berthierine chamosite corrensite and discrete chlorite from evolved verdine and evaporite associated facies in the jurassic sundance formation wyoming late jurassic sandstones of the shallow marine sundance formation contain authigenic chlorite minerals that occur as rosette like pore fillings of interstratified berthierine chamosite b c and honeycomb like pore linings of corrensite and discrete chlorite b c is nearly ubiquitous in sundance sandstones but is absent near the top of the formation whereas corrensite and discrete chlorite were detected only in uppermost sundance sandstones within 4 m of the contact with the overlying non marine morrison formation glauconite grains are common and occur as laminae along bedding planes and cross beds indicating reworking and deposition as clasts in tidally influenced regimes the mineralogical chemical and morphological properties of the b c and corrensite indicate that they are authigenic and formed during burial diagenesis from precursor minerals odinite in the case of b c and saponite in the case of corrensite and discrete chlorite odinite has been recognized in numerous shallow marine sands of the holocene verdine facies and the shallow marine conditions associated with sundance deposition would have been ideal for odinite formation saponite commonly forms in aeolian and evaporitic environments implying that the saponite precursor to corrensite and discrete chlorite formed in uppermost sundance sands that were exposed to an influx of oxidizing groundwater following regression of the sundance sea x ray diffraction xrd indicates that the proportion of 7 a b layers in b c ranges from 5 to 28 and chemical analysis by scanning electron microscope energy dispersive x ray spectrometry sem eds indicates positive correlation between b and fe fe mg the polytype of the b c is ibb and the corrensite and discrete chlorite are disordered iib these are the expected polytypes and b for sandstones exposed to burial diagenetic conditions of 3000 4000 m and temperatures 90 120 c

unsolved problems in the lowermost mantle 1 many characteristics of d layer may be attributed to the recently discovered mgsio3 post perovskite phase without chemical heterogeneities they include a sharp discontinuity at the top of d regional variation in seismic anisotropy and a steep clapeyron slope however some features remain unexplained the seismically inferred velocity jump is too large in comparison to first principles calculations and the sharpness of the discontinuity may require a chemical boundary chemical heterogeneity may play an important role in addition to the phase transformation from perovskite to post perovskite phase transformation and chemical heterogeneity and the attendant changes in physical properties such as rheology and thermal conductivity are likely to play competing roles in defining the dynamical stability of the d layer revealing the relative roles between phase transition and chemical anomalies is an outstanding challenge in the study of the role of d in thermal chemical evolution of the earth

de glauconiarum origine the glauconitic facies is widespread on present day continental shelves from 50 s to 65 n and at water depths between 50 and 500 m and is in particularly great abundance on the upper slope and outer shelf between 200 and 300 m it is also common in many ancient rocks of post late precambrian age it occurs as sand to pebble sized essentially green particles granular facies but also as a surface coating on particles and hardgrounds and as a diffuse impregnation film and diffuse facies we suggest the replacement of the term glauconite which has been interchangeably used to designate a morphological form and a specific mineral by glaucony facies and glauconitic smectite and glauconitic mica as end members of the glauconitic mineral family the widely accepted model of burst and hower for glauconitization requires a degraded micaceous 2 1 layer lattice structure parent clay mineral however detailed analysis of numerous samples of recent glaucony reveals that such a parent substrate is exceptional the model therefore requires modification generally the parent material is carbonate particles argillaceous kaolinitic faecal pellets infillings of foraminiferal tests various mineral grains and rock fragments that pass gradually into the commonly occurring green grains we show that the process of glauconitization is achieved by de novo authigenic growth of automorphous crystallites in the pores of the substrate accompanied by progressive alteration and replacement of the substrate it is this two fold evolution that causes the verdissement of granular substrates macrofossils and hardgrounds the authigenic mineral is an iron rich and potassium poor glauconitic smectite while new smectites are growing into the remaining pore space the earlier smectites are modified by incorporation of potassium producing decreasingly expandable minerals with a non expandable glauconitic mica as the end member this mineralogical diversity of the glauconitic mineral family explains the highly variable physical and chemical properties of glaucony four categories nascent little evolved evolved and highly evolved glaucony are distinguished glauconitization appears to be controlled by a delicate balance between degree of physical confinement of a particle and the amount of ionic exchange between the micro environment and ambient open marine sea water the optimum conditions for glauconitization are those of semi confinement as a result the interior of a grain is more glauconitized than its less confined periphery similarly for identical substrate types large grains 500 m provide more favourable substrates for glauconitization than lesser confined small grains on a larger scale the formation of glaucony is governed by the availability of iron and potassium and the balance between detrital influx and winnowing low accumulation rates expose grains to the open marine environment for sufficiently long times 105 106 years for highly evolved glaucony

