intraseasonal modulation of the north pacific storm track by tropical convection in boreal winter abstract the modulation of the north pacific storm track by tropical convection on intraseasonal time scales 30 90 days in boreal winter december march is investigated using the ncep ncar reanalysis and noaa satellite outgoing longwave radiation olr data multivariate empirical orthogonal function meof analysis and case compositing based upon the principal components pcs of the eofs reveal substantial changes in the structure and intensity of the pacific storm track quantified by vertically 925 200 mb averaged synoptic eddy kinetic energy seke during the course of a typical madden julian oscillation mjo event the storm track response is characterized by an amplitude varying dipole propagating northeastward as the center of the anomalous tropical convection moves eastward across the eastern indian ocean and the western central pacific a diagnosis of the seke budget indicates that the storm track anomaly is induced primarily by changes in the convergence of energy flux baroclinic conversi

three dimensional seismic imaging of paleogene dike fed submarine volcanoes from the northeast atlantic margin we use three dimensional seismic data to describe the architecture of shallow intrusive igneous sill and dike complexes intruded into the upper crust as well as the internal and external geometry of extrusive conical igneous mounds that were fed by the magma conduits the conical mounds accreted on the seabed directly above the tips of the basaltic dikes between 54 9 and 54 6 ma in plan view the mounds are broadly circular 1 1 7 km in diameter and have a relief of 50 300 m they occur above a complex network of linked cuspate shaped sills and dikes intruded into paleocene and cretaceous sediments of the faeroe shetland basin northeast atlantic margin the highly organized internal structure consisting of bulbous layers arranged concentrically around a central axis along with the clear vertical link to the underlying sills and dikes indicates that they most likely represent pillowed basaltic lava and hyaloclastite mounds that were fed by the dikes and extruded onto the seafloor mounds of similar external geometry have been described from sidescan sonar images above active spreading centers in oceanic settings however such structures are rarely recognized in passive margin settings and prior to this investigation our knowledge of their three dimensional internal geometry has been largely speculative

detection of miocene saucer shaped sills offshore senegal via integrated interpretation of seismic magnetic and gravity data the atlantic margin offshore senegal has been explored by seismic reflection and grav mag surveys high amplitude laterally transgressive seismic reflectors are found to coincide with gravimetric and magnetic highs once seismic data are integrated with potential fields modelling these reflectors can be safely interpreted as saucer shaped igneous sills up to some hundreds of metres thick some km wide the occurrence of hydrothermal vent complexes and forced folds in the stratigraphic sequence above the sills constrain the intrusion age to the miocene field observations and in situ magnetic susceptibility measurements of oligocene miocene and quaternary igneous rocks emplaced in coastal senegal support this interpretation

the dynamics of intraseasonal atmospheric angular momentum oscillations abstract the global and zonal atmospheric angular momentum aam budget is computed from seven years of national centers for environmental prediction data and a composite budget of intraseasonal 30 70 day variations during northern winter is constructed regressions on the global aam tendency are used to produce maps of outgoing longwave radiation 200 hpa wind surface stress and sea level pressure during the composite aam cycle the primary synoptic features and surface torques that contribute to the aam changes are described in the global budget the friction and mountain torques contribute about equally to the aam tendency the friction torque peaks in phase with subtropical surface easterly wind anomalies in both hemispheres the mountain torque peaks when anomalies in the midlatitude northern hemisphere and subtropical southern hemisphere are weak but of the same sign the picture is different for the zonal mean budget in which the meridional convergence of the northward relative angular moment

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

crustal assimilation in basalt and jotunite constraints from layered intrusions abstract to constrain the amount and rate of crustal contamination that is possible in basaltic and jotunitic magma and to gain an insight into the physical and thermal processes of assimilation in crustal magma chambers we have modelled published sr and nd isotopic data from three layered intrusions well exposed sequences of cumulates with no evidence of magma recharge provide direct records of concurrent assimilation and fractional crystallization afc the key to the modelling is that f the mass fraction of magma remaining in the chamber can be estimated from the thicknesses of the studied cumulate sequences this allows afc model curves to be fitted to the isotopic data by varying r the ratio of the rate of mass assimilated to the rate of mass crystallized the results of modelling show that r is nearly constant in 800 to 2000 m thick sequences of cumulates displaying up section decreases in anorthite content of plagioclase increases in whole rock sr 0 initial 87 sr 86 sr and decreases in whole rock e nd 0 initial e nd the r values of the layered sequences range from 0 12 in the fongen hyllingen intrusion over 0 20 in the bjerkreim sokndal intrusion to 0 27 in the hasvik intrusion the total amount of assimilation the bulk crust magma ratio reaches values of 0 08 0 19 and 0 28 at the level of the most contaminated samples after 60 to 80 crystallisation whereas the instantaneous crust magma ratio of the most contaminated magmas were respectively 0 14 0 46 and 0 70 for the three intrusions innumerable country rock xenoliths occur in the three layered intrusions and played a crucial role in the assimilation process the xenoliths spalled off the roofs of the magma chambers during magma emplacement and their initial temperature and composition relate to r in the intrusions in the hasvik intrusion r 0 27 the initial temperature of the country rocks was 450 c and the xenoliths were fusible metasediments and therefore produced a high fraction of partial melt that could be assimilated in the bjerkreim sokndal intrusion r 0 20 the country rocks were initially at temperatures of 640 880 c but included both refractory massif type anorthosite and fusible gneisses in the fongen hyllingen intrusion r 0 12 the country rocks were cooler 300 c and the xenoliths include refractory metabasalt dominant and fusible metapelite we argue that the refractory metabasalt and anorthosite xenoliths acted mainly as heat sinks resulting in reduced r values in fongen hyllingen and bjerkreim sokndal intrusions heating of refractory and fusible xenoliths and melting of fusible xenoliths absorbed sensible and latent heat of the magma energy balanced modelling shows that up to 75 of the heat available was absorbed by xenoliths within the magma chambers promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the envelope of country rocks alone the high r values reflect the amount of heat absorbed by heating and melting country rock within the magma chambers themselves and their constancy reflects the ready availability of fusible xenoliths

