fluvial responses to tectonics and climate change during the late weichselian in the eastern part of the pannonian basin hungary abstract fine grained sandy silty channel belt and floodplain deposits of the berettyo koros rivers a main eastern transverse tributary system of the modern tisza river in the eastern part of the pannonian basin were deposited during the late pleistocene under net subsiding conditions the palaeo drainage network pattern of a 2500 km 2 large part of the alluvial plain was reconstructed based on interpretation of airborne photographs and analysis of 18th century topographic maps which show the natural river patterns that predate the introduction of river regulation schemes the investigation showed that a large meandering river system with two main channel belts surrounding a floodbasin entered the alluvial plain from the northeast and a braided river entered the alluvial plain from the southeast detailed sedimentary logs of seven continuous corings and several sand and clay pit sections were used to characterize different alluvial units optical luminescence dating osl of 25 quartz samples and four 14 c datings showed that the sediments are of late pleniglacial to late glacial age transport directions inferred from heavy mineral analyses combined with the osl ages strongly suggest that the large meandering system represents the palaeo tisza river which supposedly flowed along the northeast southwest striking ermellek depression during the late pleniglacial the braided river can be regarded as a precursor to the fekete and feher koros rivers which entered the alluvial plain from the southeast during the late glacial the interpretation of seismic profiles field measurements of neotectonic activity and variations in thickness of sediments along the studied profile revealed that river development was largely controlled by subsidence along the ermellek depression until 14 to 16 ky and by uplift of the southeastern part of the catchment area the studied fluvial successions also document the response of the palaeo tisza and koros system to the climate changes of the weichselian late pleniglacial late glacial period much of the sand from the meandering zones was deposited during the bolling allerod and sagvar lascaux interstadials whereas some dated sand units from the braided zone represent the older and younger dryas the error ranges of osl dates which often exceed the duration of weichselian substages and subdivisions prevented an unambiguous correlation of the studied sections with the millennial scale climate changes of the last 25 ky meandering and braided river activity coexisted under different climate conditions whereas locations of the main channel belts are related to subsidence anomalies the results of our study thus clearly indicate that tectonics was the primary control on river development

the thermal history of ascending magma diapirs and the thermal and physical evolution of magmatic conduits abstract if over millions of years the positions of island arc volcanoes remain fixed relative to their source regions magma bodies must repeatedly traverse a path through the lithospheric mantle the repeated passage of such bodies thermally and chemically modifies the adjacent mantle for diapirs with radii of 3 6 km ascending at 4 10 8 to 2 10 7 m s 60 000 to 900 000 years are required to establish a magmatic conduit to crustal depths during this period wall rock temperatures increase 200 400 c below 60 km and 400 750 c between 60 and 20 km the solidification of large volumes of magma within the mantle also provides a mechanism for the introduction of kilometer scale chemical and isotopic heterogeneities into the lithospheric mantle because the crystallization sequences of magmas ascending at different stages of conduit development vary conduit thermal and physical evolution also influences magmatic differentiation these differences in crystallization sequence result from the different p t trajectories followed by individual magmatic diapirs crystal fractionation of successive diapirs with or without concurrent wall rock assimilation can therefore generate different liquid lines of descent as successive magma bodies chemically react with their wall rock and solidify at various depths the mantle also changes in composition from peridotite toward basalt depending on the stage of conduit development assimilation of wall rock could therefore yield variable magmatic differentiation trends for successive magma bodies the degree of contamination may decline as the conduit and its margin evolve physically lithospheric conduit evolution has several important ramifications for geochemical modelling first since magma ascent from source regions to crustal depths is unlikely to be isochemical radiogenic and incompatible element ratios may not always reflect characteristics of the magma source region second the earliest stage of arc magmatism e g submarine volcanism is unlikely to reflect the processes of arc magma generation rather this phase of volcanism records the interaction between ascending magma and unmodified mantle the characteristics of primary arc magmas are likely to be understood only through examining more highly developed volcanic centers supplied by mature magmatic conduits

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

the geochemistry of rock and water samples from curtis island volcano kermadec group southwest pacific abstract curtis island is the emergent summit of a dacitic volcano straddling the kermadec ridge with a well developed crater which exhibits mild solfataric activity apart from several narrow dacitic dykes the island is composed entirely of pumiceous pyroclastic deposits containing clasts which are mainly dacitic to rhyolitic in composition but include rare basalt their chemistry is comparable to that of rocks forming other islands of the kermadec group boiling and hot springs produce dilute acid sulphate waters that are believed to form from steam and h2s gas condensing and mixing with rain water this probably descends into the vent area and is then expelled hydrothermal alteration has produced a mineral suite dominated by natroalunite chlorite quartz diaspore and smectites reflecting both the high al fe and mg contents of the crater floor rocks and the low ph of the altering fluids

thermoremanent magnetization in monodomain monoclinic pyrrhotite fe7s8 the intensity and stability of thermoremanent magnetization trm acquired by fine particles 1 m of synthetic monoclinic pyrrhotite were determined a specific trm magnetization per unit inducing field of 32 with a median destructive field of 36 ka m 1 was observed the median destructive field of saturation isothermal remanence is 42 ka m 1 and the coercive force is 62 ka m 1 the intensity of trm has been compared to that predicted by the classical monodomain model adapted to the magnetic symmetry of pyrrhotite if the optically determined particle size is incorporated into the model the agreement between model and observation is poor a factor of 100 difference if a grain size 0 05 m inferred from the measurement of coercive force is adopted model and observation are in agreement within experimental error

neotectonics of the somogy hills part ii evidence from seismic sections abstract the somogy hills are located in the pannonian basin south of lake balaton hungary above several important tectonic zones analysis of industrial seismic lines shows that the pre late miocene substratum is deformed by several thrust faults and a transpressive flower structure basement is composed of slices of various palaeo mesozoic rocks overlain by sometimes preserved paleogene thick early miocene deposits middle miocene partly overlying a post thrusting unconformity partly affected by the thrusts is also present late miocene thick basin fill forms onlapping strata above a gentle paleo topography and it is also folded into broad anticlines and synclines these folds are thought to be born of blind fault reactivation of older thrusts topography follows the reactivated fold pattern especially in the central western part of the study area the map pattern of basement structures shows an eastern area where ne sw striking thrusts folds and steep normal faults dominate and a western one where e w striking thrusts and folds dominate folds in late neogene are also parallel to these directions a ne sw striking linear normal fault and associated n s faults cut the highest reflectors the ne sw fault is probably a left lateral master fault acting during after late miocene gravity anomaly and pleistocene surface uplift maps show a very good correlation to the mapped structures all these observations suggest that the main early miocene shortening was renewed during the middle and late miocene and may still persist two types of deformational pattern may explain the structural and topographic features a nw se shortening creates right lateral slip along e w faults and overthrusts on ne sw striking ones another nne ssw shortening creates thrusting and uplift along e w striking faults and transtensive left lateral slip along ne sw striking ones traces of both deformation patterns can be found in quaternary exposures and they seem to be consistent with the present day stress orientations of the pannonian basin too the alternation of stress fields and multiple reactivation of the older fault sets is thought to be caused by the northwards translation and counter clockwise rotation of adria and the continental extrusion generated by this convergence

the low temperature besnus magnetic transition signals due to monoclinic and hexagonal pyrrhotite kung suan ho and wei teh jiang kindly supplied monoclinic pyrrhotite crystals from museum and ore mineral collections respectively c s h acknowledges funding from the central geological survey and ministry of science and technology r o c through grants 104 5226904000 02 01 and most 105 2116 m 001 018 respectively a p r acknowledges funding from the australian research council through grant dp160100805

geochronology of middle and upper pleistocene loess sections in hungary abstract the application of both thermoluminescence and infrared stimulated luminescence dating to the extensively studied classical hungarian loess paleosol sequences from basaharc mende and paks provides a reliable chronological framework and climatostratigraphic reconstruction for the last interglacial glacial cycle based on this combined luminescence dating study a new chronology is proposed for the young loess in hungary luminescence dating suggests that the loess below the mf2 horizon formed during the penultimate glaciation the mf1 horizon probably formed during an interstade within oxygen isotope stage 3 for the youngest loess overlying mf1 a very high accumulation rate was determined large time gaps occur above mf2 and mf1 indicating that most of the record of the last glaciation is missing in the standard sections at basaharc mende and paks either large discontinuities or a very low accumulation rate occurred in all three type sections during the soil forming periods high resolution studies of climatic proxies using this combined luminescence dating approach provide a reliable chronological framework for loess and loess derivatives of the last glacial cycle in hungary although a precise and complete chronostratigraphic reconstruction cannot be achieved from the incomplete records found at these sites

