clumped isotope geochemistry of carbonates a new tool for the reconstruction of temperature and oxygen isotope composition of seawater abstract clumped isotope geochemistry deals with the state of ordering of rare isotopes in molecules in particular with their tendency to form bonds with other rare isotopes rather than with the most abundant ones among its possible applications carbonate clumped isotope thermometry is the one that has gained most attention because of the wide potential of applications in many disciplines of the earth sciences in particular it allows reconstructing the temperature of formation of carbonate minerals without knowledge of the isotopic composition of the water from which they were formed in addition the o isotope composition of the waters from which they were formed can be calculated using the 18 o of the same carbonate sample this feature offers new approaches in paleoclimatology for reconstructing past global geochemical cycles in this contribution two applications of this method are presented first the potential of a new analytical method of measurement of clumped isotopes on small samples of foraminifera for high resolution sst and seawater 18 o reconstructions from marine sediments is shown furthermore the potential of clumped isotope analysis of belemnites for reconstructing seawater 18 o and temperatures in the cretaceous is shown

statistical evaluation of elemental concentrations in shallow marine deposits cretaceous lusitanian basin abstract the elemental geochemistry of ancient shallow marine carbonates represents an often underexplored archive of paleoenvironmental change and diagenetic pathways complex multi parameter datasets from a lithologically heterogeneous sedimentary succession in the southern part of the lusitanian basin ericeira portugal are presented and interpreted the section comprises marly carbonate and sandstone rich coastal marine deposits of early albian to early cenomanian age principal component analysis of bulk elemental abundance ca mg sr fe and mn lead to define four geochemical clusters based on common elemental attributes siliciclastics and dolomitized limestones facies yield evidence for significant diagenetic alteration but still preserve information on paleoshoreline position mixed carbonate siliciclastics and limestone facies bear evidence for relative sea level position and relevant paleoclimatic information in their elemental record accordingly mid albian mixed carbonate siliciclastic deposits present variable degrees of continental influx specifically a sharp increase of fe and mn concentrations in relation to continental input is in agreement with periods of enhanced hydrological cycling and increased weathering on adjacent emerged lands the progressive transition towards more marine conditions is accompanied by higher ca and sr content and represented by limestone facies corresponding to a higher sea level position during late albian to early cenomanian times moreover this facies records a trend towards warmer and more arid conditions early diagenetic stabilization and or dissolution of aragonite into low mg calcite can account for major elemental variations including higher sr content strongly coupled with ca content along with lower mg fe and mn concentrations data shown here provide solid evidence that the statistical analysis of the elemental record of ancient marine deposits when combined with conventional sedimentology and thin section petrography allow for elaborate conclusions on their depositional environment and diagenetic pathways cored wells and facies specific reservoir properties can be addressed by these methodologies able to detect and quantify changes in patterns both industry and academy can therefore benefit from approaching such a complex interplay

diagenesis of an upper triassic reef complex wilde kirche northern calcareous alps austria the wilde kirche reef complex early late rhaetian grew as an isolated carbonate structure within the shallow kossen basin at the triassic jurassic boundary a single brief c 10 50 ka period of subaerial exposure occurred the preserved karst profile 70 m thick displays a vadose zone enhanced dissolution at a possible palaeo watertable 5 15 m below the exposure surface and a freshwater phreatic zone karst porosity was predominantly biomouldic primary cavities and biomoulds were enlarged and interconnected in the freshwater phreatic zone cavity networks developed preferentially in patch reef facies resubmergence of the reef complex allowed minor modification of the palaeokarst surface by sea floor dissolution and fe mn crust deposition on a sediment starved passive margin fibrous calcite fc radiaxial fibrous calcite rfc and fascicular optic calcite foc cements preserved as low mg calcite lmc are abundant in primary and karst dissolution cavities fc cement is restricted to primary porosity particularly as a synsedimentary cement at the windward reef margin fc rfc and foc contain microdolomite inclusions and show patchy non bright cathodoluminescence 18o values of non luminescent portions interpreted as near original are 1 16 to 1 82 0 close to the inferred 18o of calcite precipitated from late triassic sea water 13c values are constant 3 to 2 2 0 these observations suggest fc rfc and foc were originally marine high mg calcite hmc precipitates and that the bulk of porosity occlusion occurred not in the karst environment but in the marine environment during and after marine transgression the hmc to lmc transition may have occurred in contact with meteoric water only in the case of fc cement the most altered brightly luminescent portions of rfc foc cements yield 18o 2 44 to 5 8 0 suggesting hmc to lmc alteration at up to 34 c in the shallow burial environment at depths of 180 250 m abundant equant cements with 18o 4 1 to 7 1 0 show crisp uniform or zoned dull luminescence they are interpreted as unaltered cements precipitated at 33 36 c at 200 290 m burial depth from marine derived fluids under a slightly enhanced geothermal gradient fluids carrying the equant cements may have induced the hmc to lmc transition in the fibrous cements

shear driven upwelling induced by lateral viscosity variations and asthenospheric shear a mechanism for intraplate volcanism abstract volcanism occurring away from ridges and subduction zones does not have an obvious plate tectonic explanation but instead must arise from sub lithospheric processes that generate upwelling flow and decompression melting several convective processes such as mantle plumes convective instability edge driven convection and richter rolls produce upwelling via the action of gravity on density heterogeneities in the mantle here we investigate an alternative mechanism the shear driven upwelling sdu which instead generates upwelling solely through the action of asthenospheric shear flow on viscosity heterogeneity using a numerical flow model we examine the effect of viscosity heterogeneity on viscous shear flow induced within an asthenospheric layer we demonstrate that for certain geometries and viscosity ratios circulatory flow develops within a cavity or step embedded into the lithospheric base or within a low viscosity pocket embedded within the asthenospheric layer for asthenosphere shearing at 5 cm yr we estimate that sdu can produce upwelling rates of up to 0 2 cm yr within a continental rift 0 5 cm yr along the vertical edge of a craton or 1 0 cm yr within a pocket of low viscosity asthenosphere in the last case the pocket must feature an aspect ratio of more than 5 occupy 20 60 of the asthenosphere s thickness and be at least 100 times less viscous than the surrounding asthenosphere such viscosity heterogeneity may be associated with thermal chemical melting volatile or grain size anomalies and is consistent with tomographic constraints on asthenospheric variability we estimate that sdu may generate up to 2 5 km myr of melt that is potentially eruptible as surface volcanism this is faster than eruption rates observed at some locations of continental basaltic volcanism we conclude that sdu could provide an explanation for intraplate volcanism occurring above rapidly shearing asthenosphere for example in the basin and range region of north america

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

fibrous calcite from the ordovician of tennessee preservation of marine oxygen isotopic composition and its implications three categories of fibrous calcite from early to middle caradoc platform marginal buildups in east tennessee can be delineated using cathodoluminescent microscopy minor element chemistry and stable c o isotopic composition bright luminescent fibrous cement has elevated mn 1000 p p m negative 13c and intermediate 18o values relative to other types of fibrous calcite this cement reflects fibrous calcite that interacted with reducing mn rich fluids dully luminescent fibrous cement has elevated fe 400 p p m positive 13c and negative 18o values relative to other fibrous cements this cement was stabilized by burial fluids nonluminescent fibrous cement has low mn and fe generally below 400 p p m and positive 13c and 18o values relative to other types of fibrous calcite the latter cement is interpreted to be the best material for determining the isotopic composition of calcite precipitated in equilibrium with early to middle caradoc seawater which is 13c 1 pdb and 18o 4 to 5 pdb results from this study and ashgillian brachiopods indicate that the average 18o composition of the ordovician ocean during nonglacial periods was probably never more negative than 3 smow assuming an ordovician seawater 18o value of 1 smow holston formation fibrous cements would have precipitated at temperatures between 27 and 36 c which is near the upper temperature limit for metazoans a seawater 18o value of 2 smow yields temperatures ranging from 23 to 31 c while a 3 smow value yields temperatures of 18 26 c

spatial geochemistry of a carboniferous platform margin to basin transect balancing environmental and diagenetic factors abstract conventional one dimensional chemostratigraphy of marine carbonates assesses the chemical archive of individual stratigraphic sections and their correlation in space and time whereas this approach has shown to be of value when linking isobathymetric domains usually characterised by similar facies more caution is needed when correlations are extended across different physiographic settings and hence different facies belts here the spatial geochemical record of pennsylvanian platform margin to basin transects across a bathymetric range of about 800 m is documented and discussed in a process oriented context particularly the presence of layered palaeo water masses and their potential control on slope facies distribution and geochemical properties requires attention whereas carboniferous thermo and or chemo clines most likely affected depth related slope facies zonation it was facies change and hence variances in porosity permeability properties that controlled differential early burial diagenetic alteration specifically the lower slope related breccia facies is characterised by higher volumes of early burial carbonate cements this implies that these sediments entered the shallow burial domain with a considerable open pore space and gave way to an increased rock fluid ratio whereas the 13 c record is invariant with bathymetry the more diagenesis sensitive 18 o proxy records pronounced shifts observed across major facies boundaries from this it is concluded that although the primary controlling factor of slope facies distribution with depth is probably palaeoceanographic in nature it is differential rock fluid ratios that control the first order spatial shifts in 18 o composition these findings show that one dimensional chemostratigraphy will severely underestimate the complexity of three dimensional bathymetric data sets across platform margins this is of relevance for the interpretation of the geochemical archive of fossil carbonate platforms in general

a pennsylvanian microbial boundstone dominated carbonate shelf in a distal foreland margin picos de europa province nw spain abstract a pennsylvanian bashkirian moscovian high rising microbial boundstone dominated carbonate platform 12 000 km2 developed in the external margin of a variscan foreland basin intensely studied in the northern domain cuera area the stratigraphy and sedimentology of the southern domain the picos de europa province is here revisited distribution of the bashkirian facies valdeteja formation defines a ramp low angle platform with an internal zone central and eastern sectors of the picos de europa province consisting of huge amounts of massive cement rich microbial boundstones surrounded by an external zone comprising lower slope boundstone derived breccias and basinal lime mudstones from the bashkirian moscovian transition to the early kasimovian picos de europa formation the carbonate platform moderately aggraded acquiring a high rising and flat topped geometry and then prograded rapidly into the external zone where platform derived carbonate sediment and clay sourced from the orogen were deposited seismic scale exposures of the western margin of the shelf show angular bedding relationship between basin toe of slope slope and platform top stratal domains with slope inclination up to 32 and a topographic relief of 550 m the platform evolution above described for the picos de europa province significantly differs with respect to the cuera area where platform mostly prograded during the bashkirian and subsequently aggraded during early moscovian the evolutionary differences between these two carbonate domains which are geographically close are interpreted as changes in tectonic subsidence distribution acting as the major factor controlling the large scale platform geometries in the active foreland basin context finally the tectonic overload due to the emplacement of the cuera thrust sheets during the moscovian kasimovian transition changed the platform growth style from progradational to aggradational and consequently the stacking pattern of the platform top cycles in the picos de europa province

viscous heating in heterogeneous media as applied to the thermal interaction between the crust and mantle we have studied the effects of viscous heating involving heterogeneous media which occurs in the interaction between the mantle lithosphere crust system with the mantle lithosphere in delamination high rates of viscous heating are found in corner flow regions where the deformation rate of the flow is highest the weak lower crust is sheared between the upper crust and the mantle lithosphere giving rise to a second type of viscous heating with high dissipation values several ten to hundred times the chondritic value greater dissipation rates are found in the shear interaction between heterogeneous media with different non newtonian rheologies than in the shear heating inside a single non newtonian or newtonian homogeneous medium high spatial resolution of at least 2 5 km or finer is needed to resolve the sharp variations of viscous heating near the interface of the two media

