three dimensional mantle dynamics with an endothermic phase transition three dimensional convection for the spinel to perovskite phase change has been simulated numerically results for rayleigh ra numbers of 0 106 show intermittent layering with a strong robust plume rising through the phase boundary many descending instabilities are deflected but merging cold sheets come together at a junction a pool of cold material accumulates underneath in the phase transition zone a strong gravitational instability results which precipitates a rapid and massive discharge of upper mantle material

recent current observations in the eastern yellow sea observations of current velocity pressure and temperature in the eastern yellow sea from january 10 through april 12 1986 together with geostrophic winds calculated from surface atmospheric pressure distributions are analyzed for a study of the synoptic band response of the yellow sea to the wintertime winds north wind pulses in the winter monsoon are found to give rise to northward flow bursts in the yellow sea trough which appear to peak just as the wind relaxes and to be driven by a synoptic band north to south rise in bottom pressure synchronous with the north wind pulses the existence of a similar rise in coastal sea level indicates that the observed southward pressure rise is but an offshore extension of a setup along the west coast of south korea forced by the north wind pulses the advection by northward flow bursts of warm water from the south apparently causes the measured temperature to be substantially higher in the trough than in coastal waters at the same latitude and the observed rate of cooling in the trough to be less than half of that in coastal waters to the east a winter to spring transition is also found in the bottom pressure records obtained in the trough in which the mean north to south gradient in winter reverses suddenly in early march a similar reversal also appears in sea level records along the west coast of south korea the reversal appears to be triggered by the weakening of the north winds at the arrival of spring

the origin of low δ18o granites and related rocks from the seychelles the 750 ma seychelles granites have whole rock 18o values that range from 1 2 to 7 5 differences in 18o values between quartz feldspar and whole rock for samples that have a range of 18o values suggest that these minerals are in magmatic equilibrium and that the whole rock 18o values were little affected by post crystallization interaction with fluids two suites of granites the mahe type and the praslin type have previously been recognized on the basis of chemical and radiogenic isotope composition the former have a mean whole rock 18o value of 5 25 0 65 1 whereas the latter are much more variable with a mean whole rock 18o value of 3 00 3 27 biotite and amphibole separates from both granite types range in d from 69 to 116 and show a positive correlation with whole rock 18o values dolerite dykes that intruded the granites during or shortly after their crystallization also have whole rock 18o values mean 2 24 1 93 lower than that expected for mantle derived basaltic rocks but these values appear to be the result of fluid rock interaction we suggest that the mahe type granites are derived mainly from juvenile mafic to intermediate crust with normal d and 18o values whereas the praslin type granites are mixtures of this source and older crust that acquired its low d and 18o values by extensive interaction with meteoric water at high temperature it is unlikely that meteoric water sufficiently depleted in d and 18o was available at 750 ma because of the relatively low latitude of the region at that time we suggest that alteration of the source took place significantly before production of the granite magmas depletion in 18o of the seychelles granites does not necessarily require a regional extensional tectonic setting at 750 ma as has been proposed

stable isotope and fluid inclusion evidence for the origin of the brandberg west area sn w vein deposits nw namibia the brandberg west region of nw namibia is dominated by poly deformed turbidites and carbonate rocks of the neoproterozoic damara supergoup which have been regionally metamorphosed to greenschist facies and thermally metamorphosed up to mid amphibolite facies by neoproterozoic granite plutons the meta sedimentary rocks host damaran age hydrothermal quartz vein hosted sn w mineralization at brandberg west and numerous nearby smaller deposits fluid inclusion microthermometric studies of the vein quartz suggests that the ore forming fluids at the brandberg west mine were co2 bearing aqueous fluids represented by the nacl cacl2 h2o co2 system with moderate salinity mean 8 6 wt naclequivalent temperatures determined using oxygen isotope thermometry are 415 521 c quartz muscovite 392 447 c quartz cassiterite and 444 490 c quartz hematite at brandberg west the oxygen isotope ratios of quartz veins and siliciclastic host rocks in the mineralized area are lower than those in the rocks and veins of the surrounding areas suggesting that pervasive fluid rock interaction occurred during mineralization the o and h isotope data of quartz muscovite veins and fluid inclusions indicate that the ore fluids were dominantly of magmatic origin implying that mineralization occurred above a shallow granite pluton simple mass balance calculations suggest water rock ratios of 1 88 closed system and 1 01 open system the co2 component of the fluid inclusions had similar 13c to the carbonate rocks intercalated with the turbidites it is most likely that mineralization at brandberg west was caused by a combination of an impermeable marble barrier and interaction of the fluids with the marble the minor deposits in the area have quartz veins with higher 18o values which is consistent with these deposits being similar geological environments exposed at higher erosion levels

oxygen and hydrogen isotope stratigraphy of the rustenburg layered suite bushveld complex constraints on crustal contamination new do values for plagioclase pyroxene and olivine and limited whole rock dd values are presented for samples from the rustenburg layered suite of the bushveld complex south africa in combination with existing data these provide a much more complete composite o isotope stratigraphy for the intrusion throughout the layered suite mineral do values indicate that the magmas from which they crystallized had do values that were about 7 1 that is 1 4 higher than expected for mantle derived magmas suggesting extensive crustal contamination more limited h isotope data suggest that the oh present within whole rocks regardless of the degree of alteration is of magmatic origin and not an alteration phenomenon there appears to be no systematic change in do value with stratigraphic height and this requires the contamination to have taken place in a staging chamber before emplacement of the magma s into the present chamber large amounts 30 40 of contamination by the lower to middle crust are needed to explain these do values which is in general agreement with previous estimates based on srand nd isotope data alternatively smaller amounts of contamination 20 by sedimentary rocks or their partial melts represented by the country rock can explain the data but it is not apparent how such material could have been present at the depth of the staging chamber in the lower to middle crust

whole layer convection in a heterogeneous planetary mantle 1 geochemical observations demonstrate that earth s mantle is heterogeneous but the sizes forms and characteristics of its reservoirs are not constrained using laboratory experiments we have systematically studied a simple nonhomogeneous system where two layers of miscible fluids with different densities depths and viscosities are subjected to a destabilizing temperature contrast when the buoyancy number b i e the ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly is lower than 0 3 0 5 the whole layer regime develops with a deformed interface and convective patterns over the whole tank depth the system systematically evolves toward one fluid rayleigh benard convection because of stirring however the two isolated fluids can persist for very long time compared to the characteristic timescale of thermal convection and give rise to numerous transient behaviors of particular interest for planetary mantles are the pulsatory dynamics where the interface between the two layers deforms in large domes moving up and down quasiperiodically according to the scaling laws derived from the experiments such a mechanism could indeed provide a simple and single physical explanation for the superswells observed at present on earth and more generally for the long term episodicity in planetary interiors observed in geological records

the combined effects of continents and the 660 km depth endothermic phase boundary on the thermal regime in the mantle abstract we explore a wide range of parameter space to investigate the dynamical interaction between effects due to surface boundary conditions representing continental and oceanic lithosphere and the endothermic phase boundary at 660 km depth in simple two dimensional convection calculations phase boundary induced mantle layering is strongly affected by the wavelength of convective flows and mixed surface boundary conditions strongly increase the horizontal wavelength of convection our study shows that for mixed cases the effects of the surface boundary conditions dominate the effects of the phase boundary we show that the calculations with complete continental coverage have the most significantly decoupled lower and upper mantle flows and substantial thermal and mechanical layering unlike the free slip case where the surface heat flow decreases substantially with increasing magnitude of the clapeyron slope surface heat flow is shown to be almost independent of the clapeyron slope for mixed boundary condition cases although very different when not layered models with free and mixed surfaces have very similar planforms with very large aspect ratio flows when run with large magnitudes of the clapeyron slope we also calculate the critical boundary layer rayleigh number as a measure of the thermal resistance of the surface boundary layer our results show that the thermal resistance in the oceanic and the continental regions of the mixed cases are similar to fully free and no slip cases respectively we find that even for purely basally heated models the mantle becomes significantly subadiabatic in the presence of partial continental coverage this is due to the significant horizontal advection of heat that occurs with very large aspect ratio convection cells

sharpness of upper mantle discontinuities determined from high frequency reflections an understanding of the nature of seismic discontinuities in the earth s upper mantle is important for understanding mantle processes in particular the amplitude and sharpness of these discontinuities are critical for assessing models of upper mantle phase changes and chemical layering so far seismic studies aimed at determining the thickness and lateral variability of upper mantle discontinuities have yielded equivocal results particularly for the discontinuity at 410km depth1 2 here we present short period 0 8 2 0 s recordings of upper mantle precursors to the seismic phase p p pkppkp from two south american earthquakes recorded by the 700 station short period array in california our results show that the 410 and 660 km discontinuities beneath the indian ocean are locally simple and sharp corresponding to transi tion zones of 4 km or less these observations pose problems for mineral physics models3 5 which predict a transitional thickness greater than 6 km for the peridotite to spinel phase transition in contrast to the results of long period studies6 7 we observe no short period arrivals from near 520 km depth

is the 670 km phase transition able to layer the earth s convection in a mantle with depth dependent viscosity the effect of a viscosity stratification on phase change dynamics have been investigated with axi spherical convection models as in previous studies with a constant viscosity mantle an intermittent layering appears for a clapeyron slope from 2 mpa k to 3 mpa k a viscosity increase in lower mantle requires a more negative clapeyron slope to produce the layering this shift is sensitive to the mechanical boundary condition with a viscosity contrast of 30 a no slip top condition does not lead to layering in the range of the possible values for the clapeyron slope with a free slip condition the threshold is at 4 mpa k just below this threshold a whole mantle circulation driven by a cylindrical hot plume coexists with layered mantle domains over several billion years