miocene pliocene tectonic evolution of the slovenian periadriatic fault implications for alpine carpathian extrusion models the periadriatic line pal is a remarkable several hundred kilometer long fault system of the alpine orogen its dextral character was documented by several authors using diverse criteria but detailed kinematics and timing of movements had not been investigated along its whole length structural and paleomagnetic measurements mapping and stratigraphic and sedimentological studies have helped to unravel the miocene pliocene evolution of the slovenian segment of the pal brittle deformation was characterized by nw se to n s compression and perpendicular tension deformation has resulted in dextral strike slip faulting folding and tilting of beds the first transpressional event corresponds to the first phase of lateral extrusion of the east alpine western carpathian northern pannonian block in the early miocene 24 17 5 ma after a short period of transtension during the karpatian 17 5 16 5 ma dextral transpression reoccurred during the middle miocene to pliocene and lasted up to the quaternary middle miocene dextral slip can be connected to the second phase of extrusion the highly deformed rocks within the dextral shear zones show variable clockwise sometimes counterclockwise rotations the mechanism of rotation seems to be complex ranging from regional rotation to local folding due to pure or simple shear domino type rotation

aleutian tholeiitic and calc alkaline magma series i the mafic phenocrysts diagnostic mafic silicate assemblages in a continuous spectrum of aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the aleutian arc crust tectonic segmentation of the arc exerts a primary control on the variable mixing fractional crystallization and possible assimilation undergone by the magmas end members of the continuum are termed calc alkaline ca and tholeiitic th ca volcanic rocks e g buldir and moffett volcanoes have low feo mgo ratios and contain compositionally diverse phenocryst populations indicating magma mixing their ni and cr rich magnesian olivine and clinopyroxene come from mantle derived mafic olivine basalts that have mixed with more fractionated magmas at mid to lower crustal levels immediately preceding eruption high al amphibole is associated with the mafic end member in contrast th lavas e g okmok and westdahl volcanoes have high feo mgo ratios and contain little evidence for mixing evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma these lavas contain groundmass olivine fo 40 50 and lack ca poor pyroxene aleutian volcanic rocks with intermediate feo mgo ratios are termed transitional tholeiitic tth and calc alkaline tca tca magmas are common e g moffett adagdak great sitkin and kasatochi volcanoes and have resulted from mixing of high al basalt with more evolved magmas they contain amphibole high and low al or orthopyroxene or both and are similar to the japanese hypersthene series tth magmas e g okmok and westdahl contain orthopyroxene or pigeonite or both and show some indication of upper crustal mixing they are mineralogically similar to the japanese pigeonite series high al basalt lacks mg rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite the low pressure mineral assemblage of high al basalt results from crystallization at higher crustal levels

variable defect structures cause the magnetic low temperature transition in natural monoclinic pyrrhotite non stoichiometric monoclinic 4c pyrrhotite fe7s8 is a major magnetic remanence carrier in the earth s crust and in extraterrestrial materials because of its low temperature magnetic transition around 30 k also known as besnus transition which is considered to be an intrinsic property this mineral phase is easily detectable in natural samples although the physical properties of pyrrhotite have intensively been studied the mechanism behind the pronounced change in magnetization at the low temperature transition is still debated here we report magnetization experiments on a pyrrhotite crystal fe6 6s8 that consists of a 4c and an incommensurate 5c superstructure that are different in their defect structure the occurrence of two superstructures is magnetically confirmed by symmetric inflection points in hysteresis measurements above the transition at about 30 k the disappearance of the inflection points and the associated change of the hysteresis parameters indicate that the two superstructures become strongly coupled to form a unitary magnetic anisotropy system at the transition from this it follows that the besnus transition in monoclinic pyrrhotite is an extrinsic magnetic phenomenon with respect to the 4c superstructure and therefore the physics behind it is in fact different from that of the well known verwey transition

towards a mechanical model for the formation of the pannonian basin abstract new maps showing crustal and lithospheric thickness variations in the pannonian basin and the surrounding east alpine carpathian and dinaric mountains have been prepared on the basis of recent seismic seismologic and electromagnetic data a map presenting miocene faults of regional significance has been also constructed for the same region by using a wealth of recently available national and or more local studies it is suggested that observed crustal and lithospheric structural contrasts have been controlled primarily by neogene kinematic history of the region kinematic history is characterized by the following main patterns 1 1 indentation by adria and large scale backthrusting in the southern alps 2 2 eastward escape and extension of the eastern alps 3 3 extensional collapse in the pannonian basin area 4 4 formation of broad wrench fault systems along strike in the dinarides and southern carpathians and the western carpathians with dextral and sinistral shear respectively 5 5 compression and accretion of the external thrust and fold belt in the eastern carpathians these kinematic patterns are thought to be interrelated and all are manifestations of the late stage evolution of an overthickened orogenic wedge in order to arrive at a better understanding of the mechanism of extension which formed the pannonian basin deep crustal seismic profiles hydrocarbon exploration reflection lines and borehole data have been analyzed in the little hungarian plain this plain represents the transition zone between the alps and the pannonian lowlands and the results are illustrated by eight interpreted cross sections one result of regional importance is the clear recognition that the transdanubian central range at the southeastern flank of the little hungarian plain is composed of alpine pre senonian thrust sheets this finding marks the end of a century of debate the allochthony of the substrata of the pannonian basin can be now considered proven another result of more general interest is that these cross sections document the mode of lithospheric extension preexisting compressional detachment planes reactivate as low angle normal faults and lead to tectonic unroofing of deeply buried metamorphic terranes characterized by ductile flow along subhorizontal lineation

alpine tectonics of the pannonian basin the neogene evolution of the pannonian basin is fairly well understood in terms of back arc extension behind the carpathian thrust fold belt this middle miocene holocene basin however is superimposed on an earlier middle eocene early miocene basin traditionally called the paleogene basin of hungary although the paleogene stratigraphy of the basin is well known its structural evolution is not clear however a transtensional origin is assumed analogous to the overlying pannonian basin the existing data set on the paleogene basin can be integrated easily into a compressional basin model other intra carpathian paleogene basins such as the transylvanian and slovenian basins are now considered to be flexural in origin systematic interpretation of reflection seismic data shows that these tertiary basins are superimposed on a cretaceous alpine thrust fold belt the internal geometry of this belt in the basement of the pannonian basin indicates a close relationship to the surrounding mountain belts alps carpathians and dinarides whereas the neogene pannonian basin is in a mature stage of exploration the underlying paleogene flexural basins are much less explored furthermore the cretaceous thrust fold belts should be regarded as frontier areas for future oil and gas exploration efforts

thermal alteration of pyrite to pyrrhotite during earthquakes new evidence of seismic slip in the rock record seismic slip zones convey important information on earthquake energy dissipation and rupture processes however geological records of earthquakes along exhumed faults remain scarce they can be traced with a variety of methods that establish the frictional heating of seismic slip although each has certain assets and disadvantages here we describe a mineral magnetic method to identify seismic slip along with its peak temperature through examination of magnetic mineral assemblages within a fault zone in deep sea sediments cored from the japan trench one of the seismically most active regions around japan during the integrated ocean drilling program expedition 343 the japan trench fast drilling project fault zone sediments and adjacent host sediments were analyzed mineral magnetically supplemented by scanning electron microscope observations with associated energy dispersive x ray spectroscopy analyses the presence of the magnetic mineral pyrrhotite appears to be restricted to three fault zones occurring at 697 720 and 801 m below sea floor in the frontal prism sediments while it is absent in the adjacent host sediments elevated temperatures and coseismic hot fluids as a consequence of frictional heating during earthquake rupture induced partial reaction of preexisting pyrite to pyrrhotite the presence of pyrrhotite in combination with pyrite to pyrrhotite reaction kinetics constrains the peak temperature to between 640 and 800 c the integrated mineral magnetic microscopic and kinetic approach adopted here is a useful tool to identify seismic slip along faults without frictional melt and establish the associated maximum temperature

quaternary neotectonics of the somogy hills hungary part i evidence from field observations quaternary and directly underlying late miocene pannonian outcrops were analysed by structural tectono morphologic and sedimentologic methods to describe the main fault directions to separate mass movements from faulting and folding and to separate earthquake induced sediment deformations from other e g periglacial effects in the somogy hills this is a gentle hilly area elevated at 200 300 m above sea level located immediately south of lake balaton hungary quaternary outcrops showed several consistent directions of faulting and co depositional seismic activity three different mohr sets of faults joints could be differentiated in quaternary sediments the three sets are considered late quaternary since all cut young loess sections and have morphological expressions on the basis of the microtectonic measurements and morphotectonic investigations the following sequence of quaternary events can be proposed 1 a w nw e se compression and perpendicular extension would create e w to wnw ese oriented right lateral nnw sse to n s oriented left lateral shear zones and nw se striking normal faults some of these can be evidenced in morphology and among the individual fault measurements some reactivated faults might suggest that this field is a relatively older one but fresh topographic elements suggest that this stress field might be operational sub recently 2 a second stress field with nnw sse extensional and ene wsw oriented compressional directions could be separated this stress field could create nne ssw and nw se oriented shear fractures and ene wsw oriented conjugate normal faults flat thrusts giving ene directed shear may also be active under this field 3 a third stress field might be proposed with n s compression and perpendicular extension directions this would create ne sw and nw se oriented shear fractures which are observed in the measured fault data it is remarkable that the ne sw faults are all steep subvertical and give a very well defined fault set based on the fresh topographic expression this stress field is also sub recent the different sub recent stress fields and related fault patterns might succeed each other or might alternate through time the first and third deformations have fresh topographic expressions and cannot play synchronously the observed features suggest a compressionally active neotectonics of the study area

