application of stochastic simulation to co2 flux from soil mapping and quantification of gas release 1 conditional sequential gaussian simulations sgs have been applied for the first time to the study of soil diffuse degassing from different volcanic and nonvolcanic systems the application regards five data sets of soil co2 fluxes measured with the accumulation chamber methodology at the volcanic areas of solfatara of pozzuoli italy vesuvio cone italy nisyros greece and horseshoe lake california and at the nonvolcanic degassing area of poggio dell olivo italy the sgs algorithm was used to generate 100 realizations of co2 flux for each area probabilistic summaries of these simulations together with the information given by probability plots were used 1 to draw maps of the probability that co2 fluxes exceed thresholds specific for a background flux i e to define the probable extension of the degassing structures 2 to calculate the total co2 output and 3 to quantify the uncertainty of the estimation the results show that the sgs is a suitable tool to model soil diffuse degassing producing realistic images of the distribution of the co2 fluxes that honor the histogram and variogram of the original data moreover the relation between the sample design and the uncertainty of estimation was investigated leading to an empirical relation between uncertainty and the sampling density that can be useful for the planning of future co2 flux surveys

middle archean continent formation by crustal delamination the processes that created the first large cratonic areas such as the pilbara and the kaapvaal remain poorly understood models based on the uniformitarian extrapolation of present day arc volcanic processes to a hotter early earth have not adequately explained the observations in these terranes here we propose an alternative mechanism for the formation of the earliest continental crust the formation of continental crust may be achieved by delamination of the lower eclogitic part of an oceanic plateau like protocrust such delamination results in uplift extension and the production of tonalite trondhjemite and granodiorite ttg suites as recorded in middle archean cratons the available geologic and geophysical observations in combination with model calculations permit this scenario as an alternative to subduction based hypotheses

the adjustment of mantle plumes to changes in plate motion the relative motion of hotspots and lithospheric plates implies a velocity shear in the underlying mantle causing horizontal advection of mantle plumes as they rise toward the lithosphere consequent tilting of plumes parallel to the direction of plate motion indicates that plumes must undergo a period of readjustment after the velocity vector for plate motion is altered thus the shape of bends in the surface tracks of hotspots resulting from changes in plate motion will reflect the plume adjustment laboratory experiments as well as computations using a simple theory developed in richards griffiths 1988 for the dynamics of continuous plume conduits demonstrate that the bend in the surface track has a radius of curvature approximately equal to the maximum horizontal deflection of the conduit thus the sharpness of the bend at an age of 43ma in the hawaiian emperor volcanic chain implies that the deflection of the underlying plume in that case was small 200 km this small deflection is expected for plumes carrying large buoyancy fluxes and it indicates that tilting of the conduit is unlikely to be sufficient to cause diapiric instability

small scale heterogeneity in the mid lower mantle beneath the circum pacific area abstract data from a western united states short period seismic network are investigated in order to detect anomalous later p wave arrivals within a time window from 50 s to 170 s after direct p waves for deep and intermediate depth earthquakes at circum pacific subduction zones we interpret the anomalous arrivals as s to p scattered waves from heterogeneities in the mid lower mantle the searched regions depths from 4800 km to 2200 km cover approximately 2 of the entire lower mantle the heterogeneous scattering objects are located by array processing of the seismic waves using semblance with careful investigations of arrival time slowness and azimuth of the later phases relative to direct p waves we usually observe two or three later phases which are separated in arrival time by 5 20 s and have similar arrival azimuth and slowness this observation suggests that a few scatterers often form a cluster with a typical spacing of 100 200 km vertically more than ten clusters or groups of s to p scattering objects are detected in the mid lower mantle beneath the circum pacific regions northern kuril izu bonin mariana northern fiji and west coast of south america the s to p scattering objects are found in a depth range from 1100 to 1800 km and the most prominent objects of each region tend to be located around the middle of the depth range the amplitudes of the scattered waves range from 1 to 10 of the direct p waves with exceptionally large values as large as 40 the associated magnitudes of the changes in elasticity is not tightly constrained but probably are nearly 1 and may occasionally exceed 4 of shear velocity considering their high efficiency at converting short period waves wavelengths of 5 10 km the changes in elastic properties at the heterogeneous objects appear to occur within several kilometers seismic tomography studies have not detected prominent seismic wave speed anomalies except for those beneath south america which appear to be located in the former farallon plate and those at north of fiji which appear to be located around the pacific superplume the mid lower mantle heterogeneity should represent sharp chemical variations in major element composition of the lower mantle rocks the most plausible tectonic explanation of the variations is remnant of subducted and folded former oceanic crust which preceded the current subduction of the pacific plate the extreme estimates of low shear velocity 4 may indicate the prominent drops in the shear modulus due to ferro elastic phase transition in stishovite sio 2 our observations imply that heterogeneity of kilometer scale persists for billions of years despite of stirring due to convection and therefore is ubiquitous in the lower mantle it is qualitatively consistent with some recent convection models

methane flux and origin in the othrys ophiolite hyperalkaline springs greece abstract the occurrence and origin of methane ch4 generated by serpentinization of ultramafic rocks is of current timely interest in planetary geology astrobiology and energy resource exploration as it may contribute in particular to decipher the source of methane on mars the origin of life and the potential of abiotic hydrocarbon synthesis methane of dominant abiotic origin in serpentinized peridotites on continents ophiolites or igneous intrusions has been documented so far with complete carbon and hydrogen isotope composition in six countries in the philippines turkey oman new zealand japan and italy we report the discovery of two new sites in greece at archani and ekkara located in the othrys ophiolite massif portable sensors based on fourier transform infrared spectrometry ftir and tunable diode laser absorption spectroscopy tdlas allowed to realize that out of 21 ophiolitic springs methane is released only by four hyperalkaline ph from 10 7 to 11 3 and calcium hydroxide ca oh type waters all other 17 springs with ph

the changing nature of mantle hotspots through time implications for the chemical evolution of the mantle the melting products of mantle plumes can place constraints on the geochemical structure of the mantle and show how it has varied through time the highest temperature melts komatiites and picrites derived from each plume are assumed to represent the best available sample of the thermal boundary layer from which the plume originates a survey of the geochemistry of picrites and komatiites shows that they underwent a fundamental change at about the archean proterozoic boundary archean komatiites have depleted or neutral geochemistry whereas most protero zoic and younger picrites have originated from enriched oib type mantle although rare depleted plumes persist to the tertiary e g gorgona if plumes originate from a thermal boundary layer near the core mantle interface the composition of this layer must have changed from a mixture of depleted and neutral mantle during the archean to mainly enriched mantle in the proterozoic this change coincides with a drop in the maximum mgo content of the ma

stirring geochemistry in mantle convection models with stiff plates and slabs abstract numerical models of mantle convection are presented that readily yield midocean ridge basalt morb and oceanic island basalt oib ages equaling or exceeding the apparent 1 8 ga lead isotopic ages of trace element heterogeneities in the mantle these models feature high viscosity surface plates and subducting lithosphere and higher viscosities in the lower mantle the formation and subduction of oceanic crust are simulated by means of tracers that represent a basaltic component the models are run at the full mantle rayleigh number and take account of faster mantle overturning and deeper melting in the past more than 97 of the mantle is processed in these models including the expected excess density of former oceanic crust readily accounts for the depletion of morb source relative to oib sources a novel finding is of gravitational settling of dense tracers within the low viscosity upper mantle as well as at the base of the mantle the models suggest as well that the seismological observation of a change in tomographic character in the deep mantle might be explained without the need to postulate a separate layer in the deep mantle these results expand the range of models with the potential to reconcile geochemical and geophysical observations of the mantle