visible and near infrared reflectance spectroscopy of pyroxene bearing rocks new constraints for understanding planetary surface compositions 1 laboratory visible and near infrared reflectance spectra of solid rock slabs mineral separates and systematic mixtures were simultaneously investigated we apply an empirical approach to evaluate spectra in order to achieve qualitative and quantitative information we use cumulates mostly norites leuconorites melanorites and anorthosites belonging to the bjerkreim sokndal layered intrusion a sequence of genetically related rocks with simple textures laboratory spectra are measured on slightly polished rock slabs in the 350 to 2500 nm interval and directional hemispherical reflectance geometry composition is determined using traditional techniques other than reflectance spectroscopy we find that 1 band minima measured on rock spectra are strongly influenced by the concurrent effects due to modal abundance of the spectroscopically active mineral and mineral chemistry 2 band depths can be used for semiquantitative analyses limited to the set of rocks investigated 3 the spectral parameters derived from powdered pyroxene are in agreement with previously published calibrations 4 the mineral mixture systematics can be reasonably considered as linear when pyroxene is mixed with neutral components and 5 the empirical evaluation of solid rock surface spectra needs further insights to give a great improvement to planetary researches in addition genetic sequences of rocks should be investigated in detail to help the geological interpretation of planetary evolution therefore more laboratory and analytical studies are required in order to understand the influence of composition and petrographic textures on the spectral analysis

the atmospheric angular momentum cycle during the tropical madden julian oscillation abstract global scale wind and outgoing longwave radiation anomalies are documented during the atmospheric angular momentum aam cycle associated with the madden julian oscillation cross spectral and compositing techniques are applied to 150 and 850 mb national meteorological center wind data and outgoing longwave radiation olr data during ten northern winter and summer seasons coherent wind and olr signals are found in the tropics and in the winter hemisphere and are dominated by zonal wavenumbers 0 2 at the time of zero aam anomaly anomalous zonal winds are found over the western hemisphere tropics during northern winter while convection anomalies tend to be collocated with the seasonal mean australasian convection zonal mean olr anomalies are small as the aam anomaly increases the anomalous zonal winds move eastward and poleward while dipole convection anomalies become established over the oceanic warm pool and result in an east west shift of the australasian monsoon zonal mean olr ano

a strontium isotopic investigation of the bjerkreim sokndal layered instrusion southwest norway the 230 km 2 proterozoic bjerkreim sokndal layered instrusion has a monzonoritic bulk composition and comprises a 6 km thick broadly leuconoritic layered series ls overlain by unlayered mangerite and quartz mangerite in the bjerkreim lobe the ls comprises six megacyclic units mcu 0 ia ib ii 11 and iv in a syncline this lobe is surrounded by migmatitic gneisses the roof to the instrusion is missing the mg number opx an and whole rock initial 87 sr 86 sr isotope ratios sr 0 display broadly parallel trends through mcus ii and iii and into mcu iv with decreasing mg number 75 58 and an 50 40 accompanying increasing sr 0 0 7050 0 7085 this correlation breaks down in the upper part of mcu iv and mangerite the quartz mangerite has sr 0 values of 0 7085 abrupt reversals occur across the mcu boundaries the ls crystallized on the floor of a periodically replenished magma chamber that was continually assimilating country rock gneisses strong compositional zoning of the magma developed as a result of repeated replenishments with relatively dense primitive ferrobasaltic magma sr 0 0 7049 along the floor and the development of extensive buoyant roof melts assimilation took place on a massive scale up to 50 in the upper parts of the chamber

caught in the act the first few hours of xenolith assimilation preserved in lavas of the rockeskyllerkopf volcano west eifel germany article i nfo article history lithic sedimentary xenoliths in magmatic pyroclastic deposits in the rockeskyllerkopf volcanic complex of the quaternary west eifel volcanic field preserve evidence of the earliest stages of melting reaction with and assimilation into their silica undersaturated basanite host the xenoliths have a 0 5 1 5 mm thick surface glaze of rhyolitic glass and internally have both silica undersaturated and silica saturated glasses that are the result of in situ melting of micas and clay minerals and reaction of these melts with the surrounding quartz matrix mixing between the rhyolite glaze and host basanite resulted in the formation of a clinopyroxene and glass rich hybrid zone the glazed xenoliths are commonly found with either no coating of lava or with void space between them and the host this unusual occurrence is due to vesiculation of the glaze during eruption which led to physical separation of the xenolith from its surrounding melt simple calculations indicate that assimilation occurred at 1180 c over a period of 12 h and that all the textural features including deformation of the partially molten xenoliths occurred during transport and eruption rather than after deposition

a laboratory investigation of assimilation at the top of a basaltic magma chamber the fluid dynamic processes affecting the mechanism and amount of assimilation of the roof of a magma chamber have been studied in a series of analogue laboratory experiments an aqueous solution of na 2 co 3 was first cooled and crystallized at the top of a laboratory tank to form a solid roof layer of prescribed composition the tank was then drained and refilled with warmer solution either na 2 co 3 of the same or different composition or in two experiments kno 3 the metal top in all cases was held below the eutectic temperature for the system na 2 co 3 h 2 o the experiments were continued until substantial crystallization and in some cases complete solidification were achieved starting with a dense solid roof layer of na 2 co 3 10h 2 o and ice on the na 2 co 3 side of the eutectic and a warmer eutectic na 2 co 3 solution below there was rapid convective cooling of the liquid layer accompanied by melting of the roof producing a compositional

magma chamber processes in the tellnes ilmenite deposit rogaland anorthosite province sw norway and the formation of fe ti ores in massif type anorthosites the origin of igneous fe ti oxide ores associated with massif type anorthosites is investigated through a detailed study of the world class tellnes ilmenite deposit part of the late proterozoic 930 920 ma amc series of the rogaland anorthosite province sw norway more than 100 samples from drill cores reveal significant petrographical and compositional variations within the ore body four zones are defined based on variations in modal proportions and cumulus mineral assemblages the lower and upper central zones and the lower and upper marginal zones plagioclase and whole rock compositions discriminate the zones and display patterns interpreted as a result of mixing of either plagioclase ilmenite or plagioclase ilmenite orthopyroxene olivine cumulates with a melt of ferrodioritic jotunitic composition with a content decreasing from 80 to 20 from the margins to the central part of the ore body phase diagrams for a jotunitic parental magma reproduce the crystallization sequence at 5 kb the orthopyroxene olivine liquidus boundary is a peritectic in the bjerkreim sokndal layered intrusion and a cotectic in tellnes and this explains the differences in the sequence of crystallization of the two intrusions the high concentration of ilmenite well above cotectic proportions resulted from gravity sorting in the tellnes ore body which represents the lower part of a larger magma chamber uniform sr isotope ratios do not support magma mixing the cryptic layering of the ore body precludes injection as a crystal mush but favours in situ crystallization from an evolving magma in a sill like magma chamber the present trough shape and mineral orientations result from deformation during gravity induced subsidence and by up doming of the anorthosite fractional crystallization of a tio2 rich magma with ilmenite as an early liquidus mineral and plagioclase buoyancy are the principal mechanisms responsible for the formation of fe ti deposits in proterozoic massif type anorthosites

