effects of a perovskite post perovskite phase change near core mantle boundary in compressible mantle convection 1 numerical simulations of compressible isochemical mantle convection are used to investigate the effect of the perovskite to post perovskite phase transition at around 2700 km depth which has recently been discovered by high pressure experiments and ab initio calculations on the convective planform temperature and heat transport characteristics of the mantle the usual phase transitions at 410 km olivine spinel and 660 km spinel perovskite are also included the exothermic post perovskite phase change at 2700 km depth destabilizes the lower thermal boundary layer increasing the heat flow increasing interior mantle temperature and increasing the number and time dependence of upwelling plumes the resulting weak highly time dependent upwellings also have a smaller horizontal spacing than the plumes that occur in the absence of the phase transition while the influence of post perovskite phase change may be smaller than that of some other complexities such as compositional stratification it appears to have an important enough effect that it should not be ignored in dynamical studies of mantle convection

is the long wavelength geoid sensitive to the presence of postperovskite above the core mantle boundary 1 the analysis of seismic data represents today the primary tool in the search for the presence of postperovskite in the lowermost mantle d 00 this work aims at testing whether the inversion of gravitational data can also contribute to the detection of postperovskite in d 00 w e assume that the transition from perovskite to postperovskite is accompanied by a reduction in viscosity and test the effects of such viscosity change on the prediction of the dynamic geoid with a numerical model of subducted lithosphere our results show that the long wavelength component of the geoid is very sensitive to the presence of postperovskite areas in d 00 especially if their viscosity is significantly lower than the viscosity of the surrounding perovskite and if these areas are located close to density

volcanic biotite sanidine 40ar 39ar age discordances reflect ar partitioning and pre eruption closure in biotite the 40 ar 39 ar radioisotope system is widely used to date eruption and cooling of volcanic tephra marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained this chronometer bridges the gap between 14 c and longer lived isotopic systems that are too imprecise for dating young samples however 40 ar 39 ar ages obtained from coevally erupted biotite and sanidine do not always match here we use an independent chronometer 238 u 230 th disequilibrium to demonstrate that 40 ar 39 ar age disparity is not caused by differences in pre eruption crystallization times our findings indicate that the presence of extraneous 40 ar in biotite and its absence in sanidine may result from violations of two assumptions implicit in 40 ar 39 ar geochronology on volcanic samples 1 prior to eruption minerals are devoid of 40 ar due to rapid loss to an infinite reservoir such as the atmosphere and 2 closure to volume diffusion is geologically instantaneous and coincident with eruption we propose a mechanism whereby the presence of extraneous ar in certain minerals is explained by the relative sequence of four events in a magmatic system 1 crystallization 2 mineral closure with respect to ar diffusion 3 isotopic equilibration of magmatic and atmospheric ar and 4 quenching of the system by eruption these data have potentially far reaching implications for studies that depend on geochronological data necessitating re evaluation of interpretations based solely on biotite with no independent age control particularly in young samples where the effects are most pronounced

origin of seismic anisotropy in the d layer inferred from shear deformation experiments on post perovskite phase abstract seismic anisotropy is one of the significant features in the d layer of the earth and is thought to be derived from the lattice preferred orientation lpo of constituent materials or shape preferred orientation spo of heterogeneous materials such as melt and inclusions recent experimental and theoretical studies strongly suggest that the d layer consists mainly of a mgsio3 post perovskite phase together with ferro periclase to understand the anisotropy in the d layer we have conducted a series of simple shear deformation experiments at high temperature and pressure on polycrystalline cairo3 as an analogue of mgsio3 and measured the lpo of the post perovskite phase crystallographic orientation analysis of the deformed post perovskite phase showed strong lpos with the dominant slip system being 100 010 calculation of the elastic wave velocities considering the effect of lpos of post perovskite and ferro periclase showed as azimuthal and polarization anisotropies in the horizontal shear plane where the velocity of horizontally polarized s wave is considerably faster than that of vertically polarized s wave thus the seismic anisotropy observed in the d layer can be reasonably explained by the lpo of the mixture of post perovskite and ferro periclase where the lpo may result from the horizontal shear flow generated by the mantle convection

phase transition in mgsio 3 perovskite in the earth s lower mantle a new polymorph of mgsio3 more stable than the pbnm perovskite phase has been identified by first principles computations it has the cairo3 structure with cmcm symmetry and consists of sio3 layers intercalated by eightfoldcoordinated mg ions high temperature calculations within the quasiharmonic approximation give a volume change of f1 5 and a clapeyron slope of f7 5f0 3 mpa k at f2750 k and f125 gpa these pressure temperature p t conditions are close to those in which a phase transition in mgsio3 perovskite has been observed by in situ angle dispersive x ray diffraction measurements this transition appears to be associated with the du discontinuity d 2004 elsevier b v all rights reserved

simulation of monsoon disturbances in the lmd gcm abstract the monsoon depressions that form over india during the summer are analyzed using simulations from the laboratoire de meteorologie dynamique general circulation model this type of synoptic system often occurs with a frequency of one to two per month and can produce a strong indian rainfall two kinds of analyses are conducted in this study the first one is a subjective analysis based on the evolution of the precipitation rate and the pattern of the sea level pressure the second one is an objective analysis performed using the track program which identifies and tracks the minima in the sea level pressure anomaly field and computes the statistics for the distribution of systems the analysis of a 9 yr control run which simulates strong precipitation rates over the foothills of the himalayas and over southern india but weak rates over central india shows that the number of disturbances is too low and that they almost never occur during august when break conditions prevail the generated distu

