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

carbon isotope and magnetic polarity evidence for non depositional events within the cambrian ordovician boundary section near dayangcha jilin province china carbon isotope and magnetic polarity stratigraphic results from the cambrian ordovician boundary section at xiaoyangqiao near dayangcha jilin province china in comparison to a contemporaneous section at black mountain australia indicate strata equivalent to major portions of the australian sequence are either absent or are restricted to highly condensed intervals these intervals are correlative with regressive sea level events identified in australia and western north america suggesting regional or eustatic sea level changes strongly influenced deposition of the xiaoyangqiao sequence these results also suggest the xiaoyangqiao section is unfavourable as the site of the cambrian ordovician boundary global stratotype section and point

a laboratory model for melting erosion of a magma chamber roof and the generation of a rhythmic layering 1 a hot magma chamber can ascend by melting the roof rock a process which in turn affects the magma composition the disaggregated mineral particles which consisted the roof rock will descend in the magma chamber to form a sedimentary cumulate however the fluid dynamics leading to the formation of the sediment and how we can decipher them is unknown here we conducted a series of experiments modeling melting erosion of the roof with particle size consisting the roof rock as the parameter we find that there is a critical particle size below which the melting erosion occurs rhythmically melting erosion stops because the disaggregated particles are suspended in the magma chamber and suppress the vertical heat transfer the suspension then separates into an upper clear layer and a lower suspension layer eventually the heated stratified layers become unstable an overturn occurs and melting erosion resumes when the particles consist of two sizes such that at least one of them is smaller than the critical size a rhythmic erosion occurs particles are sorted during each erosion period and a size graded rhythmic layering is spontaneously generated we estimate that rhythmic layering can be generated from melting erosion in a basaltic magma chamber when the grain size of the roof rock is 0 6 mm assuming a vertical temperature difference of 10 c we suggest that rhythmic roof melting coupled with particle settling is one possible mechanism for generating the rhythmic layering which is commonly observed in solidified magma chambers

a rapid mechanism to remobilize and homogenize highly crystalline magma bodies the largest products of magmatic activity on earth the great bodies of granite and their corresponding large eruptions have a dual nature homogeneity at the large scale and spatial and temporal heterogeneity at the small scale this duality calls for a mechanism that selectively removes the large scale heterogeneities associated with the incremental assembly of these magmatic systems and yet occurs rapidly despite crystal rich viscous conditions seemingly resistant to mixing here we show that a simple dynamic template can unify a wide range of apparently contradictory observations from both large plutonic bodies and volcanic systems by a mechanism of rapid remobilization unzipping of highly viscous crystal rich mushes we demonstrate that this remobilization can lead to rapid overturn and produce the observed juxtaposition of magmatic materials with very disparate ages and complex chemical zoning what distinguishes our model is the recognition that the process has two stages initially a stiff mushy magma is reheated from below producing a reduction in crystallinity that leads to the growth of a subjacent buoyant mobile layer when the thickening mobile layer becomes sufficiently buoyant it penetrates the overlying viscous mushy magma this second stage rapidly exports homogenized material from the lower mobile layer to the top of the system and leads to partial overturn within the viscous mush itself as an additional mechanism of mixing model outputs illustrate that unzipping can rapidly produce large amounts of mobile magma available for eruption the agreement between calculated and observed unzipping rates for historical eruptions at pinatubo and at montserrat demonstrates the general applicability of the model this mechanism furthers our understanding of both the formation of periodically homogenized plutons crust building and of ignimbrites by large eruptions

petrology of the 1991 1995 eruption at unzen effusion pulsation and groundmass crystallization abstract effusive eruption of dacite magma 2 1 108 m3 during 1991 1995 formed a lava dome at the summit of unzen volcano japan the effusion rate was highest at the beginning 4 0 105 m3 day 4 6 m3 s and decreased roughly with time to almost zero before this pattern was repeated with a second pulse of magma supply the whole rock chemistry of lavas shows significant variation attributable to variations in phenocryst abundance the more mafic the more abundant the phenocrysts the pattern of chemical variation with time shows some difference from that of the effusion rate all phenocrysts in dacite plagioclase hornblende biotite quartz and magnetite show evidence of disequilibrium with melt although a glomerophyric aggregation of phenocrysts suggests coexistence with each other phenocrysts are isotopically heterogeneous from species to species the calculated initial melt composition was rhyodacite and was nearly constant throughout the activity in contrast the bulk phenocryst population is andesite a model explaining the textures and the isotopic heterogeneity is the capture of diorite fragments or xenocrysts by parental rhyodacite magma it is suggested that when effusion rate was high less viscous crystal poor magma exited from the reservoir groundmass glass and plagioclase microlite rims show temporal chemical variations correlating with the effusion rate the higher the effusion rate the more evolved the compositions groundmass crystallinity increased with decreasing effusion rate from 33 to 50 textures in dome lavas suggest that groundmass crystallization had been mostly completed when magma reached the conduit top the fe ti oxide temperature 880 780 c was low when the crystallinity was high micropumice erupted before dome growth provided a sample recording magmatic foam in the conduit porosity of dome lavas was lower at lower effusion rates collapse of foam magma and simultaneous escape of volatiles through the conduit top were probably responsible for the accompanying low frequency earthquakes phenocrysts were broken and the breakdown rims on hornblende phenocrysts were torn off during collapse and successive compaction when effusion waned degassing and the resultant crystallization proceeded more completely so that the magma became too viscous to flow in the conduit top and behaved as a plug resulting in a temporary halt of effusion in turn groundmass crystallization in magma below the plug increased excess pressure in the upper parts of conduit due to slow cooling the plug was scavenged when rising excess pressure overcame its effective strength then the second pulse of magma supply began strong endogenous growth and extrusion of a lava spine in the later stage probably occurred for the same reason

depositional history tectonics and provenance of the cambrian ordovician boundary interval in the western margin of the north china block comment myrow et al 2015 detailed the sedimentology biostratigraphy and chemostratigraphy of the cambrian ordovician succession in the western north china block or sino korean block at wuhai inner mongolia china and recorded an 30 m y unconformity that formed during the upper cambrian series 3 upper middle ordovician they provided a valuable description of the succession accompanied by excellent illustrations of sedimentary facies as well as detailed data on chemostratigraphy thus providing an important basis for further geologic studies in the region in addition they presented a discussion on the origin of the 30 m y unconformity and suggested that the western sino korean block would have been placed near the western himalaya during the early paleozoic where a hiatus of similar magnitude has been reported myrow et al 2006 2016 this result seems to support the theory of mckenzie et al 2011 who first suggested a close paleogeographic affinity between the sino korean block and himalaya based on detrital zircons and trilobites however the paleogeographic interpretation of myrow et al 2015 requires further discussion because of the following reasons 1 incorrect description of the unconformities in other localities of the sino korean block and 2 presence of other potential options on the formative mechanism of this unconformity including tectonic tilting and eustatic sea level change in the following section we describe each topic in detail and discuss the nature and implication of this unconformity

