evidence for the transition zone origin of some mg fe o inclusions in diamonds abstract inclusions of mg fe o fpc ferropericlase magnesiowustite were found in natural diamonds from several localities worldwide fpc is typically absent from upper mantle assemblages but expected to be common in the lower mantle in assemblages with mg fe sio3 perovskite this led to proposals that the inclusions originated in the earth s lower mantle reported here are the results from new experiments producing fpc bearing assemblages at 20 24 gpa and 1800 2000 c and the observations are explained with an analysis of the relevant phase relations these results suggest that some fpc inclusions could have originated at the base of the transition zone if the mantle temperatures were higher than 1800 c in this case a reaction of garnet with fpc could cause a seismic discontinuity near 660 km

the eclogite garnetite transformation at high pressure and some geophysical implications mineral assemblages displayed by morb and alkali poor olivine tholeiites have been investigated over the pressure interval 4 6 18 gpa at 1200 c both compositions crystallize to form normal eclogites between 4 6 and 10 gpa and there is little change in the relative proportions of garnet and pyroxene over this range however the proportion of garnet increases rapidly above 10 gpa as pyroxene dissolves in the garnet structure and pyroxene free garnetites stishovite are produced by 14 15 gpa dependent upon composition the garnetite facies for both compositions possess zero pressure densities of 3 75 g cm3 implying that subducted oceanic crust remains appreciably denser than surrounding mantle to depths exceeding 600 km it is demonstrated that the seismic velocity distributions in the mantle between 400 and 650 km are inconsistent with anderson s hypothesis that this region is of eclogitic composition

modelling the riverine δ7li variability throughout the amazon basin abstract the present study investigates the processes controlling the elementary and isotopic cycle of the lithium over the amazon basin a numerical model is developed to simulate two major processes that have been proposed as key controls of the river lithium isotopic composition weathering reactions inside the regolith accounting for secondary phase formation and interactions between riverine water and secondary phases in floodplain both processes generate fractionation of lithium isotopes batch fractionation and rayleigh distillation respectively that potentially control the riverine isotopic composition of the amazon and its tributaries a study of the model parameters shows that two different regimes are impacting the lithium isotopic composition of the rivers within the amazon catchment in the south madeira and its tributaries the lithium isotopic signature of river waters can be explained by lithium release and fractionation during weathering reactions in the regolith followed by rayleigh distillation in the floodplain increasing progressively the lithium isotopic composition in agreement with a previously published hypothesis in contrast the lithium isotopic composition of rivers located in the northern part of the amazon watershed solimoes and tributaries cannot be simulated by the model assuming the same processes than in the southern part model optimization suggests than the nature of the material being eroded and weathered is important in the north fresh source rocks of volcanic origin releases large amount of li and promotes rapid smectite precipitation allowing the riverine 7li to rise before flowing through floodplains this result suggests that the environments able to generate high riverine 7li higher than 25 are complex and not firmly identified yet

ultrahigh pressure phase relations in the system mg4si4o12mg3al2si3o12 high pressure phase relations in the system mg4si4o12 mg3al2si3o12 were investigated at pressures from 10 to 22 gpa and at a temperature of 1000 c the experimental results indicate that the maximum solubility of enstatite mg4si4o12 in pyrope mg3al2si3o12 is 80 mol enstatite at 15 gpa at pressures between 15 and 19 gpa the solubility is insensitive to pressure at 19 gpa garnet ilmenite transformation first occurs in enstatite composition at pressures above 19 gpa the compositional range of the ilmenite solid solution expands toward pyrope composition with increasing pressure based on the present experiments if pyrolite mantle is assumed the pyroxene garnet transformation appears at 350 400 km depth and garnet modified spinel and garnet spinel phase assemblages become stable successively up to 580 km depth at greater depths garnet decomposes to ilmenite garnet and hence the pyrolite mantle consists of ilmenite garnet spinel assemblage

femg interdiffusion in olivine up to 9 gpa at t 600 900 c experimental data and comparison with defect calculations abstract interdiffusion data at high pressures are needed to provide good estimates of the closure temperatures of geothermometers based on fe mg exchange in olivine in contact with other phases and to understand the pressure dependence of high temperature deformation in olivine we report here measurements of the interdiffusion coefficient of fe mg in san carlos olivine of mean composition mg mg fe 0 90 in the range of temperatures t 600 900 c and pressures p 0 5 9 gpa the measurements were performed by preparing olivine samples covered by a thin layer of fayalite these specimens were annealed in a uniaxial split sphere apparatus ussa 2000 the fe mg interdiffusion profiles were analysed by rutherford back scattering rbs the results yield a pre exponential factor d 0 7 7 10 8 cm 2 s 1 7 7 10 12 m 2 s 1 and an activation energy e 147 58 kjmol 1 or 62 58 2 sd kj mol 1 if a p o 2 1 6 correction is performed corresponding to the expression d d 0 exp e rt exp x fe exp pv rt our data are fitted best with 3 x fe fe fe mg is between zero and one in the present experiments x fe is around 0 1 p is the pressure and v is found to be 0 5 0 6 cm 3 mol 1 or 1 0 9 cm 3 mol 1 if a p o 2 1 6 correction is performed i e about zero in the p and t range investigated comparisons between the present results obtained at relatively low temperature under extrinsic conditions for diffusion with other fe mg interdiffusion data at high temperature 1125 c or more under intrinsic conditions allow us to deduce that the migration activation volume for point defects is v m 0 whereas that for formation is v f 5 5 cm 3 mol 1 this has important implications for the creep sensitivity to pressure numerical simulations involving energy minimizations and performed with the cascade and parapocs codes confirm these conclusions one finds v m 0 and v f 4 8 cm 3 mol 1 the activation energy e deduced from the experiments as well as the pre exponential factor d 0 now permit precise estimation of the closure temperatures of geothermometers based on the fe mg exchange between olivine and spinel

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

elastic wave velocities of a pyrope majorite garnet to 3 gpa abstract the pressure dependence of the elastic wave velocities for a polycrystalline specimen 100 theoretical density of majorite pyrope garnet py 62 mj 38 has been determined at room temperature to 3 gpa by ultrasonic phase comparison interferometry pressure derivatives of the adiabatic bulk modulus dk s dp 5 3 and shear modulus dgdp 2 0 are derived from the travel times of the compressional p and shear s waves these derivatives clearly demonstrate that pressure has a more pronounced effect on the elasticity of the majorite rich garnet than on the pure pyrope end member presumably in response to the coupled substitution of si and mg for al on the octahedral site in the garnet structure when combined with plausible values for the as yet unmeasured temperature derivatives these new data can be used to calculate velocity depth profiles along a 1400 c adiabat for the range of depths in which majorite is expected to be stable in the transition zone of the earth s mantle 450 600 km for shear waves these majorite gradients match those from regional seismic models much better than those for a pure olivine mantle more reliable calculations of elastic wave velocities at mantle temperatures and pressures await measurement of the temperature derivatives of elastic modulus

