carbonates in fractures of martian meteorite allan hills 84001 petrologic evidence for impact origin carbonates in martian meteorite allan hills 84001 occur as grains on pyroxene grain boundaries in crushed zones and as disks veins and irregularly shaped grains in healed pyroxene fractures some carbonate disks have tapered mg rich edges and are accompanied by smaller thinner and relatively homogeneous magnesite microdisks except for the microdisks all types of carbonate grains show the same unique chemical zoning pattern on mgco3 feco3 caco3 plots this chemical characteristic and the close spatial association of diverse carbonate types show that all carbonates formed by a similar process the heterogeneous distribution of carbonates in fractures tapered shapes of some disks and the localized occurrence of mg rich microdisks appear to be incompatible with growth from an externally derived co2 rich fluid that changed in composition over time these features suggest instead that the fractures were closed as carbonates grew from an internally derived fluid and that the microdisks formed from a residual mg rich fluid that was squeezed along fractures carbonate in pyroxene fractures is most abundant near grains of plagioclase glass that are located on pyroxene grain boundaries and commonly contain major or minor amounts of carbonate we infer that carbonates in fractures formed from grain boundary carbonates associated with plagiociase that were melted by impact and dispersed into the surrounding fractured pyroxene carbonates in fractures which include those studied by mckay et al 1996 could not have formed at low temperatures and preserved mineralogical evidence for martian organisms

velocity crossover between hydrous and anhydrous forsterite at high pressures article i nfo theelasticpropertiesofhydrousforsterite mg2 xsio4h2x arerelevanttointerpretingseismicvelocityanomalies in the earth s mantle in this study we used brillouin scattering to determine the single crystal elasticity of forsterite with 0 9 1 wt h2 o x 0 14 to 14 gpa aggregate bulk and shear moduli of hydrous forsterite increase with pressure at a greater rate than those of the corresponding anhydrous phase compared with anhydrousforsterite weobservea7 increaseinthepressurederivativeofthebulkmodulus ks0 4 50 5 and a 25 increase in the pressure derivative of the shear modulus g0 1 75 5 for forsterite with near maximum possible water content using our results we calculated the compressional vp and shear vs v elocities of forsteriteasafunctionofpressureat300 k whereasvpandvsofhydrousforsteriteare0 6 and0 4 slowerthan those of anhydrous forsterite at ambient pressure velocity crossovers at 3 4 gpa result in higher hydrous forsterite velocities at pressures corresponding to depths below 120 km at the pressure of the 410 km discontinuity vp and vs of hydrous forsterite exceed those of anhydrous forsterite by 1 1 1 and 1 9 1 respectively this implies that incorporation of water could decrease the magnitude of the velocity contrast at

fingerprinting the water site in mantle olivine nominally anhydrous minerals such as olivine contain trace amounts of water and may accommodate the entire water budget of the upper mantle here we report for the first time synthetic olivines crystallized experimentally under upper mantle conditions that reproduce the most common and intense infrared hydroxyl stretching bands at 3572 and 3525 cm 1 observed in spinel peridotite mantle olivines these bands arise from water accommodated at point defects associated with the trace element ti and we suggest that this is the most important defect site in the shallow upper mantle additional hydrated defects may occur at higher pressures we also identify bands related to water associated with fe3 these are unlikely to reflect equilibrium with the mantle and indicate water incorporation during exhumation or retrogression water must be present at the defect site appropriate for the mantle at the conditions of interest for partitioning seismic wave speed and deformation experiments on hydrous olivine to be relevant

a subsolidus olivine water solubility equation for the earth s upper mantle the pressure and temperature sensitivity of the two most important point hydrous defects in mantle olivine involving si vacancies associated to trace amounts of titanium tichu pd or exclusively to si vacancies si was investigated at subsolidus conditions in a fluid saturated natural peridotite from 0 5 to 6 gpa approximately 20 200 km depth at 750 to 1050 c water contents in olivine were monitored in sandwich experiments with a fertile serpentine layer in the middle and olivine and pyroxene sensor layers at the border textures and mineral compositions provide evidence that olivine completely recrystallized during the weeklong experiments whereas pyroxenes displayed only partial equilibration a site specific water solubility law for olivine has been formulated based on the experiments reconciling previous contradictory results from low and high pressure experiments the site specific solubility laws permit to constrain water incorporation into olivine in the subducting slab and the mantle wedge as these are rare locations on earth where fluid present conditions exist chlorite dehydration in the hydrated slab is roughly parallel to the isopleth of 50 20 ppm wt h2o in olivine a value which is independent of the pressure and temperature trajectory followed by the slab hydrous defects are dominated by si under the relevant conditions for the mantle wedge affected by fluids coming from the slab dehydration slab adjacent low viscosity seismic low velocity channel p 3 gpa in cold subduction zones at 5 5 km from the slab surface the storage capacity of the mantle wedge at depths of 100 250 km ranges from 400 to 2 000 ppm wt h2o

formation and relative ages of maskelynite and carbonate in alh84001 the morphology and stoichiometry of feldspathic glass in the martian meteorite alh84001 indicates it is maskelynite a diaplectic glass rather than a flowed glass although this glass was heterogeneously affected by a tertiary set of processes an impact event with shock pressures in excess of 31 gpa was needed to convert the original plagioclase an36ab60or4 to maskelynite carbonate is intimately associated with the maskelynite and the carbonate s radiating crystalline fabric and globular forms suggest it was produced after plagioclase was converted to maskelynite textures also suggest carbonate was produced at the expense of maskelynite in a dissolution precipitation reaction that involved a carbonic fluid this fluid system is tentatively estimated to have been active for at least a few years at temperatures 300 c based on dissolution rates of plagioclase in mildly to strongly alkaline hydrothermal systems which is the only analogue currently available this reaction does not need to be mitigated by microbial life neither are bacteria needed to produce the radiating textures and globular forms of carbonate which may instead reflect kinetic phenomena associated with crystal nucleation and growth because the carbonate was produced at the expense of maskelynite it is younger than maskelynite which has previously been shown to have last degassed 3 92 0 04 ga turner et al 1997 however the specific age of the carbonate and the carbonic fluid system remains unknown

an experimental study of water in nominally anhydrous minerals in the upper mantle near the water saturated solidus the incorporation of water in olivine and pyroxenes interlayered within fertile lherzolite compositions was explored experimentally near the wet solidus of lherzolite at 2 5 and 4 gpa the concentrations and activities of water were varied to establish the partitioning of water between nominally anhydrous minerals nams and the hydrous minerals pargasite and phlogopite the water content in nams was determined by fourier transform infrared ftir spectroscopy the main absorption bands in nams from these experiments are very similar to those generally found in natural upper mantle peridotites olivine orthopyroxene and clinopyroxene contain 32 190 290 320 and 910 980 ppm of water under the studied conditions the partition coefficients between coexisting clinopyroxene and orthopyroxene d cpx opx are 2 7 1 1 and 3 5 1 5 at 2 5 and 4 gpa respectively whereas values for coexisting orthopyroxene and olivine d opx ol are 6 7 2 and 4 7 1 1 at 2 5 and 4 gpa respectively the storage capacity in nams in a model mantle composition close to the vapour saturated solidus water rich vapour is 190 ppm at both 2 5 and 4 gpa pargasite is the most important phase accommodating significant amounts of water in the uppermost mantle its breakdown with increasing pressure at 3 gpa at the vapour saturated solidus which is at 1025 c at 2 5 gpa results in a sharp drop in the water storage capacity of peridotite from 1000 ppm to 190 ppm h 2o at pressures 3 gpa melting in fertile lherzolite begins at the vapour saturated solidus if the bulk h 2o concentration exceeds 190 ppm the author 2012 published by oxford university press all rights reserved