origin of crystal poor differentiated magmas insights from thermal gradient experiments crystal poor differentiated magmas are commonly erupted from shallow thermally zoned magma chambers in order to constrain the origin of these magmas we have experimentally investigated crystallization differentiation and crystal melt separation in presence of a thermal gradient experiments have been designed taking advantage of the innate temperature gradient of the piston cylinder apparatus and carried out on a phonolitic system at 0 3 gpa and temperature ranging from 1 050 to 800 c crystallization degree and melt composition in experimental products vary as a function of the temperature gradient in particular melt composition differentiates from tephri phonolite starting material to phonolite moving from the hotter glassy zone t 1 050 c towards the cooler heterogeneously crystallized zone t 900 c of the charge the heterogeneously crystallized zone is made up of 1 a crystal rich mushy region crystallinity 30 vol 2 a rigid crystal framework crystallinity 80 vol and 3 glassy belts of phonolitic glass at the top thermal gradient experiments picture crystallization differentiation and crystal melt separation processes occurring in a thermally zoned environment and reveal that relatively large volumes of crystal poor melt glassy belts can originate as a consequence of the instability and collapse of the rigid crystal framework analogously in thermally zoned magma chambers the development and collapse of a solidification front may represent the controlling mechanism originating large volumes of crystal poor differentiated magmas

high resolution strontium isotope stratigraphy across the cambrian ordovician transition we have analyzed 214 fossil apatite samples from nine stratigraphic sections worldwide that cover the lower cambrian to lower ordovician for their strontium isotope ratios of these samples 180 from six sections that cover the cambrian ordovician transition 5 ma reveal how the extent of isotopic alteration can differ greatly according to lithology and sample type from three limestone sections 126 euconodont samples yield the most consistent 87sr 86sr ratios and are used in this article to constrain seawater 87sr 86sr across the cambrian ordovician boundary by contrast protoconodonts paraconodonts and inarticulate brachiopods are far less likely to retain a primary 87sr 86sr signature and appear to be of only limited value for chemostratigraphy seawater 87sr 86sr fell from maximum values for the phanerozoic of more than 0 7092 during the early late cambrian to 0 7090 by the earliest ordovician high resolution sampling has permitted the recognition of sinusoidal variations with a wavelength on the order of 1 ma that are superimposed on the overall fall in 87sr 86sr these variations may be caused by high order cycles in seawater 87sr 86sr or diagenetic alteration

remobilization of andesite magma by intrusion of mafic magma at the soufriere hills volcano montserrat west indies the 1995 1999 eruption of the soufriere hills volcano montintroduction serrat has produced a crystal rich andesite containing quenchthe soufriere hills volcano montserrat located in the textured mafic inclusions which show evidence of having been molten lesser antilles island arc in the west indies began to when incorporated into the host magma individual crystals in the erupt on 18 july 1995 after about 350 years of dormancy andesite record diverse histories amphibole phenocrysts vary from young et al 1998b the eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms which began in january 1992 anhydrous reaction products plagioclase phenocrysts are commonly previous earthquake crises have occurred on montserrat reverse zoned often with dusty sieve textures reverse zoned rims are in 1896 1897 1932 1935 and 1966 1967 powell 1938 also common on orthopyroxene phenocrysts pyroxene geothermometry shepherd et al 1971 initial phreatic and phregives an average temperature of 858 20 c for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores whereas reverse zoned rims record temperatures andesitic lava dome in november 1995 pyroclastic flows from about 880 to 1050 c the heterogeneity in mineral rim generated by gravitational collapse of the unstable dome compositions zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non uniform reheating and remobilization of the resident magma eruption but periods of explosive activity producing body by intrusion of hotter mafic magma convective remobilization pumice flows and falls have also occurred robertson et results in mixing together of phenocrysts that have experienced al 1998 young et al 1998b dome growth ceased different thermal histories depending on proximity to the intruding suddenly in march 1998 and further activity up to may mafic magma the low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool highly crystalline dome and periods of ash venting with some small scale magma body beneath the soufriere hills volcano the petrological vulcanian explosive events observations in combination with data on seismicity extrusion rate the soufriere hills volcano is the only active centre and so2 fluxes indicate that the current eruption was triggered by on montserrat the volcano is a composite of at least recent influx of hot mafic magma five andesitic lava domes flanked by pyroclastic deposits rea 1974 identified four other volcanic centres all

the evolution of amphibolites from site 1067 odp leg 173 iberia abyssal plain jurassic rifting to the pyrenean compression

continuous mixing of crystal mush and replenished magma in the ongoing unzen eruption zoning profiles of magnetite phenocrysts in the successively effused dacite of the ongoing unzen eruption from 1991 wereanalyzed to estimate the time scale of magma mixing the dacite was formed by mixing of relatively high and low temperature t end member magmas the magnetite phenocrysts derived from the low t magma are reversely zoned by the mixing with high t magma a diffusion calculation for reequilibration of the reverse zonings gives the time interval from magma mixing to quenching for the mixed dacite erupted from may 1991 to may 1993 the typical diffusion time was estimated to be a few months regardless of the effused sequence for 2 yr this indicates that the mixing was continuous during the effusion the invariability of the other mixing signatures such as the thickness of reaction rims around biotite phenocrysts also supports the continuous mixing model low t end member magma is estimated by mass balance calculation to be a crystal rich mush of dacitic composition these observations lead to a model wherein the highly crystallized remnant magma of the preceding activity has been mixed with the newly injected hot magma of similar bulk composition just prior to the effusion the proposed mechanism implies that this type of magma mixing is an inevitable process in periodically erupting polygenetic volcanoes

compaction in a mantle mush with high melt concentrations and the generation of magma chambers abstract most compaction models in the partially molten mantle have addressed the case of low intergranular melt concentrations up to a few percent here we develop a mathematical and numerical formalism adapted to the two dimensional modeling of compaction in mushes with higher melt concentrations up to a few tens of percents experimental data on mantle like mushes olivine crystals basaltic melts suggest that the mush viscosity depends in a complex way on the melt concentration three rheological thresholds occur at melt concentrations of about 5 20 and 40 respectively the first threshold corresponds to the establishment of full interconnectivity of the intergranular melt the second to the formation of a very dense suspension of crystals and the last to the development of crystal clusters in the suspension the present models take into account the stiff and drastic viscosity drops associated with these rheological thresholds intergranular melt migration associated with an initial melt pulse generates a horizon of high melt crystal ratio if the melt concentration in the initial pulse presents a local excess the horizon becomes slightly tilted as a consequence melt percolates upslope inside the tilted horizon pools at its summit and generates a pocket like zone due to the higher melt concentration the upward darcy velocity in the pocket markedly exceeds that in the horizon the result is that the pocket like impregnation is rapidly disconnected from the horizon and a new pocket develops at the summit of the partially fragmented horizon eventually the intergranular melt contained in the horizon is completely redistributed into pockets increasing the background melt concentration in the mush from 5 20 and 40 leads to an increase of the maximum melt concentration of 10 40 and 100 in the transient horizons and of 25 60 and 100 in the pockets these models suggest that magma chambers with a kilometer extent naturally result from the compaction of a mantle mush with an initial melt concentration exceeding 5