application of redox sensitive proxies and carbonate clumped isotopes to mesozoic and palaeozoic radiaxial fibrous calcite cements abstract ancient marine radiaxial calcite cements are commonly exploited as archives of marine porewater properties based on the argument that they lack metabolic effects often assigned to biogenic carbonates here we critically test the significance of conventional versus with respect to these fabrics less conventional proxy data from pennsylvanian triassic and cretaceous case examples conventional proxies include cathodoluminescence carbon and oxygen isotope ratios main and trace elemental concentrations less conventionally applied proxies are clumped isotope 47 redox sensitive and rare earth elements sampled across a succession of triassic radiaxial fibrous calcites radiaxial calcites are subdivided in three groups based on their luminescence characteristics non luminescent patchy luminescent and bright luminescent luminescence patterns are in fair agreement with isotope ratios in particular with those of oxygen the data fall into or are close to the range of reconstructed marine seawater values and often plot to the positive end member of the isotopic range these results disagree with the commonly held view that isotope data from luminescent cements reflect a priori non marine values further evidence for this comes from ree concentration patterns and cerium anomalies suggesting normal marine porewater values for all except the very last generation of radiaxial calcites this implies that luminescent radiaxial calcites must not necessarily represent significant diagenetic resetting kinetic effects during precipitation and different activator elements must be considered marine and earliest burial porewater temperatures of 12 26 c are suggested by conventional calcite 18 o thermometry conversely the application of the clumped isotope thermometer to the same radiaxial calcites suggests temperatures of 180 200 c reflecting solid state resetting of fully cemented limestones under a low water rock ratio redox sensitive elements particularly zn cd u and cu are affected by kinetic processes overriding fluid eh manganese concentrations and ce anomaly data point to gradually decreasing marine porewater oxygen levels from outer to inner cement fringes judging from ree patterns and ce anomalies the cement layers in the central portions of the pore filling cement succession witnessed the end of marine precipitation and the onset of shallow marine diagenesis consequently radiaxial calcite precipitation is suggested to continue in the early shallow marine burial domain this study underscores the potential of radiaxial calcite successions as archives of marine porewater to shallow burial diagenetic pathways the combination of conventional and less conventional proxies is a clear strength of this study and documents that abiogenic carbonate archives are often underexplored

diagenesis of radiaxial fibrous calcites in a subunconformity shallow burial setting upper triassic and liassic northern calcareous alps austria multiple generations i iv of radiaxial fibrous calcite rfc cement crusts are intercalated with meteoric internal sediments and microsparitic calcite cements within karst dissolution cavities in upper triassic steinplatte and basal liassic hirlatz carbonates in the northern calcareous alps of austria stratigraphic petrographic and geochemical evidence suggest that rfc precipitated as high mg calcite from marine fluids in the shallow burial 180m environment estimated original 18o and 13c isotopic compositions of late triassic and early jurassic marine waters are 1 88 to 2 21 and 2 37 to 2 89 pdb respectively the rfc subsequently was altered to low mg calcite by meteoric or mixed meteoric marine fluids introduced during periods of subaerial exposure rfc cement generations i iv in steinplatte reef slope beds and i ii in steinplatte reef and lagoon and hirlatz samples retained near original marine oxygen and carbon isotopic compositions 1 88 and 2 89 pdb 2 11 and 2 37 pdb respectively and petrographic identity during the conversion to low mg calcite in contrast rfc generations iii and iv in the hirlatz and upper steinplatte were intensely leached and partly replaced by coarse blocky calcites of meteoric origin with attendant modification of original crystal fabrics and isotopic compositions 4 65 to 5 22 pdb 18o 2 54 to 3 14 pdb 13c these differences in diagenetic behaviour are related to contrasting processes of fluid rock systems and original microfabric variations in the rfc cements the karst affected phreatic marine burial environment of rfc formation and subsequent diagenetic alteration inferred for these rocks have not been previously documented in other ancient rocks

secular change in northwestern tethyan water mass oxygenation during the late hauterivian early aptian abstract the mesozoic is punctuated by several extreme episodes of oxygen drawdown in the ocean known as oceanic anoxic events oae beside these discrete events little is however known about longer term oxygen level fluctuations and their controls a high resolution dataset of rare earth elements ree content in carbonate deposited in the vocontian basin se france has allowed to survey the evolution of the cerium anomaly ce ce during part of the early cretaceous latest hauterivian early aptian this ratio is used as a proxy for the degree of oceanic oxygenation the cerium anomaly is compared to the coeval evolution of relative sea level organic rich occurrences and nutrient level proxies in order to infer a cause and consequence relationship between these parameters and the evolution in sea water oxygenation the long term evolution of the cerium anomaly shows that northwestern tethyan water masses have evolved from less oxygenated during the latest hauterivian early barremian interval to more oxygenated during the late barremian earliest aptian time before being strongly oxygen depleted during the early aptian oae 1a time interval this trend is correlated with both the long term trend in nutrient levels as well as with the frequency of organic rich layers within the rock record on a medium term scale sub stage level there is a good correlation between the cerium anomaly and tectono eustatism as deduced by the mn content of the basinal carbonate and sequence stratigraphic observation on surrounding shallow water areas this suggests that sea level changes by altering ocean stratification and currents as well as modulating the amount of nutrient delivered from the continent exert control on oceanic oxygenation via changes in primary productivity

growth and demise of late carboniferous carbonate platforms in the eastern cantabrian zone asturias northwestern spain abstract geometric relationships of seismic scale stratal patterns observed on aerial photographs of late carboniferous carbonate platforms in the cuera region in asturias northwestern spain allow the identification of two discrete stages of platform development with different styles of deposition the first phase represented by the bashkirian valdeteja formation is characterized by progradation of more than 10 km with steep clinoforms of up to 35 and slope heights up to 650 m during the second phase corresponding to the moscovian picos de europa formation the platform aggraded mainly vertically nearly 850 m both phases of platform growth were terminated by major relative sea level lowstands in conjunction with local filling of the adjacent basins by shales and mudstones that onlap the platform flanks calibration with outcrops suggests a subdivision into the following lithofacies directly related to the depositional profile 1 platform interior parallel well to thick bedded skeletal pack to grainstones 2 margin algal and sponge dominated bioherms with little internal structure 3 slope clinothems of medium to thick bedded margin derived breccia beds 4 toe of slope alternating grain and packstones with spiculites and 5 basin marly shales shales sandstones and calcareous mudstones relative sea level changes caused by regional tectonic movements are suggested to be responsible for the generation of the two platform phases sea level highstands triggered the nucleation and growth of the platforms whereas major sea level lowstands in combination with terrigenous infill of the basins terminated the platform growth in addition it is proposed that the termination of late carboniferous carbonate platforms is caused by the combined effect of a sea level fall subaerial exposure and erosion and finally a rapid transgression that drowned and polluted the platform with shales and mudstones local variations in the infilling of the basins controlled the variations in the style of progradation the subsidence rates for the vertically aggrading phase picos de europa formation were approximately twice that for the progradational phase valdeteja formation and may explain the aggradational versus progradational character

conditions for veining and origin of mineralizing fluids in the alpi apuane nw tuscany italy evidence from structural and geochemical analyses on calcite veins hosted in carrara marbles abstract this work deals with structural and geochemical chemical and isotopic analyses of calcite veins hosted in carrara marbles in the alpi apuane nw tuscany italy geometric features and spatial distribution of veins provided estimations of stress ratio 2 3 1 3 driving stress ratio r pf 3 1 3 and fluid overpressure si pf 3 at the time of vein formation the obtained values of 32 and r 0 43 reveal that fluid pressure was higher than the intermediate principal stress at the time of veins formation whereas the estimated si ranging from 129 to 207 mpa indicates that veins formed under supra hydrostatic to lithostatic pressure conditions carbon 13cv pdb 1 81 2 10 for veins and 1 95 2 51 for host marbles oxygen 18ov smow 28 71 29 57 for veins and 28 90 29 36 for host marbles and strontium 87sr 86sr 0 707716 0 707985 for veins and 0 0707708 0 707900 for host marbles isotope compositions in vein host marble pairs were internally quite consistent combining our structural and geochemical data a modeling approach was performed to investigate the compositional features and temperatures of calcite depositing fluids the results of our studies give evidence that 1 pore fluids in carrara marble consisting of metamorphic formation waters were re mobilized during veining event and migrated within the veins in closed system conditions 2 veins formed after ductile folding phases and before high angle brittle faulting events at temperature and pressure around 250 c and 210 mpa and finally 3 about 12 g h2o m3 marble are calculated to have been available as vein parental fluid at the time of vein formation

melt segregation from partially molten source regions the importance of melt density and source region size the compressibility of basic melt at 1 atmosphere is about an order of magnitude higher than that of mantle minerals consequently the density contrast between melt and the principal residual crystals in mantle source regions is expected to decrease with increasing source region depth the increasingly olivine normative character of primary melts produced at greater depths is also expected to result in a decrease in this density contrast with increasing source region depth once vertical permeability is established by melt generated during partial melting buoyancy driven melt percolation can under some circumstances segregate melt from the residual crystals in its source region on a geologically rapid time scale limits to this process are provided by cooling of the source region freezing melt in and rigidity of the crystalline matrix mechanically trapping melt source region size influences these limits strongly consequently small partially molten diapirs km in diameter may be able to trap large melt fractions 30 but larger source regions would be unable to do so the reduction in density contrast with pressure reduces the buoyant force driving melt percolation and provides another limit to melt segregation diapirs at depth may thus stably contain large fractions of melt but may decompress and unload their melt during ascent this effect would be enhanced in small diapirs and may be relevant to the genesis of komatiitic magma melt compression may also be a factor in explaining why the very different maximum depths inferred for typical basic melt segregation from source regions on different planets 500 km on the moon 250 km on mars 100 km on earth correspond to similar pressures 25 35 kbar at greater pressures melt may no longer be capable under ordinary conditions of segregating upwards by buoyancy this may also help to explain why depleted peridotites overlie more fertile peridotites and how deep regions of the mantle are able to remain fertile over geologic time

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

diagenesis of middle devonian carbonate mounds of the mader basin eastern anti atlas morocco abstract in the mader basin of the eastern anti atlas middle devonian carbonate mounds were established on a gently sloping ramp at moderate water depths primary stromatactis cavities in these mounds exhibit early marine shallow burial and deeper burial cementation the mound matrix was affected by neomorphic alteration stylolitization and dolomitization earliest cementation occurred in the marine environment nonluminescent to blotchy luminescent cloudy radiaxial calcites of originally high mg calcite composition coat the walls of stromatactis cavities and take up more than 50 of the primary pore space the isotopic signature d18o 2 6 0 2 pdb d13c 2 7 0 5 pdb is close to that of nonluminescent fabric retentive brachiopod shells in the same mounds and taken as the nearly pristine stable isotopic signature of mader basin seawater the high d18o values compared with middle devonian data of north america are interpreted to result from the mid latitudinal ca 35 s temperate water settings of the mader basin carbonate mounds the transition to the shallow burial realm is indicated by a typical non bright dull luminescence sequence nonluminescent scalenohedral cements pass into bright luminescent and banded luminescent scalenohedral cements which are overgrown in turn by dull to moderate luminescent blocky spar meteoric influence on that cement sequence can be excluded deeper burial diagenesis is represented by stylolitization ferroan calcite cementation and neomorphic alteration of the fine grained mound carbonates pressure solution and or outgassing of co2 are possible ion sources for ferroan calcite precipitation in the remaining pore space 18o depletion d18o 6 8 to 8 9 pdb of these cements can be calculated to elevated precipitation temperatures of 21 33 5 c and burial depths of 420 670 m microspar making up the bulk of the mounds was formed by a one step transformation of a precursor calcitic micrite low d18o values 6 9 p 2 1 pdb mottled luminescence and high fe concentrations indicate that this transformation also took place in the deeper burial realm

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km

redox conditions of the late cretaceous western interior seaway recorded by rare earth elements of bearpaw molluscan fossils understanding the causes of the low biodiversity faunas in the western interior seaway wis overwhelmingly dominated by ammonite and bivalve molluscs will help us to understand paleoenvironmental conditions in the wis in this study we examined rare earth elements rees in well preserved molluscs from the bearpaw formation to reconstruct wis redox conditions both nektonic and benthic molluscs share similar ree patterns with enrichment in light rees there is only a slight ce depletion in both types of molluscs indicating no significant fractionation of ce from the other rees a lack of significant ce anomalies in molluscs points to oxygen deficient probably dysoxic conditions in the middle to bottom part of the water column where the molluscs lived given the general lack of significant ce anomalies also in molluscs from older formations oxygen deficiency was likely prevalent in the late cretaceous in the wis a warmer climate in the cretaceous is probably a driver of such conditions as predict

modern sand rich and mud rich siliciclastic aprons alternative base of slope turbidite systems to submarine fans base of slope aprons that lack significant channel development and are fed by multiple sediment sources i e numerous slope failures or debris chutes represent an alternative facies organization to submarine fans that possess channels and are fed by canyon point sources of allochthonous sediment two styles of base of slope apron deposition are evident 1 sand rich aprons with rhythmic gradational sand and gravel sheets that evolve to basin plain turbidites as exemplified by crater lake aprons and 2 mud rich aprons composed of chaotic sediment flow lobes without gradation as exemplified by ebro margin aprons

magnesium isotope composition of sabkha porewater and related sub recent stoichiometric dolomites abu dhabi uae abstract earliest marine diagenetic sabkha type dolomites are widespread in earth s geological record potentially these carbonates may act as archives of past seawater magnesium isotope 26mg ratios at present however the fractionation of magnesium isotopes 26mg between seawater here evaporated marine porewater and sabkha dolomite is not constrained in order to explore 26mgdol mg porewater we make use of actualistic sabkha type dolomite precipitation in the gulf region trucial coast united arab emirates this paper documents and discusses the first detailed sabkha 26mg data set of mg bearing solids including stoichiometric dolomites degree of ordering 0 9 mean 26mgdol 0 79 0 41 2 n 17 and related marine pore waters the presence of dolomite crystals with a broad range of ordering in shallow cores is documented using x ray diffraction and scanning electron microscopy but individual crystals are too small