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

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

comment on petrologic evidence for low temperature possibly flood evaporitic origin of carbonates in the alh84001 meteorite by paul h warren

anomalously thin transition zone and apparently isotropic upper mantle beneath bermuda evidence for upwelling 1 the origin of the bermuda swell and volcanism remains enigmatic the lack of an associated time progressive hotspot track and absence of present day volcanic activity make it difficult to reconcile with a deep mantle plume model we analyze shear wave splitting measurements to estimate mantle flow direction and receiver function stacks to place constraints on the mantle transition zone thermal structure ks phases exhibit well resolved null arrivals no splitting beneath the swell over a range of back azimuths we find that the 410 and 660 km discontinuities are 49 5 km and 19 5 km deeper than the global average respectively leading to a transition zone thickness that is 27 4 km thinner than the average together an apparently isotropic upper mantle and a thinned mantle transition zone suggest that mantle flow is primarily vertical beneath the swell consistent with the presence of hot buoyant mantle at depth

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

forsterite to wadsleyite phase transformation under shear stress and consequences for the earth s mantle transition zone we have studied the phase transformation of forsterite to wadsleyite under shear stress at the earth s transition zone pressure and temperature conditions two step experiments were performed using a multi anvil press first we hot pressed iron free forsterite at 6 or 11 gpa and 1100 c then we deformed a slab of this starting material using a direct simple shear assembly at 16 gpa and 1400 c for 1 15 35 40 or 60 min both the starting material and the deformed samples were characterized using optical and scanning electron microscopy including measurements of crystal preferred orientations cpo by electron back scattered diffraction ebsd transmission electron microscopy tem and fourier transform infrared spectroscopy ftir the phase transformation occurs very rapidly in less than 1 min and metastable forsterite relics are not observed after deformation the grain size of wadsleyite is slightly smaller than the forsterite starting material the water contents obtained from ftir analyses in forsterite and wadsleyite are 65 124 wt ppm h2o and 114 736 wt ppm h2o respectively which are well below water solubility at similar conditions in the presence of free water wadsleyite aggregates display weak cpo patterns with 1 0 0 axes concentrated at low angle to the shear direction 0 1 0 axes perpendicular to the shear plane and nearly random 0 0 1 axes only a few dislocations were observed in wadsleyite with tem this observation is consistent with the assumption that most dislocations formed during the initial high stress stages of these stress relaxation experiments were consumed in the phase transformation probably enhancing the transformation rate cpo patterns vary as a function of the water content with increasing water content the density of 1 0 0 axes parallel to the shear direction decreases and the density of 0 0 1 axes increases viscoplastic self consistent modeling of cpo evolution using previously reported glide systems for wadsleyite i e 1 0 0 0 k l and 1 2 111 101 cannot reproduce the measured cpo unless the 0 0 1 0 1 0 system for which dislocations have not been observed by tem is also activated in addition wadsleyite grain growth suggests the participation of diffusion assisted processes in deformation calculated anisotropies for p and s waves using measured cpo are always below 1 this very low anisotropy is due to both the low finite strain achieved in the experiments which leads to weak wadsleyite cpo and to the diluting effect of added majorite the present experiments emphasize the importance of stress grain size evolution and water content in the forsterite to wadsleyite phase transformation and subsequent deformation in the transition zone

pressure and temperature dependence of h solubility in forsterite an implication to water activity in the earth interior abstract we performed experiments at 2 5 6 and 9 gpa and temperatures ranging from 1000 to 1400 c in enstatite saturated conditions in the mgo sio 2 h 2 o system using a multi anvil apparatus and determined the dependence of oh solubility in forsterite as a function of pressure and temperature the abundance of oh in forsterite was determined using polarized fourier transform infrared spectroscopy the results show that oh solubility in forsterite increases with temperature only at 2 5 gpa at 6 and 9 gpa the oh solubility reaches a maximum at temperatures of 1175 to 1250 c depending on pressure and then decreases at higher temperatures such behaviour is explained by the change of water activity in the fluid due to dissolution of silicate component using oh concentrations measured on subsolidus samples we determined the thermodynamic parameters of oh incorporation in forsterite such as change in internal energy and entropy we find almost constant e of 37 1 6 7 kj mol and s of 82 8 6 8 j mol k these parameters together with a v solids of 10 6 cm 3 mol are used to calculate the fugacity of the real fluid i e water silicates and compare it to the fugacity of pure water our results imply that estimates of water storage capacity at mantle depths greater than 80 km and above 1250 c should be substantially reduced compared to models that assume that water is a pure fluid

the role of trace elements in controlling h incorporation in san carlos olivine we conducted a series of hydroxylation experiments using mm sized cuboids cut from six different crystals of san carlos olivine with a range of trace element concentrations the cuboids were pre annealed and then hydroxylated under identical conditions ensuring that variation in the amounts of h incorporated depended only on the compositional variables the pre anneal was at 1400 c atmospheric pressure and an oxygen fugacity equivalent to log fmq 1 with the subsequent hydroxylation at 800 c and 1 5 gpa for 3 days hydrogen was incorporated into all six crystals by the four main substitution mechanisms si mg ti and triv with homogeneous h contents in the cores of the crystals indicating h diffusion rates faster than 10 11 m2 s total h as h2o in the homogeneous cores calculated by summing all the infrared absorbance bands ranges from 13 to 27 wt ppm the total h2o in the six pre annealed crystals is poorly correlated with any measured compositional variable however when the h2o associated with individual infrared bands is compared clear trends emerge the intensity of absorption bands at 3572 and 3525 cm 1 are strongly correlated with ti concentrations whose range in the six crystals exceeds an order of magnitude bands between 3400 and 3300 cm 1 correlate negatively with na but are positively correlated with the difference between molar cr3 and na this highlights a previously unrecognised role for na in suppressing h incorporation in natural olivines the results confirm the important role that the trace constituents of olivine play in h incorporation two of these trace elements na and ti tend to be similarly enriched or depleted by partial melting or metasomatism of the mantle but have opposite effects on h incorporation with ti enhancing it but na suppressing it models estimating the effect of h in olivine on mantle rheology must therefore consider carefully the availability of these trace elements

a sharp lithosphere asthenosphere boundary imaged beneath eastern north america although a rigid lithosphere about 100 km of crust and uppermost mantle moving over a weaker asthenosphere a few hundred kilometres of more deformable rock is a widely accepted aspect of plate tectonics the properties of the boundary between them are poorly understood when compressional waves p waves from an earthquake encounter an interface at an angle some of the energy is converted to shear waves or s waves these converted waves can be used to construct an image of the interface based on converted waves recorded in eastern north america the lithosphere asthenosphere boundary has been resolved as a sharp gradient that cannot be explained by thermal gradients alone indicating that the boundary is controlled by melt or high water content in the asthenosphere plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere yet the nature of the lithosphere asthenosphere boundary remains poorly understood the gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers for example if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures mantle flow models1 2 indicate that the velocity gradient at its base would occur over tens of kilometres in contrast if the asthenosphere is weak owing to volatile enrichment3 4 5 6 or the presence of partial melt7 the lithosphere asthenosphere boundary could occur over a much smaller depth range here we use converted seismic phases in eastern north america to image a very sharp seismic velocity gradient at the base of the lithosphere a 3 11 per cent drop in shear wave velocity over a depth range of 11 km or less at 90 110 km depth such a strong sharp boundary cannot be reconciled with a purely thermal gradient but could be explained by an asthenosphere that contains a few per cent partial melt7 or that is enriched in volatiles relative to the lithosphere3 4 5 6