rayleigh taylor instability of a particle packed viscous fluid implications for a solidifying magma 1 we performed laboratory experiments of rayleigh taylor instability of superposed viscous fluids where the upper layer contains denser spherical solid particles a series of experiments are made by varying the viscosity and the particle diameter and we measure the growth rate and the wave length of the instability the instability consists of fine scaled plumes which coalesce as they descend plumes are observed to form intermittently and the particle layer thins with time which finally descend as blobs we find that the growth rate can be explained by using the linear stability analysis for rayleigh taylor instability of viscous fluids by taking the effective viscosity of particle bed to be 20 times that of the fluid and scale the thickness of the upper layer by twice the particle diameter using this scaling we find that a partially solidified layer beneath the surface of a lava lake may become unstable by this mechanism

ironstones and green marine clays in the dongjeom formation early ordovician of korea abstract in the sino korean north china block the occurrence of ironstone is reported for the first time from the lower ordovician dongjeom formation korea the dongjeom formation comprises much quartzose sediment deposited on the extensive carbonate platform and is composed of three coarsening upward sequences ironstone beds are distributed on the tops of the lower two sequences suggesting that they mark approximately maximum flooding events during the relative lowstand of sea level when the dongjeom formation was deposited extensive weathering in the source areas coupled with higher order sea level rises resulted in the formation of the dongjeom ironstones the dongjeom ironstones include much green marine clays ironstones of the lowest sequence are characterized by glaucony facies and those of the middle sequence by chamosite facies chemical compositions of dongjeom glauconies reveal that they were originally nascent to slightly evolved 3 to 5 k 2 o wt and that they were affected by illitization after burial their immature compositions suggest that glauconies were formed in about 10 3 to 10 4 years or less on the seafloor the substrates for glauconitization are interpreted as detrital mineral grains regressions after glauconitization introduced iron oxides to the green grains eventually forming the iron oxide rich band under the most oxidizing conditions the distribution of glauconies in limited horizons suggests that the dongjeom glauconies are autochthonous in origin chamosites have granular shapes and are mixed with quartzose sand grains suggestive of reworking during storms the dongjeom chamosites were transformed from the precursor berthierines the occurrence of glauconies in correlative horizons in korea and north china may indicate that glauconitization in the host sediments which were deposited in far northern gondwana occurred under the influence of a coeval transgressive event

preeruptive magma viscosity an important measure of magma eruptibility 1 using a compilation of melt compositions meltwater contents temperatures and phenocryst contents the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas the basaltic to rhyolitic magmas have preeruptive viscosities over the range 101 to 108 pa s although bulk sio2 content has often been used as a qualitative measure of preeruptive magma viscosity the bulk sio2 content shows a weak correlation with magma viscosity correlation coefficient r 0 5 because of a wide range of phenocryst contents from 0 to 50 vol andesitic magmas have viscosities ranging from 102 to 107 pa s which are lower or higher than those of phenocryst poor rhyolitic magmas with 105 to 106 pa s focusing on andesitic to rhyolitic magmas the r between bulk sio2 contents and magma viscosities changes to 0 1 in contrast the melt only sio2 content from a basaltic to rhyolitic melt shows a good linear correlation with melt only viscosity r 0 9 although most of the calculated viscosities of erupted magmas fall below 106 pa s as consistent with the previous compilation study this paper describes 20 examples of highly viscous magmas with 106 pa s in most cases composed of mixtures of high silica rhyolitic melt 75 79 wt sio2 and abundant phenocrysts 30 55 vol in these highly viscous magmas 9 examples have erupted following the precursory eruption of less viscous magma suggesting that precursory dike propagation and conduit formation by the less viscous magma with 106 pa s induced the following eruption of less eruptible highly viscous magmas

melt extraction in mush zones the case of crystal rich enclaves at the sabatini volcanic district central italy abstract a peculiar feature of the sabatini volcanic district svd central italy is the occurrence of crystal poor pumices and crystal rich enclaves within the same eruptive host deposit the stratigraphic sequence of pumices and enclaves indicates the tapping of a stratified magma chamber where a crystal poor phonolitic magma lay on top of a more primitive crystal rich magma the crystal rich enclaves are genetically related to the pumices and record the evolution of a solidification front in which a more differentiated melt was produced extracted and eventually erupted we collected and analyzed crystal rich enclaves from one of the largest phonolitic eruptions at the svd and used their petrological and geochemical features to reconstruct magma differentiation and crystal melt separation in the solidification front on this basis three groups of enclaves have been identified porphyritic enclaves holocrystalline enclaves and sanidinites the mineralogical variability faithfully reproduces the spatial and temporal evolution expected of a solidification front from early to intermediate crystallization conditions porphyritic and holocrystalline type to the late stage of solidification sanidinites in which the percolation of a more differentiated melt through the crystal mush triggered the instability of the solidification front results from numerical models indicate that gravitational instability is the most efficient mechanism to explain melt extraction in mush zones of medium sized 10 km 3 short lived 10 4 years magma chambers

rheology of crystal bearing silicate melts an experimental study at high viscosities the viscosity of partially crystallized mg3al2si3o12 melts has been measured under uniaxial compression in the interval 1010 1013 poise as a function of the volume fraction of crystals these inclusions are well rounded spherulites of aluminous enstatite having the same composition as the melt and whose growth rate is negligible at the temperature of the measurements the viscosity increases by less than 1 order of magnitude for crystal fractions of 40 vol and remains newtonian up to the maximum stress exerted namely 1 kbar the einstein roscoe equation 0 1 m n provides very good fits to the measurements only if either the m or n parameter is allowed to depend on temperature for modeling of magmatic processes however the widely recommended constant values m 0 6 and n 2 5 should be adequate the rheology changes abruptly when the clustered spherulites begin to oppose shear deformation at a crystal fraction of about 40 vol the viscosity becomes non newtonian with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise as long as the crystal fraction remains lower than 70 vol the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt the deformation becomes again regular at low stresses with lower melt fractions but samples undergo extensive fracturation along the direction of uniaxial stress similar rheology changes have been observed during the isothermal crystallization of li2si2o5 melts which produces small ellipsoidal inclusions these results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions