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

delamination and detachment of a lithospheric root abstract orogens can be formed by shortening of the lithosphere on time scales much shorter than that of thermal diffusion for time scales of a few ma the major part of the uplift of an orogen which may occur during the transition from crustal shortening to extension may be due to delamination of the mantle lithosphere rather than to convective thinning the negative buoyancy forces of a lithospheric root may cause lateral shortening while the lithospheric root delaminates peels off from the crust detaches and is replaced by hot upwelling asthenosphere this provides a large input of heat into the crust geological observations show that the late orogenic phase may be characterized by volcanic activity high temperature low pressure metamorphism extension high heat flow and a rapid within a few ma increase of the topography we present dynamically and thermally consistent numerical models of the evolution of an orogenic root mantle rheology is based on laboratory data of olivine while models with the rheology of olivine do not allow for delamination due to its very high viscosity the incorporation of a quasi brittle rheology is necessary for modelling delamination of the mantle lithosphere successful delamination models are always associated with a rapid change in topography we examined the parameter range of our models and found different regimes of behaviour of the orogenic root of the mantle lithosphere no delamination delamination delamination and full detachment for a fixed viscosity of the upper crust of 1023 pa s delamination occurs for a range of viscosities of 1023 pa s to 1020 pa s for the lower crust depending on the thickness of the lithospheric root but full detachment of the delaminated lithospheric slab only occurs if the viscosity of the lower crust is greater than approximately 1021 pa s hot asthenosphere is welling up into the gap opening during delamination further implications of our models are lithospheric roots or unsupported slabs of at least 100 170 km depth extent are needed to provide sufficient negative buoyancy to allow delamination and detachment in all delamination models significant amounts of lower crustal material are subducted into the mantle in applying our models to selected orogens we argue that mantle delamination may have played an important role in the variscan and the himalayan orogenies

relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks new microstructural data on experimentally deformed wet and dry natural olivine rocks anita bay and aheim dunite together with the other reliable experimental data indicate that the experimental stress recrystallized grain size relationship in olivine rocks is largely independent of water content and temperature and is only slightly dependent on the flow properties of the material the experimental data cover a stress range of 30 300 mpa water contents from 30 ppm to 300 ppm and temperatures in the range 1100 1650 c local melt contents of up to 10 volume cannot be demonstrated to have a significant effect on the stress grain size relationship

compressible convection with constant and variable viscosity the effect on slab formation geoid and topography two dimensional cartesian finite difference models of compressible convection with constant and variable viscosity and fixed bottom temperature are presented density variations according to the adams williamson equation of state are included in the case of constant viscosity convection viscous and adiabatic heating damp the flow compressible density variations hardly influence geoid undulations however the lower thermal boundary layer becomes thinner and the mean cell temperature increases in the case of variable viscosity the nonlinear coupling between compression adiabatic and viscous heating and a temperature pressure and stress dependent rheology leads to important consequences the upwelling flow broadens and plumes are retarded the flow strongly concentrates toward the downwelling region and becomes mechanically decoupled from the interior of the cell by a low viscosity region this mechanism seems to be important for the formation of subducting slabs extrapolated to mantle conditions two low viscosity regions are predicted flanking the slab on either side and inhibiting an early dispersal and mixing of slab material into the mantle this process might be aided by an increase of negative buoyancy forces with depth as observed in the models further results are as follows increasing the dissipation number in variable viscosity convection may either damp or speed up convection depending on the rheology models with internal heating and a fixed bottom temperature show that the threshold to time dependent variable viscosity convection is drastically reduced if the anelastic liquid approximation is applied instead of the extended boussinesq approximation

mississippian paleocean chemistry from biotic and abiotic carbonate muleshoe mound lake valley formation new mexico u s a abstract it has been proposed that phanerozoic variation in the mg ca ratio of seawater has resulted in variation in the primary mineralogies of shallow water carbonates between aragonite and calcite dominated seas to test this hypothesis this study estimates the mississippian oceanic mg ca and sr ca ratios from the composition of marine carbonate cements and crinoidal carbonate both of which have been suggested to faithfully record seawater chemistry the early mississippian tournaisian visean muleshoe mound lake valley fm alamogordo nm u s a lends itself well to this endeavor because it contains abundant crinoids and marine carbonate cement stable isotope and elemental analyses as well as cathodoluminescence characteristics elucidate the diagenetic history of the bioherm and identify the least diagenetically altered materials for reconstruction of paleoseawater chemistry isotopic analyses document two diagenetic trends one meteoric and one burial diverging from values similar to literature estimates of mississippian marine calcite 1 5 18o 4 5 13c the burial diagenetic trend follows from this origin to more negative oxygen values 7 18o 3 13c the meteoric diagenetic trend is defined by a meteoric calcite line at 3 18o similarity of non luminescent marine cement and non luminescent crinoid isotopic and elemental compositions suggests that crinoidal carbonate was completely replaced by cement in the marine environment average elemental ratios for least altered marine cements are mg ca 11 mmol mol sr ca 0 2 mmol mol mn ca 0 2 mmol mol and fe ca 0 4 mmol mol early mississippian seawater is estimated to have had a mg ca ratio of 0 2 0 3 mol mol this mg ca ratio is compatible with the early mississippian being an interval of calcite sea conditions and compares well with the devonian estimate from marine cements 0 5 though it is 5 10 times lower than estimates from mass balance modeling the sr ca for early mississippian seawater was between 0 5 and 0 9 mmol mol which is much lower than previous estimates of 6 mmol mol from brachiopod calcite and argues for the development of a sr sink aragonite in the latest devonian to earliest mississippian

magnesian calcite cements and their diagenesis dissolution and dolomitization mururoa atoll the most ubiquitous syn sedimentary cements affecting mururoa atoll are composed of magnesian calcite three main types are distinguished fibrous bladed and sparitic on the basis of petrography morphology and mgco3 concentration of the constituting crystals while peloid infills a particular form of hmc chemical precipitation also exist petrographic evidence and isotopic signatures are compatible with marine precipitation mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of mg calcite cements these alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology the first stage of the partial dissolution of mg calcite induces the development of chalky white friable zones within the initially crystalline hard cement layers at ultrascale this is due to the creation of micro voids along the elongate cement fibres advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and or between superposed cement layers mg calcite dissolution is related to meteoric diagenesis during periods of quaternary exposure the creation of voids within mg calcite layers is due to the mechanical removal of previously altered calcium carbonate a process suggesting marine or non marine water flow probably in the vadose environment selective dolomitization of mururoa cements involves alternations of calcite and dolomite which form successive cement like rinds within primary cavities at mururoa these alternations are the result of selective dolomitization of the pre existing mg calcite cements rather than successive precipitation of calcite and dolomite selective dolomitization of mg calcite cements at mururoa indicates that a given cement succession is not necessarily a simple chronological sequence oxygen isotope values of dolomites are enriched in 186 by about 3 pdb within calcite dolomite pseudo alternations the dolomitizing fluid at mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization

syndepositional deformation features in high relief carbonate platforms long lived conduits for diagenetic fluids abstract syndepositional faults and fractures are known to affect early fluid flow in carbonate platforms less clear is whether they are active fluid conduits throughout the entire history of the platform strata syndeformational fractures in permian late guadalupian carbonates exposed in dark canyon guadalupe mountains new mexico u s a address this question transmitted light and cathodoluminescent petrography stable isotope and fluid inclusion analyses and clumped isotope thermometry show that there were multiple episodes of fracturing dissolution cementation and replacement in the fractures dolomite cement or dolomitized marine cements line the walls of some fractures and indicate the syndepositional reflux of evaporated permian seawaters through the fractures fine to medium crystalline luminescently zoned calcite may overlie the dolomite and marine cements line fracture walls where those phases are absent or cement karst breccia on fracture walls the 18o values of this calcite 8 8 to 14 0 vpdb and clumped isotope temperatures 16 to 32 c indicate precipitation from meteoric fluids 18osmow of 6 2 to 10 5 associated with episodic sea level lowstands during the development of high frequency depositional sequences the early calcites can themselves be fractured rotated and recemented indicating recurrent deformation and meteoric influx evaporite cements were once the dominant pore filling phase in the fractures forming both before and after the early meteoric cements the earliest evaporites formed during deposition of tansill limestone probably from the same brines that formed dolomites evaporites that postdate the early calcite probably did not form until permo triassic burial when geomechanical analysis indicates that the syndepositional fractures were likely reactivated and brines could have been sourced from overlying bedded evaporite all evaporite cements subsequently were calcitized mainly by coarse crystalline inclusion rich calcites that formed from warm 59 to 96 c fluids calculated fluid isotopic compositions 18osmow of 0 5 to 4 7 imply mixing of meteoric and oil field brines carbon isotope values 2 to 17 indicate microbial degradation of hydrocarbons in some of those fluids but not all geomechanical analysis indicates the potential for syndepositional fractures to have failed reactivated during basin and range extension and the warm basinal fluids are interpreted to have migrated upward through the fractures during that event platform margin fractures unassociated with faults witnessed cooler fluids 59 to 65 c than outer shelf fractures 70 to 96 c because outer shelf faults tapped waters from greater depths the most recent fluid flow through the fractures generated dissolution features and laminated speleogenetic calcites which are interpreted to result from intrastratal karsting associated with exhumation and weathering diagenetic features in the syndepositional fractures are equivalent to those observed in the adjacent limestones suggesting active fluid communication between matrix and fractures throughout the diagenetic history of the rocks the complex fracture paragenesis also indicates that syndepositional fractures are not only conduits for early fluid flow networks but they can also impact a rock s entire diagenetic history if reactivated by changing stress fields

numerical models of mantle lithosphere weakening erosion and delamination induced by melt extraction and emplacement continental rifting caused by extension and heating from below affects the lithosphere or cratons in various ways volcanism and melt intrusions often occur along with thinning weakening and even breaking lithosphere although mechanical necking models of the lithosphere are often applied the aspects of melting and the implications due to melt transport and emplacement at shallower depths are not well understood a two phase flow approach employing melt extraction and shallow emplacement associated with thermal weakening is developed and compared with observations the results of this comparison indicate the importance of partial melts and an asthenospheric magma source for increasing the rising rate of the lithosphere asthenosphere boundary during extension thermo mechanical physics of visco plastic flow is approximated using the finite difference method with eulerian formulation in 2d the conservation of mass momentum and energy equations are solved for a multi component crust mantle and two phase melt matrix system rheology is temperature and stress dependent in consideration of depletion and enrichment melting and solidification are controlled by a simplified linear binary solid solution model melt is extracted and emplaced in predefined depth regions emplacement zones in the lithospheric mantle and crust the compaction boussinesq approximation was applied its validity was tested against the full compaction formulation and found fully satisfactory for the case of sublithospheric melting models a simple model guided by the geodynamic situation of the rwenzori region typically results in updoming asthenosphere with melt assisted erosion of the lithosphere s base even with a conservative approach for a temperature anomaly melting alone doubles the lithospheric erosion rate in comparison with a model without melting with melt extraction and intrusion lithospheric erosion and upwelling of the lithosphere asthenosphere boundary speeds up by a factor 3 4 in an extreme case delamination may occur if weakening fully decouples a hanging mantle block models with an emplacement zone of up to approximately 70 km agree well with observations especially for a concept based on seismological and petrological data

stable isotopes and archaeological geology the carrara marble northern italy abstract the principal marble quarries of italy for the past 2000 years have been those of carrara in roman times the marble was exported all over the ancient world renaissance quarries were opened up in seravezza both the seravezza and the carrara quarries have been exploited until today the correct identification of carrara marble has always been a problem because it was traded so widely in roman times and was later used to fabricate broken or lost pieces of classical statuary many types of geochemical analysis have been tried in order to find distinctive signatures for the recognition of classical marbles including trace elements esr spectroscopy of mn thermoluminescence and stable isotopic ratios to date the most promising has been isotopic ratio analysis of oxygen and carbon in this study stable isotopic signatures were investigated as a means for distinguishing 1 the principal quarry areas of the carrara commune fossacava miseglia and torano and the seravezza commune 2 or merely the largely classical quarries of carrara from the renaissance quarries of seravezza 3 and most important the carrara quarries from other quarries of greece turkey italy and tunisia in the classical marble data base discriminant analysis da of the isotopic analyses showed that the three quarry areas of a carrara could not be told apart but that they could be told easily from seravezza in addition a mineralized carrara quarry at mandria with buff to yellow rather than white marble was also distinctive da also suggests that carrara can be told from those quarries with which it is presently confused paros in the aegean sea marmara in turkey and pentelikon near athens sr isotopic analysis showed a range in the carrara and other marbles of hettangian lower jurassic age from 0 70778 to 0 70810 which is very different from values found in paros and pentelikon this suggests that sr might make a good third discriminator for classical marble

