oxygen isotopic compositions of individual minerals in antarctic micrometeorites further links to carbonaceous chondrites abstract we report in situ measurements of oxygen isotopic abundances in individual silicate and oxide minerals from 16 antarctic micrometeorites amms the oxygen isotopic compositions of 10 olivine and 11 pyroxene grains are enriched in 16 o relative to terrestrial minerals and on an oxygen three isotope diagram they plot on the low 18 o side of the 16 o mixing line defined by calcium aluminum rich inclusions cai from chondritic meteorites amm olivine and pyroxene 18 o values range from 9 9 to 8 0 and 17 o ranges from 11 3 to 5 5 similar to values measured in individual olivine grains and whole chondrules from carbonaceous chondrites these data indicate that the mineral grains preserve their pre terrestrial oxygen isotopic compositions and provide another link between amms and carbonaceous chondrites however no clear relationship with one single subgroup of carbonaceous chondrite can be established based on their textures crystal chemistries and oxygen isotopes some coarse grained crystalline amms could originate from chondrule fragmentation whether the remaining mineral grains were formed by igneous or condensation processes is unclear no clear correlation is observed between isotopic compositions and mineral compositions of amm olivine grains suggesting that the feo and 16 o enrichment processes are not coupled in a simple way nor are any relatively large 16 o enrichments measured in any of the olivine grains however two mg al spinels and a melilite grain are 16 o enriched at the level of 18 o 17 o 40 the discovery of an 16 o enriched melilite grain in amms supports the hypothesis that refractory minerals throughout the solar nebula formed from a relatively uniformly 16 o enriched reservoir this unique 16 o rich signature of refractory minerals in primitive solar system materials suggests that they either formed from a widespread 16 o rich reservoir in the solar nebula or that an efficient mechanism such as bipolar outflows was acting to spread them from a highly localized 16 o rich region over the early solar nebula

the classification of micrometeorites due to their small size the mineralogical and chemical properties of micrometeorites mms are not representative of their parent bodies on the centimeter to meter scales that are used to define parent body groups through the petrological study of meteorites identifying which groups of mm are derived from the same type of parent body is problematic and requires particles to be rigorously grouped on the basis of mineralogical textural and chemical properties that reflect the fundamental genetic differences between meteorite parent bodies albeit with minimal bias towards preconceived genetic models specifically the interpretation of mms requires a rigorous and meaningful classification scheme at present the classification of mms is however at best ambiguous a unified petrological chemical classification scheme is proposed in the current study and is based on observations of several thousand mms collected from antarctic ice

interpretation of porosity in dacite lava domes as ductile brittle failure textures abstract pores in dacite lavas of three volcanoes in japan unzen yakedake and daisen have characteristics unlike vesicles most of the pores have ragged walls marked by the protrusion of groundmass crystals and phenocrysts the shapes of these pores range from irregular to planar many pores occur in pressure shadows adjacent to phenocrysts some pores are crack like and pass around phenocrysts and have blunt terminations most of the pores are interpreted as the result of failure of the magma by flow during ductile brittle transition this phenomenon known as cavitation is well established in hydraulics and the material sciences cavitation of the lavas in this study was promoted by steep slope of emplacement subtle mechanical contrasts between bands and strain intensification around rigid crystals and lithic fragments cavitation in lava can extract volatiles from magma causing premature embrittlement and weakening cavitation is inferred to be critical to the initiation of lava front pyroclastic flows

the leonard award address presented 1996 july 25 berlin germany the elemental composition of stony cosmic spherules five hundred stony cosmic spherules collected from deep sea sediments polar ice and the strato sphere have been analyzed for major and some minor element composition typical spherules are products of atmospheric melting of millimeter sized and smaller meteoroids the samples are small and modified by atmospheric entry but they are an important source of information on the composition of asteroids the spherules in this study were all analyzed in an identical manner and they provide a sampling of the solar system s asteroids that is both different and less biased than provided by studies of conventional meteorites volatile elements such as na and s are depleted due to atmospheric heating while siderophiles are depleted by less understood causes the refractory nonsiderophile elements appear not to have been significantly dis turbed during atmospheric melting and provide important clues on the elemental composition of millimeter sized meteoroids colliding with the earth typical spherules have cm like composition that is distinctively different than ordinary chondrites and most other meteorite types we assume that c type asteroids are the primary origin of spherules with this composition type s asteroids should also be an important source of the spherules and the analysis data provide constraints on their composition a minor fraction of the spher ules are melt products of precursor particles that did not have chondritic elemental compositions the most common of these are particles that are dominated by olivine the observed compositions of spherules are in consistent with the possibility that an appreciable fraction of the spherules are simply chondrules remelted during atmospheric entry

accretion rate of cosmic spherules measured at the south pole micrometeorites are terrestrially collected extraterrestrial particles smaller than about 1 mm which account for most of the mass being accreted to the earth1 2 compared with meteorites micrometeorites more completely represent the earth crossing meteoroid complex3 4 and should include fragments of asteroids comets mars and our moon as well as pre solar and interstellar grains3 6 previous measurements of the flux of micrometeoroids that survive to the earth s surface have large uncertainties owing to the destruction of particles by weathering7 8 9 inefficiencies in magnetic collection or separation techniques7 8 9 low particle counts10 11 poor age constraint7 8 9 12 13 or highly variable concentrating processes12 13 here we describe an attempt to circumvent these problems through the collection of thousands of well preserved and dated micrometeorites from the bottom of the south pole water well which supplies drinking water for the scott amundsen station using this collection we have determined precise estimates of the flux and mass distribution for 50 700 m cosmic spherules melted micrometeorites allowing for the expected abundance of unmelted micrometeorites14 in the samples our results indicate that about 90 of the incoming mass of submillimetre particles evaporates during atmospheric entry our data indicate the loss of glass rich and small stony spherules from deep sea deposits7 8 and they provide constraints for models describing the survival probability of micrometeoroids15 16

abundance of acidic amino acids and non protein amino acids in carbonates and muddy sediments and their relationship to diagenetic decomposition amino acid compositions of recent skeletal carbonates are usually characterized by high concentrations of the acidic amino acids aspartic acid and glutamic acid asp glu and absence or trace amount of the non protein amino acids alanine and aminobutyric acid ala aba and these characteristics are preserved to the pleistocene acidic amino acids in muddy core sediments decrease markedly as depth increases while on the other hand non protein amino acids do not indicate such a trend therefore the non protein amino acid acidic amino acid ratio increases with depth the ratio of non protein amino acids to the total amino acids amounts to 30 in pliocene mudstones decomposition rates of the acidic amino acid for shells and muddy sediments were determined by laboratory heating experiments acidic amino acids decomposed more rapidly in the muddy sediments than in shells the non protein amino acids seem to be produced by heating of powdered shells and muddy sediments it was shown in another heating experiment that both the acidic and non protein amino acids incorporated in humic acids decomposed more rapidly when mixed with montmorillonite than with ca carbonates the results of these heating experiments seem to confirm geological observations for these acidic and non protein amino acids in carbonate materials and muddy sediments

magma reservoirs from the upper crust to the moho inferred from high resolution vp and vs models beneath mount st helens washington state usa the size frequency and intensity of volcanic eruptions are strongly controlled by the volume and connectivity of magma within the crust several geophysical and geochemical studies have produced a comprehensive model of the magmatic system to depths near 7 km beneath mount st helens washington state usa currently the most active volcano in the cascade range data limitations have precluded imaging below this depth to observe the entire primary shallow magma reservoir as well as its connection to deeper zones of magma accumulation in the crust the inversion of p and s wave traveltime data collected during the active source component of the imush imaging magma under st helens project reveals a high p wave vp s wave vs velocity anomaly beneath mount st helens between depths of 4 and 13 km which we interpret as the primary upper middle crustal magma reservoir beneath and southeast of this shallow reservoir a low vp velocity column extends from 15 km depth to the moho deep long period events near the boundary of this column indicate that this anomaly is associated with the injection of magmatic fluids southeast of mount st helens an upper middle crustal high vp vs body beneath the indian heaven volcanic field may also have a magmatic origin both of these high vp vs bodies are at the boundaries of the low vp middle lower crustal column and both are directly above high vp middle lower crustal regions that may represent cumulates associated with recent quaternary or paleogene neogene cascade magmatism seismicity immediately following the 18 may 1980 eruption terminates near the top of the inferred middle lower crustal cumulates and directly adjacent to the inferred middle lower crustal magma reservoir these spatial relationships suggest that the boundaries of these high density cumulates play an important role in both vertical and lateral transport of magma through the crust

hydrolysable amino acids in sediments from the porcupine abyssal plain northeast atlantic ocean abstract the distributions of hydrolysable amino acids have been examined in replicate sediment cores from the porcupine abyssal plain in the northeastern atlantic ocean the amino acid concentrations in surficial sediments ranged from 11 4 to 12 6 mol g 1 dry sediment with a mean of 11 8 mol g 1 dry sediment and decreased with increasing sediment depth but showed a marked maximum between 20 and 40 mm albeit with significant variability in their abundance at that depth normalisation of the concentrations of hydrolysable amino acids to total organic carbon showed that the organic matter at the surface and in the sub surface maximum was relatively fresh the vertical concentration profiles of hydrolysable amino acids were also reflected by organic carbon and total nitrogen and are attributed to the transfer of relatively fresh organic matter from the sediment surface by bioturbation probably by large detritovores the depth of the zone of bioturbation i e 40 mm is consistent with the zone of maximum biological activity in the porcupine abyssal plain sediments

chondrules in antarctic micrometeorites previous studies of unmelted micrometeorites 50 m recovered from antarctic ice have concluded that chondrules which are a major component of chondritic meteorites are extremely rare among micrometeorites we report the discovery of eight micrometeorites containing chondritic igneous objects which strongly suggests that at least a portion of coarse grained crystalline micrometeorites represent chondrule fragments six of the particles are identified as composite micrometeorites that contain chondritic igneous objects and fine grained matrix these particles suggest that at least some coarse grained micrometeorites cgmms may be derived from the same parent bodies as fine grained micrometeorites the new evidence indicates that contrary to previous suggestions the parent bodies of micrometeorites broadly resemble the parent asteroids of chondrulebearing carbonaceous chondrites