a fundamental dispute a discussion of on some fundamentals of igneous petrology by bruce d marsh contributions to mineralogy and petrology 2013 166 665 690 marsh contrib miner petrol 166 665 690 2013 again claims that crystal free basalt magmas are unable to differentiate in crustal magma chambers and regards layered intrusions as primarily due to the repeated emplacement of crystal suspensions he ignores an earlier critique of his unconventional inferences latypov j petrol 50 1047 1069 2009 as well as a wealth of petrographic geochemical and experimental evidence supporting the dominant role of fractional crystallization in the solidification of layered intrusions most tellingly the cryptic variations preserved in the skaergaard and many other basaltic layered intrusions would require an exceedingly implausible sequence of phenocrystic magmas but are wholly consistent with in situ fractional crystallization a major flaw in marsh s hypothesis is that it dismisses progressive fractional crystallization within any magma chamber and hence prohibits the formation of crystal slurries with phenocrysts and melts that change systematically in composition in any feeder system this inherent attribute of the hypothesis excludes the formation of layered intrusions anywhere

the crustal tongue melting model and the origin of massive anorthosites recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their emplacement at mid crust levels recent experimental data have shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams at 10 13 kbar indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40 50 km depths in the sveconorwegian province terne boundaries have been traced in deep seismic profiles to moho offsets or to tongues of lower crustal material underthrust to depths higher than 40 km in southern norway we suggest that a lithospheric scale weakness zone the feda transition zone has channelled the rogaland anorthosites through linear delamination asthenospheric uprise and melting of a mafic lower crustal tongue

variations of atmospheric angular momentum associated with intraseasonal oscillations forced by zonally moving prescribed heating intraseasonal variations of atmospheric angular momentum aam are numerically studied with prescribed 40 day period external heating which moves eastward from 60 e to the dateline in the tropics intraseasonal oscillations are forced in a numerical model in accordance with the oscillations the variations of aam also show 40 day periods the phase relationship between the model intraseasonal oscillations and aam variations is similar to that observed further aam exhibits the largest variation in the equatorial region and a phase propagation from equatorial regions to the subtropics and from high latitudes to the subtropics as found in the real atmosphere after careful inspection of these features it was clarified that the source region for intraseasonal variations of aam is not the equatorial region but the subtropics also the equatorial region is an apparent or superficial source region since aam is transported from the subtropics in accordance with the propagation of rossby wave trains to the subtropics the phase propagation of aam from high latitudes to the subtropics can also be explained from the behavior of rossby wave trains in the midlatitudes radiating from the equatorial region thus rossby wave trains associated with intraseasonal oscillations play an important role in intraseasonal variations of aam

assimilation of crustal xenoliths in a basaltic magma chamber sr and nd isotopic constraints from the hasvik layered intrusion norway contaminated layered intrusions known thousands of recrystallized strontium and neodymium isotopic data for mafic cumulates chilled tabular xenoliths of metasedimentary origin enclosed in the cumulates margins and adjacent crustal rocks of the hasvik layered intrusion are thought to represent the remnants of the assimilated material north norwegian caledonides are reported together with new the xenoliths spalled off the roof during magma emplacement and mineralogical and whole rock analytical data to constrain the extent together with the elevated temperatures 400 600 c of the midand effect of the assimilation of crustal xenoliths in a basaltic crustal country rocks led to a high degree of assimilation in the magma chamber initial sr sr 700 ma of 0 7045 and end hasvik magma chamber 700 ma of 3 03 for the chilled margin which has a tholeiitic composition akin to the chilled rocks of the skaergaard intrusion demonstrate that the parental magma was derived from a depleted mantle source the basal cumulates 0 335 m show an up section decrease in sr sr from 0 7045 to 0 7038 and a correlative

geochemical evidence for mixing between fluids exsolved at different depths in the magmatic system of mt etna italy abstract a 4 year geochemical survey of some fumaroles at the voragine summit crater of mt etna was performed in combination with synchronous monitoring of peripheral gas emissions at the base of the volcano this was the first geochemical study at mt etna to have included the abundances of ar he and c isotopes once the effects of postmagmatic shallow processes were identified and quantitatively removed the he ar co 2 systematics of the voragine crater fumaroles and peripheral gas emissions described the same degassing path combining the carbon isotope composition with information about noble gases provided evidence that the crater fumaroles are fed from a two endmember mixture composed of a deep member coming from pressures between 200 and 400 mpa depending on time and a shallower one exsolved at 130 mpa similar mixing processes probably also occur in gases from peripheral vents the simultaneous assessment of 13 c co2 and he ar values of crater fumaroles over time has identified simple changes in the mixing proportion between the two endmembers and moreover periods during which the exsolution pressure of the deep fluid increased these periods seem to be linked to pre eruptive phases of the volcano the identified open system degassing processes are indicative of efficient bubble melt decoupling at depth whereas the mixing process requires a convective transfer of the deeply exsolved fluids toward shallower levels of magma where further vapor is exsolved in agreement with the most recent geophysical and petrological data from mt etna these observations allow inferences about a deep portion of the plumbing system 5 to 12 km b s l comprising sill like reservoirs connected by small vertical structures and a main reservoir at 2 3 km b s l that is probably fluxed by magmatic volatiles

trapped intercumulus liquid in the main zone of the eastern bushveld complex south africa lateral variations in the amount of trapped intercumulus liquid in the main zone of the eastern bushveld complex are constrained by new sr isotopic whole rock and mineralogical data from three profiles that are separated laterally by ca 100 km and represent thicknesses of 551 1 127 m of main zone gabbronorites below the pyroxenite marker an analysis of the an contents 100 ca ca na of plagioclase cores within the thornhill north roossenekal and stoffberg south profiles show similar systematic up section variations from an66 to an59 in contrast both the an contents of bulk plagioclase separates and the mg numbers 100 mg mg fet of orthopyroxenes show pronounced lateral variations from thornhill an67 61 mg 67 61 through roossenekal an64 58 mg 64 55 to stoffberg an59 55 mg 59 50 these mineralogical variations are interpreted to be the result of reaction between cumulus minerals and an increasing amount of trapped liquid from north to south modelling of the trapped liquid shift of orthopyroxene compositions suggests that the amount of trapped liquid in the cumulates increased from near 0 at thornhill through 10 30 at roossenekal to 30 45 at stoffberg a two to eightfold southward increase in whole rock concentrations of p ti y zr and ba is consistent with the trapped liquid model however the 14 fold increase in rb from thornhill to stoffberg is too great to be accounted for by trapped liquid alone but can be explained by local assimilation of partial melts of the country rock constant initial 87sr 86sr isotopic ratios of main zone plagioclase separates 0 7081 0 7085 in all the three profiles do not preclude assimilation of adjacent basaltic to rhyolitic country rock with initial ratios between 0 6924 0 7096 the southward increase in the amount of trapped liquid is ascribed to an increased cooling rate by enhanced heat loss and partial assimilation of country rock xenoliths in the distal cumulate sequence at stoffberg