estimation of ocean subsurface thermal structure from surface parameters a neural network approach 1 satellite remote sensing provides diverse and useful ocean surface observations it is of interest to determine if such surface observations can be used to infer information about the vertical structure of the ocean s interior like that of temperature profiles earlier studies used either sea surface temperature or dynamic height sea surface height to infer the subsurface temperature profiles in this study we have used neural network approach to estimate the temperature structure from sea surface temperature sea surface height wind stress net radiation and net heat flux available from an arabian sea mooring from october 1994 to october 1995 deployed by the woods hole oceanographic institution on the average 50 of the estimations are within an error of 0 5 c and 90 within 1 0 c the average rms error between the estimated temperature profiles and in situ observations is 0 584 c with a depth wise average correlation coefficient of 0 92

filling the bushveld complex magma chamber lateral expansion roof and floor interaction magmatic unconformities and the formation of giant chromitite pge and ti v magnetitite deposits his mind was like a soup dish wide and shallow irving stone on william jennings bryan a compilation of the sr isotopic stratigraphy of the bushveld complex shows that the evolution of the magma chamber occurred in two major stages during the lower open system integration stage lower critical and lower main zone there were numerous influxes of magma of contrasting isotopic composition with concomitant mixing crystallisation and deposition of cumulates larger influxes correspond to the boundaries of the zones and sub zones and are marked by sustained isotopic shifts major changes in mineral assemblages and development of unconformities during the upper closed system differentiation stage upper main zone and upper zone there were no major magma additions other than that which initiated the upper zone and the thick magma layers evolved by fractional crystallisation the lower and lower critical zones are restricted to a belt that runs from steelpoort and burgersfort in the northeast to rustenburg and northam in the west and an outlier of the lower and lower critical zone up to the lg4 chromitite layer in the far western extension north of zeerust it is only in these areas that thick harzburgite and pyroxenite layers are developed and where chromitites of the lower critical zone occur these chromitites include the economically important c 1 m thick lg6 and mg1 layers exposed around both the eastern and western lobes of the bushveld complex the upper critical zone has a greater lateral extent than the lower critical zone and overlies but also onlaps the floor rocks to the south of the steelpoort area the source of the magmas also appears to have been towards the south as the mg chromitite layers degrade and thin northward whereas the lg layers are very well represented in the north and degrade southward sr and os isotope data indicate that the major chromitite layers including the lg6 mg1 and ug2 originated in a similar way extremely abrupt and stratigraphically restricted increases in the sr isotope ratio imply that there was massive contamination of intruding melt which hit the roof of the chamber and incorporated floating granophyric liquid which forced the precipitation of chromite kruger 1999 kinnaird et al 2002 therefore each chromitite layer represents the point at which the magma chamber expanded and eroded and deformed its floor nevertheless this was achieved by in situ contamination by roof rock melt of the intruding critical zone liquids that had an orthopyroxenitic to noritic lineage the main zone is present in the eastern and western lobes of the bushveld complex where it overlies the critical zone and onlaps the floor rocks to the south and the north where it is also the basal zone in the northern lobe the new magma first intruded the northern lobe north of the thabazimbi murchison lineament interacted with the floor rocks incorporated sulphur and precipitated the platreef along the floor rock contact before flowing south into the main chamber this exceptionally large influx of new magma then eroded an unconformity on the critical zone cumulate pile and initiated the main zone in the main chamber by precipitating the merensky reef on the unconformity the upper zone magma flowed into the chamber from the southern bethal lobe as well as the tml this gigantic influx eroded the main zone rocks and caused very large scale unconformable relationships clearly evident as the gap areas in the western bushveld complex the base of this influx which is also coincident with the pyroxenite marker and a troctolitic layer in the northern lobe is the petrological and stratigraphic base of the upper zone sr isotope data show that all the pge rich ores including chromitites are related to influxes of magma and are thus related to the expansion and filling of the magma chamber dominantly by lateral expansion with associated transgressive disconformities onto the floor rocks coincident with major zone changes these positions in the stratigraphy are marked by abrupt changes in lithology and erosional features over which succeeding lithologies are draped the outcrop patterns and the concordance of geochemical isotopic and mineralogical stratigraphy indicate that during crystallisation the bushveld complex was a wide and shallow lobate sill like sheet and the rock strata and mineral deposits are quasi continuous over the whole intrusion

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

occurrence of large temperature inversion in the thermohaline frontal zone at the yellow sea entrance in winter and its relation to advection temperature inversion higher temperature at a deeper depth in winter and its relation to advection were investigated by analyzing both conductivity temperature depth data in the southern yellow sea ys and northwestern east china sea during the winter of 2002 2003 and time series data of temperature salinity and currents at a buoy station at the ys entrance significant temperature inversions occur predominantly along the thermohaline front at the ys entrance where the cheju warm current water cwcw and the cold coastal waters meet in february 2003 on the northern frontal zone along 34 n where isotherms and isohalines declined downward to the north particularly large inversions with temperature differences of larger than 2 0 c were observed to occur more in troughs than in the crests of the wave like frontal meander where the cold korean coastal water kcw advances farther southward the inversion persisted until mid april at the buoy station in the frontal zone and both temperature and salinity showed simultaneous variations in the same manner during episodic occurrences of large inversions temperature and salinity decreased sharply in the upper layer but increased concurrently in the lower layer these episodic inversions were found to be closely related to the westward advection of the kcw in the upper layer and the northward advection of the cwcw in the lower layer it is considered that these advections may play an important role in maintaining baroclinicity in the northern frontal zone which is responsible for driving the westward transversal flow across the ys entrance

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

induced compositional layering in a convecting fluid layer by an endothermic phase transition abstract an endothermic phase change in the presence of chemical heterogeneities can act as an efficient filter such that heavy material is sequestered below the phase change and light material is trapped above the phase change due to the fact that the endothermic phase change damps thermal convection it can act as a trap door for chemical heterogeneities ascending flows which are chemically dense will need more thermal buoyancy to rise through the transition than ascending flows whose composition is closer to the background composition likewise descending flows which are chemically light need more thermal buoyancy to sink through the transition the results of a suite of numerical calculations of two dimensional double diffusive convection in a fluid layer with an endothermic phase change suggests that if the 670 km seismic discontinuity has a steep enough clapeyron slope compositional layering may be induced and dynamically maintained by the transition furthermore these calculations also show that the phase change and compositional boundaries will approximately coincide

interaction of small mantle plumes with the spinel perovskite phase boundary implications for chemical mixing abstract geochemical observations of ocean island basalts show a broad variety in the chemical composition of trace elements these observations cannot be explained by different temperature and pressure conditions at the melt production region only but also demand differences in the parental material at the base of the lithosphere here we add a new explanation for this finding based on the different interaction of small and large plumes with the spinel perovskite phase boundary spb in the earth s mantle we assume that mantle plumes detaching from the core mantle boundary cmb are of variable size and excess temperature while rising through the lower mantle they form an approximately spherical plume head when arriving at the spb they are retained from further rise depending on their size and excess temperature we have investigated the interaction of a plume with the spb using a numerical approach for viscous flow to focus particularly on this interaction we simply modelled the initial plume as a hot volume of variable cross section and excess temperature volumes with a radius exceeding about 150 km and an excess temperature of more than 100 c rapidly cross the spb with decreasing size and increasing excess temperature volumes are retarded at the spb and those with a radius smaller than about 80 km stagnate entirely below the spb these results are roughly independent of the particular rheology or two dimensional versus three dimensional geometry in the present study we investigated the degree of thermal entrainment of material at the spb for strongly retarded plumes we estimate the amount of chemical mixing by tracking passive markers from the original plume volume and the regions around the spb to the base of the lithosphere the amount of entrainment of material from close to the spb depends on the retardation time and the thermal growth of the plume head at the spb plume heads which reach the base of the lithosphere after a retardation time of more than 50 ma might entrain in the numerical model large amounts of material from the sbp but they may not be characteristic for the earth since they might be distorted by large scale mantle flow however even if we limit the retardation time to reach the base of the lithosphere to 50 ma plumes originally composed of material from the cmb and the adjacent lower mantle might easily entrain up to 15 of their volume of material originating from the spb

carbon and oxygen isotope variations in diamond and graphite eclogites from orapa botswana and the nitrogen content of their diamonds abstract in eclogite xenoliths from orapa botswana 13 c of graphite varies between 4 6 and 7 8 vs pdb while that of diamonds ranges from 4 0 to 22 3 in graphite eclogites 18 o values of garnets fall between 5 2 and 7 vs smow and those of clinopyroxenes between 4 6 and 6 5 for diamond eclogites the respective ranges are wider i e 3 9 to 9 2 and 5 2 to 8 3 the 18 o enrichment above normal mantle oxygen isotopic compositions 5 to 6 5 and the wide 13 c range appear to occur in parts of the mantle in which diamond is stable eclogites with 13 c values in the range 4 to 8 and 13 o values in the normal mantle range tend to have positive garnet clinopyroxene 18 o fractionations and lower fe ca and fe mg ratios than eclogites which have simultaneously 13 c values lower than 8 and 18 o values higher than 6 5 in these eclogites the garnet clinopyroxene fractionations tend to be negative positive garnet clinopyroxene 18 o fractionations increase slightly in the sequence diamond eclogites 0 36 0 15 n 6 graphite diamond eclogites 0 46 0 36 n 3 and graphite eclogites 0 53 0 15 n 3 neither igneous fractionation processes subduction nor metasomatic alteration can satisfactorily explain all observed chemical and isotopic trends