andesites from northeastern kanaga island aleutians kanaga island is located in the central aleutian island arc northeastern kanaga is a currently active late tertiary to recent calc alkaline volcanic complex basaltic andesite to andesite lavas record three episodes series of volcanic activity series i and series ii lavas are all andesite while series iii lavas are basaltic andesite to andesite four series ii andesites contain abundant quenched magmatic inclusions ranging in composition from high mgo low alumina basalt to low mgo highalumina basalt the spectrum of lava compositions is due primarily to fractional crystallization of a parental low mgo high alumina basalt but with variable degrees of crustal contamination and magma mixing the earliest series i lavas represent mixing between high alumina basalt and silicic andesite with maximum sio2 contents of 65 67 wt later series i and all series ii lavas are due to mixing of andesite magmas of similar composition the maximum sio2 content of the pre mixed andesites magmas is estimated at 60 63 wt the youngest lavas series iii are all non mixed and have maximum estimated sio2 contents of 59 wt the earliest series i lavas contain a significant crustal component while all later lavas do not it is concluded that the maximum sio2 contents of silicic magmas the contribution of crustal material to silicic magma generation and the role of magma mixing all decrease with time furthermore silicic magmas generated by fractional crystallization at this volcanic center have a maximum sio2 content of 63 wt all of these features have also been documented at the central aleutian cold bay volcanic center brophy 1987 based on data from these two centers a model of aleutian calc alkaline magma chamber development is proposed the main features are 1 a single low pressure magma chamber is continuously supplied by primitive low alumina basalt 2 non primary high alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase 3 sidewall crystallization accompanied by crustal melting produces buoyant silicic 63 wt sio2 liquids that pond at the top of the chamber and 4 continued sidewall crystallization now isolated from the chamber wall produces silicic liquids with 63 wt sio2 that increase the thickness and lowers the overall sio2 content of the upper silicic zone it is suggested that the maximum sio2 content of 63 imposed on fractionation generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal liquid mush along the chamber wall

compaction of igneous cumulates when initially deposited mafic igneous cumulates consist of a solid matrix of mineral phases plus 50 60 interstitial liquid due to its greater density the matrix compacts squeezing the interstitial liquid upward a numerical fluid mechanical model of this process indicates that if the cumulate deposition rate is constant compaction occurs in one of two distinct models for rapid deposition the system is permeability controlled with compaction occurring only at the base of the cumulate pile for slowly deposited cumulates an additional compaction zone that is partially viscosity controlled develops at the top of the pile if the extent of compaction and thickness of this zone are known for a real system the model can be used to estimate the permeability and effective viscosity of the compacting cumulates such calculations yield a permeability of 1 5 times 10 3 a 2 f 3 1 f 2 for olivine cumulates in the muskox intrusion reliable viscosity estimates await an empirical law relating ma

measurement of equivalent doses in quartz from contemporary water lain sediments using optically stimulated luminescence the single aliquot technique and its derivatives are very attractive for the measurement of the very small doses found in young materials because they inherently avoid concerns about low dose non linearity and can offer high precisions a modification is outlined to the single aliquot regeneration and added dose sara protocol mejdahl and botter jensen 1994 quaternary geochronology quaternary science reviews 13 551 5541 to permit its application to sedimentary quartz using the optically stimulated luminescence signal this revised protocol is then used to determine the doses found in several contemporary fluvial sediments and one coastal beach sand thought to be deposi ted by recent wave action two over bank river deposits were both homogeneously well bleached with doses of 0 4 gy although most of the grains in three channel deposits from a range of catchment sizes were also well bleached contamination by a small proportion of very poorly bleached quartz grains prevented the measurement of the dose in the majority the beach deposit was homogeneously bleached and contained a dose of about 0 02 gy it is concluded that over bank sites may provide accurate dates for samples older than a few hundred years but that the heterogeneity of the channel sites limits the accuracy of dates less than a few thousand years

low temperature magnetic properties of monoclinic pyrrhotite with particular relevance to the besnus transition monoclinic pyrrhotite fe7s8 owes its ferrimagnetism to an ordered array of fe vacancies its magnetic properties change markedly around 30 k in what is known as the besnus transition plausible explanations for the besnus transition are either due to changes in crystalline anisotropy from a transformation in crystal symmetry or from the establishment of a two phase system with magnetic interaction between the two phases to help resolve this discrepancy we measured hysteresis loops every 5a degrees and backfield curves every 10a degrees in the basal plane of an oriented single crystal of monoclinic pyrrhotite at 300 k and every 2 k from 50 k through the besnus transition until 20 k between 50 and 30 k hysteresis loops possess double inflections between crystallographic a axes and only a single inflection parallel to the a axes magnetization energy calculations and relative differences of the loops show a sixfold symmetry in this temperature range we propose that the inflections stem from magnetic axis switching which is both field and temperature dependent in a manner somewhat analogous to an isotropic point where magnetocrystalline constants change their sign the besnus transition is best characterized by changes in magnetic remanence and coercivity over a 6a degrees temperature span 28 34 k with a maximum rate of change at 30 k a surprising yet puzzling finding is that the coercivity ratio becomes less than unity below the transition when fourfold symmetry arises because the changes in magnetic parameters are linked to the crystal structure we conclude the besnus transition owes its origin to a distortion of the crystallographic axes below 30 k rather than an apparition of a two phase system an isothermal magnetization of natural pyrrhotite cycled from room temperature to successively lower temperatures through the besnus transition decreases 2 4 times less than equivalent grain sizes of magnetite with less than a 10 per cent loss in remanence between 300 and 150 k in pseudo single domain psd pyrrhotite as psd monoclinic pyrrhotite carries the magnetic remanence in some meteorites it is likely that low temperature cycling in space then warming to ambient conditions at the earth s surface will have only a minor influence on palaeointensity values derived from those meteorites

paleogeographic evolution of the late miocene lake pannon in central europe abstract the paleogeographic evolution of lake pannon within the pannonian basin is reconstructed with eight maps ranging from the middle miocene to the early pliocene the maps are based on the distribution of selected biozones and specific fossils and on complementary sedimentological and seismic information our reconstruction shows that the history of lake pannon can be divided into three distinct intervals an initial stage with low water level which resulted in isolation from the sea at about 12 ma and might have led to temporary fragmentation of the lake an interval of gradual transgression lasting until ca 9 5 ma and a long late interval of shrinkage and infilling of sediments that persisted into the early pliocene the deep subbasins of the lake formed during the transgressive interval in more basinward locations than the deep basins of the preceding sarmatian age the southern shoreline running parallel with the sava and danube rivers along the northern foot of the dinarides changed very little during the lifetime of the lake while the northern shoreline underwent profound changes

crystal capture sorting and retention in convecting magma the mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet like chamber a central conclusion is that inward progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer convection is apparently confined to an essentially isoviscous isothermal region that hugs the downward growing roof zone strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity n called rigid crust n 0 5 mush 0 5 n 0 25 and suspension n 0 25 the strong increase in viscosity near the mush suspension interface acts as a capture front that overtakes and traps slowly settling crystals initial phenocrysts mostly escape capture but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself crystal laden plumes traverse the central hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone convection is never vigorous but is part of an overall intimate balance between roofward heat loss rigid crust growth crystallization kinetics and transport and sorting of sinking escaped crystals there is a strong similarity between these processes and those producing both varves and saline pan deposits it is clear that lavas lava lakes and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over all process these aspects commonly also characterize the large mafic magmatic bodies because strong compositional changes in the residual melt occur largely outward that is at lower temperatures and higher crystallinities of the capture front which is immobile and mostly within rigid crust the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited this is in broad agreement with the compositional range observed in basaltic lava lakes sills and plutons like skaergaard the tuning of convection crystallization kinetics and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions

towards a revised chronostratigraphy of loess in austria with respect to key sections in the czech republic and in hungary abstract the stratigraphic ages of some well known and characteristic paleosols in austrian and hungarian loess sections have been questioned by several workers since the mid 1970s in particular a generally accepted correlation of austrian and hungarian loess stratigraphies with the well established czech loess stratigraphy has not been put forward this paper presents 41 new thermoluminescence tl dates and the first results from amino acid chronology of key sections in these areas the results are in agreement and suggest a new interregional correlation of loess paleosol sequences for loess older than the last interglacial quartz and feldspar inclusions yield more reliable tl ages than the polymineralic fine grain fraction

authigenic or detrital origin of pyrrhotite in sediments resolving a paleomagnetic conundrum monoclinic pyrrhotite fe7s8 is widely claimed to carry magnetizations acquired during early diagenesis in anoxic sedimentary environments in contrast geochemical literature indicates that pyrrhotite formation is extremely slow below 180 c which makes it a highly unlikely carrier of early diagenetic remanences in sediments this view is confirmed by the occurrence of late diagenetic fe7s8 in ancient sediments and the general lack of fe7s8 in modern sediments horng et al c s horng m torii k s shea s j kao inconsistent magnetic polarities between greigite and pyrrhotite magnetite bearing marine sediments from the tsailiao chi section southwestern taiwan earth planet sci lett 164 1998 467 481 documented the presence of fe7s8 that carries a magnetic signal indistinguishable from that of detrital magnetite in pleistocene marine sediments from taiwan we tested the possibility that the fe7s8 could have a detrital origin by conducting a source to sink study and found fe7s8 in metamorphic rocks of the taiwan central range and in material eroded from these rocks in the transportation pathway and in the depositional sink this confirms that the fe7s8 has a detrital origin rapid transportation from source to sink e g by typhoon associated flood events probably assists preservation of the fe7s8 which might otherwise oxidize during transportation the widespread occurrence of exhumed metamorphic rocks in orogenic belts around the world makes them a likely source of fe7s8 in marginal sedimentary basins detrital fe7s8 should therefore be more routinely considered to be responsible for paleomagnetic records when it is present in sediments with partial metamorphic provenance