variationally induced breathing equations of state of pyrope grossular and majorite garnets we have completed the first non empirical fully optimized calculations of the equations of state for pyrope grossular and mgsio3 majorite garnets the static gibbs free energy of the 160 atom unit cell was minimized relative to all 27 structural parameters for the tetragonal structure zero point vibration and thermal effects were estimated with a debye like model and debye temperatures obtained from the equations of state calculated zero pressure densities differ from experimental values by less than 1 while the bulk moduli are systematically larger than those obtained spectroscopically nevertheless the calculations strongly support experimental evidence that the bulk modulus of mgsio3 majorite is close to that of pyrope and further indicate that its kt p o is not distinctly larger than that of other silicate garnets if these properties are close to those of real majorite the steep gradient of the bulk modulus in the transition zone is at best marginally consistent with a iherzolitic composition

enstatite jadeite join and its role in the earth s mantle phase relations on the enstatite jadeite join were experimentally determined at solidus temperatures and 90 152 kb and at 1400 2050 c 175 219 kb with a split sphere anvil apparatus ussa 2000 new findings include immiscibility in garnet and determination of the stability of naalsio4 calcium ferrite structure with stishovite a thermodynamic model for the enstatite jadeite join was developed to calculate a complete phase diagram for the join at 500 2500 c and 0 270 kb the results indicate that the two major discontinuities in the earth s mantle at 400 and 670 km depths could correspond respectively to the formation and the breakdown of garnet with a pyroxene composition a model for a chondritic upper mantle is proposed in which large scale chemical and mineral layering was produced by fractionation of liquidus phases in a magma ocean solidification was completed at 400 km depth by crystallization of sodium enriched residual melts which produced a pyroxene layer at 300 400 km depths

single crystal elasticity of iron bearing majorite to 26 gpa implications for seismic velocity structure of the mantle transition zone abstract single crystal elastic moduli of synthetic iron bearing majoritic garnet were measured using brillouin spectroscopy up to a pressure of 26 gpa we found that the iron bearing majoritic garnet mg 0 79 fe 0 08 al 0 30 si 0 84 o 3 remains elastically isotropic throughout the pressure range of this study the shear and adiabatic bulk moduli and their pressure derivatives at zero pressure were determined to be 0 88 7 7 gpa k s0 166 2 gpa p 0 1 28 8 k p 0 4 2 2 these values are almost indistinguishable within the experimental uncertainties from those obtained in previous studies of pure mgsio 3 majorite and other solid solutions of the majorite pyrope join our direct acoustic velocity measurements of iron bearing majorite indicate that the incorporation of iron into majoritic garnet does not drastically affect its elastic moduli and their pressure derivatives the measured pressure derivatives of iron bearing majorite are about 3 6 times lower than required to account for the high seismic velocity gradients observed in the mantle transition zone

garnet and pyroxenes in the mantle a test of the majorite fractionation hypothesis experimental data are reported for the compositions of garnets and pyroxenes that coexist with liquid in the system cao mgo al2o3 sio2 cmas at 80 160 kbar all experiments were run using the uniaxial split sphere apparatus at stony brook our data have permitted a refinement of gasparik s 1990a thermodynamic analysis of garnets and pyroxenes that occur at arid below the solidus our results should provide precise estimates of the temperature pressure conditions at which peridotite and garnet xenoliths equilibrated in the mantle an examination is also made of the composition of garnet that crystallizes as a liquidus phase it is demonstrated to be a function of both composition and pressure pressure has the effect of changing the composition of garnet from pyropic at around 80 kilobars to pyroxene like majorite at pressures in excess of 140 kbar changes that mirror those seen for garnets in the subsolidus failure to consider the effects of pressure on the composition of liquidus garnet can result in an erroneous test of the hypothesis that garnet fractionation occurred in early earth history it is shown that primitive mantle peridotite could have formed as a liquid by 35 majorite garnet fractionation from a cv chondrite like allende at 200 260 kbar these results support the hypothesis that extensive or possibly total melting occurred during the growth of the earth by accretion of planetesimals e g davies 1985 abe and matsui 1985 stevenson 1987 wetherill 1990 and that majorite fractionation could have occurred during this time ohtani and sawamoto 1987 removal of majorite to the lower mantle during the crystallization of a magma ocean is a successful way of explaining differences in mgo sio2 and cao al2o3 between mantle peridotite and chondrite

melting and subsolidus relations of sio2 at 9 14 gpa experiments were performed on amorphous silica at temperatures of 1000 2850 c and pressures of 9 14 gpa using a 2000 t uniaxial split sphere multianvil apparatus ussa 2000 the melting curve of coesite was extended from 8 to 13 7 gpa and the melting temperatures indicate a slope that is almost independent of pressure melting of stishovite was observed at 2850 c and 14 gpa the triple point for coexisting coesite stishovite and melt was located at 2800 c and 13 7 gpa the coesite stishovite phase transition was determined between 2000 and 2800 c and is described by the equation p gpa 5 8 4 0 0028 2 t c the resulting dp dt slope is more than twice that determined in the in situ x ray study of yagi and akimoto 1976 the present coesite stishovite boundary cannot be predicted by existing thermodynamic data e g fei et al 1990 which suggests some uncertainties in the extrapolation of heat capacities of coesite and stishovite at high temperatures thermodynamic extrapolations of the metastable melting curves of quartz and coesite show a strongly negative slope and thus are consistent with the proposal that the room temperature amorphization of quartz and coesite results from crossing of the metastable extension of their melting curves

oriented inclusions of pyroxene amphibole and rutile in garnet from the luliangshan garnet peridotite massif north qaidam uhpm belt nw china an electron backscatter diffraction study

chemical inhomogeneity of the mantle geochemical considerations geochemical considerations on the overall abundances for major chemical elements of the earth and ci chondrites indicate that the lower mantle is probably more enriched in silica than the upper mantle it is concluded that about 95 of the lower mantle is composed of material having the perovskite modification and that the 650 km seismic discontinuity represents a chemical boundary at which the lower mantle is about 30 to 40 richer in silicon than the upper mantle these results support those reached earlier on the basis of geophysical constraints thus about half of the earth either by weight or volume is composed of ferromagnesium silicate with the perovskite structure

melting of enstatite mgsio3 from 10 to 16 5 gpa and the forsterite mg2sio4 majorite mgsio3 eutectic at 16 5 gpa implications for the origin of the mantle a split sphere multianvil apparatus clissa 2000 has been used to determine the melting curve of enstatite mgsio3 from 10 to 16 5 gpa and the forsterile mgio majoritc mgsio3 euteetic at 16 5 gpa use of a new capsule material rhenium allowed long runs of up to 2 hours at temperatures up to 2380oc with no observed contamination of the sample and with stable temperature control melting temperatures of enstatite are up to 160oc higher than those of kato and kumazawa 1986a two invariant points have been located one for orthoenstatite linoenstatite liquid at 2230oc 11 9 gpa and one for clinoenstatite majoritc liquid at 2370oc 16 4 gpa forsteritc melts incongruently at 2350oc 16 5 gpa to liquid and anhb 0vlglnsiso24 in the system mg2sio4 mgsio3 at 16 5 gpa the forsterite majorite eutectic has been located at 34 wt mg2sio4 a composition similar to that determined by karo and kumazawa 1986a at 20 gpa but much lower in mg2sio4 than several previous estimates based on melting experiments on mantle peridotitc compositions this forstefite poor location for the cuteerie is in apparent conflict with the suggestion that the peridotitic upper mantle is a crystallized cuteeric like liquid formed at about 16 gpa