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

recent activity of nisyros volcano greece inferred from structural geochemical and seismological data this study summarizes the results of structural geochemical and seismological surveys carried out at nisyros volcano aegean sea greece during 1999 2001 field mapping and mesostructural measurements at the summit caldera lakki plain indicate that faults follow two main strikes ne sw and n s the n s striking fault depicts extensional features accommodating the left lateral component of motion of the ne sw striking main faults the ne sw preferred strike of the lakki faults and of the mineral filled veins as well as the distribution and ne sw elongation of the hydrothermal craters indicate that tectonics plays a major role in controlling the fluid pathway in the nisyros caldera the same ne sw trend is depicted by co2 anomalies revealed through detailed soil co2 flux surveys thus indicating a structural control on the pattern of the hydrothermal degassing degassing processes account for a thermal energy release of about 43 mw most of which occurs at lofos dome an area that was affected by hydrothermal eruptions in historical times the seismic study was conducted in june 2001 using a deployment specifically aimed at detecting signals of magmatic hydrothermal origin our instruments recorded local and regional earthquakes a few local long period events lp and bursts of monochromatic tremor local earthquake activity is concentrated beneath the caldera at depths generally shallower than 6 km plane wave decomposition of tremor signal indicates a shallow 200 m source located in the eastern part of the caldera conversely lp events depict a source located beneath the central part of the caldera in the area of lofos dome at depths in the 1 2 km range in agreement with geochemical and structural measurements these data suggest that both the deeper and shallower part of the hydrothermal system are subjected to instability in the fluid flow regimes probably consequent to transient pressurization of the reservoir these instabilities may be related to input of hot fluids from the deeper magmatic system as suggested by the variations in geochemical parameters observed after the 1997 1999 unrest episode the significance of seismological and geochemical indicators as precursors of hydrothermal explosive activity at nisyros is discussed

crustal evolution and mantle dynamics through earth history resolving the modes of mantle convection through earth history i e when plate tectonics started and what kind of mantle dynamics reigned before is essential to the understanding of the evolution

fluid geochemistry of nisyros island dodecanese greece abstract two distinct hydrothermal aquifers are present beneath the lakki plain a a deep aquifer characterized by temperatures higher than 290 c and chloride rich fluids and b a shallow aquifer with a chloride content close to that of sea water and temperatures in the range 170 255 c the vapor phase boiled from the shallow aquifer feeds the fumarolic vents in the southern half of the lakki plain and its liquid phase flows northeast and southwest contributing to the thermal springs located along the northern and southern coasts of the island this conceptual hydrogeochemical model of nisyros island can be used to reconstruct past hydrothermal eruptions and to estimate the risk of such events happening again in the future

mineral chemistry and density of subducted basaltic crust at lower mantle pressures subducted slabs are less dense than the surrounding mantle near the base of the transition zone 660 km depth because of the survival of garnet in former basaltic crust by this depth mantle peridotite has transformed to denser perovskitite1 2 the buoyancy of the former basaltic crust may contribute to the observed accu mulation or horizontal displacement of many slabs at the base of the transition zone3 here we report experimental confirmation of the widely held belief that the basaltic crust of slabs eventually transforms to a dense perovskititic lithology stable in the lower mantle synthetic mid ocean ridge basalt more glass subjected to pressures of 45 80 and 100 gpa in a laser heated diamond anvil cell transforms to an assemblage of aluminous mg fe silicate perovskite non quenchable casio3 perovskite stishovite and a sodic aluminous phase with the ca ferrite structure fig 1 per ovskititic more is about 0 06 g cm 3 more dense than a model lower mantle prem derived from seismological data thus even thermally equilibrated perovskititic slabs should encounter no sig nificant hindrance to subduction and convection in the lower mantle

living dead slabs in 3 d the dynamics of compositionally stratified slabs entering a slab graveyard above the core mantle boundary abstract segregation of subducted mid ocean ridge basalt morb at the cmb has been identified as a potentially important mechanism in the long term evolution of the mantle and core here three dimensional 3 d and two dimensional 2 d simulations of a compositionally stratified slab reaching the cmb are presented with the goals of characterising the resulting thermo chemical structures for comparison with seismic studies and of quantifying the fraction of morb that is able to segregate and remain near the cmb compositional stratification of the slab results in a strong torque due to the relatively high density of basalt and low density of harzburgite which tends to rotate the slab such that the basalt side faces down slab cmb interaction is characterised by heating up of the slab followed by separation of the basalt and harzburgite layers with harzburgite rising in vigorous plumes plumes form at the edges and sides of slabs at the cmb as well as in their interiors as previously observed for purely thermal slabs with plume heads dominated by depleted harzburgitic material sometimes with small amounts of entrained basalt while plume tails entrain basaltic material segregation of basalt depends strongly on the presence or absence of a preexisting dense layer at the cmb and by dimensionality a preexisting dense layer greatly increases the fraction of basalt that segregates from the slab in 3 d this fraction is in the range 0 5 0 7 with a layer present compared to 0 25 0 45 with no preexisting layer two modes of basalt segregation are observed for slabs that land basalt side up i hot harzburgite extruding from the sides and edges and ii hot harzburgite rich plumes bursting through the basalt layer as previously observed in laboratory experiments whereas for a slab that lands basalt side down iii hot basalt can peel off from its underside displaying fingering instabilities in 3 d furthermore basalt harzburgite segregation is sometimes observed in slab segments that have already been heated at the cmb and risen a few hundred km by a folding mechanism a range of interesting structures are observed in the model cmb region which may be useful in interpreting seismic observations strong lateral gradients in composition and temperature are quite common structures include plumes next to vertical slab segments inverted slab sections perched above the cmb and harzburgite curtains the presence of a stratified layer above the cmb as observed in global simulations in which a layer builds up by basalt segregation strongly suppresses plume formation 2 d simulations give a good first order guide to the dynamics obtained in fully 3 d geometry but inherently 3 d structures are missed plumes versus sheets fingering and there is a quantitative difference in the fraction of basalt segregated

