enlarged by precipitation of progressively younger and eventually ferroan calcite especially at the margins of nodular and continuous limestone beds the cathodoluminescence zoning in calcite crystals is similar to that observed in meteorically altered limestones where pore waters were initially oxidizing but became progressively more reducing with time the preservation of trace element zoning crystal growth microfabrics and 13c values in authigenic calcite suggest collectively that recrystallization did not occur during subsequent burial

isotopic reversal rollover in shale gases produced from the mississippian barnett and fayetteville formations abstract ethane propane and carbon dioxide show reversed carbon isotopic maturity trends in natural gas produced from the barnett and fayetteville shales at thermal maturities greater than 1 5 vre at this high level of thermal maturity the iso butane to n butane ratio also reverses suggesting wet gas cracking has occurred generating more gas molecules in the same volume resulting in overpressure and increased stabilized production rates in the barnett hydrothermal fluids from the nearby ouachita thrust front apparently enhanced the maturity of the fayetteville and barnett shales water hydrocarbon reactions at these high maturities may account for the isotopic reversals