nucleation crystal growth and the thermal regime of cooling magmas crystallization at the margin of a quiet cooling magma has been studied numerically taking into account the kinetics of crystallization the variables are the latent heat value the growth and nucleation functions the initial magma temperature and the thermal contrast between magma and country rock we have investigated a wide range of values for these parameters corresponding to natural conditions we show that after a highly transient stage crystallization tends toward an equilibrium between heat production latent heat release and heat loss given the small diffusivity of country rocks latent heat release is the main factor controlling the temperature evolution in order to minimize the latent heat release crystallization occurs at a temperature where nucleation is small this can be close to either the liquidus or the solidus depending on the initial conditions the main process controlling crystallization is nucleation and not crystal growth nucleation occurs as a series of sharp pulses followed by longer periods of crystal growth the nucleation pulses give birth to thermal oscillations these oscillations can be sustained if the interior magma temperature is above the liquidus independently of the heat loss mechanism we show that the phenomenon occurs on the scale of a few centimeters which corresponds to the inch scale layering of many ultrabasic complexes the model allows us to calculate crystal sizes which are in good agreement with geological observations the crucial parameters which determine crystal size variations near the margins of igneous bodies are the initial thermal conditions as well as the nucleation and growth functions in the main cooling regime close to the liquidus significant size variations can be created by small thermal disturbances

an experimental study of the kinetics of decompression induced crystallization in silicic melt 1 experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression pinatubo dacite was held at 780 c 220 mpa fo2 nno 2 h2o saturated conditions for an equilibration period decompressed to final pressures pf ranging from 175 to 5 mpa and then held for 0 3 931 hours according to the plagioclase liquidus curve in ph2o t space for the relevant melt composition these decompressions impose effective undercoolings teff of 34 266 c growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 pf 150 mpa teff 34 93 c and equilibrium crystal modes are achieved in 168 hours microlite nucleation is the dominant transformation process for 10 pf 50 mpa teff 125 241 c and chemical equilibrium is not attained by 168 hours under these conditions slow steady decompressions typically produced normally zoned euhedral and planar faceted feldspar crystals although anhedral morphologies were produced at very low pf contrary to expectation slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure although counterintuitive these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth controlled by teff and melt viscosity experienced during each decompression path analysis of liquid to solid transformation kinetics using ttt style diagrams shows that crystallization occurs most rapidly at 100 mpa by a crystal growth mechanism the next most efficient crystallization conditions are at 25 mpa in a crystal nucleation dominated regime

crystallization of interstitial liquid and latent heat buffering in solidifying gabbros skaergaard intrusion greenland new compositional profiles across plagioclase grains from the layered series ls marginal border series mbs and upper border series ubs of the skaergaard intrusion are used to understand the mechanisms of cumulate rock solidification and the fate of the interstitial liquid the data show that plagioclase crystals display three types of compositional profile over the whole intrusion as follows 1 grains with normal zoning which dominate the mbs and ubs these are interpreted as having formed at the top of a crystal mush and then buried in the cumulate pile crystallization of the interstitial melt resulted in liquid differentiation and produced normally zoned rims on plagioclase cores 2 unzoned crystals which dominate the upper part of the ls also crystallized at the top of the mush and were then buried in mush with a low interstitial liquid fraction or one experiencing convective movements that kept the liquid to a constant composition 3 crystals with a mantle of decreasing an content followed by a rim of constant composition grains showing this complex zoning mostly occur in the lower parts of the ls depending on the stratigraphic position within the intrusion the composition of the rim can be an56 an51 or an40 in the main magma body these compositions an contents correspond to those of plagioclase primocrysts e g cores at the appearance of cumulus clinopyroxene an56 fe ti oxides an51 and apatite an40 compositional buffering of plagioclase rims is interpreted as being a consequence of enhanced release of latent heat of crystallization at the appearance of new interstitial phases in the crystal mush when a new phase saturates the latent heat contribution to the global enthalpy budget of the system becomes sufficiently high to keep the interstitial melt at its liquidus temperature for a period of time that could exceed thousands of years under these conditions equilibrium adcumulus growth together with diffusion and possibly advection of chemical components result in the formation of plagioclase rims of constant composition an content efficient thermal buffering of the mush liquid depends on the porosity i e fraction of liquid within the mush and the degree of compositional homogeneity of the mush in a heterogeneous and highly porous mush saturation of the new phase occurs in the coldest part of the mush and the enhanced latent heat release at the saturation of this phase is quickly dissipated to the whole volume of liquid including the warmest part that is not yet saturated in a new phase as a consequence no thermal buffering occurs and interstitial crystallization produces grains with normal zoning the distribution of the various types of plagioclase grains throughout the skaergaard intrusion can therefore be used to infer the spatial variability in the physical properties of the crystal mush such as the residual porosity both at an intrusion wide scale and at a millimetre to centimetre scale

laboratory measurements of electrical conductivity of hydrous and dry silicic melts under pressure an interpretation of the electrical signature of molten rocks within the earth s interior in terms of nature and temperature conditions of magma requires additional laboratory data on the electrical conductivity of silicate melts this paper describes an experimental setup and presents measurements of the electrical impedance of dry and hydrous 1 3 wt h2o metaluminous obsidians determined in an internally heated pressure vessel in the range of 50 400 mpa and 350 1325 c it is shown that the temperature and pressure dependences of electrical conductivity of both hydrous and dry obsidian can be fitted by an arrhenius law applying in the melt and the glass regions this suggests that a similar transport mechanism operates in both melt and glass the determined activation energies are 70 kj mol for the dry obsidian and 65 and 61 kj mol for the 1 and 3 wt h2o melts respectively the activation volume is 20 cm3 mol combination of tracer diffusion and electrical conductivity reveals that sodium is the dominant charge carrier in the hydrous and dry obsidians the temperature and pressure effects on conductivity are therefore interpreted in terms of activation energy and activation volume for na mobility in dry and hydrous rhyolites an increase in conductivity associated with addition of water was observed and is shown to reflect the effect of water incorporation in melts on the mobility of sodium as a prospective it is anticipated that both the mobility and content of sodium could control the electrical conductivity of most terrestrial silicate melts magma under differentiation becomes more conductive due to sodium and water enrichment associated with fractional crystallization therefore its electrical signature must reveal its nature and maturity

on the heterogeneous electrical conductivity structure of the earth s mantle with implications for transition zone water content 1 we have investigated the lateral variations in mantle electrical conductivity structure using electromagnetic sounding data for this purpose we used very long time series up to 51 years of geomagnetic observatory data at six locations encompassing different geological settings to compute response functions that cover the broadest possible frequency range 3 9 to 95 2 days furstenfeldbruck fur europe hermanus her south africa langzhou lzh china alice springs asp australia tucson tuc united states north america and honolulu hon united states north pacific we inverted the response functions beneath each observatory for a local radial conductivity profile using a stochastic sampling algorithm specifically we found significant lateral variations in conductivity throughout the mantle with resolution limited to the depth range 500 1200 km at 600 km depth conductivity varies between 0 1 and 0 4 s m and increases to 1 3 2 0 s m at 800 km depth beneath all stations except her 0 5 s m at 900 km depth conductivity increases further to 1 4 2 4 s m with her hon and asp being most conductive this trend persists to a depth of 1200 km comparison with conductivity profiles constructed from laboratory measurements of mantle mineral conductivities and models of earth s mantle composition and thermal state reveal that significant thermochemical variations are at the origin of the observed heterogeneities in mantle conductivity found here because of the somewhat large error bounds on sampled conductivity profiles and the reduced sensitivity of the electromagnetic sounding data above 500 km depth constraints on transition zone water content are less conclusive although h2o contents 0 5 wt in the midtransition zone appear less likely

a reevaluation of crystal size distributions in chromite cumulates although studies have shown that igneous cumulates can form by in situ crystallization without requiring crystal settling it has not been demonstrated that crystal size distributions csds are consistent with such a process plots of crystal size fractions per unit volume vs crystal size for chromite grains from the stillwater complex show a log linear distribution with negative slope at larger sizes and a concave down distribution at smaller sizes the log linear portion of these trends is similar to previously reported trends for other silicate and oxide phases in crystallizing magmas however the lack of smaller grain sizes indicates that the original population was altered during a postnucleation crystalaging period that resulted in the loss of the smaller size fractions an interpretation consistent with textural and volume balance evidence the csd trends imply that cr was not concentrated by settling of chromite but was brought to the site of the growing chromite grains similarities between the stillwater data and csd trends for garnet in metapelites suggest that such trends are a characteristic feature of any geologic system undergoing crystal aging after an initial period of nucleation and crystal growth