pb isotopic compositions of archean komatiites and sulfides abstract pb isotopic compositions were measured in komatiites and other volcanic rocks from several archean greenstone belts and in magmatic and exhalative sulfides associated with these volcanics thin komatiite flows generally yielded pb pb isochron ages consistent with other geochronological or geological information but thicker basaltic flows gave ages that are too young these results suggest that the thin komatiite flows were hydrothermally altered during eruption and that this event is recorded by the pb pb age thicker basaltic flows resisted alteration during eruption but were affected by later events their pb pb ages probably correspond to the time of metamorphism and not to the time of emplacement magmatic and exhalative sulfides from the abitibi belt in canada fall on the same trend in the 207 pb 204 pb 206 pb 204 pb diagram as komatiites basalts sedimentary rocks and k feldspars from granites the calculated 1 value is 7 8 0 1 a similar suite of rocks and minerals from the kambalda area in australia has distinctly higher 207 pb 204 pb and a higher 1 of 8 3 nd isotopic data demonstrate that some kambalda rocks are highly contaminated with older continental crust the differences in initial pb isotopic compositions are attributed to the presence or absence and the age of underlying granitoid basement kambalda greenstones are underlain and contaminated by old 3 0 ga granitoid crust but significantly older granitoid basement appears absent from the abitibi belt the pb isotopic compositions of kambalda greenstones are dominated by the crustal component and do not represent the composition of the mantle source of these rocks the abitibi greenstones are not highly influenced by crustal contamination and their pb isotopic compositions may reflect those of archean mantle

seismic imaging of mantle transition zone discontinuities beneath the northern red sea and adjacent areas summary the dramatic asymmetry in terms of surface elevation cenozoic volcanisms and earthquake activity across the red sea is an enigmatic issue in global tectonics partially due to the unavailability of broad band seismic data on the african plate adjacent to the red sea here we report the first comprehensive image of the mantle transition zone mtz discontinuities using data from the egyptian national seismic network and compare the resulting depths of the 410 and 660 km discontinuities with those observed on the arabian side our results show that when a standard earth model is used for time to depth conversion the resulting depth of thediscontinuities increases systematicallytowards theaxisoftheafro arabiandome aad from both the west and east relative to the westernmost area the maximum depression of the 410 km discontinuity is about 30km and that of the 660 km discontinuity is about 45km the observed systematic variations can best be explained by a model involving a hydrated mtz and an upper mantle low velocity zone beneath the aad models invoking one or more mantle plumes originated from the mtz or the lower mantle beneath the study area are not consistent with the observations

melting experiments on the udachnaya kimberlite at 6 3 7 5 gpa implications for the role of h2o in magma generation and formation of hydrous olivine abstract melting experiments on kimberlite from the udachnaya pipe have been performed at 6 3 7 5 gpa and 1300 1600 c using a split sphere multianvil apparatus the water content in kimberlite varied from 2 5 to 11 6 wt and the co 2 co 2 h 2 o molar ratio was from 0 61 to 0 23 the samples were placed in a graphite container inside a pt capsule the oxygen fugacity f o 2 during the experiment was controlled by the equilibrium between graphite and water bearing carbonate silicate melt at relatively low temperatures f o 2 was close to the emog d equilibrium at higher temperatures it shifted for approximately 1 log unit to more reduced conditions an olivine garnet clinopyroxene assemblage was present at 100 c below the liquidus of the udachnaya kimberlite with 6 8 wt h 2 o at the pressure 6 3 gpa and 6 10 wt h 2 o at 7 5 gpa at 2 5 wt h 2 o the same assemblage appeared at 150 c below liquidus both at 6 3 and 7 5 gpa orthopyroxene did not form at any temperature and pressure of the experiments the presence of clinopyroxene near the liquidus was due to the calcic nature and a high degree of silica undersaturation in the udachnaya kimberlite at the supra solidus conditions garnet and clinopyroxene compositionally distinct from minerals of the megacryst macrocryst suite crystallized in equilibrium with low h 2 o carbonated melt near the liquidus of high h 2 o kimberlite the stable olivine composition was from fo 91 to fo 98 the garnet composition cao 8 wt tio 2 2 o 3 up to 2 2 wt approached that of ti bearing garnet typical of cr poor garnet megacrysts while the clinopyroxene was an analog of clinopyroxene megacrysts in kimberlite infrared absorption measurements showed that crystallized olivines contained water in the form of ti clinohumite like and oh clinohumite like defects the h 2 o content of olivine was found to depend mainly on water content in kimberlite melt and pressure olivine with 120 170 ppm h 2 o crystallized at 6 gpa in equilibrium with the melt compositionally close to the archetypical kimberlite rather than a carbonate one and water as high as 6 wt olivine with 270 300 ppm h 2 o crystallized near kimberlite liquidus only at 7 5 gpa and 9 5 10 1 wt h 2 o the experimental results imply that hydrous olivine which is typical to kimberlites including the udachnaya pipe very unlikely can crystallize from anhydrous carbonated magma judging by the conditions for multiphase saturation of the udachnaya kimberlite melt its generation was possible by partial melting of carbonated garnet wehrlite at pressures from 6 to 7 5 gpa and temperatures within 1450 1600 c in the presence of aqueous fluids thus water must have been a trigger for generation of the primary kimberlite magma

constraining olivine abundance and water content of the mantle at the 410 km discontinuity from the elasticity of olivine and wadsleyite abstract velocity and density jumps across the 410 km seismic discontinuity generally indicate olivine contents of 30 to 50 vol on the basis of the elastic properties of anhydrous olivine and wadsleyite which is considerably less than the 60 olivine in the widely accepted pyrolite model for the upper mantle a possible explanation for this discrepancy is that water dissolved in olivine and wadsleyite affects their elastic properties in ways that can reconcile the pyrolitic model with seismic observations in order to more fully constrain the olivine content of the upper mantle near the 410 km discontinuity and to place constraints on the mantle water content at this depth we determined the full elasticity of hydrous wadsleyite at the p t conditions of the discontinuity based on density functional theory calculations together with previous determinations for the effect of water on olivine elasticity we simultaneously modeled the density and seismic velocity jumps v p v s across the olivine wadsleyite transition our models allow for several scenarios that can well reproduce the density and seismic velocity jumps across the 410 km discontinuity when compared to globally averaged seismic models when the water content of olivine and wadsleyite is assumed to be equal as in a simple binary system our modeling indicates a best fit for low water contents

hydrogen solubility in al rich stishovite and water transport to the lower mantle stishovite is an important phase in subducting oceanic crust the post garnet assemblage from a precursor eclogite lithology contains up to 25 stishovite at pressures above 25 gpa this stishovite may contain up to 5 wt al2o3 we measured the hydrogen contents of stishovite samples synthesized at 20 25 gpa and 1200 1800 c from several starting materials containing up to 10 wt al2o3 ftir spectra of stishovite show major bands at 3111 3134 cm 1 with the frequencies increasing as h2o and al2o3 contents increase and several minor bands at 2659 2667 3240 3261 3312 3334 and 3351 cm 1 the h2o content of al free stishovite is in the range of 16 30 wt ppm the maximum h2o content of al bearing stishovite 4 4 wt al2o3 synthesized at 20 gpa and 1400 c is 3010 300 wt ppm most hydrogen in stishovite is associated with al 3 substitutional defects on the octahedral si 4 site the hydrogen can occupy 40 of vacancies created by incorporation of al 3 at 20 gpa this observation along with some anomalies in the ftir spectra may indicate an alternative mechanism of al 3 incorporation into stishovite via the formation of oxygen vacancies or interstitial ali defects we report the highest h2o concentrations in alstishovite to date and argue that it is the most important carrier of water into the lower mantle post garnet eclogitic assemblages 2007 elsevier b v all rights reserved

