felsic magmatic phases and the role of late stage aplitic dykes in the formation of the world class cantung tungsten skarn deposit northwest territories canada abstract a field and petro chemical classification of felsic magmatic phases fmps at the world class cantung w skarn deposit was undertaken to document the evolution of magmatism and the relationships between different fmps metasomatism and mineralization early fmps include moderately differentiated zr hf 18 26 ti zr 14 15 biotite monzogranitic plutons and early biotite rich granitic dykes and compositionally similar quartz feldspar porphyry dykes late highly fractionated zr hf 8 17 ti zr 3 13 fmps sourced from a deeper monzogranitic intrusion include 1 leucocratic biotite or tourmaline bearing dykes derived from localized entrapments of residual magma and 2 sub vertical ne trending aplitic dykes derived from a larger segregation of residual fluid and incompatible element enriched magma the aplitic dykes have textures morphologies spatial associations and a pervasive calcic metasomatic mineral assemblage ca plagioclase quartz or clinozoisite indicative of syn mineralization emplacement very late stage overpressuring and initiation of sub vertical fractures into the overlying plutonic carapace and country rocks by supercritical magmatic fluid led to an interaction with calcareous country rocks that resulted in an increased aca2 in the fluid and the concurrent precipitation of w skarn residual magma also ascended with and quenched in equilibrium with the magmatic fluid to from the aplitic dykes then was metasomatized by the fluid as it interacted with calcareous country rocks overall highly fractionated and moderately to very highly undercooled fmps at cantung provide evidence for a large and evolving felsic magmatic system at depth that segregated and maintained a stable fluid and incompatible element enriched residual magma until the latest stages of crystallization the detailed study of fmps associated with magmatic hydrothermal mineral deposits allow us to refine our understanding of these mineralizing systems and better define metallogenic and exploration models for intrusion related mineralization

geomagnetic field for 0 3 ka 2 a new series of time varying global models 1 steadily increasing numbers of archeomagnetic and paleomagnetic data for the holocene have allowed development of temporally continuous global spherical harmonic models of the geomagnetic field extending present and historical global descriptions of magnetic field evolution the current work uses various subsets of improved data compilations details of which are given in a companion paper by donadini et al 2009 and minor modifications of standard modeling strategies using temporally and spatially regularized inversion of the data and cubic spline parameterizations for temporal variations to produce five models with enhanced spatial and temporal resolution for 0 3 ka spurious end effects present in earlier models are eliminated by enforcing large scale agreement with the gufm1 historical model for 1650 1990 a d and by extending the model range to accommodate data older than 3 ka age errors are not considered as a contribution to data uncertainties but are included along with data uncertainties in an investigation of statistical uncertainty estimates for the models using parametric bootstrap resampling techniques we find common features but also significant differences among the various models indicating intrinsic uncertainties in global models based on the currently available holocene data model cals3k 3 based on all available archeomagnetic and sediment data without a priori quality selection currently constitutes the best global representation of the past field the new models have slightly higher dipole moments than our previous models virtual axial dipole moments vadms calculated directly from the data are in good agreement with all corresponding model predictions of vadms these are always higher than the spherical harmonic dipole moment indicating the limitations of using vadms as a measure of geomagnetic dipole moments

a theoretical treatment of paleointensity determination experiments on rocks containing pseudo single or multi domain magnetic particles a phenomenological theory of thermoremanent magnetization trm is applied to model the possible results of paleointensity determination experiments using the thellier method it is shown that intrinsic curvature of the arai plot for samples containing pseudo single or multi domain particles can be attributed exclusively to the presence of remanences which are unblocked below their blocking temperature thus the thermal demagnetization tail of partial trm ptrm is not a direct measure for a sample s tendency to yield a curved arai plot here it is proposed to use the advancing decay of a ptrm t1 t2 which during heating occurs already below the lower temperature t1 t2 as a more reliable pre selection criterion in addition the effect of an average magnetostatic interaction on the thellier experiment turns out to be negligible an extended thellier method which compares the thermal demagnetization of natural remanent magnetization with that of an artificial trm according to the applied trm theory should result in a perfectly linear modified arai plot even for pseudo single or multi domain samples if no alteration or viscous effects occur