radiaxial fibrous calcites a new look at an old problem abstract marine radiaxial fibrous and fascicular optic calcites are very common but poorly understood pore filling cements in paleozoic and mesozoic marine neritic and upper bathyal limestones the main diagnostic feature of these cements is their converging or diverging crystal c axis respectively the reasons for this anomalous texture are at present unknown another controversy is due to the relative lack of occurrences in cenozoic strata and their apparent absence in quaternary marine limestones despite these uncertainties marine fibrous cements are thought by some to be amongst the best proxies for the geochemistry of ancient oceans because of the absence of metabolic effects during their precipitation applying electron backscatter diffraction analysis we here for the first time document radiaxial fibrous and subordinate fascicular optic fibrous biominerals from well preserved jurassic and cretaceous low mg calcite belemnite rostra the finding of fibrous biogenic calcites in combination with the recent description of holocene and modern fibrous calcites in stalagmites represents in the view of the authors a significant advance in carbonate research here these findings are placed in their wider processes oriented context and the significance of biogenic and speloan fibrous calcite for their marine counterparts is assessed comparing the physico chemical and organomineralic properties of different precipitation sites indicates that the diagnostic variations in the crystal c axis orientation are not related to a specific nucleation environment or substratum in contrast preliminary crystallographic analyses suggest that kinetic factors during nucleation or subtle gradients in the statistical replacements of ca 2 by mg 2 in the crystal structure may lead to local strain resulting in converging or diverging crystal c axes

rare earth element evolution of phanerozoic seawater recorded in biogenic apatites rare earth element ree contents of marine biogenic apatites have been shown to record seawater compositions a data base of available and newly acquired rare earth element ree contents of marine biogenic apatites has been created to assess the past seawater ree compositions to ensure that this data base contains only the pristine ree signals altered samples characterized by very low la sm n ratios where n stands for ree ratios normalized to the nasc composition gromet et al 1984 geochim cosmochim acta 48 1984 2469 acquired during apatite recrystallization are not included in the database these data reveal a change in the tethyan seawater composition between 110 and 80 ma after the end of the cretaceous the ree chemistry of seawater remains constant until present day this seawater composition change is likely due to concurrent changes in ree scavenging processes a strong correlation between decreasing sm yb n ratios and increasing ce anomalies for samples deposited during the 80 110 ma period is observed as ce anomalies are attributed to ocean water redox conditions changes in ree scavenging could reflect an evolution from stratified and poorly oxygenated waters towards well mixed and oxygenated waters this could have resulted from changing current patterns stemming from the contemporaneous opening of the atlantic ocean a major change in middle and late cretaceous oceanic circulation linked to plate tectonics would have favored the colonization of pelagic environments

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

the diversity of tectonics from fluid dynamical modeling of the lithosphere mantle system abstract numerical modeling of the lithosphere has been carried out from many points of view most popular and prevalent is the use of the kinematic boundary conditions to move the lithosphere about in the dynamics we have approached the numerical modeling of the lithosphere mantle system by treating this as a fluid dynamical system in which crustal and mantle rheologies and thermal and compositional buoyant forces play dominant roles in delivering the dynamics in a self consistent manner we have investigated the development of extensional and compressional tectonic regimes from variations of the crustal density and compare their temporal evolutions in comparing with kinematically driven thickening of the lithosphere we find that the timescales are more self consistently determined in a full fluid dynamical treatment than in kinematic models we emphasize the delicate nature of balancing the chemical buoyancy of the crust with the thermal negative buoyancy of the mantle lithosphere as the modes and associated timescales of the delamination process are controlled critically by their relative importance the role of an actively participating buoyant crust cannot by ignored in the overall lithosphere mantle dynamics

mollusc and brachiopod skeletal hard parts intricate archives of their marine environment the biogenic carbonate hard parts of fossil bivalves cephalopods and brachiopods are among the most widely exploited marine archives of phanerozoic environmental and climate dynamics research the advent of novel analytical tools has led many workers to explore non traditional geochemical and petrographic proxies and work performed in neighbouring disciplines sheds light on the complex biomineralization strategies applied by these organisms these considerations form a strong motivation to review the potential and problems related to the compilation and interpretation of proxy data from bivalve cephalopod and brachiopod hard parts from the viewpoint of the sedimentologist and palaeoceanographer specific focus is on the complex biomineralization pathways of a given dissolved ion or food particle from its aquatic environment via the digestion and biomineralization apparatus in molluscs and brachiopods and its incorporation into a biomineral given that molluscs and brachiopods do not secrete their hard parts from seawater but rather from their mantle and periostracum this paper evaluates differences and similarities of seawater versus that of body fluids cephalopods bivalves and brachiopods exert a strong biological control on biomineralization that to some degree may buffer their shell geochemistry against secular changes in seawater chemistry disordered amorphous calcium carbonate precursor phases later transformed to crystalline biominerals may be significant in carbonate archive research due to expected geochemical offset relative to the direct precipitation of stable phases a reasonable level of understanding of the related mechanisms is thus crucial for those who use these skeletal hard parts as archives of the palaeo environment the impact of what is commonly referred to as biological factors on the geochemistry of mollusc and brachiopod hard parts is explored for conventional isotope systems such as carbon oxygen strontium and traditionally used element to calcium ratios in particular the often used 13ccarb or the mg ca and sr ca elemental proxies are fraught with problems an interesting new research field represents the analysis calibration and application of non traditional proxies to mollusc and brachiopod hard parts examples include the carbonate clumped isotope 47 approach and the analysis of the isotopes of ca mg n li s or element to ca ratios such as li ca or b ca and rare earth elements based on considerations discussed here a series of do s and don ts in mollusc and brachiopod archive research are proposed and suggestions for future work are presented in essence the suggestions proposed here include experimental work also field experiments making use of recent archive organisms or where possible a reasonable recent analogue in the case of extinct groups moreover the detailed understanding of the architecture of mollusc and brachiopod hard parts and their ultra structures must guide sampling strategies for geochemical analyses where feasible a detailed understanding of the diagenetic pathways and the application of multi proxy and multi archive approaches should form the foundation of fossil carbonate archive research the uncritical compilation of large data sets from various carbonate shelled organisms collected at different locations is not encouraged

delta 18 and delta 13 c variations in late devonian marine cements from the golden spike and nevis reefs alberta canada abstract for geologists and geochemists concerned about the chemical and thermal evolution of oceans deciphering primary marine 13c and 18o values from ancient marine carbonates can provide useful data for the physical and chemical modelling of ancient oceans although abiotic aragonite and magnesian calcite marine cements have commonly been overlooked because of their susceptibility to diagenetic alteration petrographic and chemical analysis of radiaxial fibrous calcite from two upper devonian frasnian pinnacle reefs golden spike and nevis of central alberta indicates that these magnesium rich cements preserve original marine 18o and 13c values in the pursuit of primary marine isotopic compositions it is necessary to characterize the isotopic compositions of diagenetic cement phases subaerial exposure and meteoric diagenesis of the interior of the golden spike reef is recorded by vadose cements caliche and clear non ferroan meteoric calcite spar with distinctive black and yellow cathodoluminescence cl zones 18o 13c crossplots of caliche and meteoric phreatic cement data form an inverted j curve typical of rock water reactions in meteoric systems a late phase of calcite spar with variable inclusion density and fe2 content and diffusely zoned red orange cl is associated with late fractures and stylolites and it precipitated during subsequent burial of the reef unaltered centers of radiaxial fibrous cement crystals are nonluminescent inclusion free and enriched in magnesium while diagenetically altered crystal terminations inter crystalline boundaries and crystal substrates are brightly luminescent inclusion rich microdolomite and fluid inclusions low mg calcite altered cloudy marine cements have variable 18o and invariant 13c values which define a trend that diverges from isotopically heavier unaltered marine cements the isotopic compositions of inclusionrich marine cement are coincident with those of meteoric phreatic spars although burial spars are present in all samples petrographic and chemical data suggest that portions of the marine cement crystals were altered by meteoric fluids the 18o values of unaltered radiaxial fibrous calcite are similar invariant throughout both reefs although marine cements from older deep parts of both reefs are enriched in 13c relative to younger shallow parts this records temporal changes in the 13c value of marine bicarbonate within the alberta basin variability of 18o and 13c values of unaltered marine cements is very small 0 5 for a given suite of samples making these cements ideal for h gh time resolution studies of the isotopic evolution of ancient oceans the presence of well preserved magnesium rich calcite from late devonian reefal sequences suggests that marine cements throughout the phanerozoic should be examined more carefully as they may preserve primary isotopic signatures independent confirmation of primary isotopic compositions is found in similar values for marine cements from middle frasnian reefs of the canning basin australia and the dinant synelinorium belgium the 18o values for the well preserved marine cements of this study are considerably lower than modern marine carbonate values by 3 to 4 detailed chemical and petrographic analysis indicates that diagenetic alteration is not the cause of these low 18o values in addition such a difference cannot be fully explained by warmer late devonian oceans these data suggest that late devonian seawater had a 18o value lower than modern oceans this in turn suggests that the balance of low temperature silicate weathering and high temperature seawater basalt exchange reactions must have been different from today s

the use of laboratory velocity data for estimating temperature and partial melt fraction in the low velocity zone comparison with heat flow and electrical conductivity studies peridotite is widely considered to be the dominant component of the upper most mantle seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature the strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low velocity zone of the earth the laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite i e homologous temperature dependence this observation allows us to extrapolate laboratory results to higher pressures greater depths with some confidence requiring only a knowledge of the solidus as a function of pressure using the ratios of lithospheric to asthenospheric velocities temperature and melt fraction in the asthenosphere the low velocity zone can be determined from laboratory velocity data examples of thermal structures of the upper mantle beneath oceanic plates are presented we choose three locations of geophysical interest iceland plateau pacific ocean and philippine sea where reliable seismic velocities have been determined in the upper mantle from surface wave studies a large amount of partial melt 5 vol and a higher temperature than the dry peridotite solidus are inferred in 0 5 m y asthenosphere under the iceland plateau and the active marginal basin of the east philippine sea the partial melt zone appears to extend deeper 100 km and to greater ages 20 m y in the pacific ocean region than under the slowly spreading iceland plateau partial melting is not expected in the asthenosphere older than 5 m y under the iceland plateau even though the volume fraction of partial melt is not so large 3 vol beneath the pacific plate the melting zone may extend to a distance of above 2000 km from the ridge many seamounts observed in the pacific ocean may result from this vast region of melting under the plate we suggest a linear relation between the width of melting zone and the plate velocity the laboratory results are also applied to other low velocity zones it is suggested that a velocity drop up to about 6 in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature this implies that partial melt may not exist generally but only in limited areas in the low velocity zone comparison of seismic results with theoretical models may overestimate the temperature and melt fraction if the velocity drop at subsolidus temperature is neglected temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data indicating that the upper mantle under mid ocean ridges may be fairly dry even a water or co2 undersaturated solidus 0 1 wt does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results it is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study unknown pressure frequency and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy

ultrafast upwelling bursting through the upper mantle a high resolution calculation of strongly time dependent thermal convection in the upper mantle with non newtonian temperature dependent rheology shows that for an effective rayleigh number of around 106 extremely fast upwellings at times exceeding 10 m yr can be generated a few hundred kilometers below the lithosphere there is a clear separation of timescales between this fast jet and the more slowly convecting mantle within this fast vertical shear layer is embedded a thermal boundary layer with a width of the order of 50 km the development of the fast non newtonian upwelling is characterized by the growth of the plume head to a large enough size before the plume takes off rapidly at a depth of around 350 km upon impinging the base of the lithosphere this fast plume thins the lithosphere and the flow then becomes a horizontally moving hot sheet extending out for around 1000 km this scenario is found to repeat itself at the same location about 10 myr after the first plume impingement