stress regime strain release and geothermal implicationsregional stress field and rifting in western europe in situ stress determination is one possibility to determine stress magnitudes in the earth s crust most measurements especially in the northern alpine foreland have been carried out by using the doorstopper technique stress magnitudes are presented as excess stresses a number of sites where stresses have been determined gave sufficiently high values to statistically determine stress directions for the following tectonic blocks 1 1 western alps 2 2 their immediate foreland with the rhinegraben and 3 3 the rhenish shield bordering on the rift segment of the lower rhine embayment in discussing results of the statistical analyses of stress data it is suggested that stresses measured in the western alps have mainly a topographic origin in the alpine foreland to the north of the rhenish shield topographic stresses are superimposed on active tectonic stress considering the structural and lithological differences the foreland blocks show a different response to the same tectonic stress field which trends for 1 at about 146 this active stress field causes shear rifting in the upper rhinegraben but extensional rifting in the lower rhine embayment in the rhenish shield area only local step faults are observed in direction of 1 due to the ductile behaviour of its basement

rheology of the earth s mantle a historical review abstract historical development of our understanding of rheological properties of the earth s mantle is reviewed rheological properties of the earth s mantle control most of the important geological processes such as the style of mantle convection e g stagnant lid versus plate tectonics and the nature of thermal evolution however inferring the rheological properties of the earth s mantle is challenging because of the presence of multiple mechanisms of deformation that have different dependence on time scale strain rate stress levels and other parameters through the integration of a broad range of observations including the elastic stiffness from tidal deformation and the viscosity from the concept of isostasy a gross picture of rheological stratification of the earth s mantle a strong lithosphere a weak asthenosphere and a strong layer below was proposed in the mid 19th century however not only the physical basis for such a model was weak due to the lack of proper understanding of some materials science issues such as the interplay between elastic and viscous deformation but also the lack of understanding of temperature depth relation associated with convection prevented our understanding of the rheological structure of the earth s interior major progress occurred in the first half of the 20th century in our understanding of the atomistic mechanisms of plastic deformation in solids and much of the theoretical framework on the plastic deformation of solids was established by late 1960s those developments provided a basis for scaling analyses that are critical to the applications of laboratory results to the earth s interior major progress in laboratory studies on rheological properties occurred in the mid 1960s to the early 1970s in griggs lab using a new type of solid medium high pressure deformation apparatus to pressure 2 gpa and temperature 1600 k the basic concepts such as the water weakening non linear rheology and deformation induced lattice preferred orientation were identified by their studies however large uncertainties in the stress measurements with this type of apparatus were recognized in the late 1970s and high resolution experimental studies using synthetic samples were initiated in paterson s lab in the mid 1980s using a high resolution gas medium deformation apparatus however experimental studies with such a gas medium deformation apparatus can be conducted only at low pressures

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

sample preparation effects on illite crystallinity measurement grain size gradation and particle orientation abstract specimen thickness is the main experimental factor controlling the results of illite crystallinity ic or crystallite size measurements on sedimentation slides different values obtained from thick and thin preparations are due to grain size gradation effects which may exclude larger and higher ordered grains from contributing to the diffraction orientation effects control the measured peak intensity the change from poor particle orientation in thick slides to high orientation in very thin slides is marked by an increase in specimen density diminishing non basal reflections and by a strong increase in peak intensity a plateau with constant peak breadths is observed if thin slides of well ordered platy illites are used a similar plateau can be recognized for thick preparations of specimens from less ordered materials but not from well ordered ones therefore it is suggested that ic is determined on very thin sedimentation slides with a thickness of 0 25 mg cm2 or less ultrasonic and h2o2 treatments enhance the degree of particle orientation by destruction of grain aggregates and organic compounds leading to smaller peak breadths and higher intensities

authigenic chlorites in sandstones as indicators of high temperature diagenesis arkoma foreland basin usa abstract abundant authigenic chamosite associated with higher than average porosities is present in a deep gas reservoir spiro sandstone of the arkoma basin east central oklahoma three types of chlorite can be distinguished petrographically all of which appear texturally to have formed early chlorite peloids diffuse matrix and grain coatings x ray diffraction scanning electron microscopy and electron microprobe studies show that most chlorites are composed of two distinct polytypes ib s 90 and iib a third structure ia polytype was identified in only one sample the relative percentage of the high temperature iib structure increases gradually with increasing thermal maturity from 40 at 3 5 r sub o iib chlorite forms rather thick blocky crystals distinct from the thin pseudohexagonal plates typical of low temperature ib chlorite temperature estimates based on data on vitrinite reflectance and fluid inclusions suggest that iib chlorite formed at burial temperatures 150 180 c higher contents of tetrahedral al3 and slightly higher fe fe mg ratios in iib chlorite are consistent with precipitation temperatures higher than those of the ib structure a higher temperature origin for the iib structure is also consistent with oxygen isotope data