el ventorrillo a paleostructure of popocatepetl volcano insights from geochronology and geochemistry volcan popocatepetl mexico was constructed over the remains of a volcanic paleostructure based on fieldwork 40ar 39ar dating u pb dating and geochemistry we have determined the age chemistry and location of this paleostructure and named it el ventorrillo most remnants of el ventorrillo are covered by deposits from subsequent activity of popocatepetl except for the el abanico scarp and the barranca de nexpayantla where the stratigraphy of el ventorrillo eruptive products can be investigated inception of volcanism at el ventorrillo occurred at 331 10 ka with emission of the nexpayantla andesitic lavas and continued with extrusion of the yoloxochitl 267 31 ka and microwave 227 6 ka domes intrusion of dikes occurred at 298 94 and 230 3 ka activity at el ventorrillo continued with the emission of lavas that built the el abanico scarp 193 29 to 96 8 ka and continued until the tutti frutti eruption destroyed the cone 14 1 kyr ago el ventorrillo magmas produced rocks divided into two mineralogical groups the first group contains biotite amphibole rich rocks and the second group consists of biotite amphibole free lavas the rocks that contain biotite and amphibole are older than 198 13 ka whereas the rocks with no hydrous phases are younger than 227 6 ka and contain skarn and granodiorite xenoliths we interpret the change to an anhydrous mineral assemblage and the occurrence of skarn and granodiorite xenoliths as evidence for the formation of a new shallower reservoir a granodiorite xenolith was chosen for 40ar 39ar dating and u pb zircon analyses the u pb method yielded an age of 540 110 ka and the 40ar 39ar an age of 109 24 ka these ages are interpreted to indicate granodiorite crystallization 540 110 ka which metamorphosed the calcareous basement beneath popocatepetl into skarn and an influx of magma 109 24 ka that reheated the granodiorite major and trace elements sr nd and pb isotopic compositions and textural analyses of plagioclase show that el ventorrillo magmas were moderately modified by mixing with more mafic magmas and that assimilation of the calcareous basement was negligible

a doubling of the post perovskite phase boundary and structure of the earth s lowermost mantle a new model of the deep earth allows geophysicists to measure variations in earth s temperature near the boundary between the rocky mantle and core using a recently discovered phase change in the lowest part of the earth s mantle where atoms rearrange themselves in crystals under extreme pressure and temperature the model also explains odd features of this part of the mantle such as patchy regions that reflect seismic waves generated by large earthquakes and inferences of dense partial melting combined with observations the results imply very large variations in temperature providing further evidence for circulation of material throughout the earth s mantle the thermal structure of the earth s lowermost mantle the d layer spanning depths of 2 600 2 900 kilometres1 is key to understanding the dynamical state and history of our planet earth s temperature profile the geotherm is mostly constrained by phase transitions such as freezing at the inner core boundary or changes in crystal structure within the solid mantle that are detected as discontinuities in seismic wave speed and for which the pressure and temperature conditions can be constrained by experiment and theory a recently discovered phase transition at pressures of the d layer2 3 4 is ideally situated to reveal the thermal structure of the lowermost mantle where no phase transitions were previously known to exist here we show that a pair of seismic discontinuities observed in some regions of d can be explained by the same phase transition as the result of a double crossing of the phase boundary by the geotherm at two different depths this simple model can also explain why a seismic discontinuity is not observed in some other regions and provides new constraints for the magnitude of temperature variations within d

grain size evolution in subducted oceanic lithosphere associated with the olivine spinel transformation and its effects on rheology we investigate the role of grain size reduction during the olivine spinel transformation on rheological properties of subducting slabs on the basis of a scaling model for microstructural development during nucleation and growth in this model the size of spinel grains nucleating at olivine grain boundaries is controlled by the relative rates of nucleation and growth taking into account the impingement through the collision of grains due to growth when the volume fraction of spinel reaches a certain threshold value critical volume fraction 1 10 depending on the p t conditions in the slab the new phase will form a continuous film and will significantly reduce the strength of the two phase aggregate if spinel grain size is small the size of spinel grains d at this stage is calculated and is shown to be highly sensitive to 0 size and 2 a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine bearing wedge in a cold slab possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes

a moist linear baroclinic model coupled dynamical convective response to el nino a newly developed linear baroclinic model lbm and its application to the tropical enso teleconnection is described the model based on primitive equations linearized about the observed zonally varying basic state in northern winter involves linear schemes for the cumulus convection and surface sensible and latent heat fluxes referred to as the moist lbm this enables us to solve a steady state response of the coupled dynamical convective system to a given sst anomaly which is fairly different from the conventional dry lbm linear representation of the convection is acceptable for a realistic range of sst anomalies reproducing well the rossby wave train computed in the conventional lbm forced by a tropical heating the moist lbm is used to examine the formation mechanisms of an anomalous low level anticyclone near

regional model simulations of marine boundary layer clouds over the southeast pacific off south america part ii sensitivity experiments abstract the sensitivity of a regional climate model to physical parameterizations and model resolution is investigated in terms of its simulation of boundary layer stratocumulus scu clouds over the southeast pacific specifically the physical schemes being tested include shallow cumulus convection subgrid vertical mixing cloud droplet number concentration cdnc and drizzle as described in part i the model with standard settings captures the major features of the boundary layer in the region including a well mixed marine boundary layer a capping temperature inversion scu clouds and the boundary layer regime transition from the well mixed layer near the coast of south america to a decoupled cloud layer over warmer water to the west turning off the shallow cumulus parameterization results in a dramatic increase in the simulated scu clouds while the boundary layer structure becomes unrealistic losing the decoupled regime over warm water with reduced penetrative mixing at the top of shallow cu

some mineral physics constraints on the rheology and geothermal structure of earth s lower mantle we explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of earth s lower mantle we show that mgsio3 perovskite is significantly stronger than mgo periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase periclase we calculate viscosities of the lower mantle for two cases 1 where periclase occurs as isolated grains and 2 where periclase occurs as continuous films using mineral physics data and models of two phase rheology we find that the effective viscosity for the former is about 10 1000 times higher than the latter we therefore suggest that the rheology of the lower mantle is structure and hence strain dependent leading to weakening at large strains due to the formation of continuous films of periclase overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient both mgsio3 perovskite and periclase have relatively small activation energies e g r t m with g 10 14 where r is the gas constant and t m is melting temperature and therefore the depth variation of viscosity is rather small even for a nearly adiabatic temperature gradient however the geothermal gradients consistent with the geodynamical inference of nearly depth independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood a superadiabatic gradient of up to 0 6 k km is also consistent with mineral physics and geodynamical observations