redistribution and geochemical behaviour of redox sensitive elements around s1 the most recent eastern mediterranean sapropel the youngest eastern mediterranean sapropel s1 is generally believed to have formed 7 9 14c ky b p the distribution of redox sensitive elements above in and below the sharply defined organic rich unit reveal that s 1 has already suffered heavy diagenetic alteration its present thickness based on its dark colour is only a fraction of its original thickness in one core visual evidence of s1 with an inferred initial thickness of 6 cm has been destroyed and a similar thickness appears to have been lost from the tops of another two examples this is caused by oxidation of the upper reaches of s 1 by a progressive oxidation front which commenced 5 2 5 4 14c ky ago and still appears to be active in the three cores studied diagenetic relocation of many redox sensitive elements has occurred most elements expected to be enriched in sapropels or sulphide rich sediments already have maximum concentrations above or below rather than in the present s 1 layers similarities are observed in the geochemical behaviour of as fe and p of co cu mn ni and zn and of cd mo sb ti u and v the behaviour of ba is distinctive and its immobility as barite makes it an indicator for palaeoproductivity and the original high s1 corg content the elements se and i exhibit particularly large sharp concentration peaks and together are proposed as useful indicators of the present boundary between oxic and post oxic conditions

middle proterozoic early palaeozoic evolution of central baltoscandian intracratonic basins evidence for asthenospheric diapirs abstract the three intracratonic sedimentary basins located in central baltoscandinavia namely the bothnian gulf basin the bothnian sea basin and the baltic basin developed in response to middle proterozoic and late proterozoic tectonic events separated in time by about 800 ma only the baltic basin was subsequently affected by caledonian orogenesis and mesozoic rifting crustal extension was minor or did not take place during the proterozoic basin evolution phases however according to the moho topography crustal thinning did take place this was probably a result of subcrustal magmatism on a craton wide scale the ages of granitoids which intruded during the middle proterozoic basin formation generally decrease from east to west this fact combined with the evidence provided by mantle derived flood basalt magmatism points to a moving asthenospheric diapir as the cause for basin development asthenospheric upwelling was probably also responsible for the second late proterozoic basin evolution phase as evidenced by the lack of crustal thinning and extension and the occurrence of tholeiitic intrusions in addition a late proterozoic thermally induced palaeo high located at about the position of the intracratonic basins is compatible with indications from glaciations as the ages of late proterozoic intracratonic basins also decrease from east to west across the craton the location of asthenospheric diapirism during this time interval was also moving for the fennoscandian lithosphere the presence of fundamental lithospheric weakness zones e g terrane boundaries might be an explanation for the formation of two generations of basins originating from asthenospheric upwelling at about the same location in the fennoscandian shield the spacing and size of the proterozoic intracratonic basins suggest that the asthenospheric diapirism was not deep seated therefore sublithospheric convective processes might be the cause for the asthenospheric upwellings such processes are related to rayleigh taylor instabilities in the sublithospheric mantle emplacement of an asthenospheric diapir causes a thermal bulge at the surface of the lithosphere modelling results demonstrate that erosion of the surficial high succeeded by cooling of the lithosphere can explain the accumulation of early palaeozoic sediments in the bothnian sea basin taking into account post ordovician vertical and lateral erosion of the basin fill

radiaxial calcite in lower miocene strata subsurface enewetak atoll abstract radiaxial calcite is abundant in lower miocene strata 377 819 m deep on enewetak atoll these radiaxial crystals have equant bladed and fibrous morphologies and occur as isopachous crusts 0 05 2 0 mm thick radiaxial terminations consist of fibrous subcrystals bundled together into larger acute obtuse or rounded shapes enewetak radiaxial calcite ranges from clear to very inclusion rich most inclusions are microcavities caused by organic infestations videtich 1985 alternating bands of inclusion rich and inclusion poor calcite mimic existing termination patterns radiaxial calcites fill intergranular pores sponge borings fractures and aragonite molds aragonite has been pervasively dissolved from the lower miocene strata containing radiaxial calcite banded radiaxial calcit can be traced from intergranular pores into aragonite molds radiaxial calcites have a mean magnesium concentration of 3 2 mole mgco3 with a range from 1 6 to 11 1 mole mgco3 inclusion rich bands and more deeply buried radiaxial calcite tend to have lower magnesium concentrations enewetak radiaxial calcite has 13c values of 1 3 to 2 5 pdb and 18o values of 1 8 to 0 4 pdb petrographic and stable isotopic data are consistent with precipitation of radiaxial calcite in its present form directly from seawater precipitation of radiaxial calcite after aragonite dissolution indicates precipitation distinctly after deposition probably in a burial environment dissolution of aragonite by deep marine water is suggested by 1 abundant marine cement associated with aragonite dissolution 2 a lack of meteoric cements associated with aragonite dissolution and 3 no evidence of subaerial exposure in lower miocene strata between 400 and 819 m modern pacific ocean water is undersaturated with respect to aragonite at depths where aragonite dissolution is observed 375 850 m tidal fluctuations and temperature profiles indicate that lower miocene strata containing radiaxial calcite are in open communication with modern pacific ocean water therefore aragonite dissolution and radiaxial cementation are interpreted as having occurred in seawater undersaturated with respect to aragonite circulating through the atoll variations in magnesium concentration in enewetak radiaxial calcite may be the result of original heterogeneity or differential loss of magnesium during later diagenesis in seawater the first alternative is preferred precipitation of radiaxial calcite at different rates at different degrees of saturation could cause variable magnesium concentrations more deeply buried radiaxial calcite may have precipitated more slowly in deeper less supersaturated water resulting in lower magnesium concentrations likewise slower rates of precipitation may have resultedin organic infestations and lower magnesium concentrations in inclusion rich bands enewetak radiaxial calcites suggest that two factors might be critical to development of radiaxial fabric 1 precipitation at fluctuati g rates and 2 formation in waters undersaturated with respect to aragonite

a lost tethyan evaporitic basin evidence from a cretaceous hemipelagic meta selenite red chert association in the eastern mediterranean realm

crustal accretion at high temperature spreading centres rheological control of crustal thickness abstract new determinations of lateral crustal thickness variations at anomalous oceanic spreading centres such as iceland have shown that the crust may be thinner at the ridge axis above the plume thickening towards the sides bjarnason and schmeling 2009 to understand this behaviour crustal accretion models have been carried out solving the conservation equations of mass momentum and energy with melting melt extraction and feedback of extracted material as newly formed crust for an extending lithosphere system underlain by a hot mantle plume the dynamics of rifting are thermally and rheologically controlled by the feedback due to accreted new crust four accretional modes with characteristic crustal thickness variations are identified depending on the width of the volcanic emplacement zone the accretional heating rate which can be associated with the thickness of the surface layer in which magmatic emplacement takes place and the spreading rate mode 1 zero crustal thickness at the spreading axis develops for cool accretion and a wide emplacement zone mode 2 strongly or moderately crustal thickening away from the axis develops in case of warm deep reaching accretion and wide emplacement zones mode 3 nearly constant crustal thickness develops in case of warm deep reaching accretion but narrow emplacement zones dynamic topography of mode 3 shows only a weak or no regional minimum at all near the axis modes 2 or 3 may be identified with the situation in iceland mode 4 a stagnating central crustal block evolves for cool accretion and narrow emplacement this mode disappears for increasing spreading rates no accretional mode with maximum crustal thickness above the plume at the rift axis has been found the absence of mode 1 accretion with zero crust at ridge axis on earth may be an indication that in general crustal accretion is not cold and shallow the model is also applied to other hotspot ridge settings azores galapagos and suggests modes 2 3 accretion

sector zoning in calcite cement crystals implications for trace element distributions in carbonates abstract compositional sector zoning in calcite cement crystals from a sedimentary limestone is represented by differences of mg content of over 1300 ppm or 95 and of sr and mn contents of up to 400 ppm or roughly 50 and 80 respectively between crystallographically nonequivalent time synchronous growth sectors 10 1 1 sectors are enriched with both mg and mn relative to 10 1 4 sectors and other sectors show both greater and lesser enrichment this systematic zoning pattern requires effective distribution coefficients for these trace elements to have been different for growth on different faces differential growth rate effects for nonequivalent faces are shown to be unimportant relative to differential growth face element preferences surface crystal structure and differing coordination environments are regarded as the principal factors influencing sector related elemental partitioning differences in calcite and dolomite

exploring the impact of diagenesis on isotope geochemical and microstructural alteration features in biogenic aragonite for the quaternary and neogene aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment conversely pre neogene aragonite is often diagenetically altered and calcite archives are studied instead nevertheless the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material here experiments were performed to understand biogenic aragonite alteration processes and products shell subsamples of the bivalve arctica islandica were exposed to hydrothermal alteration thermal boundary conditions were set at 100 degrees c 175 degrees c and 200 degrees c these comparably high temperatures were chosen to shorten experimental durations subsamples were exposed to different o 18 depleted fluids for durations between two and twenty weeks alteration was documented using x ray diffraction cathodoluminescence fluorescence and scanning electron microscopy as well as conventional and clumped isotope analyses experiments performed at 100 degrees c show redistribution and darkening of organic matter but lack evidence for diagenetic alteration except in delta 47 which show the effects of annealing processes at 175 degrees c valves undergo significant aragonite to calcite transformation and neomorphism the delta o 18 signature supports transformation via dissolution and reprecipitation but isotopic exchange is limited by fluid migration through the subsamples individual growth increments in these subsamples exhibit bright orange luminescence at 200 degrees c valves are fully transformed to calcite and exhibit purple blue luminescence with orange bands the delta o 18 and delta 47 signatures reveal exchange with the aqueous fluid whereas delta c 13 remains unaltered in all experiments indicating a carbonate buffered system clumped isotope temperatures in high temperature experiments show compositions in broad agreement with the measured temperature experimentally induced alteration patterns are comparable with individual features present in pleistocene shells this study represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits the limitations of a study restricted to a single test organism are acknowledged and call for refined follow up experiments

accretion tectonics of the neotethyan ermioni complex peloponessos greece late cretaceous early tertiary terrigenous and calcareous turbidites exposed in the argolis peninsula of southern greece represent an accretionary complex assembled during the closure of a late cretaceous neotethyan ocean massive sulphide bodies thrust slices of cherts and serpentinized ultramafics are found imbricated within a tectonic stack of approximately 8 km thickness basalts with overlying cenomanian maastrichtian pelagic limestones and metalliferous sediments are found distributed throughout the complex the basalts give trace element chemical signatures consistent with an origin at a mid ocean ridge or back arc basin folding and shearing is locally intense in fault zones 10 20 m across resulting in a block in matrix fabric metamorphism within the complex is of low to high diagenetic grade with grade generally increasing structurally up section palaeo slope indicators suggest that the ocean basin lay to the present se the ssw to nne emplacement is at variance with the sw vergent early tertiary thrusting of the hellenides and may be explicable in terms of transpressional tectonics along the southeastern termination of the pelagonian micro continent the processes which assembled the ermioni complex appear to have operated in a similar fashion to those seen along the margins of major ocean basins although active over much shorter periods of time

reconstructing paleoceanographic conditions during the middle ediacaran evidence from giant ooids in south china abstract the ediacaran was a period of intense environmental change in earth history transient pulses of oxygenation and shifts in ocean chemistry have been interpreted from a range of geochemical proxies including rare earth elements and yttrium rey but it is still a challenge to reconstruct paleoceanographic conditions due to the scarcity of reliable recorder in this study we explore the possibility that a newly discovered deposit of giant ooids in the nanshanping outcrop in south china contains a relatively intact record of the ree composition of ediacaran seawater providing an important window into the redox state of the surface ocean at this time these ooids are relatively large in size 1 6 mm and exhibit densely banded laminae a regional chemostratigraphic correlation shows that the giant ooid bearing succession in south china is in the upper part of the second ediacaran positive carbon isotopic shift ep2 and thus its occurrence was not associated with the shuram excursion additionally the occurrences of giant ooid bearing oolites in different blocks were not synchronous during the middle ediacaran in situ laser ablation analysis of giant ooid cortices reveals that the shale normalized rey patterns are similar to those of modern seawater including depleted light rees superchondritic y ho ratios and positive la anomalies yet no ce anomalies 1 0 0 15 in numerous ooid cortices seems to indicate a suboxidized condition of surface ocean in south china at that time the petrographic and geochemical evidence support the weak diagenetic alteration of the pure giant ooids during burial they formed in open ocean settings with energetic hydrodynamic environments and were affected by early dolomitization that might preserve mostly original petrographic fabrics and rey signatures from further diagenetic overprinting the characteristics of ce anomalies and rey compositions acquired from the giant ooids indicate that the surface ocean was suboxidized in south china during the late stage of ep2 implying the very low atmospheric o2 level at that time our results suggest that giant ooids may be a reliable analytical target that provide a snapshot into palaeoceanographic conditions during this crucial period of dynamic changes in early earth history

