alternating field characteristics of pseudo single domain 2 14 μm and multidomain magnetite abstract we report normalized af demagnetization curves of anhysteretic remanences arm s produced by 1 10 and 40 oe steady fields and of saturation isothermal remanence irm s in a suite of dispersed unannealed magnetite powders with median sizes of 2 4 6 10 and 14 m pseudo single domain or psd size range and 100 m multidomain or md size interpreted in the light of the domain structure test first proposed by lowrie and fuller 12 the relative stability trend of curves for the 2 m sample is of single domain sd type the 1 oe arm being most resistant to demagnetization followed by the 10 oe and 40 oe arm s and irm s for the 100 m sample the trend is exactly reversed and is of md type in the 4 14 m samples hitherto undescribed transitional trends between sd type and md type occur at 6 m 1 oe 10 oe and 40 oe arm s preserve an sd type trend but for all af s 75 oe irm s is more resistant than any of these remanences at 10 m this trend is unmistakable and only at 14 m do the 1 oe 10 oe and 40 oe arm curves merge we conclude 1 that the lowrie fuller test distinguishes between small md grains enhanced by psd remanence and large md grains lacking psd remanence rather than between sd and md structures per se and 2 that in the psd transition region from 6 to 14 m in magnetite irm s changes over to md type relative stability around 6 m whereas 10 oe and 40 oe arm s achieve an md type trend around 14 m in accord with the predicted field dependence of the psd threshold size our theoretical interpretation assumes that the intrinsic internal field coercive force spectra of weak field and strong field remanences are identical but that the observed external field spectrum is shifted to lower fields as a result of the internal demagnetizing field nj r of the remanence j r the effect is slight for weak field j r s but substantial for irm s since all coercivities high as well as low are shifted the result of the lowrie fuller test is determined simply by the shape of the intrinsic coercivity spectrum or the corresponding af demagnetization curve depending on the model of self demagnetization used either subexponential or sublinear af decay curves of weak field remanence will automatically lead to an md type trend whereas by either model the decay curves that characterize sd and psd remanences decaying slowly initially and then more rapidly will always produce and sd type trend

magnetic mineralogy of pelagic limestones summary due to their extremely small size and low concentration the carriers of magnetization in limestones are usually identified through indirect rock magnetic methods this paper describes a method for extracting tiny ferromagnetic grains from consolidated calcareous rocks and summarizes a scanning electron microscope sem investigation of magnetic phases from several palaeomagnetically important pelagic limestones details of surface characteristics and elemental composition are readily accessible with the sem and complement normal rock magnetic experiments in giving a more complete assessment of the major ferromagnetic phases their origin and the timing and acquisition of magnetization the results confirm many indications of the indirect methods and introduce several important new features in addition to detrital titanomagnetite diagenetic titanohaematite and iron sulphides there is a significant contribution by authigenic microcrystalline haematite detrital cosmic spherules chromian spinels and skeletal titanomaghaemite

geology and petrogenesis of the edgecumbe volcanic field se alaska the interaction of basalt and sialic crust the edgecumbe volcanic field is a holocene volcanic province located on kruzof island se alaska exposed within the 260 km2 field are basalt andesitic basalt andesite dacite and rhyodacite the rhyodacites were erupted after the basalts and before the andesites the volcanics which are al rich 14 18 wt and lack an iron enrichment trend range from tholeiites 47 wt sio2 through rhyodacites 72 but a compositional gap of approximately 9 wt separates the dacites and rhyodacites initial 87sr 86sr ranges from 0 70297 in the basalts to 0 70440 in a pyroxene andesite 18o increases across the suite 5 8 to 7 9 plagioclase an32 86 is the dominant phenocryst in all but one lava olivine fo58 86 occurs in the basic lavas 53 wt sio2 but is replaced by orthopyroxene en43 73 and clinopyroxene en31wo41 en48wo40 in the more siliceous volcanics in the basalts and rhyodacites plagioclase is weakly zoned but extreme zoning 30 mole an is characteristic of phenocrysts in the intermediate lavas fractionation of the observed phenocryst assemblages could not have produced the more silicous volcanics instead they were generated by partial melting of intrusive basement 87sr 86sr 0 70487 18o 8 7 9 3 by basaltic magma and subsequent assimilation mass balance calculations show the rhyodacites are almost pure partial melt 5 basaltic component whereas the intermediate lavas contain between 30 and 60 partial melt

samples from the crystallising boundary layer of a zoned magma chamber the spatial and chemical relationships between the melt occupying the reservoir and the mineral assemblages crystallising at the margins are reconstructed for the magma chamber which produced the 11000 yr b p tephra deposit of laacher see volcano the melt showed vertical chemical zonation immediately prior to eruption and throughout most of the magma volume only a small fraction of crystals were present the eruption also ejected crystal rich nodules ranging from mafic to felsic in composition which are samples of the materials crystallising at the boundaries of the chamber new data on nodule petrography and chemical compositions of whole rocks minerals and interstitial glasses are presented volume fraction of interstitial glass is not systematically related to mineral assemblage and varies typically between 1 and 20 vol i e the crystals interlock one exception is a group of mafic nodules with glass volume fractions between 25 and 40 vol bulk compositions of mafic nodules show strong enrichments or depletions in all major elements relative to the mafic phonolite interstitial melt felsic nodules show much less pronounced differences with their interstitial melt felsic nodules contain interstitial glasses with a range of compositions similar to that in the zoned bulk of the chamber and were probably derived from different heights on the walls mafic nodules have glass compositions similar to those at the base of the zoned liquid column and were probably derived from the floor modal mineralogy glass composition and mineral composition are systematically related in the nodules whereas in individual pumices samples derived from the main body of the chamber a broader range of mineral compositions are often found mineral assemblages were especially diverse in the upper part of the chamber it is deduced that the whole of the essentially liquid part of the chamber was emptied by the eruption that strongly contrasting mineral assemblages were forming simultaneously on the walls and floor that the gradient in crystal content in the crystallisation boundary layer was more gradual at the floor than at the walls and that the pumice mineralogy is not a simple phenocryst assemblage but is a mixture of crystals which grew from melts separated in space and or time