resistance to mantle flow inferred from the electromagnetic strike of the australian upper mantle seismic anisotropy is thought to result from the strain induced lattice preferred orientation of mantle minerals especially olivine1 2 owing to shear waves propagating faster along the a axis of olivine crystals than along the other axes this anisotropy results in birefringence or shear wave splitting 3 which has been investigated in numerous studies1 4 although olivine is also anisotropic with respect to electrical conductivity5 with the a axis being most conductive few studies of the electrical anisotropy of the upper mantle have been undertaken and these have been limited to relatively shallow depths in the lithospheric upper mantle6 7 theoretical models of mantle flow have been used to infer that for progressive simple shear imparted by the motion of an overriding tectonic plate the a axes of olivine crystals should align themselves parallel to the direction of plate motion8 9 here however we show that a significant discrepancy exists between the electromagnetic strike of the mantle below australia and the direction of present day absolute plate motion10 we infer from this discrepancy that the a axes of olivine crystals are not aligned with the direction of the present day plate motion of australia indicating resistance to deformation of the mantle by plate motion

ci chondrite like clasts in the nilpena polymict ureilite implications for aqueous alteration processes in ci chondrites abstract we have carried out a detailed petrographic and mineralogical study of carbonaceous chondrite matrix clasts in the nilpena polymict ureilite the bulk compositions of a number of clasts determined by electron microprobe show that they have close affinities to ci chondrite matrix and differ substantially from cm matrix material transmission electron microscopy studies show that the mineralogy of the clasts is also consistent with ci chondrite matrix indicated by the presence of saponitic smectite clays serpentine magnetite pentlandite pyrrhotite and ferrihydrite phases such as tochilinite and cronstedtite diagnostic of cm matrix are absent based on the oxygen isotopic composition of the matrix the phase chemistry and the bulk clast chemistry we suggest that the matrix clasts in nilpena represent material which is less altered than orgueil the difference in oxygen isotopic composition of the nilpena matrix clasts from ci chondrites may be because the clasts underwent alteration at a lower water rock ratio increased alteration of nilpena type material should result in the oxidation of fe bearing saponite and sulfides to produce ferrihydrite and more mg rich saponite such as that in orgueil matrix although the presence of ci like matrix clasts in the polymict ureilites suggests that ci matrix may have been a precursor to the ureilites in detail this seems highly unlikely the trace element and carbon isotopic compositions of ureilites are incompatible with a significant ci chondrite precursor we suggest that the ci like matrix clasts in nilpena represent a component of a late accreting veneer on the ureilite parent body

experimental evidence for crystal coarsening and fabric development during temperature cycling oscillating temperature dramatically speeds up recrystallization of a magma analog consisting of ammonium thiocyanate and ammonium chloride crystals in liquid linear growth rates increase by a factor of 10 if temperature is oscillated up and down a few degrees e g 47 3 c relative to nominally static conditions crystals pulse in size during thermal cycling over the course of hundreds of cycles larger crystals grow and smaller crystals shrink dramatically skewing the crystal size distribution crystal dissolution and growth in a pulsing thermal gradient produces a pronounced fabric with crystals of ammonium thiocyanate aligned subparallel to the direction of heat flow alignment occurs via selective dissolution and growth of crystals of diverse orientations inherited from the starting material these results have important implications for understanding how crystals grow and for interpreting texture in igneous rocks temperature cycling is likely common in magmatic systems and needs to be considered when analyzing chemical zoning of igneous crystals and rock textures

the effect of small amounts of h2o on olivine to ringwoodite transformation growth rates and implications for subduction of metastable olivine abstract it has been suggested that in relatively cold subduction zones the interior of down going lithospheric plates may be cool enough to inhibit the transformation of olivine to its high pressure polymorphs wadsleyite and ringwoodite resulting in the persistence of metastable olivine in the mantle transition zone the rapid transformation of this metastable olivine has been identified as a possible cause of deep focus earthquakes however it is known that olivine can incorporate hydrogen at high pressure and this hydrogen may enhance transformation rates we conducted a series of high pressure experiments to hydrate and then partially transform olivine to ringwoodite we measured transformed rim thickness as a function of run duration and fit a linear function to our results to determine transformation growth rates the addition of h2o to olivine appears to promote olivine to ringwoodite transformation by increasing the growth rate decreasing the activation enthalpy for growth and weakening the ringwoodite and or olivine to allow for dissipation of elastic strain sufficient to maintain constant or near constant growth rates throughout transformation the addition of approximately 300 ppmw 2h2o to olivine increases the growth rate of olivine to ringwoodite transformation at 1100 c and 18 gpa by almost a factor of 40 from 4 0 0 8 10 9 ms 1 to 1 5 0 3 10 7 ms 1 there is a corresponding decrease in the activation enthalpy of growth from 392 kj mol to 237 kj mol assuming activation volume of 0 cm3 mole analyses of hydrogen distributions in partially transformed samples suggest that the hydrogen concentration in the bulk of the olivine crystal may be less important than hydrogen at the olivine and ringwoodite interface

a crystal size distribution study of the kiglapait layered mafic intrusion labrador canada evidence for textural coarsening crystal size distributions csd of plagioclase olivine and clinopyroxene from the kiglapait intrusion labrador were measured for a series of 13 samples spanning most of the exposed intrusion all csds lack small crystals and are slightly curved to the right concave up all csds of a mineral sweep out a fan as expected where there is relatively little variation in phase proportion csd dispersion can be replicated using the communicating neighbours model of textural coarsening ostwald ripening most of the textural variation in the intrusion is the result of variable degrees of coarsening it is proposed that coarsening occurred in the outer part of the crystallisation boundary layer where release of latent heat and changes in the intercumulus liquid composition resulted in a reduction in undercooling suppression of nucleation and coarsening particularly coarse grained areas reflect focusing of circulating magmatic fluids during coarsening the effects of crystal settling were not observed in the csds even within single well developed layers they were probably concealed by the pervasive textural coarsening csds do not suggest that mechanical compaction has played an important role in the formation of texture although it is very likely to have occurred however samples from the lower part of the intrusion show textural evidence of pressure solution compaction

nature of sub volcanic magma chambers deccan province india evidence from quantitative textural analysis of plagioclase megacrysts in the giant plagioclase basalts sub volcanic magma chambers might be a widespread component of flood basalt provinces and their presence can be revealed in some cases by plagioclase megacrystic basalts in at least four levels within the deccan flood basalt sequence the generally low abundance of small plagioclase crystals increases to 5 25 with some as large as 30 mm long these giant plagioclase basalt gpb flows were formed by mixing of megacryst rich and megacryst poor magmas the crystal size distributions csd of these megacrysts mostly plot as almost straight lines on a classic csd diagram for a plagioclase growth rate of 10 10 mm s steady state magma chamber models and simple continuous growth suggest residence times of 500 1500 years however the lack of crystals smaller than 2 mm suggests that coarsening may have been involved and crystal shape can help define the environment where this happened plagioclase megacrysts are very tabular and commonly form clusters of sub parallel crystals characteristics that are also found in the plagioclase of anorthosites formed by flotation at the top of shallow magma chambers and crystallization in a high peclet number regime e g skaergaard sept iles a possible history is as follows 1 plagioclase megacrysts crystallize in a convecting magma chamber just below the lava pile 2 currents sweep the crystals to the top of the chamber where they accumulate as a result of their buoyancy the crystals coarsen in response to the continuous supply of hot magma 3 new magma sweeps through the plagioclase mush mingles and mixes then erupts to form the gpbs the residence time recorded by the megacrysts in the gpbs is that of the magma chamber where the megacrysts formed not that of the magmas that make up the megacryst poor part of the gpbs or the other megacryst poor lavas lavas with megacrysts similar to the gpbs are uncommon but widespread galapagos surtsey etc and suggest the presence of sub volcanic magma chambers elsewhere

density of hydrous magma abstract magmas in the interior of the earth have been geophysically observed the generation of magmas is closely related to h2o because the water induced decrease in the melting temperature of mantle rocks is an essential process which forms hydrous magmas due to h2o incorporation into the molten rocks knowledge on physical properties of hydrous magmas such as density can give us better understanding of the magma behavior inside the earth the density of hydrous magma can be controlled by the partial molar volume of h2o v h 2 o and its pressure and temperature dependences the new compression behavior of v h 2 o using vinet equation of state is proposed based on compiled data since h2o is one of the most compressible oxides in the magma at lower pressure a rapid densification of hydrous magma can be observed at shallower mantle the dense hydrous magma can be gravitational stable above and below mantle transition zone