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

dolomite dissolution kinetics in aqueous solutions in the presence of organic and inorganic additives at 25 c and pco 2 1 atm abstract the steady state dissolution rate of dolomite was studied at 25 c and 1 atm co2 total pressure in deionised water in the absence control and presence of twenty seven different organic and inorganic additives at low concentration 1 mm these additives are most likely present at low concentrations

vapor transfer prior to the october 2004 eruption of mount st helens washington dome lavas from the 2004 eruption of mount st helens show elevated li contents in plagioclase phenocrysts at the onset of dome growth in october 2004 these cannot be explained by variations in plagioclase melt partitioning but require elevated li contents in coexisting melt a fact confirmed by measurements of li contents as high as 207 g g in coexisting melt inclusions similar li enrichment has been observed in material erupted prior to and during the climactic may 1980 eruption and is likewise best explained via pre eruptive transfer of an exsolved alkali rich vapor phase derived from deeper within the magma transport system unlike 1980 however high li samples from 2004 show no evidence of excess 210pb 226ra implying that measurable li enrichments may occur despite significant differences in the timing and or extent of magmatic degassing diffusion modeling shows that li enrichment occurred within 1 yr before eruption and that magma remained li enriched until immediately before eruption and cooling this short flux time and the very high li contents in ash produced by phreatomagmatic activity prior to the onset of dome extrusion suggest that vapor transfer and accumulation were associated with initiation of the current eruption overall observation of a high li signature in both 1980 and 2004 dacites indicates that li enrichment may be a relatively common phenomenon and may prove useful for petrologic monitoring of mount st helens and other silicic volcanoes lithium diffusion is also sufficiently rapid to constrain vapor transfer on similar time scales to short lived radionuclides

mineralogy of tagish lake an ungrouped type 2 carbonaceous chondrite in this paper we describe the recovery handling and preliminary mineralogical investigation of the tagish lake meteorite tagish lake is a type 2 carbonaceous chondrite which bears similarities to ci1 and cm chondrite groups but is distinct from both abundant phyllosilicates as well as chondrules however sparse and common olivine grains in the matrix preclude any other classification the bulk density of tagish lake 1 67 g cc which is far lower than ci or cm chondrites 2 2 2 3 and 2 6 2 9 g cc respectively or any other meteorite for that matter we have identified two lithologies a dominant carbonate poor lithology and a less abundant carbonate rich lithology the meteorite is a breccia at all scales we have noted similarities between tagish lake and some clasts within the enigmatic meteorite kaidun possibly there are genetic relationships here worth exploring in the paper we describe a clast of cm1 material within tagish lake which is very similar to a major lithology in kaidun

igneous rims on micrometeorites abstract melting of micrometeorites mms due to atmospheric entry heating causes significant changes in the textures mineralogies and compositions of particles that obscure their primary properties and greatly complicate interpretation of these extraterrestrial materials despite the abundance of melted mms the nature of melting processes in these materials is poorly constrained in this study mineralogical textural and compositional data on 77 mms with igneous rims are presented which suggest that fusion of micrometeorites during atmospheric entry occurs initially by surface melting textural and mineralogical evidence are presented that demonstrate unequivocally that igneous rims crystallized from a melt surrounding a largely unmelted core and establish melting as a gradational process the compositions of igneous rims on fine grained mms fgmms are broadly similar to those of the unmelted core except for depletions in volatile and moderately volatile elements produced by partial evaporation and suggest the formation of the rims by melting of the fine grained matrix core enrichments in fe si ni si and mn si in igneous rims compared with unmelted cores within fgmms are suggested to occur due to the migration of fe s eutectic liquids from the core of the particle into the surface melt layer the sulphide liquids are probably generated in the core of the particle under reducing conditions resulting by the pyrolysis of carbonaceous materials the presence of igneous rims on fgmms is enabled by the thermal decomposition of phyllosilicates and therefore indicates that fgmms were hydrated particles prior to atmospheric entry in contrast the compositions of igneous rims on coarse grained mms cgmms are difficult to reconcile with those of the unmelted core and instead closely resemble those of fgmms the igneous rims of cgmms are therefore suggested to form by melting of fine grained matrix that was present on the exterior of these particles prior to atmospheric entry coarse grained mms with igneous rims thus were originally composite micrometeoroids which consist of both fine grained and coarse grained materials and are thought to be samples of chondrule like objects the abundance of composite mms to cgmms allows a first estimate of the mean chondrule radius within the parent bodies of mms of 625 m to be made

numbers types and compositions of an unbiased collection of cosmic spherules abstract abstract micrometeorites collected from the bottom of the south pole water well spww may represent a complete well preserved sample of the cosmic dust that accreted on earth from 1100 1500 a d we classified 1588 cosmic spherules in the size range 50 800 pm the collection has 41 barred olivine spherules 17 glass spheres 12 cryptocrystalline spherules 11 porphyritic olivine spherules 12 relic grain bearing spherules 3 scoriaceous spherules 2 i type spherules 1 ca al ti rich cat spherules and 1 g type spherules we also found bubbly glass spherules spherules with glass caps and ones with sulfide coatings particles that are absent from other collections a classification sequence of the stony spherules scoriaceous relic grain bearing porphyritic barred olivine cryptocrystalline glass and cat is consistent with progressive heating and evaporation of fe from chondritic materials the modem day accretion rate and size distribution measured at the spww can account for the stony spherules present in deep sea collection through preferential dissolution of glass and small stony spherules however weathering alone cannot account for the high accretion rate of i type spherules determined for two deep sea collections the spww collection provides data to constrain models of atmospheric entry heating and to assess the effects of terrestrial weathering

the structure and composition of microspheres from the kilauea volcano hawaii microspheres from the plume of the kilauea volcano hawaii display previously unrecognized structural and compositional features indicating that the spheres are concentrically differentiated and at least partially crystalline the surfaces of some spheres display evenly spaced hexagonal sub micrometer grains whose compositions suggest that they may be spinels distinctive prismatic and dendritic structures on the surfaces of the microspheres are remarkably consistent through the population and indicate that the spheres have crystalline outer shells indications of chemical etching are present probably from exposure to hf in the volcanic plume the structures and compositions of the microspheres suggest that they differentiated and then solidified during rapid cooling as they passed through the gradients of temperature chemical composition and redox conditions established in the plume as air diluted the gaseous species evolved from magma

the origin of mount st helens andesites abstract mount st helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants the mafic lavas include a transitional olivine plagioclase basalts with low k 2 o and incompatible trace element abundances b calc alkaline olivine plagioclase clinopyroxene basalts enriched in k 2 o tio 2 and incompatible trace elements and c calc alkaline olivine plagioclase basaltic andesites with incompatible trace element contents transitional between the two basalt types intermediate lavas include a tholeiitic two pyroxene andesites b calc alkaline plagioclase two pyroxenes olivine amphibole mafic andesites 56 59 wt sio 2 and c calc alkaline plagioclase two pyroxenes amphibole high silica andesites 61 62 wt sio 2 eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches it is unlikely that any of the andesites or dacites are derived by fractional crystallization of the recognized basaltic variants formation of the andesites simply by contamination or assimilation fractional crystallization of basaltic magma is also improbable more plausibly the andesites represent mixing between basaltic and dacitic end member magmas each of which may be somewhat heterogeneous or vary in composition with time in this model efficient mixing must occur in some parts of the magma plumbing system while some conduits or storage reservoirs must be effectively isolated

distribution and compositions of magmatic inclusions in the mount helen dome lassen volcanic center california insights into magma chamber processes abstract variations in spatial abundances compositions and textures of undercooled magmatic inclusions were determined in a glaciated pleistocene lava dome mt helen 0 6 km3 at the lassen volcanic center lvc southernmost cascades spatial variations were determined by point counting at 86 locations separated by 100 m on the dome major and trace element compositions of host rocks and inclusions at 12 locations along the flow length of the dome were obtained important results include the following 1 inclusion abundances range from 3 19 vol with the highest values generally located along the little eroded northwestern margin and flow front of the dome 2 host rock compositions are markedly uniform across the dome 65 4 0 4 wt sio2 indicating that the degree of inclusion disaggregation was uniform despite large spatial variations in inclusion abundances 3 inclusion sizes range from a maximum of 1 m across to mm sized crystal clots of phenocrysts plus adhering ca rich plagioclase microphenocrysts 4 inclusions have variable macroscopic textures indicating that partial undercooling both prior to and following entrapment in cooler dacitic host magma were important processes 5 inclusions are variably fractionated magmas with large variations in ni 79 11 ppm and cr 87 7 ppm contents that are lower than presumed mantle derived melts furthermore large ranges in incompatible trace elements indicate that inclusion compositions also reflect deep processes involving either melting of variable mantle source rocks or assimilation fractional crystallization 6 inclusions are variably mixed magmas 56 61 wt sio2 that contain up to 50 host dacitic magma 7 correlations between ni and cr contents in hosts and inclusions from individual outcrops indicate that the effect of inclusion disaggregation and magma mingling on host dacitic magma was local e g these features are interpreted to reflect protracted recharge of diverse composition mafic magmas into the base of a shallow magma reservoir containing a lower layer of compositionally evolving hybrid mafic magma and an upper layer of rhyodacitic magma complex interactions among magmas in the upper and lower layers resulted in formation of mafic inclusions by both pre and post entrapment undercooling mechanisms followed by buoyant rise and accumulation near the roof of the reservoir the main stage of inclusion disaggregation probably occurred in the conduit during eruptive ascent and largely ceased upon surface eruption we infer that endogenous growth concentrated inclusions along the margins and top of the dome as more inclusion poor dacite was progressively tapped from deeper regions of the reservoir