deformation across the mantle transition zone a theoretical mineral physics view abstract the dynamics of the earth s mantle is still poorly constrained due to the lack of understanding the transfer of matter between the upper and the lower mantle and their convective vigor the transition zone tz might play a crucial role as the interface connecting the upper to the lower mantle here we examine the rheology of the main tz minerals wadsleyite ringwoodite and majorite garnet based on a mineral physics approach using the results of lattice friction modeling and dislocation glide mobilities together with the available data on self diffusion in the tz minerals we quantify their plastic deformation by diffusion and dislocation creep from theoretical plasticity models we show that pure climb creep is expected to contribute to the plasticity of the tz without the need of significant diffusion related hydrolytic weakening matching well the geophysical observations our model results predict that crystallographic preferred orientations cpo might only develop along with stress concentrations as present around cold subducting slabs which can be locally weaker than the surrounding tz despite their lower temperatures

dynamic topography gravity and the role of lateral viscosity variations from inversion of global mantle flow lateral viscosity variations lvvs in the mantle influence geodynamic processes and their surface expressions with the observed long wavelength geoid free air anomaly gravity gradient in three directions and discrete high accuracy residual topography we invert for depth and temperature dependent and tectonically regionalized mantle viscosity with a mantle flow model the inversions suggest that long wavelength gravitational and topographic signals are mainly controlled by the radial viscosity profile the pre cambrian lithosphere viscosity is slightly one order of magnitude higher than that of oceanic and phanerozoic lithosphere plate margins are substantially weaker than plate interiors and viscosity has only a weak apparent dependence on temperature suggesting either a balancing between factors or a smoothing of actual higher amplitude but short wavelength lvvs the predicted large scale lithospheric stress regime compression or extension is consistent with the world stress map thrust or normal faulting both recent compiled high accuracy residual topography and the predicted dynamic topography yield 1 km amplitude long wavelength dynamic topography inconsistent with recent studies suggesting amplitudes of 100 to 500 m such studies use a constant positive admittance transfer function between topography and gravity in contrast to the evidence which shows that the earth has a spatially and wavelength dependent admittance with large negative admittances between 4000 and 10 4 km wavelengths

regionalized models for the thermal evolution of the earth abstract traditional models for the heat loss in oceanic and continental regions are combined into a regionalized model for the thermal evolution of the earth the need for regionalization is obvious when one considers that the mantle loses 3 to 4 times as much heat per unit area in oceanic regions than in continental areas the present day rate of heat loss together with a geochemical estimate of the concentration of heat producing elements in the earth fixes the response time of the thermally convecting mantle the response time in turn can be used to select the most reasonable representation for mantle convection in terms of the sensitivity of viscosity on temperature and layering versus mantle wide circulation present geochemical estimates of the bulk composition of the earth are most easily reconciled with the observed heat flow if the mantle is layered and its rheology is slightly less temperature dependent than generally assumed the layered system can produce sufficiently high temperatures to explain the high magnesian komatiites of the archean one difficulty with the models is that they predict widespread melting at shallow depth in the early stages of earth history but do not address how such melting affects and alters the heat transfer mechanisms

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

thermochemical piles in the lowermost mantle and their evolution abstract two large low shear wave velocity provinces llsvps lie beneath southern africa and southern pacific ocean these llsvps may be thermochemical in origin studying the dynamic evolution of these thermochemical piles will help us understand the earth s present structure and history we investigate the entrainment and evolution of an initially isolated high viscosity thermochemical pile in detail the entrainment rate of the dense pile increases with time until it being totally entrained the pile s survival time does not monotonically varies with its viscosity the chemical pile obstructs the horizontal flow along cmb and turns it into upwelling flow this may help explain the observation of plume generation zones the morphology and location of the dense pile oscillate with time a high viscosity pile can keep its location and morphology roughly unchanged for hundreds of millions years if the convective structure in the surrounding mantle keeps steady