mineral chemistry and alteration of rare earth element ree carbonates from alkaline pegmatites of mount malosa malawi in this study the mineral chemistry and alteration of bastnasite ce parisite ce and synchysite ce was investigated by empa tg dtg chns and scxrd techniques relevant chemical data on light and volatile elements and x ray diffraction data useful to evaluate the effect of oh content on the unit cell volume of such carbonates are provided significant amounts of oh in both the parisite ce and synchysite ce were found and these data are the first direct evidence of hydration for these carbonates single crystal x ray diffraction data showed that hydration causes an increase in the unit cell volume for parisite and synchysite parisite ce and synchysite ce oh end members have not been described in nature so far and the amount of water must be taken into account for evaluating changes in stability field conditions textural relationships and replacement processes are discussed

seismic signature of a hydrous mantle transition zone abstract although water has a major influence on tectonic and other geodynamic processes little is known about its quantity and distribution within the deep earth in the last few decades laboratory experiments on nominally anhydrous minerals nams of the transition zone have shown that these minerals can contain significant amounts of water up to 3 3 wt in this study we investigate if it is possible to use seismic observations to distinguish between a hydrous and anhydrous transition zone we perform an extensive literature search of mineral experimental data to generate a compilation of the water storage capacities elastic parameters and phase boundary data for potentially hydrous minerals in the transition zone and use thermodynamic modelling to compute synthetic seismic profiles of density v p and v s at transition zone temperatures and pressures we find that large uncertainties on the mineral phase equilibria ca 2 gpa and elastic properties produce a wide range of seismic profiles in particular there is a lack of data at temperatures corresponding to those along a 1300 c adiabat or hotter which may be expected at transition zone pressures comparing our hydrous transition zone models with equivalent profiles at anhydrous conditions we see that the depths of the 410 and 660 discontinuities cannot at present be used to map the water content of the transition zone due to these uncertainties further while average velocities and densities inside the transition zone clearly decrease with increasing water content there is a near perfect trade off with increases in temperature it is therefore difficult to distinguish thermal from water effects and the conventional view of a slow and thick transition zone for water and slow and thin transition zone for high temperature should be regarded with caution a better diagnostic for water may be given by the average velocity gradients of the transition zone which increase with increasing water content but decrease for increasing temperature however the significance of this effect depends on the degree of water saturation and partitioning between the nams since seismology is better able to constrain the thickness of the transition zone than velocity gradients our study indicates that the most useful input from future mineral physics experiments would be to better constrain the phase relations between hydrous olivine and its high pressure polymorphs especially at high temperatures additionally the uncertainties on the mineral seismic properties could be reduced significantly if the experimentally observable correlations between bulk and shear moduli and their corresponding pressure derivatives would be published

mantle transition zone discontinuities beneath the baikal rift and adjacent areas like most other major continental rifts the baikal rift zone brz in siberia ispresumably underlain by a hot and partially molten mantle which has a reduced seismicvelocity relative to surrounding areas recent seismic tomography studies however gaveconflicting results about the depth extent and even the existence of the low velocityanomaly beneath the brz suggesting that additional constraints are needed here wepresent results from stacking of about 1700 radial p to s receiver functions from asingle long running seismic station tly located at the sw tip of lake baikal a clearuplift of the 410 km discontinuity d410 with a magnitude of about 47 km relative tothe south margin of the siberian platform is observed beneath the rift currently availableseismic results suggest that the uplift is unlikely to be caused by addition of water tomantle transition zone mtz silicates but is the result of a 550 c reduction in temperaturein the vicinity of the d410 in addition the 660 km discontinuity d660 shows adownward trend toward the rift from the south suggesting that the entire mtz might havea low temperature beneath the rift the thickening of the mtz suggests a high velocityanomaly of about 2 in the mtz and rules out the possibility that the rifting is caused bya mantle plume originated in or beneath the mantle transition zone

hydroxyl in olivines from mantle xenoliths in kimberlites of the siberian platform from a total of 335 olivine crystal grains crystallographically orientated platelets and where possible parallelepipeds were prepared chemically analysed by electron microprobe examined under the polarisation microscope and studied by polarised ftir microscope absorption spectrometry in the oh vibrational range 3 000 3 800 cm 1 the 335 crystal grains were extracted from 174 different specimens of yakutian upper mantle material including 97 xenoliths that represent all the rock types occurring in all the kimberlites of the siberian shield the other specimens were mega and macrocrysts or inclusions in diamonds and garnets analysis of the polarised oh spectra allowed distinction between hydroxyl in non intrinsic separate inclusions nsi and in isolated local or condensed extended defects intrinsic to the olivines ild or ced respectively as the two latter types cannot be distinguished by vibrational spectroscopy and as they are presumably interconnected by intracrystalline condensation reactions of the type n ild ced n we propose to symbolise them as ild ced of the total of 70 polarised oh bands that were found in the whole set 17 are caused by nsi 53 by ild ced total mean integrated oh band intensities int total with int a b c int 3 were determined from the spectra they yielded the contents of structurally unallocated water using the recent calibration of the ir method bell et al 2003 the range is 0 a b at 3 327 3 328 cm 1 an energy typical of oh in hydrous wadsleyite we assume this phase to be present as nis in the respective olivines possibly as relic phase 4 weak bands between 3 175 and 3 260 cm 1 polarised along c are tentatively assigned to molecular water nsi with relatively strong hydrogen bonds to the matrix we did not find larger clusters of molecular water i e liquid like water with its characteristic broad band centred at ca 3 400 cm 1 we did also not find any humite minerals as an nsi of the 53 oh bands intrinsic to olivine the 29 most abundant and strong ones were subject to further analysis in terms of oh bearing structural defects ild ced nearly all these bands are strongly polarised along a two bands at 3 672 and 3 535 cm 1 are assigned to boron related defects ild ced b five bands at 3 573 3 563 3 541 3 524 and 3 512 cm 1 are intensity correlated and are assigned to si depleted titan clinohumite like defects ild ced thl the other so far unidentified oh of ild ced are suggested to originate from oh in different types of mg fe depleted defects recently predicted and discovered by tem these are called ild ced kwk eight mostly strong bands of them occur at energies higher than 3 573 cm 1 ild ced kwk h 13 strong to medium strong bands occur below 3 500 cm 1 ild ced kwk l such intrinsic defects may occur alone ild ced thl and ild ced kwk h or in different combinations with each other ild ced kwk h ild ced thl ild ced kwk h ild ced kwk l and ild ced kwk h ild ced thl ild ced kwk l though there are indications that the occurrences of such types and combinations of the intrinsic oh bearing defects in the olivines are related to the types and genetic peculiarities of their host rocks straightforward and simple correlations do not exist the reasons for this and also for the great number of varieties of intrinsic ild ced are discussed