origin of methane and light hydrocarbons in natural fluid emissions a key study from greece abstract greece a country characterised by intense seismic and volcanic activity has a complex geodynamic and geological setting that favours the occurrence of many gas manifestations in this study we address the origin of ch 4 and light hydrocarbons in cold and thermal emissions discharging along the hellenic territory also we investigate their possible relationship with the main geochemical composition of the gases and the different geological settings of the sampling sites for this purpose we collected 101 new samples that were analysed for their chemical o 2 n 2 ch 4 co 2 he ne ar h 2 h 2 s and c 2 c 6 hydrocarbons and isotopic r r a 13 c co 2 13 c ch 4 and 2 h ch 4 composition results show that ch 4 presents a wide range of concentrations from 13 c ch 4 from 79 8 to 45 0 vs v pdb 2 h ch 4 from 311 to 301 vs v smow greece was subdivided in four geologic units external eh and internal ih hellenides hellenic hinterland hh and active volcanic arc va and a decreasing ch 4 concentration from eh to hh was recognized whereas ch 4 showed intermediate concentrations in va the ch 4 c 2 h 6 c 3 h 8 ratios from 1 5 to 93 200 coupled with ch 4 isotopic features suggest that the light alkanes derive from different primary sources and are affected by secondary processes an almost exclusive biotic mainly microbial origin of ch 4 can be attributed to eh gases cold gases at ih have mainly a thermogenic origin although some gases connected to continental serpentinization may have an abiogenic origin methane in gases bubbling in thermal waters of ih hh and va and fumarolic gases of the va seem to have an abiogenic origin although their chemical and isotopic characteristics may have been produced by secondary oxidation of thermogenic ch 4 a process that in some of the sampled gases causes extremely positive isotopic values 13 c ch 4 up to 45 0 vs v pdb and 2 h ch 4 up to 301 vs v smow

hydrothermal methane fluxes from the soil at pantelleria island italy abstract chemical and isotopic analyses of the main gas manifestations of the island of pantelleria italy were used to gain insight on the origin of the released methane results indicate that the most probable origin is through abiogenic reactions within the hydrothermal system methane and co 2 flux measurements from the soils were made with the accumulation chamber method in an area of about 0 015 km 2 within the main fumarolic area of the island favara grande the 23 measurements range from 34 to 3550 mg m 2 d 1 for ch 4 and from 0 6 to 379 g m 2 d 1 for co 2 the relationships between ch 4 and co 2 fluxes and the ch 4 co 2 ratios in the gases collected between the depths of 25 and 100 cm provide evidence for methanotrophic processes within the soils methane output for the surveyed area was calculated to be 2 5 t a 1 and extrapolated to about 5 10 t a 1 for the entire volcanic hydrothermal system of the island previous higher estimates of the ch 4 output at pantelleria etiope et al 2007 j volcanol geotherm res 165 76 86 were based on soil co 2 output and ch 4 co 2 ratios in fumarolic gases the present work provides the first direct ch 4 flux data and it suggests that methanotrophic activity in the soil could be substantial in reducing the ch 4 emission to the atmosphere

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

phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle abstract phase transformations in a representative oceanic crust composition morb have been studied at pressures up to 28 gpa at 1200 c and 1500 c using a multi anvil apparatus the assemblage of majorite casio 3 rich perovskite stishovite which is displayed by the morb composition at pressures above 20 gpa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 gpa this al phase has a complex composition and is also relatively sodium rich its x ray diffraction pattern is similar to that observed for mgal 2 o 4 calcium ferrite type and is tentatively indexed on the basis of an orthorhombic cell yielding a zero pressure density of 3 87 2 g cm 3 majorite garnet was found to remain as the most abundant phase at the highest pressure studied 28 gpa and does not appear likely to transform to perovskite until pressures exceed 29 30 gpa these results imply that the basaltic crust component of subducted oceanic lithosphere would be 0 1 0 2 g cm 3 less dense than surrounding mantle pyrolite between the depths of 660 800 km thereby confirming an earlier hypothesis of anderson moreover the underlying layer of harzburgite would be 0 05 g cm 3 less dense than pyrolite throughout most of this depth interval these buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle

what caused the rise of atmospheric o2 abstract oxygenic photosynthesis appears to have evolved well before o 2 levels increased in the atmosphere at around 2 4 ga this has led to numerous suggestions as to what may have kept o 2 suppressed and then eventually allowed it to rise these suggestions include changes in the recycling of carbon and sulfur relative to water or hydrogen a switch from dominantly submarine to dominantly subaerial volcanism gradual oxidation of the continents and a concomitant decrease in reduced metamorphic gases a decline in deposition of banded iron formations a decline in nickel availability and various proposals to increase the efficiency of photosynthesis several of these different mechanisms could have contributed to the rise of o 2 although not all of them are equally effective to be considered successful any proposed mechanism must make predictions that are consistent with the carbon isotope record in marine carbonates which shows relatively little change with time apart from transient but occasionally spectacular excursions the reasons for this constancy are explored here but are not fully resolved in the process of making these comparisons a self consistent redox balance framework is developed which will hopefully prove useful to others who may work on this problem and to astronomers who may one day try to decipher spectral signatures of oxygen on earth like exoplanets

methanotrophy in geothermal soils an overlooked process the example of nisyros island greece abstract a multidisciplinary field campaign was carried out at nisyros island greece hydrothermal gases were sampled and analysed and ch4 and co2 fluxes from the soils were measured with the accumulation chamber method the sampling area lakki plain covers an area of about 0 08 km2 and includes the main fumarolic areas of kaminakia stefanos ramos lofos and phlegeton flux values measured at 130 sites range from 3 4 to 1420 mg m 2 d 1 for ch4 and from 0 1 to 383 g m 2 d 1 for co2 the fumarolic areas show very different ch4 degassing patterns kaminakia showing the highest ch4 output values about 0 8 t a 1 from an area of about 30 000 m2 and phlegeton the lowest about 0 01 t a 1 from an area of about 2500 m2 the total output from the entire geothermal system of nisyros should not exceed 2 t a 1 previous indirect estimates of the ch4 output at nisyros based on soil co2 output and ch4 co2 ratios in fumarolic gases were more than one order of magnitude higher the present work further underscores the utmost importance of direct ch4 flux data because indirect methods totally disregard methanotrophic activity within the soil ten soil samples were collected for ch4 consumption experiments and for metagenomic analysis seven of the soil samples showed small but significant ch4 consumption up to 39 7 ng g 1 h 1 and were positive for the methanotrophs specific gene pmoa confirming microbial ch4 oxidation in the soil notwithstanding the harsh environmental conditions high temperature and h2s concentrations and low ph