influence of grain size evolution on convective instability 1 grain size one of the most important microstructural properties of materials evolves during creep deformation to minimize the free energy of polycrystalline aggregates we apply a model of grain size evolution to the study of convective instability of cooling boundary layers the grain size evolution model is coupled to a composite rheology where the deformation rate is the sum of that due to dislocation creep and grain size sensitive diffusion creep the onset of convection is sensitive to grain growth rates and the initial grain size the formation of convective instabilities is enhanced by stresses induced by plate motions therefore small scale convection is more likely to occur beneath fast moving plates in finite amplitude convection grain size evolution leads to high viscosity in regions where convective stresses are low and can induce viscosity contrasts exceeding one order of magnitude such viscosity contrasts are sufficient to influence the dynamics of convection often leading to domains which remain isolated from the well mixed convecting fluid a composite viscosity including diffusion creep which has a lower activation energy than dislocation creep reduces the effective temperature dependence of viscosity

can hotter mantle have a larger viscosity in the diffusion creep regime which is believed to be appropriate for the lower mantle of the earth the viscosity of rocks depends on the grain size the grain size is normally assumed to be constant in mantle convection models however it is controlled by the kinetics of polymorphic phase transformations and post transformation grain growth and can vary substantially a simple scaling analysis demonstrates one particular feature of the interaction of rheology convection and kinetics the mantle resembles a temperature dependent viscosity fluid with an extremely unusual apparent activation enthalpy for the viscosity it can take almost any value including negative so that hotter regions can have a larger viscosity or the averaged viscosity of the mantle can increase during earth s secular cooling this affects mantle dynamics and evolution and should also be taken into account in interpretating the viscosity variations in the mantle

thermal evolution of earth with magnesium precipitation in the core vigorous convection in earth s core powers our global magnetic field which has survived for over three billion years in this study we calculate the rate of entropy production available to drive the dynamo throughout geologic time using one dimensional parameterizations of the evolution of earth s core and mantle to prevent a thermal catastrophe in models with realistic urey ratios we avoid the conventional scaling for plate tectonics in favor of one featuring reduced convective vigor for hotter mantle we present multiple simulations that capture the effects of uncertainties in key parameters like the rheology of the lower mantle and the overall thermal budget simple scaling laws imply that the heat flow across the core mantle boundary was elevated by less than a factor of two in the past relative to the present another process like the precipitation of magnesium bearing minerals is therefore required to sustain convection prior to the nucleation of the inner core roughly one billion years ago especially given the recent upward revision to the thermal conductivity of the core simulations that include precipitation lack a dramatic increase in entropy production associated with the formation of the inner core complicating attempts to determine its age using paleomagnetic measurements of field intensity because mantle dynamics impose strict limits on the amount of heat extracted from the core we find that the addition of radioactive isotopes like potassium 40 implies less entropy production today and in the past on terrestrial planets like venus with more sluggish mantle convection even precipitation of elements like magnesium may not sustain a dynamo if cooling rates are too slow

anomalously high porosity and permeability in deeply buried sandstone reservoirs origin and predictability porosity and permeability generally decrease with increasing depth thermal exposure and effective pressure however a significant number of deep 4 km approximately 13 000 ft sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology composition and texture age and burial temperature history in sandstones containing anomalously high porosities such porosities exceed the maximum porosity of the typical sandstone subpopulation major causes of anomalous porosity and permeability were identified decades ago however quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature the focus of this article is on quantification and predictability of three major causes of anomalously high porosity 1 grain coats and grain rims 2 early emplacement of hydrocarbons and 3 shallow development of fluid overpressure grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital quartz grains currently prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets in the absence of adequate empirical data sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability begin page 302 in a given play or prospect these constraints include thermal history and sandstone grain size and composition the overall effect of hydrocarbon emplacement on reservoir quality is controversial it appears that at least some cements quartz and illite may continue to precipitate following emplacement of hydrocarbons into the reservoir our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify prior to drilling the potential impact of hydrocarbon emplacement on porosity and permeability the best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited tertiary or quaternary sandstones our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths the models also indicate that the potential for porosity preservation is greatest in ductile grain rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks the case for significant porosity preservation associated with fluid overpressures in pre tertiary basins however is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss

steady plumes produced by downwellings in earth like vigor spherical whole mantle convection models if mantle thermal upwellings plumes are the cause of volcanic hot spots then observations suggest that plumes are relatively fixed with nonuniform distribution and limited lifetimes to date fixity of upwellings has only been shown in models of convection at either low vigor or with layering though studies where the lower mantle has high viscosity do frequently show upwellings with much lower drift velocities than the surface velocity since more vigorous convection traditionally shows more time dependence fixity of upwellings has not been expected for nonlayered convection at earth like vigor rather we might expect slow but increasing drift velocities i have undertaken numerical models of whole mantle convection in three dimensional spherical geometry at approaching earth like vigor surprisingly these simulations show prominent steady virtually fixed plumes arising from self organization controlled by the smaller but numerous cold downwellings if downwelling subduction dominates earth s interior dynamics then this work suggests that plume fixity need not require layering the regular spacing and permanence of model upwellings contrast with observations of hot spots though in these models which do not simulate plates the fixity and regular spacing result from the freedom of downwellings to occur everywhere except where plumes reach the surface i suggest that on earth this feedback is weakened by the presence of plates but that whole mantle thermal convection can still produce relatively fixed plumes though we might expect them to be weaker more mobile and transient

the combined influences of variable thermal conductivity temperature and pressure dependent viscosity and core mantle coupling on thermal evolution abstract most convection studies of thermal history have not considered explicitly the thermal interaction between the mantle flow and the core we have investigated the influences of variable thermal conductivity and variable viscosity temperature and pressure dependent on the boundary layer and thermal characteristics of the d layer and the evolution of the thermo mechanical profiles of horizontally averaged viscosity and thermal conductivity viscosity contrast due to temperature dependence of up to 30 000 has been considered our results show clearly that variable thermal conductivity though small in magnitude as compared to variations in the viscosity does exert a significant delaying influence on mantle cooling thereby keeping the urey ratio low reducing the growth of the bottom thermal boundary layer and changing the viscosity profiles over time a higher temperature at the core mantle boundary increases the overall time dependent behavior of the thermal boundary layers enhanced radiative conductivity results in faster cooling opposite to the effect of the phonon conductivity component and a superadiabatic temperature gradient in the deep lower mantle finally the initial value of the core mantle boundary temperature can be inferred to wield a strong influence on the subsequent mantle thermal evolution in this model with both variable thermal conductivity and viscosity we may conjecture that other rheological and conductivity complexities such as grain size dependence of mantle properties would also have an impact on the current state of the mantle resulting from the primordial thermal condition

ordovician increase in extent and depth of bioturbation implications for understanding early paleozoic ecospace utilization the infaunal habitat was colonized in the early phanerozoic primarily by soft bodied organisms to understand the early biotic history of this environment it is necessary to examine the overall record of bioturbation or ichnofabric we present here results documenting trends in ichnofabric from ordovician strata of the great basin in california nevada and utah collected by a method devised to semiquantitatively measure the extent of early paleozoic utilization of infaunal ecospace when combined with similar cambrian data a two phase stepwise pattern of increasing bioturbation contemporaneous with the cambrian and ordovician radiations of skeletonized metazoans is apparent these data represent evidence independent of the body fossil record of increasing ecospace utilization in the early paleozoic

sediment ingestion by worms and the production of bio clays a study of macrobiologically enhanced weathering and early diagenetic processes the experiments reported here show that a range of early diagenetic clay minerals can develop within ingested sand in the guts of creatures at a rate that is at least 2 or 3 orders of magnitude greater than in an abiotic environment early diagenesis in marine environments may be strongly influenced by the occurrence of sediment ingestion and excretion by animals using a set of artificial marine experiments and the common lugworm arenicola marina i t is shown that sediment ingestion by macrobiota represents a new way in which to precipitate clay minerals or bio clays significant quantities of clay minerals can be the product of biological interaction within sediment tanks were constructed with artificially layered sediment and natural seawater the experiments were conducted at room temperature for a total duration of 9 months the silicate material fed to the worms was an unweathered and thus clay mineral free basalt from iceland less than 2 lm fractions from original control and faecal samples were periodically analysed using x ray diffraction xrd and fourier transform infrared spectroscopy ftir faecal casts underwent mineral alteration as denoted by loss of primary minerals especially plagioclase new minerals in faecal casts from the lugworms included kaolinite illite quartz and possibly chlorite inorganic weathering of similar parent basalt would probably produce an identical mineral assemblage but many more times slowly than with the macrobiotic mechanism demonstrated here

interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data clay mineral crystallinity and crystallite domain size data determined by x ray diffraction xrd are methods extensively used in the characterization of very low grade metamorphic conditions however the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained a consequence of the different machine conditions measurement methods and sample preparations used during analysis a calibration approach to the standardization of data using rock chip standards is presented which allows data sets produced by different research groups to be directly and quantitatively compared a standardized scale the crystallinity index standard cis is proposed with illite crystallinity anchizonal boundary limits of 0 25d 2 and 0 42d 2 and equivalent illite crystallite sizes of 52 and 23 nm respectively determined by the warren averbach method calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments and should improve the accuracy and reliability of correlations made between xrd data and other indicators of very low grade metamorphism hence increasing the value of such clay mineral studies