bentonites in the chalk of central eastern england and their relation to the opening of the northeast atlantic abstract within the post cenomanian chalk sequence of central eastern england about 20 thin persistent marl horizons occur these marls contain from 30 to 65 non carbonate material the bulk of which is composed of a well crystallised mg rich smectite with a notable paucity of illite and quartz the chalk matrix above and below the marls contains up to 2 0 of non carbonate matter which again is dominated by the presence of an mg rich smectite but which contains a larger proportion of illite and quartz than occurs in the marls for both sets of non carbonate materials the excellent crystallinity and mg rich nature of the smectite strongly suggest that it was formed by the in situ alteration of an aeolian transported volcanic ash i e it is a true bentonite quartz and illite in the chalk matrix are probably detrital components relative to a standard shale non carbonate materials from the chalk marl horizons exhibit a highly anomalous trace element pattern with accentuated levels of zr nb and th but depleted levels of ba rb and k this trace element chemistry corroborates a volcanogenic origin for the smectite insofar as it is not explicable in terms of any realistic sedimentary process the chemistry is best interpreted in terms of the magnetic processes responsible for the formation of the original ash precursor of the marls it appears that this precursor had a pantelleritic composition and therefore originated from a source overlying an area where incipient continental rifting was taking place micromorphological examinations prove that residues from the chalk marls contain small amounts of pyroclastic debris including pumice and fragments of sideromelane glass the morphology of the fragments suggests that the volcanic eruptions responsible for the ejection of the original ash into the air were of the shallow water phreatomagmatic surtseyan type the distribution of marls in the chalk of eastern england suggests that the ashfalls responsible for their origin were of the tropospheric type and were drawn by westerly winds from a centre to the west or northwest of mainland britain of the numerous volcanic centres that lie in offshore areas to the west the anton dohrn seamount in the rockall trough appears to a likely centre of origin for the ashfalls however ashfalls responsible for the formation of the traces of smectite in the chalk matrix were probably of the general background stratospheric type and may have been drawn from more distant centres volcanism at the anton dohrn seamount was related to tensional effects in the continental crust that immediately preceded the formation of the rockall trough and the separation of greenland from northwestern europe during tertiary times activity at the centre and cretaceous bentonites derived therefrom therefore represent the earliest stages in the development of the thulean igneous province

genesis and evolution of regoliths evidence from trace and major elements and sr nd pb u isotopes abstract regoliths encompass different materials from the fresh bedrock to the top of the organic horizons the occurrence and evolution of these materials are determined by deposition erosion and weathering processes that are specific for each region the origin and interaction of the regolith layers are still important issues in the study of critical zone functioning studies that attempt to understand the processes responsible for the structure and the evolution of regoliths often focus either on the soil or on the bedrock compartment and often from a narrowly focused approach using just a single tool such as major element patterns or mineralogy this limits the understanding of the complete regolith composition and evolution the present study proposes the combination of mineralogy major and trace element concentrations and sr nd pb u isotopes as an extremely useful procedure for tracking the spatial and temporal origin of regolith materials and understanding their stratification this multi tracer approach was applied on a regolith profile from the south western edge of the rhenish massif which is characterised by the occurrence of slate bedrocks and overlying pleistocene periglacial slope deposits ppsd the data allowed the differentiation of three compartments within the regolith 1 the organic compartment where low 206pb 207pb and 87sr 86sr isotopic ratios reflected the impact of atmosphere derived components 2 the ppsd compartment whose upper horizons were characterised by the presence of atmosphere derived particles rich in cd sn sb hg and pb and by relatively low 206pb 207pb and 87sr 86sr ratios whereas the lower horizons showed nb enrichments in their matrix and a mineralogical composition pointing to ancient volcanic events and 3 the weathered slate compartment whose weathering progression and genetic relationship with the ppsd were particularly well tracked with 87sr 86sr and 143nd 144nd isotope ratios 234u 238u activity ratios and positive ce anomalies the results prove the efficiency of radiogenic isotope measurements combined with the analysis of major and trace elements to complete regolith studies moreover these isotopes and elements are shown to be potential geochemical tracers of exchange processes at the water mineral interface this approach allows for the first time the description of an evolution scheme of a complete slate regolith which covers a large part of the rhenish massif

phase equilibria of the lower laacher see tephra east eifel germany constraints on pre eruptive storage conditions of a phonolitic magma reservoir laacher see volcano erupted about 12 900 years ago resulting in a tephra blanket widely dispersed in central europe the eruption tapped a chemically and mineralogically strongly zoned phonolitic magma reservoir in order to better constrain the pre eruptive storage of that magma hydrothermal phase equilibria experiments have been performed on a powdered sample from the highly evolved phonolite of the lower laacher see tephra llst experiments were run at water pressures of 75 175 mpa and temperatures of 725 800 c stability fields of sanidine and amphibole the most common crystal phases in llst suggest that the magma last equilibrated at temperatures of 750 760 c and pressures of 115 145 mpa those conditions suggest the magma should have contained between 5 and 6 wt water which agrees well with water contents of 5 7 wt found in glass inclusions that agreement along with the observation that cl and s vary with the composition of the inclusions suggest that the laacher see magma was saturated at depth with a water rich fluid and thus was stored at pressures near those estimated from our experiments such pressures roughly correspond to depths of 5 6 km clinopyroxene and plagioclase did not occur in the experimental runs although present in the natural pumice sample it seems likely that these phases are not inherent to the llst they were formed in other parts of the chemically stratified column and later redistributed a recalculation of the magma density together with a new estimate of the temperature and pressure range suggests a steep density gradient and a thin withdrawal layer which is typical for volatile enriched magma bodies and confirms earlier models of convective fractionation in the laacher see magma reservoir