density of morb eclogite in the upper mantle in order to study the role of eclogite subducted oceanic crust in geodynamics we carried out high pressure experiments on natural mid oceanic ridge basalt morb using a multi anvil apparatus we determined phase relations of morb as a function of pressure and temperature in the temperature range of 1200 1700 c and pressures from 5 to 14 gpa the present results differ from a previous high pressure study of morb earth planet sci lett 77 1986 245 conducted at 1200 c as follows 1 coesite coe or stishovite st appears throughout the studied pressure range and 2 the clinopyroxene garnet cpx gar ratio is smaller particularly in the low pressure range 12 gpa our results show that the calculated bulk density of subducted oceanic crust increases discontinuously at around 9 gpa due to the coesite stishovite phase transition the density is considerably higher than estimates based on the previous experiments at pressures less than 12 gpa thus the role of eclogite in driving subduction should be more important than previously thought on the other hand the density difference between peridotite and eclogite in ascending mantle plumes becomes smaller as temperature increases a result of the increase in clinopyroxene garnet sio2 ratio in eclogite with increasing temperature as eclogite is still denser than peridotite even under subsolidus conditions recycled oceanic crust entrained in ascending plumes must be sustained by the upward flow of ambient peridotite in mantle plumes

hydro isostasy and the sea level isobase of 5500 b p in north queensland australia abstract previous studies of holocene sea level in north queensland have been based on a variety of sea level related deposits with interpretations of mid holocene levels ranging from near zero up to 4 m above present we report data from 8 carefully controlled new sites located from the palm group northwards and extending into the gulf of carpentaria field evidence is from coral reef flats and prograded coastal deposits the highest sea level on inner shelf islands and mainland coast occurred 5500 yrs ago the results of this and of certain previous studies are reduced to a common reference surface or isobase which varies systematically in elevation across the region the isobase correlates strongly with predicted hydro isostatic deformation caused by postglacial sea level rise

compositional change of majoritic garnet in a morb composition from 7 to 17 gpa and 1400 to 1600 c experimental phase relations of an average normal mid ocean ridge basalt n morb composition from 7 gpa to 17 gpa and 1400 to 1600 c reveal the compositional variation of garnet in equilibrium with pyroxene the na2o content of majoritic garnet in equilibrium with pyroxene and stishovite increases up to 2 5 wt with increasing pressure and temperature na enters the garnet structure via the substitution mg fe ca al na si ti the pyroxene garnet partition coefficient dcp ga for na decreases with increasing pressure however clinopyroxene coexisting with garnet becomes more na rich as pressure increases these relationships are independent of the feo content in garnet our experiments confirm that rare majoritic and na rich garnet inclusions in diamonds have formed at transition zone pressure in lithologies of basaltic bulk composition

pyroxene exsolution topotaxy in majoritic garnet from 250 to 300 km depth we report here the detailed microstructure and chemistry of pyroxene exsolution from a polycrystalline garnet porphyroblast of the western gneiss region wgr garnet peridotite otroy norway for both clinopyroxene cpx and orthopyroxene opx the same basic crystallographic relationship is found with the host garnet 100 py 112 grt 010 px 110 grt and 001 px 111 grt for the majority 90 of its intracrystalline pyroxene rods in addition this pattern is also exhibited by some interstitial opx and a subpopulation of both pyroxenes shows a different pattern or no discernible pattern the results provide quantitative microstructural evidence demonstrating an exsolution precipitation origin of both the intracrystalline cpx and opx and the small interstitial opx crystals the reconstructed precursor majoritic garnet taking into account both the intracrystalline pyroxenes and interstitial opx was characterized by si 3 07 cation per formula unit that corresponds to a minimum pressure of 7 7 gpa 250 km depth we also deduce from the observation of opx being the majority of intracrystalline precipitates and 100 of the interstitial ones that the precursor majoritic garnet probably originated from a pressure less than 10 gpa 300 km depth a multistage decomposition hypothesis is proposed for this wgr majoritic garnet during exhumation of the peridotite from 250 to 300 km depth to explain the topotaxy and chemistry of the exsolved pyroxenes

a model for the layered upper mantle abstract numerical modeling of mantle convection by liu 1994 science 264 1904 1907 favors a two layer convection if the results are reinterpreted for the correct phase relations in mg fe 2sio4 the resulting chemical isolation of the upper and lower mantle suggests a highly differentiated and layered upper mantle to account for the discrepancy between the observed compositions of mantle xenoliths and the cosmic abundances of elements it is shown that a layered upper mantle with a hidden reservoir can have a structure consistent with the observed seismic velocity profiles and an average bulk composition corresponding to the cosmic abundances the evolution of the upper mantle and the origin of komatiites are discussed in the context of the proposed model

divalent cation diffusion in mg2sio4 spinel ringwoodite β phase wadsleyite and olivine implications for the electrical conductivity of the mantle mg fe and mg ni interdiffusion coefficients have been measured in single crystal olivine alpha mg ni fe sub 2 sio sub 4 polycrystalline beta phase beta mg ni fe sub 2 sio sub 4 and silicate spinel gamma mg ni fe sub 2 sio sub 4 at 1473 k between 1 and 14 gpa under controlled thermodynamic conditions in olivine d sub mg ni sup alpha ranges from 10 sup 17 m sup 2 s sup 1 at 1 gpa to 4x10 sup 18 m sup 2 s sup 1 at 9 gpa and d sub mg fe sup alpha ranges from 3x10 sup 15 m sup 2 s sup 1 at 1 gpa to 10 sup 15 m sup 2 s sup 1 at 4 gpa at 9 gpa the cation diffusion rates in gamma spinel are approx 3 orders of magnitude higher than those in olivine between 9 and 14 gpa d sub mg ni sup gamma decreases from 8x10 sup 15 to 9x10 sup 16 m sup 2 s sup 1 and d sub mg fe sup gamma decreases from 2x10 sup 13 to 5x10 sup 14 m sup 2 s sup 1 between 10 and 14 gpa the diffusion coefficients for both fe and ni are similar in magnitude and pressure dependence between beta phase and gamma spinel the activation volumes for mg ni diffusion are 3 2 and 6 7 cm3 mol 1 in the alpha and gamma beta phases respectively and for mg fe diffusion are 5 4more and 6 1 cm3 mol 1 in the alpha and gamma beta phases respectively our results demonstrate that extreme differences in transport properties are expected to occur across the 400 km discontinuity a model for electrical conductivity in the upper mantle involving two mechanisms for electrical conduction with one mechanism being associated with a hydrogen or defect related conduction process and the other involving a straightforward application of the nernst einstein equation to our cationic diffusion data reproduces geomagnetic sounding results for the upper mantle and shallow transition zone this modeling yields an order of magnitude increase in electrical conductivity across the 400 km discontinuity with little or no change in conductivity occurring near 520 km depth our modeling suggests that divalent cation diffusion is the dominant mechanism of charge transport between 400 and 670 km depth and that the high pressure olivine polymorphs dominate the electrical behavior of the deep upper mantle comparison between field based deep mantle conductivity profiles derived from magnetotelluric studies and extrapolation of our model suggests that a discontinuity in electrical transport properties exists at the base of the transition zone as well similarly given the dramatic differences in chemical diffusion between different polymorphs of olivine it is possible that other transport property mediated processes such as viscous flow could dramatically change across the 400 and 670 km discontinuities such changes are less likely to occur at 520 km depth c 2000 american geophysical union less