late holocene isotope hydrology of lake qinghai ne tibetan plateau effective moisture variability and atmospheric circulation changes a sub centennial resolution record of lacustrine carbonate oxygen isotopes 18oc from the closed basin lake qinghai on the ne tibetan plateau shows pronounced variability over the past 1500 years changes in 18oc in hydrologically closed lakes are often interpreted in terms of changing effective moisture under this interpretation our record would imply increasing effective moisture during the little ice age lia compared to the medieval warm period mwp however independent evidence from other archives strongly suggests the asian summer monsoon was stronger during the mwp and weakened during the lia controls other than effective moisture the balance of water inputs over evaporative loss must therefore have contributed to the 18oc values we propose the lia signal in lake qinghai resulted from a reduction in evaporation caused by colder air temperatures coupled with a decrease in oxygen isotope composition of input waters as a result of an increase in the relative importance of westerly derived precipitation our results caution against simplistic interpretations of carbonate oxygen isotope records from hydrologically closed lakes and suggest all possible controlling factors must be taken into account in order to avoid misleading palaeoclimatic reconstructions

revised chronostratigraphy of the kenai group from 40ar 39ar dating of low potassium bearing minerals cook inlet basin alaska thirty seven tephra beds primarily from coal partings in the sterling and beluga formations were successfully dated by the 40ar 39ar method providing a new and revised understanding of the chronostratigraphy of late tertiary strata within cook inlet basin meticulous sample preparation multiple analyses and statistical evaluation of the data were required for these low k plagioclase and hornblende bearing tephras dating of subsurface core material provides the first subsurface to outcrop tie in cook inlet between well 212 24 in the beluga river unit and deposits in the clam gulch diamond creek and fox creek areas the new 40ar 39ar chronostratigraphic framework place the age of upper part of the kenai group strata between 4 6 and 9 4 ma and support the 8 ma interpretation of the boundary between the homerian and clamgulchian paleobotanical stages the 49 ma age from core data in pioneer unit suggests the tyonek formation is older than previously thought or that these units belong to an older form

the nature of the 660 kilometer upper mantle seismic discontinuity from precursors to the pp phase global seismic network data were used to image upper mantle seismic discontinuities stacks of phases that precede the pp phase thought to be underside reflections from the upper mantle discontinuities at depths of 410 and 660 kilometers show that the reflection from 410 kilometers is present but the reflection from 660 kilometers is not observed a continuous lame s constant lambda and seismic parameter at the 660 kilometer discontinuity explain the missing underside p reflections and lead to a p wave velocity jump of only 2 percent whereas the s wave velocity and density remain unchanged with respect to previous global models the model deemphasizes the role of lame s constant lambda with regard to the shear modulus and constrains the mineralogical composition across the discontinuity

can hotter mantle have a larger viscosity in the diffusion creep regime which is believed to be appropriate for the lower mantle of the earth the viscosity of rocks depends on the grain size the grain size is normally assumed to be constant in mantle convection models however it is controlled by the kinetics of polymorphic phase transformations and post transformation grain growth and can vary substantially a simple scaling analysis demonstrates one particular feature of the interaction of rheology convection and kinetics the mantle resembles a temperature dependent viscosity fluid with an extremely unusual apparent activation enthalpy for the viscosity it can take almost any value including negative so that hotter regions can have a larger viscosity or the averaged viscosity of the mantle can increase during earth s secular cooling this affects mantle dynamics and evolution and should also be taken into account in interpretating the viscosity variations in the mantle

petrologic diversity in mount st helens dacites during the last 4 000 years implications for magma mixing mount st helens has explosively erupted dacitic magma discontinuously over the last 40 000 years and detailed stratigraphic data are available for the past 4 000 years during this last time period the major element composition of the dacites has ranged from mafic 62 64 wt sio2 to felsic 65 67 wt sio2 temperature has varied by about 150 c 770 920 c and crystallinity has ranged between 20 and 55 water content of these dacites has also fluctuated greatly although the source for the dacitic magmas is probably partial melting of lower crustal rocks there is strong physical evidence such as banded pumices thermal heterogeneities in single pumices phenocryst disequilibrium contrasts between compositions of glass inclusions and host matrix glass and amphibole reaction rims that suggests that magma mixing has been prominent in the dacitic reservoir indeed we suggest that the variations in major and trace element abundances in mount st helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas magma mixing has also controlled the composition of andesites erupted at mount st helens and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite or possibly rhyodacite and basalt the flux of the felsic endmember to the reservior appears to have been relatively constant whereas the flux of basalt may have increased in the past 4 000 years as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period

inherited argon in a pleistocene andesite lava 40ar 39ar incremental heating and laser fusion analyses of plagioclase by using 40 ar 39 ar incremental heating and laser fusion techniques xenocrystic plagioclase was discovered in a late pleistocene andesitic lava that erupted through the andean cordillera inherited argon in the xenocrysts is as much as 450 times older than the host lava the age of which is independently known and is an obstacle to dating the lava by using a whole rock sample the xenocrysts are impossible to identify from petrography or chemical parameters such as their k ca ratios holocrystalline groundmass carefully separated from plagioclase and other phenocrysts gives an accurate 40 ar 39 ar age for the lava the xenocrysts could not have been degassed for more than several days in the magma and probably were assimilated from paleozoic rocks buried under kilometers of mesozoic and tertiary arc rocks composing the cordillera in central chile the required magma ascent velocity on the order of kilometers day is extraordinarily high compared to the 10 4 km day minimum implied by the 226 ra excess in continental arc lavas these data permit magma migration and storage in the mantle and lower crust for as much as thousands of years followed abruptly by rapid ascent to the surface