petrology and geochemistry of antarctic micrometeorites abstract the petrology and geochemistry of twentythree chondritic dust particles with masses of 1 47 g sizes 100 400 m were recovered from blue ice near cap prudhomme antarctica and studied by inaa asem empa and optical microscopy sample selection criteria were irregular shape and for a subsample black color with the aim of studying as many unmelted micrometeorites mms as possible of thirteen unmelted mms six were phyllosilicate dominated mms and seven were coarsegrained crystalline mms consisting mainly of olivine and pyroxene the remaining ten particles were largely melted and consisted of a foamy melt with variable amounts of relic phases scoriaceous mms thus of the black particles selected an astonishing portion 40 by number consisted of largely unmelted mms although unmelted most phyllosilicate mms have been thermally metamorphosed to a degree that most of the phyllosilicates were destroyed but not melted the original preterrestrial mineralogy is occasionally preserved and consists of serpentine like phyllosilicates with variable amounts of cronstedtite tochilinite like oxides olivine and pyroxene the crystalline mms consist of olivine low ca pyroxene tochilinite like oxides and occasional ni poor metal relics in scoriaceous mms consist of the same phases mineral compositions and the coexistence of phyllosilicates with anhydrous phases are typical of cm and cr type carbonaceous chondrites however the olivine pyroxene ratio 1 and the lack of carbonates sulfates and of very fe poor refractory element rich olivines and pyroxenes sets the mms apart from cm and cr chondrites the bulk chemistry of the phyllosilicate mms is similar to that of cm chondrites however several elements are either depleted ca ni s less commonly na mg and mn or enriched k fe as br rb sb and au in mms as compared to cm chondrites similar depletions and enrichments are also found in the scoriaceous mms we suggest that the depletions are probably due to terrestrial leaching of sulfates and carbonates from unmelted mms the overabundance of some elements may also be due to processes acting during atmospheric passage such as the recondensation of meteoric vapors in the high atmosphere most mms are coated by magnetite of platy or octahedral habit which is rich in mg al si mn and ni we interpret the magnetites to be products of recondensation processes in the high 90 km atmosphere which are therefore probably the first refractory aerominerals identified

tomographic inversion for three dimensional velocity structure at mount st helens using earthquake data tomographic inversion is applied to 17 659 p phase observations at 21 stations from 2023 earthquakes in the vicinity of mount st helens to study the three dimensional velocity structure block size for the inversion is 2 km horizontally and 2 km or more vertically locations of hypocenters are assumed known and are based on a reference one dimensional layered velocity structure a conjugate gradient technique lsqr is used to invert the large sparse system of equations augmented by regularization with a laplacian roughening matrix resolution is estimated by computing the impulse response of the inversion for various critical locations and uncertainties of the estimates are determined by a jackknife approach the results of the inversion show a remarkable correlation with known geological and geophysical features the spirit lake and spud mt plutons are characterized by high velocity regions extending to approximately 9 km depth the st helens seismic zone a band of diffuse seismicity extending nnw from the volcano is evident as a prominent low velocity lineation the change in character of the velocity anomalies south of st helens corresponds well with the near cessation of seismic activity there a low velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations

magnetization of polar ice a measurement of terrestrial dust and extraterrestrial fallout abstract laboratory induced remanent magnetization of polar ice constitutes a measurement of the magnetization carried by the ferromagnetic dust particles in the ice this non destructive technique provides a novel kind of information on the dust deposited on the surface of polar ice sheets measurements made on ice samples from greenland north grip ice core and antarctica vostok and epica dome c ice cores allowed the recognition of a fraction of magnetic minerals in ice whose concentration and magnetic properties are directly related to that of insoluble dust the source of this fraction of magnetic minerals thus appears closely related to terrestrial dust transport and deposition and its magnetic properties are informative of the dust provenance areas the rock magnetic properties of the dust may reflect distinct changes of dust source areas from glacial to interglacial periods in agreement with and adding further information to the isotopic 87 sr 86 sr and 143 nd 144 nd analyses a second magnetic fraction consists of particles of nanometric size which are superparamagnetic at freezer temperature and whose concentration is independent of the mass of aerosol dust found in the ice the source of these nanometric sized magnetic particles is ascribed to fallout of meteoric smoke and their concentration in ice was found to be compatible with the extraterrestrial fallout inferred from ir concentrations the diameter of the smoke particles as inferred from magnetic measurements is in the range of about 7 20 nm

the textures and compositions of fine grained antarctic micrometeorites implications for comparisons with meteorites abstract micrometeorites recovered from the earth s surface constitute the most abundant interplanetary dust now falling to earth we studied eighty nine fine grained antarctic micrometeorites fg amms to evaluate their state of alteration and to identify the nature of their precursor materials fine grained amms are divided into melted and unmelted groups and subdivided on the basis of textures related to atmospheric entry heating the textures of melted particles reflect crystallisation after entry heating and those of unmelted amms broadly resemble cl cm and cr chondrite matrix the matrix compositions of the fg amms closely resemble those of cm2 chondrites matrices provide a more reliable comparison to meteorites than bulk compositions because components such as chondrules and cais are not present in representative proportions within individual amms divergences from cm like matrix compositions are generally minor and probably arose during entry heating by the dissociation of volatile bearing phases at subsolidus temperatures and evaporation and loss of immiscible metallic liquids at higher temperatures depletions in ni and mg relative to cm matrix are tentatively attributed to terrestrial weathering no conclusive evidence for contamination during atmospheric residence was observed contrary to theory textural evidence suggests that bow shocks and high thermal gradients existed during deceleration of micrometeoroids in the atmosphere

production and detachment of oxide crystal shells on bubble walls during experimental vesiculation of andesitic magmas the mutual affinity between bubbles and oxide crystals especially magnetite is well established and their tendency to remain in contact once they become connected either by nucleation of one upon the other or by attachment has led to models of oxide transport via bubbles in natural melts however despite the widespread acceptance of bubble oxide association there is little direct textural evidence for these processes we present results from a series of decompression experiments on andesitic melts during which aggregates of bubbles and oxides formed because of hydrogen loss through the capsule walls causing oxidation of the melt experimental charges were imaged using 3d x ray computed tomography that revealed complex bubble oxide aggregates with small oxide crystals coating part of the outer bubble surfaces in a shell like morphology these shells have smooth inner and rugose outer surfaces sometimes additional concentric shells or partial shells can be found around bubbles in the glass between the bubble wall and another shell we quantified the volumes of bubbles and oxides and the oxides compositions we measured the surface area where the bubbles and oxides are in contact thus quantifying their interface in 3d and used these measurements to investigate the process of oxide shell formation the complexity of the oxide textures when studied in 3d reveals a range of bubble oxide interactions from continuous generation detachment and disintegration these processes carry important implications on why such textures seem to have a low preservation potential in natural environments nevertheless we have found natural samples that resemble our experimental results in a range of rock compositions from different geological environments that could have formed either due to rapid oxidation via the fluid phase or by bubbles harvesting different crystals

partially melted lithic megablocks in the yardea dacite gawler range volcanics australia implications for eruption and emplacement mechanisms lithic megablocks ranging from 200 m felsic volcanic unit in the mesoproterozoic gawler range volcanic province grv of south australia throughout its vast extent the yardea dacite shows typical lava like features in that it is massive columnar jointed and evenly porphyritic with 30 40 crystals in a spherulitic and granophyric groundmass in addition flow banding is present at many locations the megablocks are abundant at two sites 50 km apart but isolated megablocks and smaller 6 cm lithic clasts are also scattered throughout the unit at both sites the megablocks are matrix supported non graded randomly oriented and show no evidence of being confined to a particular stratigraphic level in the dacite the most abundant and largest megablocks are granitoids derived from older basement and from early crystallised plutons of the hiltaba suite which is broadly coeval and comagmatic with the grv the granitoid megablocks have been partially melted most likely prior to eruption when resident in the thermal aureole of the yardea dacite magma chamber the lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas consistent with the lava like character of the host dacite using reasonable estimates of megablock density magma density and magma viscosity we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks implying that they could have been entrained and erupted effusively all but the largest and least melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow the rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber dislodging megablocks that were later entrained and effusively erupted