electromagnetic constraints on a melt region beneath the central mariana back arc spreading ridge 1 an electrical resistivity profile across the central mariana subduction system shows high resistivity in the upper mantle beneath the back arc spreading ridge where melt might be expected to exist although seismic data are equivocal on the extent of a possible melt region the question arises as to why a 2 d magnetotelluric mt survey apparently failed to image any melt we have run forward models and inversions that test possible 3 d melt geometries that are consistent with the mt data and results of other studies from the region and that we use to place upper bounds on the possible extent of 3 d melt region beneath the spreading center our study suggests that the largest melt region that was not directly imaged by the 2 d mt data but that is compatible with the observations as well as the likely effects of melt focusing has a 3 d shape on a ridge segment scale focused toward the spreading center and a resistivity of 100 m that corresponds to 0 1 1 interconnected silicate melt embedded in a background resistivity of 500 m in contrast to the superfast spreading southern east pacific rise the 3 d melt region suggests that buoyant mantle upwelling on a ridge segment scale is the dominant process beneath the slow spreading central mariana back arc a final test considers whether the inability to image a 3 d melt region was a result of the 2 d survey geometry the result reveals that the 2 d transect completed is useful to elucidate a broad range of 3 d melt bodies

mantle transition zone beneath a normal seafloor in the northwestern pacific electrical conductivity seismic thickness and water content abstract we conducted a joint electromagnetic em and seismic experiment to reveal the mantle structure beneath a normal seafloor at 130 145 ma in the northwestern pacific where the seafloor is relatively flat and the underlying mantle is expected to be normal free from tectonic perturbations in the experiment we deployed state of the art instruments in two arrays from 2010 2015 here we report the result of analyses of the em and seismic data for investigating the mantle transition zone mtz structure the em data analysis revealed that an electrical conductivity structure below both arrays was approximated by an average 1 d model of the north pacific and showed a possible downward increase in conductivity at the top of the mtz from the p wave receiver function analysis perturbations in the mtz thickness from a global average were estimated to be 20 km and 2 km below the northern and southern arrays respectively from which temperature profiles in the mtz below these two arrays were then estimated we jointly interpreted the profiles of electrical conductivity and thus estimated temperature with reference to the experimental values of the effects of water on the electrical conductivities of mtz minerals wadsleyite and ringwoodite from mineral physics the upper bound of the water content below the northern array was determined to be 0 4 wt or 0 04 wt depending on different results of mineral physics and that below the southern array was determined to be slightly smaller the lower bound of the water content was not constrained by our data our results indicate that the mtz beneath the normal seafloor in the northwestern pacific is drier than subduction zones and may be a water poor region in a plum pudding mantle model

evolution of crystal sizes in the series of dissolution and precipitation events in open magma systems article i nfo we propose a model that describes the evolution of crystal sizes and crystal size distributions csd of igneous phenocrysts in a sequence of dissolution and crystallization events this model is based on the assumption that crystal dissolution is rate limited by diffusion in melt while crystal growth is controlled by the slower kinetic of new nucleation and growth as a result the dissolution rate is inversely proportional to crystal size coming into effect through the curvature of the crystal s surface but the growth rate does not depend on the crystal size closed form analytical solution of equation for csd is obtained we apply results of modeling to quartz and zircon two prime minerals in silicic igneous systems that are widely used in geochemical and isotopic investigations the time series of multiple solution reprecipitation episodes generate concave downward csds and this result fits well with experimental and natural observations on the abundant concave down csd in silicic igneous rocks we suggest that maturation of crystal populations with sizes above several micrometers can not be caused by a size effect on the solubility of the crystals ostwald ripening but is rather driven by thermal oscillations in experiments and in nature the model predicts that mean crystal size increases with time proportionally to t 0 20 which is very close to the published experimental results for quartz maturation with the exponent of 0 19 0 22 our proposed model gives an opportunity to use natural csds for interpretation of pre eruptive magma history when solubilities and diffusion data are available for constituent elements of the dissolving mineral in particular we present time estimates for maturing zircon populations in large volume ignimbrites and estimate that it takes 100 1000 yrs to mature an initially exponential csd to a lognormal csd

origin of megacrysts in granitoids by textural coarsening a crystal size distribution csd study of microcline in the cathedral peak granodiorite sierra nevada california microcline megacrysts in the cathedral peak granodiorite and other parts of the tuolumne intrusive suite were formed by textural coarsening ostwald ripening of earlier formed crystals the early formed crystals nucleated and grew in an environment of increasing undercooling probably during the ascent of the magma emplacement of the magma into warm host rocks promoted textural coarsening crystals smaller than a certain size the critical size dissolved in the interstitial melt whilst large crystals grew microcline was most sensitive to this effect as the magma temperature was buffered close to its liquidus for a long period by the release of latent heat of crystallization positive feedback between textural coarsening and magma permeability channelled the flow of interstitial melt to produce a heterogeneous distribution of megacrysts megacryst growth was halted when cooling resumed at the end of the intrusive cycle k feldspar nucleation was then renewed and k feldspar crystals grew to form part of the groundmass it was the particular thermal history of this pluton that promoted textural coarsening chemically similar plutons that lack megacrysts probably did not have the pause during cooling that was necessary for the development of this texture

magma accumulation rates and thermal histories of plutons of the sierra nevada batholith ca zircon u pb geochronology results indicate that the john muir intrusive suite of the central sierra nevada batholith california was assembled over a period of at least 12 ma between 96 and 84 ma bulk mineral thermochronology u pb zircon and titanite 40ar 39ar hornblende and biotite of rocks from multiple plutons comprising the muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure assembly of intrusive suites in the sierra nevada and elsewhere over millions of years favors growth by incremental intrusion estimated long term pluton assembly rates for the john muir intrusive suite are on the order of 0 001 km3 a 1 which is inconsistent with the rapid magma fluxes that are necessary to form large volume magma chambers capable of producing caldera forming eruptions if large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites it is doubtful that the intrusive suites represent cumulates left behind following caldera forming eruptions

very long period magnetotellurics at tucson observatory estimation of impedances eleven years 1932 1942 of electric potential and magnetic measurements at the tucson observatory represent a unique very long period magnetotelluric mt data set we report here on a careful reanalysis of this data using modern processing techniques we have developed and used novel methods for separating out the quasi periodic daily variation fields and for cleaning up outliers and filling in missing data in the time domain mt impedance tensors estimated using the cleaned and filled data and using robust frequency domain methods are well determined and smoothly varying for periods between 4 hours and 10 days at longer periods the electric field data are swamped by large amplitude incoherent noise particularly after the third year of the experiment although we find no evidence for contamination of any field components by oceanic motional induction at tidal periods the mt impedance estimates do show evidence of small systematic biases due to finite spatial scale geomagnetic sources at harmonies of the daily variation period these periods are thus removed from the time series and not used in further analysis we show that the resulting impedance tensor is well modeled by a real frequency independent distortion of a scalar impedance which is consistent with non inductive distortion of the electric fields by local surface geology to estimate the undetermined static shift of the mt impedance we compare the long period mt results to equivalent mt impedances determined from 46 years of geomagnetic data combining the geomagnetic and undistorted mt impedances results in scalar impedance estimates for periods 0 17 t 91 days of unprecedented precision however for periods less than one day the phase and amplitude of this impedance while individually consistent are not mutually consistent with any one dimensional conductivity distribution the inconsistency probably results from a combination of subtle multidimensional effects and systematic biases

a sporadic low velocity layer atop the western u s mantle transition zone and short wavelength variations in transition zone discontinuities teleseismic receiver function analysis of data from six dense arrays in the western u s is used to investigate mantle transition zone mtz discontinuities and the prevalence of a low velocity layer atop the 410 km discontinuity 410 lvl negative polarity ps arrivals indicative of a low velocity layer with a top 25 60 km above the 410 are identified in 8 11 out of 18 stacks of receiver functions from highly sampled back azimuth corridors the 410 lvl is interpreted as partial melt resulting from upwelling of hydrated mantle across a water solubility contrast at the 410 the 669 km mean depth of the 660 km discontinuity 660 and the magnitude of 660 topography suggest variable hydration locally approaching saturation in addition to 150 k lateral temperature variations beneath five arrays mean amplitudes of p410s and p660s increase monotonically with period from 2 to 10 s however greater variations are observed in the frequency dependence of p410s compared to p660s implying 410 thickness is more heterogeneous variable 410 thickness is attributed to changes in hydration modulating the width of the olivine to wadsleyite transition interval frequency dependence of p660s amplitudes suggests a broad velocity gradient consistent with multivariate phase changes in the olivine and garnet systems sporadic detection of the 410 lvl the magnitude and length scales of mtz discontinuity topography and inferred variations in hydration support the occurrence of vigorous small scale convection in the western u s mantle comparison of receiver functions with body wave tomography suggests small scale convection driven by sinking slab segments and lithospheric instabilities contributes to the intermittent nature of the 410 lvl