two lips and two earth system crises the impact of the north atlantic igneous province and the siberian traps on the earth surface carbon cycle the links between the siberian traps and the end permian mass extinction and between the north atlantic igneous province naip and the paleocene eocene thermal maximum petm demonstrate a critical role for large igneous provinces lips in the disruption of the earth surface carbon cycle escc high precision ages for both volcanic provinces and the associated environmental crises show that in both cases the crisis was contemporaneous with the volcanism the naip comprises two phases the earlier phase 1 c 61 ma and the much more voluminous phase 2 c 56 ma linked to the opening of the ne atlantic the latter triggered the petm the largest cenozoic hyperthermal the siberian traps are significantly more voluminous than the naip and triggered the end permian mass extinction the masses of volcanic co 2 emitted from these provinces may have been much greater than previously suggested as substantial gas may come from intrusive bodies deep within the crust precursory warming due to the accumulation of volcanic co 2 in the atmosphere likely triggered the release of low 13 c methane hydrate although the masses of methane hydrate alone may have been insufficient to account for the observed temperature rises the organic c was likely strongly supplemented by magmatically derived carbon and thermogenic carbon released during emplacement of sills and dykes into c rich sedimentary units more data are required on the volcanic flux rates in order to refine the cause effect relationships between lips and the escc

deformation structures associated with the trachyte mesa intrusion henry mountains utah implications for sill and laccolith emplacement mechanisms abstract deformation structures in the wall rocks of igneous intrusions emplaced at shallow crustal depths preserve an important record of how space was created for magma in the host rocks trachyte mesa a small oligocene age intrusion in the henry mountains utah is composed of a series of stacked tabular sheet like intrusions emplaced at 3 3 5 km depth into sandstone dominated sedimentary sequences of late palaeozoic mesozoic age new structural analysis of the spatial distribution geometry kinematics and relative timings of deformation structures in the host rocks of the intrusion has enabled the recognition of distinct pre syn and late stage emplacement deformation phases our observations suggest a two stage growth mechanism for individual sheets where radial growth of a thin sheet was followed by vertical inflation dip slip faults formed during vertical inflation they are restricted to the tips of individual sheets due to strain localisation with magma preferentially exploiting these faults initiating sill sheet climbing the order in which sheets are stacked impacts on the intrusion geometry and associated deformation of wall rocks our results offer new insights into the incremental intrusion geometries of shallow level magmatic bodies and the potential impact of their emplacement on surrounding host rocks

diffusive isotopic contamination of mafic magma by coexisting silicic liquid in the muskox intrusion shifts in 87 sr 86 sr and 143 nd 144 nd ratios measured in cumulates from the upper levels of the muskox mafic intrusion indicate that isotopic and bulk chemical exchange were decoupled across a mafic silicic liquid interface during crystallization of the intrusion modeling of diffusive exchange between liquid layers demonstrates that isotopic compositions of silicate liquids in layered magma chambers may be strongly affected by this process on time scales of 10 3 to 10 4 years diffusive contamination can be used to place constraints on the physical processes and time scales of magmatic systems

early cenozoic saucer shaped sills of the faroe islands an example of intrusive styles in basaltic lava piles abstract the early cenozoic igneous activity of the north atlantic igneous province that generated widespread sill complexes in sedimentary basins at the nw european margins also generated various intrusive systems in the contemporaneous basaltic lava pile of the faroe islands the faroe island basalt group comprises seven formations with a total thickness of about 6 5 km of which four major formations are built up of tholeiitic lava flows each being several hundred metres thick and three thinner formations are mostly built up of volcaniclastic lithologies each being a few metres to a few tens of metres thick the largest sills are exposed as partly saucer shaped bodies in the three uppermost formations where inner gently dipping basal sill sections gradually give way to more steeply inclined discordant outer rims that commonly cut several hundred metres into overlying lava flows numerous subvertical and moderately inclined dykes ranging in thickness from c 0 5 to c 4 0 m intersect the areas affected by sill intrusion but only inclined dykes or sheets have been positively identified as sill feeders locally controlled rotations of least principal stress axes 3 during initial sill intrusion or propagation may have been an important contributing factor in determining the overall geometry of the investigated intrusions

possible feedback path for low frequency atmospheric oscillations abstract equatorial low frequency oscillations with periods of 1 2 months are being intensively studied by many investigators a strong equatorial dipole pattern is observed in which atmospheric variables such as temperate wind and pressure are out of phase between the indian ocean indonesia region and the western pacific while it is generally thought that the oscillations make a complete circuit around the earth from their excitation region in the equatorial indian ocean western pacific the signal is more difficult to observe over the south america atlantic africa sector using analyses of four year of satellite derived microwave radiance data evidence is presented here for the possibility of a feedback route in the southern hemisphere the observed propagation path extends from the central equatorial pacific across lower south america and heads equatorward after passing south of africa the response route finally reenters the equatorial indian ocean with the correct phase to enhance the primary e

implications of simple flexure theory for the formation of saucer shaped sills saucer shaped dolerite sills have been identified in several sedimentary basins they are characteristically sub circular in plan with an inner sill connected to a steeply inclined arcuate sheet extending upwards from the edge of the inner sill and in some cases have raggedly developed flat outer rims by applying elastic plate theory to the overburden we obtain an expression for the maximum radius of a circular sill in terms of emplacement depth excess magma pressure elastic constants of the overburden and the tensile strain at failure this expression predicts that the radius of the flat base of a saucer shaped sill increases with emplacement depth as observed in the western karoo basin south africa some saucer shaped sills display an elliptical morphology from consideration of the flexural strains around the periphery of a horizontal elliptical sill we infer that saucer shaped elliptical sills have been fed from below by feeder dykes oriented along the major axis of the ellipse flat outer rims would have developed at the final stage of each intrusive episode when the excess magma pressure was decreasing towards equilibrium with the overburden load consequently flexural strains would also have decreased so continued intrusion might preferentially occur along a bedding plane