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

high pressure single crystal elasticity of wadsleyite and the seismic signature of water in the shallow transition zone abstract earth s transition zone at depths between 410 km and 660 km plays a key role in earth s deep water cycle since large amounts of hydrogen can be stored in the nominally anhydrous minerals wadsleyite and ringwoodite mg fe 2 sio 4 previous mineral physics experiments on iron free wadsleyite proposed low seismic velocities as an indicative feature for hydration in the transition zone here we report simultaneous sound wave velocity and density measurements on iron bearing wadsleyite single crystals with 0 24 wt h 2 o by comparison with earlier studies we show that pressure suppresses the velocity reduction caused by higher degrees of hydration in iron bearing wadsleyite ultimately leading to a velocity cross over for both p waves and s waves modeling based on our experimental results shows that wave speed variations within the transition zone as well as velocity jumps at the 410 km seismic discontinuity both of which have been used in previous work to detect mantle hydration are poor water sensors instead the impedance contrast across the 410 km seismic discontinuity that is reduced in the presence of water can serve as a more robust indicator for hydrated parts of the transition zone

complete miscibility between silicate melts and hydrous fluids in the upper mantle experimental evidence and geochemical implications the phase relationships between silicate melts and hydrous fluids were studied by direct visual observation in an externally heated diamond anvil cell complete miscibility of silicate melt and water was observed for a wide range of melt compositions including nepheline jadeite dacite haplogranite and ca bearing granite some evidence for complete miscibility was also observed in the system basalt h2o the critical temperatures in all systems decrease rapidly with pressure at 15 kbar the critical temperature for nepheline is around 550 c for jadeite around 800 c and for granitic compositions it is close to 900 c in general the critical temperatures appear to increase with silica content in the system our results suggest that there is complete miscibility between silicate melts and water in most of the upper mantle except at very shallow depths this means that a water saturated solidus cannot be defined any more in the deeper parts of the upper mantle very silica rich melt inclusions found in spinel lherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water and silicate components the decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate melt is not yet closed while the decomposition of phengite and lawsonite occurs far beyond the critical curve accordingly the fluids released by the breakdown of these minerals should have very different properties highly mobile hydrous fluids containing little dissolved silicate should be produced by amphibole breakdown while the decomposition of lawsonite and phengite will lead to much more silicate rich and less mobile fluid phases

two dimensional numerical studies on the effects of water on martian mantle evolution induced by magmatism and solid state mantle convection 1 numerical models of magmatism and mantle convection beneath a stagnant lithosphere are presented to understand how water affects martian mantle evolution magmatism is modeled as an upward permeable flow of the basaltic magma generated by decompression melting first a global partially molten layer gpml develops and generates a basaltic crust a layer of compositionally buoyant residue of melt extraction in the uppermost mantle and a denser layer with a higher content of the basaltic component in the deep mantle the gpml extracts most of the water initially contained in the upper layer but some water remains in the lower layer of the mantle subsequently hot plumes ascend from the lower layer to induce magmatism the water allows plume magmatism to continue for a long duration up to 5 gyr depending on the initial water content and the detail of the initial temperature distribution in the mantle provided that the mantle is initially not too hot just after planetary formation the plume magmatism is sufficiently active to cause significant crustal growth and dehydration of the crust and mantle in the early evolutionary stage when the internal heating is strong the amount of extracted water is equivalent to a water layer of up to several hundred meters in depth water can also enhance the extraction of heat producing elements hpes from the mantle reduce the heat flux from the mantle to the crust and make the lithosphere thicker both crustal growth and dehydration eventually subside as the hpes decay

hydrogen incorporation and retention in metamorphic olivine during subduction implications for the deep water cycle incorporation of hydrogen into metamorphic olivine during dehydration reactions in the subducting oceanic lithosphere provides a mechanism to replenish the deep mantle with water fourier transform infrared spectroscopy of metamorphic olivines formed at 2 5 gpa and 550 c through the reaction antigorite brucite olivine chlorite water shows water contents between 100 and 140 ppm h2o associated exclusively with silicon vacancies similar to the highest values found in peridotite xenoliths brucite involvement in the olivine forming reaction ensures h2o saturation and a low si activity favoring hydrogen incorporation into si vacancies the mapped water distribution in olivine is consistent with growth zoning and there is no evidence of water gain or loss thus even for metamorphic timescales of several million years at 550 c no ionic diffusion modification is observed in agreement with recent experimental findings metamorphic olivines formed by this dehydration reaction may contribute considerable amounts of water to the deep water cycle additionally olivine with abundant h in si vacancies are expected to be rheologically weaker than anyhdrous mantle olivine and might provide a weak interface between slabs and mantle wedges in subduction zones at conditions beyond the stability of hydrous phases

high resolution transition zone structures of the gorda slab beneath the western united states implication for deep water subduction 1 the current data set from usarray provides an unprecedented opportunity to investigate mantle transition zone structures beneath the western united states we have made transition zone images with the common converted point ccp stacking method more than 9600 high quality receiver functions were stacked with reference to two different three dimensional tomography models and a one dimensional velocity model where the gorda plate passes through the transition zone the 410 discontinuity has been elevated 25 km and the 660 discontinuity has been depressed 35 km we interpret the transition zone topography in terms of mineral physics results in several different ways noting in particular that recent measurements on the clapeyron slope for the ringwoodite to perovskite phase transition under dry conditions give a phase boundary slope of 1 3 to 0 4 mpa k the 35 km deflection of the 660 discontinuity observed in the receiver functions seems to be the evidence that the subducted slab can carry abundant water from the surface to the transition zone and in the transition zone the water in the slab may be fully saturated e g the water content is 2 0 wt analyses of the velocity perturbations in the tomography models and the transition zone thickness indicate that the deep water is likely well confined within the subducted slab we infer that the presence of water in the subducted gorda slab might have contributed 15 km and the thermal anomaly in the slab might have contributed 20 km to the depression of the 660 discontinuity

effects of hydration on thermal expansion of forsterite wadsleyite and ringwoodite at ambient pressure single crystal x ray diffraction has been used to measure the thermal expansion coefficients of forsterite wadsleyite ringwoodite and their hydrous forms at ambient pressure from temperatures as low as 133 k to as high as 919 k second order polynomial fitting to ln v v0 vs t was applied to derive the expansion coefficients in the form of a1 t a0 the single crystal of anhydrous wadsleyite persisted up to 859 k hydrous forsterite was observed to dehydrate at 919 k whereas hydrous wadsleyite started to dehydrate at 655 k the crystal of ringwoodite with 0 20 wt broke down at 911 k and the two ringwoodite samples with 0 74 and 2 4 wt h2o were observed to break down with an irreversible unit cell expansion at 808 and 606 k respectively from room temperature to high temperatures in this study the mean thermal volume expansion coefficients are 36 4 5 and 38 1 9 10 6 k 1 respectively for anhydrous and hydrous forsterite 28 5 5 and 35 8 8 10 6 k 1

hydrous melting and partitioning in and above the mantle transition zone insights from water rich mgo sio2 h2o experiments abstract hydrous melting at high pressures affects the physical properties dynamics and chemical differentiation of the earth however probing the compositions of hydrous melts at the conditions of the deeper mantle such as the transition zone has traditionally been challenging in this study we conducted high pressure multianvil experiments at 13 gpa between 1200 and 1900 c to investigate the liquidus in the system mgo sio2 h2o water rich starting compositions were created using platinic acid h2pt oh 6 as a novel water source as mgo sio2 ratios decrease the t x h 2 o liquidus curve develops an increasingly pronounced concave up topology the melting point reduction of enstatite and stishovite at low water contents exceeds that predicted by simple ideal models of hydrogen speciation we discuss the implications of these results with respect to the behaviour of melts in the deep upper mantle and transition zone and present new models describing the partitioning of water between the olivine polymorphs and associated hydrous melts

olivine hydration in the deep upper mantle effects of temperature and silica activity 1 although water controls the biology and geology of the surface hydrogen is perhaps the most poorly constrained compositional variable in the bulk earth its concentration in the upper mantle appears to be controlled by its solubility as hydroxyl in the nominally anhydrous silicate phases olivine pyroxene garnet wadsleyite and ringwoodite here we describe a series of experiments showing that the solubility of h2o in olivine at 12 gpa increases with temperature to 8900 ppm by weight at 1250 c and decreases at higher temperature with the onset of melting sample characterization by infrared spectroscopy indicates that the primary hydration mechanism is the substitution of 2h for mg2 similar results obtained from samples coexisting with clinohumite low silica and with clinoenstatite high silica indicate that silica activity has minimal effect on hydration under these conditions single crystal x ray diffraction measurements constrain the volume of hydration and indicate significant m site vacancies hydrogen thus appears to become a geochemically compatible element as depths approach 400 km