structure of axisymmetric mantle plumes results of high resolution numerical calculations of axisymmetric thermal plumes in an infinite prandtl number fluid with thermally activated viscosity are used to infer the structure of axisymmetric subsolidus thermal plumes in the earth s lower mantle we calculate the velocity and temperature distribution for axisymmetric convection above a heated disk in an incompressible fluid cylinder 2400 km in height and 1200 km in diameter initially at a uniform temperature t the lower boundary representing the plume source region near the core mantle boundary is stress free and maintained at temperature tc t the upper boundary is permeable allowing the plume to escape the cylinder we determine the sensitivity of plume structure to viscosity variations c ranging from 102 to more than 104 and temperature differences t tc t in the range 400 850 k starting plumes consist of a large leading diapir and a trailing conduit connected to the thermal boundary layer above the heated surface the leading diapir expands during ascent to about 400 km diameter at 2200 km height in steady state the plume consists of a narrow high velocity conduit imbedded within a broader thermal halo the width of both these structures is proportional to height above the heated boundary and inversely proportional to t and the viscosity ratio heat transport in the plume is nearly independent of viscosity variations a simplified boundary layer model yields a heat transfer law nu ra where nu is the plume nusselt number and ra is the rayleigh number based on t and the radius of the heated circular region feeding the plume calculated steady state plume heat transports fit this law to within a few percent we present several calculations of plumes with heat transport in the range 100 400 gw similar to the advective heat transport by the hawaiian hotspot at the top of the lower mantle plumes with large viscosity variations have high velocity conduits 50 60 km in diameter and 125 km diameter thermal halos a centerline temperature anomaly of about 500 k and a centerline velocity of about 75 cm yr 1 plumes with weak viscosity variations are approximately twice as broad and have centerline temperature anomalies less than 160 k at the same height and much smaller velocity large viscosity variations and a minimum heat transport are both

stable isotope study of the archaean rocks of the vredefort impact structure central kaapvaal craton south africa the vredefort dome in the kaapvaal craton was formed as a result of the impact of a large meteorite at 2 02 ga the central core of archaean granitic basement rocks is surrounded by a collar of uplifted and overturned strata of the witwatersrand supergroup exposing a substantial depth section of the archaean crust orthogneisses of the core show little variation in whole rock 18o value with the majority being between 8 and 10 with a mean of 9 2 n 35 quartz and feldspar have per mil differences that are consistent with o isotope equilibrium at high temperatures suggesting minimal interaction with fluids during subsequent cooling these data refute previous suggestions that the outer granite gneiss ogg and inlandsee leucogranofels ilg of the core represent middle and lower crust respectively granulite facies greenstone remnants from the ilg have 18o values that are on average 1 5 higher than the ilg host rocks and are unlikely therefore to represent the residuum from the partial melting event that formed the host rock witwatersrand supergroup sedimentary rocks of the collar which were metamorphosed at greenschist to amphibolite facies conditions generally have lower 18o values than the core rocks with a mean value for metapelites of 7 7 n 45 overall through an 20 km thick section of crust there is a general increase in whole rock 18o value with increasing depth this is the reverse of what is normal in the crust largely because the collar rocks have 18o values that are unusually low in comparison with metamorphosed sedimentary rocks worldwide the collar rocks have d values ranging from 35 to 115 average 62 n 29 which are consistent with interaction with water of meteoric origin having a d of about 25 to 45 we suggest that fluid movement through the collar rocks was enhanced by impact induced secondary permeability in the dome structure

a parametric model for the yellow sea thermal variability a thermal parametric model has been developed for analyzing observed regional sea temperature profiles based on a layered structure of temperature fields mixed layer thermocline and deep layers it contains three major components 1 a first guess parametric model 2 high resolution profiles interpolated from observed profiles and 3 fitting of high resolution profiles to the parametric model the output of this parametric model is a set of major characteristics of each profile sea surface temperature mixed layer depth thermocline depth thermocline temperature gradient and deep layer stratification analyzing nearly 15 000 yellow sea historical 1950 1988 temperature profiles conductivity temperature depth station 4825 expendable bathythermograph 3213 bathythermograph 6965 from the naval oceanographic office s master oceanographic observation data set by this parametric model the yellow sea thermal field reveals dual structure one layer vertically uniform during winter and multilayer mixed layer thermocline sublayer during summer strong seasonal variations were also found in mixed layer depth thermocline depth and thermocline strength

a detailed map of the 660 kilometer discontinuity beneath the izu bonin subduction zone dynamical processes in the earth s mantle such as cold downwelling at subduction zones cause deformations of the solid state phase change that produces a seismic discontinuity near a depth of 660 kilometers observations of short period shear to compressional wave conversions produced at the discontinuity yield a detailed map of deformation beneath the izu bonin subduction zone the discontinuity is depressed by about 60 kilometers beneath the coldest part of the subducted slab with a deformation profile consistent with the expected thermal signature of the slab the experimentally determined clapeyron slope of the phase transition and the regional tectonic history

silicic magma petrogenesis in iceland by remelting of hydrothermally altered crust based on oxygen isotope diversity and disequilibria between zircon and magma with implications for morb terra nova 24 227 232 2012 abstract petrogenesis of silicic magmas in iceland has fundamental significance for understanding the relative importance of fractional crystallization of mantle derived basalt and partial melting of hydrothermally altered basaltic crust in formation of the earliest continental crust first results of in situ oxygen isotope investigation of zircons in large volume silicic eruptive products of three volcanoes in iceland askja torfajokull and hekla demonstrate isotope diversity and disequilibria and long u th zircon pre eruptive residence of 103 104 year this suggests that zircons did not grow from their host melts but instead were inherited from older magma batches and leftover cumulates with generally low and variable 18o values this study demonstrates that segregation of cubic kilometres of silicic magma is faster than mineral diffusive or recrystallization time scales estimated at 103 years and it suggests that partial melting of hydrothermally altered and oxidized oceanic crust is the mechanism that best explains silicic rocks in iceland and early earth environments

conditions for plumes to penetrate the mantle phase boundaries at a depth of 660 km in the earth s mantle the spinel perovskite phase boundary is a prominent barrier for mantle convection this is due to the negative clapeyron slope of the phase equilibrium curve which leads to an elevation of the phase boundary within hot upwellings causing negative buoyancy forces we have investigated the conditions for rising plumes either to penetrate and pass the spinel perovskite phase boundary or to stick and spread below it by studying the fundamental physics of this process the plume heads were simply modeled as hot three dimensional 3 d spheres or 2 d cylinders a simple calculation balancing the positive thermal and the negative phase bouyancy forces leads to a better parameterization using two dimensionless quantities in addition to the phase buoyancy parameter we defined a deflection parameter relating the elevation of the phase boundary to the plume head radius to account for the geometrical shape of a plume head this parameterization is further tested with numerical models that include the effects of thermal diffusion latent heat the olivine spinel phase boundary at a depth of 410 km and temperature and or phase dependent viscosity structure for laboratory estimates of the slope 3 mpa k and density increase at the spinel perovskite phase boundary 250 kg m 3 our models predict that plumes with excess temperatures of 50 600 c will stick at the top of the lower mantle if their head radii are less than 100 km plumes will penetrate into the upper mantle if plume head radii exceed 100 km while the style of plume penetration or spreading at the top of the lower mantle strongly depends on the viscosity structure the conditions for penetration do not all rising hot volumes with nonpenetrating conditions stick at the top of the lower mantle and spread laterally independent of their viscosity structure for weakly nonpenetrating conditions heat diffusion increases the radius of the hot volume and leads to penetration during a secondary stage for strongly nonpenetrating conditions spreading at the top of the lower mantle drives a mechanically coupled counterflow in the upper mantle which is stable for a very long time for volumes with an excess temperature of more than 350 c heat diffusion across the phase boundary will eventually inhibit this counterflow and stabilize a thermally coupled flow which might entrain some material of the hot volume however our results suggest that the spinel perovskite phase boundary is unlikely to inhibit the penetration of mantle plumes of the size thought to have generated many flood basalt provinces and hotspot chains

instabilities from phase transitions and the timescales of mantle thermal evolution we have investigated the effects of mantle phase transitions on the earth s thermal evolution by numerical modelling our results show that the tendency of layering grows with increasing vigor of convection a self consistent model which couples the convective dynamics of the mantle to the secular cooling of the core is constructed for a temperature dependent effective viscosity we found that the thermal history of the earth is characterized by two timescales one is associated with layered convection in the early period which is followed by a transition period with dramatic overturns as the rayleigh number decreases sufficiently low whole mantle convection prevails and a faster cooling timescale ensues

on the penetration of the 660 km phase change by mantle downflows we present a simple analytic model of the inter action of cold convective downwelling currents with an dothermic phase change the model describes the ponding and lateral spreading of downflows along the phase transi tion interface a simple comparison of the vertical forces on the ponding material provides a necessary condition for a downflow to penetrate the phase boundary this con dition is fundamentally dependent on the geometry of the downflow for planar downwellings the model predicts a minimum ponding time before the structure can pen etrate the phase boundary for columnar axisymmetric downflows there is no minimum time of spreading required before penetration can proceed the model thus provides an explanation for the observation that in numerical mod els of three dimensional convection with an endothermic phase change cylindrical downflows penetrate the phase interface while planar ones do not since descending slabs in the earth s mantle display a wide spectrum of geome tries between planar and cylindrical given various trench curvatures as well as intersections of two or more subduc tion zones this phenomenon may explain in part why some slabs appear to extend into the lower mantle while others are deflected at the 660 km discontinuity