experiments on the evolution of laccolith morphology in plan view abstract the ultimate shape of a laccolith is the result of competition between internal magmatic phenomena e g overpressure and the process of solidification and the state of the external host rock e g thickness of the overburden the presence of faults and regional tectonic stresses intrusion morphology can be utilized as an inferential tool to better understand emplacement dynamics and history one specific shape aspect rarely considered is the intrusion s ellipticity which is the ratio of its long to short axes in plan view here with a series of simple analog experiments that integrate the effects of flow along pre existing fractures and solidification during emplacement an assessment is performed of the existing models for formation of a laccolith s plan morphology i e growth dependent and feeder dependent the results of these experiments suggest that neither model fully captures the behavior of laccolith ellipticity evolution and an updated model of formation that takes into account the control of the feeder dike homogenization of overburden bending moments and the processes involving solidification is presented furthermore experimental results are compared with ellipticity measurements of natural laccoliths there are proportionately more high ellipticity laccoliths in nature than in experiments this is interpreted as an induced morphological effect by magma viscosity crustal heterogeneities and or incremental assembly

roof assimilation at fast spreading ridges an investigation combining geophysical geochemical and field evidence 1 the role of assimilation at the roof of the axial magma chamber amc at fast spreading mid ocean ridges is investigated using field geophysical and geochemical data field observations from ophiolites indicate that roof assimilation is a common process but provide little constraint on the amount of material assimilated arguments based on geophysical data that include the subsidence of the layer 2a 2b boundary across the width of the amc and the depth of large axial summit troughs require at least 50 100 m of assimilation however significant variations in the depth of the amc on the east pacific rise epr over short along axis distances suggest up to 500 m fluctuations in the depth of the amc may be common on short timescales geochemical modeling of the over enrichment of chlorine in epr basalts suggests that 20 of the oceanic crust may go through a cycle of crystallization alteration then assimilation this is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation assimilation most likely occurs on a number of different timescales ranging from decadal due to perturbations in the thermal regime related to diking to longer period fluctuations controlled by magma supply from the mantle large scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation and for the interpretation of the geochemistry of the oceanic crust

testing the validity of the petrological hypothesis no phenocrysts no post emplacement differentiation an extravagant hypothesis no phenocrysts no post emplacement differentiation has been put forward by marsh in a series of papers for the development of mafic ultramafic intrusions this hypothesis is based on an assertion that the majority of these intrusions are structureless and undifferentiated because they lack residual granitic rocks to explain this the hypothesis postulates that phenocryst free magmas are not able to differentiate in crustal chambers because all evolved interstitial liquid is locked in solidification fronts producing compositionally uniform magmatic bodies layered well differentiated intrusions are attributed to the successive emplacement of magma pulses with phenocrysts of different phase and chemical compositions rather than to slow magma cooling and fractional crystallization as conventional models imply such phenocryst laden magma pulses are supposed to be derived from an underlying magmatic mush column however structureless undifferentiated mafic ultramafic bodies simply do not exist in nature all wellstudied mafic ultramafic bodies with or without residual granitic rocks that crystallized from parental magmas of non eutectic composition tend to reveal clear evidence of internal compositional differentiation in terms of crystallization sequences e g ol opx opx pl opx pl cpx mineral compositions e g an in plag en in cpx and compatible incompatible major and trace element geochemistry e g mg number cr rare earth elements this is especially evident in layered intrusions that represent the key evidence against the hypothesis essentially by denying the ability of magma to differentiate in intrusive bodies the hypothesis forbids magmatic differentiation in any sub chamber related to the entire magmatic mush column as a result magma pulses in which phenocrysts progressively change in composition cannot be derived from the column to form layered intrusions the hypothesis is thus contradictory baseless and fundamentally flawed it should be abandoned in favour of a classical fractional crystallization model based on the pioneering experiments of bowen and amply confirmed over almost a century by subsequent studies of layered intrusions the classical model was is and will most probably remain the best explanation for the origin of differentiated magmatic bodies

geometry and growth of sill complexes insights using 3d seismic from the north rockall trough doleritic sill complexes which are an important component of volcanic continental margins can be imaged using 3d seismic reflection data this allows unprecedented access to the complete 3d geometry of the bodies and an opportunity to test classic sill emplacement models the doleritic sills associated with basaltic volcanism in the north rockall trough occur in two forms radially symmetrical sill complexes consist of a saucer like inner sill at the base with an arcuate inclined sheet connecting it to a gently inclined commonly ragged outer rim bilaterally symmetrical sill complexes are sourced by magma diverted from a magma conduit feeding an overlying volcano with an elongate concave upwards trough like geometry bilaterally symmetrical sills climb away from the magma source from which they originate both sill complex types can appear as isolated bodies but commonly occur in close proximity and consequently merge producing hybrid sill complexes radial sill complexes consist of a series of radiating primary flow units with dimensions up to 3 km each primary flow unit rises from the inner saucer and is fed by primary magma tube primary flow units contain secondary flow units with dimensions up to 2 km each being fed by a secondary magma tube branching from the primary magma tube secondary flow units in turn are composed of 100 m scale tertiary flow units a similar branching hierarchy of flow units can also be seen in bilaterally symmetrical sill complexes with their internal architecture resembling an enlarged version of a primary flow unit from a radial sill complex this branching flow pattern as well as the interaction between flow units of varying orders provides new insights into the origin of the structures commonly seen within sill complexes and the hybrid sill bodies produced by their merger the data demonstrate that each radially symmetrical sill complex is independently fed from a source located beneath the centre of the inner saucer grows by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature these features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening the branching hierarchy of flow units within bilaterally symmetrical sill complexes is broadly similar to that of primary flow units within a radially symmetrical sill complex suggesting that the general features of the laccolith emplacement model also apply