elastic moduli of jadeite enstatite majorite 1 the adiabatic bulk ks and shear g moduli of a polycrystalline majorite garnet solid solution with composition na0 92mg2 08 mg0 02al1 06si0 92 si3o12 were measured at ambient conditions by brillouin spectroscopy ks and g were found to be 171 6 3 5 gpa and 103 5 2 0 gpa respectively assuming a linear dependence of the elastic moduli on composition for the mg2si2o6 enstatite majorite en mj naalsi2o6 jadeite majorite jd mj join the calculated ks and g of the fictive jd mj end member are 178 and 125 gpa respectively at ambient conditions the shear velocity change associated with the pyroxene to majorite transition increases dramatically with increasing na content in the system in contrast there is virtually no shear velocity increase associated with the pyroxene to majorite transition in mgsio3 if this relationship holds at high pressures and temperatures it suggests that for mineral assemblages containing an abundance of large radius cations such as ca and na the pyroxene garnet reaction will produce higher shear velocity gradients than assemblages that are relatively deficient in these elements such chemical effects could play a significant role in explaining high seismic velocity gradients in the transition zone

stability of the assemblage albite plus forsterite at high temperatures and pressures with petrologic implications the maximum limits of the assemblage albiteforsterite have been determined experimentally at high pressures and temperatures at subsolidus temperatures albite plus forsterite is replaced at high pressures by jadeitic clinopyroxene and enstatitic orthopyroxene the boundary for this reaction lies within experimental uncertainity of that for jadeite albite nepheline melting of albite forsterite at high pressures produces enstatite liquid which is different from the low pressure eutectic behavior melting rates are very slow and several hundred hours are required to establish equilibrium near the solidus the subsolidus boundary for albite plus forsterite lies near that for sanidine plus forsterite but with a shallower slope which more closely matches that of anorthite plus forsterite both albite plus forsterite and anorthite plus forsterite are replaced at high pressures by an assemblage containing clinopyroxene plus orthopyroxene unlike sanidine plus forsterite which is replaced by a feldspathoid plus orthopyroxene the presence of sodium enlarges the depth region over which plagioclase lherzolite can stably exist it may also stabilize alkali feldspar plus olivine in crustal rocks

partitioning of rare earth elements between casio3 perovskite and coexisting phases constraints on the formation of casio3 inclusions in diamonds minerals with casio3 composition were found as inclusions in diamonds and are considered to be originally of perovskite structure to constrain their genesis and consequently the extent of circulation of mantle material rare earth element ree partitioning between casio3 perovskite and coexisting majoritic garnet 20 gpa 1520 c or mgsio3 perovskite 25 gpa 1600 c was determined by combining the technologies of high pressure synthesis and trace element analysis using ion probe it is consistent with previous experiments and confirms that casio3 perovskite is the main ree depository especially of the light ree kce capv gt is 1900 and decreases gradually to 18 5 of kyb capv gt for capv mgpv it decreases gradually from 57 7 of kce to about 10 of kyb estimated ree concentrations in the source lithology of the casio3 inclusions according to these partitioning coefficients either peridotitic or eclogitic paragenesis show strong enrichment in light ree e g cen 18 163 very different from normal mantle peridotite and subducted oceanic crust it is proposed that interaction with carbonatic melt in the deep mantle may have played an important role in the formation of these casio3 inclusions in diamonds as well as in their ascending transportation

phase relations and formation of sodium rich majoritic garnet in the system mg 3 al 2 si 3 o 12 na 2 mgsi 5 o 12 at 7 0 and 8 5 gpa the pseudo binary system mg3al2si3o12 na2mgsi5o12 modelling the sodium bearing garnet solid solutions has been studied at 7 and 8 5 gpa and 1 500 1 950 c the na bearing garnet is a liquidus phase of the system up to 60 mol na2mgsi5o12 nagrt at higher content of nagrt in the system enstatite up to 80 mol and then coesite are observed as liquidus phases our experiments provided evidence for a stable sodium incorporation in garnet 0 3 0 6 wt na2o and its control by temperature and pressure the highest sodium contents were obtained in experiments at p 8 5 gpa near the liquidus t 1 840 c the equilibrium concentration of na2o in garnet is 0 7 0 8 wt 6 mol na2mgsi5o12 with the temperature decrease na concentration in grt increases and the maximal na2mgsi5o12 content of 12 mol 1 52 wt na2o is gained at the solidus of the system t 1 760 the data obtained show that most of natural diamonds with inclusions of na bearing garnets usually containing 1 wt na2o may crystallize at a pressure range of 7 0 8 5 gpa however the upper pressure limit for the formation of naturally occurring na bearing garnets is restricted by the eclogite garnetite bulk composition

phase relations on the diopside jadeite hedenbergite join up to 24 gpa and stability of na bearing majoritic garnet abstract phase relations on the diopside di hedenbergite hd jadeite jd system modeling mineral associations of natural eclogites were studied for the compositions mol di70jd30 di50jd50 di30jd70 di20hd80 and di40hd10jd50 using a toroidal anvil with hole 7 gpa and a kawai type 6 8 multianvil apparatus 12 24 gpa we established that di hd and jd form complete series of solid solutions at 7 gpa and melting temperatures of pure di 1980 c and jd 1870 c for that pressure were estimated experimentally the melting temperature for the di50jd50 composition at 15 5 gpa is 2270 c the appearance of garnet is clearly dependent on initial clinopyroxene composition at 1600 c the first garnet crystals are observed at 13 5 gpa in the jadeite rich part of the system di30jd70 whereas diopside rich starting material di70jd30 produces garnet only above 17 gpa the proportion of garnet increases rapidly above 18 gpa as pyroxene dissolves in the garnet structure and pyroxene free garnetites are produced from diopside rich starting materials in all experiments garnet coexists with stishovite st at a pressure above 18 gpa pyroxene is completely replaced by an assemblage of majorite maj st casio3 perovskite ca pv in ca rich systems whereas maj is associated with almost pure jd up to a pressure of 21 5 gpa above 22 gpa maj and st are associated with naalsio4 with calcium ferrite structure cf we established that an hd component also spreads the range of pyroxene stability up to 20 gpa in the di70jd30 system at 24 gpa an assemblage of maj ca pv mgsio3 with ilmenite structure mg il was obtained the experimentally established correlation between na si and al contents in maj and pressure in grt maj pyroxene assemblages may be the basis for a majorite geobarometer the results of our experiments are applicable to the upper mantle and the transition zone of the earth 400 670 km and demonstrate a wide range of transformations from eclogite to perovskite bearing garnetite in addition the mineral associations obtained from the experiments allowed us to simulate parageneses of inclusions in diamonds formed under the conditions of the transition zone and the lower mantle