the kinetics of the reaction of majorite plus ferropericlase to ringwoodite implications for mantle upwellings crossing the 660 km discontinuity we have measured the kinetics of reaction between mgo and majoritic garnet at 20 gpa and 1773 2123 k as a proxy for the reaction between perovskite and ferropericlase during mantle upwelling across the 660 km seismic discontinuity ringwoodite forms a layer between mgo and garnet and in the case of aluminous garnets the interface between ringwoodite and garnet develops a fingering instability resulting in a complex intergrowth at this interface by contrast the mgo ringwoodite interface is always planar for an initial planar mgo garnet interface two thicknesses are therefore defined 1 a layer thickness x1x1 which is the maximum thickness of ringwoodite which forms a plane parallel bounded layer next to the mgo and 2 an interface thickness x2x2 which is the maximum extent of the intergrowth region away from the ringwoodite layer the growth of both of these regions can be described by apparent rate constants kiki which are arrhenius with view the mathml sourceln k10 6 36 0 25 m2 s and e1 456 40 kj mole1 456 40 kj mol for the ringwoodite layer and view the mathml sourceln k20 9 2 3 3 m2 s and e2 371 53 kj mole2 371 53 kj mol for the intergrowth region the fingering instability is caused by the incompatibility of aluminium in ringwoodite and its low chemical diffusivity in garnet which results in an increase of surface area at the ringwoodite garnet interface to minimise the aluminium concentration at the interface the intergrowth region contains a fine grained mixture of ringwoodite and garnet which coarsens very slowly with time this might result in a transient weakening of upwelling regions of mantle just above the 660 km seismic discontinuity allowing some viscous decoupling between the upper and lower mantle

non newtonian stagnant lid convection and magmatic resur facing on venus abstract mantle convection on venus is likely to occur in the regime known as stagnant lid convection we investigate this regime for internally heated convection with temperature and pressure dependent power law viscosity dislocation creep scaling relationships obtained for large aspect ratio convection are different from steady state square box calculations but agree well with scaling theory and boundary layer stability analysis results for arrhenius viscosity and pressure dependent viscosity show that the efficiency of heat transport is sensitive to the viscosity function at the bottom of the lid new scaling relationships are applied to parameterized convection calculations of the thermal history of venus assuming that plate tectonics could not occur during evolution the onset of convection beneath the lid is delayed even for initial potential temperatures near the solidus during the conductive regime melting is suppressed due to the development of a thick cold lid at the surface after convection begins the lid becomes thinner and the planet undergoes a period of widespread melting and volcanism the timing of the beginning and ending of melting depends on various factors such as the initial conditions and mantle rheology the episode of melting predicted by the models can be reconciled with the cessation of global resurfacing on venus 300 800 myr ago the models yield present day lithospheric thicknesses around 200 km which is similar to previously suggested estimates

the effects of phase transition kinetics on subducting slabs the effects of kinetics on the non equilibrium aspects of the olivine to spinel transition in a descending slab have been studied numerically a one dimensional model consisting of the kinetic equations and the heat diffusion equation with latent heat release has been constructed numerical results show that the position and sharpness of the kinetic phase boundary is determined by the surface tension and the activation volume for slow slab velocities less than 6 cm yr near equilibrium conditions are found finger like structures emanating from the phase boundaries are obtained in this regime these phase boundary prolusions may cause earthquakes for slab velocities of around 10 cm yr the metastable olivine region may be pushed down to a depth of about 600 km with a sharp phase boundary a direct transition from olivine to perovskite may be feasible

can sharp seismic discontinuities be caused by non equilibrium phase transformations a long posed problem of the seismic discontinuities at the depths of 410 and 660 km in earth s mantle is whether these discontinuities are caused by phase transitions alone or together with chemical layering among several tests the sharpness of the boundary is one of the most crucial recent data suggest that the transition region thickness is less than 4 km for both the 410 and the 660 km discontinuities this may be smaller than predicted by phase equilibria we suggest that if there is a sufficiently large nucleation barrier for phase transformation and if the transformation front cannot move faster than the convective flow the transformation in rising or descending material does not take place until the metastable overshoot becomes equal to the nucleation barrier an avalanche like transformation following this overshoot occurs in a very narrow region and can be the cause of a sharp seismic discontinuity in this case the topography of the phase boundary is also substantially modified