some phase equilibrium constraints on the origin of proterozoic massif anorthosites and related rocks experimental data in the range of 1 bar to 13 kbar enable us to introduction map the liquidus equilibria relevant to proterozoic massif anorthere is hardly a consensus on the petrogenesis of thosites and related mafic rocks massif anorthosites are widely proterozoic massif anorthosites but many investigators believed to have formed by accumulation of plagioclase into highbelieve in a petrogenetic scheme that involves at least al basaltic liquids mantle derived basaltic liquids fractionating two major stages 1 extensive crystallization asat pressures sufficiently high 10 13 kbar to crystallize the highly similation of a mantle derived magma ponded at or aluminous orthopyroxene megacrysts typically observed in anorthosite near the base of the crust that produces suspensions of massifs reach plagioclase saturation at low normative silica contents plagioclase in fe rich high al gabbroic liquids followed peritectic like equilibria e g liq opx plag cpx sp by 2 intrusion of these suspensions into the mid upper and a thermal divide on the plagioclase pyroxene liquidus surface crust where they form complex magma chambers e g ensure that mantle derived liquids become nepheline normative with duchesne 1984 emslie 1985 ashwal 1993 wiebe further crystallization and crustal assimilation at depth such 1994 a key component of the scheme is the presence liquids cannot produce the full range of troctolitic noritic to trocin most massifs of high al orthopyroxene megacrysts tolitic gabbroic mineral assemblages observed in anorthosite massifs haom emslie 1975 that appear to preserve a record without extensive low pressure granite assimilation conversely the of the high pressure stage maquil duchesne 1984 array of plausible anorthosite parental liquids not only lies along longhi et al 1993 the structures of massifs vary from the trace of the plagioclase two pyroxene cotectic from 10 to 13 those that are composites of multiple diapiric intrusions kbar but also straddles the thermal divide on the plagioclase harp lake emslie 1980 rogaland duchesne 1984 pyroxene liquidus surface this condition requires mafic source to intrusive bodies with well developed layering of regions such as lower continental crust or foundered mafic plutons leucocratic troctolites and leuconorites or leucogabbros for liquids parental to massif anorthosites and associated mafic overlain by masses of anorthosite michikamau emslie intrusions 1970 laramie frost et al 1993 so there may be

assimilation and fractionation in adjacent parts of the same magme chamber vandfaldsdalen macrodike east greenland the vandfaldsdalen macrodike which lies in the skaergaard region of east greenland is a remarkably zoned fossil magma chamber with a granophyric cap overlying cumulate gabboros the intrusion is distinctly bimodal with a large compositional discontinuity at the contact between the gabbro and granophyre although the exposed part of the macrodike is in contact with tertiary basalts and sediments the granophyre originated by assimilation of xenoliths derived from the underlying archean basement sr and nd isotopic ratios throughout the cumulate sequence are remarkably similar indicating insignificant contamination of the gabbro by the granophyre modelling of the compositional effects of cooling and crystallization indicate that the cumulate pile resulted from fractional crystallization with the complicating effects of trapped liquid and post cumulus fractionation the uppermost rocks in the mafic part of the chamber sio2 62 feo 12 4 resulted from about 85 fractional crystallization a transgressive sill of strongly fractionated magma sio2 67 feo 8 8 formed from extracted intercumulus liquid that was the result of 90 fractional crystallization of the original magma mass balance indicates that typical granophyre is made up of about 75 dissolved xenoliths by weight and 25 mantle derived basaltic magma the magmas were not measurably affected by material exchange across the interface between the gabbro and granophyre this magma chamber evolved by both assimilation and fractional crystallization but the residual liquids formed by fractional crystallization were unaffected by assimilation heat exchange between were unaffected by assimilation heat exchange between the two parts of the chamber was obviously important but there was insignificant material exchange the inability of fractional crystallization and assimilation to affect the same liquid is related to the dynamic behavior of this particular magma chamber particularly the buoyancy of granophyre relative to evolving tholeiitic magma

the latitude height structure of 40 50 day variations in atmospheric angular momentum abstract quasi periodic variations in the relative angular momentum of the atmosphere on time scales of around 40 50 days have been observed by langley et al 1981 a description of the two dimensional latitude height structure of the winds responsible for these changes is constructed here from five years of nmc twice daily global analyses using cross spectral and amplitude phase eigenvector techniques we find these variations are associated with wavelike motions in the tropical upper troposphere that propagate and downward in phase within the tropics a coherently connected midlatitude northern hemisphere component is also present whose phase is essentially independent of height we believe the tropical component to be the zonally averaged part of the motions described by madden and julian 1971 1972 the northern hemisphere midlatitude component may be a direct response to the tropical motions or both motions may be the common response to an as yet unidentified tropical forcing

re os isotopic evidence for a lower crustal origin of massif type anorthosites massif type anorthosites are large igneous complexes of proterozoic age they are almost monomineralic representing vast accumulations of plagioclase with subordinate pyroxene or olivine and fe ti oxides the 930 myr old rogaland anorthosite province in southwest norway1 represents one of the youngest known expressions of such magmatism the source of the magma and geodynamic setting of massif type anorthosites remain long standing controversies in precambrian geology with no consensus existing as to the nature of the parental magmas or whether these magmas primarily originate in the earth s mantle or crust at present massif type anorthosites are believed to have crystallized from either crustally contaminated mantle derived melts that have fractionated olivine and pyroxenes at depth2 or primary aluminous gabbroic to jotunitic melts derived from the lower continental crust3 here we report rhenium and osmium isotopic data from the rogaland anorthosite province that strongly support a lower crustal source for the parental magmas there is no evidence of significantly older crust in southwest scandinavia and models invoking crustal contamination of mantle derived magmas fail to account for the isotopic data from the rogaland province initial osmium and neodymium isotopic values testify to the melting of mafic source rocks in the lower crust with an age of 1 400 1 550 myr

the 1985 86 intraseasonal oscillation and the role of the extratropics abstract an intraseasonal oscillation that occurred in the 1985 86 northern winter is documented in this study the tropical convection of this event is dominated by the mixture of a standing oscillation over the maritime continent and an eastward moving feature from the indian ocean into the central pacific the time evolution of the upper tropospheric circulation patterns instead of propagating eastward along the equator as suggested in the existing composites of the intraseasonal oscillation is characterized by a series of wave patterns in the northern hemisphere and does not complete the cycle around the globe the familiar moist kelvin wave explanation for the intraseasonal oscillation receives little support from diagnosis of this event using zonal wind height field streamfunction and potential vorticity only in the lower troposphere near the date line is the convincing evidence for its existence found a scenario for the intraseasonal oscillation which is suggested by the analysis includes

determining magma flow in sills dykes and laccoliths and their implications for sill emplacement mechanisms three dimensional seismic data from the faeroe shetland basin provides detailed information on the relationships between sills dykes laccoliths and contemporaneous volcanic activity the data shows that sills are predominantly concave upwards being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet such morphologies are only partially modified by pre existing faults sills can be sourced from dykes or the steep climbing portions of deeper sills both sills and dykes can provide magma to overlying volcanic fissures and sills can be shown to feed shallow laccoliths magma flow patterns as revealed by opacity rendering suggest that sills propagate upwards and outwards away from the magma feeder as an individual sill can consist of several leaves emplaced at different stratigraphic levels and as a sill or dyke can provide magma to volcanic fissures other sills and laccoliths the data suggests that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement instead the data suggests that the presence of lithological contrasts particularly ductile horizons such as overpressured shales may permit sill formation at any level below the neutrally buoyant level