evolution of the lower saxony basin abstract the lower saxony basin is a 300 km long and 65 km wide late jurassic and early cretaceous trough which parallels the northern margin of the rhenish massif this basin became inverted during the senonian and paleocene the lower saxony basin is the most important oil province of germany past production amounts to 200 10 6 t of oil 1 5 10 9 bbl the lower saxony basin is superimposed on the late carboniferous variscan foredeep basin and the frontal structures of the variscan fold belt during the permian to middle jurassic the area formed part of the northwest european basin differential subsidence of the lower saxony basin was initiated during the late jurassic and persisted till albian time this involved reactivation of permo carboniferous fracture systems by divergent wrenching a major aptian deep seated laccolith is evident in the western part of the lower saxony basin during the senonian and paleocene sinistral convergent wrenching induced uplift of the basin floor and folding of its sedimentary fill the margins of the inverted lower saxony basin are thrust over the adjacent stable platform areas the differential subsidence of the lower saxony basin can be related to a period of major crustal extension in the north sea rift basin inversion coincides with important orogenic events in the alpine system and is thought to have been induced by intra plate compressional stresses

magma mixing versus xenocryst assimilation the genesis of trachyandesites in sancy volcano massif central france sr and nd isotopic compositions have been determined on basaltic and acid trachyandesites bta ata from the sancy volcano mont dore massif france these represent more than 80 of the lavas erupted during its activity between 0 9 and 0 2 ma these lavas have been recently interpreted as the result of two component magma mixing during and after repeated injections of basaltic magmas in trachytic reservoirs magmatic heterogeneities in the ata s large to small enclaves banded lavas megacrysts testify to the mingling event complete mixing is supposed to have been achieved in the hybrid bta s which contain sanidine plagioclase and clinopyroxene megacrysts in disequilibrium with their host the megacrysts are interpreted as relicts of the trachytic end member isotopic data on basic comagmatic enclaves and host ata matrix samples from three different cycles of mingling succession of heterogeneous pyroclastics heterogeneous ata lava flows or domes and occasionally homogeneous bta lava flows are not incompatible with two component mixing but could just reflect the heterogeneity of the analysed samples however the bta s have sr contents and sr isotopic ratios which are too high to be simple binary mixing products between the postulated end members three hypotheses are considered to explain this discrepancy 1 the analysed end members are not those involved in bta genesis 2 some crustal contamination occurred during and after the mixing event 3 sr rich sanidine xenocrysts with radiogenic 87sr86sr have been assimilated and digested in the bta s in this third hypothesis that i favour it is not necessary to resort to magma mixing to explain the genesis of the bta s assimilation of xenocrysts by basaltic hawaiitic or mugearitic magmas accounts for both mineralogical disequilibria and isotopic characteristics of these lavas

evolution of the laacher see magma chamber evidence from sims and tims measurements of uth disequilibria in minerals and glasses abstract u th disequilibrium was measured by secondary ion mass spectrometry th and thermal ionization mass spectrometry u in mineral separates from pumices and cumulate nodules from the laacher see volcano 12 5 0 5 kyr corrected 14 c age in east eifel germany phonolitic glasses all plot to the left of the equiline with higher u th observed in the most felsic glasses measured partitioning of u and th in the mineral phases is consistent with the observed increase in u th with progressive fractional crystallization apparent ages of crystallization inferred from internal isochrons in the pumices from the main part of the magma chamber are about 13 3 kyr indistinguishable from the eruption age this implies that the residence time of these phenocrysts did not exceed 1 2 kyr older less precise ages for the upper part of the magma chamber 30 kyr suggest mixing of older crystals from cumulates and or longer residence times mineral isochrons for cumulate nodules are not well defined with ages ranging from 10 to 30 kyr suggesting in situ cumulate formation 10 20 kyr prior to the eruption the time required to explain the 230 th 232 th difference between parental basanite and laacher see phonolite is inferred to be about 100 kyr together these observations favor a cooling model where differentiation of parental basanite takes place in a deep magma body over about 100 kyr followed by emplacement of the resulting mafic phonolite into an upper crustal chamber and continued differentiation toward evolved phonolite the time scale for the development of zonation in the laacher see magma chamber is estimated to be 10 20 kyr at most

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

late cretaceous and cenozoic intra plate compressional deformations in the alpine foreland a geodynamic model abstract the mesozoic grabens and wrench induced troughs of western and central europe developed in response to triassic early cretaceous intra plate tensional stresses which affected the arctic north atlantic and the tethys borderlands during the rifting phase preceding the opening of the respective oceans this resulted in the fragmentation of the continental crust of the alpine foreland during the late cretaceous and tertiary alpine orogeny collision related compressive stresses exerted on the foreland induced the reactivation of these fracture systems causing the inversion of mesozoic grabens and the uplifting of major basement blocks at distances up to 1300 km to the north of the present alpine deformation front the structural style of these inverted basins and basement uplifts is indicative of compressional and transpressional deformations the total amount of crustal shortening associated with these deformations is unlikely to exceed a few tens of kilometres these displacements require a coupling between the foreland and the orogen at the alpine a subduction zones and within the foreland a decoupling at intra crustal levels between the crust and mantle and or at deeper lithospheric levels the example of the alpine foreland indicates that compressive stresses inducing major deformations can be transmitted from a collisional plate boundary through continental lithosphere over distances of up to 1300 km circumstantial evidence suggests that collision related compressive stresses not large enough to give rise to significant deformations can be transmitted through continental lithosphere over even greater distances