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

mesozoic evolution of nw africa implications for the central atlantic ocean dynamics the central atlantic ocean opened during the early jurassic and represents the oldest portion of the atlantic ocean although the american margin has been well studied the onshore evolution of its african counterpart is poorly understood we investigated the evolution of a c 1300 km transect across the reguibat shield morocco mauritania algeria in the northern west african craton using low temperature thermochronology fourteen samples were dated using apatite fission track analysis nine of these samples were also dated using u th sm he analysis fission track ages range from 118 10 to 497 61 ma with mean track lengths between 11 2 0 4 and 12 5 0 2 m u th sm he single grain ages range from 32 3 to 396 32 ma through forward and inverse thermal modelling we demonstrate that the craton underwent kilometre scale exhumation between the early middle jurassic and the late cretaceous based on our new results published data on nw africa and data from the conjugate eastern north american passive margin we show that this post rift early middle jurassic to early cretaceous exhumation affected both margins to a similar areal extent and simultaneously transient mantle dynamic support is suggested to account for the major erosional phase recorded on both margins supplementary material the method for thermal modelling the thermal modelling for all samples and the relationship between apatite chemistry and u th he ages are available at https doi org 10 6084 m9 figshare c 3770918

thermochemical remanence acquisition in single domain particle ensembles a case for possible overestimation of the geomagnetic paleointensity 1 single domain sd particle ensembles are the best studied systems in rock magnetism yet even for this simple system a complete theoretical treatment of the acquisition of thermochemical remanent magnetization tcrm is missing on the basis of the sd theory of chemical remanent magnetization it is generally taken for granted that tcrm acquisition must be less efficient than acquisition of a thermoremanent magnetization trm and that consequently the paleofield intensity is inevitably underestimated when a tcrm is interpreted as a trm here it is shown that this conclusion holds only when a continuous chemical change e g grain growth takes place during remanence blocking in cases where an initial trm at a later time and at lower temperature is modified by chemical processes the final tcrm can result in considerable paleofield overestimation when interpreted as a trm this is demonstrated for two different scenarios first partial grain dissolution after trm acquisition and second low temperature oxidation these mechanisms are not unlikely to occur in nature and numerical experiments indicate that both would lead to perfectly straight arai plots which makes such sd tcrms indistinguishable from true trms in the thellier experiment

exponential decay of concentration variance during magma mixing robustness of a volcanic chronometer and implications for the homogenization of chemical heterogeneities in magmatic systems abstract the mixing of magmas is a fundamental process in the earth system causing extreme compositional variations in igneous rocks this process can develop with different intensities both in space and time making the interpretation of compositional patterns in igneous rocks a petrological challenge as a time dependent process magma mixing has been suggested to preserve information about the time elapsed between the injection of a new magma into sub volcanic magma chambers and eruptions this allowed the use of magma mixing as an additional volcanological tool to infer the mixing to eruption timescales in spite of the potential of magma mixing processes to provide information about the timing of volcanic eruptions its statistical robustness is not yet established this represents a prerequisite to apply reliably this conceptual model here new chaotic magma mixing experiments were performed at different times using natural melts the degree of reproducibility of experimental results was tested repeating one experiment at the same starting conditions and comparing the compositional variability we further tested the robustness of the statistical analysis by randomly removing from the analysed dataset a progressively increasing number of samples results highlight the robustness of the method to derive empirical relationships linking the efficiency of chemical exchanges and mixing time these empirical relationships remain valid by removing up to 80 of the analytical determinations experimental results were applied to constrain the homogenization time of chemical heterogeneities in natural magmatic system during mixing the calculations show that when the mixing dynamics generate millimetre thick filaments homogenization timescales of the order of a few minutes are to be expected