strong remanent magnetization in pyrrhotite a structurally controlled example from the paleoproterozoic tanami orogenic gold province northern australia abstract rocks associated with an orogenic gold system in the paleoproterozoic granites tanami inlier of northern australia exhibit strong natural remanent magnetism identifiable in regional aeromagnetic data petrographical analysis indicates the two dominant magnetic minerals in the rocks of this region are multidomain magnetite and monoclinic pyrrhotite chemical analysis using an electron microprobe has determined that the magnetite and pyrrhotite are stoichiometrically pure without significant elemental substitution alternating field demagnetization curie temperature and hysteresis analysis indicates that the remanently magnetized phase is monoclinic pyrrhotite with low magnetic susceptibility in contrast multidomain magnetites lack remanence and have high magnetic susceptibilities pyrrhotite is preserved in the rock mass along with other sulfide minerals as millimeter scale veins parallel to regionally developed penetrative mylonitic shear bands c planes formed during sulfide remobilization in the waning cooling stages of hydrothermal au deposition in contrast magnetite grains exhibit porphyroblastic textures consistent with formation during prograde to peak metamorphism indicating formation prior to pyrrhotite earlier in the orogenic cycle because of the low curie temperature ca 325 c of pyrrhotite it is therefore likely that remanent magnetization effects observed in aeromagnetic images of the tanami inlier are associated with structures intimately related to the late stages of au mineralization during retrograde greenschist metamorphism after the peak thermal point of the orogenic cycle

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

pleistocene evolution of the danube in the carpathian basin the present day drainage system of the carpathian basin originates from the gradual regression of the last marine transgression brackish pannonian sea the flow directions of the rivers including the danube are determined by the varying rates and locations of subsidence within the region the danube which forms the main axis of the drainage network first filled the depression of the little plain lake and then further southward the slavonian lake from the end of the pliocene the crustal movements which caused the uplift of the transdanubian mountains forced the danube to flow in an easterly direction towards the antecedent visegrid gorge and into the subsiding basins of the great plain climatic changes during the pleistocene had the effect of forming up to seven fluvial terraces the uplift of the mountains is demonstrated by the deformation of the terraces while the subsidence of the plains is proven by an accumulation of several hundred metres of sediment the river only occupied its present position south of budapest in the latest pleistocene

can high alumina arc basalt be derived from low alumina arc basalt evidence from kanaga island aleutian arc alaska the coexistence of both high mgo low alumina basalt lab and low mgo high alumina basalt hab magmatic inclusions within a single andesitic host lava on kanaga island provides the opportunity to rigorously evaluate the presence or absence of a fractionation relation between the two basalt magma types traditional mass balance calculations pearce ratio arguments comparative phenocryst modes and compositions and incompatible element rb ba zr abundances all indicate that selective clinopyroxene and orthopyroxene fractionation from the plagioclase clinopyroxene orthopyroxene titanomagnetite saturated lab magma yielded the plagioclase phyric hab magma the same data are consistent with a model of low pressure hab generation calling upon selective mafic crystal fractionation along the margins of a crystallizing and convecting lab magma chamber the ability to demonstrate that nonliquid hab magma can be produced from lab through crystal fractionation eliminates many of the arguments used in support of a primary eclogite melting origin for hab

tectonic controls on tholeiitic and calc alkaline magmatism in the aleutian arc the tectonic position of aleutian arc volcanic centers and their magmatic differentiation trends calc alkaline or tholeiitic appear to correlate from 160 w to 175 e the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates the tholeiitic volcanoes are large primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow closed system differentiation the calc alkaline volcanoes are smaller more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers the chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma probably derived from mantle peridotite conditions for the derivation of the two trends through crystal fractionation support the proposed model the tholeiitic magmas show characteristics i e no hydrous phenocrysts fe enrichment trend parallel ree patterns vitrophyric and esites and dacites consistent with low pressure high temperature crystallization in large shallow magma chambers the calc alkaline magmas show characteristics i e some hydrous phenocrysts no fe enrichment trend nonparallel rare earth patterns porphyritic lavas consistent with higher pressure and lower temperature of crystallization than the tholeiitic series tertiary plutons also show both calc alkaline and tholeiitic trends and appear to be chemically similar to the quaternary volcanoes the larger plutons are calc alkaline and probably represent the extensive 1002 km magma chambers of the small calc alkaline volcanoes the small shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series

geochemistry of basalts from central and southern new hebrides arc implication for their source rock composition abstract the basaltic rocks from the central and southern islands of the new hebrides aneityum tanna erromango efate emae tongoa and epi have geochemical features typical of island arc volcanics they are enriched in lile and depleted in zr hf nb and ta compared to n type morb the rocks were derived from a similar upper mantle source as n type morb but with a higher degree of partial melting in addition their source was enriched in lile k rb sr ba and lree probably by migrating hydrous fluids released during the dehydration of the subducted oceanic slab the basalts from futuna island which is located farther from the trench display characteristics typical of calc alkaline rocks the futuna basalts were generated from a different lile enriched upper mantle source it seems that this upper mantle source was modified by interaction with partial melts from the subducted oceanic lithosphere

unusual magnetic properties of sedimentary pyrrhotite in methane seepage sediments comparison with metamorphic pyrrhotite and sedimentary greigite

rock magnetism in the global palaeomagnetic database gpmdb abstract the iaga global palaeomagnetic database initiated in 1988 is supposed to contain directional and statistical palaeomagnetic data and additional information on rock magnetic measurements for assessment of the reliability of the palaeomagnetic results the rock magnetic information content of the database for 1969 1988 was analysed as a main result it could be shown that at present only about half of the palaeomagnetic results are supported by rock magnetic measurements however there is a tendency towards the improvement of this figure the increase of investigations on sediments is clearly associated with the introduction of more sensitive magnetometers since the early 1970s trends could also be found for some rock magnetic techniques e g isothermal remanent magnetization acquisition curves

magnetic characterization of non ideal single domain monoclinic pyrrhotite and its demagnetization under hydrostatic pressure up to 2 gpa with implications for impact demagnetization here we present a comprehensive magnetic characterization of synthesized non ideal single domain sd monoclinic pyrrhotite fe 7 s 8 the samples were in the form of a powder and a powder dispersed in epoxy non ideal refers to a powder fraction of predominantly sd size with a minor contribution of small pseudo single domain grains such non ideal sd pyrrhotite was found to be a remanence carrier in several types of meteorites carbonaceous chondrites snc which justifies the usage of synthetic compositions as analogous to natural samples data were collected from 5 to 633 k and include low field magnetic susceptibility 0 thermomagnetic curves major hysteresis loops back field remanence demagnetization curves first order reversal curves forcs alternating field and pressure demagnetization of saturation isothermal remanent magnetization sirm low temperature data such as zero field cooled and field cooled remanence datasets together with room temperature sirm cooling warming cycles as well as xrd and mossbauer spectra the characteristic besnus transition is observed at 33 k forc diagrams indicate interacting sd grains the application of hydrostatic pressure up to 2 gpa using nonmagnetic high pressure cells resulted in the demagnetization of the sample by 32 38 repeated cycling from 1 8 gpa to atmospheric pressure and back resulted in a total remanence decrease of 44 after 3 cycles pressure demagnetization experiments have important implications for meteorite paleomagnetism and suggest that some published paleointensities of meteorites with non ideal sd monoclinic pyrrhotite as remanence carrier may be lower limits because shock demagnetization was not accounted for

osl ages and origin of late quaternary sediments in the north transdanubian hills hungary timing of neotectonic activity abstract the transdanubian hills south of lake balaton is a gently elevated area with well developed dissected morphology a new highway constructed through the area provided an excellent opportunity to make field observations and date the neotectonic movements deformed and undeformed fluvial and eolian sequences were dated using optically stimulated luminescence on quartz complemented by radiocarbon age data the distribution of the equivalent doses measured on small aliquots indicates that the sediments were poorly bleached before deposition and therefore the osl ages calculated by the mean equivalent doses represent the maximum depositional ages of the sediments to determine the source area of the sediments microscopic study was applied the oldest dated samples are 49 39 ka fluvial sands at maximum on the top of the sand a forest steppe type paleosol formed which interfingers with the coeval fluvial channel sand body these sediments are younger than about 30 ka loess like sediments were deposited later than 25 and 10 8 ka and fluvial sand was deposited a maximum of 22 and 9 ka the fluvial sediments except the youngest were transported from nnw of the transdanubian central range indicating that lake balaton did not exist during the late pleistocene and the drainage network was quite different from the recent situation field structural observations and osl dating showed that a deformation period started later than 49 ka in the study area while folding started after 22 ka gentle late stage uplift at maximum 9 ka ago was identified which might have been active sub recently