volatile composition of the phonolitic laacher see magma 12 900 yr bp implications for syn eruptive degassing of s f cl and h 2 o a volatile rich and chemically zoned phonolitic magma reservoir was tapped successively during the eruption of laacher see volcano ca 12 900 years bp and produced a tephra sequence consisting of phenocryst poor highly evolved phonolite at the base and phenocryst rich more mafic phonolite at the top the stratospheric volatile loading was estimated by comparing pre and post eruptive s f cl and h2o contents of undegassed glass inclusions and partially degassed matrix glasses glass inclusions 150 1490 ppm s and host matrix glasses 150 820 ppm s both document a strong s decrease during progressive magmatic differentiation which is interpreted to be partially caused by crystallization of hauyne the s6 stotal ratio of the pre eruptive melt increased with differentiation from 8 to 71 as indicated by s k wavelength shift measurements in glass inclusions sulfate rich highly evolved phonolitic magma was erupted during plinian and sulfide rich more mafic phonolitic magma during late phreatomagmatic phases f and cl became enriched during late stages of differentiation glass inclusions 690 4060 ppm f 1770 4400 ppm cl matrix glasses 680 3660 ppm f 2130 4330 ppm cl the most differentiated melts maximum 13 wt na2o occur only as matrix glass and are extremely f enriched 5080 8780 ppm but cl depleted 460 2820 ppm suggesting that f was retained in the melt whereas some cl was lost during pre and or syn eruptive degassing the h2o contents of glass inclusions increase irregularly with differentiation 2 5 5 7 wt matrix glasses are h2o depleted 0 2 2 8 wt compared to most glass inclusions showing that most h2o was released to the atmosphere by explosive degassing a mass balance calculation yields a syn eruptive volatile release of 1 9 tg stotal 6 6 tg cl and 403 tg h2o from the melt this is a minimum estimate since s and cl could have accumulated in a separate fluid phase as indicated by fluid inclusions in hauyne and pre eruption h2o contents close to saturation level at the likely pressure temperature conditions in the laacher see magma reservoir we estimate that at least ca 20 tg so2 were injected into the stratosphere causing a significant negative climate forcing as reflected by several paleoclimate proxies and as shown by recent modeling

new aspects on the formation of hydrocarbon source rocks episodic volcanism favors as shown by examples from europe the usa and brazil the formation of organic rich sediments hydrocarbon source rocks the examples examined from europe comprise the lower carboniferous oil shale group scotland upper carboniferous coal measures western germany upper jurassic kimmeridge shale north sea area lower cretaceous blattertonstein northwest germany and eocene messel oil shale rheingraben examples from the usa are devonian mississippian gas shales permian phosphoria shale middle cretaceous mowry shale eocene green river shale and miocene monterey shale examples from brazil are permian irati oil shale and oligocene paraiba valley oil shale volcanism is reflected during sedimentation 1 by exhalation of so2 and or co2 2 volcanic ashfalls 3 by generation of anoxia and preservation of organic matter and 4 by halmyrolytic alteration of volcanic ashes into highly surface active clay minerals montmorillonite

incipient blueschist metamorphism and metasomatism in the tavsanli region northwest turkey volcano sedimentary rocks in an imbricate tectonic zone around a peridotite massif have been studied northeast of the town of tavsanli in northwest turkey basic volcanic rocks which are the dominant rock type in this zone show incipient blueschist metamorphism and associated metasomatism while the igneous textures of the volcanic rocks are retained augites are partially to completely replaced by sodic pyroxene and plagioclase is albitised resulting in rocks with 6 8 wt na2o the volcanic rocks are cross cut by numerous veins of calcite aragonite quartz pumpellyite albite lawsonite and sodic pyroxene pelagic limestones which are interbedded with the basic volcanic rocks consist of coarse aragonite grains showing partial replacement by calcite the occurrence of aragonite lawsonite and albite indicates conditions of metamorphism for the whole zone in the range of 5 8 kb and 150 200 c metasomatism probably related to high pressure serpentinization has occurred contemporaneously with the incipient high pressure metamorphism

structure and dynamics of the laacher see magma chamber eifel germany from major and trace element zoning in sanidine a cathodoluminescence and electron microprobe study microtextures and zoning patterns in sanidine phenocrysts from the phonolitic laacher see tephra germany were investigated in order to constrain processes occurring in the magma chamber before eruption the laacher see tephra unit is chemically zoned and has been inferred to represent the product of eruption of an inverted layered magma chamber samples from various levels in the deposit were investigated we used a combination of textural studies optical microscopy and back scattered electron bse imaging electron microprobe emp analysis of major minor and trace elements ca na k al si ba sr fe and ti and element mapping the samples studied contain two feldspar phases and the ternary composition of the sanidine thus constrains the temperature of crystallization whereas its trace element content reflects the melt composition the large diversity in textures found in the sanidine crystals can be classified into three types composite c type pseudooscillatory po type and resorbed patchy r type trace elementpoor lamellar composite alkali feldspars c type are found in samples inferred to represent the top and the middle part of the magma chamber they grew as composite crystals from the melt at temperatures as low as 650 c in a highly differentiated and volatilerich boundary layer at the magma chamber roof or wall pseudooscillatory zoning with resorption surfaces po type is found in sanidinesfromsamplesinferredtorepresentthemiddlepartandthebase of the magma chamber repeated large an variations 1 4 mol reflect temperature variations of 100 300 ca ssociated with changes in water content variations of minor and trace elements ba sr and ti in sanidine decoupled from the pseudo oscillations of the major elements reflect chemical changes in the melt and are damped by chemicaldiffusioninthemelt bothmajorandminorelementvariations inpo typecrystalsareinterpretedastheprogressiveinfluenceofamore mafic hotter melt this may be partly explained by the settling of crystals through the thermal and chemical gradient existing in the magma chamber however the additional role of magma recharge may be required to explain the large temperature variations resorbed patchy zoned crystals r type found mainly in samples corresponding to the middle part of the magma chamber reflect early growth in a differentiated boundary layer followed by resorption and overgrowthinthemainmagmabody manyofthelaacherseesanidine crystals did not crystallize for the most part in the melt in which they were erupted in the presence of preserved major and trace element zonation in the magma this observation indicates crystal dispersion within a layered magma chamber without large scale mixing and overturn