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

re examination of crystal ages in recent mount st helens lavas implications for magma reservoir processes u series data for recent mount st helens lavas suggest that crystallization preceded eruption by more than 0 5 ka but are complicated by possible evidence of crystal recycling and or addition of radium to the liquid after crystallization we report new ion and electron microprobe trace and major element data for plagioclase and pyroxene in these recent mount st helens lavas and use these data to reassess 226ra 230th crystal ages by taking into account differences in the partitioning behavior of radium and barium and the effects of impurities in mineral separates revised 226ra 230th model crystallization ages are 2 4 ka for plagioclase with the exception of the 1982 dacite and 0 15 5 7 ka for pyroxene in contrast to previous interpretations no late stage addition of ra to the liquid after precipitation of the minerals is required the variability of ba concentrations measured in plagioclase is too large to be consistent with progressive crystallization from the same liquid or with diffusive re equilibration of xenocrysts with a new host liquid ba heterogeneity limits the residence time of the crystals in a magma at high temperatures and also suggests that in most cases ra th ages have not been significantly modified by ra diffusion into or out of the crystals high 226ra ba in plagioclase in the 1982 dacite relative to the host liquid likely reflects crystallization processes that precluded bulk crystal liquid chemical equilibrium one possibility is that of growth entrapment of surface enrichments during rapid crystallization which could lead to less discrimination between ra and ba than predicted by calculated bulk partition coefficients 226ra 230th crystal ages for the castle creek andesite and basalt that are younger than 230th 238u ages of the same crystals could be explained by mixing of crystals into melts with different 230th 232th ratios by combinations of older and younger crystal growth within the same magma or for the basalt by diffusion of ra into crystals at high magmatic temperatures average plagioclase ages in most of the samples of 2 ka imply that some significant mass fraction of the crystals in each flow was present simultaneously beneath the volcano this observation could be consistent either with simultaneous storage of physically distinct magma batches or with incorporation of a population of similarly aged crystals into each successive magma batch

modeling tropical convergence based on the moist static energy budget abstract the vertically integrated moist static energy equation provides a convenient starting point for the construction of simple models of the time mean low level convergence in the tropics a vertically integrated measure of the moist static stability the gross moist stability proves to be of central importance minima in this quantity mark the positions of the tropical convergence zones we argue that the positions of these minima are determined by the time mean moisture field which is in turn closely tied to the time mean surface temperature

response of the south asian summer monsoon to global warming mean and synoptic systems abstract recent diagnostics with the geophysical fluid dynamics laboratory climate model version 2 1 gfdl cm2 1 coupled model s twentieth century simulations reveal that this particular model demonstrates skill in capturing the mean and variability associated with the south asian summer monsoon precipitation motivated by this the authors examine the future projections of the mean monsoon and synoptic systems in this model s simulations in which quadrupling of co2 concentrations are imposed in a warmer climate despite a weakened cross equatorial flow the time mean precipitation over peninsular parts of india increases by about 10 15 this paradox is interpreted as follows the increased precipitation over the equatorial western pacific forces an anomalous descending circulation over the eastern equatorial indian ocean the two regions being connected by an overturning mass circulation the spatially well organized anomalous precipitation over the eastern equatorial indian ocean forces twin antic

a critical evaluation of young near zero k ar ages an evaluation of the precision and resolution of the unspiked k ar dating method is presented with particular regard to the statistical significance of ages that are measured near or at the detection limit of the technique near zero historical ages can be measured by the unspiked k ar technique with a precision that is essentially controlled by the precision with which the 40ar 36ar of the sample can be resolved from the present day atmospheric value of 295 5 the best analytical precision on the isotopic ratio is 0 05 1 by this technique which currently limits the lower detection limit of unspiked k ar ages to samples featuring at least 0 14 of radiogenic 40ar the corresponding youngest resolvable k ar age depends on the k content and atmospheric contamination of the sample total fusion analysis of high k refractory minerals like sanidine is not practicable via k ar and the lowest resolvable age for medium k samples more amenable to complete fusion is around 1 5 ka on a single run basis it is argued that near zero age measured with a probability density straddling or narrowing the time origin cannot be handled without accounting for the non negativity constraint imposed by the physical requirement of a positive age the pertinent equations are derived both for the single run case and for the case of independent replicates made on a single sample we show that pooled k ar replicates can theoretically reduce the nominal uncertainty of individual unspiked ages typically 1 5 ka 2 to a value that is close to the smallest 40ar 39ar isochron age uncertainty achievable on sanidine in the 0 2 ka range 0 2 ka 2 however this performance is obtained at the cost of prohibitively large sample statistics n 15 for medium k feldspars datable via k ar coupled with the inability of the k ar approach to obviate the problems of excess fractionated 40ar and or xenocrystic contamination this makes the 40ar 39ar technique the method of choice for dating historical events by the k ar scheme

the age of the steens reversal and the columbia river basalt group abstract the columbia river basalt group crbg eruptions have a well defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale 40 ar 39 ar ages are presented from lavas erupted in the r 0 through n 1 magnetozones of the crbg and in the transition between r 0 and n 0 four ages from transitionally magnetized lava flows at steens mountain catlow peak and poker jim ridge with a weighted mean age 16 58 0 10 ma 1 and the more precise age 16 654 0 025 ma of the normally magnetized oregon canyon tuff at the top of the catlow peak section show that the oldest crbg magnetozone r 0 correlates with the c5cr chron bayesian statistical analysis applied to data from four flows at catlow peak using the mean age of the steens reversal gives a best and preferred age of the steens reversal of 16 73 0 13 0 08 ma 95 confidence depending on the geomagnetic polarity time scale model the eruption rate from n 0 through r 2 0 34 0 45 ma in the middle and the bulk of the crbg emplacement averaged 0 30 0 41 km 3 a and peaked at a rate 1 1 2 to 4 1 2 times higher during r 2

tree ring evidence for a two year interval between recent prehistoric explosive eruptions of mount st helens distinctive annual ring patterns in trees in a forest stand 10 km east of mount st helens indicate that the volcano9s voluminous layer we tephra was erupted after the a d 1481 growth season late in 1481 or early during 1482 combined with earlier research that established a d 1480 as the date of eruption of the underlying similar layer wn tephra the new tree ring data provide evidence that these two major explosive tephra eruptions about 500 yr ago were separated by about 2 yr during that eruptive sequence a voluminous explosive eruption was therefore followed by more than 1 yr of minor explosive activity before another voluminous eruption occurred this record suggests that mount st helens might produce a second large explosive eruption during its current period of activity examinations of the tephra deposits that correlate with narrowring patterns in trees at mount st helens also demonstrate that distinctive ring patterns can form in trees across a large range of tephra layer thicknesses the examinations also suggest that tephra coarseness interacts with tephra layer thickness in determining the effects that tephra fallout will have upon forest trees