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

carbonaceous micrometeorites from antarctica over 100 000 large interplanetary dust particles in the 50 500 micrometers size range have been recovered in clean conditions from approximately 600 tons of antarctic melt ice water as both unmelted and partially melted dehydrated micrometeorites and cosmic spherules flux measurements in both the greenland and antarctica ice sheets indicate that the micrometeorites deliver to the earth s surface approximately 2000x more extraterrestrial material than brought by meteorites mineralogical and chemical studies of antarctic micrometeorites indicate that they are only related to the relatively rare cm and cr carbonaceous chondrite groups being mostly chondritic carbonaceous objects composed of highly unequilibrated assemblages of anhydrous and hydrous minerals however there are also marked differences between these two families of solar system objects including higher c o ratios and a very marked depletion of chondrules in micrometeorite matter hence they are chondrites without chondrules thus the parent meteoroids of micrometeorites represent a dominant and new population of solar system objects probably formed in the outer solar system and delivered to the inner solar system by the most appropriate vehicles comets one of the major purposes of this paper is to discuss applications of micrometeorite studies that have been previously presented to exobiologists but deal with the synthesis of prebiotic molecules on the early earth and more recently with the early history of the solar system

partial melting at the earth s surface implications for assimilation rates and mechanisms in subvolcanic intrusions abstract the relatively recent routine availability of isotopic and elemental analytical results from large sample suites of igneous rocks has revolutionized the study of magmatic processes large uncertainties remain however with regard to the highly variable rates of magma differentiation and ascent through the crust although the repose interval prior to an eruption may be equivalent to magma residence time at a certain level in the crust this cannot be assumed a priori even where this happens to be the case uncertainties in determining such intervals for prehistoric eruptive sequences may be large with respect to process rates consequently independent insights from geologically well constrained situations must be sought this paper first presents documentation of an example wherein fragments of dacite incorporated by a mafic lava flow on the surface were melted to the point that they viscously deformed in response to stresses in this flow the transformation from the solid state to a largely molten state is inferred to have occurred within a few weeks these results are then discussed in the context of examples from the literature that support rapid melting and digestion of xenoliths in magma it is suggested that the rate of crustal assimilation may be sufficiently rapid that it can operate within the durations of extended eruptive episodes years decades which are generally much shorter than typical repose periods at arc volcanoes decades millennia

the isotopic and chemical evolution of mount st helens isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the mt st helens volcano suggest three different episodes 1 40 000 2500 years ago eruptions of dacite with end 5 esr 10 variable 18o 206pb 204pb 18 76 ca sr 60 rb ba 0 1 la yb 18 2 2500 1000 years ago eruptions of basalt andesite and dacite with end 4 to 8 esr 7 to 22 variable 18o thought to represent melting of differing mantle crust reservoirs 206pb 204pb 18 81 18 87 variable ca sr rb ba la yb and high zr 3 1000 years ago to present day eruptions of andesite and dacite with end 6 esr 13 18o 6 variable206pb 204pb ca sr 77 rb ba 0 1 la yb 11 none of the products exhibit eu anomalies and all are lree enriched there is a strong correlation between87sr 86sr and differentiation indices these data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet but not feldspar followed by intrusion of this crustal reservoir by mantle derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re equilibration of pb isotopic compositions than other components suggesting that pb is being transported via a fluid phase the nd and sr isotopic compositions lie along the mantle array and suggest that the mantle underneath mt st helens is not as depleted as morb sources there is no indication of seawater involvement in the source region

magmatic na rich phlogopite in a suite of gabbroic crustal xenoliths from volcan san pedro chilean andes evidence for a solvus relation between phlogopite and aspidolite magmatic na rich phlogopite 1 5 wt na 2 o is present as a late crystallizing mineral in two groups of texturally and mineralogically distinct gabbroic xenoliths at volcan san pedro 36 s chile an andean arc volcano phlogopites are characterized by high 100 mg mg fe up to 83 and high cr 2 o 3 contents up to 0 4 wt and they are always found surrounding variably resorbed olivine pyroxenes cr spinel and in some cases plagioclase we interpret these micas as the result of open system processes involving infiltration of water rich evolved melts with high na na k and reaction with refractory minerals the highest 100 na na k 70 and na 2 o concentrations 5 wt in phlogopite appear to require reaction with liquids of unrealistically high na na k if no other factor is considered this together with the observation that phlogopites consist of alternating na rich and na poor cleavage parallel bands can be best interpreted by the presence of a solvus between the aspidolite na and phlogopite k end members the high proportions up to 15 vol of na rich phlogopite in two different groups of gabbroic xenoliths suggest that it might be a more common and abundant mineral than has been previously recognized and that it may be used as an indicator of open system processes

late quaternary glacier response to humidity changes in the arid andes of chile 18 29 s abstract today no glaciers exist between 18 30 s and 27 s even on mountains much higher than 6000 m these dry high mountain environments are very sensitive to changes in climatic boundary conditions particularly humidity since temperatures are far below freezing here we present a reconstruction of climatic changes since the last glacial maximum for the chilean dry andes of the southern hemisphere we reconstructed regional equilibrium line altitudes ela for three different moraine stages representing extensive past glaciations in this currently unglaciated region comparison of the regional pattern of the ela with modern climate conditions allows us to draw implications about the paleoclimatic conditions during the best preserved moraine stage ii glaciations in the northern as well as in the southern part of the research area our results suggest humid conditions in the northern part 18 24 s during late glacial times with strongly increased convective precipitation during austral summer the temporal coincidence of glaciers in the mountains and high lake levels on the altiplano tauca phase is evident to the south no simple shift of the westerlies is implied by our glacier reconstructions the northern limit of effective moisture for glacier formation did not shift significantly but stayed in the area of the present day northernmost glaciers of the westerlies at about 27 s but further south precipitation increased significantly accompanied by at least 2 3 c colder conditions the age of this glaciation is uncertain but probably of late glacial or last glacial maximum lgm times overall glaciers in the dry andes of south america clearly responded to large changes in the humidity not primarily the temperature regime with all its consequences on the environment hydrology vegetation etc illustrating the different regional responses to global climatic change

triple oxygen δ18o δ17o hydrogen δ2h and iron δ56fe stable isotope signatures indicate a silicate magma source and magmatic hydrothermal genesis for magnetite orebodies at el laco chile

hydrogen isotope investigation of amphibole and glass in dacite magmas erupted in 1980 1986 and 2005 at mount st helens washington in active shallow sub volcanic magma conduits the extent of the dehydrogenation oxidation reaction in amphibole phenocrysts is controlled by energetic processes that cause crystal lattice damage or conditions that increase hydrogen diffusivity in magmatic phases amphibole phenocrysts separated from dacitic volcanic rocks erupted from 1980 to 1986 and in 2005 at mount st helens msh were analyzed for dd water content and fe3 fe2 and fragments of glassy groundmass were analyzed for dd and water content changes in amphibole dd values through time are evaluated within the context of carefully observed volcanic eruption behavior and published petrological and geochemical investigations driving forces for amphibole dehydrogenation include increase in magma oxygen fugacity decrease in amphibole hydrogen fugacity or both the phenocryst amphibole dd value c 57o and 2 wt h2o in the white fallout pumice of the may 18 1980 plinian eruptive phase is probably little modified during rapid magma ascent up an 7 km conduit younger volcanic rocks incorporate some shallowly degassed dacitic magma from earlier pulses based on amphibole phenocryst populations that exhibit varying degrees of dehydrogenation pyroclastic rocks from explosive eruptions in june october 1980 have elevated abundances of mottled amphibole phenocrysts peaking in some pyroclastic rocks erupted on july 22 1980 and extensive amphibole dehydrogenation is linked to crystal damage from vesiculation and pyroclastic fountain collapse that increased effective hydrogen diffusion in amphibole multiple amphibole dd populations in many 1980 pyroclastic rocks combined with their groundmass characteristics e g mixed pumice textures support models of shallow mixing prior to or during eruption as new volatile rich magma pulses blended with more oxidized degassed magma amphibole dehydrogenation is quenched at the top surface of msh dacite lava lobes but the diversity in the ddamph populations in original fresh lava flow surfaces may occur from blending magma domains with different ascent histories in the sub volcanic environment immediately before eruption multi stage open system magma degassing operated in each parcel of magma rising toward the surface whereas the magma below 7 km was a relatively closed system at least to the october 1986 eruption based on the large population of minimally dehydrogenated rim free amphibole in the lavas magma degassing and possibly h isotope exchange with low dd fluids around the roof zone may have accompanied the 1 5 km upward migration of the 1980 magma body the low ddamph c 188 to 122o oxy amphibole phenocrysts in lava spines extruded in may 2005 reflect dehydrogenation as ascending viscous magma degassed and crystallized and fractures that admitted oxygen into the hot solidified lava spine interior facilitated additional iron oxidation

new field evidence bearing on the origin of the el laco magnetite deposit northern chile a reply the el laco magnetite deposit has been interpreted as lava flows and feeder dikes formed from iron oxide magma but more recently as a product of metasomatic replacement open pit exposure created at laco sur during the 1990s reveals that the massive magnetite contains magnetite veined blocks and smaller fragments of altered andesitic volcanic rock clearly supporting the replacement origin open space growth of magnetite and pyroxene along the walls of chimney structures and veins within magnetite indicates precipitation from aqueous fluid rather than from gas alone the native sulfur bearing cristobalite alunite alteration widespread at el laco and broadly contemporaneous with magnetite formation is recognized as part of a steam heated horizon generated in the vadose zone above a paleowater table consequently the magnetite replacement must have taken place beneath the paleowater table but probably 300 m beneath the paleosurface thus magnetite deposition at or immediately beneath the paleosurface is precluded the main outstanding question is how hot hypersaline brine the probable ore forming fluid at el laco attained such shallow crustal levels within a high volcanic edifice without undergoing the extensive groundwater dilution documented from most shallowly preserved hydrothermal systems

further evidence of the complexity of the racemization process in fossil shells with implications for amino acid racemization dating abstract simulated aging experiments with shells of ostrea anadara and katelysia spp from modern holocene and pleistocene marine sediments in southern south australia give further insight into the difficulties in using d l ratios of amino acids for quantitative age determinations evidence is given that the progress of racemization as monitored by changes in d l ratios with time is not only non linear but also may be reversed demonstrating a decrease in the observed degree of racemization the observed changes in d l ratio with time differ according to the age and the genus of shell and the amino acid examined katelysia spp gives less variable values than ostrea or anadara and similarly valine racemization and isoleucine epimerization are far less variable than aspartic acid racemization