geochemical and palaeontological studies of the δ13c anomaly in boreal and north tethyan cenomanian turonian sediments in germany and adjacent areas in two sections of the upper cenomanian and lower turonian of northwest germany boreal realm north sea basin and southeast germany north tethys regensburg area stable isotopes 13c 18o acid soluble manganese carbonate contents and foraminifera were investigated in addition other sections across norttwest and middle europe are reviewed with respect to the detailed stratigraphy of a positive 13c anomaly which was found earlier to be of global importance in the archaeocretacea foraminiferal zone cenomanian turonian transition the 13c excursion was found to be isochronous with the lower limit marked by the extinction of the foraminiferal genus rotalipora the upper limit is found immediately above the entry of the foraminiferal praeglobotruncana helvetica bolli in the macrofossil zonation the base is marked by the occurrence of the belemnite actinocamax plenus blainville in the upper part the metoicoceras geslinianum ammonite zone the upper limit falls in the watinoceras coloradoense ammonite zone just above the entry of the inoceramid genus mytiloides in some cases the 13c excursion refers to a maximum of a more or less well expressed oscillation of the 13c values with minima in the lowermost cenomanian and the upper turonian in other cases 13c is constant with a marked spike at the position of the anomaly the locally different structures of the 13c trends point to incomplete mixing of water masses between basins in northwest europe a manganese anomaly associated with the 13c excursion in the anglo paris basin was found at a higher position in northwest germany and is apparently absent from the german north tethys in both cases high mn contents appear to be associated with a markedly increased terrigenous input expressed by an increase in grainsize of the acid insoluble residue the stratigraphical value of the mn anomaly is therefore limited the palaeoecological conditions are discussed in terms of foraminiferal diversity and the occurrences of macrofossils a shift to higher 18o values coincident with the 13c excursion is possibly a response to cooling of ocean water for which evidence is provided by a low diversity globular planktonic assemblage the correlation of 18o and foraminiferal diversity is suggested to provide a tool for bathymetric interpretations of sections but needs further investigations

evolution of magma mixing in an alkaline suite the grande cascade sequence monts dore french massif central geochemical modelling abstract trachyandesitic lavas from an alkaline suite grande cascade sequence monts dore french massif central have been investigated for evidence of mechanical magma mixing a wide variety of textures are present in the sequence as pillow like enclaves bandings and emulsified facies the grande cascade flows and pyroclasts range from basalt to trachyte 44 67 wt sio 2 pillow like enclaves intricate commingling and local diffusive mixing at the mafic felsic interface in emulsified types show that mafic inclusions were incorporated in their trachytic host as blobs of magma such inclusions were probably dispersed throughout the trachytic magma by mechanical mixing different stages of the evolution of a mechanical mixing process were recognized in relation with the stratigraphy and chronology of the products the most heterogeneous features enclaves bandings are restricted to the bottom of the sequence where basalt and trachyte coexist at the top texturally homogeneous trachyandesites occur relict phenocrysts are observed in both basaltic enclaves oligoclase fe rich clinopyroxene sanidine of the trachyte and trachytic hosts mg rich olivine mg rich clinopyroxene ca rich plagioclase of basalt the intermediate rocks basic trachyandesites contain xenocrysts of mg rich olivine clinopyroxenes oligoclase sanidine together with reversely zoned plagioclase and pyroxene phenocrysts this assemblage is presumed to result from the incorporation of xenocrysts related both to mafic and felsic endmembers all mineral assemblages are out of equilibrium reversed compositional zoning profiles in plagioclase and clinopyroxene crystals were produced by the mixing event we are able to identify the compositional characteristics of the components that were mixed a primary alkaline basalt mg olivine labrador mg clinopyroxene magnetite and a silicic trachyte oligoclase fe clinopyroxene sanidine are the endmembers of mixing observed chemical variations in major and trace elements can be modelled as pure binary mixtures between basaltic and trachytic endmembers using the best mixing line method provost and allegre 1979 the calculations emphasize that trachyandesites are the result of complete hybridization between basalt and trachyte the data on mesostasis show that melt mixing and not only crystal transfers has produced the intermediate lavas of the grande cascade sequence textural criteria petrography mineral chemistry and geochemical data major and trace elements support the magma mixing model