dusty and honeycomb plagioclase indicators of processes in the uchino stratified magma chamber izu peninsula japan abstract two types of melt inclusion bearing plagioclase coexist in the uchino basalt izu peninsula japan 1 dusty plagioclase with an ab rich core mantled by a dusty zone consisting of more an rich plagioclase and fine melt inclusions and 2 honeycomb plagioclase with a central core containing larger melt inclusions the dusty plagioclase probably forms through partial dissolution of ab rich plagioclase as dusty zones truncate the concentric zonal structure of the ab rich inner core the honeycomb plagioclase is formed by skeletal growth under supercooling conditions this is indicated by the euhedral geometry of the boundary between the inner inclusion free and the outer inclusion bearing zones of the honeycomb plagioclase which has a concentric inner core bimodal distribution of the chemical compositions of phenocryst cores and assemblages in crystal clots suggest that hybridization of basaltic and andesitic magmas took place before crystallization of phenocryst rims the core rim boundaries of phenocrysts derived from the andesitic magma and the basaltic magma have a rounded and a euhedral geometry respectively this suggests the andesitic magma was heated and the basaltic magma was cooled prior to hybridization these features are consistent with the development of a thermal diffusive interface between overlying andesitic magma and underlying basaltic magma in a stratified magma chamber

dolomitization of a carbonate platform during late burial lower to middle cambrian shady dolomite virginia appalachians abstract outer platform carbonates of the lower to middle cambrian shady dolomite virginia appalachians record a lengthy and complex diagenetic history ranging from marine cementation to burial diagenesis during late paleozoic overthrusting analysis of petrographically well preserved marine cements has provided an estimate of the isotopic composition of lower cambrian marine carbonate 18o 7 5 to 6 1 pdb 13c 0 2 to 0 8 pdb 87sr 86sr 0 70869 to 0 70975 establishing a base line with which to assess later diagenetic carbonate meteoric dissolution and early calcite cement postdate marine cementation and formed during subaerial exposure the platform margin carbonates were extensively dolomitized dolomitization postdates calcite cementation and the initial development of stylolites and tectonic fractures several episodes of dolomite replacement and cementation are evident zone 1 dolomite consists of rare relict dolomite cores which were replaced and overgrown by zone 2a dolomite the dominant replacement phase zone 2a dolomite is depleted in 18o 18o 10 2 to 7 0 and enriched in 13c 13c 1 0 to 1 6 relative to marine carbonate reflecting precipitation from warm formation fluids buffered by the host carbonate uniformly low sr contents 25 to 65 ppm and nonradiogenic 87sr 86sr 0 70900 to 0 70971 also attest to pore water equilibration with the host rock replacement dolomitization was followed by widespread carbonate dissolution which formed vuggy porosity and local solution breccias the host dolomite locally is encrusted by mississippi valley type mvt occurrences of ore mineralization consisting of sphalerite galena and pyrite this major generation of ore minerals is overgrown by three generations of dolomite cement the first dolomite cement zone 2b has similar 18o composition 13o 10 3 to 7 4 but slightly depleted 13c values 13c 0 4 to 1 1 relative to zone 2a dolomite and has similar sr contents 30 to 45 ppm and 87sr 86sr ratios 0 70933 to 0 70966 zone 2b dolomite cement exhibits petrographic isotopic and geochemical affinities with zone 2a replacement dolomite implying that the two dolomite types formed from fluids of similar composition and temperature primary fluid inclusions in zone 2b dolomite indicate precipitation from warm minimum 120 to 150 c brines 23 26 wt nacl equiv these temperatures imply that the dolomitizing fluid was enriched in 18o due to extensive water rock interaction with host carbonate which also buffered the fluid 13c sr and 87sr 86sr content zones 3 and 4 dolomite cement are depleted in 18o 13o 13 8 to 11 3 relative to the previous dolomites implying a temperature increase to 150 to 210 c the late dolomite cements have 13c compositions 13c 0 7 to 0 9 ranging from those similar to the earlier dolomites to more depleted values zones 3 and 4 dolomite cement are enriched in sr 60 to 135 ppm which is radiogenic 87sr 86sr 0 71025 to 0 71445 these later cements thus record fluids that were no longer buffered by the host carbonate dolomite cements locally were overgrown by sphalerite followed by authigenic quartz and calcite cement secondary three phase fluid inclusions in quartz and calcite indicate hot 175 to 225 c saline 30 to 33 wt nacl equiv fluids temperatures and pressures interpreted from the fluid inclusions are compatible with estimated burial depths in excess of 5 km the dolomites formed during deep burial coeval with late paleozoic alleghanian age thrusting the mg required for burial dolomitization was derived from pressure solution of structurally lower platform interior dolomites that were overridden by the shady dolomite thrust sheet tectonic uplift and deformation resulted in regional fluid flow which transported this mg to the shady dolomite outer platform during burial dolomitization