transient mantle layering and the episodic behaviour of venus due to the basalt barrier mechanism abstract numerical models of mantle convection that include the basalt barrier mechanism are explored for venus the basalt barrier mechanism is due to the positive buoyancy of subducted basaltic crust between the mantle depths of 660 and 750 km the inclusion of this mechanism in models of earth s evolution has been shown to cause episodic mantle layering early in earth history and we explore whether it can also operate on venus the models presented here include a moderately mobile lithosphere which is not representative of the current state of venus but this allows us to exclude the effects of episodic lithosphere mobility and thus to isolate the effect of the basalt barrier this is a step in a systematic approach to models with a mostly static lithosphere we find the basalt barrier does yield episodically layered mantle convection in some venus models the likelihood of episodic layering is increased by venus high surface temperature and by its less mobile or immobile lithosphere surprisingly secondary differences from earth including the lower gravity density and mantle depth also promote episodic layering the models suggest that mantle layering and overturns may still be likely to occur in venus the breakdown of mantle layering and consequent mantle overturns would lead to dramatic episodes of volcanism formation of large amounts of crust and tectonic activity on the planet s surface as has been inferred to have happened on venus around 500 ma ago from surface morphology and cratering these results thus suggest that a transient layering of the mantle by the basalt barrier mechanism and mantle overturns may be part of the explanation for venus s recent resurfacing

mixing by time dependent convection abstract two dimensional numerical flow models are employed to study the mixing of passive heterogeneities in the earth s mantle both simple kinematic models of time dependent flow and dynamic convection models are studied initially small blocks of tracers are inserted into the flow and their progressive dispersal is monitored through time some of the tracers also serve as infinitesimal strain markers three different regimes of mixing are observed a all heterogeneities are rapidly strained and stirred according to a turbulent mixing law where the size of a blob decreases exponentially with time b most heterogeneities are only slowly strained and possibly follow a laminar mixing law with a linear decrease with time of the size of a heterogeneity c the size reduction proceeds rapidly in part of the fluid but some poorly stirred islands persist for a long time all three regimes are found in various kinematic flow models while in the dynamic convection models only regime a is observed the effective strain rate in the exponential turbulent mixing law is more than an order of magnitude smaller than the average flow strain rate when the results from the dynamic convection models with variable viscosity are scaled to the upper mantle mixing over small length scales would be achieved after a few hundred million years for whole mantle convection with a significant increase of viscosity with depth mixing times could be in the order of 1 2 ga in the latter case it appears marginally possible that part of the isotopic heterogeneity observed in oceanic basalts has evolved within the convecting mantle rather than in physically separated reservoirs

co2 ch4 ratio in fumaroles a powerful tool to detect magma degassing episodes at quiescent volcanoes 1 long time series of fumarolic compositions at campi flegrei italy mammoth mountain california panarea italy and nisyros greece show rapid increases up to orders of magnitude of the co2 ch4 ratio systematically with the occurrence of volcanic unrest periods these easily detected anomalies originate with the arrival of ch4 poor magmatic fluids in the shallower levels of the volcanoes the data suggest that volcanoes are characterized by magmatic activity at depth also in periods of apparent quiescence the activity is constituted by the pulsing release of large amount of fluids which either cause unrest periods seismicity and ground deformation or possibly could precede volcanic eruption this type of volcanic activity can be monitored trough the classical geophysical techniques together with the systematic sampling and analysis of fumaroles

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

controls on density stratification in the early mantle previous numerical models of convection in the hot early mantle incorporating simulated subducted oceanic crust and some viscosity stratification developed thermal and compositional stratification with cooler strongly depleted mantle above hotter slightly enriched mantle the robustness of these results is explored through variation of some important parameters the stratification is minor at present mantle temperatures but stronger at higher mantle temperatures because mantle viscosities are reduced the results are not much affected by variations in plate velocity nor by whether internal heat sources are directly associated with the mafic component nor by 20 of heat entering from below however stratification was reduced somewhat at higher temperatures by increasing the number of tracers indicating that the effect was moderately overestimated in a previous paper the present results still indicate that oceanic crust would be no more than 6 8 km thick at high mantle temperatures because of the strong depletion of the upper mantle when the effect of a phase transformation in the transition zone is included the stratification is moderately enhanced and crustal thicknesses of 3 5 km result at 1650 c oceanic crustal thickness is highly variable in space and time with some plates having essentially zero crust for significant periods the new results support the previous conclusions that the stratification may explain the strong depletion of incompatible elements documented for the earliest mantle and that early plate tectonics may have been more viable that hitherto thought

structure and dynamics of sheared mantle plumes 1 an extensive series of laboratory experiments is used to investigate the behavior of sheared thermal plumes the plumes are generated by heating a small circular plate on the base of a cylindrical tank filled with viscous fluid and then sheared by rotating a horizontal lid at the fluid surface the motion of passive tracers in the plumes is visualized by the release of several dye streams on the hot plate we systematically examine the dependence of the convective flow on four dimensionless numbers a velocity ratio a rayleigh number the viscosity ratio and an aspect ratio we identify and delineate two transitions in the convective behavior from a regime where the plume can spread upstream against the shear to a regime where the entire plume is advected downstream and from a regime of negligible cross stream circulation to a regime with significant cross stream circulation and thermal entrainment our analysis of the steady profiles of the plumes shows that they initially rise with a constant vertical rise velocity this rise velocity depends on the buoyancy flux and ambient viscosity but is almost independent of the centerline plume viscosity which suggests that most of the thermal plume has a viscosity that is much closer to the ambient viscosity than the centerline viscosity as the plumes approach the lid they decelerate as the viscous drag on them steadily increases the lateral spreading of the plumes under the lid is found to be well described by similarity solutions derived for the spreading of compositional plumes on a rigid surface if the effective viscosity of the thermal plumes is taken to be the ambient viscosity rather than the centerline viscosity a similar theoretical model is found to roughly predict the upstream spreading of thermal plumes at low shear but it breaks down at moderate to high shear where the entire plumes are advected downstream when our results are applied to the earth we find that mantle plumes are mostly divided into only two flow regimes in the upper mantle plumes under slow moving plates experience upstream flow and negligible cross stream circulation while plumes under faster moving plates including all pacific plumes experience significant cross stream circulation and are advected downstream we also demonstrate that geochemical heterogeneities in a plume s source region will result in an azimuthally zoned plume and in an asymmetric geographical distribution of geochemical heterogeneities in the erupted hot spot basalts as is seen in the hawaiian galapagos marquesas and tahiti society island chains for individual mantle plumes we determine their diameter and vertical rise velocity as well as the extent of upstream spreading and the rate of lateral spreading under the lithosphere

diffuse soil emission of hydrothermal gases co2 ch4 and c6h6 at solfatara crater campi flegrei southern italy abstract measurements of soil fluxes of hydrothermal gases with special emphasis on c 6 h 6 as well as chemical composition of mono aromatic compounds in fumaroles and air were carried out in april 2012 at the solfatara crater campi flegrei southern italy to investigate the distribution and behavior of these species as they migrate through the soil from their deep source to the atmosphere soil fluxes of co 2 ch 4 and c 6 h 6 exhibit good spatial correlation suggesting that diffuse degassing is mainly controlled by local fractures the calculated total output of diffuse c 6 h 6 from solfatara is 0 10 kg day 1 whereas fluxes of co 2 and ch 4 are 79 10 3 and 1 04 kg day 1 respectively a comparison between soil gas fluxes and fumarole composition reveals that within the crater soil ch 4 is significantly affected by oxidation processes which are more efficient for low gas fluxes being dependent on the residence time of the uprising hydrothermal gases at shallow depth benzene degradation mainly proceeding through oxidation via benzoate seems to be strongly controlled by the presence of a shallow so 4 2 rich aquifer located in the central and southwestern sectors of the crater suggesting that the process is particularly efficient when so 4 2 acts as terminal electron acceptor so 4 reduction relatively high c 6 h 6 c 7 h 8 ratios typical of hydrothermal fluids were measured in air close to the main fumarolic field of solfatara crater here c 6 h 6 concentrations whose detection limit is 0 1 g m 3 are more than one order of magnitude higher than the limit value for ambient air 5 g m 3 this suggests that hydrothermal fluids have a strong impact on air quality in the immediate surroundings of the fumarolic vents significant concentrations of endogenous mono aromatics were also detected in air samples collected from the northern and western sides of the crater where these gas compounds are mostly fed by diffuse degassing through the crater bottom soil