long term preservation of early formed mantle heterogeneity by mobile lid convection importance of grainsize evolution abstract the style of tectonics on the hadean and archean earth particularly whether plate tectonics was in operation or not is debated one important albeit indirect constraint on early earth tectonics comes from observations of early formed geochemical heterogeneities 142nd and 182w anomalies recorded in hadean to phanerozoic rocks from different localities indicate that chemically heterogeneous reservoirs formed during the first 500 myrs of earth s history survived their remixing into the mantle for over 1 gyrs such a long mixing time is difficult to explain because hotter mantle temperatures expected for the early earth act to lower mantle viscosity and increase convective vigor previous studies found that mobile lid convection typically erases heterogeneity within 100 myrs under such conditions leading to the hypothesis that stagnant lid convection on the early earth was responsible for the observed long mixing times however using two dimensional cartesian convection models that include grainsize evolution we find that mobile lid convection can preserve heterogeneity at high mantle temperature conditions for much longer than previously thought because higher mantle temperatures lead to larger grainsizes in the lithosphere these larger grainsizes result in stronger plate boundaries that act to slow down surface and interior convective motions in competition with the direct effect temperature has on mantle viscosity our models indicate that mobile lid convection can preserve heterogeneity for 0 4 1 gyrs at early earth mantle temperatures when the initial heterogeneity has the same viscosity as the background mantle and 1 4 gyrs when the heterogeneity is ten times more viscous than the background mantle thus stagnant lid convection is not required to explain long term survival of early formed geochemical heterogeneities though these heterogeneities having an elevated viscosity compared to the surrounding mantle may be essential for their preservation

grain size variations in the earth s mantle and the evolution of primordial chemical heterogeneities 1 chemical heterogeneities with relatively high viscosity are known to be poorly mixed by mantle convection one of the factors which can cause viscosity variations is the grain size analysis of various physical processes in the earth s mantle suggests that variation in the grain size can be extremely large even outside of subducting slabs the largest grains are formed in the early molten earth where ostwald ripening is controlled by liquid state diffusion the smallest grains are formed due to recrystallization by the spinel perovskite phase transformation in downwelling flow and subsequent ostwald ripening the viscosity contrast between primordial large grain size material and recrystallized material is likely to be many orders of magnitude preliminary numerical simulations show that strongly time dependent mantle convection can process primordial large grain size heterogeneities through the spinel perovskite phase boundary only after a few overturns this reduces the viscosity contrasts and leads to rapid mixing by mantle convection a more steady flow in the lower mantle may lead to the formation of stagnant poorly mixed regions which may never have had a chance to cross the 660 km discontinuity and be recrystallized by the spinel perovskite phase transformation heterogeneities can also avoid grain size reduction and mixing if their density is slightly larger than the surrounding mantle in this case they accumulate at the core mantle boundary

experimental production of clay rims by macrobiotic sediment ingestion and excretion processes using controlled experiments that simulate interlayered sand and mud in marine conditions it is shown that clay rims develop on sand grains during the digestive processes of the polychaete worm arenicola marina the clay size minerals also undergo accelerated weathering and mineral authigenesis synthetic mud crushed slate and clean eolian sand of known composition and texture were horizontally layered in experimental and control tanks natural seawater was added to these tanks along with several a marina fecal casts were collected at regular time intervals and these were analyzed texturally using electron microscopy and mineralogically using x ray diffraction xrd total bioturbation of the sand and mud layering by a marina was achieved after only eight weeks of feeding activity sand grains in the fecal casts developed clay mineral rims a feature absent in the original material and not found in the control tanks thin composite curvilinear structures assumed to be detached clay rims were also produced over a period of many months the synthetic mud proved to be unchanged in the control tank but it was significantly different in the fecal casts from the experimental tank that contained the worm a marina the original minerals were degraded and new authigenic clay minerals formed it is postulated that a combination of low ph and the biologically active microenvironment in the guts of annelid worms may radically accelerate mineral dissolution and clay mineral precipitation processes during digestion it is likely that mucous produced by the worms not only led to the aggregation of the fine grained material into fecal casts but also adhered the fine sediment onto sand grains thus creating clay mineral rims in this case the solid part of the clay rim will be a combination of the primary material crushed slate and the neoformed clay minerals