geomagnetic paleointensity between 1300 and 1750 a d derived from a bread oven floor sequence in lubeck germany geomagnetic paleointensities have been determined from a single archaeological site in lubeck germany where a sequence of 25 bread oven floors has been preserved in a bakery from medieval times until today age dating confines the time interval from about 1300 a d to about 1750 a d paleomagnetic directions have been published from each oven floor and are updated here the specimens have very stable directions and no or only weak secondary components the oven floor material was characterized rock magnetically using thellier viscosity indices median destructive field values curie point determinations and hysteresis measurements magnetic carriers are mixtures of sd psd and minor md magnetite and or maghemite together with small amounts of hematite paleointensity was measured from selected specimens with the double heating thellier method including ptrm checks and determination of trm anisotropy tensors corrections for anisotropy as well as for cooling rate turned out to be unnecessary ninety two percent of the thellier experiments passed the assigned acceptance criteria and provided four to six reliable paleointensity estimates per oven floor mean paleointensity values derived from 22 oven floors show maxima in the 15th and early 17th centuries a d followed by a decrease of paleointensity of about 20 until 1750 a d together with the directions the record represents about 450 years of full vector secular variation the results compare well with historical models of the earth s magnetic field as well as with a selected high quality paleointensity data set for western and central europe

archaeomagnetic results on three early iron age salt kilns from moyenvic france variations of the earth fs magnetic field during the first millennium bc in western europe remain poorly constrained especially archaeointensity changes three salt kilns moa mob and moc sampled in moyenvic lorraine eastern france have been studied to provide new reference data each kiln has been dated by radiocarbon to originate from the early iron age or hallstatt period between viii and vth century bc rock magnetic experiments and hysteresis results suggest the predominance of pseudo single domain psd ti poor magnetite archaeomagnetic directions obtained by thermal and alternating field demagnetizations have high mean inclination close to 70 and declination between 19 and 31 a first set of classical thellier thellier experiments was conducted on 46 samples with a laboratory field almost parallel to the direction of the characteristic remanent magnetization chrm only 24 of these specimens present a linear nrm trm plot for other specimens nrm trm plots are concave up with positive ptrm checks the very large dispersion observed between the determined palaeointensity values suggests some artefacts have not been fully recognized a second set of thellier experiments was conducted on 34 sister specimens with the laboratory field applied quasi perpendicular to the chrm in these cases mineralogical evolutions during heating and chemical remanent magnetization acquisitions have been clearly recognized despite positive ptrm checks the concave up shapes of nrm trm plots appear mainly due to mineralogical alteration rather than to the presence of psd md grains for the entire set of samples the success rate of the palaeointensity determinations is very low with 80 per cent of the samples rejected nevertheless reliable mean archaeointensities have been obtained for two of the three kilns moa 80 1 14 5 et and mob 86 6 6 9 et at the latitude of paris the high field strength and the archaeomagnetic directions determined combined with previous published data provide further evidence for important changes of the earth magnetic field in europe during the first half of the first millennium bc these large variations of the geomagnetic field during the iron ages indicate that archaeomagnetism is highly suitable for dating of structures from this period