40ar39ar geochronology of rhyolites erupted following collapse of the yellowstone caldera yellowstone plateau volcanic field implications for crustal contamination abstract single crystal laser probe 40 ar 39 ar dating of 133 grains of sanidine and plagioclase has enabled us to resolve the eruption ages of the upper basin member rhyolites the lava flows and related tuffs that erupted within the yellowstone caldera shortly after its collapse 630 ky ago on eruption of the lava creek tuff two lavas and a tuff that erupted from the eastern ring fracture zone yield an eruptive age of 481 8 ka whereas two flows from the western ring fracture zone yield eruptive ages of 516 7 and 198 8 ka most of the units contain old xenocrysts explaining why previous attempts at dating these earliest post caldera units by the conventional k ar method yielded poorly resolved and in some cases anomalous ages the tuff shows the most severe contamination grains from a single pumice lapilli in the tuff show as large an age range as those from bulk vitrophyre indicating that the xenocrysts were incorporated in the magma prior to its near surface explosive fragmentation diffusion calculations indicate that the xenocrysts could not have remained in the magma for more than a few years without degassing and giving ages indistinguishable from the phenocrysts thus the contamination represented by the xenocrysts probably occurred during fracturing and conduit propagation rather than during caldera collapse which took place more than 100 ky earlier the apparent ages of xenocrysts and their compositions as determined by electron microprobe suggest that the eocene absaroka volcanics are the main contaminant with a single xenocryst probably coming from precambrian basement rocks most of the xenocrysts are difficult to distinguish optically or chemically from feldspar phenocrysts illustrating the necessity of single crystal to date many young volcanic rocks accurately

stability and compressibility of the high pressure phases of al2o3 up to 200 gpa implications for the electrical conductivity of the base of the lower mantle we have used a laser heated diamond anvil cell to investigate the stability and compressibility of cmcm cairo3 type postperovskite structure al2o3 at pressures up to 200 gpa a phase transformation from the pbcn rh2o3 ii type to the cairo3 type structure was observed at 130 gpa which is consistent with previous theoretical studies the observed cairo3 type structure in al2o3 is the same as that in mgsio3 post perovskite the main mineral of earth s lowermost mantle we also calculated the raman shifts of cairo3 type al2o3 and mgsio3 using density functional perturbation theory the similarity of the crystal structures and raman spectra of cairo3 type al2o3 and mgsio3 suggests that the other physical properties of the two phases could be similar as well based on the high electrical conductivity of cairo3 type al2o3 we predicted a profile of electrical conductivity at the bottom of the lower mantle which can explain earth s rotation period changes of a few milliseconds in earth s length of day on decadal timescales if the exchange of angular momentum between the solid mantle and fluid core occurs by an electromagnetic coupling between the conducting core and mantle 2006 elsevier b v all rights reserved

coupled model simulations of boreal summer intraseasonal 30 50 day variability part 1 systematic errors and caution on use of metrics boreal summer intraseasonal 30 50 day variability bsisv over the asian monsoon region is more complex than its boreal winter counterpart the madden julian oscillation mjo since it also exhibits northward and northwestward propagating convective components near india and over the west pacific here we analyze the bsisv in the cmip3 and two cmip2 coupled ocean atmosphere models though most models exhibit eastward propagation of convective anomalies over the indian ocean difficulty remains in simulating the life cycle of the bsisv as few represent its eastward extension into the western central pacific as such few models produce statistically significant anomalies that comprise the northwest to southeast tilted convection which results from the forced rossby waves that are excited by the near equatorial convective anomalies our results indicate that it is a necessary but not sufficient condition that the locations the time mean monsoon heat sources and the easterly wind shear be simulated correctly in order for the life cycle of the bsisv to be represented realistically extreme caution is needed when using metrics such as the pattern correlation for assessing the fidelity of model performance as models with the most physically realistic bsisv do not necessarily exhibit the highest pattern correlations with observations furthermore diagnostic latitude time plots to evaluate the northward propagation of convection from the equator to india and the bay of bengal also need to be used with caution here incorrectly representing extratropical tropical interactions can give rise to apparent northward propagation when none exists in association with the eastward propagating equatorial convection despite these cautions the use of multiple cross checking diagnostics enables the fidelity of the simulation of the bsisv to be meaningfully assessed

under the volcano a new dimension in ar ar dating of volcanic ash a significant part of the history of the greek island of kos has been decoded from a handful of volcanic ash by argon argon laser probe dating of single crystals of the minerals sanidine plagioclase and quartz on a 3 d argon isotope plot the data define six straight lines corresponding to four distinct volcanic events from 161 to 1728 ka all four events produced plagioclase crystals whose ages and ca k ratios are consistent with the known evolution of the rest of the aegean arc sanidine appeared only in the final eruption at 161 ka the joint occurrence of quartz and plagioclase at a 925 ka event implies that quartz did not reside for millennia in the magma as has been suggested for quartz from the bishop tuff california the step heated cooling age of a schist fragment from the ash provides the first radiometric evidence for 250 ma crust under the volcano