temperature dependence and mechanism of hydrogen incorporation in olivine at 12 5 14 0 gpa 1 the concentration and incorporation mechanisms of hydrogen into mg fe 2sio4 olivine have been studied under water saturated conditions at 12 5 14 0 gpa and 1100 2000 c the hydrogen content of olivine fo90 fo95 coexisting with enstatite and hydrous melt increases from about 4600 ppmw at 1100 c to a maximum of 6250 ppmw at 1200 c above 1400 c the hydrogen content of olivine decreases non linearly reaching 160 240 ppmw in the 1800 2000 c range where there is a high melt fraction and no enstatite polarized infrared spectra of the recovered olivines exhibit oh absorption bands in both the 3450 3650 cm 1 range group i and 3160 3450 cm 1 range group ii results indicate that hydration of olivine is associated with both mg and si site vacancies microprobe analyses of the samples show a clear deficit of si in olivine the water storage capacity of olivine is probably no more than 1 5 2 0 times less than wadsleyite at 1400 c and transition zone pressures

the partitioning of water between olivine orthopyroxene and melt synthesised in the system albite forsterite h2o coexisting forsterite enstatite and silicate glass were synthesised from a water bearing basalt analogue in programmed cooling experiments at 1 0 1 5 2 0 and 2 5 gpa hydrous species in each phase were characterised and quantified using infrared absorbance spectroscopy al 3 related hydroxyl defects with vibrational energies between 3400 3200 cm 1 dominate the hydroxyl budget of synthetic forsterite the vibrational characteristics of these defects indicate that similar absorptions in febearing systems result from fe 3 related oh species this interpretation resolves a previously recognised but unexplained relationship between oxygen fugacity and hydroxyl speciation in experimentally annealed olivines and suggests that the intensities of bands at 3400 3300 cm 1 could form the basis of an oxybarometer oh defect speciation in enstatite is similar to that in aluminous crystals synthesised at high water fugacity and many natural orthopyroxenes hydroxyl speciation in orthopyroxene is not therefore controlled by water activity total water contents of synthetic enstatites from any single experiment vary enormously this heterogeneity is attributed to variable contributions from a non intrinsic broad band hydrous component intrinsic hydroxyl concentrations dissolved in enstatite are identical in all crystals from an experiment the partitioning of water between forsterite enstatite and coexisting silicate melt is independent of pressure temperature and total water content over the range of conditions studied as long as the influence of these parameters on phase compositions is taken into account dh2o fo melt 0 0001 0 0003 n 4 and is insensitive to pressure or olivine aluminium contents dh2o en melt is strongly controlled by al2o3 concentration and ranges from 0 003 0 016 n 6 two samples contain both forsterite and enstatite and yield dh2o en fo 25 1

the effect of silica activity on the incorporation mechanisms of water in synthetic forsterite a polarised infrared spectroscopic study pure forsterite crystals were grown from hydrous melts using controlled cooling experiments at 2 0 gpa and varying the bulk mg si ratio from 2 0 to 1 5 oriented single crystals were then studied by polarised infrared spectroscopy the spectra of the samples with the lowest silica activity asio2 contain the main oh bands in the range 3 620 3 450 cm 1 only in contrast the spectra of the samples synthesised with the highest asio2 contain additional pleochroic bands at 3 160 3 220 and 3 600 cm 1 the variations are interpreted in terms of protonated silicon vacancies being dominant at low asio2 and mg vacancies dominant at high asio2 xenolithic mantle olivines generally do not have the spectrum expected for orthopyroxene buffered conditions suggesting that they re equilibrated with their host melts during ascent but mantle olivine from the zabargad peridotite massif probably is in equilibrium with the coexisting orthopyroxene

effects of shock and martian alteration on tissint hydrogen isotope ratios and water content abstract the tissint meteorite a picritic shergottite fell to earth in morocco on the 18th of july 2011 and is only the fifth martian meteorite witnessed to fall hydrogen isotope ratios and water contents are variable within different minerals in tissint ringwoodite and shock melt pockets contain elevated d h ratios relative to terrestrial values d 761 4224 these high ratios in recrystallized phases indicate significant implantation of hydrogen from the d rich martian atmosphere during shock in contrast although olivine has detectable water abundances 230 485 ppm it exhibits much lower d h ratios d 88 to 150 suggesting this water was not implanted from the martian atmosphere the minimal terrestrial weathering experienced by tissint gives confidence that the olivine hosted water has a martian origin but its high concentration indicates direct inheritance from the parental melt is improbable especially given the low pressure of olivine crystallisation incorporation of a low d crustal fluid or deuteric alteration during crystallisation could explain the relatively high water contents and low d h ratios in tissint olivine

effect of oxygen fugacity on the h2o storage capacity of forsterite in the carbon saturated systems abstract high pressure experiments have been performed in the systems mg 2 sio 4 c o h and mg 2 sio 4 k 2 co 3 c at 6 3 gpa and 1200 to 1600 c using a split sphere multi anvil apparatus in the mg 2 sio 4 c o h system the composition of fluid was modeled by adding different amounts of water and stearic acid the f o 2 was controlled by the mo moo 2 or fe feo oxygen buffers several experiments in the mg 2 sio 4 c o h system and all experiments in the mg 2 sio 4 k 2 co 3 c system have been conducted without buffering the f o 2 forsterite in the system mg 2 sio 4 k 2 co 3 c does not reveal oh absorption bands in the ir spectra while forsterite coexisting with carbon bearing fluid and silicate melt at log f o 2 from fmq 2 to fmq 5 from 2 to 5 log units below fayalite magnetite quartz oxygen buffer contains 800 1850 wt ppm h 2 o the maximum concentrations were detected at 1400 c and fmq 3 5 we observed an increase in the solidus temperature in the system mg 2 sio 4 c o h from 1200 to above 1600 c with log f o 2 decreasing from fmq 2 to fmq 5 the increase of the solidus temperature and the broadening of the stability field of the h 2 o h 2 ch 4 subsolidus fluid phase at 1400 1600 c explain the high h 2 o storage capacity of forsterite relative to that crystallized from carbon free oxidized hydrous silicic melt at temperatures above 1400 c liquidus forsterite precipitated along with diamond from oxidized fmq 1 carbonate silicate melt and from silicate melt dissolving the moderately reduced c o h fluid from fmq 2 to fmq 3 5 formation of diamond was not detected under ultra reduced conditions fmq 5 at 1200 1600 c olivine co precipitating with diamond from dry carbonate silicate or hydrous silicic fluid melt can provide information on the h 2 o contents and speciation of the diamond forming media in the mantle the conditions for minimum post crystallization alteration of olivine and its hydrogen content are discussed

in situ infrared spectra of oh in olivine to 1100 c the infrared spectra of hydrated san carlos olivine were measured from room temperature to 1100 c at 1 bar using a heating stage the spectra show that even at the highest temperatures studied there are still two well separated groups of oh bands centered between 3200 and 3300 cm 1 and between 3500 and 3600 cm 1 respectively the distinction between group i 3450 cm 1 and group ii bands 3450 cm 1 is therefore still meaningful at upper mantle temperatures however a prominent band at 3612 cm 1 already loses intensity by 100 c and nearly disappears by 300 c suggesting that it corresponds to a particular h environment that only formed during cooling of the sample and that it is not stable at high temperature a similar band is prominent in many olivines from mantle xenoliths and previously it has sometimes been assigned to oh groups surrounding si vacancies we show that structural models of water dissolution in olivine derived from infrared spectra at ambient conditions may not be fully applicable for the upper mantle