postulated restite fragments from karoo picrite basalts their bearing on magma segregation and mantle deformation karoo picrite basalts from the letaba formation of se zimbabwe and the ne transvaal contain rare but widespread multigrain aggregates of high pressure orthopyroxene these are of two types interpreted as a crystallization products of the host liquids b restite fragments refractory residue from the zone of partial melting the latter are characterized by strong lattice preferred orientation fabrics produced by simple flattening it is speculated that such fabrics are generated either by the upward flow of magma through refractory residue or by earlier solid flow in the axial region of a diapir ultimately magma segregation appears to be sufficiently vigorous to entrain restite fragments and the apparent degree of partial melting may increase as restite is dissolved during ascent

stable isotope profile across the orthoamphibole isograd in the southern marginal zone of the limpopo belt south africa abstract in the southern marginal zone of the limpopo belt the transition from granulite to amphibolite facies metamorphism is defined by a change from orthopyroxene to orthoamphibole bearing assemblages within metasedimentary rocks of the supracrustal bandelierkop formation oxygen isotope fractionations among constituent minerals within both amphibolite and granulite facies metasedimentary rocks are of similar magnitude implying a similar retrograde thermal history temperatures calculated from these fractionations can be arranged in the following order quartz garnet quartz orthoamphibole quartz pyroxene quartz plagioclase quartz biotite the observed temperature sequence is consistent with retrograde metamorphism under closed system conditions and oxygen closure of garnet orthoamphibole and orthopyroxene at or close to peak metamorphism while quartz plagioclase and biotite crystallized from and or re equilibrated via a melt fluid phase down to temperatures of 560 c small fluid rock ratios are implied by the simultaneous closure of quartz plagioclase and biotite given that quartz is the dominant oxygen bearing phase during retrogressive exchange and assuming garnet closure to oxygen diffusion at the time of formation then the quartz garnet fractionations may provide minimum peak temperature estimates of 736 52 c coarse grained concordant felsic veins within the metasediments texturally interpreted as anatectites are in isotopic equilibrium with the host rock the closed system defined here only on the basis of oxygen isotope systematics does allow for fluid melt extraction at high temperatures in contrast to the metasediments oxygen isotope systematics for mafic and ultramafic rocks of the bandelierkop formation suggest open system behaviour for at least some of the rocks such rocks are commonly cross cut by coarse grained felsic veins which are not in isotopic equilibrium with the host rock and contain quartz whose 18 o values are indistinguishable from those observed in the metasedimentary rocks the stable isotope data are consistent with a model involving anatexis of metasedimentary rocks at peak metamorphic temperatures partial extraction of the anatectic melts and their crystallization elsewhere in the sequence is suggested by the mineralogy and isotopic composition of large discordant pegmatites together with the isotopic disequilibrium and open system behaviour observed in some mafic and ultramafic rocks in the pelitic rocks the release of small amounts of fluid from residual melts and or somewhat larger amounts from partially collected melts during cooling may account for many of the retrograde mineralogic features observed for these rocks without perturbing the oxygen isotope systematics such retrograde features include the cordierite orthopyroxene symplectite after garnet and the incipient hydration of orthopyroxene and cordierite within the granulites and possibly the regional retrogression of granulites to form orthoamphibole gneisses retrograde kyanite within these rocks thus suggests an isobaric cooling path similar 13 c values of graphite separated from amphibolite and granulite facies metasediments together with significant oxygen and carbon isotope heterogeneity among charnockites metasediments banded iron formations and most mafic and ultramafic rocks are not consistent with a retrogressive influx of extraneous co 2 rich fluids as the cause of regional retrogression and orthoamphibole formation in the gneisses nor with charnockitization the data also argue against a pervasive co 2 infiltration as the cause of granulite facies metamorphism

seasonal thermocline in the china seas and northwestern pacific ocean climatological seasonal variations of the thermocline in the china seas and northwestern pacific ocean were studied using historical data from 1930 through 2001 707 624 profiles the quantitative roles of surface thermal buoyancy b q haline buoyancy flux b p and total buoyancy flux b against the wind induced mixing tau in different seasons and regions were also explored using the buoyancy ratio r b q b p and the monin obukhov depth ratio delta respectively the thermocline has obvious seasonal variations in the study area north of 20 degrees n there is no thermocline along the west coast of the bohai sea bs yellow sea ys and northern east china sea from december to march resulting from surface cooling and wind mixing the significantly different variation of the thermocline strength on and off the chinese shelf is mainly caused by the fact that the thermal stratification is enhanced by bottom tidal mixing on the shelf the delta indicates that the thermocline depth on the chinese shelf is mainly dominated by b in summer while it is dominated by tau in winter it reveals an opposite feature in the kuroshio region the dominating factor is b in winter associated with the large heat buoyancy loss there south of 20 degrees n the dominating factor is similar to that on the shelf with the more obvious b dominant characteristic during the monsoon transition periods the r demonstrates that b is mainly controlled by b q all year round with some sporadically b p dominated regions in the tropical area in winter and in the bs and eastern ys in september

melting instabilities in the transition zone abstract melting in the transition zone could result from the rapid passage of hot upwelling through the perovskite spinel phase transition a large temperature increase could be produced above the perovskite to spinel transition by both viscous heating and a nonlinear positive feedback between temperature dependent viscosity and latent heat release the rise times for these instabilities have been estimated to be about 10 ma these instabilities were triggered by the episodic downwelling instabilities which preceded the flush events of cold material in layered mantle convection with phase transitions these results would suggest that melting in the transition zone would have had a greater tendency to occur in the past and might have caused the generation of komatiites in the archean

an extended kalman filter to assimilate satellite altimeter data into a nonlinear numerical model of the tropical pacific ocean method and validation a new implementation of the extended kalman filter scheme is developed for the purpose of assimilating observations into a high resolution nonlinear numerical model of the tropical pacific ocean it is characterized by two successive stages 1 approximation of the error covariance matrix by a singular low rank matrix which leads to making corrections only in those directions for which the error is not naturally attenuated by the system in this respect it is quite similar to the approach adopted by cane et al 1996 and 2 modification of these directions over time according to the model dynamics thus reflecting the evolutive nature of the filter the filter is initialized by a method based on the empirical orthogonal functions obtained from free runs of the model the reduction of the error covariance matrix avoids the overwhelming burden of computing the temporal evolution of the prediction error with all the degrees of freedom of the full state vector this assimilation scheme is implemented in the gent and cane 1989 primitive equation reduced gravity model this model has the advantage of using a coordinate in the vertical which allows a geographical refinement of the vertical resolution for thermocline gradients it is forced by the florida state university database winds and the chen et al 1994 hybrid scheme parameterization is assumed for the mixed layer the domain of application is the tropical pacific ocean basin between 120 e and 80 w longitude and 30 s and 30 n latitude the resolution is finest at the equator and steadily increases away from the equator in a first application validation twin experiments are conducted in which observations are assumed to be synthetic altimeter data sampled according to the topex poseidon mission features the method is shown to be efficient under various conditions and in particular to be capable of transferring information in the vertical from the surface only altimeter data to the deepest ocean layers and therefore of adequately constraining the thermal and the velocity profiles within the 300 or 400 m upper ocean in a second application real topex poseidon data are used and the results evaluated against independent in situ data namely tropical ocean global atmosphere toga tao mooring observations improvements in the model behavior are clear in terms of the time variability of the tropical ocean it therefore seems that the reduced state kalman filter with empirical orthogonal function eof initialization can be applied with success to such a primitive equation model of the pacific for the assimilation of altimeter data however at this stage the evolutivity of the filter seems to have a limited impact on the performances which was not the case for tests in the strongly nonlinear midlatitudes pham et al 1998a

solitary waves in mantle plumes numerical calculations in two dimensions show that solitary wave disturbances can propagate along thermal plumes in a homogeneous viscous fluid with a thermally activated rheology similar to the mantle comparison of the numerical results with analytic results from an idealized two fluid model indicates that the two fluid model can be applied to solitary waves in the thermally activated plume a two fluid model of solitary waves on cylindrical conduits is accordingly applied to mantle plumes to estimate the propagation speeds time durations and pulse lengths of solitary waves in mantle plumes as a function of background mantle viscosity plume flux plume density deficit plume viscosity and the volume of material transported by the solitary wave it is inferred that mantle plumes may be episodic via solitary waves and that these disturbances might account for observed 0 10 m y variations in the durations and spacings of episodes of enhanced hotspot volcanic activity solitary waves in mantle plumes could be generated by interactions among plumes and between plumes and the large scale time dependent mantle circulation