partial assimilative recycling of the mafic plutonic roots of arc volcanoes an example from the chilean andes at long lived arc volcanic centers open system interactions between new magma and the plutonic roots of the arc will increase with time relative to interactions with older crust assimilative recycling of mafic plutonic arc roots will have different isotopic consequences than incorporation of aged continental crust in the tatara san pedro complex 30 comagmatic basaltic lavas are characterized by xenocrysts and microxenoliths of olivine augite and plagioclase derived from solidified cumulates wide but poorly correlated ranges of incompatible and compatible major and trace elements and limited isotopic variability this mineralogical chemical signature reflects assimilation wherein grain boundary melting and disaggregation of xenoliths led to blending of incompatible element enriched melts derived mainly from hornblende phlogopite and plagioclase in combination with variable retention of xenocrysts hence variable but high compatible element concentrations

the fusion crusts of stony meteorites implications for the atmospheric reprocessing of extraterrestrial materials fusion crusts develop on all meteorites during their passage through the atmosphere but have been little studied we have characterized the textures and compositions of the fusion crusts of 73 stony meteorites to identify the nature of meteorite ablation spheres mas and constrain the processes operating during the entry heating most chondrite fusion crusts are porphyritic and are dominated by olivine glass and accessory magnetite whereas those of the achondrites are mainly glassy chondrite fusion crusts contain sulphide droplets with high ni contents 55 wt the partially melted substrate of ordinary chondrites underlying the outer melted crusts are dominated by silicate glass and composite metal sulphide and cr bearing fe oxide droplets that form as coexisting immiscible liquids enstatite chondrite substrates contain cr and mn bearing sulphides the substrates of the carbonaceous chondrites comprise a sulphide enriched layer of matrix the compositions of melted crusts are similar to those of the bulk meteorite however differences from whole rock suggest that three main processes control their chemical evolution 1 the loss and reaction of immiscible fe rich liquids 2 mixing between substrate partial melts and bulk melts of the melted crust and 3 the loss of volatile components by evaporation and degassing data from fusion crusts suggest that mas produced at low altitude have compositions within the range of those of silicate dominated cosmic spherules that are formed by the melting dust particles meteorite ablation spheres produced at high altitude probably have compositions very different from bulk meteorite and will resemble cosmic spherules derived from coarse grained precursors

oxygen isotopes in cosmic spherules and the composition of the near earth interplanetary dust complex abstract a long standing controversy in the micrometeorite community regards the relative contribution of primitive asteroids or comets and of evolved asteroids to the interplanetary dust cloud we compiled and studied a large set of oxygen isotopic data from the literature on cosmic spherules from different collections covering different influx periods within the last 1 myr cosmic spherules micrometeorites melted during atmospheric entry are the most abundant micrometeorites in worldwide collections according to several models they are representative of the composition and origin of micrometeorites 50 m in size spherule statistics 136 spherules 50 2280 m in size indicate that at least 20 of the micrometeoroid complex is fed by asteroids observed in the inner asteroid belt the ordinary chondrite and secondarily the hed parent asteroids likely belonging to the s type and v type spectral classes respectively another 60 or more is related to primitive objects of the solar system with carbonaceous chondrite compositions either primitive asteroids belonging to the c d or p type spectral classes in the outer asteroid belt or comets contribution from terrestrial planets has not been identified yet oxygen isotopes also document that the composition of the micrometeoroid complex is different from that of macroscopic meteoroids since the latter is dominated by materials from evolved and differentiated asteroids rather than primitive asteroids or comets cosmic spherule statistics show that the contribution of ordinary chondrite material to the composition of the micrometeoroid complex increases with micrometeorite size thereby documenting a continuum between meteorites and micrometeorites the transition in terms of relative abundance of the two cosmic spherule populations occurs around 500 m in size

olivine settling in cosmic spherules during atmospheric deceleration an indicator of the orbital eccentricity of interplanetary dust a new type of cosmic spherule is reported with textures suggesting settling of olivine during atmospheric deceleration numerical simulations of entry heating reveal that relict forsterite which survives melting can settle over the 1 2 s of flight at high entry angles and entry velocities up to 16 km s 1 enhanced crystallisation of phenocrysts by heterogeneous nucleation on accumulated relict forsterites is the most likely origin of the observed cumulate textures in cosmic spherules such textures in cosmic spherules reveal interplanetary dust with higher encounter velocity with the earth that correspond to orbital eccentricities 0 3 the relative abundance of cumulate spherules suggests 14 of ordinary chondrite related s iv type asteroid dust over the last 800 kyr had relatively high orbital eccentricity owing to secular and planetary perturbations the textures of cosmic spherules collected from sediments can therefore be used to trace dust orbital variations with time which may influence terrestrial climate

geology of the mount st helens area record of discontinuous volcanic and plutonic activity in the cascade arc of southern washington the quaternary edifice of mount st helens volcano was built upon a deeply eroded terrane of gently folded and altered volcanic and plutonic rocks that represent the core of the tertiary cascade magmatic arc these rocks constitute an east dipping homoclinal sequence several kilometers thick of subaerially erupted mafic to silicic flows and volcaniclastic strata k ar ages from this section range from about 28 to 23 ma late oligocene and earliest miocene which corresponds to an apparent lull in cascade volcanism to the north of mount rainier volcanism was essentially continuous during this period of time and neither a well defined base nor top of the section is exposed within the mapped area basalt and basaltic andesite dominate the lower part of the mid tertiary section whereas andesitic and dacitic rocks comprise most of the upper part this section was intruded by numerous mafic to silicic dikes sills and irregular plutonic bodies most no more than a few million years younger than their host rocks and subjected to pervasive burial metamorphism and widespread hydrothermal alteration large areas of hornfelsed rock surrounding even relatively small intrusions indicate that the proportion of plutonic rock becomes significantly greater at shallow depth beneath the existing erosion surface a large granitic pluton intruded the mid tertiary section north of spirit lake at about 21 ma the earl porphyry copper deposit occurs within the pluton but appears too young 17 ma to be genetically related to it in contrast to the rather continuous and voluminous oligocene to early miocene activity volcanism since then in the mount st helens area has been localized and volumetrically minor products of three younger eruptive periods have been recognized a sequence of 15 m y old pyroxene andesite flows resting unconformably on mid tertiary strata south of mount st helens widespread shallow dikes and sills of pyroxene andesite between 10 and 8 m y old and compositionally diverse rocks erupted during the past 3 m y the quaternary lavas are more potassic than the tertiary lavas and typically contain phenocrysts of hornblende and biotite which are absent from the older rocks a number of tertiary structures define a broad nne trending zone that may reflect a deep seated lithospheric flaw that has controlled the locus of cascade magmatism in southern washington for the past 25 m y mount st helens lies within this zone at the intersection of the nnw striking st helens seismic zone shz and an ene trending alignment of pleistocene silicic plug domes no surface breakage has been detected along the shz which is apparently very young the linear zone of silicic vents is probably controlled by a fault that has been interpreted from seismic records to occur directly beneath the volcano this zone parallels the directions of regional maximum horizontal compressive stress and north america juan de fuca plate convergence mount st helens is an example of a low volume tectonically controlled magmatic system in an early stage of development

extraterrestrial water in micrometeorites and cosmic spherules from antarctica an ion microprobe study the d h ratios and water contents were measured by ion microprobe analysis in 52 individual antarctic micrometeorites amms and 10 antarctic cosmic spherules acss containing nuggets of iron hydroxide cops phase in amms d values vary from 366 to 249 and water contents lie between 0 4 3 7 wt the cops nuggets in cosmic spherules have high water contents 2 to 8 wt and exhibit d values from 144 to 167 which is indicative of an extraterrestrial origin of their constituent water the silicate portion of acss also contain extraterrestrial h equivalent to 0 l to 1 2 wt water deuterium exchange experiments were performed with isotopically spiked water these experiments demonstrate that water in mineral phases of amms and acss is indigenous and does not result from contamination during residence in antarctic ice the frequency distribution of d h ratios in amms allows us to further narrow the relationship between amms and carbonaceous chondrites to cm and ci chondrites but contrasts with that of stratospheric interplanetary dust particles idps of similar sizes from 10 to 50 m the relatively narrow range of d h ratios measured in amms as well as in acss which are more resistant and thus less susceptible to collection biases suggests that d rich idp like particles are very rare in our amms collections this indicates that these d rich grains might constitute a minor fraction of the micrometeorite flux in the interplanetary medium and that possible collection biases in antarctica would not be responsible for their strong depletion in the amms collections

the genesis of silicic arc magmas in shallow crustal cold zones abstract a number of currently popular models for the genesis of evolved arc magmas from basaltic andesite to dacite invoke repeated intrusion partial melting and differentiation at the base of the crust however several observations suggest that this may be the exception rather than the norm 1 geobarometry often indicates shallow pressure 0 1 0 3 gpa evolution 2 incongruent melting of amphibolite at elevated pressures should yield magmas in equilibrium with high pressure phases like garnet but rare earth element patterns almost ubiquiously preclude this 3 compositionally zoned caldera forming eruptions suggest differentiation at near surface depths 4 u series data most commonly indicate differentiation over millennia time scales this requires rapid cooling that in turn is most easily explained by relatively small magma volumes undergoing crystal fractionation within the shallow i e cool crust to further test these ideas we combined published experimental data for liquidus equilibria with appropriate silicic arc magma compositions on projections of the ternary liquidus system nepheline silica olivine recent data for tongan silicic lavas plot either on or close to low pressure 1 atm cotectics for the rocks phenocryst phases suggesting low pressure differentiation using our own and published data from arc volcanoes around the world we find that the majority are consistent with differentiation at shallow depths regardless of total crustal thickness combined with the typical timescales of differentiation we estimate that the volumes of magma stored during differentiation in shallow crustal zones are usually on the order of only a few km 3 there is also a clear role for mixing and recharge that involves magmas that are more deeply sourced and primitive in character typically evolved basalts and basaltic andesites whether the latter differentiated in the lower crust or at the crust mantle boundary has important implications for the constitution and average composition of arc crust at present we can only conclude that evidence for more silicic arc magma generation at these depths is generally lacking