syn tectonic emplacement of the late mesozoic laojunshan granite pluton in the eastern qinling central china an integrated fabric and geochronologic study abstract the porphyritic monzogranite of laojunshan eastern qinling in central china forms a w e elongated pluton characterized by a varying abundance of cm scale potassium feldspar kfs megacrysts with or without plagioclase mantles mafic microgranular enclaves mmes and associated mafic dykes the measurements of the shape preferred orientation of kfs megacrysts and the tiling of megacrystic pairs throughout the pluton suggest that these megacrysts constitute magmatic foliations subparallel to the pluton s margin possibly resulting from eastward non coaxial magmatic flow felsic mafic magma mixing and mingling were important during magma emplacement as indicated by abundant mmes coeval mafic dykes and disequilibrium textures within kfs megacrysts quantitative analyses of the crystal size distribution and the in situ chemistry of kfs megacrysts revealed that the temperature fluctuation in the deep magma chamber and the resultant textural coarsening are the most likely important mechanisms for the growth of kfs megacrysts the syn tectonic and two main stage emplacement of the pluton in a sinistral transtensional regime is inferred by the mechanical coupling between magmatic submagmatic solid state fabrics in the pluton and wall rock deformations as well as zircon u pb dating results which produce eastward younging emplacement ages spanning from 122 ma to 119 ma combined with other studies our work supports the idea that the qinling dabie orogen was in a sinistral transtensional regime during the early cretaceous which may correspond to the eastward extrusion tectonics in eastern china

quantitative petrological evidence for the origin of k feldspar megacrysts in dacites from taapaca volcano chile k feldspar megacrysts are common in granitoids but relatively rare in chemically equivalent volcanic rocks dacites from taapaca volcano have euhedral sanidine megacrysts up to 5 cm long small crystals where present are rounded growth of the megacrysts engulfed plagioclase and amphibole crystals crystal size distributions csd of sanidine megacrysts are hump shaped all these data show that megacrysts developed from the host magma by coarsening this was enabled by the cycling of magma temperature around the sanidine liquidus temperature in response to injections of more mafic magma and subsequent magmatic overturns plagioclase crystals enclosed in the megacrysts are small and have short steep straight csds which contrasts with the csds of plagioclase in the groundmass which are shallower and extend to larger sizes this shows that plagioclase was also coarsened approximately synchronously with sanidine in response to the same temperature conditions

conductivity discontinuities in the upper mantle beneath a stable craton we present evidence for approximate collocation of seismic and electrical transitions in the upper mantle more than two years of very long period magnetotelluric mt data were recorded at a lakebottom observatory in the central canadian shield after processing to contend with non stationary source effects and removal of galvanic distortion the underlying structure is 1d for periods of one hour to four days the response was extended to periods of 100 days by appending geomagnetic depth sounding data to the mt curves minimum structure linearised inversion nonlinear extremal inversion and a new genetic algorithm for nonlinear hypothesis testing reveal discrete jumps in conductivity at depths near the major upper mantle seismic discontinuities the jumps occur over limited depth ranges

stability of hydrous melt at the base of the earth s upper mantle seismological observations have revealed the existence of low velocity and high attenuation zones above the discontinuity at 410 km depth at the base of the earth s upper mantle1 2 it has been suggested that a small amount of melt could be responsible for such anomalies3 4 5 the density of silicate melt under dry conditions has been measured at high pressure4 6 7 8 9 10 11 12 13 and found to be denser than the surrounding solid thereby allowing the melt to remain at depth but no experimental investigation of the density of hydrous melt has yet been carried out here we present data constraining the density of hydrous basaltic melt under pressure to examine the stability of melt above the 410 km discontinuity we infer that hydrous magma formed by partial melting above the 410 km discontinuity may indeed be gravitationally stable thereby supporting the idea that low velocity or high attentuation regions just above the mantle transition zone may result from the presence of melt

multiple plagioclase crystal populations identified by crystal size distribution and in situ chemical data implications for timescales of magma chamber processes associated with the 1915 eruption of lassen peak ca products of the 1915 lassen peak eruption reveal evidence for a magma recharge magma mixing event that may have catalyzed the eruption and from which four compositional members were identified light dacite black dacite andesitic inclusion and dark andesite crystal size distribution textural and in situ chemical major and trace element and sr isotope data for plagioclase from these compositional products define three crystal populations that have distinct origins phenocrysts long axis40 5 mm that typically have core an contents between 34 and 36 mol microphenocrysts long axis between 0 1 and 0 5 mm that have core an contents of 66 69 and microlites long axis50 1mm with variable an core contents from 64 to 52 phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling based on textural compositional and isotopic data for the magma represented by the dacitic component magma recharge was not an important process until just prior to the 1915 eruption average residence times for phenocrysts are in the range of centuries to millennia microphenocrysts formed in a hybrid layer that resulted from mixing between endmember reservoir dacite and recharge magma of basaltic andesite composition high thermal contrast between the two end member magmas led to relatively high degrees of undercooling which resulted in faster crystal growth rates and acicular and swallowtail crystal habits some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution precipitation seen in both crystal textures and rim compositions average microphenocryst residence times are of the order of months microlites may have formed in response to decompression and or syn eruptive degassing as magma ascended from the chamber through the volcanic conduit chemical distinctions in plagioclase microlite an contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions we suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing as a consequence melt and microlites among these three products have similar compositions the results of this study highlight the integrated use of crystal size distribution textural and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies

an experimental study of ostwald ripening of olivine and plagioclase in silicate melts implications for the growth and size of crystals in magmas we carried out an experimental study to characterize the kinetics of ostwald ripening in the forsterite basalt system and in the plagioclase an65 andesite system eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution csd with time t the experiments were performed in a 1 atmosphere quench furnace at 1 250 c for plagioclase and 1 300 c for olivine very contrasted coarsening kinetics were observed in the two series in the plagioclase series the mean grain size increased as log t from 3 m to only 8 7 m in 336 h the kinetic law in log t means that ostwald ripening was rate limited by surface nucleation at plagioclase liquid interfaces in the olivine series the mean grain size increased as t1 3 from 3 m to 23 2 m in 496 h a kinetic law in t1 3 is expected when ostwald ripening is rate limited either by diffusion in the liquid or by grain growth dissolution controlled by a screw dislocation mechanism the shape of olivine csds in particular their positive skewness indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism not by diffusion as the degrees of undercooling t or supersaturation involved in ostwald ripening are essentially 1 c the mechanisms of crystal growth identified in our experiments are expected to be those prevailing during the slow crystallisation of large magma chambers we extrapolated our experimental data to geological time scales to estimate the effect of ostwald ripening on the size of crystals in magmas in the case of plagioclase ostwald ripening is only efficient for mean grain sizes of a few microns to 20 m even for a time scale of 105 years it can however result in a significant decrease of the number of small crystals per unit volume and contribute to the development of convex upwards csds for olivine the mean grain size increases from 2 3 m to 70 m in 1 year and 700 m in 103 years a mean grain size of 3 mm is reached in 105 years accordingly the rate of grain size dependent processes such as compaction of olivine rich cumulates or melt extraction from partially molten peridotites may significantly be enhanced by textural coarsening

electrical conductivity of molten basalt and andesite to 25 kilobars pressure geophysical significance and implications for charge transport and melt structure electrical conductivities of molten hawaiian tholeiite and crater lake andesite were measured between 1200 c and 1400 c at atmospheric pressure and at pressures up to 17and 25 kbar respectively isobaric plots of log versus 1 t is electrical conductivity are linear with the exception of the zero pressure tholeiite melt data conductivities decrease with increasing pressure in both melts with the andesitic melt exhibiting a greater pressure dependence between 5 and 10 kbar abrupt decreases in the slopes of isothermal log versus p plots i e decreases in activation volume are observed for both rock melts this discontinuity probably reflects changes in melt structure as opposed to changes in conduction mechanism in each pressure range the data for each rock melt can be described reasonably well by an equation of the form 0 exp ea p v kt where 0 is a preexponential constant ea is the activation energy and v is the activation volume a qualitative model involving depolymerization of the melt with increased pressure leading to increased efficiency of packing can explain the observed discontinuity in activation volume as well as the observed pressure dependences of other melt physical properties such as viscosity and density conductivity versus melt fraction curves for partially molten peridotite are reevaluated using high pressure tholeiitic melt conductivities and crystalline conductivity values recently determined by other workers minimum melt fraction estimates of 5 10 are required to explain upper mantle regions of anomalously high electrical conductivity in terms of a partial melting hypothesis