numerical models of thermally and mechanically coupled two layer convection of highly viscous fluids summary numerical solutions for thermal convection of two superposed viscous fluid layers with the same thicknesses as the earth s upper and lower mantles and separated by a fixed horizontal interface are obtained at the onset of convection and in the nonlinear regime several heating configurations and thermal boundary conditions are considered including an isothermal bottom boundary a prescribed heat flux bottom boundary and internal heating with an insulated lower boundary at the onset of convection and with the same material properties in each layer the lower layer is more unstable than the upper layer and long horizontal wavelengths proportional to the lower layer thickness predominate for layers with sufficiently different material properties the upper layer can be more unstable and short horizontal wavelengths proportional to the upper layer thickness can predominate at the onset of convection when the material properties are chosen to make the two layers equally unstable there are two stability curve rayleigh number ra versus wavelength minima with the lower minimum at long wavelength linear stability results typify weakly nonlinear convection except when both layers are equally unstable when the layers have unequal stability the stable layer has a conductive temperature field and its flow is viscously driven by the convective flow in the unstable layer when the two layers are equally unstable both layers are separately convecting even at rayleigh numbers of only twice the critical value for convection onset at large enough supercritical ra the lower layer convects with a long wavelength cell even when linear stability theory predicts short wavelength cells the upper layer always begins convecting with short wavelength cells however the upper layer cell over the lower layer downwelling plume increases in size with increasing ra thus horizontal wavelength plate dimension for example cannot be used to discriminate between whole mantle and layered mantle convection mechanical coupling of the layers downwelling plume over upwelling plume occurs for all cases with the same material properties in each layer and for the case with the same ra in each layer when the material properties of the layers are sufficiently different to produce short wavelength cells at the onset of convection the upper layer has two thermally coupled cells downwelling plume over downwelling plume and a shear zone at the interface over both lower layer plumes a large 50 per cent of total temperature difference across the interface occurs in all cases examined implying that two layer mantle convection should also have a large temperature change across the 670 km discontinuity however temperature dependent viscosity may reduce the magnitude of this temperature difference interpretations of geoid anomalies over subduction zones and seismic traveltime anomalies of slabs should account for possible thermal coupling of separate upper and lower mantle convective systems

mantle plume interaction with an endothermic phase change high spatial resolution numerical simulations of mantle plumes impinging from below on the endothermic phase change at 660 km depth are used to investigate the effects of latent heat release on the plume phase change interaction both axisymmetric and planar upflows are considered and the strong temperature dependence of mantle viscosity is taken into account for plume strengths considered a clapeyron slope of 4 mpa k 1 prevents plume penetration of the phase change plumes readily penetrate the phase change for a clapeyron slope of 2 mpa k 1 and arrive in the upper mantle considerably hotter than if they had not traversed the phase change for the same amount of thermal drive i e the same excess basal temperature axisymmetric plumes are hotter upon reaching the upper mantle than are planar upwellings heating of plumes by their passage through the spinel perovskite endothermic phase change can have important consequences for the ability of the plume to thermally thin the lithosphere and cause melting and volcanism

petrogenesis of the gross spitzkoppe topaz granite central western namibia a geochemical and nd sr pb isotope study abstract the early cretaceous gross spitzkoppe granite is one of several igneous complexes that formed in the damaraland of namibia during an episode of continental rifting and magmatism associated with opening of the south atlantic this is a classic setting of rift related anorogenic magmatism showing the typical association of mantle derived mafic and alkaline rocks along with silicic units the gross spitzkoppe granite is an important example of the latter because its peraluminous composition and crustal isotopic signature makes it well suited to characterize the crustal contribution to magmatism the gross spitzkoppe stock gss is composed of three texturally distinct topaz bearing biotite monzogranites average 76 wt sio 2 30 wt rb sr and 0 5 wt f iron rich biotite is the only mafic silicate in proximity to the stock are numerous felsic dikes rhyodacites to rhyolites the elevated high field strength elements hfse contents and high ga al ratios of the granites and felsic dikes are typical of a type granites the granites and dikes show similar geochemical differentiation trends and the compositions of the more evolved dikes overlaps with the granites this and similar initial isotope ratios suggest that the dikes and granites have a common origin major and trace element modeling confirms that the composition of the gross spitzkoppe granite can be produced by fractional crystallization from magma with the average felsic dike composition rb sr isotope data for nine whole rock samples of the gss yield an age for emplacement of the complex at 125 1 ma initial 87 sr 86 sr ratios vary from 0 710 to 0 716 neodymium isotope compositions of the granites and dikes yield initial e nd 125 values from 5 6 to 6 4 the pb isotopic ratios k feldspar or age corrected whole rock fall in the range 206 pb 204 pb 18 70 18 82 207 pb 204 pb 15 66 15 71 and 208 pb 204 pb 38 57 39 07 these geochemical and isotopic characteristics are consistent with derivation of the gross spitzkoppe granitic magma from a granulite facies metasedimentary source batch melting calculations suggest that low 10 20 degree partial melting of a felsic granulitic source is capable of producing melt compositions similar to those of the felsic dikes with further fractional crystallization being required to produce the granites the source rocks are thought to be damara belt metasediments that were melt depleted in the early paleozoic by extraction of the s type damara granites the most likely scenario for crustal melting is by basaltic underplating of the source region in the lower crust the range in initial nd and sr values is attributed to minor mixing of basaltic magma with the crustal melts although sr was also affected by minor contamination from the radiogenic damara granites near the level of emplacement

phase boundary deflections at 660 km depth and episodically layered isochemical convection in the mantle numerical simulations have been performed using a multiphase anelastic axisymmetric spherical mantle convection model as part of an ongoing effort to explore the ability of the endothermic phase transition at 660 km depth to cause the circulation to assume a layered style in particular model solutions have been constructed for a rayleigh number of 107 internal heating corresponding to 50 heating from within and 50 heating from below and clapeyron slopes for the 410 km and 660 km phase boundaries set to 3 0 and 2 8 mpa k respectively in this regime the flow exhibits a substantial degree of radial layering wherein the radial mass flux is reduced significantly at 660 km depth this layered regime of flow is episodically disrupted by massive localized avalanches of fluid across the 660 km boundary uiat recur at intervals separated by hundreds of millions of years the degree of layering is related to the magnitude of the 660 km phase boundary deflection away from its average depth in these earth like simulations we find that the average magnitude of such phase boundary deflections is similar to the average magnitude of seismically observed deflections of this horizon

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

the oxygen isotope composition of karoo and etendeka picrites high δ18o mantle or crustal contamination oxygen isotope compositions of karoo and etendeka large igneous province lip picrites and picrite basalts are presented to constrain the effects of crustal contamination versus mantle source variation olivine and orthopyroxene phenocrysts from lavas and dykes mg 64 80 from the tuli and mwenezi nuanetsi regions of the ca 180 ma karoo lip have 18o values that range from 6 0 to 6 7 they appear to have crystallized from magmas having 18o values about 1 1 5 higher than expected in an entirely mantle derived magma olivines from picrite and picrite basalt dykes from the ca 135 ma etendeka lip of namibia and karoo age picrite dykes from dronning maud land antarctica do not have such elevated 18o values a range of 18o values from 4 9 to 6 0 and good correlations between 18o value and sr nd and pb isotope ratios for the etendeka picrites are consistent with previously proposed models of crustal contamination explanations for the high 18o values in tuli mwenezi picrites are limited to 1 alteration 2 crustal contamination and 3 derivation from mantle with an abnormally high 18o previously a variety of models that range from crustal contamination to derivation from the enriched mantle lithosphere have been suggested to explain high concentrations of incompatible elements such as k and average end and esr values of 8 and 16 in mwenezi nuanetsi picrites however the primitive character of the magmas mg 73 combined with the lack of correlation between 18o values and radiogenic isotopic compositions mgo content or mg is inconsistent with crustal contamination thus an 18o enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition is indicated high 18o values are accompanied by negative nb and ta anomalies consistent with the involvement of the mantle lithosphere whereas the high 18o themselves are consistent with an eclogitic source magma 18o values about 1 higher than expected for mantle derived magma are also a feature of the bushveld mafic and ultramafic magmas and the possibility exists that a long lived 18o enriched mantle source has existed beneath southern africa a mixed eclogite peridotite source could have developed by emplacement of oceanic lithosphere into the cratonic keel during archaean subduction

numerical approach to problems of gravitational instability of geostructures with advected material boundaries summary we present a numerical approach for solving 2 d mantle flow problems where the chemical composition changes abruptly across intermediate boundaries the method combines a galerkin spline technique with a method of integration over regions bounded by advected interfaces to represent discontinuous variations of material parameters it allows direct approximation of a natural free surface position instead of a posteriori calculation of topography from the normal stress at the upper free slip boundary we formulate a model where a viscous incompressible fluid filling a square box is divided into layers not necessarily horizontal by advected boundaries across which the density and viscosity change discontinuously no slip or free slip conditions are assumed at the model sides the suggested approach being eulerian avoids the difficulties due to material discontinuities at intermediate boundaries like the moho or the earth s surface and is also free from the deficiencies of the lagrangian approach always resulting in mesh distortion we present two geophysical cases analysed by this technique the first case concerns the formation of sedimentary basins under the effects of heavy bodies sinking in the asthenosphere and of load due to sedimentary infills the second case demonstrates the evolution of salt diapirs and shows how their growth is affected by a laterally inhomogeneous sedimentary layer this numerical approach is well suited for problems of gravitational instability with discontinuities of density and viscosity across advected boundaries

the excess temperature of plumes rising from the core mantle boundary we study the rise of conduit type thermal plumes from the core mantle boundary cmb in a simplified numerical model viscosity is a strong function of temperature and pressure assuming that all plumes start with the same excess temperature at the cmb we find that their temperature in the upper mantle is controlled by the plume flux and is insensitive to variations in viscosity or other material parameters or to the kind of flow law newtonian or non linear the reduction of excess temperature with height is stronger for weaker plumes plumes with a buoyancy flux of less than 1000 kg s should have cooled so much that they would not melt beneath old lithosphere the existence of such hot spots possibly indicates that not all plumes rise from the cmb