radiaxial fibrous calcites as low magnesian calcite cement precipitated in a marine meteoric mixing zone radiaxial fibrous calcite rfc has previously been interpreted as a marine or replacive cement study of the dongjeom formation early ordovician korea shows that rfc can form in marine meteoric mixing zones as a low magnesian calcite lmc cement rfc in the shallow marine dongjeom formation occurs in arenaceous limestones at the top of a transgressive facies overlying a regressive facies it shows well developed growth zonation and lighter oxygen isotope values and more radiogenic strontium isotope ratios than those of early ordovician marine calcite such petrographic and chemical evidence indicates that the rfc was precipitated as a primary lmc cement in a marine and meteoric mixing zone owing to the unique environment of formation the dongjeom rfc is characterized by growth zonal fabric comprising alternating subzones which may indicate precipitation from varying fluids in addition this study documents the importance of substrate for development of rfc early nucleation for rfc occurred mainly on microcrystalline skeletal grains and internal sediments whereas on homogeneously altered substrates thin coated banding structure developed ultimately forming coarse crystalline spar this suggests that microcrystalline substrates are preferred sites for nucleation of rfc

seismic wave dispersion and attenuation in aheim dunite an experimental study summary a machine has been developed which provides for the study of both shear modulus dispersion and internal friction in geological materials through the observation of forced torsional oscillations of low frequency 10 1000 mhz and strain amplitude 1000 c the eventual goal is an understanding of the dispersion and attenuation of seismic shear waves in the earth s upper mantle measurements have been made on cylindrical specimens of an olivine rich rock from aheim norway which contains in addition to olivine about 10 per cent pyroxene and 5 10 per cent of hydrous silicate phases dominantly clinochlore serpentine and talc in order to separate aspects of the mechanical behaviour associated with the olivine aggregate from those attributable to the hydrous phases and or their dehydration products the specimens were either previously fired under controlled oxygen fugacity at 1200 c for 24 hours in order to effect complete dehydration within the olivine stability field or simply oven dried at 110 c linearity of the mechanical behaviour has been demonstrated by the amplitude independence of the results for the strain amplitude range 10 8 10 6 and by the quantitative consistency between the observed modulus dispersion and that calculated from the measured internal friction through the kramers kronig relations of linear theory both the pressure dependence at room temperature and the temperature dependence at 300 mpa of the shear modulus and internal friction have been investigated much of the variation of the shear modulus is attributed to changes of crack porosity associated with changes in pressure and temperature and with in situ dehydration temporal evolution of q 1 towards a lower asymptotic value over periods of hours of exposure to given conditions of high pressure and temperature is tentatively attributed to the gradual diminution of enhanced anelastic relaxation associated with regions of decaying stress concentration at asperities on cracks and or grain boundaries at the highest pressures and temperatures achieved in this study 300 mpa and 1000 c marked dispersion of the shear modulus amounting to 5 per cent between periods of 3 and 100 s and concomitant strong internal friction varying with oscillation period to approximately as 0 011to1 6 are observed the energy dissipation appears to be concentrated within rather than at the boundaries between the olivine grains these results provide the clearest indication yet that solid state probably intragranular anelastic relaxation in ultramafic rocks gives rise to losses comparable with those observed seismologically in the earth s upper mantle although the mechanistic basis for the observed anelasticity remains to be established

mantle flow and the evolution of the lithosphere abstract the evolution of the lithosphere is mainly controlled by time dependent forces due to 1 plate tectonic processes and 2 sublithospheric mantle flow plate tectonic processes like continental collision may provide strong thermal disturbances and after completion may trigger secondary convection beneath the lithosphere without such mantle flows lateral variations of temperature and associated variations of lithospheric thickness hl or seismic velocities will equilibrate after time scales which are considerably shorter than the geologic ages of several provinces in europe we will discuss the role of sublithospheric mantle flows on the evolution of the lithosphere and the asthenosphere and compare the results with observations from the european lithosphere steady state convection models with a rheology based on laboratory data on lherzolite show that there exists a simple relationship between mantle heat flow and hl however steady state may be reached only after transition times of the order of 1 ga during such times hl shows strong lateral variations if such variations were inherited from plate tectonic events the role of sublithospheric convection would be to prolong their lifetime considerably at higher heat flows sublithospheric mantle convection becomes strongly time dependent with a time scale of 25 50 ma on this time scale variations of hl are small altogether we observe no strong direct correlation between upwelling mantle flows and thin lithosphere in some cases the correlation is even reversed on the other hand the presence or absence of a partially molten asthenosphere may be strongly affected by sublithospheric convection and may show strong lateral variations as well these variations in space and time can be correlated with the laterally heterogeneous lvz with seismic tomography data and with intraplate volcanism in europe mantle convection mantle diapirs and variations in thickness of the lithosphere may exert forces on the lithosphere and crust they will lead to bending of the lithosphere changes in surface topography and thinning or thickening of the crust induced by lateral flows within the lower crust depending on the time scale of loading and unloading of the lithosphere and the relaxation time associated with lower crustal flows characteristic relations between moho depth and surface topography are predicted comparison with observed correlations between moho depth surface topography and lithospheric thickness of european crustal age provinces show some trends in agreement with the proposed crustal deformation model

large scale waveform inversions of surface waves for lateral heterogeneity 2 application to surface waves in europe and the mediterranean linear surface wave scattering theory is used to reconstruct the lateral heterogeneity under europe and the mediterranean using surface wave data recorded with the network of autonomously recording seismographs nars the waveform inversion of the phase and the amplitude of the direct surface wave leads to a variance reduction of approximately 40 and results in phase velocity maps in the period ranges 30 40 s 40 60 s and 60 100 s a resolution analysis is performed in order to establish the lateral resolution of these inversions using the phase velocity perturbations of the three period bands a two layer model for the s velocity under europe and the mediterranean is constructed the s velocity perturbations in the deepest layer 100 200 km are much more pronounced than in the top layer 0 100 km which confirms that the low velocity zone exhibits pronounced lateral variations in both layers the s velocity is low under the western mediterranean while the s velocity is high under the scandinavian shield in the deepest layer a high s velocity region extends from greece under the adriatic to northern italy several interesting smaller features such as the massif central are reconstructed one of the spectacular features of the reconstructed models is a sharp transition in the layer between 100 and 200 km near the tornquist tesseyre zone this would indicate that there is a sharp transition at depth between central europe and the east european platform the waveform inversion of the surface wave coda leads to good waveform fits but the reconstructed models are chaotic this is due both to a lack of sufficient data for a good imaging of the surface wave energy on the heterogeneities and to an appreciable noise component in the surface wave coda

a benchmark comparison for mantle convection codes summary we have carried out a comparison study for a set of benchmark problems which are relevant for convection in the earth s mantle the cases comprise steady isoviscous convection variable viscosity convection and time dependent convection with internal heating we compare nusselt numbers velocity temperature heat flow topography and geoid data among the applied codes are finite difference finite element and spectral methods in a synthesis we give best estimates of the true solutions and ranges of uncertainty we recommend these data for the validation of convection codes in the future

use of clumped isotope thermometry to constrain the crystallization temperature of diagenetic calcite we describe an approach to estimating the crystallization temperatures of diagenetic calcites using clumped isotope thermometry a paleothermometer based on the 13 c 18 o bond enrichment in carbonates application of this thermometer to calcified gastropod shells and calcite cements in an early eocene limestone from the colorado plateau reveals a record of calcite precipitation and replacement at temperatures varying from 14 to 123 c the early eocene host sediments were never deeply buried but they experienced a significant thermal pulse associated with the emplacement of a late miocene basalt flow the combination of independent constraints on thermal history with clumped isotope thermometry petrographic including cathodoluminescence observations and oxygen isotopic data provides an improved basis for estimation of the temperature and timing of diagenetic events and fluid sources the petrography and calcite 18 o values taken alone suggest that the aragonite to calcite transformation of gastropod shell material occurred simultaneously with early formation of cements and lithification of the matrix in the same sample however addition of clumped isotope thermometry demonstrates that this phase transformation of shell material occurred at temperatures of 94 123 c in a highly rock buffered microenvironment i e with the isotopic composition of fluid buffered by coexisting carbonate millions of years after lithification of the matrix and formation of initial low temperature 14 19 c calcite cements within shell body cavities clumped isotope temperatures in excess of reasonable earth surface conditions recorded by later formed cements demand that cement growth occurred in association with the lava emplacement our results illustrate the potential for clumped isotope thermometry to constrain conditions of diagenesis and guide interpretations that would not be possible on the basis of conventional stable isotopic and petrographic data alone and demonstrate how petrographic characterization of clumped isotope thermometry samples can benefit paleoclimate studies

bottom water oxygenation history in southeastern arabian sea during the past 140 ka results from redox sensitive elements the concentrations of multiple redox sensitive elements such as re u mo cd v sb and tl were determined in sediments from the southeastern arabian sea 9 o 21 n 71 o 59 e to understand the bottom water oxygenation history throughout the past 140 ka the enrichment of redox sensitive elements re u cd and sb above average crustal abundances suggests that the last glacial maxima 17 48 ka stadials of marine isotope stage mis 5 5b and 5d and glacial termination gt ii 133 ka were associated with suboxic bottom water conditions sediments deposited during these suboxic conditions show the highest re content up to 54 ppb normalized to a carbonate free basis which is highly enriched over average continental crust 0 4ppb and these sediments appear to be the major sink for the global mass balance estimation marine isotope stages 1 3 4 and interstadials of mis 5 5a 5c 5e were all associated with near oxic conditions overall the lack of enrichment of mo and v above crustal abundance and a high re mo ppm ppm ratio avg 18 2x10 3 suggest that sediments of the southeastern arabian sea never contained free h2s during the last 140 ka these changes in the bottom water oxygen content can be related to the oceanic circulation pattern during this time and in part are reflected in relationships between the timing of redox changes and paleoproductivity proxies

marine red staining of a pennsylvanian carbonate slope environmental and oceanographic significance abstract red stained platform facies are a common feature of many carbonate settings throughout the geological record although the mechanisms involved in red staining of subaerially exposed or argillaceous peri platform limestones are reasonably well understood the environmental and oceanographic significance of red carbonates often remains uncertain here sedimentological sequence stratigraphic geochemical paleontological and quantitative bathymetric data from pennsylvanian red intervals across a well exposed carbonate platform top and slope are documented and interpreted in a process oriented context on the upper slope 80 350 meters below the shelf break red intervals alternate with gray mainly microbial algal boundstones on the lower slope 350 600 meters below the shelf break redeposited red stained mud builds matrix supported breccia tongues interbedded with predominantly redeposited clast supported carbonate debris the presence of large volumes of fibrous calcite biocementstones as well as firmgrounds point to low sedimentation rates or omission in terms of sequence stratigraphy red intervals occur within maximum flooding intervals and reflect near platform drowning elevated 18o values 2 3 and an essentially cool water heterotrophic biotic association in red intervals on platform top and slope suggest deposition during sea level highs associated with colder water masses and high nutrient levels the most likely drivers are an elevated thermocline and upwelling the red staining is the result of iron oxidation which occurred during early diagenesis likely by iron bacteria these red intervals provide an important bathymetric benchmark against which other mainly paleozoic red facies can be tested and calibrated

rare earth and trace elements of microbialites in upper jurassic coral and sponge microbialite reefs major trace y zr pb th and u rare earth element ree and sm nd isotope analyses were carried out on microbialites and detrital carbonate sediments collected in upper jurassic coral and sponge microbialite reef settings selected bioconstructions developed in various palaeogeographic settings and depositional environments on northwestern tethys and eastern atlantic margins pure carbonate lagoon carbonate dominated deep shelf to epicontinental basin and mixed carbonate siliciclastic shallow ramp and lagoon the proximity to terrigenous sources directly controlled ree patterns of carbonate samples lateral and intrareef sediments and to a lesser degree stromatolitic microbial crusts are more susceptible to contamination by terrigenous material than are microbialites of thrombolitic fabric in mixed carbonate siliciclastic settings microbialites display a flat shale normalized ree pattern in pure carbonate lagoonal settings microbialites have shale normalized ree patterns similar to those of modern seawaters with a negative ce anomaly ce ce up to 0 14 positive la and gd anomalies and light ree depletion relative to heavy ree these data imply that similar ree chemistry than modern seawater already occurred on the northwestern tethys margin at the upper jurassic strong negative ce anomalies b0 4 imply microbialite formation in well oxygenated seawaters both in coral reefs of pure carbonate lagoonal environments pagny sur meuse and sponge bioherms of deep shelf settings plettenberg absence of positive ce anomalies rules out an influence of alkaline and high ph seawaters thus ree patterns and nd isotopic signatures which are in perfect agreement with existing results for tethysian seawater confirm that microbialites can be used as proxies of palaeoenvironmental seawater chemical characteristics