water in the mantle results from electrical conductivity beneath the french alps 1 a deep magnetotelluric sounding in the french alps provided a vertical electrical conductivity profile between 200 1000 km two prominent features are observed first the conductivity in the depth range 400 800 km is smaller than the conductivity of a pyrolite mantle obtained from laboratory results for a normal geotherm second the data do not require the conductivity to change throughout the transition zone 410 660 km in this part of the mantle a temperature of 350 450 c less than normal explains the magnetotelluric conductivity profile at 200 400 km our model favors a cold mantle with 1000 1500 ppm of water dissolved in olivine if correct this model suggests that the subducted slab is dehydrated before reaching the transition zone

electromagnetic detection of a 410 km deep melt layer in the southwestern united states a deep seated melt or fluid layer on top of the 410 km deep seismic discontinuity in earth s upper mantle as proposed in the transition zone water filter hypothesis may have significant bearing on mantle dynamics and chemical differentiation the geophysical detection of such a layer has however proved difficult magnetotelluric and geomagnetic depth sounding are geophysical methods sensitive to mantle melt here we use these methods to search for a distinct structure near 410 km depth we calculate one dimensional forward models of the response of electrical conductivity depth profiles based on mineral physics studies of the effect of incorporating hydrogen in upper mantle and transition zone minerals these models indicate that a melt layer at 410 km depth is consistent with regional magnetotelluric and geomagnetic depth sounding data from the southwestern united states tucson the 410 km deep melt layer in this model has a conductance of 3 0 x 10 4 s and an estimated thickness of 5 30 km this is the only regional data set that we have examined for which such a melt layer structure was found consistent with regional seismic studies we infer that the hypothesized transition zone water filter occurs regionally but that such a layer is unlikely to be a global feature

crystal sizes in evolving silicic magma chambers crystal size distributions csds of quartz and zircon phenocrysts in individual pumice clasts from several voluminous ash flow tuffs provide a quenched snapshot view of conditions in preclimactic magma chambers a common feature of these csds is a concave down lognormal shape in contrast to the reported linear csds in more mafic systems this feature is interpreted to be a general result of surface controlled size dependent growth by a layer nucleation in silicic magmas at low supersaturation specific csds may be important for interpreting nucleation and crystal growth conditions and mechanisms in magmas erupted as large ash flow tuffs and smaller volume volcanic units and for fingerprinting different magma batches layers in products of the same eruption

experimental study on the effects of temperature cycling on coarsening of plagioclase and olivine in an alkali basalt we conducted experiments on an alkali basalt at 1 atmosphere in order to investigate the effects of temperature cycling on crystal coarsening experiments at 1 150 c near the ni nio buffer indicate that coarsening of plagioclase and olivine crystals is greatly accentuated by temperature cycling for a given experiment duration crystal number density decreases with temperature cycle amplitude and average crystal size increases with increasing amplitude we observed little correlation between cycle period and crystal number density or average crystal size we suggest that dissolution and size proportional crystal growth during repeated heating and cooling decrease crystal number density and increase average crystal size these experiments indicate that the texture of silicate minerals can be modified by temperature cycling and that phenocrysts may develop quicker in silicate magmas when the magma temperature is cycled

dissolution crystallization kinetics recorded in the 2002 2003 lavas of stromboli italy on 28 december 2002 new vents opened on the flanks of stromboli just below the summit craters interrupting the persistent activity of the volcano with a 7 month long effusive eruption we here report on the plagioclase size distribution plgsd in lava samples collected following the chronology of the 2002 2003 eruption data reveal a linear plgsd similar to that found in samples of normal stromboli activity indicating that the switch from strombolian explosive to effusive activity is not associated with changes in texture nevertheless the crystal size distribution slopes and intercepts exhibit slight sinusoidal temporal variations that are here ascribed to a magma supply mechanism able to induce resonance in the crystal size distribution with an amplitude that depends on the supply rate

a geophysical perspective on mantle water content and melting inverting electromagnetic sounding data using laboratory based electrical conductivity profiles abstract this paper applies electromagnetic sounding methods for earth s mantle to constrain its thermal state chemical composition and water content we consider long period inductive response functions in the form of c responses from four stations distributed across the earth europe north america asia and australia covering a period range from 3 9 to 95 2 days and sensitivity to 1200 km depth we invert c responses directly for thermo chemical state using a self consistent thermodynamic method that computes phase equilibria as functions of pressure temperature and composition in the na 2 o cao feo mgo al 2 o 3 sio 2 model system computed mineral modes are combined with recent laboratory based electrical conductivity models from independent experimental research groups yoshino 2010 and karato 2011 to compute bulk conductivity structure beneath each of the four stations from which c responses are estimated to reliably allocate water between the various mineral phases we include laboratory measured water partition coefficients for major upper mantle and transition zone minerals this scheme is interfaced with a sampling based algorithm to solve the resulting non linear inverse problem this approach has two advantages 1 it anchors temperatures composition electrical conductivities and discontinuities that are in laboratory based forward models and 2 at the same time it permits the use of geophysical inverse methods to optimize conductivity profiles to match geophysical data the results show lateral variations in upper mantle temperatures beneath the four stations that appear to persist throughout the upper mantle and parts of the transition zone calculated mantle temperatures at 410 and 660 km depth lie in the range 1250 1650 c and 1500 1750 c respectively and generally agree with the experimentally determined temperatures at which the measured phase reactions olivine spinel and spinel ferropericlase perovskite occur the retrieved conductivity structures beneath the various stations tend to follow trends observed for temperature with the strongest lateral variations in the uppermost mantle for depths 300 km conductivities appear to depend less on the particular conductivity database conductivities at 410 km and at 660 km depth are found to agree overall with purely geophysically derived global and semi global one dimensional conductivity models both electrical conductivity databases point to 2 o in the upper mantle for transition zone minerals results from the laboratory database of yoshino 2010 suggest that a much higher water content up to 2 wt h 2 o is required than in the other database karato 2011 which favors a relatively dry transition zone 2 o incorporating laboratory measurements of hydrous silicate melting relations and available conductivity data allows us to consider the possibility of hydration melting and a high conductivity melt layer above the 410 km discontinuity the latter appears to be 1 regionally localized and 2 principally a feature from the yoshino 2010 database further there is evidence of lateral heterogeneity the mantle beneath southwestern north america and central china appears wetter than that beneath central europe or australia

long period 30 days 1 year electromagnetic sounding and the electrical conductivity of the lower mantle beneath europe the c response connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic variations and forms the basis for deriving the conductivity depth profile of the earth time series of daily mean values at 42 observatories typically with 50 years of data are used to estimate c responses for periods between 1 month and 1 yr the z y method is applied which means that the vertical component is taken locally whereas the horizontal components are used globally by expansion in a series of spherical harmonics in combination with results from previous analyses the method yields consistent results for european observatories in the entire period range from a few hours to 1 yr corresponding to penetration depths between 300 and 1800 km 1 d conductivity models derived from these results show an increase in conductivity with depth z to about 2 s m 1 at z 800 km and almost constant conductivity between z 800 and z 2000 km with values of 3 10 s m 1 in good agreement with laboratory measurements of mantle material below 2000 km the conductivity is poorly resolved however the best fitting models indicate a further increase in conductivity to values between 50 and 150 s m 1