effects of depth dependent properties on the thermal anomalies produced in flush instabilities from phase transitions abstract the effects of depth dependent thermal expansivity viscosity and thermal conductivity on mantle convection with phase transitions have been examined with two dimensional finite element simulations in an aspect ratio four the model includes a core mantle thermal coupling boundary condition which takes into play the secular cooling of the core by the overlying mantle flow initial surface rayleigh numbers between 2 10 7 and 10 8 have been considered with time the surface rayleigh number decreases to a value upon which a transition takes place from layered to single cell convection this tumultuous period is marked by large scale coherent breakthrough of cold material trapped in the transition zone all the way to the base of the mantle and a violent reaction of hot plume in the upper mantle both hot and cold anomalies have large magnitudes cold anomalies with temperatures exceeding 1000 k are found at the base of the mantle the magnitude of the cold anomalies is largest with all three depth dependent properties the timescales of this catastrophic event are between 20 and 50 myr with the longest being produced by the model with depth dependent expansivity and viscosity results from these simple two dimensional cartesian models represent a lower bound to the larger cold thermal anomalies potentially capable of being generated in spherical shell convection models

mantle phase transitions and layered convection numerical simulations with an anelastic spherical axisymmetric mantle convection model have been conducted to address the question of the radial mixing length in the general circulation of the mantle continuing debate centers on the question as to whether or not the 670 km seismic discontinuity which we now understand to exist as a consequence of an endothermic phase change of mantle mineralogy from the spinel phase to a mixture of perovskite and periclase in combination with the 400 km discontinuity associated with the exothermic phase change from olivine to spinel will impose a sufficient barrier to the circulation so as to induce layering we argue herein that the mantle must currently be converting in a partially layered style but that the degree of layering is highly time dependent moreover in the perhaps not too distant past the propensity to layering was greater possibly to the extent that soon after planetary formation mantle mixing occurred in two distinct reservoirs as the planet cool

effects of multiple phase transitions in a three dimensional spherical model of convection in earth s mantle numerical models of mantle convection that incorporate the major mantle phase changes of the transition zone reveal an inherently three dimensional flow pattern with cylindrical features and linear features that behave differently in their ability to penetrate the 670 km discontinuity the dynamics are dominated by accumulation of cold linear downwellings into rounded pools above the endothermic phase change at 670 km depth resulting in frequent avalanches of upper mantle material into the lower mantle the effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushing material through the 670 km phase change resulting in smaller and more frequent avalanches and a wider range of morphologies large quantities of avalanched cold material accumulate above the coremantle boundary cmb resulting in a region of strongly depressed mean temperature at the base of the mantle the 670 km phase change has a strong effect on the temperature field with three distinct regions being visible 1 the upper mantle containing linear downwellings and pools of cold material in the transition zone and characterized by a high amplitude long wavelength spectrum 2 the midmantle containing quasi cylindrical avalanche conduits and characterized by a low amplitude broad spectrum and 3 the deep mantle containing large pools of cold avalanched material and characterized by a high amplitude ultra red i e long wavelength spectrum the effect on the velocity field is very different flow penetration across the 670 km phase change is strongly wavelength dependent with easy penetration at long wavelengths but strong inhibition at short wavelengths thus when comparing numerical models with long wavelength seismic tomography diagnostics based on the density field such as the radial correlation function are much more sensitive to the effects of phase transitions than those based on the velocity field the amplitude of the geoid is not significantly affected by the partial layering because the contribution from the strong heterogeneity in the transition zone is almost perfectly balanced by the contribution from deflection of the 670 km discontinuity avalanches are associated with geoid lows however a more complex viscosity structure is required to correctly match the sign of the geoid over slabs in earth

a novel approach to double spike pb pb dating of carbonate rocks examples from neoproterozoic sequences in southern africa application of the 207 pb 204 pb double spiking technique to correct for mass fractionation led to marked improvement in the quality of pb isotope data and resulting ages obtained on low grade metamorphosed neoproterozoic carbonate rocks from the pan african gariep and saldania belts in southern namibia and south africa carbonate rock powders were dissolved using 1 n acetic acid residue fractions were separated from the supernatant carbonate fractions and analysed separately marbles from the lower pickelhaube formation gariep belt yielded distinctly different 207 pbr 206 pb isochron ages of 728 32 mswds 1 5 and 545 13 ma mswds 7 4 for the carbonate and residue fractions respectively the former is interpreted as dating early diagenesis whereas the latter is in excellent agreement with the timing of regional metamorphism in contrast both the carbonate and residue fractions from the bloeddrif member holgat formation gariep belt and from the kombuis member matjies river formation saldania belt yielded indistinguishable 207 pbr 206 pb isochron ages the isotope data from both fractions combined define an isochron age for the bloeddrif member marbles of

extreme crustal oxygen isotope signatures preserved in coesite in diamond the anomalously high and low oxygen isotope values observed in eclogite xenoliths from the upper mantle beneath cratons have been interpreted as indicating that the parent rock of the eclogites experienced alteration on the ancient sea floor1 recognition of this genetic lineage has provided the foundation for a model of the evolution of the continents whereby imbricated slabs of oceanic lithosphere underpin and promote stabilization of early cratons2 early crustal growth is thought to have been enhanced by the addition of slab derived magmas leaving an eclogite residuum in the upper mantle beneath the cratons3 but the oxygen isotope anomalies observed in eclogite xenoliths are small relative to those in altered ocean floor basalt and intermediate stage subduction zone eclogites and this has hindered acceptance of the hypothesis that the eclogite xenoliths represent subducted and metamorphosed ocean floor basalts we present here the oxygen isotope composition of eclogitic mineral inclusions analysed in situ in diamonds using an ion microprobe secondary ion mass spectrometer the oxygen isotope values of coesite a polymorph of sio2 inclusions are substantially higher than previously reported for xenoliths from the subcratonic mantle but are typical of subduction zone meta basalts and accordingly provide strong support for the link between altered ocean floor basalts and mantle eclogite xenoliths

global distribution of seamounts from seasat profiles bathymetrie profiles and contour charts have been used to study the distribution of seamounts in the deep ocean basins but only a small fraction of the seafloor has been sampled by ships at the present exploration rate it will take several centuries to map significant portions of the seafloor topography satellite altimetry which maps the topography of the equipotential sea surface is a promising tool for studying the gravity fields of seamounts because all ocean basins can be sampled in a couple of years using a model of a gaussian shaped seamount loading a thin elastic lithosphere we develop a new technique for measuring basic characteristics of a seamount from a single satellite altimeter profile the model predicts that the seamount diameter is equal to the peak to trough distance along the vertical deflection profile and that the overall diameter of the signature reveals the age of the lithosphere when the seamount formed moreover the model suggests that these two measurements are relatively insensitive to the cross track location of the seamount we confirm these model predictions using seasat altimeter profiles crossing 14 well surveyed seamounts in the pacific we then apply the measurement technique to 26 106 million kilometers of seasat profiles resulting in a new global set of seamount locations approximately one quarter of the seamounts identified in seasat profiles were previously uncharted modeling suggests that there is no direct relationship between the size of a seamount and its signature in the geoid therefore the set of locations is not a straightforward sampling of the total seamount population but is weighted toward seamounts which are poorly compensated a preliminary analysis indicates considerable variations in population density and type across the oceans most notable among them are the absence of seamounts in the atlantic variations in population density across large age offset fracture zones in the pacific the prevalence of small signatures in the indian ocean and the existence of linear trends in the large seamounts of the west pacific

mantle phase transitions and layered chaotic convection we report preliminary results from a sequence of very high resolution simulations of the mantle convection process based upon an axisymmetric anelastically compressible spherical model with both olivine spinel and spinel post spinel phase transitions our analyses strongly suggest that at earth like rayleigh numbers near 107 the circulation may be quite strongly layered although a very long timescale transient exists that is characterized by brief excursions into the whole mantle state we suggest the use of a new diagnostic tool that may be employed to determine whether or not the layered state is characteristic of the present day earth