dehydration of glaucophane in the system na2o mgo al2o3 sio2 h2o and the effects of nacl co2 and silicate bearing aqueous fluids

study of the mggeo 3 polymorphs orthopyroxene clinopyroxene and ilmenite structures by calorimetry spectroscopy and phase equilibria

stability of clinopyroxene at pressure temperature conditions of the transition region abstract the stability of diopside at conditions of 16 19 gpa and 1250 1900 c was determined using a 6 8 type multi anvil apparatus at pressures of 17 18 gpa diopside undergoes a decomposition reaction to an assemblage of casio3 perovskite ca mg sio3 garnet or ilmenite depending on temperature the clapeyron slope for the breakdown reaction download download full size image is 2 mpa k 1 when coupled with a re examination of clinopyroxene stability in other more complex peridotite systems the experimental data are consistent with a recent proposal that the 520 km discontinuity in the transition region is caused by the exsolution of casio3 rich perovskite from garnet rich in majorite component

acid compositions in a veined lower mantle as indicated by inclusions of k na hollandite sio2 in diamonds abstract diamonds from juina area brazil are known by their superdeep origin related to recycling of oceanic crust basalts and pelagic sediments in the lower mantle chemical and structural data on mineral inclusions support this hypothesis we have studied the inclusions of five diamonds from juininha alluvial deposits part of the juina province and we have found for the first time in terrestrial rocks a paragenetic assemblage only observed in high pressure experiments and or extraterrestrial rocks the results suggest the presence of a veined type lower mantle where an assemblage of hydrous basaltic origin mgfeal perovskite is crosscut by pockets and or veins of intermediate to acid compositions dominantly constituted by sio 2 k na hollandite the silicate paragenesis kna hollandite and sio 2 can be produced by melting of a pelagic metasedimentary source subducted together with the oceanic slab down to the uppermost lower mantle such a veined type lower mantle puts in evidence a derived source modified by melting dehydration of multicomponent from a subducted slab

mineralogy of the martian interior up to core mantle boundary pressures in order to determine the mineralogy of the martian interior along a high temperature areotherm multianvil experiments have been performed with a model martian mantle composition up to 23 5 gpa the dreibus and wanke 19851 martian mantle composition yields an upper mantle that consists of olivine clinopyroxene orthopyroxene garnet at pressures up to 9 gpa above 9 gpa orthopyroxene is no longer present the transition zone is marked by the appearance of spinel at 13 5 gpa up to 15 gpa clinopyroxene and majorite coexists with phase and or spinel by 17 gpa clinopyroxene is entirely replaced by majorite and the modal abundance of spinel increases at the expense of phase the dominant assemblage throughout most of the transition zone is spinel majorite two experiments completed in the perovskite stability field indicate that the lower mantle consists of mg fe silicate perovskite magnesiowustite and majorite casio 3 perovskite is not present in these experiments both the presence of a martian lower mantle i e an mg fe silicate perovskite bearing zone and the phase assemblage stable in the martian lower mantle are very sensitive to the temperature profile of the interior a low temperature profile may stabilize stishovite in the lower mantle or it may lead to the absence of the lower mantle because of the higher transition pressure required for forming perovskite at lower temperatures regardless of the temperature profile assumed the martian upper mantle and transition zone will account for a larger proportion of the planet s interior than is the case for the earth s interior because of the smaller size of mars

na bearing majoritic garnet in the na2mgsi5o12 mg3al2si3o12 join at 11 20 gpa phase relations structural peculiarities and solid solutions abstract na majorite na 2 mgsi 5 o 12 namaj an end member of sodium rich majoritic garnet was synthesized and garnet pyroxene transition was studied in kawai type multi anvil experiments at 11 20 gpa and 1500 2300 c na majorite was obtained at 16 gpa and 1500 c its stability spreads to the high temperature region with pressure 1900 c at 17 gpa and 2100 c at 19 5 gpa experimental study of the mg 3 al 2 si 3 o 12 prp na 2 mgsi 5 o 12 join at 11 20 gpa demonstrated the following phase assemblages changing with pressure garnet two pyroxenes enstatite rich and na pyroxene garnet pyroxene enstatite jadeite na pyroxene stishovite garnet stishovite garnet naalsio 4 with calcium ferrite type structure stishovite for a given bulk composition prp 50 namaj 50 a regular increase of both na and si concentrations and consequently namaj mole portion in garnets was observed from 8 5 to 20 gpa the concentration of na 2 o increased from 1 52 to 5 71 wt which correspond to a namaj content of 40 mol the highest na 2 o content 12 24 wt was registered at 18 5 gpa and 1600 c for the prp 20 namaj 80 starting composition experiments at 18 5 gpa and 1600 c on the pyrope na majorite join allowed us to study mixing peculiarities for sodium rich majoritic garnet the transition from cubic ia 3 d to tetragonal i 4 1 acd symmetry was observed for the composition with 80 mol na 2 mgsi 5 o 12 which is consistent with the similar change of the structure in the pyrope majorite mg 4 si 4 o 12 system significant na majorite solubility in pyrope as well as findings of natural garnets with high na concentrations 1 wt na 2 o allow us to consider na bearing majoritic garnet as an important concentrator of sodium in the deep upper mantle and transition zone

transformation of enstatite diopside jadeite pyroxenes to garnet the high pressure stability of enstatite en diopside di jadeite jd pyroxenes has been investigated experimentally with a split sphere anvil apparatus ussa 2000 on the enstatite pyrope join the compositions of garnet coexisting with enstatite were determined at 100 165 kbar and 1450 1850 c the results indicate complete solubility between enstatite and pyrope in the system cao mgo al2o3 sio2 cmas the compositions of coexisting pyroxenes and garnet were determined at 100 165 kbar and 1250 1750 c at 157 kbar 1650 c garnet with the composition en79di21 mol forms on the en di join in the system na2o mgo al2o3 sio2 nmas the compositions of coexisting pyroxenes and garnet were determined at 60 160 kbar and 1200 1850 c on the en jd join the first garnet has the composition en48jd52 at 135 kbar 1650 c and en53jd47 at 140 kbar 1500 c on the di jd join the first garnet with the composition di63jd37 forms around 170 kbar 1650 c in the en di jd system the first appearance of garnet with the composition en42di9jd49 is estimated at 133 kbar 1650 c the new pyroxene with the composition namg0 5si2 5o6 napx transforms to garnet at 154 kbar 1650 c the experimental results indicate that the transformation of a twopyroxene assemblage to garnet and residual pyroxene in the en di jd system could occur at pressures consistent with the 400 km seismic discontinuity and in a pressure interval of 0 3 kbar