hydration adjacent to a deeply subducting slab the roles of nominally anhydrous minerals and migrating fluids subduction related transport of water into the mantle has significant dynamical and geochemical implications dehydration of hydrous phases within the slab can introduce water into the transition zone and lower mantle potentially hydrating nominally anhydrous minerals nam and impacting the viscosity and density structure of the mantle over a wide area we present models of fluid transport and mantle hydration in the vicinity of a deeply subducting slab focusing on the fate of water released by deep dehydration reaction in the subducted serpentinized mantle a sharp decrease in water storage capacity across the lower boundary of the transition zone may produce secondary dehydration of hydrated nam leading to precipitation of a hydrous fluid and heterogeneous hydration of the transition zone rapid fluid migration relative to the solid flow field can lead to a broad region of diffuse hydration within the upper mantle wedge and the potential for localized melt regions at the top of the transition zone coincident with fluid pathways slower fluid migration instead implies that the fluid phase can be transported deep into the lower mantle water stabilized in nam and as a free fluid can initiate upwelling within and above the transition zone a less abrupt change in water storage capacity across the base of the transition zone leads to high nam water contents in a channel adjacent to the slab where viscosity is reduced however seismic and electromagnetic observations of hydration in the transition zone are most compatible with a sudden drop of water storage capacity

experimental quantification of plagioclase crystal size distribution during cooling of a basaltic liquid an experimental study has been conducted to constrain how thermal history controls crystal size distribution csd of plagioclase in cooling basalts data from all experiments are dominated by a log linear segment of decreasing number density with increasing crystal size consistent with observations in many natural rocks the slope of the csd is found to be a function of cooling rate faster cooling leading to greater slopes at constant cooling rate the csds flatten as temperature decreases and are sometimes kinked characteristics consistent with independent textural observations that crystal agglomeration contributes significantly to crystal growth a downturn is observed toward small size which suggests ostwald ripening furthermore we find that thermal history above the liquidus has a major influence on csds and on the temperatures of phase appearance we conclude that near the liquidus heterogeneous nucleation typically dominates

origin of anorthosite by textural coarsening quantitative measurements of a natural sequence of textural development the textures of plagioclase crystals within large olivine oikocrysts preserve the sequence of the formation of anorthosite such textures have been quantified in a troctolite anorthosite from the proterozoic lac st jean anorthosite complex canada crystal size distributions csds indicate that initially plagioclase nucleated and grew in an introduction environment of linearly increasing undercooling producing a straight the textures of plutonic rocks are one of their most line csd during this phase latent heat of crystallization was striking aspects but they have been little used to elucidate largely removed by circulation of magma through the porous crystal petrological processes as compared with chemical and mush by about 25 solidification the crystallinity was such that isotopic methods where used discussion has tended it reduced but did not eliminate the circulation of magma resulting to be qualitative in contrast to quantitative chemical in the retention of more latent heat within the crystal pile the methods hence reducing the weight of this approach temperature rose until it was buvered by the solution of plagioclase indeed it is not possible to verify physical models if close to the liquidus temperature of plagioclase nucleation of quantitative data are not available quantitative textural plagioclase was inhibited and conditions were suitable for textural studies of volcanic rocks are becoming more common coarsening of both plagioclase and olivine to occur ostwald and these methods are applied here to the development ripening in this process small crystals were resorbed whereas of anorthosite larger crystals grew from both material recycled by the resorption of quantitative textural studies of plutonic rocks really crystals smaller than the critical size and new material brought in started with jackson s studies of the stillwater complex by the circulating magmatic fluid the results of this process resemble in 1961 jackson 1961 he measured crystal size disthose of high temperature metasomatism but there is no necessity for tributions but did not record the area that he measured a magmatic fluid with a diverent composition the shapes of the hence his data cannot be completely compared with plagioclase csds fit better the communicating neighbours equation of modern data his pioneering work was not really followed textural coarsening rather than the classic lifshitz slyozov wagner up and since then most studies have looked only at equation as do other examples drawn from the literature if olivine average or typical crystal sizes these data are much started to nucleate at a higher temperature than plagioclase then less revealing of petrological processes than crystal size during the textural coarsening phase olivine would have been more distributions partly because they only give one parameter undercooled than plagioclase and would have had a higher maximum for each sample the few existing studies of crystal size growth rate in these conditions olivine would coarsen more rapidly distributions in plutonic rocks have not treated than plagioclase and engulf it hence the order of crystallization plagioclase determined from the textures would be the reverse of the order of one problem especially pertinent to quantitative texfirst nucleation of the two phases from equilibrium phase diagrams tural studies is the way that many igneous petrologists maintenance of the temperature near the plagioclase liquidus may look at rocks they commonly try to reconstruct how a rock has formed by examining the final fully crystallized also inhibit the nucleation and growth of other phases

an interplanetary dust particle with links to ci chondrites abstract w7013f5 is a chondritic hydrated interplanetary dust particle whose composition and mineralogy is nearly identical to that found in the ci chondrites transmission electron microscope observations show that the phyllosilicates in w7013f5 consist largely of a coherent intergrowth of mg fe serpentine and fe bearing saponite on the unitcell scale this distinctive intergrowth of phyllosilicates has only been observed previously in the ci chondrites other secondary minerals in w7013f5 include mg fe carbonates magnetite and pentlandite the mineral assemblage in w7013f5 is generally not as oxidized as that in the ci chondrites the presence of kamacite in w7013f5 indicates that the particle is extraterrestrial and a thin amorphous rim surrounding the particle provides evidence that it is not a piece of a meteorite that fragmented during transit through the atmosphere the apparent lack of hydrated idps with ci mineralogy and chemistry may indicate that ci type dust producing asteroids are uncommon in the asteroid belt

first in situ observation of crystallization processes in a basaltic andesitic melt with the moissanite cell the evolution of texture and crystal size distribution of igneous rocks is mainly determined by processes such as nucleation grain growth and ostwald ripening however the role played by these processes on the final crystal size distribution is difficult to quantify with conventional methods here we present the first direct in situ observation of a crystallizing supercooled basaltic andesitic liquid in order to directly assess the role of grain scale processes occurring during magma solidification the experiments were conducted at 900 c and 1 bar with a new moissanite cell specifically designed for long term high temperature experiments the method allowed the direct continuous observation of the crystallizing melt until the crystal fraction approached 0 7 investigation of the evolving texture revealed that individual crystals followed distinct growth histories characterized by intervals of relatively uniform free growth abrupt size increase due to grain coalescence and growth reduction due to impingement the latter played a dominant role at higher crystal fractions despite this growth dispersion average growth rates calculated for single grains showed only little variation 4 9 10 6 to 1 9 10 6 mm s no clear relation between growth rate and crystal size was observed crystal coarsening through a combination of overgrowth and coalescence can also account for the counterclockwise rotation and the drop off at smaller size fractions of the recovered crystal size distributions in our experiments these results suggest that crystal size distributions of natural rocks do not record the original nucleation rates

crystal size distribution systematics and the determination of magma storage times the 1959 eruption of kilauea volcano hawaii abstract the theory of crystal size distribution csd theory is based on a steady state population balance that monitors the flux of crystals growing into and out of specificed size categories in precipitating solutions the conservation equation describing this balance permits crystal growth and nucleation rates to be determined directly from crystal size distribution data in this investigation it is shown that csd analysis can alternatively be used to calculate the residence time of crystals in the system or in volcanologic terms the magma storage time prior to eruption if the characteristic crystal growth rate can be independently determined or estimated the crystal size data needed for storage time determinations are easily obtained from thin sections of glassy eruption samples and the mathematical calculations are relatively simple analysis of the errors inherent in this new technique predicts storage estimates accurate to within an order of magnitude

electrical conductivity of wadsleyite at high temperatures and high pressures abstract the electrical conductivity of wadsleyite aggregates has been determined under the broad range of thermodynamic conditions using the impedance spectroscopy for a frequency range of 10 2 to 10 6 hz two branches are observed in the complex impedance one at high frequency range showing a half circle originated at z real part of impedance z imaginary part of impedance 0 in the z z plot and another branch in the low frequency range the results from high frequency semi circles correspond to the electric properties of a sample whereas the results from a low frequency branch correspond to the electrode effects from the analysis of the results from the semi circles we have identified two distinct mechanisms of electrical conduction having different activation enthalpies and different sensitivity to oxygen fugacity and water content one mechanism dominating at water poor condition has a high activation enthalpy 147 kj mol and the conductivity increases with oxygen fugacity we suggest that electrical conduction in this regime is due to charge transfer involving ferric iron polaron conduction under water rich conditions electrical conductivity increases with water content but decreases with oxygen fugacity and the activation enthalpy is smaller 88 kj mol we infer that electrical conduction in this regime is due to protons the activation enthalpy in this regime is insensitive to water content and the conductivity is proportional to water content c w as c w r with r 0 72 the value of r is smaller than one suggests that minority defects such as h m or h are responsible for electrical conduction our results show that a completely dry transition zone is incompatible with most of the geophysical observations on the mantle transition zone and some water 0 1 0 3 wt in the pacific is required to explain the observed electrical conductivity

low velocity zone atop the 410 km seismic discontinuity in the northwestern united states the seismic discontinuity at 410 km depth in the earth s mantle is generally attributed to the phase transition of mg fe 2sio 4 from the olivine to wadsleyite structure variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations for example a cold anomaly would elevate the 410 km discontinuity because of its positive clapeyron slope whereas a warm anomaly would depress the discontinuity but trade offs between seismic wave speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities and structure often appears very complicated here we simultaneously model seismic refracted waves and scattered waves from the 410 km discontinuity in the western united states to constrain structure in the region we find a low velocity zone with a shear wave velocity drop of 5 on top of the 410 km discontinuity beneath the northwestern united states extending from southwestern oregon to the northern basin and range province this low velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic receiver function observations observed above the region we interpret the low velocity zone as a compositional anomaly possibly due to a dense partial melt layer which may be linked to prior subduction of the farallon plate and back arc extension the existence of such a layer could be indicative of high water content in the earth s transition zone