excess argon in melt inclusions in zero age anorthoclase feldspar from mt erebus antarctica as revealed by the method abstract historically erupted 1984 anorthoclase phenocrysts from mt erebus yield k ar and 40 ar 40 ar apparent ages as old as 700 ka indicating the presence of excess argon 40 ar 39 ar furnace step heating results from anorthoclase reveal a positive correlation between the cl k ratio and apparent age because chlorine up to 1700 ppm is present in melt inclusions but not in the anorthoclase crystal lattice this correlation suggests that the excess argon is associated with melt inclusions trapped within the anorthoclase during rapid crystal growth confirmation of the source of excess argon comes from step heating experiments on multiple anorthoclase aliquots separated from two phenocrysts and one glass aliquot prepared from the matrix of a volcanic bomb the anorthoclase phenocrysts were crushed and hf etched to yield aliquots containing 30 10 and 1 melt inclusions the step heated anorthoclase with 30 and 10 melt inclusions yielded the highest cl k ratios and apparent integrated ages of 640 30 ka and 179 16 ka respectively the anorthoclase with i melt inclusions yielded significantly lower cl k ratios and apparent integrated ages of 48 8 ka and 50 30 ka the step heated volcanic glass yielded the least variable cl k ratios and a total gas age of 101 16 ka argon released from the anorthoclase and the trapped melt inclusions can be distinguished by differences in their degassing behavior allowing geologically more reasonable ages to be obtained melt inclusions exposed on the exterior of anorthoclase grains principally degas during furnace extraction at temperatures less than 1200 c inclusions entirely within anorthoclase grains principally degas at temperatures greater than 1200 c when incongruent melting of the anorthoclase allows melt inclusion hosted excess argon to escape anorthoclase aliquots prepared with less than 1 inclusions can be fitted with a plateau for heating steps below 1200 c to yield ages as young as 8 2 ka whereas steps above 1200 c yield ages in excess of 100 ka however anorthoclase aliquots containing 10 30 melt inclusions yield ages in excess of 200 ka for heating steps below 1200 c minimizing the effects of the excess argon from melt inclusions relies on sample preparation and step heating fine crushing and treatment with hydrofluoric acid removes many of the larger melt inclusions small melt inclusions which remain within the anorthoclase degas primarily at temperatures above 1200 c temperatures below 1200 c yield the most accurate ages attempts at post analytically correcting for the chlorine correlated excess argon are hindered by the variations in 40 ar e cl within and between samples elevated 40 ar e cl ratios in bubbles within the melt inclusions as deduced from in vacuo crushing experiments are the most likely cause for some or all of a sample s total 40 ar e cl variation in addition relative solubilities of argon and chlorine within phonolitic melts may be partly responsible for variations in 40 ar e cl

ar diffusion and solubility measurements in plagioclases using the ultra violet laser depth profiling technique we describe the first direct measurements of ar diffusion and solubility in plagioclases using ultra violet uv laser ablation depth profiling and noble gas mass spectrometer analyses of experimentallytreated 599 1000 8c 50 200 mpaofar crystalfragmentsoflabradoriteandoli goclase labradorite 40 ar gain diffusion profiles were measured yielding an activation energy of 26 72 4 58 kcal mol 21 118 0 19 16 kj mol 21 and a frequency factor of 9 77 10 29 8 79 10 28 28 79 10 29 c m 2 s 21 95 confidence the ar solubility in labradorite was measured yielding a value of 0 2 ppb bar 21 which is similar to or lower than many rock forming minerals the labradorite diffusion parameters indicate ar closure temperatures of 211 8c for a spherical diffusion geometry and 243 8c for a planar diffusion geometry for 100 mm diameter grains with cooling rates of 10 8 cm a 21 the data indicate that labradorite is less ar retentive than k feldspar at low temperatures but more ar retentive than k feldspar at high temperatures corroborating previous work on plagioclase the relatively slow ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems supplementary material details of the uv laser depth profiles obtained from the labradorite and oligoclase samples used in this study are available at www geolsoc org uk sup18608

a 3100 year history of argon isotopic and compositional variation at el chichon volcano abstract el chichon volcano has produced at least 11 eruptive events in the past 8000 years all of which display a relatively constant trachyandesitic composition the constancy of the eruptive products attests to the tapping of a long lived magma chamber and suggests a system held in steady state between the influx of basaltic magma and differentiation of resident magma we have sampled eruptive products from eight of these events subjecting their dominant phenocryst phases plagioclase and hornblende to argon isotopic analysis in order to investigate the dynamics of a steady state magmatic system plagioclase from the older eruptions 1500 yr bp 1600 yr bp 2000 yr bp and 3100 yr bp display variable enrichment of 40 ar excess argon whereas hornblende from the same eruptions shows little or no enrichment in contrast both plagioclase and hornblende phenocrysts from the younger eruptions in 1982 550 yr bp 900 yr bp and 1250 yr bp have argon isotopic ratios near atmospheric values isochron analysis of these mineral phases rules out xenocrystic contamination as the source of excess argon while mafic enclaves and isotopic and compositional zoning evidence frequent recharge events suggesting the source of this argon is most likely the same as the source of this basaltic magma correlation with cl points to melt vapor inclusions as the primary host for ar within the phenocrysts argon isotopic variations point to a disturbance of the system between 1500 yr bp and 1250 yr bp while compositional evidence for a major mafic input is present in the 900 yr bp eruption indicating a complex relationship between recharge and isotopic signature of eruptive products the amount of excess argon within a plagioclase phenocryst are a function of variations in melt and vapor inclusion abundance time elapsed between melt vapor inclusion entrapment and eruption variations in ar abundance in melt itself a function of vapor exsolution timing and time variations in ar isotopic composition of the melt subdued behavior of hornblende is due to slower diffusion and minimal inclusions

40ar 39ar laser probe ages of bishop tuff quartz phenocrysts substantiate long lived silicic magma chamber at long valley united states laser probe ar 40 ar 39 dating of quartz phenocrysts with rhyolite glass inclusions from the bishop tuff air fall and ignimbrite deposits reveals that the long valley magma system existed as a long lived silicic magma chamber throughout most of the pleistocene sanidine phenocryst and matrix glass analyses show that the bishop tuff eruption occurred at 759 1 to 761 1 ha initial and radiogenic ar isotope ratios indicate isotopic equilibrium between the sanidine phenocrysts and their host melt at the time of eruption the quartz phenocrysts in contrast the most abundant phenocryst phase of the bishop rhyolite crystallized trapped their glass inclusions and became a closed system with respect to ar at 1 89 0 03 to 2 3 0 3 ma consequently the bishop rhyolite magma already resided in the long valley basement and had formed most of its quartz phenocrysts similar to 1 1 m y before its principal eruption providing important constraints on the longevity of large silicic magma chambers

extreme u th disequilibrium in rift related basalts rhyolites and granophyric granite and the timescale of rhyolite generation intrusion and crystallization at alid volcanic center eritrea received september 20 2005 accepted june 14 2006 advance access publication july 14 2006rhyolite pumices and co erupted granophyric granite xenolithsyield evidence for rapid magma generation and crystallization priorto their eruption at 15 2 2 9 ka at the alid volcanic center inthe danikil depression eritrea whole rock u and th isotopicanalyses show