portrait of a giant deep seated magmatic conduit system the seiland igneous province abstract the seiland igneous province sip northern norway contains 5000 km2 of mafic and ultramafic intrusions with minor alkaline carbonatite and felsic rocks that were intruded into the lower continental crust at a depth of 25 to as much as 35 km the sip can be geochemically and temporally correlated to numerous dyke swarms throughout scandinavia at 560 610 ma and is linked to magmatic provinces in w greenland and ne america that are collectively known as the central iapetus magmatic province cimp revised mapping show that the sip exposes 85 90 layered tholeiitic alkaline and syeno gabbros 8 10 peridotitic complexes 2 5 carbonatite syenite and diorite that formed within a narrow gravimetric data implies an average thickness of igneous rocks of 4 5 km and also features six deep lithospheric roots of ultramafic rocks extending min 9 km into the crust together the root structures represent the main volcanic conduits conveying thousands of km3 of mafic ultramafic melts from the asthenosphere to the lithosphere the ultramafic complexes were predominantly emplaced into the layered gabbros at four major igneous centres respectively nordre brumandsfjord melkvann kvalfjord and reinfjord all complexes are situated in a right way up position and are steep sided forming large plugs a marginal hybrid zone forms at the contact with country rock and transitions gradually from olivine mela gabbro over pyroxenites that grades in to an olivine clinopyroxenite zone which is followed by a wehrlite zone and finally the centre of the complexes comprises pure dunite from pyroxenite to dunite olivine changes from fo72 to fo85 and clinopyroxene from di80 to di92 i e the complexes observe a reverse fractional crystallisation sequence with time parental melt compositions modelled from early dykes indicate komatiitic to picritic melts with 16 22 wt mgo cr of 1594 ppm and ni of 611 ppm which were emplaced at 1450 1500 c melt compositions calculated from clinopyroxene compositions from reinfjord are oib like with lree enriched over hree the high abundance of carbonatites and lamproites demonstrates the volatile rich nature of the mantle source region and is further corroborated by the unusually high abundance of magmatic sulphides 0 5 1 and carbonated and hydrous assemblages c 1 throughout the region in reinfjord they are also closely associated with pge cu ni reef deposits essentially the ultramafic complexes in the sip comprises deep seated transient magma chambers that facilitated mixing and homogenisation of a rich diversity of fertile asthenospheric melts en route to the upper parts of the continental crust

high temperature metamorphism and the role of magmatic heat sources at the rogaland anorthosite complex in southwestern norway the rogaland complex covers 1000 km in southwestern norway and consists mainly of anorthosite massifs and the layered bjerkreim sokndal lopolith bsl these rocks intrude charnockitic migmatites containing intercalated marbles and garnetiferous migmatites high temperature mineral isograds pigeonite osumilite and orthopyroxene in the metamorphic basement are subparallel to and increase in grade towards the intrusive complex p t estimates from the country rocks show a roughly linear increase in temperature towards the bsl consistent with the distribution of isograds the peak p t conditions at 20 and 2 5 km from the contact at 5 kbar range from 700 to 41000 c field relations and age determinations link the high t metamorphism and the magmatism the two dimensional thermal modelling indicates that heat from a single magmatic cooling unit is not sufficient to produce the array of isograds and the peak metamorphic temperatures two magmatic episodes separated by 3myr however can account for the high temperature metamorphism in this model the emplacement and crystallization of the anorthosite produces a regional thermal gradient from 750 to 600 c after a brief hiatus a second smaller body bsl provides an additional thermal input that results in an array of high temperature isograds and country rock temperatures 41000 c

time scales of magmatic processes new insights from dynamic models for magmatic assimilation we present a computational model that predicts the consequences of time dependent isenthalpic assimilation fractional crystallization afc our model combines an existing paradigm for fractional crystallization based on equilibrium thermodynamics with a kinetic model for predicting rates of mineral dissolution the isenthalpic constraint directly links the sensible heat of the magma and the latent heat of crystallization to the energy necessary for assimilation thus we predict liquid lines of descent rates of crystallization rates of cooling and ratios of assimilation to crystallization r as a function of time model simulations predict the time scale of afc processes weeks to years and show the extent to which temperature and composition of the assimilant control these time scales

a hybrid composite dike suite from the northern arabian nubian shield southwest jordan implications for magma mixing and partial melting of granite by mafic magma abstract the arabian nubian shield is an exemplary juvenile continental crust of neoproterozoic age 1000 542 ma the post collisional rift related stage 610 to 542 ma of its formation is characterized among others by the intrusion of several generations of simple and composite dikes this study documents a suite of hybrid composite dikes and a natural example of partial melting of granite by a mafic magma from the northernmost extremity of arabian nubian shield in southwest jordan the petrogenesis of this suite is discussed on the basis of field petrographic geochemical and rb sr isotopic data these dikes give spectacular examples of the interaction between basaltic magma and the granitic basement this interaction ranges from brecciation partial melting of the host alkali feldspar granite to complete assimilation of the granitic material field structures range from intrusive breccia angular partially melted granitic fragments in a mafic groundmass to the formation of hybrid composite dikes that are up to 14 m in thickness the rims of these dikes are trachyandesite latite with alkali feldspar ovoids up to 1 cm in diameter while the central cores are trachydacite to dacite and again with alkali feldspar ovoids and xenoliths from the dike rims the granitic xenoliths in the intrusive breccia have been subjected to at least 33 partial melting a seven point rb sr isochron from one of these composite dikes yields an age of 561 33 ma and an initial 87 sr 86 sr ratio of 0 70326 0 0003 2 and mswd of 0 62 geochemical modeling using major trace rare earth elements and isotopes suggests the generation of the hybrid composite dike suite through the assimilation of 30 to 60 granitic crustal material by a basaltic magma while the latter was undergoing fractional crystallization at different levels in the continental crust

reappraisal of uranium series isotope data in kamchatka lavas implications for continental arc magma genesis uranium series isotopes can be used to determine constraints on the timescale of slab dehydration and melt production at subduction zones however interpretations of u th ra data suggest very different timescales of slab dehydration here we present new u th ra data from kamchatka along with a number of alternative models for production of radioactive disequilibrium variations in 226ra 230th and 231pa 235u activity ratios are best explained by crystal fractionation with host rock assimilation for a duration of less than c 6000 years the association of the largest 226ra excesses with high sr th in the most primitive lavas suggests that ra th fractionation is controlled by slab dehydration less than 10 ka ago we show that u th data can be explained by dynamic melting of a recently ka metasomatized mantle wedge dynamic melting of an oxidized source metasomatized several hundreds of thousands of years ago cannot produce significant 231pa excess because 238u 230th disequilibrium is inferred to be controlled by partial melting there is no requirement for multi stage slab dehydration commencing 150 ka we suggest that ra th disequilibria constrain the timing of slab dehydration whilst u th fractionation is dominated by partial melting at least at the kamchatka arc