phase transformations in diopside camgsi2o6 at pressures up to 25 gpa phase transformations in diopside have been studied at pressures up to 25 gpa and at temperatures of 1100 and 1500 c by a multiple anvil apparatus the following phase assemblages are found to be stable at given pressures diopside 15 and 18 gpa mg2sio4 spinel stishovite casio3 perovskite 19 and 20 gpa mgsio3 ilmenite casio3 perovskite 22 gpa and mgsio3 perovskite casio3 perovskite 24 5 gpa electron microprobe analysis reveals that the solubility of the mgsio3 molecule in casio3 cubic perovskite is limited to between 2 5 the solubility of the casio3 molecule in mgsio3 orthorhombic perovskite is more restricted being of the order of 2 or less these results are contradictory to earlier work in which diopside is reported to transform into a cubic perovskite structure of the same composition above 22 23 gpa

shock conditions recorded in nwa 8159 martian augite basalt with implications for the impact cratering history on mars nwa 8159 is an augite rich martian basalt formed by cooling of a relatively evolved ca rich ti poor and lree depleted lava under relatively oxidizing conditions during the early amazonian in addition to its distinct igneous petrogenesis and high fo 2 nwa 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase an 50 65 in this study mineral transformations within and adjacent to shock veins in nwa 8159 were investigated using scanning electron microscopy raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite the transformation of olivine to ahrensite fe ringwoodite along shock vein margins and tissintite and coesiteformed from igneous mineral labradorite and silica grains entrained as clasts within shock veins has been documented in this study we report on a previously unidentified mineral assemblage of ca na majoritic garnet sodic clinopyroxene and stishovite crystallized from shock melt this mineral assemblage indicates a crystallization pressure of approximately 16 gpa which is within the range of previous shock pressure estimates for this meteorite 15 23 gpa the presence of a majoritic garnet bearing assemblage throughout veins up to 0 6 mm wide indicates that the sample remained at high pressure throughout the melt vein quench based on thermal models the sample must have remained at high pressure for 100 ms this shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as tissint or zagami 30 gpa 10 20 ms and would seem to imply a relatively large impact event recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact the shock conditions experienced by nwa 8159 therefore do not require an impact event distinct from other shergottites rather our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site the shock history of nwa 8159 provides a picture of mars consistent with that derived from remote observation that of a random cratering process that samples a geologically long lived and complex planet

an experimental study of the garnet perovskite transformation in the system mgsio3mg3al2si3o12 phase transformations in the system mgsio3 mg3al2si3o12 have been studied at pressures between 22 and 28 gpa at 1500 c using a multi anvil apparatus majorite garnet with less than about 10 wt al2o3 was found to completely transform into an orthorhombic perovskite structure over a pressure interval between 24 and 27 gpa whereas pyrope garnet disproportionated into sub aluminous perovskite plus corundum solid solutions at pressures greater than 26 5 gpa alumina contents in the perovskite structure are dependent on temperature and increase from 11 wt at 1500 c to 17 wt at 1800 c at a pressure of 27 gpa the alumina content of perovskite also in creases with increasing pressure such that aluminous perovskite with the pyrope composition 25 mol al2o3 should be formed by about 30 40 gpa these observations are consistent with our recent experimental results on a pyrolite composition which demonstrated that no separate aluminous phase exists in the upper part of the lower mantle the entire inventory of al2o3 in pyrolite can be accommodated in solid solution in magnesium silicate perovskite

the eclogite garnetite transformation in the morb h2o system abstract to decipher phase relations of oceanic crust in the coldest slab at the mantle transition zone multi anvil experiments were conducted in the morb h 2 o system at pressures of 10 19 gpa and temperatures of 700 1500 c garnet and stishovite were recognized in all run charges above 15 gpa garnet drastically increases nasi na 2 msi 5 o 12 component m ca mg fe 2 jadeite occurs instead of omphacite na k hollandite containing 7 mol naalsi 3 o 8 and ca perovskite with 60 mol catio 3 were observed at p 17 gpa with decomposition of omphacite and increase of modal ratio of garnet there is a sharp increase of density at 440 km the density increase due to appearance of ca perovskite at 570 km is estimated approximately 100 km shallower than that of previous estimation

phase relations in the transition zone phase relations in the systems mgo al2o3 sio2 and cao mgo sio2 have been investigated experimentally at 166 226 kbar pressures and 1350 2500 c temperatures with a split sphere anvil apparatus ussa 2000 mgsio3 ilmenite is stable to lower pressures 175 kbar than found previously casio3 perovskite forms at p bar 15t c 151 000 three new high pressure phases are reported a phase with the diopsidic composition the cm phase a high pressure polymorph of al2o3 and superphase b with the tentative composition mg10si3o14 oh f 4 the invariant point for coexisting majorite mg4si4o12 perovskite mgsio3 beta phase mg2sio4 and liquid has been located at 224 kbar 2430 c with the eutectic melt composition on the enstatite forsterite join at 40 44 wt of forsterite the melting temperature of mgsio3 perovskite at 225 kbar is predicted to be 2600 c the new data made possible the calculation of a temperature pressure phase diagram for the system cao mgo al2o3 sio2 cmas in the pressure range 0 280 kbar the mineralogy of the earth s mantle and the possible mechanisms for producing large seismic velocity gradients in the transition zone are discussed on the basis of the derived phase relations

slow si al interdiffusion in garnet and stagnation of subducting slabs abstract si 4 m 2 2al 3 m mg fe ca interdiffusion rates in majoritic garnet have been determined at pressure of 17 gpa and temperatures of 1550 1700 c using a kawai type high pressure apparatus the diffusion profiles were measured across the interface between pyropic garnet and majoritic garnet samples by an analytical transmission electron microscope and the rate of the si 4 m 2 2al 3 interdiffusion was determined to be a d si al m 2 s 1 2 10 10 exp 364 kj mol 1 rt we found that the diffusion rate is low compared to the si diffusion rate in other silicate minerals relevant to the deep mantle slow kinetics of the pyroxene garnet transformation due to the low diffusion rate indicates that large low density metastable regions may exist in the slab greatly contributing to the slab stagnation around the transition region stagnant slabs possibly stay in this region over 10 8 yr because of the sluggish pyroxene garnet transformation but may eventually fall into the lower mantle after the completion of the transformation

crystal chemistry of ca bearing majorite single crystals of ca bearing majorite a garnet with composition cao 49mg2sl mgsi sipi2 have been synthesized at 18 2 gpa and 2050 0c this sample is the first silicate garnet to display ordering on both octahedral and dodecahedral sites behavior that may increase the compositional flexibility of garnet affect element partitioning at high pressure and stabilize the garnet structure in the transition zone and upper portion of the lower mantle the garnet is tetragonal space group 14 a z 8 a 11 5816 9 c 11 5288 13 a v 1546 39 29 a3 and like mgsio majorite displays twinning by twofold rotation about 110