thellier paleointensity theory and experiments for multidomain grains 1 we extend theories of thermoremanent magnetization trm and partial trm ptrm in multidomain md grains to model thermal demagnetization and ptrm acquisition steps in thellier paleointensity determination because of the interleaving of zero field and in field heating cooling steps to increasing temperatures the initial state for any step is complex and theoretical modeling is more intricate than for ptrm production or thermal demagnetization separately at low to moderate temperature t trm lost exceeds ptrm regained causing convex down arai plots sagging below the ideal single domain sd line at moderate to high t ptrm acquisition outweighs trm loss as t approaches tcurie ptrm gain exactly equals trm loss and the arai plot becomes ideal when ptrms are produced perpendicular to the original trm and measured directly rather than by differencing field on and field off results there is less deviation from ideality at low to moderate t our theory agrees semiquantitatively with results for parallel and perpendicular ptrms for large md magnetites 135 m smaller md magnetites 6 and 20 m have less curved arai plots and the smallest magnetites 0 6 and 1 m have almost linear plots positive ptrm checks demonstrate that curved arai plots of md grains are reproducible unlike curved plots for rocks that alter physicochemically in the thellier experiment while negative ptrm tail checks indicate undemagnetized ptrm residuals low temperature demagnetization improves linearity only slightly practical applications of this work include using the predicted threshold t below which no net ptrm is produced in a parallel thellier experiment to screen data used for paleointensity fits straight line fits through low and medium t points in arai plots of md 135 m grains overestimated the paleofield by as much as 100 and for small pseudo single domain psd 0 6 and 1 m grains overestimated by about 25 however by using linear segments of medium to high t data with f values 0 5 it may be possible to obtain reasonable paleointensity estimates even for larger psd 6 and 20 m and md grains middle to high t fits for 0 6 m grains gave paleointensities within 4 of the correct value utilizing essentially the entire data set f 0 9 perpendicular data always gave superior linear fits orienting samples with their natural remanent magnetizations perpendicular to the laboratory field is therefore recommended for rocks containing psd and md grains however double heatings are preferable to single heatings because they allow ptrm tail checks to be carried out

melting kinetics of a plagioclase feldspar partial melting experiments on plagioclase feldspar have been carried out to investigate textures and kinetics of the melting a labradorite single crystal was heated at one atmosphere pressure and temperatures within its melting interval as a function of time so called honeycomb fingerprint or sieve textures were produced except for the runs just below the liquidus the melting was initiated by heterogeneous nucleation of melt at the surface and or interior cracks and possively dislocations of the crystal the pattern of the melt is dendritic with a few m arm spacing after the melt develops throughout the crystal the volumes of melt and residual crystal become larger and smaller respectively without changing the arm spacings the melt is homogeneous and has the approximate temperature dependent liquidus composition irrespective of the time there are compositional gradients in the residual crystal after short periods of melting the an content of the crystals increases with increasing time until it finally reaches equilibrium with the melt after several thousands minutes of heating it is concluded that the enlargement of the melt the main process of the melting is controlled by diffusion in the crystal the fact that partial melts have the composition of the equilibrium liquidus even from the first several minutes strongly suggests that the local equilibrium at the crystal liquid interface is satisfied during the melting some of the honeycomb fingerprint and sieve textures found in xenoliths and phenocrysts of sodic plagioclase in volcanic rocks would be caused by heating events such as magma mixing during which temperatures of magmas were temporarily higher than the solidus of some of the minerals

a paleoweathering profile from griqualand west south africa evidence for a dramatic rise in atmospheric oxygen between 2 2 and 1 9 bybp a core drilled near wolhaarkop in griqualand west south africa intersected highly oxidized kuruman iron formation below red beds of the gamagara formation the lateral equivalents of the kuruman iron formation in this drill hole consist largely of siderite ankerite magnetite greenalite and quartz the oxidation of the kuruman iron formation in wol 2 occurred almost certainly during weathering prior to the deposition of the gamagara formation the date of this weathering episode is bracketed between about 2 2 and 1 9 bybp by the age of the ongeluk lavas in the transvaal sequence below the unconformity and by the age of the hartley lavas in the olifantshoek group above the unconformity the ratio of iron to sio2 in the several facies of the weathered kuruman iron formation in wol 2 is nearly the same as that in their unweathered equivalents since sio2 loss during weathering was almost certainly minor the similarity of the fe sio2 ratio in the weathered and unweathered bif indicates that nearly all the feo in the kuruman iron formation was oxidized and retained as feo3 during weathering such a high degree of iron retention is best explained by an o2 content of the atmosphere or 0 03 atm at the time of weathering such an o2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt or 2 2 bybp but is consistent with the highly oxidized nature of the 1 85 by flin flon paleosol the new data suggest that po2 rose dramatically from about 1 percent pal present atmospheric level to or 15 percent pal between 2 2 and 1 9 bybp