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the

the bjerkreim sokndal layered intrusion rogaland sw norway evidence from marginal rocks for a jotunite parent magma abstract the late proterozoic bjerkreim sokndal layered intrusion bksk consists of andesine anorthosite leuconorite troctolite norite gabbronorite jotunite mangerite quartz mangerite and charnockite the sequence of appearance of cumulus minerals and their compositions suggest a parent magma that was evolved had plagioclase olivine on the liquidus was sufficiently tio2 rich for hemo ilmenite to crystallise early and low in cao and cao al 2 o 3 compared to basalts as reflected by the sodic plagioclases and the delayed appearance of cumulus augite fine to medium grained jotunites found along the northern contact of the bksk consist of plagioclase an45 53 inverted pigeonite mg 55 50 sparse augite mg 69 59 fe ti oxides k feldspar quartz and apatite they are basic to intermediate rocks with relatively high feototal high tio2 low mgo mgo feo moderate al2o3 and low cao and normative diopside the jotunites have compositions that are consistent with the parental magma for the lower part of the bksk layered series and are interpreted as being marginal chills similar but slightly more differentiated jotunite magmas were subsequently emplaced into the bksk and the surrounding region as broad dykes and small plutons jotunite is a minor rock type in most massif type anorthosite provinces but may have an important petrological significance

assimilation of felsic crust by basaltic magma thermal limits and extents of crustal contamination of mantle derived magmas basaltic magma is the most widely used probe of mantle chemistry and structure chemical characterization of the sources for mantle derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks the most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization afc because of the energetic link between these processes predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma country rock system using the melts software package we modeled isenthalpic heat balanced afc between two common types of basaltic magma and several types of felsic crust at 1 kbar our simulations show that during the early stages of isenthalpic afc the ratio of rates of assimilation to crystallization r may be substantially greater than one 2 0 2 7 allowing assimilation of a mass of country rock up to 5 18 of the initial magma mass with only 3 7 crystallization the second stage of afc beginning with plagioclase and or pyroxene saturation is characterized by lower r 0 5 1 0 the initial high r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses under certain conditions even small degrees of crystallization of olivine alone coupled with cooling of the magma can accommodate relatively large amounts of crustal assimilation and cause large shifts in isotopic and trace element geochemical indices with little differentiation

a sr nd and pb isotopic investigation of the transition between two megacyclic units of the bjerkreim sokndal layered intrusion south norway abstract the bjerkreim sokndal layered intrusion forms part of the proterozoic south rogaland igneous complex of southwest norway the intrusion has been divided into two parts an anorthositic to gabbro noritic layered series and a generally more massive mangeritic quartz mangeritic to charnockitic upper part on the basis of repeated lithological sequences the layered series has been divided into a number of megacyclic units which are interpreted as reflecting magma chamber replenishment initial 940 ma sr nd and pb isotope ratios for plagioclases separated from samples crossing the transition between megacyclic unit iii and megacyclic unit iv range from 87 sr 86 sr i 0 7048 143 nd 144 nd i 0 51156 end 2 6 and 206 pb 204 pb i 17 50 to 87 sr 86 sr i 0 7061 143 nd 144 nd i 0 51143 end 0 1 and 206 pb 204 pb i 17 73 the isotopic compositions vary systematically with stratigraphic height and the interplay between the three isotopic systems is used to divide the stratigraphic profile into two parts replenishment and post replenishment across the replenishment interval isotopic variations are consistent with mixing between a resident and a replenishing magma the replenishing magma was characterized by 87 sr 86 sr i 0 7048 143 nd 144 nd i 0 51156 end 2 6 and 206 pb 204 pb i 17 50 the positive end of this magma indicates that its source region was either in the mantle or was itself recently derived from the mantle in contrast the resident magma had 87 sr 86 sr i 0 7061 143 nd 144 nd i 0 51143 end 0 1 and 206 pb 204 pb i 17 73 indicating that it had assimilated crustal material in addition the isotopic correlations across the replenishment interval indicate that prior to mixing the concentrations of nd and pb in the resident magma had been strongly reduced probably by fractionation of apatite above the replenishment interval nd and pb isotopic variations are apparently decoupled from sr isotopic variations two mechanisms are proposed for achieving this the first is heterogeneous mixing of up to 20 of a third magmatic component into an already isotopically stratified magma chamber the second is the presence of 10 20 of post cumulus plagioclase that crystallized from a migrating intercumulus melt

understanding re os systematics and model ages in metamorphosed archean ultramafic rocks a single mineral to whole rock investigation abstract 187rhenium 187os model ages are used to infer the timing of crust mantle differentiation events and evolution of lithospheric mantle however ancient samples often have long and complex histories of metamorphism metasomatism and deformation that may be problematic if these processes caused disturbance the re os system such processes have been shown to disturb the re os systematics of ophiolitic chromitites but the effect on stratiform chromitites has not previously been investigated here we present a detailed petrographic and re os isotopic study of chromitites chromite bearing meta peridotites and single chromite grains from the ujaragssuit nun t layered ultramafic intrusion southwest greenland which has experienced intense deformation and at least two episodes of metamorphism up to amphibolite facies we report the first ever re os isotope and platinum group element concentration data for single chromite grains achieved by single grain dissolution and isotope dilution micrometric os ir ru sulphide and sulpharsenide inclusions in chromite occur in the massive chromitites these accessory phases host a significant portion of the os ir and ru in these samples the remaining os ir ru budget along with pt and pd appears to be homogeneously distributed within the chromite occurring either in the lattice itself as evenly distributed sub micrometric alloy inclusions in chromite unresolvable using the techniques applied in this study or a combination of both rhenium is hosted in silicates as predicted by previous studies both single grain and whole rock re os isotope systematics yield unfeasibly young re os model ages due to their radiogenic 187os 188os compositions this could result from country rock contamination of the original melt from which the ujaragssuit nun t intrusion crystallised and or from disturbance of the re os isotope systematics of these rocks during regional metamorphic events at 2 8 and or 3 75 ga we conclude that it is vital to establish a complete sample history before interpreting re os isotope data and the validity of model ages as the re os system is sensitive to disturbance it is feasible that in some cases single chromite grains or chromite mineral separates may offer a more robust and meaningful isotopic record than silicates because re is preferentially hosted in silicates relative to chromite alternatively the 190pt 186os decay system applied to whole rock samples may provide a more reliable chronometer than 187re 187os model ages in samples that have experienced crustal contamination and or metamorphic disturbance