volcanogenic hydrothermal iron rich materials from the southern part of the central indian ocean basin abstract we report the occurrences of magnetite rich spherules ti rich particles and possibly nontronite recovered at 19 s and 76 e from the pelagic clay in the central indian ocean basin ciob it is proposed that the magnetite spherules were formed as a result of volcanic activity in this part of the ciob while the presence of nontronite suggests localized hydrothermal activity the magnetic spherules and the associated materials are compared with similar occurrences 555 km to the north in the vicinity of the 76 30 e fracture zone considering radiolarian age dating of the sediment the age of the volcano hydrothermal episode responsible for the formation of these materials seems to be between 425 and 650 ka it is inferred that there could have been widespread local volcanism in the ciob

small antarctic micrometeorites a mineralogical and in situ oxygen isotope study we have investigated the texture bulk chemistry mineralogy as well as the anhydrous minerals oxygen isotopic composition of 67 small antarctic micrometeorites amms collected at cap prudhomme antarctica and belonging to the currently poorly studied size fraction 25 50 m when compared to larger 50 400 m micrometeorites collected at the same site in antarctica with the same techniques no significant differences are found between the two populations we therefore conclude that the population of cap prudhomme amms is homogeneous over the size range 25 400 m in contrast small amms have different textures mineralogy and oxygen isotopic compositions than those of stratospheric interplanetary dust particles idps because small amms 50 m overlap in size with idps the differences between these two important sources of micrometeorites can no longer be attributed to a variation of the micrometeorite composition with size physical biases introduced by the collection procedures might account for these differences

melted micrometeorites from antarctic ice with evidence for the separation of immiscible fe ni s liquids during entry heating we report the discovery of four large 50 m cosmic spherules css and a single scoriaceous micrometeorite smm that contain evidence for the separation of immiscible fe ni s liquids during atmospheric entry heating the particles contain segregated fe rich regions dominated by either ni s bearing fe oxides or iron sulphides and have textural relations that suggest these separated from the silicate portions of the particles as metallic liquids the oxides which may be hydrous are thought to result from alteration of metal and sulphide the compositions of the silicate portions of the css are equivalent to spherules without fe rich regions implying that metallic liquids are exsolved during the heating of most spherules but completely separate the single smm has a very different composition from other scoriaceous particles and the occurrence of an exsolved metallic liquid probably indicates extreme reduction during entry heating the pyrolysis of carbonaceous materials is the most likely explanation for reduction and suggests that the precursor material of this particle was unusually c rich this smm might be therefore an appropriate candidate for a large melted anhydrous or smectite interplanetary dust particle idp the exsolution of immiscible fe ni s liquids during entry heating will result in systematic changes in the compositions of the remaining silicate melt

natural partial melting of syenite blocks from ascension island blocks of coarse grained syenite included in a trachyandesite lava flow on ascension island exhibit features attributable to partial melting the liquid so formed has been quenched to a glass and the chemical variation of the glass is considered to depend on the amount of melting the mineralogy and modal proportions of each block especially the presence or absence of quartz and the chemical composition of the minerals subjected to melting partial melting of quartz syenites such as these could have produced magmas of composition similar to that of the comendites and granites found on ascension but differences in major element and minor element chemistry between the glass and the comendites and granites suggest otherwise

silicate microspherules intercepted in the plume of etna volcano a possible volcanic origin has been suggested for the micrometre sized spherules that have been discovered in polar snows or ice cores1 5 and in the stratosphere6 7 however although such particles especially when black and magnetic have been identified among the tephra deposits of some volcanoes3 8 12 their volcanic emission has never been directly observed here we describe micro spherules intercepted in the plume of mount etna sicily during moderate recurring volcanic activity their study using analytical transmission and scanning electron microscopy atem and asem demonstrates the simultaneous presence in the plume of glassy silicate microspherules of various chemical compositions 47 98 sio2

multiple precursors of secondary mineralogical assemblages in cm chondrites we report a petrographic and mineralogical survey of tochilinite cronstedtite intergrowths tcis in paris a new cm chondrite considered to be the least altered cm identified to date our results indicate that type i tcis consist of compact tochilinite cronstedtite rims surrounding fe ni metal beads thus confirming kamacite as the precursor of type i tcis in contrast type ii tcis are characterized by complex compositional zoning composed of three different fe bearing secondary minerals from the outside inwards tochilinite cronstedtite and amakinite type ii tcis present well developed faces that allow a detailed morphological analysis to be performed in order to identify the precursors the results demonstrate that type ii tcis formed by pseudomorphism of the anhydrous silicates olivine and pyroxene hence there is no apparent genetic relationship between type i and type ii tcis in addition the complex chemical zoning observed within type ii tcis suggests that the alteration conditions evolved dramatically over time at least three stages of alteration can be proposed characterized by alteration fluids with varying compositions 1 fe and s rich fluids 2 s poor and fe and si rich fluids and 3 s and si poor fe rich fluids the presence of unaltered silicates in close association with euhedral type ii tcis suggests the existence of microenvironments during the first alteration stages of cm chondrites in addition the absence of mg bearing secondary minerals in paris tcis suggests that the mg content increases during the course of alteration

immiscible iron and silica rich melts and magnetite geochemistry at the el laco volcano northern chile evidence for a magmatic origin for the magnetite deposits abstract the pliocene el laco volcano northern chile hosts a large iron oxide deposit that occurs as stratiform bodies with underlying feeder zones dykes hosted by hydrothermally altered andesite lava flows phenocrysts in the andesite hosting the mineralization have abundant melt inclusions displaying strong evidence for melt immiscibility most of the pyroxene and the plagioclase phenocrysts contain abundant fe rich melt inclusions with low ti magnetite pyroxene apatite and minor anhydrite the glassy groundmass of these melt inclusions is chemically similar to sub alkaline rhyolitic rocks e g 70 sio 2 si al k na rich large resorbed plagioclase phenocrysts also host another group of melt inclusions with a similar silica rich groundmass but hosting evenly distributed immiscible and complex fe rich spheroidal globules that are enriched in ti p mg ca and s these immiscible globules and the ti poor magnetite daughter crystals in the melt inclusions in pyroxene phenocrysts are thought to be the parental iron oxide melt liquid after which their coalescence leads to the formation of magnetite melts from which the magnetite that formed the el laco iron oxide deposits crystallized and were emplaced the mineralogy and chemistry of these melt inclusions tracks the chemical evolution of the fe ox melts and supports a magmatic origin for the el laco magnetite this is mainly recognized by the continuum of compositions between volcanic separate immiscible fe oxide melts and magnetite ores the trace element composition of the fe rich melt inclusions and of the magnetite in the host andesite and the ore bodies reveals a systematic depletion in most of the elements that are compatible in magnetite the ti content of the magnetite also records the separation and crystallization of the iron rich melt typically the magnetite in the orebodies and the melt inclusions is ti poor compared with titanomagnetite found in the andesite groundmass and that hosted by the hydrothermally altered andesite that is interpreted as being attributed to its crystallization under oxidizing and sulfur rich conditions the cause of this unusual chemical behaviour is probably due to the mechanism s governing the crystallization of the iron rich melt including the temperature changes in chemistry prior to eruption and an increase in the redox conditions of the silicate melt magma oxidation facilitates the prevalence of ferric iron and formation of ti poor magnetite preventing the incorporation of ti in the exsolving iron rich melt the likely reason for the formation of these large ti poor magnetite deposits is likely the contamination of the parental andesitic magma by oxidized crustal rocks and concomitant crystallization of magnetite under oxidizing conditions something that had a significant impact on the elemental distribution

subaerial explosive deposition of magnetite apatite mineralization the artillero deposit pena colorada district colima mexico abstract the artillero magnetite deposit is a satellite orebody within the pena colorada iron district mexico the mineralization consists of a stratiform orebody formed of subaqueous pyroclastic magnetite interbedded with welded rhyodacite this was likely deposited in a shallow maar crater capping breccia pipes and feeder dikes all hosted by coherent cretaceous rhyodacite all of these features suggest that this mineralization was formed by the crystallization of iron rich melts in a subaerial environment the formation of the magnetite ore in artillero and pena colorada at 90 11 ma and 81 3 ma u pb zircon and apatite respectively is coeval with the deposition of the rhyodacite host rocks 84 1 ma u pb zircon the likely source of the iron rich melts was the mantle wedge having a small but noticeable crustal contamination due to the input of fluids derived from dehydrating slab or continental crust incorporated via subduction erosion which is reflected in an increase in the 87sr 86sr values of the guerrero volcanic arc iron rich melts ascended through strike slip structures related to compression extension processes caused by the post laramide inversion the shallow emplacement of the melts favored the separation of large amounts of low density aqueous fluids which increased wetting properties of the iron rich magma favored explosive extrusion and formed maar diatreme complexes