depositional facies and stratal geometry of an upper carboniferous prograding and aggrading high relief carbonate platform cantabrian mountains n spain seismic scale continuous exposures of an upper carboniferous bashkirian moscovian carbonate platform n spain provide detailed information about the lithofacies and stratal geometries quantified with differential global positioning system measurements of microbial boundstone dominated steep prograding and aggrading platform margins progradational and aggradational platform to slope transects are characterized by distinct lithological features and stratal patterns that can be applied to the understanding of geometrically comparable high relief depositional systems the bashkirian is characterized by rapid progradation at rates of 415 970 m my 1 characteristic outerplatform facies are high energy grainstones with coated intraclasts ooids and pisoids moderate energy algal skeletal grainstones to packstones and lower energy algal packstone and boundstone units the moscovian aggradational phase is characterized by aggradation rates of 108 m my 1 coated grain shoals are less common whereas crinoidal bars nucleated in well circulated settings below wave base boundstones form a belt 30 300 m wide at the platform break and interfinger inwards with massive algal skeletal wackestones mud rich banks the progradational phase has divergent outer platform strata with basinward dips of 12 to 2 steep clinoforms with dips of 20 28 are 650 750 m in relief and possibly sigmoidal to concave in the lower part the basinward dipping outer platform strata might be depositional for less than 6 consistent with lithofacies deepening seaward the basinward dip is attributed to the downward shift of upperslope boundstone forced by late highstand and relative sea level fall and to compaction induced differential subsidence during progradation the aggradational phase is characterized by horizontally layered platform strata clinoforms steepen to 30 45 reaching heights of 850 m and are planar to concave the evolution from progradation to aggradation at the bashkirian moscovian boundary is attributed to increased foreland basin subsidence and decreased boundstone accumulation rates progradation was primarily controlled by boundstone growth rather than by highstand shedding from the platform top within the major phases aggradational progradational increments are produced by third to fourth order relative sea level

application of the ce anomaly as a paleoredox indicator the ground rules much attention has been paid in recent years to the potential application of the ce anomaly measured in various marine phases as a paleoceanographic indicator of widespread marine anoxia in this paper we present and discuss results from recent studies of present day rare earth element ree distributions and hence ce anomaly distributions in the marine environment which are particularly pertinent to paleoceanography subsequently we review and discuss the validity of the recent literature in which ce anomalies have been employed as paleoredox indicators

the influence of second scale convection on the thickness of continental lithosphere and crust abstract the effect of sublithospheric convection on the thickness and structure of the continental lithosphere is studied in numerical models assuming different rheologies newtonian non newtonian and temperature and pressure dependent heat fluxes and heating modes bottom versus internal heating the stagnant regions near the top of the models are identified with the lithosphere we distinguish a seismic and thermal lithosphere controlled by temperature an elastic lithosphere controlled by viscosity and a mechanical lithosphere controlled by strain rate strong lateral thickness undulations with a thick lithosphere above the downwelling regions and a thin lithosphere above the upwelling regions develop the thickness of the elastic lithosphere is about two thirds of the mechanical lithosphere while the ratio of the elastic to the thermal lithosphere varies between 0 4 and 0 8 the time dependence of some models with internal heating and a surface heat flow of 20 mw m 2 is characterized by long periods of the order of 1 ga close to steady state and short periods 100 ma of changing cell patterns and lithospheric thicknesses models showing thick lithospheric roots suggest that the mantle beneath some old shields may be associated with cold slowly downwelling convective flow rather than being firmly attached to the lithosphere the missing gravity and topography signals from such regions can be explained by elastic bending of the lithosphere and by dynamic adjustment of the moho observed lithospheric thickness variations in europe and their minimum to maximum spacing agree well with the models suggesting sublithospheric convection to be the cause compared to other observed features thickness undulations of the lithosphere seem to provide the strongest indications for sublithospherical convection relatively low viscosities in the lower crust may lead to a long term decoupling of surface topography from moho topography and mantle dynamics the moho depth is capable of adjusting to mantle stresses within relaxation times of the order of 100 ma while surface topography may flatten out such a mechanism may be important in regions with a deep moho such as fennoscandia or with a shallow moho such as the pannonian basin neither region showing a pronounced surface topography

accurate trace element analysis of speleothems and biogenic calcium carbonates by la icp ms the high spatial resolution of laser ablation inductively coupled plasma mass spectrometry la icp ms microanalysis of speleothems and biogenic calcium carbonates makes it possible to improve our understanding of past climatic conditions however there are analytical problems associated with this method such as molecular interferences elemental fractionation and mass load dependent matrix effects when using nist silicate reference glasses for calibration our study with a sector field icp ms shows that many masses are affected by interferences such as 24mg by 48ca or 31p by 15n16o elemental fractionation and mass load dependent matrix effects have been detected for both 213 and 193 nm nd yag lasers they are small for refractory lithophile elements in particular for the 193 nm laser 5 for chalcophile siderophile trace elements with low boiling points however these effects are large up to 40 and 20 for the 213 and 193 nm lasers respectively and variable for the nist glasses and carbonates in this paper a protocol for precise and accurate la icp ms analysis of calcium carbonates is established isotopes for interference free measurements at low m m 300 and medium m m 4000 mass resolution have been identified the nist glasses are suitable for calibration of refractory lithophile element concentrations however matrix matched calibration with carbonate reference materials such as usgs macs 1 or macs 3 is necessary for accurate analysis of chalcophile siderophile elements with low boiling points analytical reproducibility rsd is a factor of 2 better using the 193 nm laser than the 213 nm laser as shown by the analysis of macs 1 and macs 3 la icp ms analyses yield results that agree with the reference values within relative uncertainties of ca 5 10 at the 95 confidence level we have applied la icp ms for the determination of trace element concentrations in calcite and aragonite layers of a stalagmite and found large variations for mg zn sr and u in ostracod shells the concentrations of some trace elements e g sr and ba vary significantly indicating the potential for paleoclimate research

variable viscosity convection and partial melting in the continental asthenosphere convection beneath continents may be associated with the generation of partially molten zones at depth experimentally determined non newtonian temperature and pressure dependent creep laws for olivine were used to model upper mantle convection in the presence and absence of a melt phase in our models convection is chaotic and exhibits a strong time dependence using model dry and water undersaturated solidus relations partially molten regions develop at asthenospheric depths forming low viscosity regions the existence of these regions is linked to convection and consequently varies in space and time with an episodicity of 5 to 10 m y in good agreement with observations based on seismic tomography and the episodicity and volumetric abundance of surface volcanics for an average continental heat flux of about 53 3 mw m2 maximum melt fractions range between 0 and 2 dry solidus and 2 and 4 water undersaturated solidus on the basis of experimental deformation studies we employed an empirical relationship between stress and grain size to assess the grain size variations possible in the upper mantle the grain sizes obtained millimeters to centimeters are in good agreement with grain sizes observed in natural examples the smallest grain sizes occur along narrow zones of high stress within the deepest part of the lithosphere the largest grain sizes are found in the overlying lithosphere and in partially molten asthenospheric regions as our models scale well to natural observations these results appear to preclude diffusion creep n 1 as a viable upper mantle steady state deformation mechanism

synsedimentary diagenesis in a cryogenian reef complex ubiquitous marine dolomite precipitation abstract analysis of the synsedimentary diagenetic phases from a pervasively dolomitised cryogenian 650 ma reef complex south australia reveals a fundamentally different style of marine diagenesis to that of phanerozoic carbonates textural evidence from dolomitised and undolomitised lithologies of the oodnaminta reef complex indicates that depositional components were precipitated initially as calcium carbonate aragonite and calcite dissolution textures and preserved fabrics in ooids indicate an originally aragonitic mineralogy early fibrous marine cements that have well preserved fibrous crystallographic fabrics with a length fast optical character are likely to have precipitated as high magnesium calcite however following this very early phase of diagenesis the dominant marine process occurring in these reefs was dolomite precipitation this involved the dolomitisation of earlier depositional and diagenetic phases as well as the precipitation of fibrous dolomite cements these fibrous marine cements include the newly described radial slow fascicular slow radiaxial slow and rhombic dolomite cements the cements have optical properties chemical zonation and cathodoluminescent characteristics indicating that they were direct marine precipitates dolomite precipitation during marine diagenesis in this reef complex suggests that the oceans of the cryogenian were chemically different to those of the phanerozoic abundant dolomite formation appears to be linked to anoxic magnesium rich seawater during this time marine dolomite precipitation under these conditions may explain the abundance of dolomite in the precambrian sedimentary record

diffusion in solid earth systems recent years have seen a rapid expansion in the acquisition and use of information on diffusive transport in phases relevant to the solid earth crystals melts and fluids although far from complete the data base on diffusion coefficients is now sufficiently large that broad constraints can be placed upon the length and time scales of many natural transport phenomena in which diffusion plays a role conversely observations of diffusion progress in specific natural samples can be used to extract time temperature information for a variety of geologic and geochemical processes ranging from sediment burial and crustal erosion to fluidmediated reactions and biosignature retention despite this undeniable progress several major challenges remain that largely define the frontiers of research in solid earth diffusion perhaps foremost among these is the need to address and understand the multi scale multi path aspects of diffusion in many systems a complication that is not limited to polyphase materials individual mineral grains can exhibit clear indications of multi path behavior even when visible evidence of such paths is lacking many other diffusion frontiers are linked in one way or another to this multi scale issue they include diffusion of molecular h2o and the effect of h species on diffusion in minerals and rocks diffusive fractionation of multiple isotopes of a single element diffusion at the extreme conditions of the deep earth reconciliation of observations from natural samples and laboratory studies and development of theoretical approaches to predict

crustal accretion and dynamic feedback on mantle melting of a ridge centred plume the iceland case abstract a major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation in the case of iceland crustal thickness as large as 20 to 40 km has been reported crustal seismic velocities are high and have to be explained by thermal or chemical effects in the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting to study this question we solve the convection equations for a ridge centred plume with non newtonian rheology including melting melt extraction associated with deposition of cold crust at the surface of the model and using a simplified approach for compaction the assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km depending on the buoyancy flux of the plume crustal thicknesses between 10 and 40 km are modelled showing characteristic dipping structures resembling the rift ward dipping basaltic layers of east and western iceland comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg s based on the observed crustal thickness of iceland our models predict a plume buoyancy flux of 1140 kg s in the second part we study the accretion of the icelandic crust by a thermo mechanical model in more detail based on the navier stokes the heat transport and the mass conservation equations including volumetric sources hot 1200 c molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions a deep widespread emplacement of dykes and sills including crustal underplating b magma chambers at shallow to mid crustal level and c surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres we identify the material from the different source regions by a marker approach varying the relative dominance of these source regions characteristic crustal structures evolve showing shallow dipping upper crustal layers with dip angles between 10 and 15 the thermal structure of the crust varies between cold crust shallow source region dominating and hot crust deep source region dominating we use observations of maximum depth of seismicity to constrain the depth of the 650 c isotherm and seismological inferences on the lower crust to constrain temperatures in that region the best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion

ree3 and mn2 activated cathodoluminescence in lateglacial and holocene stalagmites of central europe evidence for climatic processes combined visual cathodoluminescence cl and spectral analyses of cl reveals periodic enrichments of rare earth elements ree3 and manganese mn2 within the laminations of eight calcitic lateglacial to postglacial stalagmites in the annual layers the enrichment of trace elements can be correlated with the autumn winter laminae which are strongly pigmented and rich in organic carbon during the holocene they occur especially in the atlantic stage and in subrecent recent times the enrichment of ree3 and mn reflects times of more intense weathering which presumably prevailed during the atlantic warm and humid climate in subrecent recent times especially the last 100 years these enrichments may have been at least partially anthropogenically induced