analog experiments on melting and contamination at the roof and walls of magma chambers abstract a series of analogue laboratory experiments were carried out to model melting and mixing processes occurring at the roof and sides of a magma chamber when hot dense mantle derived magmas are emplaced in the deep crust the experiments investigated the influence of a viscosity contrast between crustal melts and input magma the presence of crystals in the input magma and roof geometry the crust was modelled by aqueous ice or a cold low melting point wax arranged as a flat roof or in an upside down u the input magma was modelled by a hot dense aqueous solution of nano3 or feso4 which was injected into the cavity between the walls the experiments revealed the potentially important role that side wall processes may have in crustal melting and mantle magma contamination in this environment the walls melted back much faster than the roof and the melts from the opposite walls ponded next to each other under the roof and mixed with each other only in the central region when the input solution was able to crystallise it did so preferentially in the active side wall boundary layers the tiny dense crystals which were dragged up in the melt boundary layer and then dropped into the interior fluid enhanced hybridisation of the melt and contamination of the input fluid and through their influence on the boundary layer structure reduced the melting rate

mixing of stratified liquids by the motion of gas bubbles application to magma mixing mafic magma underlying more silicic magma in a reservoir can exsolve volatiles as it crystallizes forming bubbles that segregate upwards to the interface with the overlying silicic magma we describe laboratory experiments designed to investigate how gas bubbles migrate into the more viscous upper layer in one regime bubbles move individually across the interface entraining a small quantity of lower layer fluid and the two layers become progressively stirred into a homogeneous mixture in a second regime bubbles form a thin foam layer at the interface which becomes gravitationally unstable giving rise to two phase plumes which rise and produce a coarse mixture of the liquids for a given viscosity ratio the transition between these regimes occurs when the gas flux is greater than a critical value which we found to decrease with increasing viscosity ratio we estimate the critical gas flux for magmatic conditions and how vesiculation may vary with pressure degree of crystallization and the composition of volatile components mafic inclusions which are often found in silicic to intermediate lavas can form in the plume regime analysis of the instability of the foam predicts wavelengths of between 1 cm and 1 m which is consistent with the observed sizes of inclusions volatiles from the mafic magma may be efficiently transported by this mechanism into the viscous silicic magma as bubble plumes despite low stokes velocities for individual vesicles at small viscosity ratios or low gas fluxes the bubbling regime can produce hybrid magmas

rb sr and k ar dating of precambrian clays and glauconies abstract isotopic dating of sedimentary material requires a different approach from the isotopic dating of plutonic volcanic and high grade metamorphic rocks the present contribution presents mineralogical criteria such as clay composition illite or glauconite crystallinity indices illite or glauconite polymorphic types sedimentological criteria such as mechanisms of genesis as well as chemical criteria such as the sr composition and content of the carbonates associated with the studied clays and glauconies these criteria help in the interpretation of the numerical results and should always be considered in attempts to obtain radiometric ages of sedimentary clays and or glauconies it is emphasized that a difference between measured and expected age seldom means that the chronometer used is inaccurate but it commonly means that a parameter was not recognized or even disregarded a critical review of the uncertainties typically related to sedimentary dating is also proposed they may be grouped into four types 1 genetic uncertainties dependent on the genetic mechanisms of the studied minerals 2 historic uncertainties dependent on the alteration events imprinted in the sediments since deposition weathering diagenesis and or tectono metamorphism 3 stratigraphic uncertainties dependent on the precision of the sample location in the sedimentary sequence and 4 analytic uncertainties dependent on the representativeness and the preparation of the samples as well as the accuracy of the measurements a reliability test of the rb sr and k ar methods as applied to dispersed precambrian clays and glauconies is discussed from an extensively studied sequence from west africa it shows that the k ar system of these minerals is much more sensitive to later modifications than the rb sr system therefore the two techniques may be complementary in geochronological studies of precambrian sediments

the recommended cambrian ordovician global boundary stratotype of the xiaoyangqiao section dayangcha jilin province china this paper provides a brief outline of the current detailed inter disciplinary work on the xiaoyangquiao section trying to expose all the aspects for reference tied to the global single stratigraphic point gssp concept for defining the cambrian ordovician boundary the 45 m critical interval of this section outcrops very well along the steep bank of a stream and is free from folding faulting intrusions and has not been affected by weathering colour alternation of conodonts and acritarchs and crystallinity indices of illite all indicate a maximum thermal grade of 100 c the lithofacies being of great lateral persistence for a hundred kilometres consists mainly of a rhythmical sequence of lime mudstone and shales deposited in a moderately deep outer shelf environment of quiet water well below the normal storm wave base chemical investigation of the rocks demonstrates strong positive correlation between a12o3 content and those of k2o mgo fe2o3 tio2 be cr co ni zn and ba indicating these components are tied to the clay fractions a stable depositional environment is demonstrated by the uniform chemistries through the boundary interval close to the boundary itself p2o5 contents are low indicating continuous sedimentation at fairly substantial rates the major biological events the biostratigraphic framework and the stratigraphic range of conodont graptolite trilobite and acritarch taxa are illustrated briefly with diagrams following the majority views of the calgary plenary session the boundary level is to be chosen at a point marked by the first appearance datum fad of the selected conodont taxon or taxa in the vicinity of the level close to but below the first influx of nematophorous graptolites the following four points marked by the incoming of conodont taxon or taxa are recommended for consideration of the golden spike 1 fad of cordylodus intermedius at 5 28 m below the first influx of nematophorous graptolites 2 fad of hirsutodontus simpler cordylodus drucei albiconus postcostatus at 5 23 m 3 fad of semiacontiodus lavadamensis utahconus utahensis monocostodus sevierensis at 3 85 m 4 fad of cordylodus lindstromi at 2 23 m for the following reasons the fad of h simplex c drucei a postcostatus is favoured 1 the taxa are all distinct and widely dispersed 2 intensive evolutionary change took place in conodonts graptolites trilobites and acritarchs prior to or after this point all the fossil groups occur together providing correlation with many regions throughout the world 3 the point is in a position between the previously widely accepted boundary levels based on graptolites and trilobites 4 the proposed point lies within a thin laterally persistent rhythmical sequence the fad of cordylodus lindstromi is also a favourable point sharing many advantages mentioned above but this point is less satisfactory in being defined by the fad of a single taxon c lindstromi which also has an extremely small population size an isochron age of 500 7 7 4 ma is determined from clay fractions of mudstones 8 5 m below the proposed point by means of the rubidium strontium method the nd signature determined from conodonts trilobites hyolithids and acrotretid brachiopods has a mean value of 6 7 comparable with that of the coeval oceanic water mass occupying southeastern north america and europe and indicating that northeastern china bordered the same ocean the mean tdm model age determined was 1 26 ga at the time of sedimentation compatible with the mean tdm model age of approximately 1 1 ga for the pacific ocean today the relatively low value of the tdm model age indicates a substantial input from young orogenic volcanic island arcs and terranes