petrology and fe ti oxide reequilibration of the 1991 mount unzen mixed magma a dacitic magma 64 5 wt sio2 a mixture of phenocryst rich rhyodacite and an aphyric mafic magma was erupted during the recent 1991 1995 mount unzen eruptive cycle the experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas and the nature of the mixing process the preeruption rhyodacitic magma was at a temperature of 790 20 c according to fe ti oxide phenocryst cores and at a depth of 6 to 7 km 160 mpa according to al in hornblende geobarometry the mafic magma that mixed with the rhyodacite is found as andesitic 54 to 62 wt sio2 enclaves in the erupted magma and was essentially aphyric when intruded phase equilibria indicate that an aphyric andesite at 160 mpa is 1030 c h2o saturated and possibly as high as 1130 c 2 wt h2o the composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt sio2 using these compositions and temperatures the temperature of the unzen magma after mixing is estimated to be at least 850 to 870 c the groundmass fe ti oxide microphenocrysts and those in pargasite bearing reaction zones around biotite phenocrysts both give 890 20 c temperatures the oxide oxide contacts give temperatures of 910 20 c the 900 30 c postmixing temperatures are consistent with phase equilibria experiments which show that the magma was not above 930 c at 160 mpa our fe ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption which is a shorter time than is calculated using available diffusion data there is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma

seismostratigraphy and thermal structure of earth s core mantle boundary region we used three dimensional inverse scattering of core reflected shear waves for large scale high resolution exploration of earth9s deep interior d and detected multiple piecewise continuous interfaces in the lowermost layer d beneath central and north america with thermodynamic properties of phase transitions in mantle silicates we interpret the images and estimate in situ temperatures a widespread wave speed increase at 150 to 300 kilometers above the coremantle boundary is consistent with a transition from perovskite to postperovskite internal d stratification may be due to multiple phase boundary crossings and a deep wave speed reduction may mark the base of a postperovskite lens about 2300 kilometers wide and 250 kilometers thick the core mantle boundary temperature is estimated at 3950 200 kelvin beneath central america a site of deep subduction the d is relatively cold t 700 100 kelvin accounting for a factor of two uncertainty in thermal conductivity core heat flux is 80 to 160 milliwatts per square meter mw m 2 into the coldest d region and 35 to 70 mw m 2 away from it combined with estimates from the central pacific this suggests a global average of 50 to 100 mw m 2 and a total heat loss of 7 5 to 15 terawatts

summary of pre 1980 tephra fall deposits erupted from mount st helens washington state usa mount st helens has been a prolific source of tephra fall deposits for about 40 000 years these tephra deposits 1 record numerous explosive eruptions 2 form important regional time stratigraphic marker beds and 3 record repeated changes in composition within and between eruptive periods

twenty first century projected summer mean climate in the mediterranean interpreted through the monsoon desert mechanism the term monsoon desert mechanism indicates the relationship between the diabatic heating associated with the south asian summer monsoon rainfall and the remote response in the western sub tropics where long rossby waves anchor strong descent with high subsidence in cmip5 twenty first century climate scenarios the precipitation over south asia is projected to increase this study investigates how this change could affect the summer climate projections in the mediterranean region in a linear framework the monsoon desert mechanism in the context of climate change would imply that the change in subsidence over the mediterranean should be strongly linked with the changes in south asian monsoon precipitation the steady state solution from a linear model forced with cmip5 model projected precipitation change over south asia shows a broad region of descent in the mediterranean while the results from cmip5 projections differ having increased descent mostly in the western sector but also decreased descent in parts of the eastern sector local changes in circulation particularly the meridional wind promote cold air advection that anchors the descent but the barotropic rossby wave nature of the wind anomalies consisting of alternating northerlies southerlies favors alternating descent ascent locations in fact the local mid tropospheric meridional wind changes have the strongest correlation with the regions where the difference in subsidence is largest there decreased rainfall is mostly balanced by changes in moisture omega and in the horizontal advection of moisture

magmatic processes of unzen volcano revealed by excess argon distribution in zero age plagioclase phenocrysts abstract excess argon 40 ar is one of the most important problems in k ar geochronometry because it renders incorrect the simple assumption that all initial argon in rock samples is derived from the atmosphere using the laser microprobe analytical method we investigate and reveal the source and trapping site of excess argon in zero age plagioclase phenocrysts extracted from the lavas erupted during 1990 1995 activity of the unzen volcano kyushu japan by combining the results of laser microprobe analysis and conventional stepwise heating of separated plagioclase phenocrysts we reveal that the majority of the excess argon is retained in melt inclusions in the dusty zone of the phenocrysts we propose a model for the isotopic evolution of argon in the phenocryst bearing magma of the unzen volcano by taking into consideration the noble gas data from unzen volcanic gases and the chemical composition of the melt inclusions in the phenocrysts in our model plagioclase phenocrysts crystallize from or are captured by a low temperature t rhyodacitic magma this magma has a comparatively high 40 ar 36 ar ratio for an arc magma due to assimilation of old cretaceous granitic wall rock mantle derived argon is replenished in the rhyodacitic magma over the life of the volcano when the magma chamber is recharged at intervals by high t juvenile magma the plagioclase phenocrysts crystallize rapidly trapping excess argon 40 ar 36 ar 970 during successive dissolution crystallization events caused by intrusion of the high t magma or water rich fluids the phenocryst bearing mixed magma ascends at a relatively rapid rate following a final injection of the high t magma and interacts with shallow groundwater causing the 40 ar 36 ar ratio of the magma to decrease to the atmospheric ratio contamination of the phenocrysts by atmospheric argon in the surrounding magma is enhanced by rapid short circuit diffusion in which dislocations and or micro cracks in the plagioclase crystals are used for argon transport the remnant magma has been continuously degassing for a decade since the recent 1990 1995 eruptive activity without significant change to 3 he 4 he and 40 ar 36 ar ratios