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

a joint interpretation of electromagnetic and seismic tomography models suggests the mantle transition zone below europe is dry abstract the three dimensional 3d electrical conductivity distribution in the mantle transition zone mtz below europe is resolved by directly inverting geomagnetic response functions from 12 selected european observatories comparison of the conductivity image with the results of global seismic tomography reveals a good correlation between low conductivity and high velocity anomalies at depths of 400 700 km beneath the mediterranean subduction zone in central europe interpretation of the 3d conductivity model combined with seismic tomography indicates that the european mtz is drier than the mtz beneath the western pacific subduction zone even though the subducted slab material is stagnating similarly in both regions

a sporadic low velocity layer atop the 410 km discontinuity beneath the pacific ocean waveforms of ss precursors recorded by global stations are analyzed to investigate lateral heterogeneities of upper mantle discontinuities on a global scale a sporadic low velocity layer immediately above the 410 km discontinuity lvl 410 is observed worldwide including east asia western north america eastern south america the pacific ocean and possibly the indian ocean our best data coverage is for the pacific ocean where the lvl 410 covers 33 50 of the resolved region lateral variations of our lvl 410 observations show no geographical correlation with 410 km discontinuity topography or tomographic models of seismic velocity suggesting that the lvl 410 is not caused by regional thermal anomalies we interpret the lvl 410 as partial melting due to dehydration of ascending mantle across the 410 km discontinuity which is predicted by the transition zone water filter hypothesis given the low vertical resolution of ss precursors it is possible that the regions without a clear lvl 410 detection also have a thin layer therefore the strong lateral heterogeneity of the lvl 410 in our observations suggests partial melting with varying intensities across the pacific and further provides indirect evidence of a hydrous mantle transition zone with laterally varying water content

an experimental investigation of texture evolution during continuous cooling abstract textural evolution during a controlled cooling experiment has been quantified using bulk textural characterizations mean crystal length total numbers of crystals and modal olivine content and crystal size distributions csds the experiments involved subliquidus melting of an olivine glass mixture followed by cooling at a constant rate 92 c h 1 through the crystallization interval measured changes in population density n l for small 7 mm s 1 the csd trends for larger 0 3 mm skeletal crystals are broadly consistent with an exponentially increasing nucleation rate and with a uniform growth rate of 2 0 10 5 mm s 1 however the textural development of these crystals cannot be explained solely in terms of homogeneous nucleation and growth additional factors influencing the development of textures in the skeletal zone of the charges include grain impingement and development of skeletal overgrowths on the top of a layer of seed crystals csd data is ideal for analyzing texture development because it is sensitive to the detailed structure of the crystal population and can be used to detect variations in behavior between different size classes here population density trends for various crystal size classes are used as a tool for estimating the kinetics of crystal growth and nucleation in a cooling igneous system this type of analysis can be used to gain insight into crystallization processes from csds without relying on assumptions regarding csd shape examples of the use of population density trends in the evaluation of natural systems are provided for the case of crystallization in porphyritic olivine chondrules and in komatiite flows

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

aqueous alteration in the kaba cv3 carbonaceous chondrite the kaba cv3 carbonaceous chondrite has undergone pervasive aqueous alteration that produced fe bearing saponite in chondrules and matrix saponite in chondrules is coarse grained and shows a crystallographic orientation relationship such that c of saponite parallels a of enstatite matrix saponite exhibits a variety of textures including coarse and fine grained clusters isolated packets and sparse lamellar replacements of fe rich olivine the coarse grained clusters were probably derived from altered chondrules that were fragmented during regolith gardening the fine grained clusters and isolated packets formed from material released during the partial dissolution of matrix silicates much saponite is associated with submicron fe ni sulfides suggesting that conditions were not oxidizing during aqueous alteration we believe that the alteration occurred after accretion and on the kaba parent body because saponite is present throughout the sample by analogy to terrestrial occurrences the saponite in kaba probably formed at low temperatures 100 c the distribution of saponite in kaba is determined by local bulk compositions enstatite in chondrules has been altered more extensively than the fine grained olivine in matrix the alteration products in kaba resemble those in other aqueously altered carbonaceous chondrites such as the mokoia cv3 chondrite and in certain ci chondrites there are however significant differences kaba lacks the abundant high al phyllosilicates in chondrules and in cais reported from mokoia also lacking in kaba is the serpentine and ferrihydrite found in orgueil alteration products in kaba matrix are dissimilar from those in co3 chondrites even though their prealteration mineralogies were similar an observation that probably reflects different temperatures during alteration

density and thermal expansion of a peridotite melt at high pressure abstract density of a feo rich peridotite ma a model composition of martian mantle melt was investigated by high pressure experiments conducted at 6 3 18 0 gpa and 1800 2570 c we adopted the sink float method i e the sinking and flotation of a certain crystal density marker in the silicate melt is judged from its position in the run product recovered from high pressure and high temperature conditions the density marker moves in a vertical direction driven by the density difference between the melt and the marker we used olivine and diamond for the density marker the third order birch murnaghan equation of state was applied for a feo rich peridotite melt and its thermal expansion coefficients at 1980 c and bulk moduli at 1980 c and 2330 c were determined from the constraints obtained by sinking and flotation of olivine and diamond assuming that the pressure derivative of bulk modulus k equals 10 1 as the same value as that of fayalite melt agee 1992a the thermal expansion coefficients at 1980 c are 6 2 2 0 10 5 k 1 at 8 1 gpa and 5 9 1 8 10 5 k 1 at 15 1 gpa and the isothermal bulk moduli are 17 9 3 3 gpa at 1980 c and 17 7 4 5 gpa at 2330 c the density relations between the martian mantle melt and the liquidus crystals obtained in the present study suggest the enrichment of olivine at 600 km depth and of majorite at the bottom of the mantle in the crystallizing martian magma ocean

magma oceans in the inner solar system theory and observations point to the occurrence of magma ponds or oceans in the early evolution of terrestrial planets and in many early accreting planetesimals the apparent ubiquity of melting during giant accretionary impacts suggests that silicate and metallic material may be processed through multiple magma oceans before reaching solidity in a planet the processes of magma ocean formation and solidification therefore strongly influence the earliest compositional differentiation and volatile content of the terrestrial planets and they form the starting point for cooling to clement habitable conditions and for the onset of thermally driven mantle convection and plate tectonics this review focuses on evidence for magma oceans on planetesimals and planets and on research concerning the processes of compositional differentiation in the silicate magma ocean distribution and degassing of volatiles and cooling

northeastern pacific mantle conductivity profile from long period magnetotelluric sounding using hawaii to california submarine cable data we present results of a long period magnetotelluric mt investigation of the electrical structure beneath the eastern north pacific the electric field data consist of 2 years of continuously recorded voltages across an unpowered 4000 km long submarine telephone cable haw 1 extending from point arena california to oahu hawaii the electric field measurements are coherent to some degree with magnetic field measurements from honolulu observatory at periods of 0 1 to 45 days this coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length significantly diminishing their effect robust controlled leverage mt response estimates and their jacknife confidence limits are computed for the haw 1 to honolulu data an equivalent scalar mt response obtained from honolulu magnetic variations data is used to correct the haw 1 mt response for static shift and to extend the mt response estimate to periods of 100 days the composite response function satisfies necessary and sufficient conditions for consistency with a one dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km inversion of the mt response reveals a conductive zone 0 05 0 1 s m between 150 and 400 km depth and a positive gradient below 500 km these observations are consistent with previous mt studies in the north pacific this upper mantle conductivity is too high to be explained by solid state conduction in dry olivine using reasonable mantle geotherms calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that h dissolved in olivine possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy provide sufficient conductivity enhancement to explain the inversion results the high conductivity may also be explained by the presence of gravitationally stable partial melt comparison of the haw 1 results with long period mt studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes in particular the upper mantle beneath the pacific ocean is considerably more conductive than that beneath the canadian shield and similar in conductivity to that beneath the basin and range