offshore and onshore seepage of thermogenic gas at katakolo bay western greece abstract considerable seepage of natural gas occurs throughout the katakolo bay both at sea and on land along the ionian coast of peloponnesus western greece explosive levels of ch4 and toxic concentrations of h2s accumulating in the ground pose a severe hazard for humans and tourist infrastructures a wide offshore and onshore gas survey including marine remote sensing underwater exploration by a towed instrumented system compositional and isotopic analyses and flux measurements of gas allowed us to assess that a gas seepage takes place along two main normal faults b offshore side scan sonographs recorded at least 823 gas bubble plumes over an area of 94 200 m2 at depths ranging from 5 5 to 16 m c offshore and onshore seeps release the same type of thermogenic gas 13cch4 34 to 36 d offshore gas showed increased stable carbon isotopic ratio of co2 and propane which suggests enhanced biodegradation of hydrocarbons e isotopic data combined with thermogenic gas generation modeling and maturity plots suggest that the gas is related to a deep petroleum system with jurassic carbonate reservoirs triassic source rocks and triassic evaporites f h2s 34s 2 4 is produced by thermochemical sulfate reduction in deep anhydrites in contact with hydrocarbon rich carbonates g due to the shallow depth more than 90 of ch4 released at the seabed enters the atmosphere consistent with theoretical bubble dissolution models with a mean plume output of 0 12 kg d 1 total offshore ch4 output was estimated in the range of 33 to 120 t y 1 and h in the onshore area at least 50 gas vents in the harbor and a large seep on the adjacent faros hill emit in total about 89 t ch4 y 1 katakolo results to be one of the biggest thermogenic gas seepage zones in europe

poisson s ratio in the lower mantle beneath alaska evidence for compositional heterogeneity 1 using waveform cross correlation we have measured 17 000 p and s wave differential travel times from earthquakes and receivers near a great circle path from japan across alaska to north america we jointly invert the data for variations in shear and bulk sound speed and derived poisson s ratio variations in the mantle beneath 1000 km depth we explore the model space using different levels of regularization and our conclusions are based on a range of models that give acceptable data fits the correlation between vs and vp variations is good to 1500 km depth but gradually degrades in the bottom 1000 km whereas the ratio r lnvs lnvp is between 1 0 and 1 8 at most depths our data suggest that there is no significant correlation between bulk and shear wave speed but they appear anticorrelated in several regions we estimate the effect on the poisson s ratio of changes in temperature iron content and the magnesiowustite mw to perovskite pv ratio at all depths the effect of temperature is largest and that of the pv to mw ratio smallest our data cannot resolve the trade off between thermal and compositional effects but explaining the poisson s ratio variability by temperature alone would require unrealistic perturbations and it would not explain the inferred anticorrelation between shear and bulk sound speed our results suggest that despite r being significantly smaller than 2 5 we need a combination of thermal and compositional effects in order to explain the elastic parameter variations in the deep mantle beneath alaska with realistic values of t of 300 800 k variation in xfe of 4 and pv enrichment or depletion of up to 10 just sufficient for explaining the data

discovery of a huge sector collapse at the nisyros volcano greece by on land and offshore geological structural data abstract this study uses on land and offshore geological and structural data to demonstrate that a huge lateral collapse involved the se flank of nisyros volcano the collapse beheaded the summit part of the volcano and also involved the submarine portion of the slope producing a large debris avalanche deposit with a volume of about 1 km3 which has been recognized on the sea floor on land stratigraphic and structural data indicate that a thick succession of lava flows nikia lavas was emplaced in a huge horseshoe shaped depression open seaward and extending below the sea the magma feeding system in the volcano pre dating and following the collapse was structurally influenced by a dominant ne sw direction which is perpendicular to the newly recognised sector collapse the ne sw structural trend is consistent with the regional tectonic structures found offshore around nisyros and with the related nw se extension direction we suggest that the lateral magma pressure produced by repeated magma injections along tectonic discontinuities contributed to destabilise the volcano flank the occurrence of a pyroclastic deposit that mantled the scar left by the collapse suggests that a magma batch might have been injected inside the volcano and triggered the collapse the lavas of the pre collapse edifice have been deposited in alternating submarine and subaerial environments suggesting that vertical movements might also be a major triggering mechanism for large lateral collapses recognition of this phenomenon is particularly important in recent active island or coastal volcanoes as it can trigger tsunamis

diffuse emission of co2 and convective heat release at nisyros caldera greece abstract the diffuse emission of co2 from the south east sector of nisyros caldera lakki plain has been measured during a detailed survey 1400 soil co2 flux measurements performed in october 2018 the gas emissions are fed by hydrothermal sources and in minor part by the soil biogenic activity whose mean co2 flux 4 g m 2 d 1 is here estimated for the first time the total amount of hydrothermal co2 reaches 92 8 t d a value that is slightly higher than that estimated with the same method between 1999 and 2001 74 7 t d the gas is emitted by different diffuse degassing structures ddss including volcanic hydrothermal structures craters and domes and ne sw and nw se trending tectonic lineaments even if the total co2 emission is not particularly high at nisyros close to the median of co2 emissions measured in volcanoes worldwide the process is very energetic the thermal energy associated with the shallow condensation of the steam in the ddss reaches 60 mw while we estimate at 134 270 mw the total amount of thermal energy involved in the convective rising of the deep geothermal liquids that transport the gas from the depth toward the surface this large flux of energy could dramatically increase during future earthquakes by addition of heat and mass from a deep hydrothermal reservoir potentially triggering hydrothermal explosions as it happened several times in the past few centuries

the effect of depth dependent viscosity on convective mixing in the mantle and the possible survival of primitive mantle the effect depth dependent viscosity has on convective mixing and sampling or degassing of primitive mantle beneath ridges is explored in two dimensional models higher relative viscosities in the deep mantle decrease convection velocities and strain rates and prolong the residence time of material in the deep mantle if the average viscosity of the lower mantle is at least 100 times the viscosity of the upper mantle then some mantle material could have survived from very early in the earth s history if in addition the depth of degassing under ridges has been less than 75 km on average over earth history then helium isotopic systematics are qualitatively consistent with whole mantle convection