a regional archeomagnetic model for europe for the last 3000 years scha dif 3k applications to archeomagnetic dating 1 the available european database of archeomagnetic field values and instrumental data has been used to produce a regional model for the geomagnetic field in europe for the last 3000 years from 1000 b c to 1900 a d connecting with the epoch covered by the igrf models this new model scha dif 3k constitutes an improvement with respect to the previous regional archeomagnetic model scha di 00 f which used relocated values and was only valid for the last 2000 years the new model has been obtained by least sums of absolute deviation inversion of paleomagnetic data using spherical cap harmonics for the spatial representation of the field and sliding windows in time an algorithm has been developed to jointly model the three archeomagnetic elements declination inclination and intensity the resulting model provides the direction and intensity of the earth s magnetic field over the european continent northern africa and western asia for the last 3000 years the fit to the european archeomagnetic database is more accurate than that provided by global archeomagnetic models in addition this model represents a step forward in archeomagnetic dating studies since the relocation error is avoided and can also be used to study the rapid changes of the geomagnetic field archeomagnetic jerks that have been recently proposed

oxide sulphide relationships in sodalite bearing metasomatites of the epembe swartbooisdrif alkaline province north west namibia the south eastern part of kunene intrusive complex kic namibia angola is host to volumetrically significant and economically important concentrations of sodalite in the area around swartbooisdrif north west namibia the mineralisation was formed by metasomatic exchange with carbonatites of the epembe swartbooisdrif alkaline province this process led to the breakdown of ore minerals initially present in various rock types of the kic and caused the formation of new opaque phases in the sodalite bearing metasomatites a detailed investigation of textures and chemical compositions of the fe ti oxides and sulphides has allowed evaluation of the complex ore forming processes related to the polyphase magmatic and metasomatic history of the sodalite deposit the predominant opaque phases in the various rock types of the kic are ilmenite and titano magnetite which are highly concentrated in the so called magnetite plugs it is clear from the textural evidence that most of the ilmenite and titano magnetite although of orthomagmatic origin recrystallised under subsolidus conditions conformably their respective chemical compositions and phase relations represented in the system feo 1 2fe2o3 tio2 point to re equilibration at temperatures below 600 c ilmenite and titano magnetite were affected by later deformation and decomposed by various reactions related to or outlasting the metasomatic process oxidation of ilmenite led to the formation of symplectitic aggregates of rutile and secondary magnetite carbonatisation of the fe ti oxides produced rutile and the siderite and rhodochrosite components in ankerite pyrite in part together with rutile and secondary magnetite was formed by sulphidation of the fe ti oxides conspicuous aggregates of granular or lamellar intergrowths of pyrite with hematite and or magnetite are interpreted as products of contemporaneous sulphidation and oxidation of former igneous pyrrhotite rarely observed pyrrhotite with pentlandite lamellae is probably not an igneous relic but was formed during the metasomatic event smaller amounts of chalcopyrite bornite digenite chalcocite galena ferroan siegenite millerite and polydymite testify to different cooling stages during or after metasomatism applying the phase relations in the simplified system fe co ni s o we were able to reconstruct a semi quantitative t f s2 f o2 path for the ore forming processes