the open system geochemical evolution of alkalic cap lavas from haleakala crater hawaii usa lavas ranging from alkalic basalt to mugearite are exposed in a continuous stratigraphic section in haleakala crater on the island of maui these lavas define a series of discrete magma batches separated by sharp compositional breaks characterized by upsection increases in incompatible element e g k2o nb ree th ba rb contents al2o3 cao ratios and la sm ratios and complementary decreases in compatible element e g mgo ni cr v sc contents within magma batches there are systematic upsection decreases in incompatible element contents al2o3 cao ratios and la sm ratios and systematic increases in compatible element contents the observed compositional cyclicity can be explained as the result of alternating periods of low and high magma chamber recharge rates the upsection interbatch compositional breaks represent periods of low or negligible magma chamber recharge during which eruption is suppressed and the geochemical effects of crystal fractionation dominate those imposed by magma mixing at the initiation of recharge the influx of mafic magma into the magma chamber produces an eruption resulting in partial evacuation of the magma chamber following eruption recharge magma mixes with magma remaining in the magma chamber continued recharge and mixing during these eruptive periods cause the compositions of lavas to become more mafic with time thus during a recharge period the bulk composition of the magma chamber approaches that of the recharge magmas the most mafic lavas i e the youngest lavas within each magma batch typically are relatively evolved mg 48 mgo 7 wt and therefore cannot represent primary magmas evidence for polybaric fractionation suggests that these lavas represent recharge magmas that have undergone crystal fractionation in a deeper level reservoir prior to injection into the shallow level haleakala magma chamber the observed fine scale geochemical cyclicity is superimposed over the larger scale systematic changes in isotope ratios and highly incompatible trace element ratios that are attributed to time dependent changes in the proportions of distinct mantle components in the source of these lavas these contrasting scales of temporal geochemical variability illustrate how shallow level magma chamber processes have imposed short time period 2000 years compositional controls on magmas whose compositions were also changing over relatively long time periods 800 000 years due to source related effects the decrease in magma supply rates during the alkalic cap phase of hawaiian volcanism probably results in increased magma chamber residence times thus creating the greater magmatic diversity observed the specifics of the petrologic and geochemical evolution of individual hawaiian volcanoes during this period differ at least in part because of differences in magma supply rates

eruption and emplacement of a basaltic welded ignimbrite during caldera formation on gran canaria the 14 1 ma old composite ignimbrite cooling unit p1 45 km3 on gran canaria comprises a lower mixed rhyolite trachyte tuff a central rhyolite basalt mixed tuff and a slightly rhyolite contaminated basaltic tuff at the top the basaltic tuff is compositionally zoned with a an upward change in basalt composition to higher mgo content 4 3 5 2 wt b variably admixed rhyolite or trachyte commonly 5 wt and c an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments the basaltic tuff is divided into three structural units i the welded basaltic ignimbrite which forms the thickest part c 95 vol and is the main subject of the present paper ii poorly consolidated massive bomb and block rich beds interpreted as phreatomagmatic pyroclastic flow deposits and iii various facies of reworked basaltic tuff tuff unit i is a basaltic ignimbrite rather than a lava flow because of the absence of top and bottom breccias radial sheet like distribution around the central tejeda caldera thickening in valleys but also covering higher ground and local erosion of the underlying p1 ash a gradual transition from dense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding the shape of the welding profile is typical for emplacement temperatures well above the minimum welding temperature a similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections in proximal sections the base is dense where it was emplaced on hot felsic p1 tuff the intensity of welding especially at the base and the presence of spherical particles and of mantled and composite particles formed by accretion and coalescence in a viscous state imply that the flow was a suspension of hot magma droplets the flow most likely had to be density stratified and highly turbulent to prevent massive coalescence and collapse model calculations suggest eruption through low pyroclastic fountains 1000 m high with limited cooling during eruption and turbulent flow from an initial temperature of 1160 c the large volume of 26 km3 of erupted basalt compared with only 16 km3 of the evolved p1 magmas and the extremely high discharge rates inferred from model calculations are unusual for a basaltic eruption it is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic p1 magma chamber and its ring fissure conduits evolution of the entire p1 eruption was controlled by withdrawal dynamics involving magmas differing in viscosity by more than four orders of magnitude the basaltic eruption phase was initially driven by buoyancy of the basaltic magma at chamber depth and continued degassing of felsic magma but most of the large volume of basalt magma was driven out of the reservoir by subsidence of a c 10 km diameter roof block which followed a decrease in magma chamber pressure during low viscosity basaltic outflow

authigenic magnetite formation in suboxic marine sediments the resolution and reliability of magnetostratigraphy and reconstructed time series of geomagnetic field behaviour depend critically on where remanence is acquired and fixed in the sediment column and whether the magnetization is altered chemically after deposition if authigenic magnetic minerals are formed at depth or if the magnetic carriers are changed after deposition then the nature and timing of magnetic events can be affected by depth offsets in remanence acquisition and mixing of detrital and authigenic signals using palaeo and rock magnetic and sediment geochemical analyses we have studied how early diagenesis affects the magnetic properties of suboxic hemipelagic sediments here we report evidence of the formation of fine grained authigenic magnetites near the commonly observed brown tan green colour boundary which marks the transition from fe oxidizing to fe reducing conditions we propose that biogenic magnetite produced by magnetotactic bacteria forms as part of the microbially mediated sequence of reactions involved in the oxidation of organic matter the magnetite is created as a metabolic by product of the microorganisms use of iron redox transitions as a source of energy active magnetite formation appears to be restricted to a zone between the levels of nitrate and iron reduction

rock magnetic properties and facies change in cenomanian turonian boundary sediments from nw germany abstract rock magnetic properties of sediments from the werne 27 konrad 101 and anderten muhle 1 borehole sections nw germany covering the cenomian turonian boundary are related to lithofacies white coccolith limestones of lower upper cenomanian age have very low nrm intensities below 0 1 ma m 1 at the base of the cenomanian turonian boundary succession lithofacies changes very distinctly to multicoloured marls and marly limestones the nrm intensities of these sediments are up to several hundred ma m 1 with a very distinctive intensity peak occurring within the plenus bed sequence the lithofacies change is also reflected in a drastic bulk susceptibility increase coercitivity and blocking temperature spectra and irm acquisition characteristics reveal that the magnetic mineralogy is dominated by minerals of the magnetite group presumably derived from ash falls