deep earth recycling in the hadean and constraints on surface tectonics the hadean mantle was efficiently heated from high internal heat production high rates of impact bombardment and primordial heat from accretion as a result a strong case is made for extremely high internal temperatures low internal viscosities and extremely vigorous mantle convection previous studies of mixing in such high rayleigh number convective environments indicate that chemically heterogeneous mantle anomalies should have efficiently remixed into the mantle on timescales of less than 100 myr however 142nd and 182w isotope studies indicate that heterogeneous mantle domains survived without mixing for over 2 gyr at odds with expected mixing rates similarly concentrations of platinum group elements in archean komatiites purportedly due to the later veneer of meteoritic addition on the earth only achieve current levels at 2 7 ga indicating a time lag of almost 1 to 2 gyr in mixing this material thoroughly into the mantle previous studies have sought to explain slow archean mantle mixing via mantle layering due to endothermic phase changes or anomalously viscous blobs of material with limited efficacy here we pursue another explanation for inefficient mantle mixing in the hadean tectonic regime a number of lines of evidence suggest that resurfacing in the archean was episodic and extending these models to hadean times implies the hadean was characterized by long periods of tectonic quiescence we explore mixing times in 3d spherical cap models of mantle convection which incorporate vertically stratified and temperature dependent viscosities we show that mixing in stagnant lid regimes is at the extreme over an order of magnitude less efficient than mobile lid mixing and for plausible rayleigh numbers and internal heat production the lag in hadean convective recycling can be explained the attractiveness of this model is that it not only explains the long lived 142nd and 182w anomalies but also posits an explanation for the delay between accretion of the late veneer between 4 5 to 3 8 ga on a stagnant surface and its fully mixed signature apparent in elevated pges in 2 7 ga komatiites it also provides an explanation for the 400 myrs of immobility of the mafic protolith from which the jack hill zircons were sourced and retards early heat loss from the mantle providing a solution to the archean thermal catastrophe of parameterized earth evolution models

gas geochemistry and co2 output estimation at the island of milos greece abstract twenty gas samples have been collected from the natural gas manifestations of milos island the majority of which is found underwater along its coast furthermore three anomalous degassing fumarolic areas kalamos paleochori and adamas have been recognized on land almost all the gases are co2 dominated with co2 ranging from 88 to 99 vol for the samples taken underwater while the on land manifestations show a wider range 15 98 due to air contamination methane reaches up to 1 0 vol h2 up to 3 2 vol and h2s up to 3 5 vol indicating a hydrothermal origin of the gases the isotope composition of he points out to mantle contributions up to 45 while the c isotope composition of co2 from 1 9 to 1 3 vs v pdb with most of the values around 0 5 suggests a prevailing limestone origin isotope composition of ch4 ranging from 18 4 to 5 0 vs v pdb for c and from 295 to 7 vs v smow for h points to a geothermal origin with sometimes evident secondary oxidation processes additionally co2 flux measurements showed high values in the three fumarolic areas up to 1100 1500 and 8000 g m2 d at kalamos paleochori and adamas respectively with the highest co2 flux values up to about 23 000 g m2 d being measured in the sea at kanavas with a floating chamber the south western part of the island was covered with a lower density prospection revealing only few anomalous co2 flux values up to 650 g m2 d the total output of the island 30 5 t d is typical of quiescent closed conduit volcanoes and comparable to the other volcanic geothermal systems of the south aegean active volcanic arc nisyros kos nea kameni methana and sousaki

the internal activity and thermal evolution of earth like planets abstract we model the internal thermal evolution of planets with earth like composition and masses ranging from 0 1 to 10 earth masses over a period of 10 billion years we also characterize the internal activity of the planets by the velocity of putative tectonic plates the rate at which mantle material is processed through melting zones and the time taken to process one mantle mass the more massive the planet the larger its processing rate which scales approximately as m 0 8 1 0 the processing times for all the planets increase with time as they cool and become less active as would be expected the surface heat flow scales with planet mass all planets have similar declines in mantle temperature except for the largest in which pressure effects cause a larger decline the larger planets have higher mantle temperatures over all times the less massive the planet the larger the decrease in core temperature with time the core heat flow is also found to decrease more rapidly for smaller planet masses finally rough predictions are made for the time required to generate an atmosphere from estimates of the time to degas water and carbon dioxide in mantle melting zones the degassing times depend strongly on the initial temperature of the planet but for the temperatures used in our model all the planets degas within 32 ma after their formation

hydrothermal eruptions of nisyros dodecanese greece past events and present hazard abstract the detailed analysis of the craters of hydrothermal eruptions and related products present on nisyros island demonstrates the ephemerality of these morphological forms in other words the mere recognizable existence of the craters and associated deposits implies recency of hydrothermal activity the minimum temperature required to cause the explosive phenomenon and possibly the depth of the reservoir which can be evaluated on the basis of the correlation between the diameter of the crater and the depth of explosion as proposed by fytikas and marinelli 1976 are therefore closely representative of the current hydrothermal circulation both field evidence and historical records indicate that all the deposits of hydrothermal eruption recognized on nisyros island were emplaced as debris flows almost all the ballistic ejecta were entrained in these debris flows and either redeposited far from their landing sites or involved in later crater collapse and erosion this emplacing mechanism implies that the original products were characterized by a water content higher than about 5 by weight steam driven hydrothermal eruptions one of which took place in 1871 originated deposits of limited dispersion as no sign of these erodible products can be found in the field today surface geology and fluid geochemistry together with subsurface information e g primary and hydrothermal lithologies distribution of temperature with depth physical chemical characteristics of deep water bearing zones indicate that two distinct hydrothermal aquifers are present underneath the southeastern part of the caldera floor both aquifers were probably involved in the most important historically documented hydrothermal eruptions which occurred in 1873 at that time violent earthquakes fractured the brittle aquiclude separating the two aquifers and caused a sudden transfer of fluids from the deep to the shallow aquifer thus triggering the hydrothermal eruptions hydrothermal eruptions will probably occur in future and this hazard must be taken into serious consideration the southern half of lakki plain where all past eruptions took place and active fumaroles are concentrated is the zone at highest risk at present gas geochemistry represents an effective tool to detect changes in the p t conditions of the shallow aquifer and particularly the phenomena of pressure build up that may lead to a hydrothermal eruption

on the emergence of plate tectonics at present young oceanic lithosphere is positively buoyant and it does not become negatively buoyant until it is older than about 20 m y if in the past the mantle was hotter the oceanic crust would have been thicker and the lithosphere9s age of neutral buoyancy greater on the other hand the hotter mantle would have had a lower viscosity and convected faster so the average age of oceanic plates at subduction would have been less than the present 100 m y at some time in the past average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone it is estimated here that this condition occurred when the mantle was only about 50 c hotter than at present and as recently as 0 9 1 4 b y ago transformation of basaltic crust to eclogite might have enhanced the vigor of the plates but possibly only modestly and intermittently in earlier times plate tectonics could have operated but more slowly so that it could not by itself have accomplished the necessary rate of heat removal from the earth a different tectonic mode must also have operated perhaps subcrustal delamination or dripping plate tectonics would have gradually taken over from the earlier mode plumes are a omplementary mode of mantle convection driven by a lower thermal boundary layer and so they could have operated in conjunction with both regimes of the upper boundary layer