unravelling the complexity of magma plumbing at mount st helens a new trace element partitioning scheme for amphibole volcanoes at subduction zones reside above complex magma plumbing systems where individual magmatic components may originate and interact at a range of pressures because whole rock compositions of subduction zone magmas are the integrated result of processes operating throughout the entire plumbing system processes such as mixing homogenisation and magma assembly during shallow storage can overprint the chemical signatures of deeper crustal processes whereas melt inclusions provide an effective way to study the uppermost 10 15 km of the plumbing system challenges remain in understanding magma intrusion fractionation and hybridisation processes in the middle to lower crust 15 30 km depth which commonly involves amphibole crystallisation here we present new insights into the mid crustal plumbing system at mount st helens usa using multiple regression methods to calculate trace element partition coefficients for amphibole phenocrysts and thus infer the trace element compositions of their equilibrium melts the results indicate vertically distributed crystal fractionation dominated by amphibole at higher pressures and in intermediate melts and by plagioclase at lower pressures variations in nb zr and ree concentrations at intermediate sio2 contents suggest repeated scavenging of partially remelted intrusive material in the mid crust and mixing with material from geochemically diverse sources amphibole is an effective probe for deep crustal magmatism worldwide and this approach offers a new tool to explore the structure and chemistry of arc magmas including those forming plutonic or cumulate materials that offer no other constraints on melt composition

bulk mineralogy of individual micrometeorites determined by x ray diffraction analysis and transmission electron microscopy abstract bulk mineralogy of individual fine grained micrometeorites from 50 to 200 m in diameter was determined on the basis of the powder x ray diffraction patterns and the observation of internal textures by a transmission electron microscope tem x ray diffraction analysis of 56 micrometeorites indicated that 42 11 and 3 samples are olivine rich pyroxene rich and phyllosilicate rich micrometeorites respectively among the phyllosilicate rich micrometeorites one contains saponite and other two contain serpentine no samples contain both saponite and serpentine we found that saponite rich micrometeorite was weakly heated which results in shrinkage of 001 basal spacing of saponite down to 9 7 a and that cronstedtite which is commonly contained in cm chondrites occurs in serpentine rich micrometeorites micrometeorites that consist entirely of anhydrous minerals and amorphous phases are predominant in the samples studied the major phases of such micrometeorites are olivine low ca pyroxene magnetite and fe sulfide and the average abundances are 65 17 11 and 7 wt respectively when the total abundance of the four minerals are normalized to 100 wt the relative mineral abundance varies greatly between samples low ca pyroxene olivine ratios range from 0 to 3 5 with a mean of 0 3 tem observations of inner portions of some micrometeorites revealed that they are aggregates of very small equigranular grains 100 nm of olivine magnetite or low ca pyroxene olivine magnesiowustite the textures are very similar to those of hydrous carbonaceous chondrite that was experimentally heated to temperature below melting point thus suggesting that the micrometeorites had been hydrous particles but were decomposed by the brief heating upon atmospheric entry it is newly found that magnesiowustite was formed in micrometeorites instead of magnetite as a product of phyllosilicate decomposition under low oxygen fugacity the decomposed hydrous micrometeorites gave two types of characteristic x ray diffraction patterns 1 broad olivine and magnetite reflections or 2 variable intensities of magnesiowustite reflections together with magnetite low ca pyroxene and olivine reflections twenty nine olivine or pyroxene rich micrometeorites showed such diffraction patterns thus suggesting that more than half of micrometeorites investigated must be decomposed hydrous particles the results confirmed that hydrous dust particles are much more abundant in the interplanetary space than in the micrometeorites recovered on the earth

oxygen isotopes in deep sea spherules oxygen isotopic compositions have been measured on several size fractions of deep sea spherules of extraterrestrial origin the silicate spherules have an isotopic composition unlike that of any known macrometeorite their pre terrestrial compositions may have been similar to those of c3 chondrites or the anhydrous component of c2 chondrites the latter being preferred on chemical grounds metallic particles oxidize in the upper atmosphere and sample a region for which no previous oxygen isotope data exist this part of the atmosphere above about 100 km is apparently strongly enriched in the heavy isotopes of oxygen

the magmatic to magmatic hydrothermal evolution of the el laco deposit chile and its implications for the genesis of magnetite apatite deposits

meteoritic event recorded in antarctic ice during systematic sampling of volcanic ash tephra layers at a well known antarctic meteorite collection site the allan hills main ice field a band of unusually dark and rounded many spheroidal particles was discovered this debris layer bit 58 extends parallel to the stratigraphy of the ice established from the tephra bands apparently marking a single depositional event the shapes internal texture major element composition and levels of cosmogenic nuclides of particles from within bit 58 all strongly suggest that this material represents ablation debris from the passage of a large h group ordinary chondrite preliminary cosmogenic isotope dating suggests an age of 2 8 ma implying that the east antarctic ice sheet has been stable since that time the relationship of the bit 58 layer to known impact events is not clear

heating experiments simulating atmospheric entry heating of micrometeorites clues to their parent body sources depending on their velocity entry angle and mass extraterrestrial dust particles suffer certain degrees of heating during entry into earth s atmosphere and the mineralogy and chemical composition of these dust particles are significantly changed in the present study pulse heating experiments simulating the atmospheric entry heating of micrometeoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temperature experienced by the particles during entry heating fragments of the ci chondrites orgueil and alais as well as pyrrhotites from orgueil were used as analogue material the experiments show that the volatile elements s zn ga ge and se can be lost from 50 to 100 m sized ci meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles it is concluded that depletions of these elements relative to ci as observed in micrometeorites are mainly caused by atmospheric entry heating besides explaining the element abundances in micrometeorites the experimentally obtained release patterns can also be used as indicators to estimate the peak heating of dust particles during entry using the abundances of zn and ge and assuming their original concentrations close to ci a maximum heating of 1100 1200 c is obtained for previously analyzed antarctic micrometeroites thermal alteration also strongly influenced the mineralogy of the meteorite fragments while the unheated samples mainly consisted of phyllosilicates these phases almost completely transformed into olivine and pyroxene in the fragments heated to 800 c therefore dust particles that still contain hydrous minerals were probably never heated to temperatures 800 c in the atmosphere during continued heating the grain size of the newly formed silicates increased and the composition of the olivines equilibrated applying these results quantitatively to antarctic micrometeorites typical peak temperatures in the range of 1100 1200 c during atmospheric entry heating are deduced this temperature range corresponds to the one obtained from the volatile element concentrations measured in these micrometeorites and points to an asteroidal origin of the particles

diagenetically altered fossil micrometeorites suggest cosmic dust is common in the geological record abstract we report the discovery of fossil micrometeorites from late cretaceous chalk seventy six cosmic spherules were recovered from coniacian 87 1 ma sediments of the white chalk supergroup particles vary from pristine silicate and iron type spherules to pseudomorphic spherules consisting of either single phase recrystallized magnetite or fe silicide pristine spherules are readily identified as micrometeorites on the basis of their characteristic mineralogies textures and compositions both magnetite and silicide spherules contain dendritic crystals and spherical morphologies testifying to rapid crystallisation of high temperature iron rich metallic and oxide liquids these particles also contain spherical cavities representing weathering and removal of metal beads and irregular cavities representing vesicles formed by trapped gas during crystallization both features commonly found among modern antarctic iron type i type cosmic spherules on the basis of textural analysis the magnetite and fe silicide spherules are shown to be i type cosmic spherules that have experienced complete secondary replacement during diagenesis fossilization our results demonstrate that micrometeorites preserved in sedimentary rocks are affected by a suite of complex diagenetic processes which can result in disparate replacement minerals even within the same sequence of sedimentary beds as a result the identification of fossil micrometeorites requires careful observation of particle textures and comparisons with modern antarctic collections replaced micrometeorites imply that geochemical signatures the extraterrestrial dust are subject to diagenetic remobilisation that limits their stratigraphic resolution however this study demonstrates that fossil pseudomorphic micrometeorites can be recognised and are likely common within the geological record

experiments on phosphate silicate liquid immiscibility with potential links to iron oxide apatite and nelsonite deposits the formation of phosphorus iron oxide p fe immiscible melts and their possible connection to the genesis of kiruna type and nelsonite deposits was experimentally investigated by adding phosphoric acid h3po4 water and sulfur to andesite at 100 450 mpa 500 900 c at the ninio and magnetite hematite fo2 buffers using internally heated gas vessels the addition of up to 8 02 wt of h3po4 to the andesite causes crystallization of apatite at higher concentrations of h3po4 whitlockite crystallizes and at concentrations above 11 4 h3po4 at 800 c 385 mpa an immiscible p fe melt forms adding sulfur at low fo2 ninio causes an additional immiscible fe s melt to form increasing the fo2 to the hematite magnetite buffer causes the sulfur rich melt to shift in composition to a ca s o melt and the coexisting p fe melt to incorporate large amounts of so4 immiscible p fe melts can form at temperatures above 1100 c down to 600 c at 400 mpa mass balance calculations show that some experimentally produced p fe rich immiscible liquids may result in mineral assemblages similar to those found at some kiruna type deposits such as actinolite rich dikes and apatite rich veins depending on the geological conditions and the composition the fractionation of a p fe melt may result in the formation of nelsonites at high pressures high temperatures and low fo2 or kiruna type deposits at lower temperatures and higher fo2

magnetite spherules in pyroclastic iron ore at el laco chile the el laco iron deposits in northern chile consist of magnetite or martite and minor hematite pyroxene and apatite the orebodies are situated on a volcanic complex and resemble lavas and pyroclastic deposits but a magmatic origin is rejected by some geologists who regard the ores as products of hydrothermal replacement of volcanic rocks this study describes spherules of magnetite in the ore at laco sur and outlines a previously unrecognized crystallization process for the formation of spherical magnetite crystal aggregates during volcanic eruption mining at laco sur the second largest deposit at el laco shows that most of the ore is friable and resembles pyroclastic material hard ore with vesicle like cavities occurs subordinately the friable ore is a porous aggregate of 0 01 0 2 mm magnetite octahedra with only a local stratification defined by millimeter thin strata of apatite films of iron phosphate are common on magnetite crystals and vertical pipes called gas escape tubes are abundant in the ore a sem study reveals that magnetite spherules in the range 0 05 0 2 mm occur in most samples of friable ore from the central lower part of the deposit the proportion of spherules in a sample varies from high to nil but overall the spherule content is low in the ore the spherules are aggregates of octahedral crystals or single octahedra that have been rounded by stepwise subparallel growth of magnetite with a systematic slight shift in orientation of successive steps the shape of the spherules demonstrates that they formed unattached to any surface growth from hot magmatic gas saturated in iron in a volcanic plume and deposition as ash fall can account for the features of the spherule bearing friable ore