high hydrogen solubility in al rich stishovite and water transport in the lower mantle abstract stishovite is an important phase in subducting oceanic crust the post garnet assemblage from a precursor eclogite lithology contains up to 25 stishovite at pressures above 25 gpa this stishovite may contain up to 5 wt al 2 o 3 we measured the hydrogen contents of stishovite samples synthesized at 20 25 gpa and 1200 1800 c from several starting materials containing up to 10 wt al 2 o 3 ftir spectra of stishovite show major bands at 3111 3134 cm 1 with the frequencies increasing as h 2 o and al 2 o 3 contents increase and several minor bands at 2659 2667 3240 3261 3312 3334 and 3351 cm 1 the h 2 o content of al free stishovite is in the range of 16 30 wt ppm the maximum h 2 o content of al bearing stishovite 4 4 wt al 2 o 3 synthesized at 20 gpa and 1400 c is 3010 300 wt ppm most hydrogen in stishovite is associated with al 3 substitutional defects on the octahedral si 4 site the hydrogen can occupy 40 of vacancies created by incorporation of al 3 at 20 gpa this observation along with some anomalies in the ftir spectra may indicate an alternative mechanism of al 3 incorporation into stishovite via the formation of oxygen vacancies or interstitial al i defects we report the highest h 2 o concentrations in al stishovite to date and argue that it is the most important carrier of water into the lower mantle post garnet eclogitic assemblages

formation of carbonates in the tatahouine meteorite the tatahouine meteorite in southern tunisia shows terrestrial contamination that developed during 63 years of exposure on earth s surface samples collected on the day of the fall in 1931 contained fractures with no secondary minerals whereas samples collected in 1994 contain calcite aggregates 70 to 150 micrometers and rod shaped forms 100 to 600 nanometers in length and 70 to 80 nanometers in diameter on the fractures carbon isotope analysis of the carbonates within the tatahouine meteorite delta13c 2 0 per mil pee dee belemnite standard pdb and the underlying ground delta13c 3 2 per mil pdb confirm their terrestrial origin

ar ar chronology of the martian meteorite alh84001 evidence for the timing of the early bombardment of mars abstract alh84001 a cataclastic cumulate orthopyroxenite meteorite from mars has been dated by ar ar stepped heating and laser probe methods both methods give ages close to 3 900 ma the age calculated is dependent on assumptions made about 39ar recoil effects and on whether significant quantities of 40ar from the martian atmosphere are trapped in the meteorite if as suggested by xenon and nitrogen isotope studies martian atmospheric argon is present then it must reside predominantly in the k rich phase maskelynite independently determined 129xe abundances in the maskelynite can be used to place limits on the concentration of the atmospheric 40ar these indicate a reduction of around 80 ma to ages calculated on the assumption that no martian atmosphere is present after this correction the nominal ages obtained are 3940 50 3870 80 and 3970 100 ma by stepped heating and 3900 90 ma by laser probe 1 statistical errors giving a weighted mean value of 3 920 ma ambiguities in the interpretation of 39ar recoil effects and in the contribution of martian atmospheric 40ar lead to uncertainties in the ar ar age which are difficult to quantify but we suggest that the true value lies somewhere between 4 050 and 3 800 ma this age probably dates a period of annealing of the meteorite subsequent to the shock event which gave it its cataclastic texture the experiments provide the first evidence of an event occurring on mars coincident with the time of the late heavy bombardment of the moon and may reflect a similar period of bombardment in the southern highlands of mars whether the age determined bears any relationship to the time of carbonate deposition in alh84001 is not known such a link depends on whether the temperature associated with the metasomatic activity was sufficient to cause argon loss from the maskelynite and or whether the metasomatism and metamorphism were linked in time through a common heat source

ftir spectrum of phenocryst olivine as an indicator of silica saturation in magmas fourier transform infrared ftir absorption spectra of hydroxyl were measured on olivine phenocrysts from hydrous basaltic melts that originated in island arc tectonic settings the basaltic melts encompass a wide range of silica activities from orthopyroxene saturated hypersthene normative to nepheline normative compositions the intensities and wavenumber placement of hydroxyl absorption bands correlate with the degree of silica saturation of the parent melt from which the olivine crystallized olivines from silica undersaturated nepheline normative melts absorb ir radiation in the wavenumber range 3430 3590 cm 1 group 1 in contrast olivines from orthopyroxene saturated boninitic melts exhibit hydroxyl absorption bands in the wavenumber range 3285 3380 cm 1 group 2 olivines crystallized at intermediate silica activities exhibit a combination of the two groups of hydroxyl ir bands where the proportion of group 2 bands increases with increasing silica saturation of the parent melt the positions of hydroxyl absorption peaks observed here for natural samples are consistent with previous measurements on experimentally annealed olivines thus protonation experiments can be employed to make spectroscopically dry olivine structures visible by ir yielding information on the silica saturation of the parental magmas hydroxyl concentrations in the studied olivines were estimated to be 1 2 ppm corresponding to an olivine melt partition coefficient of similar to 1 0 0 3 x 10 4

ftir spectroscopy of ti chondrodite ti clinohumite and olivine in deeply subducted serpentinites and implications for the deep water cycle separated olivine grains from a deeply subducted serpentinized wehrlite changawuzi in the western tianshan ultrahigh pressure belt china were analysed with unpolarized transmission ftir and the ti contents were determined using la icp ms the major bands in the olivine spectra display striking similarities to ti clinohumite and are interpreted as oh in lamellae the quantification of the water content in lamellae requires calibration of the ir absorption for such bands we have obtained a new absorption coefficient for ti clinohumite of 0 125 0 017 ppm cm2 based on polarized ftir measurements on three orthogonal sections through a large single crystal of ti clinohumite from val malenco which has a known water content of 1 53 wt the resulting water content in olivine using this calibration factor ranges from 440 to 2 600 ppm and correlates positively with the ti content that ranges from 130 to 1 400 ppm for the quantification of the water content in ti chondrodite and ti clinohumite that are associated with olivine we developed a new method using attenuated total reflectance ftir spectroscopy ti chondrodite and ti clinohumite display similar ir bands at 3 562 3 525 and 3 583 3 586 cm 1 as in olivine the water content and ti content correlate in both ti clinohumite and ti chondrodite indicating an intergrowth of these minerals which has been confirmed by tem analyses our results confirm previous suggestions that there is a strong correlation between the ti content of ultramafic rocks and their capacity to transport water to the deeper mantle in subduction zones beyond conditions where hydrous phases are stable

the infrared signature of water associated with trivalent cations in olivine abstract forsterite crystals were synthesised under water saturated conditions at 1400 c and 1 5 gpa doped with trace amounts of either b al sc ti v cr mn fe co ga y zr in sm gd dy tm or lu the common and intense hydroxyl stretching bands in the infrared spectra of spinel peridotite olivine at 3572 and 3525 cm 1 were only reproduced in the presence of ti those samples where the trace element substitutes as the trivalent cation on the mg 2 site were identified from a systematic variation in concentration with the trivalent ionic radius the hydroxyl region of all samples is essentially identical except for between 3300 and 3400 cm 1 this region is characterised by one or more bands with the energy of the most intense feature being correlated with the ionic radius of the trivalent cation the integrated intensity of these hydroxyl bands also correlates with the concentration of the trivalent cation these correlations provide unambiguous evidence that bands or peaks in this region correspond to water at defect sites associated with trivalent cations trivalent peaks are sometimes observed in samples of mantle olivine and most likely indicate water associated with fe 3 the water at this site is not incorporated under normal mantle conditions and should not be included in estimates of the water capacity of mantle olivine these results emphasise the importance of identifying the infrared signature of different water substitution mechanisms