first description of phanerozoic radiaxial fibrous dolomite abstract the petrographic analysis and crystallographic analysis of concretionary carbonate cements coal balls from carboniferous paralic swamp deposits reveal the presence of length fast radiaxial fibrous dolomite rfd a fabric not previously reported from the phanerozoic this finding is of significance as earlier reports of phanerozoic radiaxial fibrous carbonates are exclusively of calcite mineralogy dolomite concretions described here formed beneath marine transgressive intervals within palustrine coal seams this is of significance as seawater was arguably the main source of mg2 ions for dolomite formation here data from optical microscopy cathodoluminescence electron backscattered diffraction x ray diffraction and geochemical analyses are presented to characterize three paragenetic dolomite phases and one calcite phase in these concretions the main focus is on the earliest diagenetic non stoichiometric degree of order 0 41 0 46 phase i characterized by botryoidal dolomite constructed of fibres up to 110 m wide with a systematic undulatory extinction and converging crystal axes petrographic and crystallographic evidence clearly qualifies phase i dolomite as radiaxial fibrous conversely fascicular optical fabrics were not found carbon isotope ratios 13c are depleted between 11 8 and 22 1 as expected for carbonate precipitation from marine pore fluids in organic matter rich paralic sediment oxygen isotope 18o ratios range between 1 3 and 6 0 the earliest diagenetic nature of these cements is documented by the presence of ubiquitous non compacted fossil plant remains encased in phase i dolomite as well as by the complex zoned luminescence patterns in the crystals and is supported by crystallographic and thermodynamic considerations it is argued that organic matter and specifically carboxyl groups reduced thermodynamic barriers for dolomite formation and facilitated mg caco3 precipitation the data shown here reveal a hitherto unknown level of complexity with respect to radiaxial fibrous carbonates and are of importance for those concerned with dolomite and carbonate petrography in general

the influence of olivine metastability on the dynamics of subduction abstract both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine mo dynamically consistent models of subducting slabs in which the kinetics of the olivine to spinel wadsleyite or ringwoodite transition is treated in a simplified way are carried out to evaluate the buoyancy effect of mo on the subduction velocity assuming slab thicknesses according to the cooling half space model we find that mo forms in slabs with ages greater than approximately 70 myr and that a significant slowing down of subduction velocity occurs for ages greater than 100 myr because a decrease in the subduction velocity for old lithospheric ages is not observed our models suggest that the amount of mo in slabs is less than approximately 5000 km2 in cross section and that for old lithosphere the cooling half space model is not applicable

isotope stratigraphy of the european carboniferous proxy signals for ocean chemistry climate and tectonics abstract carboniferous brachiopods from western europe and the former ussr have been utilized as carriers of isotopic proxy signals for paleoceanography and paleoclimatology during the assemblage of pangea this is of particular interest because this time interval coincides with the transition from greenhouse to icehouse conditions the 18 o values 3 ma running means show an overall 18 o increase from about 7 1 to 3 1 during the carboniferous the rise in the 18 o is stepwise with major shifts in the mid tournaisian and at the visean serpukhovian transition the comparable 13 c record shows a tournaisian peak at 5 a visean oscillation at about 2 5 1 a serpukhovian rise of more than 3 a plateau around 5 during the bashkirian and moscovian a 4 drop during the kasimovian and another rise of about 3 during the gzhelian the 87 sr 86 sr record shows an early carboniferous decline from 0 7083 to a mid visean minimum of 0 7077 followed by a rapid serpukhovian rise to a bashkirian and moscovian plateau at 0 7087 and a slight decline during the kasimovian and gzhelian the covariance of isotopic trends and the coincidence with geological phenomena indicates that the long term variations in ocean chemistry and climate may be coupled to the tectonic evolution of the earth system the namurian witnesses the onset of hercynian orogeny initiation of the main phase of carboniferous glaciation formation of the extensive coal deposits and the rise in 87 sr 86 sr 13 c and 18 o all this suggests that a tectonically controlled climate change resulting in cooling and increased carbon storage in the organic reservoir was the major triggering factor for all the above phenomena the late carboniferous glacial episode was interrupted by a warm interval in the stephanian the carbon and oxygen isotope data indicate that this episode may have been restricted to the early kasimovian a proposition at odds with the poorly dated record of glacial deposits that are interpreted as indicating a warm climate for the entire stephanian in addition the pattern of short lived peaks in 13 c and 18 o in the tournaisian and possibly visean may be interpreted as a reflection of short lived glacial episodes a proposition consistent with geologic records

partial melt below iceland a combined interpretation of seismic and conductivity data a combined interpretation of published seismic and electric data is carried out to impose constraints on the state of melting below central iceland these data indicate high electrical conductivity and low vp velocity regionally below 6 km s for the crust mantle transition region while the mantle below shows lower conductivity but still anomalous low velocity the velocity data are treated by utilizing gravity data to quantify the elastic modulus decrease due to partial melt a comprehensive set of theoretical models on elasticity anelasticity and electrical conductivity of partially molten rock is applied to this data set the geometry the fraction and the interconnection of the melt phase are varied systematically the conductivity models are based on published laboratory data on conductivity of solid and melt phases of rocks anisotropy and possible unexplored conductivity mechanisms near the melting temperature are not considered here by means of the melt models the data can be explained most easily as follows in a depth range roughly between 20 and 50 km only a small percentage of melt 2 at the most 5 if melt pockets are also present occurs in thin films possibly with a distribution of aspect ratios and a low degree of interconnection if in the crust mantle transition region zones of low p wave velocity 6 km s are correlated with those of high conductivity 0 08 s m melt fractions between 10 and 20 are likely and the degree of interconnection should be high thin films can be excluded as a dominant melt geometry in that region apart from improved constraints on melt geometry the results are in good agreement with published models based only on conductivity data however they disagree with published models based entirely on seismic data the discrepancy is now resolved by the combined interpretation

fractures in mudrocks advances in constraining timing and understanding mechanisms abstract recent advances in fracture timing techniques have led to new insights about sequences and mechanisms of fracture formation in mudrocks methods to constrain fracture timing in mudrocks include i field evidence including crosscutting relationships and evidence of compaction ii petrographic evidence including signs of soft sediment deformation and diagnostic mineral assemblages or textures iii stable isotopic evidence iv thermometric dating using burial history modelling and v radiometric age dating in this contribution we briefly review each method with salient examples from the literature many documented examples of mudrock hosted fractures formed during early burial a non intuitive finding that demands a re assessment of our assumptions about what types of materials can develop opening mode fractures in general the style and distribution of fractures varies dramatically based on fracture timing highlighting the importance of constraining timing for predicting fracture patterns and interpreting their mechanisms including the role of fluid flow

deep temperatures and lithospheric thickness along the european geotraverse abstract one dimensional steady state heat conduction models in the crust and upper mantle for 24 areas along the european geotraverse egt show that temperatures are higher beneath young geological regions at the moho they range from 450 to 825 c heat flowing out from the mantle varies from 18 to 72 mw m2 and the contribution of crustal heat flow density evaluated by the correlation between seismic velocity and the abundance of radioactive elements accounts for about 48 of the surface heat flow density the temperature gradient below moho from about 5 to 23 mk m is low where the surface heat flow density reaches its minimum values the depth at which the geotherms reach 0 85 of the mantle melting temperature is greater in the precambrian and caledonian areas with a maximum value of about 195 km under the early proterozoic scandinavian shield in the hercynian and alpine areas this depth is generally lower higher values maximum 150 km coincide with the orogenic belts and the continental block of sardinia and corsica under the ligurian sea and the sardinian channel the lithospheric thickness decreases remarkably to about 50 km globally the lithosphere thickness is controlled by the mantle heat flow density which decreases according to a power law with an increase in the lithosphere asthenosphere depth

low limit of mn2 activated cathodoluminescence of calcite state of the art in the literature the lower limit for mn2 activated cathodoluminescence cl of calcite is variously reputed to over a very wide range of values above 10 ppm mn our spectroscopic investigations of the cl response in natural calcite reveal that below 10 ppm manganese content mn2 activation is also present using the quantitative high resolution spectral analysis of cl qhrs cl an activation by mn2 in the range of 700 ppb is proved which cannot be determined visually so if not quenched the minimum mn2 content for mn2 activation is one atom in the irradiated calcite crystal lattice volume as the intrinsic background blue luminescence is used to determine non altered biogenic calcite the limit of mn2 activation plays an important role in the interpretation of diagenetic processes our results of spectroscopic analyses require a revision of current opinions about the diagenesis of calcite as revealed by cl investigation

on the lehmann discontinuity the lehmann discontinuity 200km in the upper mantle is proposed to result from a change in the preferred orientation of olivine as a result of a change in deformation mechanism in the shallow upper mantle deformation is due to dislocation creep and results in an anisotropic structure whereas in the deeper upper mantle diffusion creep becomes dominant due to a smaller activation volume resulting in an isotropic structure for anisotropy due to dislocation glide corresponding to a horizontal flow the resultant anisotropy will yield vsh vsv and vph vpv thus for waves propagating nearly vertically e g scs waves there will be discontinuous jumps in the s and p wave velocities at this discontinuity in this model the depth of the lehmann discontinuity depends on the deformation conditions such as stress level geotherm water fugacity and grain size giving a natural explanation for its observed variation with tectonic setting

crystal habit geochemistry and cathodoluminescence of magnesian calcite marine cements from the lower slope of little bahama bank the petrography and geochemistry of marine magnesian calcite cements recovered from the sea floor on the lower thermocline lower slope of little bahama bank address three issues concerning inorganic carbonates 1 crystal habit of marine magnesian calcite cements 2 cation composition of marine precipitates and 3 origins of cathodoluminescence in calcite bladed and blocky limpid spar cements formed contemporaneously within the same intraparticle pore as shown by banded cathodoluminescence traced without interruption through both cement habits bladed and blocky cements exhibit a positively covariant linear relationship for mg and sr having low proximal early formed compositions and high distal late formed compositions linear covariance of mg and sr is interpreted to indicate that the concentrations of these cations were controlled by kinetic effects as they formed with early precipitation occurring more rapidly than later precipitation precipitation rates apparently did not affect cement habit limpid isopachous bladed spar cements the distal third of which are brightly cathodoluminescent have a wide range of mg and sr compositions which is almost completely spanned by both distal cathodoluminescent and proximal noncathodoluminescent parts of cements occurring within the same pore mg and sr do not have a linear relationship these cements are interpreted to have recrystallized in deep cool sea water during recrystallization magnesium was lost and in some cases manganese the principal cathodoluminescence excitor was presumably gained

deep water basins within the mesozoic carbonate platform of argolis greece in the past the classic argolis area of the central southern hellenides was interpreted as comprised of essentially in situ carbonate platforms and basins or alternatively of far travelled thrust sheets derived from a mesozoic ocean basin sited well to the east vardar ocean our fieldwork however demonstrates that at two key localities an important unit of middle triassic early jurassic deep water redeposited carbonates asklipion unit passes laterally into shallow water platform carbonates pantakrator unit the deep water limestones represent basins within the platform rather than far travelled thrust sheets while other slices of deep water limestone record a platform margin further east the basins originated during middle triassic ladinian rifting of the neotethys and are floored by extrusive rocks of intermediate composition highly condensed ammonitico rosso pelagic limestones accumulated on an inferred volcanic seamount during the mid late triassic anisian carnian the platforms slowly subsided in early jurassic time while the basins partly filled with redeposited carbonates adhami limestone shallow water carbonates locally transgressed the margins of the deeper water basins the platforms were submerged in the mid jurassic toarcian bathonian with the deposition of pink pelagic ammonitico rosso followed by marked subsidence and ribbon radiolarite deposition below the carbonate compensation depth e g koliaki chert in early late jurassic time kimmeridgian the area was regionally compressed eohellenic phase and the intraplatform basins founded on thinned continental crust were inverted and thrust up to several kilometres over the adjacent platform units the deformed continental margin then flexurally subsided ahead of emplacing jurassic ophiolites largely olistostromes migdhalitsa unit after taking account of up to 90 degrees clockwise palaeorotation of assumed neotectonic age kinematic indicators and facies trends suggest the ophiolite was derived from a pindos ocean basin originally sited to the west early cretaceous transgression resulted in the deposition of shallow water limestone and was followed by late cretaceous deep water limestone then terrigenous flysch accumulation in a foreland basin related to final suturing of the neotethyan ocean later the area was dissected by a series of mainly e w trending high angle neotectonic normal faults related to the aegean back arc extensional system

the compaction of igneous and sedimentary rocks in many problems of interest to geology and soil mechanics a fluid moves through a solid matrix which is also being deformed in most cases of interest the effective viscosity of the matrix is many orders of magnitude greater than that of the percolating fluid and both form interconnected networks in three dimensions the partial differential equations governing such behaviour can now be obtained and a variety of simple model problems have been investigated some of the more surprising solutions contain solitary waves and compaction fronts some of the less exotic solutions are presently of more geological interest the mobility of volatile rich melts at melt fractions as small as 0 1 has important consequences for trace element geochemistry the geometry of layered intrusives appears to result from differential compaction of the crystal mush and the rates required can be used to estimate a viscosity of 3 x 10 18 pa s for an olivine matrix this value agrees excellently with laboratory experiments when proper account is taken of the grain size dependence compaction in sedimentary rocks results in overpressure when the porosity becomes sufficiently small and can also lead to the development of secondary porosity