mineralogy of phyllosilicate rich micrometeorites and comparison with tagish lake and sayama meteorites abstract four phyllosilicate rich micrometeorites mms were investigated by a synchrotron radiation x ray diffraction technique and transmission electron microscopy three are saponite rich mms and one is a serpentine rich one in the saponite rich mms we could not find serpentine and vice versa in the serpentine rich mm in the saponite rich mms major constituent minerals are saponite fe and ni bearing sulfides and magnetite two saponite rich mms contain fine grained magnesiowustite rich aggregates the aggregates consist of

evidence from gravity data for an intrusive complex beneath mount st helens on the basis of gravity data aided by aeromagnetic magnetotelluric side looking radar and geologic information we tentatively identify a large shallow intrusive complex beneath mount st helens the complex is roughly 5 6 km thick and has apparently intruded a buried and compressed mesozoic forearc sedimentary sequence the lateral extent of the intrusive complex is uncertain and we give three alternative models varying from about 18 by 10 km to as much as 18 by 22 km the western boundary of the inferred mount st helens intrusion abuts several of a number of tertiary and younger plutons that crop out in the area the mount st helens intrusion apparently is adjacent to sedimentary rock or shallow volcanic rock along most of the remainder of its boundary a ringlike drainage pattern around mount st helens suggests subsidence caused by removing magma from deep beneath mount st helens and adding this mass to the mount st helens edifice and its underlying intrusion our analysis indicates that the average bulk density of the volcanic rock comprising mount st helens is about 2 15 g cm3

experimental simulation of atmospheric entry of micrometeorites depending on their velocity entry angle and mass micrometeorites suffer different degrees of heating during their deceleration in the earth s atmosphere leading in most cases to significant textural mineralogical and chemical modifications one of these modifications is the formation of a magnetite shell around most micrometeorites which until now could not be reproduced neither theoretically nor experimentally the present study was designed to better understand the entry heating effects on micrometeorites and especially the formation of the magnetite shell fragments of the murchison and orgueil meteorites were used as analogue material in flash heating experiments performed in a high temperature furnace effects of temperature heating duration and oxygen fugacity were investigated these experiments were able to reproduce most of the micrometeorites textures from the vesicular fine grained micrometeorites to the totally melted cosmic spherules for the first time the formation of a magnetite shell could be observed on micrometeorite analogues we suggest that the most plausible mechanism for the formation of this shell is a peripheral partial melting with subsequent magnetite crystallization at the surface of the micrometeorite furthermore with this study it is possible to estimate the atmospheric entry conditions of micrometeorites such as the peak temperature and the duration of flash heating

inclusions in mount st helens dacite erupted from 1980 through 1983 abstract inclusions of plutonic metavolcanic and volcanic rocks are abundant in dacite pumice and lava from the 1980 1986 eruption sequence at mount st helens point counts of inclusions exposed in talus blocks from the dome from 1980 through 1983 show that inclusions form approximately 3 5 vol of the lava eighty five percent of the inclusions are medium grained gabbros with an average diameter of 6 cm additional rock types include quartz diorite hornfelsic basalt dacite andesite and vein quartz disaggregated inclusions are common and define shear planes within the dome these fragmented inclusions may significantly contaminate analyses of the dacite the gabbroic inclusions are of four distinct types all with mineral assemblages consistent with crystallization pressures of less than 9 kb textures and major element compositions indicate that most of the gabbroic inclusions are cumulates the most abundant inclusion type is laminated gabbronorite which contains up to 9 interstitial glass derived from partial melting the presence of quartz veins and hornblende bearing veins within sheared zones in the laminated gabbronorite indicates that the source of these inclusions was holocrystalline rock that had been penetrated by water rich fluids the gabbronorite contained sufficient water to be susceptible to partial melting when the magma that fed the 1980 1986 eruption sequence was emplaced nearby various types of gabbroic inclusions including the laminated gabbronorite are common in mount st helens lavas of approximately the last 3000 years this coincides with the interval in which mount st helens first erupted basalt and basaltic andesite lavas these observations together with the fact that the gabbroic inclusions are compositionally unlike any of the tertiary intrusive rocks in the mount st helens area strongly suggest that the inclusions are related to the introduction of basalt to the mount st helens magmatic system the source of the gabbros could be a layered mafic pluton formed through crystal accumulation from multiple batches of basaltic magma emplaced at mid to upper crustal depths beneath the volcano the prevalence of explosive eruptions at mount st helens may play a part in bringing the inclusions to the surface the eruptive products of the cataclysmic eruption of may 18 1980 contain notably fewer inclusions than the pyroclastic flows and dome lavas erupted subsequently this suggests that the may 18 eruption shattered conduit wall rock that was subsequently stoped into the magma and carried to the surface later in the eruption series

factors controlling compositions of cosmic spinels application to atmospheric entry conditions of meteoritic materials abstract during their deceleration through the earth s atmosphere meteoritic materials i e interplanetary dust particles micrometeorites and meteorites experience thermal shocks which may alter their pristine mineralogy texture or chemical characteristics among these changes one of the most ubiquitous is the formation of spinels resulting from partial melting and subsequent crystallization of the meteoritic material these cosmic spinels differ from terrestrial spinels by their high ni and fe3 contents and show large variations in composition in order to better understand the factors controlling their chemistry pulse heating experiments simulating atmospheric entry of extraterrestrial objects were carried out using orgueil samples as proxies of meteoritic material covering a large range of experimental conditions temperature 500 c we also show that due to their fast crystallization kinetics cosmic spinels can record through their composition i e al2o3 contents and feo fe2o3 ratio the diverse conditions of the atmosphere crossed by the extraterrestrial object during its fall towards the earth s surface chemistry of cosmic spinels is thus a powerful tool for constraining the entry conditions in the earth s atmosphere of any extraterrestrial object including altitude of deceleration entry angle and incident velocity these in turn may provide valuable information on the origin of the extraterrestrial material

tektites and microtektites key facts and inferences abstract tektites are generally small black rounded silicate glass bodies found scattered over several widely separated areas of the earth s surface called strewn fields four major tektite strewn fields are known 1 australasian 2 ivory coast 3 czechoslovakian and 4 north american tektites from these four strewn fields have been dated at 0 7 1 15 and 34 ma respectively most tektites are splash forms i e spheres teardrops dumbbells etc but some have undergone a second period of melting to produce ablated forms tektites with a blocky layered appearance are called muong nong types microtektites microscopic tektites belonging to the australasian ivory coast and north american strewn fields have been found in deep sea sediments the geographic occurrence and abundance of microtektites at each site have been used to determine the size and mass of each strewn field the australasian ivory coast and north american strewn fields contain 100 million 20 million and 1 billion tonnes of glass respectively although tektites resemble obsidian they can be distinguished from terrestrial volcanic glasses by their composition and petrography tektites for example are drier than volcanic glasses and have higher feo fe 2 o 3 ratios in addition tektites unlike volcanic glasses contain lechatelierite particles sio 2 glass and are devoid of primary crystallites although some authors favor a lunar volcanic origin most of the compositional and petrographic data indicate a terrestrial impact origin the major trace and isotopic compositions of tektites indicate that the parent material was a terrestrial sedimentary deposit and the presence of lechatelierite coesite and shocked relict inclusions favors an impact origin furthermore two tektite strewn fields have been associated with known impact craters and tektites from one strewn field have been found associated with impact ejecta unlike the cretaceous tertiary boundary layer microtektite layers are not associated with mass extinctions or iridium anomalies

new insights for the formation of kiruna type iron deposits by immiscible hydrous fe p melt and high temperature hydrothermal processes evidence from el laco deposit

amino acid abundances and stereochemistry in hydrothermally altered sediments from the juan de fuca ridge northeastern pacific ocean amino acid abundances and stereochemistry in hydrothermally altered sediments from the juan de fuca ridge northeastern pacific ocean

attenuation and scattering tomography of the deep plumbing system of mount st helens we present a combined 3 d p wave attenuation 2 d s coda attenuation and 3 d s coda scattering tomography model of fluid pathways feeding systems and sediments below mount st helens msh volcano between depths of 0 and 18 km high scattering and high attenuation shallow anomalies are indicative of magma and fluid rich zones within and below the volcanic edifice down to 6 km depth where a high scattering body outlines the top of deeper aseismic velocity anomalies both the volcanic edifice and these structures induce a combination of strong scattering and attenuation on any seismic wavefield particularly those recorded on the northern and eastern flanks of the volcanic cone north of the cone between depths of 0 and 10 km a low velocity high scattering and high attenuation north south trending trough is attributed to thick piles of tertiary marine sediments within the st helens seismic zone a laterally extended 3 d scattering contrast at depths of 10 to 14 km is related to the boundary between upper and lower crust and caused in our interpretation by the large scale interaction of the siletz terrane with the cascade arc crust this contrast presents a low scattering 4 6 km2 hole under the northeastern flank of the volcano we infer that this section represents the main path of magma ascent from depths greater than 6 km at msh with a small north east shift in the lower plumbing system of the volcano we conclude that combinations of different nonstandard tomographic methods leading toward full waveform tomography represent the future of seismic volcano imaging