replenished magma chambers effects of compositional zonation and input rates several recent publications have reported investigations of the dynamical effects and geological consequences of the replenishment of a homogeneous magma chamber by the rapidinjection of hot relatively dense fresh magma the new magmaforms a separate layer at the base of the chamber whereit cools and crystallizes until the density of the residual liquid in the lower layer becomes equal to that of the upperlayer at which time the fluid in the two layers becomes intimately mixed the fluid dynamical aspects of this modelwere confirmed by laboratory experiments using aqueous solutions this paper reports three experimental variations on the basic theme which examine the effects of the rate of injection into homogeneous and compositionally stratified chambers first the rapid injection of a hot dense layer beneath a fluid with a lower temperature and a vertical compositional gradient is considered a series of layers gradually builds up from the base of the upper gradient region while the lower layer cools crystallizes and finally overturns in this situation however the rise of the lower layer fluid is restrained by the density gradient and mixing takes place with only part of the upper fluid second the slow influx of a hot dense liquid beneath a cold less dense homogeneous layer is considered a considerable quantity of the input fluid coolsand crystallizes immediately releasing some lighter fluid to mix with the fluid in the homogeneous layer with time some dense input fluid spreads along the bottom so that locally the situation is the same as the one considered in the previous studies with a local overturning occurring when the density of the lower fluid has decreased sufficiently the deposition of crystals continues and some rather fine chimney like structures can develop which bear a resemblance to the black smokers recently discovered on the sea floor third a combination of the first two variations is studied by the consideration of the slow influx of hot dense liquid beneath a uniformly cooler fluid with a vertical compositional gradient the input fluid is constrained in its rise by the density gradient as the influx continues the additional thermal energy is also confined and causes the crystals to redissolve and build up a lower layer as was observed in the case of a rapid injection beneath a compositional gradient

zoning and exsolution in alkali feldspars from laacher see volcano western germany constraints on temperature history prior to eruption compositional zoning and exsolution patterns of alkali feldspars in carbonatite bearing cognate syenites from the 6 3 km3 d r e phonolitic laacher see tephra lst deposit in western germany 12 9 ka are reported these rocks represent the cooler outer portion and crystal rich products of a cooling magma reservoir at upper crustal levels major and trace element difference between cores and rims in sanidine crystals represent two generations of crystal growth separated by unmixing of a carbonate melt trace element differences measured by la icp ms are in accordance with silicate carbonate unmixing across the core rim boundary we extracted gray scale profiles from multiple accumulations of back scattered electron images gray scales directly represent k na ratios owing to low concentrations of ba and sr 30 ppm diffusion gradients are modeled to solve for temperature using known pre eruptive u th zircon ages 0 20 ky of each sample schmitt et al j petrol 51 1053 1085 2010 estimated temperatures range from 630 c to 670 c for the exsolution boundaries a diffusive homogenization model is constrained by the solvus temperature of 712 725 c and gives short time scales of only 15 50 days based on our results we present a model for the temperature time history of these rocks the model also constrains the thermal variation across the cooling crystal rich carapace of the magma reservoir over 20 ka and suggests a thermal reactivation of cumulates the cooling carapace and probably the entire system only a few years prior to the explosive eruption of the remaining molten core of the phonolitic magma reservoir

evidence for magma mixing within the laacher see magma chamber east eifel germany abstract the final pyroclastic products of the late quaternary phonolitic laacher see volcano east eifel w germany range from feldspar rich gray phonolite to dark olivine bearing rocks with variable amounts of feldspar and al augite megacrysts petrographically and chemically homogeneous clasts occur along with composite lapilli spanning the compositional range from phonolite mgo 0 9 to mafic hybrid rock mgo 7 0 for all major and trace elements both a basanitic and a phonolitic phenocryst paragenesis occur within individual clasts the phonolite derived phenocrysts are characterized by glass inclusions of evolved composition rare inverse zoning and strong resorption indicating disequilibrium with the mafic hybrid matrix basanitic magnesian clinopyroxene and olivine in contrast show skeletal normally zoned overgrowths indicative of post mixing crystallization in accord with petrographical and other chemical evidence mass balance calculations suggest mixing of an evolved laacher see phonolite containing variable amounts of mineral cumulates and a megacryst bearing basanite magma magma mixing occurred just prior to eruption hours of the lowermost magma layer of the laacher see magma chamber but did not trigger the volcanic activity

the bag tephra a widespread tephrochronological marker in middle europe chemical and mineralogical investigations the bag tephra is a widespread tephra layer interbedded in quaternary loess deposits along the danubian valley of hungary and slovakia its age is poorly defined between 788 and 380 ka b p the glass and mineral composition micropumice clasts of phono tephrite and blocky shards of tephri phonolite associated with two kinds of clinopyroxene fassaitic diopside and salite is very distinctive this tephra could be used as a chronological marker as soon as its age is refined the probable origin is the middle italian volcanic area

constraints on the pre eruptive magmatic history of the quaternary laacher see volcano germany ba zonation patterns in sanidine phenocrysts from mafic and intermediate phonolite and crystal rich cumulates from the laacher see volcano 12 9 ka in western germany document diffusion times suggestive of periodic recharge events throughout the magma reservoir s entire lifespan of 24 ky phenocrysts analysed from samples that formed late at the base of the compositionally zoned magma reservoir by mixing and mingling between a resident phonolite magma and recharging basanite show resorption and thin 2 10 m late stage ba rich overgrowth short diffusion profiles across these boundaries give diffusion times of 1 5 3 years at most which are interpreted to be the maximum duration between the most recent recharge by the basanite and eruption the lack of such late overgrowth in samples from other parts of the phonolite reservoir suggests that effect of this mixing and mingling was limited to the crystal rich base sanidines in the cumulates by contrast are generally devoid of zoned crystals only rare cumulate crystals with resorbed outer boundaries and very thin overgrowths a few microns with very sharp compositional changes imply the remobilization of cumulates only months before eruption based on the diffusion timescales and storage temperatures obtained in a previous study we present a genetic model for the conditions and timing of storage and re activation of the magma system prior to the eruption of laacher see which is the largest volcanic event in central europe since the last glaciation