quality of ptrm acquisition in pyrrhotite bearing contact metamorphic limestones possibility of a continuous record of earth magnetic field variations abstract the rockmagnetic and palaeomagnetic signal in pyrrhotite bearing limestones of different contact metamorphic settings were investigated related to intrusions ranging from small sills to large magmatic complexes magnetic susceptibility the pyrrhotite magnetite ratio and thermal modelling serve as an investigative tool to define three metamorphic zonations a contact zone of a mixed magnetic assemblage and low susceptibilities a pyrrhotite bearing transitional zone where full thermoremanent magnetisations trms are acquired due to temperatures above the curie temperature of pyrrhotite t c po and a marginal zone containing pyrrhotite and magnetite generated at temperatures below t c po the fact that trms can consist of independent ptrms is successfully tested by modified thellier experiments it is shown that a metamorphic environment with low fluid circulation provides a scenario for the recording of independent ptrms multicomponent behaviour of the nrm residing in samples from the transitional zone can therefore be interpreted as a continuous record of earth magnetic field variations

late pleistocene to holocene palaeoenvironmental variability in the north west spanish mountains insights from a source to sink environmental magnetic study of lake sanabria we present a source to sink environmental magnetic study of a sediment core from lake sanabria north west iberian peninsula and rocks of its catchment the results indicate the occurrence of magnetite and probably also pyrrhotite in sediments accumulated between ca 26 and 13 cal ka bp in a proglacial lake environment these minerals also appear to dominate the magnetic assemblage of palaeozoic rocks from the lake catchment this indicates that sedimentation was then driven by the erosion of glacial flour which suffered minimal chemical transformation due to a rapid and short routing to the lake a sharp change in magnetic properties observed in the lake sediments between 13 and 12 6 cal ka bp reflects the rapid retreat of glaciers from the lake catchment sediments from the upper half of the studied sequence accumulated after 12 6 cal ka bp in a lacustrine environment with strong fluvial influence contain magnetite and smaller amounts of maghemite and greigite we suggest that greigite grew authigenically under anoxic conditions caused by enhanced accumulation of organic matter into the lake the occurrence of maghemite in these sediments suggests pedogenic activity in the then deglaciated lake catchment before the erosion and transportation of detrital material into the lake

magnetic anisotropy behaviour of pyrrhotite as determined by low and high field experiments summary here we report on the sources of magnetic anisotropy in pyrrhotite an iron sulphide present in many rocks as an important carrier of the natural remanent magnetization while the magnetic hysteresis parameters of pyrrhotite are well known the existing database concerning its anisotropy behaviour is patchy and ambiguous therefore a collection of 11 seemingly single crystals of natural pyrrhotite was scrutinized before embarking on the anisotropy determinations the set of single crystals was extensively characterized rock magnetically by measuring curie temperatures hysteresis loops irm acquisition curves and forc diagrams the latter three all at room temperature first the variation of the low field susceptibility as function of applied field and grain size was evaluated for fields ranging from 1 to 450 a m 1 existing grain size dependent data and the present larger crystals show a logarithmic grain size dependence this enables estimating the grain size for unimodal pyrrhotite distributions in rocks measured trends are better fitted with an exponential function than with a rayleigh law style function based on the rock magnetic characterization and the behaviour of the anisotropy of magnetic susceptibility six samples of the original 11 were selected for the high field anisotropy determinations within the basal plane those data were acquired with a torque cantilever type magnetometer as expected most single crystals showed a pure 6 curve within their basal plane because of the easy axis configuration in some crystals however lower harmonic terms overlapped the 6 term this may be the dominant source of the observed variation in magnetic anisotropy properties torque data of three of the six samples were of sufficient quality to allow evaluation of k 1 re evaluation of existing torque data and including the present newly derived determinations yields for the anisotropy constant of pyrrhotite within the basal plane k 1 2 7 0 2 10 4 jm 3 this is over an order of magnitude more precise than the sparse existing k 1 data only the value reported by mikami and co authors in 1959 agrees with the new determination with this firmly established k 1 value meaningful anisotropy models are now possible for pyrrhotite bearing rocks

the origin of reversed geochemical zoning in the northern new hebrides volcanic arc integration of petrographic and geochemical data on each island in the banks group northern vanuatu new hebrides has revealed a decrease in k2o and related incompatible elements across the islands away from the new hebrides trench the correlation with depth to the benioff zone is therefore the reverse of that typically found in island arcs ree evidence and source modelling indicates that this variation represents a progressive depletion in lree hree in the upper mantle laterally away from the trench and a progressive increase in partial melting of the source in the same direction these variations in chemistry are attributed to an earlier west dipping miocene subduction system and the variation in degree of partial melting to the location of the islands with respect to the active back arc basin an enrichment of the upper mantle in the miocene is ascribed to the ascent of hydrous fluids enriched in incompatible elements from the west dipping benioff zone and subsequent reaction with the overlying upper mantle the model thus envisages approximately contemporaneous development of the central chain volcanoes and the backarc basin in the late pliocene with partial melting of hydrous laterally zoned upper mantle resulting from the convective thermal regime associated with the rifting apart of the back arc basin the role of the present subduction regime in magma generation is considered to be very limited

evidence for aqueous activity on comet 81p wild 2 from sulfide mineral assemblages in stardust samples and ci chondrites abstract the discovery of nickel copper and zinc bearing iron sulfides from comet 81p wild 2 wild 2 represents the strongest evidence in the stardust collection of grains that formed in an aqueous environment we investigated three microtomed tem sections which contain crystalline sulfide assemblages from wild 2 and twelve thin sections of the hydrothermally altered ci chondrite orgueil detailed structural and compositional characterizations of the sulfide grains from both collections reveal striking similarities the stardust samples include a cubanite cufe 2 s 3 grain a pyrrhotite fe ni 1 x s pentlandite fe ni 9 s 8 assemblage and a pyrrhotite sphalerite fe zn s assemblage similarly the ci chondrite sulfides include individual cubanite and pyrrhotite grains cubanite pyrrhotite assemblages pyrrhotite pentlandite assemblages as well as possible sphalerite inclusions within pyrrhotite grains the cubanite is the low temperature orthorhombic form which constrains temperature to a maximum of 210 c the stardust and orgueil pyrrhotites are the 4c monoclinic polytype which is not stable above 250 c the combinations of cubanite and pyrrhotite as well as pyrrhotite and pentlandite signify even lower temperatures the crystal structures compositions and petrographic relationships of these sulfides constrain formation and alteration conditions taken together these constraints attest to low temperature hydrothermal processing our analyses of these minerals provide constraints on large scale issues such as heat sources in the comet forming region aqueous activity on cometary bodies and the extent and mechanisms of radial mixing of material in the early nebula the sulfides in the wild 2 collection are most likely the products of low temperature aqueous alteration they provide evidence of radial mixing of material e g cubanite troilite from the inner solar system to the comet forming region and possible secondary aqueous processing on the cometary body

genesis of dacitic magmatism at batur volcano bali indonesia implications for the origins of stratovolcano calderas abstract batur is an active stratovolcano on the island of bali indonesia with a large well formed caldera whose formation is correlated with the eruption about 23 700 years ago of a thick ignimbrite sheet our study of the volcanic stratigraphy and geochemistry of batur shows the formation of the caldera was signalled by a change in the composition of the erupting material from basaltic and andesitic to dacitic the dacitic rocks are glassy possess equilibrium phenocryst assemblages and display compositional characteristics consistent with an origin by crystal liquid fractionation from more mafic parent magmas in a shallow chamber possibly at 1 5 km depth and 1000 1070 c however although separated by a gap of 6 wt sio 2 the dacitic rocks are clearly related in their minor and trace element geochemistry to those basalts and basaltic andesites erupted after the caldera was formed rather than to the andesites erupted immediately before the dacites first appeared we infer from this and published experimental modelling of the possible crystallization behaviour of basaltic magma chambers that a magmatic cycle involving caldera formation began independently of the previous activity of batur by formation of a new closed system magma chamber beneath the volcano fractional crystallization possibly at the walls of the chamber led to the early production of derivative siliceous magmas and consequently to caldera formation while most of the magma retained its original composition the postcaldera batur basalts represent the largely undifferentiated core liquids of this chamber this model contrasts with the traditional evolutionary model for stratovolcano calderas but may be applicable to the origins of calderas similar to that of batur particularly those in volcanic island arcs