the stability and composition of phengitic muscovite and associated phases from 5 5 to 11 gpa implications for deeply subducted sediments abstract the stability and composition of phengitic muscovite was investigated from 5 5 11 gpa 700 1150 c in synthesis experiments performed in a multianvil apparatus starting materials consisted of natural minerals with a bulk composition similar to that of a k deficient intermediate dioctahedral trioctahedral mica phengitic muscovite was found to be stable from 5 5 11 gpa at 900 c phengite melting occurs between 1075 1150 c at 7 8 gpa and 1000 1050 c at 10 gpa at 10 11 gpa 800 c octahedral cation deficient ocd muscovite and k hollandite are observed rather than phengite the average phengite content of muscovite is positively correlated with pressure ranging from 3 65 si pfu at 5 5 gpa to 3 81 si pfu at 11 gpa the maximum phengite content of stable muscovite appears to be 3 80 3 85 si pfu most phengite examined exhibits at least minor solid solution towards phlogopite averaging 2 04 0 06 2 octahedral cations pfu the hydrous phases phengite lawsonite topaz oh and mg pumpellyite occur between 6 8 gpa 700 900 c with increasing pressure lawsonite and mg pumpellyite dehydrate to form garnet between 8 9 gpa with increasing temperature mg pumpellyite lawsonite and topaz oh devolatilize to form garnet and kyanite between 900 1000 c at 7 8 gpa phengitic muscovite and topaz oh would be stable in hydrous sediments in cool mature subduction zones to depths exceeding 360 km while lawsonite and mg pumpellyite would be stable to 240 300 km allowing the transport of h2o contained in these phases deep into the upper mantle in warmer subduction zones the dehydration of mg pumpellyite lawsonite and topaz oh as well as the melting of phengite would result in fluid release at depths of 180 240 km the presence of phengite at far greater depths than the zone of melt generation beneath arcs 100 150 km requires a mechanism such as the partitioning of k be b ba and rb into migrating fluids rather than the simple dehydration of phengite beneath arcs in order to provide for the transfer of these slab signature elements from phengite into arc magmas

ultradeep ultramafic mantle xenoliths transmission electron microscopy preliminary results abstract the defect microstructure of ultradeep garnets from the jagersfontein kimberlite pipe south africa was investigated by transmission electron microscopy these crystals exhibit a high density of dislocations most of which form well organized and regularly spaced subgrain boundaries free dislocations are also detected most of them have a burgers vector 1 2 111 shortest lattice repeat but a few 100 burgers vectors are also characterized especially in junctions small needle shaped pyroxene exsolutions are also detected they exsolved from the si rich garnets during assent to the earth s surface their nucleation and growth required bulk diffusion and this process may have affected the original dislocation microstructure it is believed however that the observed dislocation pattern still reflects the original deformation regime of garnets at very high pressure and temperature and this dislocation pattern strongly suggests that garnets were highly ductile finally a number of precipitates of a dense amorphous phase of pure carbon are also detected they might result from the amorphization of small diamonds exsolved from the garnet as the pressure was progressively released

investigation of the martensitic like transformation from mg2geo4 olivine to its spinel structure polymorph a coherent mechanism for the transformation of olivine to its spinel structure polymorph has been investigated in mg 2 geo 4 olivine at conditions ranging from 6 gpa to 18 gpa at 300 to 1250 c using a multi anvil apparatus this transformation mechanism which produces plates of spinel lying parallel to the 100 planes in olivine becomes increasingly important above 10 gpa at temperatures between 300 and 400 c the primary effect of temperature above 400 c is to increase the length of the lamellae the primary effect of pressure is to increase the number of lamellae that have nucleated the distribution of transformation is very inhomogeneous lamellae first appear in patches associated with grain edges and corners as the degree of transformation increases patches become more extensive and more heavily populated until the majority if not all grains contain large numbers of lamellae that span large portions of their host grains the mechanism that produces the lamellae requires three factors high pressure overstep of the equilibrium boundary some level of differential stress either microscopic or macroscopic and high enough temperature to facilitate rearrangement of the cations it is most consistent with a martensitic like transformation mechanism the results of this study suggest that olivine may exhibit similar behavior in cold rapidly subducting lithospheric slabs if conditions are too cold for olivine to transform by reconstructive nucleation and growth it could instead progressively transform by the martensitic like mechanism as conditions deviate further and further from the equilibrium phase boundary

excess silica in omphacite and the formation of free silica in eclogite silica lamellae in eclogitic clinopyroxene are widely interpreted as evidence of exsolution during decompression of eclogite however mechanisms other than exsolution might produce free silica and the possible mechanisms depend in part on the nature and definition of excess silica excess silica may occur in both stoichiometric and non stoichiometric pyroxene although the issue has been debated we show that all common definitions of excess silica in non stoichiometric clinopyroxene are internally consistent interchangeable and therefore equivalent the excess silica content of pyroxene is easily illustrated in a three component condensed composition space and may be plotted directly from a structural formula unit or recalculated end members in order to evaluate possible mechanisms for the formation of free silica in eclogite we examined the net transfer reactions in model eclogites using a thompson reaction space we show that there are at least three broad classes of reactions that release free silica in eclogite i vacancy consumption in non stoichiometric pyroxene ii dissolution of ti phases in pyroxene or garnet iii reactions between accessory phases and either pyroxene or garnet we suggest that reliable interpretation of the significance of silica lamellae in natural clinopyroxene will require the evaluation not only of silica solubility but also of titanium solubility and the possible roles of accessory phases and inclusions on the balance of free silica

ultradeep greater than 300 kilometers ultramafic upper mantle xenoliths geophysical discontinuities in earth s upper mantle and experimental data predict the structural transformation of pyroxene to garnet and the solid state dissolution of pyroxene into garnet with increasing depth these predictions are indirectly verified by omphacitic pyroxene exsolution in pyropic garnet bearing xenoliths from a diamondiferous kimberlite conditions for silicon in octahedral sites in the original garnets are met at pressures greater than 130 kilobars placing the origin of these xenoliths at depths of 300 to 400 kilometers these ultradeep xenoliths support the theory that the 400 km seismic discontinuity is marked by a transition from peridotite to eclogite

origin of mantle peridotite and komatiite by partial melting theoretical calculations and some preliminary experimental observations show that the liquidus and solidus temperatures for each of a wide range of upper mantle peridotite and komatiite compositions become coherent at high pressures this suggests that these materials may have been liquids in eutectic like equilibrium rather than peritectic like with mantle assemblages it is shown that the major element geochemistry of 83 mantle peridotites and 61 komatiites define a trend which is not primarily due to an olivine control process rather it is interpreted to represent the pressure induced compositional trace of eutectic liquids in equilibrium with mantle assemblages from 4 to 15 gpa if the bulk earth is chondritic in composition these phase equilibrium constraints would imply that the upper mantle had formed from the whole mantle as an ultrabasic partial melt the transition zone and lower mantle being the complementary eclogite and pyroxenite residua this is consistent with mineralogical interpretations of seismic data for the present day earth which call for a peridotite upper mantle and pyroxene like transition zone and lower mantle