a comprehensive mass flux scheme for cumulus parameterization in large scale models abstract observational studies indicate that a mass flux approach may provide a realistic framework for cumulus parameterization in large scale models but this approach through the introduction of a spectral cloud ensemble leads normally to rather complex schemes in this paper the question is addressed whether much simpler schemes can already provide realistic values of the thermal forcing by convection under various synoptic conditions this is done through verifying such a scheme first on data from field experiments for periods of tropical penetrative convection gate marshall islands tradewind cumuli atex bomex and extratropical organized convection sesame 79 and then in a nwp model the scheme considers a population of clouds where the cloud ensemble is described by a one dimensional bulk model as earlier applied by yanai et al in a diagnostic study of tropical convection cumulus scale downdrafts are included various types of convection are represented i e penetrative convection in c

seismic anisotropy in the deep mantle boundary layers and the geometry of mantle convection an attempt is made to explore the geodynamical significance of seismic anisotropy in the deep mantle on the basis of mineral physics the mineral physics observations used include the effects of deformation mechanisms on lattice and shape preferred orientation the effects of pressure on elastic anisotropy and the nature of lattice preferred orientation in deep mantle minerals in dislocation creep regime many of these issues are still poorly constrained but a review of recent results shows that it is possible to interpret deep mantle seismic anisotropy in a unified fashion based on the solid state processes without invoking partial melting the key notions are i the likely regional variation in the magnitude of anisotropy as deformation mechanisms change from dislocation to diffusion creep or superplasticity associated with a change in the stress level and or grain size in the convecting mantle with a high rayleigh number and ii the change in elastic anisotropy with pressure in major mantle minerals particularly in mg fe o the results provide the following constraints on the style of mantle convection i the sh sv anisotropy in the bottom transition zone and the sv sh anisotropy in the top lower mantle can be attributed to anisotropy structures lattice preferred orientation and or laminated structures caused by the horizontal flow in this depth range suggesting the presence of a mid mantle boundary layer due to partially layered convection ii the observed no significant seismic anisotropy in the deep mantle near subduction zones implies that deformation associated with subducting slabs is due mostly to diffusion creep or superplasticity and therefore slabs are weak in the deep mantle and hence easily deformed when encountered with resistance forces and iii the sh sv anisotropy in the cold thick portions of the d layer is likely to be due to horizontally aligned shape preferred orientation in perovskite plus magnesiowustite aggregates formed by strong horizontal shear motion in the recent past

excess argon in mount st helens plagioclase as a recorder of magmatic processes excess argon in plagioclase crystals from young 4000 ybp tephra layers from mount st helens washington illustrates the importance of argon isotopes in understanding magmatic processes 40ar 39ar step heating identifies two distinct argon reservoirs in the plagioclase a high cl k low ca k reservoir with atmospheric 40ar 36ar and a low cl k higher ca k reservoir with variable 40ar 36ar the first is probably glass whereas the second is true plagioclase felsic dacite plagioclases have little or no excess argon but those from more mafic layers have significantly higher 40ar 36ar indicating a non atmospheric source these variations are seen within a layer that contains both mafic and felsic dacite and are inconsistent with either xenocrystic or restitic origin for plagioclase the magma chamber exhibits long term open system degassing behavior punctuated by short term fluctuations from influx of high 40ar 36ar basalt the relative timing between mixing and eruption can affect the amount of excess argon recorded in plagioclase

development of finite strain in the convecting lower mantle and its implications for seismic anisotropy 1 seismological observations have revealed patches of seismic anisotropy in regions related to mantle upwelling and paleosubduction within an otherwise isotropic lower mantle a combination of numerical modeling and mineral physics is used to constrain the source of anisotropy in these regions in an effort to better understand lower mantle dynamics and mineral physics specifically it is investigated whether lattice preferred orientation lpo can explain the anisotropy observed in regions of paleosubduction since lpo is caused by dislocation creep and is destroyed by diffusion creep we can develop deformation mechanism maps to determine which regions allow for the development of a mineral fabric strain is then calculated in these regions and is related to mineral physics experiments combined with high pressure elastic constants of lower mantle minerals in order to assess the predicted seismic anisotropy uncertainties in rheological parameters such as the transition stress between dislocation creep and diffusion creep necessitate a full evaluation of the parameter range the effect of variations in transition stress activations parameters and strength of slabs on fabric development is investigated it is shown that lpo is a likely candidate for the cause of lowermost mantle anisotropy in regions of paleosubduction

implications of post perovskite transport properties for core mantle dynamics abstract recent evidence on perovskite to post perovskite phase change in the lowermost mantle suggests that post perovskite piles or lens should be present in the relatively cold downwelling areas while the roots of the hot upwelling plumes consist of perovskite post perovskite is often believed to be deformed predominantly by the dislocation creep and there are some hints that the activation parameters of the dislocation creep in post perovskite induce lower viscosity than that of perovskite at the same pressure and temperature conditions that can even result in a viscosity paradox in the lowermost mantle viscosity in cold downwellings transformed to post perovskite might be lower than the viscosity of warm perovskite plumes such a viscosity structure was indeed recently suggested by the geodynamical inversions of the geoid rheologically weak areas at the base of the subducted slabs may have important consequences for the slab deformation in the d we investigate the dynamics of the cold slabs transformed to post perovskite in the lowermost mantle by means of the simulations of thermal convection in a 2d cartesian model of the lower mantle with composite rheology including diffusion creep and dislocation creep different creep parameters are used for the perovskite lower mantle and for the post perovskite lens or layer respectively while perovskite is considered to be deformed via linear diffusion creep post perovskite is modeled either as a constant viscosity material with viscosity ranging from 1020 pa s to 1023 pa s or it is deformed purely by the dislocation creep the presence of the rheologically distinct post perovskite not only strongly influences the slab deformation above the core mantle boundary cmb but also results in different dynamic regimes of the cmb region characterised by e g the different length scales of the upwelling plumes depending on the creep parameters of post perovskite further the presence of very low viscosity lens or a layer above the cmb strongly enhances the cmb heat flux in the cold slab areas according to the amount of post perovskite present in the d and the rheology of post perovskite the heat flux across the cmb can vary considerably with time which may have important implications for the core mantle coupling and the character of the earth s geomagnetic field