on the distribution and separation of crystals in convecting magma abstract fractionation of crystals from magma stands squarely as the central principle of igneous petrology and the critical balance is the competition between crystal settling or rising velocity and magmatic convective velocity a ratio we call s the convective velocity is intimately related to the rate of heat transfer from the system although the theory of sediment distribution in rivers establishes the critical parametric balance and it has been often applied to igneous systems this method turns out to be valid only for small concentrations of crystals and it is not clear if it is accurate for other than turbulent flows where the prandtl number is near unity a generalized theory of particle crystal or bubble behavior is presented scaling of the generalized result shows for the magmatic conditions of small crystallinities small particle reynolds number and newtonian viscosity that the exact distribution of crystals in a convecting magma can be found analytically regardless of the flow field critical retention occurs only for 0 s s 0 for sinking crystals retention is centered in the region of maximum updraft for floating particles retention occurs in regions of downflow which for a simple cellular convective pattern produces a torus shaped retention region against the walls about the midsection of the body for a stack of flattened convective cells which in the crudest sense is analogous to double diffusive convection the zones of retention form depending on the exact flow field either a stack or staggered stack of pillows through the central region crystals settling from top to bottom around this stack take a tortuous path and may exhibit a complex but regular compositional zoning additional effects of fluids possessing a yield strength turbulence particle shape and wall and particle dispersive effects are more qualitatively evaluated for each flow field the amount of retention and the effective settling velocity of all crystals is found as a function of s the critical parameter s controlling retention can be estimated from a parametric representation involving stokes s law and the rayleigh number this shows that some retention probably occurs in all magmas and it increases with approach of the magma to the earth s surface although both crystal settling rate and convection decrease with increasing magmatic viscosity the overall effect is to increase retention decrease s magmas having a viscosity near and beyond about 10 4 poise and chambered in the nearsurface environment should show large degrees of crystal retention retention is however never complete for magmas and the flux of settling crystals leading to differentiation varies as a function of crystallinity this flux or flux probability p f is zero both at the liquidus and the point of maximum packing of crystals which for basalts is about 60 the importance of differentiation is regulated by the amount of time spent in the high crystal flux state and a relative measure of this time is given by the thermal probability p t introduced previously thus the probability of differentiation p d is given by the product p f p t which shows that differentiation must occur over a restricted range of crystallinities from about 20 to 45 for basalts and from about 10 to 30 for rhyolitic magmas the most basic lessons learned here are that the full range of crystallinities is not equally accessible to crystal fractionation and the most fundamental information describing the dynamic state and behavior of magma may well be the distribution of crystal sizes found in lavas

stress induced late stage subsidence anomalies in the pannonian basin abstract subsidence sedimentation and tectonic quiescence of the pannonian basin was interrupted a few million years ago by tectonic reactivation this recent activity has manifested itself in uplift of the western and eastern flanks and continuing subsidence of the central part of the pannonian basin low to medium magnitude earthquakes of the carpathian pannonian region are generated mostly in the upper crust by reverse and wrench fault mechanisms there is no evidence for earthquakes of extensional origin 2 d model calculation of the subsidence history shows that a recent increase in magnitude of horizontal compressional intraplate stress can explain fairly well the observed quaternary uplift and subsidence pattern we propose that this stress increase is caused by the overall europe africa convergence in late pliocene consumption of subductible lithosphere at the eastern margin of the pannonian basin was completed and the lithosphere underlying the pannonian basin became locked in a stable continental frame consequently extensional basin formation has come to an end and compressional inversion of the pannonian basin is in progress

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

mineralogy geochemistry and petrogenesis of kurile island arc basalts whole rock major and trace element and mineral chemical data are presented for basaltic rocks from the main evolutionary stages of the kurile island arc nw pacific an outer inactive arc contains a cretaceous lower tertiary sequence of tholeiitic calcalkaline and shoshonitic basalts the main arc miocene quaternary is dominated by weakly tholeiitic with lesser alkalic basalts the mineralogy of kuriles basalts is characterised by an rich plagioclases a continuous transition from chromites to titanomagnetites pyroxenes with low fe3 contents and without strong fe enrichment abundance of groundmass pigeonites and the absence of amphiboles there is an increase in k2o contents both along arc northwards and towards the reararc side the basalts show an exceptionally wide but continuous range of k2o contents 0 1 4 7 which correlate with other lil element contents tholeiitic basalts with low lil element contents la yb and th u but high k rb p2o5 la and zr nb were derived from depleted lherzolitic mantle which had suffered fluid metasomatism by k rb cs sr ba pb and h2o only alkali basalts are also thought to be derived from depleted mantle but melt metasomatism involved addition of all lil elements to a garnet lherzolite mantle the kuriles basalts and their mantle sources range continuously between these two end member compositions the metasomatic fluids melts were probably released by early dehydration and later melting within subducted oceanic lithosphere though the process is not adequately constrained

determination of slip rate by optical dating of fluvial deposits from the wangsan fault se korea abstract the time integrated slip rate in fault zones can be determined if the deformed deposits are reliably dated here we report optically stimulated luminescence osl ages of late pleistocene fluvial deposits cut by the wangsan fault southeastern korea which displaces a hanging wall block of about 28 m five sandy samples of the deformed quaternary deposits were dated by quartz osl using the single aliquot regenerative dose sar protocol three samples taken from the footwall block show stratigraphically consistent osl ages of 54 7 76 5 and 90 6 ka from top to bottom two samples collected from the same layer in the hanging wall block show reproducible osl ages of 81 5 and 82 5 ka which are also in good agreement with the stratigraphic relationships our osl ages yield an average sedimentation rate of the quaternary deposits as around 0 04 mm a 1 and a minimum value of time integrated slip rate as 0 52 mm a 1 this minimum slip rate is considerably higher than those reported earlier for quaternary faults in southeastern korea the youngest osl age 54 7 ka constrains the maximum value of the recurrence interval of the fault movement

luminescence dating of neotectonic activity on the southwestern coastal plain taiwan abstract the southwestern coastal plain of taiwan is heavily urbanised with a population of more than 6 million the area is known to be subsiding and the resulting basin is filled with thick fluvial deposits of at least 200 m thickness in the past century three large earthquakes have occurred in the area two caused surface ruptures and resulted in large property loss and more than 2000 casualties there is an urgent need for an understanding of the dynamics and recurrence intervals of this neotectonic activity but little is known of the chronology of the late pleistocene deposits recently more than 20 cores longer than 250 m were taken from the coastal plain as part of a large hydrogeological investigation and basic data on lithology hydrogeology and palaeobiology were collected the base of these cores is beyond the 14 c age limit and so the application of luminescence dating to these sediments has been investigated optically stimulated luminescence methods have been applied to quartz sand sized grains extracted from 29 samples dosimetry based on gamma spectrometry is also compared with icp ms and xrf analyses in the age range up to 40 ka radiocarbon ages are compared with the luminescence results to give confidence that the initial bleaching of these sediments was sufficient the luminescence ages are then discussed and differential rates of basin subsidence are deduced it is clear from these data that the study area is tectonically active and it may be that regions of similar subsidence rate correlate with identifiable geological structures

electrical and magnetic properties of sulfides the metal sulfides exhibit a great diversity of electrical and magnetic properties with both scientific interest and practical applications these properties apply major constraints on models of the electronic structure or chemical bonding in sulfides vaughan and rosso 2006 this volume the pure and doped synthetic equivalents of certain sulfide minerals have actual or potential applications in the electronics industries optical devices photovoltaics photodiodes and magnetic recording devices sulfides are also components of many thin film devices and have been extensively investigated as part of the nanotechnology revolution certain electrical and magnetic properties of sulfide minerals mean they contribute to geomagnetism and paleo magnetism and provide the geophysical prospector with exploration tools for metalliferous ore deposits to the mineral technologist these same properties provide methods for the separation of the metal bearing sulfides from associated waste minerals after mining and milling and before extraction of the metal by pyrometallurgical or hydrometallurgical treatment in this chapter the theory and measurement of electrical and magnetic properties are outlined along with spectroscopic and diffraction studies that can provide insights into magnetic behavior are discussed a brief review of electrical and magnetic studies of major sulfide minerals includes some examples of the applications of sulfide electrical and magnetic properties including special consideration of the properties of sulfide nanoparticles most of the available data presented are for pure synthetic binary and ternary sulfides as very small concentrations of impurities can dramatically affect electrical properties leading to problems of interpreting data from natural samples although data for several of the commonly found sulfides are discussed in this chapter no attempt is made at a comprehensive coverage the section below on theory and measurement of electrical and magnetic properties draws on the account given in vaughan and craig 1978 to which readers are referred for further details it

magnetic properties of synthetic analogues of pyrrhotite ore in the grain size range 1 24 µm summary iron and sulphur in proportions appropriate to fe 7 s 8 were reacted in evacuated quartz tubes for 24 hr at 500 c quenching produces predominantly hexagonal pyrrhotite whereas further prolonged annealing at 250 c produces monoclinic pyrrhotite we mainly report the properties of material cooled from 500 ca t 10 c min 1 which is predominantly monoclinic but probably contains hexagonal pyrrhotite together with pyrite fes 2 this material which we refer to as non ideal pyrrhotite may be a better analogue of pyrrhotite ore than ideal synthetic monoclinic pyrrhotite the samples were characterized by x ray mossbauer evect and thermomagnetic analyses the slow cooled non ideal monoclinic pyrrhotite exhibits thermal stability intermediate between monoclinic and hexagonal pyrrhotites one striking observation is that the deviation from ideal monoclinic pyrrhotite increases with decreasing particle size particles about 1 mm in size have a low saturation magnetization value of about 6 am2 kg 1 indicating a lower concentration of ferrimagnetic monoclinic pyrrhotite saturation magnetization 18 am2 kg 1 than particles of about 6 mm and above saturation magnetization 12 am2 kg 1 on the other hand magnetization process parameters coercive force ratio of saturation remanence to saturation magnetization coercive force of remanence median destructive and inductive fields the magnetic susceptibility follow well behaved power law dependences on particle size similar to monoclinic pyrrhotite magnetic domain patterns in the slow cooled non ideal pyrrhotite are similar in type and style to those observed in synthetic monoclinic pyrrhotite and in natural pyrrhotite particles in rocks dynamic observations of domain walls in changing external alternating and direct magnetic fields are consistent with macroscopic obser vations of magnetization change with field the critical size for the monodomain multidomain transition in synthetic monoclinic pyrrhotite at about 1 mm is essentially the same as that previously determined for natural pyrrhotite grains in rocks the observations suggest that the domain walls lie normal to the c axis of the structure consistent with the known magnetocrystalline anisotropy symmetry of monoclinic pyrrhotite the type and style of the magnetic microstructures of hexagonal pyrrhotite are quite diverent from those of monoclinic pyrrhotite the domain patterns are on a much smaller scale wavy and sometimes resemble those of a hexagonal material such as magnetoplumbite or cobalt