origin of mantle rapakivi feldspars experimental evidence of a dissolution and diffusion controlled mechanism experiments designed to simulate the dissolution of alkali feldspar during magma mixing produced plagioclase mantles that are texturally and compositionally similar to those in some hybrid volcanic rocks in hydrous dacite melt 69 sio2 at 0 8 gpa 850 c orthoclase or93 and sanidine or30 partially dissolved and were mantled by sodic plagioclase an25 30 although plagioclase nucleated epitaxially as a thin shell on the alkali feldspar surface near the time of initial resorption plagioclase subsequently grew inward mostly in the form of parallel blades toaard the receding dissolution surface orthoclase dissolved at a rate approximately proportional to the square root of run duration indicating diffusional control plagioclase grew inward within a static boundary zone of melt that formed between the original crystal dacite interface and the dissolution surface during orthoclase dissolution this boundary zone rapidly and simultancously gained na by diffusion from dacite and lost k by diffusion into dacite ca diffused more slowly into this zone from which non feldspar species were mostly excluded plagioclase was stable where sufficient ca had diffused in that the boundary zone melt intersected the plagioclase saturation liquidus plagioclase subsequently grew toward the receding dissolution surface as the ca compositional gradient and hence the site of plagioclase saturation stepped inward crystallization of plagioclase in the form of parallel blades allowed continued diffusive exchange of melt components between the dissolution surface and the host melt bladed growth also served to maintain at blade tips proximity of plagioclase to the dissolution surface thereby apparently preserving locally a thin zone of low variance melt in natural systems mantling of alkali feldspar by plagioclase will occur in a similar manner when a p t or x are changed to induce alkali feldspar dissolution b sufficient ca is available in the host melt to drive by diffusion boundary zone melt compositions to plagioclase saturation and c temperatures are low enough to stabilize sodic plagioclase and to maintain a coherent boundary zone these reqjirements are satisfied in volcanic systems when alkali feldspar is juxtaposed during mixing with hybrid melts of dacitic composition mantled feldspars in some intrusive systems i e rapakivi granites may form by a similar dissolution and diffusion controlled mechanism textural evidence of a similar origin may be obscurred in intrusive rocks however by products of late stage magmatic and subsolidus processes

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

multidomain behavior during thellier paleointensity experiments results from the 1915 mt lassen flow abstract paleointensity experiments using the thellier method on samples of the 1915 mt lassen dacite flow yielded concave and s shaped nrm trm curves on arai plots departure of these curves from the ideal straight line corresponding to the known geomagnetic intensity of 54 t for the region in 1915 is significant the magnetic properties of these rocks change very little during heating and hysteresis experiments that suggest pseudo single domain to multidomain md magnetic grains are confirmed by microscopic observations thus the curvature appears to be due to md like behavior rather than to irreversible alteration of magnetic minerals in support of this interpretation the slope of the line through just the initial and the final point yields an average value of 52 t that is in good agreement with the known field intensity using the slope of only the lower or higher temperature points which is never advisable for persistently curved arai plots such as these would lead to estimates that are 30 80 too high or too low s shaped nrm trm curves are atypical in paleointensity studies and the usual explanation in terms of md unblocking temperatures less than blocking temperatures cannot account for the convex curvature of the higher temperature part rather the convex curvature can be explained by the predominance of remanence with unblocking temperatures greater than blocking temperatures the so called tail of ptrm acquired in the higher temperature part of the ptrm spectrum during the thellier experiment

crystallization processes of quartz in a granitic magma cathodoluminescence zonation pattern controlled by temperature and titanium diffusivity abstract quartz from a granitic pluton is found to have formed through sequential growth events under different mechanisms and crystallization temperatures which can provide new insights into magmatic processes of granitic magmas that were eventually consolidified into plutons the events were identified using 1 the description of crystal shape and occurrence 2 the study of the internal structure with cathodoluminescence cl and 3 derivation of the crystallization temperatures based on titaniq thermometry the magmatic quartz crystals from the toki granite central japan are characterized as having the following internal structures oscillatory zonation no oscillatory zonation with luminescence gradation gradational zonation and heterogeneous cl the quartz crystals with oscillatory zonation were formed in the temperature range of about 800 c to below 700 c which is referred to as oscillatory zoning temperature ozt conditions the cl zonation pattern was controlled by the temperature conditions and titanium diffusivity in the melt magma the crystallization process of quartz within the toki granite reveals the cooling processes of the granitic pluton the lithofacies with a high frequency of oscillatory zoned quartz underwent slower cooling under the ozt conditions than those in other lithofacies

new experimental approach to understanding igneous texture thin sections containing a magma like mixture of three crystalline ammonium compounds and intercrystalline melt can be prepared and observed microscopically during crystallization melting and deformation the aim of such observations is to establish links between active processes in the crystal melt mixture and the textural signatures of the processes the hope is to improve the basis for process interpretation of textures in rocks although the value of the experimental system in this regard is still unknown three unexpected processes are seen creation of growth centers for later blocky grains by isothermal decomposition of early dendritic grains migration of grain boundaries early in the crystallization history and growth of porphyroblast like grains by phase boundary migration with abundant melt nearby textural metamorphism begins in this material well before its melt present history is complete