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

progressive crustal contamination of the bushveld complex evidence from nd isotopic analyses of the cumulate rocks we report the first nd isotopic data on the cumulate rocks of the bushveld complex south africa we analysed 17 whole rock samples covering 4700 m of stratigraphy through the lower critical and main zones of the intrusion at union section north western bushveld complex the basal ultramafic portions of the complex have markedly higher nd t 5 3 to 6 0 than the gabbronoritic main zone nd t 6 4 to 7 9 the rocks of the upper critical zone have intermediate values these results are in agreement with new nd isotope data on marginal rocks and sills in the floor of the complex that are generally interpreted as representing chilled parental magmas and with published sr isotopic data all of which show a larger crustal component in the upper part of the intrusion in contrast the concentrations of many highly incompatible trace elements are decoupled from the isotopic signatures the basal portions of the complex have higher ratios of incompatible to compatible trace elements than the upper portions the variations of isotopic and trace element compositions are interpreted in terms of a change in the nature of the crustal material that contaminated bushveld magmas those magmas that fed into the lower part of the complex had assimilated a relatively small amount of incompatible trace element rich partial melt of upper crust whereas magmas parental to the upper part of the complex had assimilated a higher proportion of the incompatible trace element poor residue of partial melting

the effects of temperature dependent viscosity on mantle convection with the two major phase transitions abstract we have studied the consequences of including temperature dependent viscosity in mantle convection with both of the major phase transitions included we have employed a two dimensional cartesian model in a box of aspect ratio four the time dependent equations have been cast within an extended boussinesq framework in which each of the phase changes has been simulated by an enhanced thermal expansivity both latent heat release and viscous dissipation have been considered two different forms of temperature dependent viscosity the arrhenius and the frank kamenetzky forms have been employed we have considered both models with a constant temperature equilibrium boundary condition at the core mantle boundary and models with a time varying non equilibrium temperature boundary condition from cooling of the core the effects of temperature dependent viscosity are to enhance the tendency of the convective system to be layered because of the influences of the latent heat release and the mechanical heating on creating a sharp viscosity drop across the transition zone we have found that there is a non linear coupling between latent heat release from the interaction of the hot plume with the endothermic phase transition and the temperature dependent viscosity this positive feedback process can give rise to the production of very high temperatures in the transition zone which may cause melting in the region between the two major phase transitions flushing events produced by the non equilibrium boundary condition are much more dramatic than those generated by the equilibrium boundary condition the non linear coupling between latent heat release and temperature dependent viscosity is also stronger in the non equilibrium models comparison with models without the presence of latent heat release and viscous dissipation shows that the models without the heating terms produce a greater degree of mass flux across the endothermic phase boundary because of the lack of the viscosity stratification in the interior of the convective flow results of this moded can be applied to other types of endothermic phase transition found in the lithosphere or to the spinel perovskite transition in the deep portion of the martian mantle

geochemistry across an exposed section of archaean crust at vredefort south africa with implications for mid crustal discontinuities abstract the central region of the vredefort structure consists of a semi circular multi layered sequence of crystalline rocks which are nearly vertical in attitude and which increase in metamorphic grade towards the core of the structure together with the overlying precambrian strata this sequence provides a cross section through almost the entire crustal section of the kaapvaal craton 36 km the upper part of the vredefort crystalline crust consists of a 3 0 ga sequence of differentiated felsic rocks in amphibolite facies the lower crust consists of a complex and heterogeneous both chemically and isotopically high grade metamorphic terrain of charnockites granulites mafic and felsic and supracrustal rocks the upper crust is separated from the lower crust by the vredefort discontinuity a brittle ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock petrographic chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution this is indicative of different geological environments prior to their present juxtaposition we speculate that the upper and the lower parts of the vredefort crystalline crust were juxtaposed during intracratonic thrusting 2 8 ga ago and that the entire 36 km section of crust including the vredefort discontinuity were rotated and uplifted into their present vertical position during the 2 0 ga vredefort catastrophe

catastrophic overturn of the earth s mantle driven by multiple phase changes and internal heat generation the effects of phase changes and strong internal heat generation may combine to bring about brief but extremely intense episodes of rapid thermal convection in the earth s mantle numerical calculations using realistic thermodynamic properties for the exothermic olivine spinel and endothermic spinel perovskite magnesiowustite phase transitions suggest the transition region of the earth s mantle may act as a capacitor for subducting slabs slab material accumulates in the transition region until a threshold level of thermal buoyancy is reached and is then rapidly discharged into the lower mantle in my calculations this occurs as a catastrophic burst of convection lasting 10 myr with elevated heat transfer rates lasting 100 myr such episodes may be analogs to superplume activity which has been hypothesized to give rise to an intense episode of intra plate volcanism and stabilize the geodynamo against reversals the topography on the two phase change boundaries is found to be negatively correlated with the correlation becoming more negative during periods of rapid convection furthermore the results of this study suggest the transition region of the earth s mantle could be 250 k cooler on average and consequently more viscous than the surrounding mantle

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

are bushveld u type parent magmas boninites or contaminated komatiites three distinct categories of magmas bushveld u type parent magmas boninites and siliceous high magnesium basalts from archaean greenstone belts share the distictive geochemical characteristics of high mgo 9 19 low tio2 less than 1 and high sio2 greater than 52 boninites are generally thought to form by hydrous melting of metasomatized previously depleted upper mantle while siliceous high magnesium basalts shmb in greenstone belts have recently been recognized as the products of combined fractionation and crustal contamination of komatiites both these mechanisms can apparently give rise to similar end products and both mechanisms have been proposed for the petrogenesis of bushveld u type magmas

mantle plumes pinched in the transition zone abstract we have employed an axisymmetric spherical shell model with the two major phase transitions and variable viscosity to study the dynamical processes associated with both regular mantle plumes and superplumes which can develop spontaneously in strongly time dependent situations this model is heated both from below and from within and the viscosity varies according to the temperature pressure and the phase boundaries we have studied the physical processes due to the interactions between the different types of plumes and superplumes with the transition zone which can shed light on some new understanding of intraplate volcanism the interaction between the plume and the two major phase transitions gives rise to the production of a reservoir of plume material localized in the transition zone the spinel to olivine phase transition induces the ejection of very fast plume material within a very thin vertical channel towards the surface the spreading of this plume is controlled by the upper mantle circulation stable plumes which extend throughout the mantle for several hundred million years can produce volcanism with an age progression of the seamounts other plumes which are bent and move relative to the global circulation may generate chaotic surface volcanism with a very limited age progression of the seamounts plume plume collision produces a superplume growing from the core mantle boundary cmb the interaction between superplumes and the perovskite to spinel phase transition generates a thick thermal boundary layer in which the upper mantle can serve as the site for the launching of new smaller plumes stress fields associated with the superplume have the largest magnitude at the cmb and just below the 670 km discontinuity they may be associated with seismic anisotropy found there

a numerical study on dynamic mechanisms of seasonal temperature variability in the yellow sea 1 using in situ observations and numerical modeling this study investigates the dynamical mechanisms of seasonal variability of water temperature in the yellow sea ys observations indicate that bottom temperature lags 3 4 months behind surface temperature in reaching a maximum in the central ys wave tide circulation coupled model simulates this time lag and indicates that the diffusion process is a key factor governing the temperature variation below the surface layer based on the diffusion equation of temperature a scheme is developed to estimate the vertical diffusion coefficient at an observation station located at 36 00 n 124 00 e the diffusion coefficients from april to september are estimated by using the temperature data from 1954 to 1985 the mean diffusion coefficient mdc in the upper layer from 0 m to 15 m is almost one order of magnitude larger than those in the middle layer from 20 to 40 m except in april in the middle layer the mdc is inversely proportional to the squared buoyancy frequency and the mean value of mdc averaged from june to september is 0 28 cm2 s 1 the inverse proportionality agrees with the osborn s relation which has been used to estimate the diapycnal diffusivity

interaction of the upwelling plume with the phase and chemical boundary at the 670 km discontinuity effects of temperature dependent viscosity abstract numerical simulations have been performed to investigate the interaction of upwelling plumes with the 670 km discontinuity temperature dependent rheology is employed to introduce the low viscosity and strength of the hot plume the 670 km discontinuity is taken as a phase and or chemical boundary the condition for penetration of the plume into the upper mantle is examined by varying the value of the clapeyron slope of the phase transition and the compositional density difference of the chemical boundary possible styles of the mantle convection are classified into six types depending on the nature of the 670 km discontinuity the plume in the lower mantle can penetrate into the upper mantle and reach the bottom of the lithosphere when the clapeyron slope is greater than 3 to 4 mpa k for the pure phase boundary and when the compositional density difference is smaller than 0 5 1 5 for the pure chemical boundary the plume can barely penetrate into the upper mantle when the 670 km discontinuity possesses even a small amount of the compositional density contrast less than 0 5 1 0 together with the phase transition of the experimentally determined clapeyron slope of 3 1 mpa k this result suggests that the mantle may be of uniform composition or at most has a weak compositional layering if the origin of the plume is in the deep mantle

multiple phase transitions and the style of mantle convection abstract multiple phase transitions based on experimental and theoretical results have been incorporated into an extended boussinesq model of mantle convection a formulation is used which facilitates the numerical computations of convection with multiple phase transitions and a triple point near the 670 km discontinuity the propensity for layered convection is enhanced by including the additional phase transitions the location of the triple point of the spinel to perovskite phase transitions plays an important role in controlling the fate of the ascending hot plumes a plume with a core temperature above this triple point temperature t c tends to pass easily through the phase boundary at 670 km plumes with a core temperature lower than t c are blocked by the multiple phase transitions both depth dependent thermal expansivity and internal heating increase the tendency of the convective system to layering the dynamical time scales in the transition zone are much shorter than those of the top and bottom boundary layers simulations with increasing rayleigh number ra from 10 6 to 5 10 7 show unambiguously the increased tendency toward layering with larger ra in the early earth when ra was higher and internal heating was several times stronger layered convection might have been the preferred mode of mantle convection with time as both ra and radioactive heating decreased the style of mantle convection would have become less layered

archean crustal structure of the kaapvaal craton south africa evidence from the vredefort dome abstract crystalline archean basement rocks in the core of the vredefort dome present a profile through a substantial part of the middle and lower crust of the kaapvaal craton previously this profile has been subdivided into two terranes with allegedly distinct lithologies and tectonometamorphic histories that were juxtaposed along a crustal scale late archean brittle ductile thrust zone lithological and structural mapping across the dome indicates however that the basement lithologies share a common polyphase tectonic history culminating in high grade metamorphism and melting at 3 1 ga no evidence was found of the postulated tectonic terrane boundary but the alleged boundary does coincide with a 1 2 km wide transition zone between upper amphibolite facies migmatitic gneisses and more restitic granulite facies gneisses the implications of these results for archean regional tectonic models for the kaapvaal craton are discussed