the mid hungarian line a zone of repeated tectonic inversions abstract the mid hungarian line is a major tectonic feature of the intra carpathian area separating two terranes of different origin and tectonic structure although this tectonic line was known from borehole records it has not been described in seismic sections the study presents interpreted seismic lines crossing the supposed trace of the mid hungarian line these seismic sections show north dipping normal faults and thrust faults as well as cross cutting young strike slip faults a complex tectonic history is deduced including intra oligocene early miocene thrusting middle miocene extension local late miocene inversion and late miocene pliocene normal faulting and left lateral wrenching in the light of our seismic study we think that the best candidate for the mid hungarian line is a north dipping detachment fault beneath large masses of neogene volcanics the auxiliary structures to the north seen on seismic sections suggest that it moved as a south vergent thrust fault during the palaeogene early miocene which later was reactivated as a set of normal faults the northern alcapa unit overrode the southern tisza dacia unit along this fault zone the same relative positions are observed in the northern termination of the line other structures along the supposed trace of the line are north dipping normal or strike slip faults which frequently were reactivated as smaller thrust faults during the late neogene palaeogene early miocene thrusting along the line might be the result of the opposite tertiary rotations of the two major units as suggested by palaeomagnetic measurements and earlier models

late quaternary dynamics of the tisza river evidence of climatic and tectonic controls abstract the focus of this study is to understand a dramatic avulsion event on the tisza river during the late pleistocene the river course switched by about 80 km from the east to west of the great hungarian plain ghp through the zahony bend to its present meander belt the aim of this study is to date this zahony avulsion based upon radiocarbon and pollen samples from six cores in the polgar study area situated in the west of the ghp at the middle course of the tisza river in addition a grain size composition and heavy mineral analysis has been performed the results of these analyses reveal a sequence of paleochannels and have been plotted on a high resolution digital elevation model illustrating the paleochannel form and age relationships the study suggests that the age of this major avulsion event is significantly older than it was previously supposed instead of 10 to 11 ka it is according to our new data 16 18 ka and definitely predates the last glacial maximum lgm the river cut into the previous contiguous surface and formed at least one climatic terrace then drifted gradually westward as a response to a tilt or the differential subsidence of tectonic origin

domain structure of natural fine grained pyrrhotite in a rock matrix diabase the domain structure of natural pyrrhotite grains in a devonian diabase was studied with the bitter pattern technique examples are shown of the domain structure of large d 100 m true multidomain grains intermediate 10 d 40 m pseudo single domain grains and ore grains with less than four domains down to the transition from the two domain to the single domain state plate shaped domains prevail for all sizes of ore grains closure domains are only rarely formed non magnetic inclusions and surface irregularities impede the domain wall mobility as expected resulting in an often quite considerable spread of muscopic coercive forces in the large grains intergrowths of chemically only slightly different phases of pyrrhotite which cannot be distinguished in a reflected light muscope seem to be responsible for entirely stationary pseudo domain walls in low dc fields domain wall displacements as well as spin rotations within the basal plane of easy magnetization are probably the dominant magnetization processes in pseudo single domain and multidomain particles

mid pleistocene lavas from the seguam volcanic center central aleutian arc closed system fractional crystallization of a basalt to rhyodacite eruptive suite in contrast to adjacent volcanic centers of the modern central aleutian arc seguam island developed on strongly extended arc crust k ar dates indicate that mid pleistocene late pleistocene and holocene eruptive phases constitute seguam this study focuses on the petrology of the mid pleistocene 1 07 07 ma turf point formation tpf which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst poor andesite to rhyodacite lavas increasing whole rock feo mgo from basalt to dacite the anhydrous plag ol cpx opx mt phenocryst assemblage groundmass pigeonite and the reaction ol liq opx preserved in the mafic lavas indicate a tholeiitic affinity thermometry and comparison to published phase equilibria suggests that most tpf basalts crystallized plag ol cpx mt at 1160 c between about 3 5 kb 1 2 h2o andesites crystallized plag cpx opx mt at 1000 c between 3 4 kb with 3 5 h2o and dacites crystallized plag cpx opx mt at 1000 c between 1 2 kb with 2 3 h2o all lavas crystallized at f o 2 close to the nno buffer mineral compositions and textures indicate equilibrium crystallization of the evolved lavas petrographic evidence of open system mixing or assimilation is rare mgo cao al2o3 cr ni and sr abundances decrease and k2o na2o rb ba zr and pb increase with increasing sio2 50 71 lree enrichment ce yb n 1 7 0 2 characterizes most tpf lavas total ree contents increase and eu anomalies become more negative with increasing sio2 relative to other aleutian volcanic centers tpf basalts and basaltic andesites have lower k2o na2o tio2 rb ba sr zr y and lree abundances 87sr 86sr ratios 0 70361 0 70375 and ratios of 206pb 204pb 18 88 18 97 207pb 204pb 15 58 15 62 208pb 204pb 38 46 38 55 are the highest measured for any suite of lavas in the oceanic portion of the aleutian arc conversely end values 5 8 to 6 7 are among the lowest from the aleutians sr nd and pb ratios are virtually constant from basalt through rhyodacite whereas detectable isotopic heterogenity is observed at most other aleutian volcanic centers major and trace element ree and sr nd and pb isotopic compositions are consistent with the basaltic andesitic andesitic dacitic and rhyodacitic liquids evolving from tpf basaltic magma via closed system fractional crystallization alone fractionation models suggest that removal of 80 wt cumulate 61 plag 17 cpx 12 opx 7 ol and 3 mt can produce 20 wt rhyodacitic residual liquid per unit mass of parental basaltic liquid petrologic and physical constraints favor segregation of small batches of basalt from a larger mid crustal reservoir trapped below a low density upper crustal lid in these small magma batches the degree of cooling crystallization and fractionation are functions of the initial mass of basaltic magma segregated the thermal state of the upper crust and the magnitude of extension tholeiitic magmas erupted at seguam evolved by substantially different mechanisms than did calc alkaline lavas erupted at the adjacent volcanic centers of kanaga and adak on unextended arc crust these variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew at seguam intra arc extension promoted eruption of voluminous basalt and its differentiates unmodified by interaction with lower crustal or upper mantle wallrocks

preliminary results of long term slip rates of 1999 earthquake fault by luminescence and radiocarbon dating for the purpose of deriving the long term slip rates of different segments along the surface rupture of 1999 chi chi earthquake radiocarbon and luminescence methods tl and osl were applied to the tectonically influenced geomorphic features and deposits we obtained six age estimates on pottery driftwood and fluvial sediments we found that the osl signals of the samples collected from the coarse fluvial deposits had not been totally bleached due to rapid deposition based on the cumulative vertical offsets the vertical slip rates were computed for each site our results reveal that the slip rate of the northern segment is relatively higher than that of the southern segment which is consistent with the coseismic offsets that occurred in the 1999 earthquake

the loess profile at albertirsa hungary improvements in loess stratigraphy by luminescence dating abstract a very detailed upper pleistocene record is exposed in a loess palaeosol sequence at the section at albertirsa in hungary luminescence dating was carried out to establish a more reliable chronological framework for the hungarian loess record the lowermost palaeosol of the section probably an equivalent of the mf2 horizon at the mende key section is superimposed on penultimate glacial loess an irsl age estimate of about 65 ka bp was determined for the lower part of this palaeosol indicating soil formation late in oxygen isotope stage ois 5 or early in ois 3 a significant hiatus most likely occurs in the lower part of the lower fossil soil the loess between the upper and and lower palaeosol yielded a mean luminescence age of about 50 ka bp the upper palaeosol is probably an equivalent of the mf1 horizon at the mende type section formed between 37 and 25 ka bp and so this palaeosol represents the hengelo and or denekamp interstadial of northwestern european stratigraphy the loess is intercalated by a humic horizon which yielded a mean luminescence age estimate between 21 and 19 ka at the albertirsa section

thermoluminescence ages for three sections in hungary abstract samples were collected from sections at mende paks and tapiosuly ages for the loess above the mf1 soil at mende were in agreement with the radiocarbon dates on charcoal fragments from the soil ages for loess from tapiosuly were also in agreement with radiocarbon ages for younger soils however tl age estimates for loess above or below the mf2 soil at each of the three sites suggested that this soil had an age of around 76 ka it has not been confirmed whether these ages are underestimated however they are older than previously thought on the basis of other tl studies they imply that the underlying mb soil is much older than the last interglacial