systematic retrograde metamorphism of sillimanite staurolite schists new salem area massachusetts the new salem retrograde metamorphic zone occupies an approximately triangular area covering 26 km 2 in sillimanite staurolite grade rocks largely pelitic schists of the devonian littleton formation acadian prograde metamorphism at 6 kbar and 600 c produced the common assemblage quartz muscovite biotite garnet staurolite iunenite albite graphite sillimanite pyrrhotite systematic hydration of the prograde assemblages by an influx of h 2 o produced the retrograde assemblages chlorite k feldspar celadonitic musco vite sphene anatase pyrite common and chlorite chloritoid musco vite anatase sphene rare retrograde sillimanite out staurolite out and garnet out isograds have been mapped representing the completion of important retrograde hydration reactions these isograds plus the thicknesses of chlorite rims on garnets 0 and 0 2 mm can be used to subdivide the area into six concentric retrograde metamorphic zones designated r1 to r6 during retrograde metamorphism the exchange reactions k na mn fe mg and fe mg proceeded while more or less consistent tie lines between the sheet silicates and ilmenite were maintained biotite and chlorite became more fe rich owing to staurolite garnet and ilmenite breakdown biotite chlorite and ilmenite became more mn rich owing to garnet breakdown coexisting biotite and muscovite became more k rich during biotite breakdown muscovite also became progressively more celadonite rich chlorite biotite and ilmenite are homogeneous in single thin sections muscovite is zoned with respect to m 2 m 2 al iv and k k na ratios which are higher in muscovite rims there is no evidence for re equilibration of staurolite garnet or albite during retrograde metamorphism temperature and pressure estimates for retrograde metamorphism are poorly constrained to 3 5 kbar and 280 c each kilometre of vertical extent of the retrograde zone required an influx of 0 26 km 3 of h 2 o at stp water may have been derived from a large volume of surrounding rock and concentrated in the study area by a pressure shadow effect around the north end of the prescott intrusive complex

paleointensity estimates from ignimbrites an evaluation of the bishop tuff 1 ash flow tuffs or ignimbrites typically contain fine grained magnetite spanning the superparamagnetic to single domain size range that should be suitable for estimating geomagnetic field intensity however ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history we examined three stratigraphic sections through the 0 76 ma bishop tuff where independent information on postemplacement cooling and alteration is available as a test of the suitability of ignimbrites for paleointensity studies thermomagnetic curves suggest that low ti titanomagnetite tc 560 c 580 c is the dominant phase with a minor contribution from a higher tc phase s significant remanence unblocking above 580 c suggests that maghemite and or titano maghemite is an important contributor to the remanence in most samples we obtained successful paleofield estimates from remanence unblocked between 440 c and 580 c for 46 of 89 specimens 15 sites at two of three total localities these specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43 0 t 3 2 equivalent to a vadm of 7 8 1022 am2 the most densely welded sections of the tuff have emplacement temperatures inferred to be as high as 660 c suggesting that the remanence may be primarily thermal in origin though a contribution from thermochemical remanence cannot be excluded these results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations

petrographic classification of crystal morphology the terms euhedral subhedral and anhedral have limited value as an expression of crystal morphology relating to conditions of crystal formation and rock forming processes the basis of this traditional nomenclature is the presence or absence of crystal faces implying that if a crystal has faces its overall shape is likely to reflect crystal structure experimental reproduction of growth dissolution and melting textures along with an ever increasing awareness of the importance of incomplete and malformed crystals in natural systems is the impetus for presentation here of a more detailed morphological classification of crystals at the petrographic scale the classification is binary based on the degree to which crystals are an expression of crystal structure and on the degree of completeness unlike the traditional scheme the proposed classification is sensitive to faceted crystals that do not have ideal shape and to incomplete crystals variously described as skeletal dendritic cellular