trilobite faunal successions across the cambrian ordovician boundary intervals in korea and their correlation with china and australia abstract the cambrian ordovician boundary in korea lies within the choson supergroup exposed in the taebaeksan basin and has been recognized in the taebaek and yongwol groups the taebaek and yongwol groups show slightly different trilobite faunal successions across the putative cambrian ordovician boundary intervals in the taebaek group saukiid dominated missisquoia onychopyge and kainellid dominated faunas occur successively in the basal part of the tongjom formation while the remaining part of the tongjom formation is poorly fossiliferous the succeeding tumugol formation yields fairly diverse trilobite faunal assemblages of late tremadocian age this implies that the cambrian ordovician boundary in the taebaek group should be placed somewhere within the interval yielding the kainellid dominated fauna in the yongwol group an uppermost cambrian fatocephalus fauna is more or less contemporaneous with the saukiid dominated fauna of the taebaek group while three tremadocian trilobite faunas i e yosimuraspis kainella and shumardia zones in ascending order are well represented in the mungok formation the occurrence of jujuyaspis in the yosimuraspis zone provides a firm basis for locating the cambrian ordovician boundary at the base of the mungok formation these cambrian ordovician trilobite faunas of korea are in general well correlated with those of china and australia

simultaneous crystallization and melting at both the roof and floor of crustal magma chambers experimental study using nh4cl h2o binary eutectic system abstract thermal and compositional evolution of magmas after emplacement of basalt into continental crust has been investigated by means of fluid dynamic experiments using a cold solid mixture with eutectic composition and a hot liquid with higher salinity in the nh 4 cl h 2 o binary eutectic system the experiments were designed to simulate cases where crystallization of a basalt magma is accompanied by melting at both the roof and floor of a crustal magma chamber the results show that thermal and compositional convection occur simultaneously in the solution the thermal convection is driven by cooling at the roof and the compositional convection is driven by melting and crystallization at the floor the roof was rapidly melted by the convective heat flux which resulted in formation of a separate eutectic melt layer the upper liquid layer with negligible mixing of the underlying liquid the lower liquid layer on the other hand a mushy layer formed at the floor the compositional convection at the floor carried a low heat flux so that the heat transfer at the floor was basically explained by simple heat conduction the thermal boundary layer in the lower liquid layer at the interface with the upper liquid layer became thicker with time and subsequently temperature decreased upward throughout the lower liquid layer compositional gradient with nh 4 cl content decreasing upward formed by compositional convection in the lower liquid layer the formation of these gradients resulted in formation of double diffusive convecting layers in the lower liquid layer the upward heat transfer was suppressed when compared with the case where the liquid region is homogenized by vigorous convection these experimental results imply that when a basalt magma is emplaced in continental crust floor melting does not always enhance the cooling of the magma but it may even reduce the total heat loss from the magma to the crusts due to suppression of convection by formation of a stabilizing compositional gradient

paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite abstract rare earth element concentrations in biogenic apatite of conodonts fish debris and inarticulate brachiopods were determined in over 200 samples from cambrian to modern sediments tests for experimental bias caused by the chemicals used to separate fossils from the rock matrix and for interlaboratory interexperiment or interspecies related variations clearly show that no resolvable fractionation of ree occurs incorporation of ree in biogenic apatite of recent fish debris occurs near the sediment water interface soon after deposition and reflects characteristics of seawater the original ree signature apparently survives subsequent burial and diagenesis cerium variations in fish debris from modern environments are controlled by redox potential ce is fractionated by co precipitation with metallic oxides under oxidizing conditions this fractionation produces a negative ce anomaly ce anom in seawater that is reflected in the sedimented fish debris conversely no fractionation of ce occurs under reducing conditions resulting in ce concentrations that are normal to enriched in anoxic seawater and fish debris deposited under local or basinwide anoxic conditions extrapolation of observations of the redox control of ce anom in modern oceans to fossil apatite indicates that anoxic conditions were prevalent in lower paleozoic and lower triassic oceans whereas in the upper paleozoic the world ocean was generally oxidizing

the contribution of experimental volcanology to the study of the physics of eruptive processes and related scaling issues a review abstract the experimental approach has become a major tool increasingly used by volcanologists in recent decades to investigate the physics of eruptive processes in complement to field and theoretical works researchers have developed various methodologies to study volcanic phenomena at reduced length scale the works involve natural or analogue materials and their types range from first order tests to identify fundamental processes and make qualitative comparison with field observations to more sophisticated experiments in which precise data obtained in a controlled environment can be used to validate outputs of theoretical models scaling is a central issue to ensure dynamic similarity between the small scale experiments and the large scale volcanic phenomena when natural conditions cannot be simulated due to inherent length scale difference and or technical limitations in this respect dimensionless numbers are used to map physical regimes and to define scaling laws which allow experimental results to be extrapolated to natural scale this review presents the variety of experimental studies conducted to investigate subterraneous and aerial volcanic phenomena involving in particular fluid particle mixtures we focus on the major scaling issues and the physical regimes investigated and we also highlight in a historical perspective some of the major advances achieved through experimental studies finally we conclude on some perspectives for future works

experiments on upward migration of a liquid rich layer in a granular medium implications for a crystalline magma chamber 1 we perform a series of experiments to investigate the situation in which a melt rich layer formed by a magma intrusion ascends through a crystalline magma chamber the initial condition is such that a heavier granular layer overlies a liquid layer the particles consisting the upper granular layer are in a jammed state and only the particles near the interface can move to form a dilated boundary layer the dilated layer detaches from the upper granular layer and forms downwelling plumes which drive a cellular convection within the liquid rich layer the convection erodes the upper granular layer and the liquid rich layer migrates upwards with time this upward migration of the liquid rich layer differs from the previously known mechanisms of liquid transport permeable flow in which the liquid migrates at the darcy velocity the stokes settling in which the individual particle settles and diapirs formed by the rayleigh taylor instability we find that the velocity of the upward migration of the liquid rich layer can be scaled by the volumetric flux of the liquid ascending through the narrow channel between the particles the upward migration of the liquid rich layer is faster than the darcy velocity in a mushy magma chamber whose crystals are in a jammed state neither the stokes settling nor the rayleigh taylor instability can occur we propose that the upward migration of the melt rich layer observed in our experiments can become an efficient mechanism of melt transport in a crystalline magma chamber