magnetic inclusions in diamonds abstract we demonstrate that the pyrrhotite that commonly occurs as inclusions in diamonds is capable of carrying strong and stable remanent magnetizations this suggests that with suitable samples it may be possible to use such inclusions to obtain information about the geomagnetic field such as the paleointensity during key intervals of geologic time the pyrrhotite inclusions examined here exhibit a strong and pronounced anisotropy of magnetic remanence that is coupled to the crystal structure of the diamond while this anisotropy of remanence may complicate efforts to obtain paleointensity data we propose that it may provide a valuable way to uniquely identify individual diamonds that contain even minute inclusions of pyrrhotite

convective fractionation a mechanism to provide cryptic zoning macrosegregation layering crescumulates banded tuffs and explosive volcanism in igneous processes a large array of igneous and volcanic features has characteristics that are recognized in other disciplines as conclusive and direct evidence of convection in stratified and or solidifying melts e g macroscopic segregation cryptic variation zoning in magma chambers mineral layering in mafic intrusives crescumulates fingering in the vertical and horizontal banding in pyroclastics rollover with attendant flashing of volatiles explosive volcanism etc some quantitative and qualitative aspects of convection in solidifying and or stratified melts e g mineral layer widths such as are observed in the skaergaards are examined to show consistency with field evidence convective fractionation does not possess the physical implausibilities of gravitational segregation crystal settling neither is the field evidence as ambiguous if interpreted in terms of convective fractionation which can explain amongst other things heavier material overlying lighter convective fractionation may operate on larger scales in the interior of planets

luminescence and amino acid racemization chronology of the loess paleosol sequence at suttő hungary abstract the loess paleosol sequences in hungary provide an excellent upper pleistocene climate archive of the carpathian basin loess deposits up to 20 m thick cover the sutt travertine complex located in the very northern part of hungary next to the right bank of the danube river the loess is intercalated with two weakly developed greyish horizons two thinner brownish steppe like soils and a pedocomplex including a reddish brown clay enriched horizon covered by a chernozem like paleosol infrared optically stimulated luminescence irsl dating was applied on polymineral fine grain material to determine the depositional age of the loess the uppermost loess yields irsl age estimates ranging from 15 to 40 ka the loess on top of the pedocomplex yields irsl age estimates ranging from 48 to 60 ka the pedocomplex most likely correlates with the marine oxygen isotope stage mis 5 interglacial period the loess from below the pedocomplex was deposited during the penultimate glaciation as evidenced by amino acid racemization aar results further independent age control is provided by radiocarbon dating for the upper part of the profile and by uranium series 230th 234u ages correlating the travertine below the loess with mis 7 8 the magnetic susceptibility record through the sequence shows a strong correlation with the loess layers and soil horizons

wind erosion under cold climate a pleistocene periglacial mega yardang system in central europe western pannonian basin hungary abstract mega yardangs are known to form under extremely arid warm or in some instances moderate climates here we report on the mega yardang field of the pannonian basin hungary which was uniquely formed under periglacial probably semi arid conditions and discuss its implications on factors controlling yardang formation in general the yardang field is composed of advanced streamlined forms and of less aerodynamic ridges separated by well developed corridors the length of the largest features exceeds 60 km their height varies around 150 m together with ventifacts deflation hollows and wind blown sand areas the yardang field is part of an integrated eolian feature system driven by northerly to northwesterly winds dating of ventifact surfaces with in situ produced cosmogenic nuclides places the onset of significant wind erosion to the early pleistocene 1 5 ma osl dated young loess from which well developed yardangs were sculpted and wind blown sands show that eolian action continued until the early holocene the long temporal range of wind erosion compared to average yardang formation rates and the rarity of advanced yardang forms indicate intermittent and relatively short most probably millennial scale deflation periods since the early holocene the system has been inactive and demonstrates that yardangs can persist in a recognizable form for several millennia even under less arid conditions the main control on yardang formation appears to be the existence of strong unidirectional winds in the pannonian basin facilitated mainly by the channeling effect of topographic lows in the surrounding mountain chains yardangs can form even under semi arid climates however the rarity of these occurrences suggests that this requires a special association of environmental conditions yardang formation is facilitated by homogeneous lithology namely by widespread weakly consolidated sandy sediments the distribution of yardang fields is controlled by vertical neotectonic movements which provided uplifted areas to be sculptured and topographic obstacles that sheltered downwind sediments faulting can be excluded as a controlling factor in mega yardang formation nor is the role of jointing supported

composition gaps critical crystallinity and fractional crystallization in orogenic calc alkaline magmatic systems the recognition of a three way correlation between magmatic sio2 content critical crystallinity and the size magnitude of crystal fractionation generated composition gaps in calc alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps to explain this correlation it is proposed that fractionation generated composition gaps are caused by 1 simultaneous interior i e non substrate crystallization and vigorous chamberwide convection which leads to progessive crystal suspension 2 cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma and 3 eventual buoyancy driven crystal liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap this mechanism implies that many if not most magma bodies are characterized by interior crystallization and vigorous convection conditions which are not universally agreed upon at present given that such conditions characterize natural magma bodies fractional crystallization through crystal settling in low velocity boundary layers should be an important mechanism of fractional crystallization in a crystallizing and convecting body of magma composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention as a given volcanic plumbing system matures with time average settling retention ratios within individual magma bodies should increase due to higher average wall rock temperatures it follows that within a given volcanic center early stage volcanism should be more likely to display fractionation generated composition gaps than later stage volcanism such a temporal evolution has been documented at at least two aleutian calc alkaline volcanic centers

domain wall dynamics in 4c pyrrhotite at low temperature summary monoclinic 4c pyrrhotite fe7s8 is ferrimagnetic due to an ordered defect structure with alternating vacancy and vacancy free sublattices its low temperature magnetic transition near 35k is characterized by the distinct increase in coercivity and remanent magnetization the increase of these parameters has been attributed to changes in the domain wall structure we present static and dynamic magnetization data of a powder sample to study the domain wall dynamics across the low temperature transition the amplitude dependent ac susceptibility and the ferromagnetic resonance spectroscopy indicate that the hardening of the domain wall pinning at the transition occurs simultaneously with the decrease in initial saturation remanent magnetization these two effects are explained by the enhanced inhomogeneity of the bulk material caused by the persistency of the ordered vacancies and by newly formed defects due to localized distortion of fe ii sites in the vacancy free sublattice the generated localized defects are the link between the domain wall dynamics and the low temperature transition in 4c pyrrhotite

the cold bay volcanic center aleutian volcanic arc the cold bay volcanic center has experienced two major stages of eruptive activity early m series acitivity produced bimodal hi alumina basalt and calc alkaline andesite lavas while later fpk series activity produced only calc alkaline andesite the spectrum of basalt compositions is believed to be due to high pressure 8 kb fractionation at or near the base of the crust abundant mineralogical and geochemical evidence support a lower pressure mixing origin for all andesites inspection of the mineralogical data has shown that the earliest m series andesites were produced by mixing of basalt 53 wt sio2 and silicic andesite 60 5 to 62 5 wt while later fpk series andesites resulted from the mixing of basaltic andesite 53 to 56 wt and less silicic andesite 58 5 to 60 0 wt the major element and trace element geochemical data are consistent with a low pressure fractionation origin for the silicic endmember magmas and support the temporal variations in both mafic and silicic endmember compositions the complete lack of crustal inclusions in all lavas is taken as evidence for a minimal crustal melting and or assimilation role in the origin of the silicic endmembers many of the features of all andesites including the important long term convergence of endmember magma compositions are consistent with the process of liquid fractionation accompanied by large scale magma mixing a deduced upper limit of 62 5 wt sio2 for the silicic endmember magmas suggests that liquid fractionation in the absence of major crustal melting cannot produce more silicic magmas a possible explanation is the presence of a rheological barrier based on the concept of critical crystallinity marsh 1981 which prohibits more silicic liquids from being extracted from a crystal liquid suspension

three dimensional interpretation of the ktb gravity and magnetic anomalies a combination of a closely spaced surface gravity survey with a high resolution helicopter aeromagnetic survey as well as borehole gravity and magnetometer measurements allowed a detailed three dimensional 3 d modeling of the anomalies at the ktb drill site the models could be constrained by new evidence from a 3 d seismic survey and by structural geology and petrophysical data from drill cores and cuttings the source body for the positive gravity anomaly consist of high density metabasite the vertical derivative of the bouguer anomaly does not resemble the aeromagnetic anomaly in all areas indicating that parts of the metabasites are more or less nonmagnetic surprisingly and confirming the observation in other deep drill holes into continental crystalline basement rocks pyrrhotite is the dominant magnetic mineral below a depth of about 300 m magnetite mainly occurs in the depth intervals 360 520 m and 7300 7900 m the lower interval causes the anomalous vertical gradient of 60 nt km for the geomagnetic field the occurrence of strongly magnetic minerals in the borehole down to about 3000 m correlates with the lithology while in the deeper parts it is more related to fissures and fault zones where chemical processes reduction oxidation are active