the cenomanian turonian oceanic anoxic event i stratigraphy and distribution of organic carbon rich beds and the marine δ13c excursion marine strata deposited during late cenomanian and early turonian time display lithological faunal and geochemical characteristics which indicate that significant parts of the world ocean were periodically oxygen deficient at or very close to the cenomanian turonian boundary between 90 5 and 91 5ma ago oxygen deficiencies were particularly marked over a period of 1ma this short lived episode of oceanic oxygen deficiency has been termed the cenomanian turonian oceanic anoxic event oae the widespread distribution of anoxic sediments deposited synchronously during such a short lived event indicates that such sediments are not simply the product of coincidental local climatic or basinal water mass characteristics but are the result of a global expansion and intensification of the cenomanian turonian oxygen minimum zone related to feedback between sea level rise and regional palaeoceanography the palaeoceanography of the cenomanian turonian oae is discussed in detail in a companion paper by arthur et al 1987 from authors

petrotectonics of lawsonite eclogite exhumation insights from the sivrihisar massif turkey 1 the sivrihisar massif of the tavsanli zone of turkey is 1 of less than 10 known lawsonite eclogite localities worldwide rocks of the sivrihisar massif consist of eclogite and blueschist in contact with metasedimentary host rocks and record decreasing maximum pressure conditions across three wnw ese striking belts from 16 24 kbar in the northern halilbagi belt to 14 16 kbar in the karacaoren belt and 8 10 kbar in the kertek belt where present sodic amphibole phengite chlorite and quartz define a pervasive s0 s1 foliation garnet omphacite and lawsonite define stretching lineations and kinematic indicators d1 and d2 structures are similar in the halilbagi and karacaoren belts but differ to those in the kertek belt d3 structures are uniform across the massif including fibrous calcite that occurs parallel to f3 fold axes shear sense indicators from field observations and asymmetric type i cross girdle of quartz c axes obtained from electron backscatter diffraction ebsd show top to the south thrusting throughout much of the massif d1 and d2 structures are interpreted to have formed during exhumation by extrusion along a 5 c km gradient the halilbagi and karacaoren belts were juxtaposed possibly as deep as 45 km within the subduction channel and exhumed by the arrival of the anatolide microcontinent at approximately 70 ma homogeneity of f3 axes and calcite fibers across the massif suggests that assembly occurred at blueschist conditions before exhumation through the aragonite calcite transition 350 c above 8 kbar

stable isotope relations in an open magma system laacher see eifel frg 18o 16o and d h ratios have been measured for matrix glasses and phenocrysts from the zoned phonolitic laacher see tephra sequence 11000 y b p east eifel volcanic field frg to study open system behaviour of the associated magma system mineral and glass 18o values appear to be largely undisturbed by low temperature secondary alteration record isotopic equilibrium and confirm previous conclusions based on radiogenic isotope evidence of early small scale crustal assimilation during differentiation of parental magmas in a crustal magma chamber one sanidine glass pair possibly documents the late stage influx of meteoric fluids into the topmost magma layer prior to eruption a sealing carapace of chilled magma which itself was strongly contaminated prevented large scale fluid exchange up to the point prior to eruption when this carapace was fractured and meteoric water gained access to parts of the magma system d h measurements of various glass types glass inclusions dense and pumiceous glass and amphiboles gave conflicting results suggesting a combination of degassing volatile exchange with country rocks and hydration stable isotope ratios for primitive parental magmas 18o 5 5 to 7 0 and mantle megacrysts 18o 5 5 to 6 0 d 21 to 38 for amphiboles and phlogopite resp suggest a rather variable fluid composition for the sub eifel mantle

magnetic properties of marine limestones numerous paleomagnetic investigations have been carried out on limestones but their rock magnetic properties have often been neglected in this review geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized and laboratory techniques for the identification of the magnetic mineralogy are evaluated the conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence nrm probably by a postdepositional alignment process the effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation other remanence components can be acquired during diagenesis in the soft sediment right after deposition or much later even in the indurated limestone the magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration magnetic techniques are more convenient direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract a particularly useful technique involves combined observations of isothermal remanent magnetization irm acquisition in strong fields up to at least 4 t and the subsequent destruction of this irm during stepwise or continuous thermal demagnetization the magnetic minerals most commonly identified by these techniques are magnetite goethite hematite and maghemite these magnetic minerals occur in different combinations in the marine limestones of central europe and the mediterranean realm pyrite is common in certain limestones where it can be the precursor of an unwelcome late goethite pyrrhotite may be responsible for unstable magnetization in the swiss helvetic limestones but otherwise is rare thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures 300 400 c due to the breakdown of a metastable mineral such as maghemite goethite or pyrrhotite at high temperatures above 500 c pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite this necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent vrm behavior of the newly formed magnetite vrm is a widespread component of nrm in many limestones but it can usually be removed effectively by standard magnetic cleaning techniques the susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles the susceptibility is anisotropic and shows a classical depositional pattern of the distribution of principal axes however the probable postdepositional mechanism of nrm acquisition especially in bioturbated sediments implies that the anisotropy pattern results from postdepositional compaction of the sediment