conodonts from the cambrian ordovician boundary interval in the southeast margin of the sichuan basin china abstract conodonts from the cambrian ordovician transition at the liangcun section in xishui county guizhou and at the huangcao section in wulong county chongqing are examined for the first time both sections are located at the southeast margin of the sichuan basin a total of 1367 specimens were recovered representing 30 species and 15 genera based on the ranges of conodonts generalized from these two sections and another six sections previously studied in the same region three conodont zones cordylodus proavus monocostodus sevierensis and cordylodus angulatus zones are recognized the index species of the cambrian ordovician boundary at the global stratotype section and point gssp iapetognathus fluctivagus and its substitute in china iapetognathus jilinensis are not observed in the study sections therefore it is impossible to determine the cambrian ordovician boundary exactly however it probably lies within the lower part of m sevierensis zone the upper part of the loushanguan group correlating with the gssp in canada and the dayangcha section in china chronological sequences of the fad first appearance datum of c angulatus chosonodina herfurthi and rossodus manitouensis are not obvious in the study so the c angulatus zone here is correlated with zones defined by c angulatus ch herfurthi and r manitouensis in the lower yangtze platform

nd isotopic study of upper cambrian conodonts from korea and implications for early paleozoic paleogeography abstract correlation of the korean peninsula with the neighboring south and north china blocks has been one of the most important and long debated issues in east asian tectonics the eastward extension of the chinese collisional belt between the south and north china blocks into the korean peninsula is still in dispute the lower paleozoic sedimentary rocks in south korea the joseon supergroup accumulated in the okcheon belt which has been proposed as the suture zone or the block boundary between the south and north china blocks in korea the joseon supergroup comprises five different lithostratigraphic units and has been proposed to represent two tectonostratigraphic blocks bounded by the honam shear zone hsz in the okcheon belt among them the yeongweol unit in the western part of the hsz is correlated with south china and the duwibong unit in the eastern part of the hsz with north china to test the above geodynamic interpretation this study analyzed the nd isotopic composition of upper cambrian conodonts from the duwibong and yeongweol units the difference of the nd t values between the upper cambrian duwibong and yeongweol units is about 3 units compared to a wide range of the nd 0 values in the modern oceanic basins this slight difference can be regarded as a variation in a same paleo watermass the nd t values of the two units are consistent with coeval values from north china and this suggests that the duwibong and yeongweol units shared the same oceanic watermass with the north china considering current early paleozoic paleogeographic reconstructions the nd isotopic signatures of north china duwibong and yeongweol units may be independent of laurentia and baltica and thus can be indicative of another unique oceanic watermass in contrast to the previous tectonic interpretations the same paleoceanographic signatures shown by the duwibong and yeongweol units and north china and recent developments suggest that the yeongweol unit in the early paleozoic joseon supergroup is not correlated with south china but correlated with north china and the korean peninsula and north china block may have been included in a larger continental block the sino korean block the collisional belt between the south and north china blocks may not extend into the korean peninsula

re os age and depositional environment for black shales from the cambrian ordovician boundary green point western newfoundland chemical and isotopic signatures for black shales serve as potential proxies for reconstruction of paleoenvironmental conditions here we bring rock eval major and trace element and re os isotopic data together to examine the environmental record at the cambrian ordovician global stratotype section and point gssp at green point in western newfoundland canada the green point shales are oil mature and contain type ii organic material of marine origin a re os isochron for the shales provides the first radiometric age for shale deposition at the gssp at 484 16 ma with an initial 187os 188os ratio of 0 74 0 05 model 3 age mswd 21 n 13 2 uncertainties factor analysis of the geochemical data set shows association of most trace elements with total organic carbon toc and s contents ensuring an authigenic origin for most elements and hence their validity for evaluating the paleo redox state relatively high enrichment factors for redox sensitive elements e g re u and mo compared to average shale but comparatively low enrichment compared to modern black sea sediments suggest deposition in anoxic but not euxinic waters comparison of lower ordovician shale geochemistry data sets at a global scale leads us to suggest that anoxic conditions and warm oceanic regimes were restricted to the margins of laurentia and baltica whereas depositional basins with colder waters e g avalonia and gondwana were less reducing these outcomes underscore the important role of paleogeography in regulating oceanic conditions and marine life

the permeability controlled accumulation of primary magma the initial accumulation of primary magma occurs just after the mantle has become permeable the accumulation is caused by the compaction of the residuum which either may be controlled by the rate of creep or by the rate of flow of the interstitial melt experimental results suggest that the rate of compaction is controlled by the permeability and a model for the accumulation process is worked out on this basis the compaction causes the formation of a lower compaction boundary and an upper layer of melt the ascending mantle of plumes and convection currents will form layers of melt situated 20 100 m apart the type of partial melting for this accumulation is critical melting

non newtonian behavior of plagioclase bearing basaltic magma subliquidus viscosity measurement of the 1707 basalt of fuji volcano japan abstract laboratory measurements of viscosity were done for basalt erupted in 1707 ad from fuji volcano japan using a concentric cylinder rotational viscometer at temperatures of 1297 1157 c 1 atm pressure and fo 2 near the ni nio buffer on cooling elongated plagioclase crystals with a mean length width ratio of ca 8 5 appeared at 1237 c followed by olivine at 1157 c at progressively lower temperatures the total crystal volume fraction increased monotonously to ca 0 25 viscosity increased from 38 9 to 765 pa s at a shear strain rate of 1 s 1 this basalt magma behaves as a newtonian fluid at temperatures greater than 1217 c but shear thinning behavior occurs at temperatures less than 1197 c because of the suspended plagioclase crystals this behavior is well approximated as a power law fluid at the onset of shear thinning the crystal volume fraction was between 0 06 and 0 13 which is attributed to the pronounced lath shape of plagioclase crystals the relative viscosity increases monotonously with increase of crystal volume fraction at a constant shear strain rate and with decrease of shear strain rate at a constant crystal volume fraction a modified form of the krieger dougherty equation is introduced herein it enables us to describe the dependencies of relative viscosity on both the crystal volume fraction and shear strain rate and consequently the onset of shear thinning behavior

viscosity of high crystal content melts dependence on solid fraction 1 the rheological properties of suspensions containing high solid fractions are investigated attention is focused on viscosity of silicate and magmatic melt systems a general empirical equation which describes the relative viscosity of suspensions as a function of suspended solid fraction is proposed in the limit of very dilute solid concentrations it reduces to the einstein equation the proposed relationship is satisfactorily applied to reproduce available experimental data relative to silicate melts moreover the extrapolation of the model to very high concentrations is compared with experimental observations on partially melted granite