electrical conductivity of hydrous silicate melts implications for the bottom up hydration of earth s upper mantle abstract the upwelling of the hydrous mantle transition zone triggers dehydration induced partial melting atop the 410 km discontinuity here we investigate the electrical conductivity of hydrous silicate melts in the 200 400 km depth range and explore whether melting at the 410 km depths is responsible for the hydration of the upper mantle our experimental electrical conductivity data demonstrate that the mantle at 180 350 km depths is mostly melt free confirming the h2o under saturated conditions however the residual mantle from partial melting atop the 410 km discontinuity may contain various possible amounts of water according to the initial mantle transition zone and melt concentrations this residual h2o could contribute to the hydration of the upper mantle either through diffusion or material replacement by upwelling our calculations suggest that the diffusion may not be responsible for the hydration of the upper mantle to present h2o concentration of 50 200 ppm wt melting of the upwelling mantle transition zone with less than 1500 ppm wt h2o produces residual peridotites with 200 ppm h2o at the 410 km discontinuity continuous upwelling of such hydrous residues would gradually replace the dry upper mantle with depleted residual hydrous peridotites in less than 260 ma in this study we propose a bottom up hydration mechanism for the earth s upper mantle driven by dehydration melting at the 410 km discontinuity the hydrous partial melting at the top of the asthenosphere appears to be a consequence of h2o saturation in the upwelling residual peridotites

formation of k feldspar megacrysts in granodioritic plutons by thermal cycling and late stage textural coarsening k feldspar megacrysts in granite and granodiorite plutons are generally inferred to be early crystallizing phases grown to large sizes when the magma was mostly liquid owing to their large size euhedral form and features that suggest deposition by magmatic sedimentation however phase equilibrium experiments and natural examples of crystallization and partial melting demonstrate that k feldspar is one of the last phases to nucleate and that most crystal growth must occur after the magma has exceeded 50 crystallization and is thus largely incapable of flow and sedimentation megacryst size distributions compositions and textural relationships from the cretaceous tuolumne intrusive suite california reveal that the gradational transition from equigranular to megacrystic granodiorite likely occurred via textural coarsening caused by thermal cycling experimental and theoretical studies demonstrate that rising temperature induces relatively more melting in small crystals than in large ones whereas linear growth rates during cooling are similar thus during thermal cycling material is transferred from small crystals to larger ones megacryst growth via thermal cycling during incremental emplacement is consistent with the required late growth of k feldspar explains the presence of megacrysts in the inner parts of thetuolumne intrusive suite and elsewhere and may be a common process in formation of megacrystic granitic rocks

water induced convection in the earth s mantle transition zone 1 water enters the earth s mantle by subduction of oceanic lithosphere most of this water immediately returns to the atmosphere through arc volcanism but a part of it is expected as deep as the mantle transition zone 410 660 km depth there slabs can be deflected and linger before sinking into the lower mantle because it lowers the density and viscosity of the transition zone minerals i e wadsleyite and ringwoodite water is likely to affect the dynamics of the transition zone mantle overlying stagnant slabs the consequences of water exchange between a floating slab and the transition zone are investigated in particular we focus on the possible onset of small scale convection despite the adverse thermal gradient i e mantle is cooled from below by the slab the competition between thermal and hydrous effects on the density and thus on the convective stability of the top layer of the slab is examined numerically including water dependent density and viscosity and temperature dependent water solubility for plausible initial water content in a slab 0 5 wt an episode of convection is likely to occur after a relatively short time delay 5 20 ma after the slab enters the transition zone however water induced rheological weakening is seen to be a controlling parameter for the onset time of convection moreover small scale convection above a stagnant slab greatly enhances the rate of slab dehydration small scale convection also facilitates heating of the slab which in itself may prolong the residence time of the slab in the transition zone

crystal size distribution analysis of plagioclase in experimentally decompressed hydrous rhyodacite magma article i nfo this study presents crystal size distributions csd of plagioclase forming during decompression experiments of hydrous rhyodacite magma samples were annealed at 130 mpa subjected to continuous decompression at either 2 mpa h 1 or 0 5 mpa h 1 and then quenched along their respective decompression paths at 20 mpa intervals samples display concave up curved csd plots which result from a combination of two crystal populations those which formed during the anneal period and those which formed during decompression the csds also display a decrease in crystal number density at 3 7 m a size range that is easily resolvable with the imaging strategy employed the downturn at small sizes is presumably due to insufficient compensation for the intersection probability effect in converting 2d measurements to 3d size distributions crystal nucleation and growth rates derived from csds using standard assumptions are compared with values obtained using 2d measurements of bulk crystal populations batch methods csd calculated nucleation rates are substantially low relative to the batch values in fact csd derived volumetric nucleation rates may underestimate actual nucleation rates by up to two orders of magnitude in contrast growth rates from csds are consistently higher than batch rates although plagioclase growth rates are relatively constant during decompression at a given rate the average growth rate in the rapidly decompressed series is approximately five times faster than the crystal growth rate in the slowly decompressed series because crystal growth rate depends on decompression rate csds are incapable of revealing decompression timescales or magma ascent rates without independent knowledge of crystal growth rate published by elsevier b v

density of basaltic melt at high pressure and stability of the melt at the base of the lower mantle abstract we made density measurements of model morb and picritic melts up to about 15 gpa using the floating diamond method the compression behavior of the melts is fitted by the vinet equation of state and extrapolated to higher pressure a density crossover between the morb melt and the mantle peridotite is suggested at the base of the lower mantle thus the ultra low velocity zone in the d layer may be explained by the existence of gravitationally stabilized basaltic melt formed by melting of the accumulated slab materials this region might become unstable due to progressive melting of slab materials and or supply of volatiles

melting relations of peridotite and the density crossover in planetary mantles melting relations of primitive peridotite were studied up to 25 gpa the change of the liquidus phase from olivine to majorite occurs at 16 gpa we confirmed the density crossover of the feo rich peridotite melt and the equilibrium olivine fo90 at 7 gpa sinking of equilibrium olivine fo95 in the primitive peridotite melt was observed up to 10 gpa the compression curves of feo rich peridotitic and komatiite melts reported in this and earlier work suggest that the density crossover in the earth s mantle will be located at 11 12 gpa at 2000 c consistent with an previous estimation by c b agee and d walker the density crossover can play a key role in the moon and the terrestrial planets such as the earth venus and mars majorite and some fraction of melt could have separated from the ascending diapir and sunk downwards at the depths below the density crossover this process could have produced a garnet rich transition zone in the earth s mantle the density crossover may exist in the feo rich lunar mantle at around the center of the moon the density crossover which exists at the depth of 600 km in the martian mantle plays a key role in producing a fractionated mantle which is the source the parent magmas of the snc meteorites

rethinking the emplacement and evolution of zoned plutons geochronologic evidence for incremental assembly of the tuolumne intrusive suite california new u pb geochronologic data indicate that the tuolumne intrusive suite california was assembled over a period of at least 10 m y between 95 and 85 ma and that the half dome granodiorite intruded over a period approaching 4 m y simple thermal considerations preclude the possibility that a magma chamber the size of the half dome pluton could have existed as a liquid at shallow crustal depths for that long rather field evidence for sheeting along the margins of the suite the range of ages and the regular decrease of ages toward the center of the suite and within individual plutons suggest incremental assembly geochronologic evidence for incremental assembly is consistent with the failure of geophysical methods to detect large magma chambers with more than 20 melt even in active volcanic areas because it is unlikely that the individual plutons composing the tuolumne ever coexisted as liquid rich magmas the chemical evolution of the suite cannot be the result of simple fractionation and or mixing between exposed units but instead must reflect processes occurring during magma generation

an overview of seasonal snow metamorphism the grains in seasonal snow undergo rapid and radical transformations in size shape and cohesion these grain characteristics affect all of the basic properties of snow snow is characterized as either wet or dry depending on the presence of liquid water wet snow is markedly different at low and high liquid contents dry snow is characterized as either an equilibrium form or a kinetic growth form that is it is either well rounded or faceted of course many snow grains display either transitional features between two of these categories or features which arise from other processes snow is classified depending on the dominant processes of its metamorphism