the effect of oxygen fugacity on hydroxyl concentrations and speciation in olivine implications for water solubility in the upper mantle the effect of oxygen fugacity on hydroxyl speciation and solubility in san carlos olivine was investigated in a series of piston cylinder experiments at 2 0 gpa and 1100 and 1300 c the presence of enstatite was intended to fix silica activity and experimental fo2 in most runs was regulated by lining the inner walls of the sample containing pt capsules with either fe ni or re foil polarised infrared absorbance spectra show that olivine annealed in capsules lined with fe foil only contain hydroxyl defects with vibrational energies between 3650 and 3400 cm 1 relatively high frequencies spectra of crystals annealed in capsules lined with ni or re foil show that these samples contain hydroxyl defects with vibrational energies between 3650 and 3400 cm 1 high frequency and at 3355 and 3325 cm 1 low frequency the concentrations of hydroxyl defects responsible for absorptions at 3355 and 3325 cm 1 change as a function of oxygen fugacity and are therefore interpreted as fe 3 related oh centres hydroxyl solubility in our experiments is substantially less than current solubility laws might suggest these differences may be attributed to variations in experimental procedure but they also demonstrate that the incorporation of hydroxyl in olivine must be betterunderstoodbeforeanaccuratemodeltodescribewatersolubilityinthedeepearthcanbeconstructed ourdatashowthat when experiments conductedat identicalp tconditionsarecompared olivine annealedat lowf o2 below fe feo contains lessthanhalf as much water as crystals from relatively more oxidised experiments because natural mantle conditions approximate to those of our low fo2 experiments current solubility laws which are based on data from relatively high f o2 experiments overestimate the amount of water that can be hosted in olivine in the upper mantle from a consideration of defect equilibria we propose a mechanism for the incorporation of oh in olivine in a way that is appropriate for the actual range of conditions expected in the mantle 2007 elsevier b v all rights reserved

h2o and co2 in kimberlitic fluid as recorded by diamonds and olivines in several ekati diamond mine kimberlites northwest territories canada abstract surface dissolution features on diamonds and fourier transform infra red spectroscopy ftir of phenocrystal and xenocrystal olivines from kimberlites contain a record of magmatic fluid in kimberlite magmas we investigated composition and behavior of kimberlitic fluid and the effect of volatiles on the eruption style and geology of kimberlites using microdiamonds and olivine concentrates from six kimberlite pipes with different lithologies and the character of diamond resorption ekati diamond mine northwest territories canada the study showed a clear correlation between the resorption style of diamond population of the kimberlites and the type of infrared ir spectra of their olivines four kimberlites have high quality diamonds with smooth regular surface features and high h 2 o content of the olivines indicating the presence of h 2 o rich fluid during the emplacement fast ascent rates of fluid rich magma can explain explosive eruption and filling the pipes with volcaniclastic kimberlite facies conversely grizzly and leslie kimberlites have diamonds with complex sharp features diminishing diamond quality and indicating loss of the fluid the slower ascent rates and less explosive eruption of the fluid free magmas produced kimberlite pipes filled with magmatic facies kimberlite distinctive peaks in olivine ir spectra at 3356 and 3327 cm 1 were found to correlate with the presence of hydrous magmatic fluid character of diamond morphology suggests that during the whole ascent of all six kimberlites the magmatic fluid when present had a high h 2 o co 2 ratio

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

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

experimental determination of the effect of h2o on the 410 km seismic discontinuity abstract due to the high solubility of h 2 o in the mineral wadsleyite it has been recognized that the presence of h 2 o in the mantle could influence the pressure and pressure interval of the olivine to wadsleyite phase transformation that is generally considered to be the cause of the 410 km seismic discontinuity we have performed multianvil experiments in the mg 2 sio 4 h 2 o system in order to quantify the expansion of the wadsleyite stability field as a result of the presence of h 2 o at 1200 c we find that the wadsleyite stability field expands to lower pressures by approximately 1 gpa under h 2 o rich fluid saturated conditions at 1400 c the expansion is approximately 0 4 gpa and no expansion can be detected at 1600 c a thermodynamic model based on recent h 2 o solubility measurements and crystallographic observations predicts an expansion of the wadsleyite stability field that is in good agreement with the experimental results when these results are combined with phase relations in the mg 2 sio 4 fe 2 sio 4 system it can be shown that significant effects on the pressure and pressure interval of the olivine to wadsleyite transformation i e on the depth and width of the 410 km discontinuity are only expected at lower than ambient mantle temperatures 2 o concentrations that are substantially greater than 0 2 wt areas of the mantle where the 410 km discontinuity appears to occur over a depth interval of over 20 km can only be explained if the temperature in these regions is at or below 1200 c and water concentrations are close to the level where olivine would become h 2 o saturated i e 0 5 wt the presence of ferric fe in the mantle however may also play a role in the broadening of the 410 km discontinuity that is as yet unquantified

electrical conductivity anisotropy of dry and hydrous olivine at 8 gpa the effects of dissolved h2o on the electrical conductivity and its anisotropy in olivine fo90 at 8 gpa were investigated by complex impedance spectroscopy at nominally anhydrous conditions conduction along 1 0 0 and 0 0 1 is slightly higher than along 0 1 0 in contrast to observations made at lower pressures in earlier studies increasing h2o content increases conductivities but activation energies are lower and h2o concentration dependent the use of polarized ftir spectroscopy to determine h2o concentrations reveals a weaker than expected effect that water has on olivine conductivity and distinguishes our results from earlier studies based on analyses using non polarized infrared spectroscopy we show that at h2o concentrations of a few hundred wt ppm or less that the dominant conduction mechanism at mantle temperatures continues via small polarons such as that observed for anhydrous olivine our results also suggest that at depths greater than 200 km the presence of h2o may not be necessary to explain regions in the upper mantle where both electrical and seismic anisotropy are observed this can be explained by differences in the pressure dependence of the activation energy for conduction along each of the three crystallographic axes however while electrical anisotropy of anhydrous olivine remains weak at 8 gpa it is nevertheless enhanced by elevated concentrations several hundred wt ppm of dissolved h2o at these conditions dominated by proton hopping conductivity along 0 1 0 is highest approximately an order of magnitude greater than along 1 0 0 additionally at 1000 wt ppm and 1500 c an isotropic conductivity derived from the data is about 1 order of magnitude higher than that for nominally anhydrous olivine thus in regions of the mantle characterized by anomalously high conductivities and both electrical and seismic anisotropy significant amounts of dissolved hydrogen can be expected

evolution of grain sizes and orientations during phase transitions in hydrous mg2sio4 transformation microstructures in mantle minerals such as mg fe 2sio4 are critical for predicting the rheological properties of rocks and the interpretation of seismic observations we present in situ multi grain x ray diffraction experiments on hydrous mg2sio4 at the p t conditions relevant for deep cold subducting slabs up to 40 gpa and 850 c at a low experimental strain rate 4 10 6s 1 we monitor the orientations of hundreds of grains and grain size variations during the series of forsterite wadsleyite ringwoodite phase transformations microtextural results indicate that the and an intermediate phase grow incoherently relatively to the host phase consistent with a nucleation and growth model the and phase exhibit orientation relationships which are in agreement with previous ex situ observations the and intermediate show texturing due to moderate differential stress in the sample both the and transformation induce significant reductions of the mean sample grain size of up to 90 that starts prior to the appearance of the daughter phase apart from the in the newly formed and phases the nucleation rate is faster than the growth rate inhibiting the formation of large grains these results on grain orientations and grain size reductions in relation to transformation kinetics should allow refining existing slab strength models