modelling of melt segregation processes by high temperature centrifuging of partially molten granites ii rayleigh taylor instability and sedimentation summary the present experimental study deals with the laboratory modelling of two different mechanisms of gravitational percolation in partially melted rocks 1 diapiric percolation of heavy material and 2 the sedimentation of heavy particles these two mechanisms of mass transport in partially melted rocks result in different scales of the segregation process in the melt crystal matrix a centrifuge furnace was used to simulate the percolation of the heavy particle layer through the partially molten granite at temperatures of up to 1000 c samples of beauvoir granite massif central france grain size 0 16 0 5 mm with an initial degree of partial melting 45 per cent were used as a matrix a layer of pt powder suspended in a melt of the same composition as the partially melted matrix was placed on the top of the granite sample after centrifuging for various times up to 2 x lo4 s x ray images of samples were obtained and the evolution of the percolation process of heavy suspension in the partially molten granite was monitored from the pt particle distribution the diapiric or finger regime of percolation starts when the growth rate of a raleigh taylor instability of the heavy layer is faster than the stokes sedimentation velocity of individual particles in the upper layer this relationship is a complex function of the size and initial concentration of heavy particles as well as the ratio of particle to crystal size the permeability of the matrix and the heterogeneity scale in the partially melted matrix at small concentrations several per cent and at large concentrations where close packing of heavy particles results in an anomalous viscosity increase in the upper heavy layer stokes sedimentation is dominant in the vertical percolation of the heavy material the sinking velocity of the diapir decreases when the size of heavy particles in it becomes comparable with the size of crystals in the partially melted granite in this situation the vertical sinking of the diapir is not stable and the horizontal instability of the vertical mass transport starts to become important mass transport via diapiric percolation results in more efficient crystal melt segregation of partially melted rocks the percolation of individual particles provides only local melt crystal flow on a scale comparable with the heavy particle size the diapiric percolation provides a much larger scale of partial melt segregation with a length scale comparable with the diapir size

late cambrian biogeography conodont bioprovinces from korea abstract although the collisional belt between the north and south china blocks has been well defined in china the location of its eastward extension in the korean peninsula is highly controversial the cambro ordovician sedimentary rocks in south korea the joseon supergroup accumulated in the ogcheon belt which has been proposed as the suture zone between the north and south china blocks or the block boundary the joseon supergroup comprises five different lithostratigraphic units and may be divided into two tectonic blocks by the honam shear zone hsz in the ogcheon belt based on the affinities of the cambrian trilobite faunas previous studies correlated the yeongweol unit in the western part of the hsz with south china and the duwibong unit in the eastern part of the hsz with north china to test the above geodynamic interpretation this study analyzed and compared the degree of the provinciality of upper cambrian conodonts from the yeongweol and duwibong units and north and south china the quantitative test for the upper cambrian provincialism shows high bioprovincial affinities between the yeongweol and duwibong units and both conodont faunas from the two units display higher bioprovincial affinities with that of north china rather than with that of south china their high conodont faunal similarity suggests that the korean peninsula might have been included in a larger continental block including the north china and the korean peninsula the sino korean block in early paleozoic time this result is supported by recent studies on lower paleozoic biogeography stratigraphy and sedimentology that suggest that the yeongweol unit of the joseon supergroup is more closely correlated with north china than with south china

a review of the late cambrian furongian palaeogeography in the western mediterranean region nw gondwana abstract the cambrian ordovician transition of the western mediterranean region nw gondwana is characterized by the record of major erosive unconformities with gaps that range from a chronostratigraphic stage to a series the hiatii are diachronous and involved progressively younger strata along the gondwanan margin from sw morocco to ne montagne noire they can be related to development of a multi stage rifting further north currently connected to the opening of the rheic ocean and concomitant erosion on southern rift shoulders the platforms of this margin of gondwana occupied temperate water mid latitudes and were dominated by siliciclastic sedimentation while carbonate factories were only episodically active in the montagne noire platform the upper cambrian is devoid of significant gaps in the southern montagne noire and the iberian chains there the sedimentation took place in a transgressive dominated depositional system with common offshore deposits and clayey substrates and was bracketed by two major regressive trends the late cambrian is also associated with the record of volcanic activity e g in the cantabrian and ossa morena zones and the northern montagne noire and widespread development of a tectonic instability that led to the episodic establishment of palaeotopographies and record of slope related facies associations several immigration events are recognized throughout the latest middle cambrian late cambrian and tremadocian the trilobites show a stepwise replacement of acado baltic type families e g the conocoryphid paradoxidid solenopleurid assemblage characterized by i a late languedocian latest middle cambrian co occurrence of middle cambrian trilobite families with the first anomocarid dorypygid and proasaphiscid invaders ii a late cambrian immigration replacing previous faunas composed of trilobites aphelaspidids catillicephalids ceratopygids damesellids eulomids idahoiids linchakephalids lisariids onchonotinids and pagodiids linguliformean brachiopods acrotretids obolids scaphelasmatids siphonotretids and zhanatellids echinoderms mitrates glyptocystitid cystoids and stromatocystoids and conodonts belonging to the lower peltura zone and iii the subsequent input of new trilobites asaphids calymenids catillicephalids nileids and remopleurids which marks the base of the proteuloma geinitzi zone associated with pelmatozoan holdfasts oryctoconus and a distinct input of late tremadocian conodonts paltodus deltifer zone the biogeographic distribution of latest middle and late cambrian trilobites supports brachiopod data indicating strong affinities between the western mediterranean region east gondwana north china korea south china australia and antarctica and kazakhstania during the late languedocian which became significantly stronger during the late cambrian this major shift may suggest modification in oceanic circulation patterns throughout gondwana across the middle late cambrian transition

cambrian ordovician conodonts from slump deposits of the argentine precordillera new insights into its passive margin development fil voldman gustavo gabriel consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet cordoba centro de investigaciones en ciencias de la tierra universidad nacional de cordoba facultad de ciencias exactas fisicas y naturales centro de investigaciones en ciencias de la tierra argentina