dynamical effects of a temperature and pressure dependent lower mantle rheology on the interaction of upwellings with the transition zone we have examined the consequences of a temperature and pressure dependent lower mantle viscosity on the interaction of upwellings with the two major phase transitions the viscosity used is only temperature dependent in the upper mantle but assumes both temperature and pressure dependences in the lower mantle the activation enthalpy increases with depth dependence scaled after experimental melting curves of lower mantle components we have employed a two dimensional cartesian box with an aspect ratio of five and a mantle layer thickness of 2900 km using an extended boussinesq model with a depth dependent thermal expansivity latent heat release and viscous dissipation have been included in the temperature equation we have considered models both without and with a viscosity jump a factor of 8 across the 660 km boundary the range of averaged rayleigh numbers ra considered lies between 2 106 and 107 a major effect of this temperature and pressure dependent rheology is the stabilization of the upwelling plumes and the increase of viscous heating in the bottom portion of these plumes the impingement of these heated plumes at the endothermic phase boundary causes the development of secondary plumes to rise into the upper mantle shear heating together with the latent heat release produces large temperature increases in the plumes passing through the transition zone which may cause melting in the deep upper mantle although the average rayleigh number in the case of temperature and pressure dependent viscosity is around a factor of 5 lower than in a case with purely temperature dependent viscosity the resulting flow is only slightly less layered which is contrary to the earlier finding of christensen and yuen 1985 the tendencies for the development of secondary plumes and high temperatures inside the upwellings are enhanced by the viscosity jump at the 660 km boundary in this situation few nearly steady vigorous upwellings exist in the lower mantle while in the upper mantle vigorous time dependent convection takes place for an averaged rayleigh number of 107 the system becomes layered with many thin plumes appearing in the lower mantle

development of diapiric structures in the upper mantle due to phase transitions solid state phase transitions in time dependent mantle convection can induce diapiric flows in the upper mantle when a deep mantle plume rises toward phase boundaries in the upper mantle the changes in the local thermal buoyancy local heat capacity and latent heat associated with the phase change at a depth of 670 kilometers tend to pinch off the plume head from the feeding stem and form a diapir this mechanism may explain episodic hot spot volcanism the nature of the multiple phase boundaries at the boundary between the upper and lower mantle may control the fate of deep mantle plumes allowing hot plumes to go through and retarding the tepid ones

the effects of multiple phase transitions on venusian mantle convection the effects of multiple phase transitions on the dynamics of the venusian mantle have been investigated with a two dimensional finite element method a depth dependent thermal expansivity has been employed in a purely basal heating configuration with an aspect ratio of four the addition of the olivine spinel transition promotes layered convection more so than models with a single spinel perovskite phase change the shifts in the phase transition depths and the presence of a conductive lid increase the tendency of the venusian mantle circulation to flow through the transition zone the potentially lower rayleigh number in venus from lack of volatiles will also enhance whole mantle convection the style of convection in venus may have changed from layered convection in the past to whole mantle convection today this transition might have been responsible for the major resurfacing event on venus

temperature inversion in china seas temperature inversion was reported as a common phenomenon in the areas near the southeastern chinese coast region a west and south of the korean peninsula region b and north and east of the shandong peninsula region c during october may in the present study based on hydrographic data archived from 1930 through 2001 319 029 profiles the inversion was found to be remarkable with obvious temporal and spatial variabilities in both magnitude and coverage with higher probabilities in region a up to about 60 and region c 40 50 than in region b 15 20 the analysis shows that seasonal variation of the net air sea heat flux is closely related to the occurrence time of the inversion in the three areas while the yangtze and yellow river freshwater plumes in the surface layer and ocean origin saline water in the subsurface layer maintain stable stratification it seems that the evaporation excessive precipitation flux makes little contribution to maintaining the stable inversion advection of surface fresh water by the wind driven coastal currents results in the expansion of inversion in regions a and c the inversion lasts for the longest period in region a october may sustained by the taiwan warm current carrying the subsurface saline water while evolution of the inversion in region b is mainly controlled by the yellow sea warm current

archean emplacement of eclogitic components into the lithospheric mantle during formation of the kaapvaal craton re os data for selected whole rock eclogites and eclogitic sulfide inclusions in diamonds from seven kimberlites on the kaapvaal zimbabwe cratons yield ages around 3 ga the presence of such old eclogitic components in the subcontinental lithospheric mantle is more common than previously thought overlap of these ages with those of many cratonic peridotites demonstrates that incorporation of eclogitic materials accompanied or closely followed cratonic lithosphere stabilization many of these eclogitic components have fractionated c n o and s isotopic compositions which could have been created at low temperatures implying that some style of archean subduction involving emplacement of oceanic lithosphere was part of craton keel development

mantle plume heads and the initiation of plate tectonic reorganizations we present results of numerical simulations which examine the plausibility of a reorganization of plate motions brought about by the interaction of a hot low viscosity plume head with the underside of a tectonic plate a numerical model of highly viscous fluid flow driven by thermal buoyancy is employed to examine the interaction of mantle plumes with surface plates in a two dimensional cartesian geometry plate like boundary conditions applied to the upper surface consist of a large active plate with fixed speed plus a smaller passive plate whose speed and direction of motion are permitted to react to tractions induced by the underlying mantle flow driven by the active plate the passive plate is also subject to applied end loads intended to simulate incipient subduction the mantle plume is initiated with a hot patch on the lower boundary and viscosity varies with temperature following an arrhenius rheology the calculations performed with fixed aspect ratio 4 1 and fixed rayleigh number ra 106 where ra refers to the strength of the starting plume were found to depend on three parameters the strength of the viscosity variation the size of the applied end load and the speed of the active plate these calculations indicate that plate motions can be sharply influenced by the lubricating effects of mantle plume heads even to the point of causing a reversal in the direction of plate motion

japan sea thermohaline structure and circulation part ii a variational p vector method abstract the second part of this work investigates the seasonal variabilities of the japan east sea jes circulation using the u s navy generalized digital environmental model gdem climatological temperature and salinity dataset public domain on a 0 5 0 5 grid a variational p vector method was developed to invert the velocity field the gdem for the jes was built up on historical 1930 97 136 509 temperature and 52 572 salinity profiles the climatological mean and seasonal variability of the current systems are well inverted especially the tsushima warm current and its bifurcation the east korean warm current ekwc the japan nearshore branch the confluence of the ekwc and the north korean cold current near the korean coast and flows northeastward along the subpolar front and a mesoscale anticyclonic eddy in the ulleng tsushima basin furthermore this method has the capability to invert flow reasonably well across the shallow straits such as the tsushima korea tsugaru and soya strait

oxygen isotope heterogeneity of the mantle beneath the canary islands a discussion of the paper of gurenko et al gurenko et al contrib mineral petrol 162 349 363 2011 report laser assisted fluorination lf and secondary ionization mass spectrometry sims 18o 16o datasets for olivine grains from the canary islands of gran canaria tenerife la gomera la palma and el hierro as with prior studies of oxygen isotopes in canary island lavas e g thirlwall et al chem geol 135 233 262 1997 day et al geology 37 555 558 2009 geochim cosmochim acta 74 6565 6589 2010 these authors find variations in 18ool 4 6 6 0 beyond that measured for mantle peridotite olivine mattey et al earth planet sci lett 128 231 241 1994 and interpret this variation to reflect contributions from pyroxenite peridotite mantle sources furthermore gurenko et al contrib mineral petrol 162 349 363 2011 speculate that 18ool values for la palma olivine grains measured by lf day et al geology 37 555 558 2009 geochim cosmochim acta 74 6565 6589 2010 may be biased to low values due to the presence of altered silicate possibly serpentine the range in 18ool values for canary island lavas are of importance for constraining their origin gurenko et al contrib mineral petrol 162 349 363 2011 took a subset 39 sims analyses from 13 grains from a single el hierro lava eh4 of a more extensive dataset 321 sims analyses from 110 grains from 16 canary island lavas to suggest that 18ool is weakly correlated r2 0 291 with the parameter used by gurenko et al earth planet sci lett 277 514 524 2009 to describe the estimated weight fraction of pyroxenite derived melt xpx with this relationship end member 18o values for himu peridotite 18o 5 3 0 3 and depleted pyroxenite 18o 5 9 0 3 were defined although the model proposed by gurenko et al contrib mineral petrol 162 349 363 2011 implicates similar pyroxenite peridotite mantle sources to those proposed by day et al geology 37 555 558 2009 geochim cosmochim acta 74 6565 6589 2010 and day and hilton earth planet sci lett 305 226 234 2011 there are significant differences in the predicted 18o values of end member components in the two models in particular day et al geochim cosmochim acta 74 6565 6589 2010 proposed a mantle source for la palma lavas with low 18o 5 rather than higher 18o c f the himu peridotite composition of gurenko et al in contrib mineral petrol 162 349 363 2011 here we question the approach of using weakly correlated variations in 18ool and the xpx parameter to define mantle source oxygen isotope compositions and provide examples of why this approach appears flawed we also provide reasons why the lf datasets previously published for canary island lavas remain robust and discuss why lf and sims data may provide complementary information on oxygen isotope variations in ocean island basalts oib despite unresolved small scale uncertainties associated with both techniques