heterogeneity and time dependence in 3d spherical mantle convection models with continental drift abstract feedback between continents and large scale mantle flow through thermal blanketing has long been surmised as a mechanism for continental drift and wilson cycles paleomagnetism provides evidence for extensive continental displacements 10 000 km on time scales of 100 200 million years comparable to an intrinsic overturn in whole mantle convection here we model continental motions in vigorous 3d spherical convection models focusing on the effects of continent size mantle heating mode and a strong increase in lower mantle viscosity continents covering 30 10 and 3 of earth s surface representative of the former supercontinent pangea present day asia and antarctica respectively are introduced into simple end member mantle convection models characterized by pure core or internal heating and uniform or layered mantle viscosity supercontinents promote temperature anomalies on the largest scales spherical harmonic degrees 1 and 2 primarily through the organization of the long wavelength convective planform inherent in models with a high viscosity lower mantle bottom heating can promote long wavelength heterogeneity by clustering plumes beneath the continent however in isoviscous models small scale structure persists away from the continent regardless of the heating mode supercontinents respond to long wavelength heterogeneity by following great circle paths with variations in velocity on time scales of 1 billion years smaller continents are unable to promote long wavelength structure and the resulting motions are governed by bursts in velocity on time scales of the order of 100 million years continental velocities are roughly a factor of 3 smaller than those in oceanic regions an observation that may help explain the observed difference in the speed of predominantly continental or oceanic plates

superplumes plates and mantle magmatism in two dimensional numerical models 1 two dimensional numerical models are systematically presented for magmatism in a convecting mantle internally heated by incompatible radioactive elements plate tectonics is self consistently modeled as a part of convection of newtonian fluid with temperature and stress history dependent viscosity driven by thermal and compositional buoyancy while magmatism is modeled as an upward migration of basaltic magma generated by decompression melting ridge magmatism actively takes place to chemically differentiate the mantle and a superplume is well developed in the lowermost mantle as a broad hot region enriched in basaltic component provided that the lithosphere is mechanically strong enough to inhibit spontaneous initiation of subduction by its own weight and that the internal heating rate is sufficiently high hot narrow plumes frequently rise up from the superplume to induce deep magma generation and hot spot magmatism which in turn frequently induces new plate margins the frequent plate margin formation keeps the activity of plate tectonics rather steady the thermal and chemical state of the mantle remains rather steady too as the internal heating rate becomes lower however the narrow plumes from superplume become sporadic and significantly colder deep magma generation becomes rare and new plate margin formation becomes less frequent as a consequence the activity of plate tectonics and the thermal and chemical state of the mantle become fluctuating much with time the mantle structure and plume behavior modeled here match with many observations for those of the earth from the archean to the present

chemical stratification in a two dimensional convecting mantle with magmatism and moving plates 1 a self consistent numerical model is presented for magmatism in a convecting mantle with moving plates mantle convection is modeled as a thermal chemical convection of binary eutectic material with newtonian rheology in a two dimensional rectangular box internally heated by radioactive elements viscosity is assumed to depend on stress history as well as temperature and pressure to self consistently reproduce moving plates the barrier effect of the solid solid phase transitions at depths around 660 km to convective flow across the 660 km phase boundary is also taken into account magmatism is modeled as a permeable flow of basaltic melt produced upon decompression melting of the convecting material through the coexisting matrix magmatism makes mantle chemically stratified with the shallower part occupied by chemically buoyant residue of magma and the deeper part occupied by hot but chemically dense materials enriched in basaltic component even under the influence of moving plates and subducting slabs the magmatism is induced by moving plates ridge volcanism mantle overturn and hot plumes uprising from the 660 km phase boundary or the lower mantle depending upon the internal heating rate and the strength of the 660 km barrier a weaker 660 km barrier leads to more continuous plate tectonics when plates move subducting slabs penetrate deep into the lower mantle and induce broad thermal and chemical heterogeneity at depth in the lower mantle the chemically stratified mantle with moving plates is compared to the earth s mantle

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

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

comment on mixing by time dependent convection by u christensen commentaire et reponse concernant l article de u christensen earth planet sci lett 95 382 394 1989

geochemical processing in a three dimensional regional spherical shell model of mantle convection 1 a three dimensional regional spherical model of processing of oceanic crust through subduction stirring and remelting confirms the main results from previous two dimensional models and from three dimensional models with low rayleigh number and only depth dependent viscosity the new model features high rayleigh number strongly temperature dependent viscosity a set of realistic surface plates and oceanic crust simulated by passive tracers it confirms that the processing timescale is the main determinant of the age distributions and mean ages of mid ocean ridge basalt morb and ocean island basalt oib samples and also of the present degree of processing of the mantle which is 99 in this model the melting depth decreases with time accounting for its temperature dependence and so the processing timescale increases with time an approximate processing theory reveals that morb and oib ages depend mainly on the recent processing timescale whereas the degree of processing depends mainly on the initial processing timescale

stagnant slab a review a stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above across or below the 660 km discontinuity this phenomenon has now been widely recognized beneath subduction zones around the circum pacific and in the mediterranean collaboration of seismic and electromagnetic observations mineral physics measurements and geodynamic modeling has begun to provide a consistent picture of stagnant slab

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

carbon bearing gas geothermometers for volcanic hydrothermal systems abstract the genetic relationship between carbon bearing species co co 2 ch 4 c 2 h 6 c 3 h 8 c 2 h 4 and c 3 h 6 was investigated in volcanic hydrothermal gases emitted from nisyros greece vesuvio la solfatara campi flegrei and pantelleria all italy apparent carbon isotopic temperatures of the ch 4 co 2 system are 360 c at nisyros 420 460 c at vesuvio 450 c at la solfatara and 540 c at pantelleria these temperatures are confirmed by measured propene propane and h 2 h 2 o concentration ratios ch 4 and co 2 equilibrate in the single liquid phase prior to the onset of boiling whereas propene and propane attain equilibrium in the saturated water vapor phase boiling in these high enthalpy hydrothermal systems might occur isothermally once vapor has been extracted from the parental liquid co co 2 responds most sensitively to the temperature gradient encountered by the ascending gases our results imply that the ch 4 co 2 isotopic geothermometer can provide reliable information about temperatures of deep hydrothermal liquids associated with volcanism propene propane and h 2 h 2 o concentration ratios should be measured along with the carbon isotopic composition of co 2 and ch 4 to provide independent constraints on the geological significance of the apparent carbon isotopic temperatures