grain size distribution and rheology of the upper mantle abstract the physical processes that govern the grain size of rocks in the upper mantle are examined the analysis is based on the experimental data on creep recrystallization and grain growth in dunites and on a theoretical model for the thermomechanical structure of the cooling moving lithosphere the grain size of rocks is shown to be determined by the in situ stress only at the deeper part where the temperature is high enough to allow significant strain rate above this depth the microstructures record the thermomechanical history of rocks rather than the in situ stress in the case of the oceanic lithosphere where the thermomechanical history is best known the following features of grain size distribution are found at the uppermost mantle where the amount of grain growth is limited the grain size is determined by the initial value and the growth rate and where the effect of grain growth dominates it increases with depth when the amount of grain growth becomes large and the grain size reaches the steady state size corresponding to the ambient stress while the rock is hot enough to deform the grain size is then determined by the applied stress this grain size is however frozen when the rock gets cool and the strain rate becomes too small to induce any further dynamic recrystallization thus at the intermediate depth region the grain size records the fossil frozen stress at which the microstructures of rock have been frozen since the frozen stress increases with age the grain size in this depth interval decreases with depth finally the grain size below this level reflects the in situ stress and increases with depth its extent being dependent on the nature of return flow in the deep mantle thus the grain size versus depth relation may show a sigmoid curve the qualitative features of this curve may be similar also in the case of the continental lithosphere if a similar thermal event i e the intrusion of hot material and subsequent cooling occurs the results are quite consistent with the observed depth variation of olivine grain size in peridotite nodules ave lallemant et al 1980 the present model suggests that the depth of minimum grain size 65 and 150 km at the continental rift zone and the shield region respectively corresponds to that where the mechanical properties of the upper mantle change from elastic to ductile at tectonic stress levels 1 mpa and in the geological time scale this result leads to a new definition of the thickness of lithosphere in terms of its rheological properties this thickness is about twice as large as that inferred from the flexure of lithosphere but approximately equal to seismic thickness the model suggests the importance of grain growth as well as dynamic recrystallization and plastic flow in determining the texture of upper mantle rocks and therefore seismic anisotropy

evolution of grain sizes and orientations during phase transitions in hydrous mg2sio4 transformation microstructures in mantle minerals such as mg fe 2sio4 are critical for predicting the rheological properties of rocks and the interpretation of seismic observations we present in situ multi grain x ray diffraction experiments on hydrous mg2sio4 at the p t conditions relevant for deep cold subducting slabs up to 40 gpa and 850 c at a low experimental strain rate 4 10 6s 1 we monitor the orientations of hundreds of grains and grain size variations during the series of forsterite wadsleyite ringwoodite phase transformations microtextural results indicate that the and an intermediate phase grow incoherently relatively to the host phase consistent with a nucleation and growth model the and phase exhibit orientation relationships which are in agreement with previous ex situ observations the and intermediate show texturing due to moderate differential stress in the sample both the and transformation induce significant reductions of the mean sample grain size of up to 90 that starts prior to the appearance of the daughter phase apart from the in the newly formed and phases the nucleation rate is faster than the growth rate inhibiting the formation of large grains these results on grain orientations and grain size reductions in relation to transformation kinetics should allow refining existing slab strength models

implications of post perovskite transport properties for core mantle dynamics abstract recent evidence on perovskite to post perovskite phase change in the lowermost mantle suggests that post perovskite piles or lens should be present in the relatively cold downwelling areas while the roots of the hot upwelling plumes consist of perovskite post perovskite is often believed to be deformed predominantly by the dislocation creep and there are some hints that the activation parameters of the dislocation creep in post perovskite induce lower viscosity than that of perovskite at the same pressure and temperature conditions that can even result in a viscosity paradox in the lowermost mantle viscosity in cold downwellings transformed to post perovskite might be lower than the viscosity of warm perovskite plumes such a viscosity structure was indeed recently suggested by the geodynamical inversions of the geoid rheologically weak areas at the base of the subducted slabs may have important consequences for the slab deformation in the d we investigate the dynamics of the cold slabs transformed to post perovskite in the lowermost mantle by means of the simulations of thermal convection in a 2d cartesian model of the lower mantle with composite rheology including diffusion creep and dislocation creep different creep parameters are used for the perovskite lower mantle and for the post perovskite lens or layer respectively while perovskite is considered to be deformed via linear diffusion creep post perovskite is modeled either as a constant viscosity material with viscosity ranging from 1020 pa s to 1023 pa s or it is deformed purely by the dislocation creep the presence of the rheologically distinct post perovskite not only strongly influences the slab deformation above the core mantle boundary cmb but also results in different dynamic regimes of the cmb region characterised by e g the different length scales of the upwelling plumes depending on the creep parameters of post perovskite further the presence of very low viscosity lens or a layer above the cmb strongly enhances the cmb heat flux in the cold slab areas according to the amount of post perovskite present in the d and the rheology of post perovskite the heat flux across the cmb can vary considerably with time which may have important implications for the core mantle coupling and the character of the earth s geomagnetic field