analytical electron microscopy of a synthetic peridotite experimentally deformed in the β olivine stability field a synthetic harzburgite prepared by isostatic hot pressing of finely ground natural olivine admixed with 3 enstatite was deformed at high temperature and high pressure such that the olivine phase boundary was traversed both residual and new phases their grain sizes their deformation induced lattice defects and their compositions were then characterized by analytical electron microscopy owing to large heterogeneities in temperature pressure and deformation conditions contrasting microstructures were observed some areas consist essentially of untransformed olivine while the polymorph is the main constituent of other areas crystals of the polymorph are generally free of dislocations except at the grain boundaries of the larger grains where slip bands were nucleated probably induced by stress concentrations the activated glide systems are 021 100 the orthopyroxene grains have been transformed to very fine grained clinopyroxene and have reacted with chromite to form garnet with a majoritic component compared to olivine crystals of the phase contain significantly higher concentrations of fe and trivalent cations all crystals are free of inclusions confirming the conclusion from previous infrared spectroscopic analysis that the high oh concentration in this specimen is dissolved in the phase coupled with these previously measured oh concentrations our new measurements suggest that incorporation of hydrogen into the phase and substitution of trivalent cations into the octahedral sites may be coupled such increased complexity of mineral chemistry points to a need for detailed investigation of the spectrum of compositions stable in the structure and coexisting phases in order to better estimate the modal mineralogy of the transition zone

computation of seismic profiles from mineral physics the importance of the non olivine components for explaining the 660 km depth discontinuity the recent increasing number of experimental works leads us to review the elastic properties and mineralogical transformations of mantle minerals the updated data set is used to compute seismic profiles for two petrological models along three adiabatic temperature profiles these profiles are chosen to stress out the influence of non olivine minerals on seismic parameters and to represent cold and horizontally averaged temperature profiles of the earth s mantle in a first part starting compositions of pyrolite and piclogite and a single layer convection are assumed the results clearly point out the importance of the non olivine part of the mineralogy two scenarios are found to explain the 660 km depth discontinuity

melting of forsterite mg2sio4 from 9 7 to 16 5 gpa a multianvil apparatus has been used to determine the pressure temperature melting curve of forsterite from 9 7 to 16 5 gpa at 10 1 gpa a singular point occurs that marks the change from congruent melting at lower pressures to incongruent melting forsterite periclase liquid at higher pressures the melting curve also passes through two invariant points at one 15 6 gpa 2310 c the phases forsterite periclase anhydrous b and liquid coexist and the melting reaction changes from forsterite periclase liquid at lower pressures to forsterite anhydrous b liquid at higher pressures at the other 16 7 gpa 2315 c the phases forsterite modified spinel anhydrous b and liquid coexist and the melting reaction changes from forsterite anhydrous b liquid at lower pressures to modified spinel anhydrous b liquid at higher pressures the simon equation p gpa 2 44 t k 2171 11 4 1 fits both our melting curve data and the lower pressure data of davis and england 1964 at low pressures the melting curve of forsterite lies at higher temperatures than that of enstatite but the two curves cross at 13 3 gpa because of the lower dt dp slope of the forsterite melting curve this causes the forsterite enstatite eutectic to shift toward forsterite as pressure increases but our data are consistent with earlier findings that the shift is not sufficient to support an origin for the mantle by eutectic like melting at high pressures

melting experiments on the enstatite pyrope join at 80 152 kbar melting temperatures of the assemblage pyroxene garnet on the enstatite pyrope join were experimentally determined at 11 different pressures between 80 and 152 kbar with a split sphere anvil apparatus ussa 2000 the compositions of pyroxene garnet and melt coexisting along the solidus were also determined comparison with previous data at lower temperatures revealed a large temperature dependence of the compositions of garnet coexisting with pyroxene it is proposed that the observed variation in the garnet composition is caused by disorder in garnet the obtained alumina contents of orthopyroxene coexisting with garnet on the solidus at 80 110 kbar contributed to an improvement of the garnet peridotite thermobarometry an internally consistent set of thermodynamic parameters was derived which allows calculation of the temperature pressure phase diagram for the enstatite pyrope join at 20 260 kbar

thermodynamic properties and phase relations in mantle minerals investigated by first principles quasiharmonic theory this research has been motivated by geophysics and materials physics the objective of this research has been to advance materials theory and computations for high pressure and high temperature applications to the point that it can make a difference in our understanding of the earth understanding of the mineralogy composition and thermal structure of the earth evolves by close interaction of three fields seismology geodynamics and mineral physics earth s structure is imaged by seismology which obtains body wave velocities and density throughout the earth s interior fig 1 interpretation of this data relies on knowledge of aggregate properties of earth forming materials either measured in laboratory or calculated in many cases by both however the conditions of the earth s interior especially at the core may be challenging for important experiments and materials computations have emerged to contribute decisively to this field earth s evolution is simulated by geodynamics but these simulations need as input information about rheological and thermodynamic properties of minerals including phase transformation properties such as clapeyron slopes our research has concentrated on the phases of the earth s mantle particularly the deep mantle whose conditions are more challenging for experiments the mantle accounts for 83 of the earth s volume in this article we will review the essential first principles approach used to investigate solids at high temperatures and pressures summarize their performance for mantle minerals fig 2 and point to critical results that have stirred us in the current research path the success is remarkable but some limitations point the way to future developments in this field figure 1 a velocities and density profiles kennett et al 1995 b earth s cross section and c composition and mineralogy of earth s layers figure 2 phases proportions versus depth and or pressure ol mg fe 2sio4 olivine also known as phase mg fe

experimental evidence for the origin of two kinds of inclusions in diamonds from the deep mantle abstract the conditions of origin for the type iii inclusions in diamonds from brazil hutchison thesis 1997 and the napx en inclusion from china wang and sueno miner j 18 1996 9 16 were experimentally determined using a split sphere anvil apparatus ussa 2000 the type iii inclusions formed at a minimum pressure and temperature of 25 gpa and 2000 c while the origin of the napx en inclusion was close to 23 gpa and 1900 c both determinations suggest that the temperature at the corresponding depths is about 300 c higher than predicted by most geotherms for a convecting mantle without a thermal boundary layer at 660 km both kinds of inclusions required rapid single stage transport by carbonate melt to the earth s surface which is consistent with the depths of origin for this melt greater than 660 km the unusual composition of the napx en inclusion is the result of metasomatism by carbonate melt enriched in na k and mg and depleted in si and al since this melt is not kimberlitic in composition exchange of material between the melt and the mantle was necessary for the melt to become kimberlitic by the time it reached the surface the resulting metasomatism taking place over a long period of time could cause major changes in the mineral and chemical composition and the structure of the earth s mantle and thus play an important role in its evolution

evidence for immiscibility in majorite garnet from experiments at 13 15 gpa abstract the compositions of coexisting aluminous garnet clinoenstatite and either jadeitic clinopyroxene or a second silica rich garnet were experimentally determined in the system na2o cao feo mgo al2o3 sio2 at 1450 1650 c and 13 15 gpa using a split sphere anvil apparatus ussa 2000 the pyroxene content of the aluminous garnet increases from 50 at 13 gpa to 100 at 15 gpa for bulk compositions with a high na ca ratio the trends of the observed variation in the compositions of both garnet and clinopyroxene exhibit a major change with increasing pressure at 14 gpa this is consistent with the stabilization of a second silica rich garnet which replaces jadeitic clinopyroxene in the assemblage with aluminous garnet and clinoenstatite the transformation from clinopyroxene to silica rich garnet is almost isochemical and occurs near the composition na0 8ca0 3 fe mg 2 25al0 5si4 15o12 due to the observed immiscibility in the garnet solution the transformation from pyroxene to garnet could produce a sharp seismic discontinuity in the earth s mantle at 400 km depth