sm nd in marine carbonates and phosphates implications for nd isotopes in seawater and crustal ages this study explores the possibility of establishing nd isotopic variations in seawater over geologic time calcite aragonite and apatite are examined as possible phases recording seawater values of nd modern biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have nd concentrations of from 0 2 to 70 ppb showing that primary marine caco 3 contains little ree and that nd ca is not greatly enhanced relative to seawater during carbonate precipitation very young marine limestone and dolomite containing no continental detritus have 200 ppb nd all the carbonates are lree enriched 0 16 sm nd 0 45 modern and very young atlantic and pacific carbonates have nd in the range of shallow atlantic and pacific seawater respectively implying that they derive their ree from local seawater the nd in well preserved carbonate fossils is 4 10 4 ppb much greater than in their modern counterparts but like the high values found for carbonates in other studies we believe the high ree contents at the 500 ppb level in some detritusfree carbonates are due to ree rich fe hydroxide in on the carbonate in favorable cases such material may record seawater nd values however introduction of extraneous ree may obscure the original isotopic composition of pure caco 3 because of its very low intrinsic primary ree abundance modern biogenic apatite is also shown to have very low ree content 150 ppb nd but appears to quickly scavenge ree from seawater inorganically precipitated apatite from phosphorites has high concentrations of seawater derived ree young phosphorite apatite from the atlantic and pacific oceans has nd in the range of the seawater from these oceans older apatite samples of similar age from different localities bordering common oceans record similar values of nd t sedimentary apatite has sr t values in good agreement with the curves for 87 sr 86 sr of seawater as a function of time individual conodonts from a single formation yield the same sr t and nd t other workers have shown that sedimentary apatite preserves seawater ree patterns these characteristics suggest that sedimentary apatite can be used to determine nd t in ancient seawater the seawater values so inferred range between 1 7 and 8 9 over the last 700 my and lie in the range of modern seawater showing no evidence for drastic changes high values of seawater nd t in the triassic and latest precambrian may correlate with the breakup of large continental landmasses the initial nd t 15 0 of a 2 ae old phosphorite implies the presence of 1 5 ae old continental crust at 2 ae ago the approach outlined here can be used to constrain the age of the exposed crust as a function of time

ion microprobe and id tims u pb dating on zircon grains from leg 173 amphibolites evidence for permian magmatism on the west iberian margin in this paper we report evidence for permian magmatism at the ocean continent transition zone of the west iberian margin by dating magmatic intrusion of amphibolite precursors drilled at site 1067 during odp leg 173 the p t conditions recorded by the amphibolites indicate that the latter was emplaced at the bottom of a 25 km thick crust subhedral to rounded and colourless zircon grains were obtained from samples at the base of the cored section id tims dating on these zircon grains yielded a lower intercept age of 247 5 ma and a poorly defined upper intercept around 2 ga in situ dating of single zircons by ion microprobe cameca ims 1270 provided an age of 246 5 ma this permian age is interpreted as a minimum estimate for the emplacement of the gabbroic melts at the base of the hercynian crust during the early stage of rifting between the iberia newfoundland conjugate margins

the growth of the continent through geological time studied by nd isotope analysis of shales abstract we determined the nd isotopic composition and the sm nd ratios in a series of australian shales ranging from 0 2 gy to 3 3 gy the first result of this study is the constancy of the sm nd ratio in these shales as in granitoids secondly the initial 143nd 144nd ratio gives a regular curve decreasing through geological time both results confirm that shales are representative samples of the continental crust when insoluble elements like ree are studied we calculated their nd model ages of crustal differentiation the model ages regularly decrease with the stratigraphic ages and after 2 gy the curve flattens and tends to an asymptotic value at around 1 8 gy the significance of the shales is that they represent a mixture of continental materials and we consider the model age of such a mixture directly linked with the mean age of the continental portion feeding the sedimentary basin from these results we deduce a quantitative model of the growth curve of the continental source of the shales taking into account the effects of erosion which selectively sample recent mountains relative to shield areas we propose that the results obtained here are representative of the whole gondwana continent having studied the case of the australian shales we try to extend our study first we applied our quantitative approach to the recent results obtained by o nions et al on north atlantic provinces the continental growth curves obtained by our inversion procedure are quite distinct from the australian shales showing the regional character of shales with these two cases studied we try to compare our results with the already developed model for continental growth the consideration of the surface conservative versus surface non conservative models clearly show the non uniqueness of the geological interpretation of the growth curve on the other hand we have calculated for each case the recycling rate versus geological time by comparing the growth curve with the hurley rand province age curve such recycling increases with time in agreement with the data obtained on nd and sr initial ratios on granitoids

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions

experiments on metal silicate plumes and core formation short lived isotope systematics mantle siderophile abundances and the power requirements of the geodynamo favour an early and high temperature core formation process in which metals concentrate and partially equilibrate with silicates in a deep magma ocean before descending to the core we report results of laboratory experiments on liquid metal dynamics in a two layer stratified viscous fluid using sucrose solutions to represent the magma ocean and the crystalline more primitive mantle and liquid gallium to represent the core forming metals single gallium drop experiments and experiments on rayleigh taylor instabilities with gallium layers and gallium mixtures produce metal diapirs that entrain the less viscous upper layer fluid and produce trailing plume conduits in the high viscosity lower layer calculations indicate that viscous dissipation in metal silicate plumes in the early earth would result in a large initial core superheat our experiments suggest that metal silicate mantle plumes facilitate high pressure metal silicate interaction and may later evolve into buoyant thermal plumes connecting core formation to ancient hotspot activity on the earth and possibly on other terrestrial planets

convection and particle entrainment driven by differential sedimentation when a suspension of small particles is overlain by a clear fluid whose density is greater than that of the interstitial fluid but less than that of the bulk suspension the settling of the dense suspended particles can lead to vigorous convection in the overlying fluid this novel situation is investigated experimentally and theoretically a sharp interface is observed between the convecting upper region and a stagnant lower region in which there is unimpeded sedimentation at low reynolds number there is no transport of fluid from the upper region into the lower though there is mixing of both buoyant fluid and entrained particles from the lower region into the upper the interface between the two regions is found to descend at a constant velocity systematic laboratory measurements have determined how this velocity depends on the densities of the layers and the distributions of settling velocities of the particles a theoretical description is developed which calculates the evolution of the density of the lower region due to differential sedimentation of polydisperse particles buoyancy arguments based on the calculated density profile are used to placc upper and lower bounds on the amount of particle entrainment into the upper layer and on the rate of fall of the interface between the convecting and sedimenting regions the theoretical predictions are in good agreement with the experimental observations the analysis of the interaction between convection and sedimentation in the system considered here may be particularly relevant to the description of evolving crystal rich layers in magma chambers and of silt laden outflow from rivers and has a wide range of other industrial environmental and geological applications

a new miniaturized extraction chromatography method for precise u pb zircon geochronology abstract in this paper we describe a new method for the concomitant separation of u and pb in zircons for radiometric dating purposes the proposed separation scheme is based on a blend of two extraction chromatographic materials referred to as tru spec and sr spec respectively these afford a straightforward separation of u and pb with high yield good purity and satisfactory blank levels on micro columns using very small volume 5 l of mixed resins owing to the selectivity and high capacity of the specific extraction chromatographic materials for u and pb this new technique could be further scaled down the validity of the method is illustrated by measurements of u pb isotopic ratios and determination of corresponding ages in 0 2 0 8 mg fragments of the zr91500 zircon standard