investigation of 3 d basin structures in the izmit bay area turkey by single station microtremor and gravimetric methods summary we investigate 3 d basin structures and site resonance frequencies in the izmit bay area of turkey by new geophysical surveys based on 239 single station microtremor and 405 point gravity measurements a fundamental resonance frequency map of the izmit bay was produced from the main peak in the horizontal to vertical component spectral ratio hvsr of microtremors the hvsr analysis of the microtremor data reveals single double broad peaked or no peak type hvsr curves varying in accordance with the surface geology and spatial extent of the three basins present in the izmit bay area in the deepest sections of the izmit golcuk and derince basins of the izmit bay the fundamental resonance frequencies are dominantly 0 24 0 30 hz these resonance frequencies should be taken into consideration along with higher mode frequencies to construct earthquake resistant structures in the izmit bay area the 3 d gravimetric bedrock depth map of the izmit basin shows that the basin has an asymmetric shape with its deepest section coinciding with the surface trace of the north anatolian fault the northern shoulder of the basin has a gentle dip on the kocaeli peneplain side and the southern shoulder is much steeper and it is bounded by the samanli mountains we derive a power law relationship that correlates the fundamental site resonance frequencies with the sedimentary cover thickness obtained from the gravity and shear wave velocity data in the izmit bay we use this relationship to estimate bedrock depths beneath golcuk and derince basins our estimation of maximum basin depths is 1400 m for the izmit and is 800 m for the golcuk and derince basins finally we have analysed a converted sp phase from a local earthquake recording made at site cmp to calculate and verify the sediment thickness estimations obtained from our gravimetric and microtremor analyses this calculation shows close agreement with that of the gravimetric and microtremor results our results show that the basins in the izmit bay area have a very thick sedimentary cover with very low shear velocities underlined by hard bedrock forming a sharp impedance contrast we anticipate that these results will be a key contribution to the quantitative assessment of seismic hazard for the izmit bay area before the occurrence of strong earthquakes in the marmara region

constraining the geometries of small scale heterogeneities a case study from the mariana region a new array based method for seismic waveform data is developed to detect small scale heterogeneities in the mantle and constrain their possible locations and sizes detecting small scale heterogeneities using scattered waves has been challenging because scattered phases usually suffer from low signal to noise ratios and determining the shapes of such heterogeneities is even more difficult because of isochronal artifacts in the proposed method the problem of the low signal to noise ratio is addressed by adopting dual bootstrap stacking which can detect weak signals even with a limited number of seismic traces as well as by ensuring reproducibility among multiple events the locations of detected scatterers and their sizes and shapes are then estimated using a new kind of hierarchical cluster analysis that could minimize the effect of isochronal artifacts the new method is applied to a previously analyzed data set of mariana earthquakes to facilitate a comparison with conventional methods in the mid to lower mantle beneath the mariana trench region a total of 39 scatterers are identified via s to p scattering with their potential volumes ranging from 8 104 to 1 106 km3

lower mantle scattering profiles and fabric below pacific subduction zones abstract the earth s mantle is compositionally evolving principally through the process of mid ocean ridge basalt production 95 by lithospheric area but also by way of arc magmatism 5 and continental growth reymer a schubert g 1984 phanerozoic addition rate to the continental crust and crustal growth tectonics 3 63 77 rowley d b 2002 rate of plate creation and destruction 180 ma to present geol soc am bull 114 927 933 convection eventually must mix these products back into the mantle but the details of the mixing process are unknown we report here new observational data documenting the location of differentiated material in the present day lower mantle and scattering power profiles with depth small scale 10 km objects which efficiently scatter short period s waves are present in the mantle lying below the pacific ocean rim at depths between 1100 and 1800 km we also observe a reduction in scattering power with depth indicating that the objects represent bodies richer in differentiated basalt than the surrounding mantle the scattering objects appear to be deformed to various degrees and usually form clusters separated from each other by a distance of 100 to 200 km the clusters probably represent folds of the order of 100 km probably corresponding to early stage of mixing process because the scattering power does not increase with depth neither does a permanent barrier to flow exist in the mid mantle nor does the intrinsic density of basalt prevent re entrainment into mantle flow from the bottom of the mantle

mantle plumes and flood basalts enhanced melting from plume ascent and an eclogite component new numerical models of starting plumes reproduce the observed volumes and rates of flood basalt eruptions even for a plume of moderate temperature arriving under thick lithosphere these models follow the growth of a new plume from a thermal boundary layer and its subsequent rise through the mantle viscosity structure they show that as a plume head rises into the lower viscosity upper mantle it narrows and it is thus able to penetrate rapidly right to the base of lithosphere where it spreads as a thin layer this behavior also brings the hottest plume material to the shallowest depths both factors enhance melt production compared with previous plume models the model plumes are also assumed to contain eclogite bodies inferred from geochemistry to be recycled oceanic crust previous numerical models have shown that the presence of nonreacting eclogite bodies may greatly enhance melt production it has been argued that the eclogite derived melt would react with surrounding peridotite and refreeze however recent experimental studies indicate that eclogite derived melts may have reached the earth s surface with only moderate or even minor modification combined with an assumed 15 vol component of eclogite our models yield a sharp peak in melting of about 1 3 myr duration and volumes of melt that encompass those observed in flood basalt provinces

shear wave velocity structure of the izmit bay area turkey estimated from active passive array surface wave and single station microtremor methods summary to provide quantitative information on the shear wave velocity structure of the izmit bay area we conduct active passive array surface wave and single station microtremor measurements at 60 sites we process these array measurements to produce combined rayleigh wave dispersion curves that span a broad frequency range 0 5 20 hz we also make use of horizontal to vertical spectral ratio hvsr curves obtained from the single station microtremor measurements to benefit from their close relation to the ellipticity of rayleigh waves using the dispersion curve sensitivity to the absolute velocities and the shape of the hvsr curve sensitivity to the velocity contrasts of the velocity depth model we have applied a combined inversion technique to derive better constrained shear velocity models at each site the derived shear velocity models are used to provide a vs30 site classification map and information on the sediment bedrock structure for the izmit bay area the vs30 map shows that the entire shoreline regions including artificial infill areas of the izmit bay have vs30 values less than 200 m s 1 locally as low as 80 m s 1 the older sediment areas of the izmit basin have vs30 values ranging between 250 and 350 m s 1 the highest vs30 values are associated with rock sites reaching about 950 1300 m s 1 we also present a high correlation between the vs30 values and the phase velocity of rayleigh wave corresponding to the 40 m wavelength c40 the important implication of this correlation is that vs30 may be estimated from c40 without inverting the dispersion curve the inferred sediment bedrock interface along a cross section shows an antisymmetric v shaped basin with a sedimentary cover thickness reaching about 1200 m at the deepest part of the izmit basin this deepest part coincides in the map view where the north anatolian fault zone crosses the basin in the east west direction the sedimentary cover thickness is found to be 750 m in the golcuk derince basin

soil co2 emissions at furnas volcano sao miguel island azores archipelago volcano monitoring perspectives geomorphologic studies and land use planning application accepted for publication in journal of geophysical research copyright 2010 american geophysical union