deformation of forsterite polycrystals at mantle pressure comparison with fe bearing olivine and the effect of iron on its plasticity abstract the rheology of polycrystalline forsterite was investigated in the deformation dia d dia using in situ x ray diffraction at pressure between 3 1 and 8 1 gpa temperature in the 1373 1673 k range and at steady state strain rate ranging from 0 5 10 5 to 5 5 10 5 s 1 microscopic observations of the run products show characteristic microstructures of the so called dislocation creep regime in wet conditions based on the present data at 1473 k the pressure effect on forsterite plasticity is quantified using an activation volume v fo 12 1 3 0 cm3 mol 1 a comparison between the strain rates of san carlos olivine and forsterite specimens deformed together indicates that at the experimental conditions they compare with each other within less than half an order of magnitude this comparison also allows for the determination of the stress exponent of forsterite of nfo 2 3 0 6 our results combined with data from the literature indicate a clear trend of increasing stress exponent with fe content in olivine

stability metastability and elastic properties of a dense silica polymorph seifertite 1 dense silica polymorphs with sixfold coordinated si have been found in snc and lunar meteorites and may be important minerals for silica rich components in the lower mantle however the stable crystal structure in the lower mantle and properties of dense silica remain controversial under stable heating and quasi hydrostatic stress conditions we found that the cacl2 type undergoes a phase transition to the pbo2type seifertite at 130 140 gpa and 2500 k our data suggest that this phase transition occurs at a greater depth than the perovskite postperovskite transition in the lowermost mantle the molar volume measured at 1 bar is the smallest among the reported silica polymorphs therefore having the highest calculated density and in excellent agreement with recent first principles calculations the greater molar volume of seifertite found in the shergottite meteorite and previous experiments supports a metastable synthesis of the phase outside its stability field our data combined with the hugoniots of silica polymorphs also rule out the possibility of the formation of seifertite in the meteorite within its stability field we found very little change in bulk sound speed across the cacl2 type seifertite transition if shear wave velocity decreases at the transition to seifertite as suggested by some computational studies this silica transition may provide an alternative explanation for the discontinuities with a shear wave velocity decrease found at depths greater than the d discontinuity

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

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

low strength of earth s uppermost mantle inferred from tri axial deformation experiments on dry olivine crystals abstract we have performed tri axial compression experiments on single crystals of san carlos olivine with various orientations at temperatures relevant for the uppermost mantle between 800 and 1090 c the experiments were carried out at a confining pressure of 300 mpa in a high resolution gas medium mechanical testing apparatus at various constant strain rates from 7 10 6 s 1 to 1 10 4 s 1 they yield differential stresses ranging from 88 to 754 mpa unpolarized infrared spectroscopy analyses indicate that hydrogen concentration in the olivine lattice is very low e 1 0 2 10 6 exp 450 60 10 3 rt 1 15 4 1 2 2 where t is the absolute temperature r is the gas constant and is the differential stress in gpa subsequently small fractions of melt or incorporation of hydrogen in olivine may not be required to reconcile natural observations indicating a limited strength of earth s uppermost mantle lithosphere

diffusion creep of dry melt free olivine 1 deformation experiments were conducted on fine grained 3 6 m fully synthetic fo90 olivine aggregates in a gas medium apparatus at 300 mpa confining pressure and temperatures of 1150 1360 c the strain rates of the solution gelation derived and therefore genuinely melt free dry samples are about two orders of magnitude lower than the strain rates for nominally melt free aggregates at the same pressure and temperature conditions and grain size benchmark deformation tests with anita bay dunite and mild steel reproduce published data the creep strength of melt added sol gel olivine is similar to the published creep strength of dry melt bearing olivine derived from natural rocks nonlinear least squares fits to the melt free deformation data give an activation energy of 484 kj mol a stress exponent of 1 4 and a grain size exponent of 3 over a range of stresses from 15 to 210 mpa these results suggest that small amounts of melt may be similarly effective in reducing the creep strength of upper mantle rocks as small amounts of water however a possible contribution of grain boundary composition to the observed differences in rheology in the absence of melt cannot be conclusively ruled out by the current experiments

transmission electron microscopy of coesite inclusions in the dora maira high pressure metamorphic pyrope quartzite abstract inclusions composed of relict coesite and retrograde quartz in garnet of a pyrope quartzite from the dora maira massif were examined in transmission electron microscopy although coesite has suffered very little plastic deformation the burgers vectors of dislocations could be identified they are 100 001 110 i e a c and a b and other symmetrically almost identical vectors the 110 plane could be identified as a slip plane the dislocations are very straight pointing to a high peierls stress some small prismatic dislocation loops with burgers vector 010 are observed possibly formed by water related defects retrograde quartz occurs at the margin of coesite with a palisade texture and as veins within coesite palisade quartz exhibits a high density of dislocations pinned on bubbles and brazil twins parallel to 10 1 1 planes the quartz veins are aligned parallel to the 100 and 021 composition planes of twins in coesite crystals these observations strongly suggest that back transformation of coesite into quartz starts at grain and twin boundaries and that coesite loses water to quartz during transformation

hydrolytic weakening of olivine at mantle pressure evidence of 100 010 slip system softening from single crystal deformation experiments abstract to date most of the experiments performed to quantify the hydrolytic weakening of olivine single crystals have been performed at low confining pressure and low water fugacities in order to determine if the degree of hydrolytic weakening changes at higher water fugacities higher water contents we have performed a series of high pressure p and high temperature t deformation experiments in hydrous condition on san carlos olivine fo92 and forsterite fo100 single crystals in a deformation dia apparatus d dia deformation was carried out in axial compression along the 1 1 0 c crystallographic reference frame direction in order to activate 1 0 0 0 1 0 dislocation slip system alone at p ranging from 4 to 7 gpa and t 1200 c water was supplied during the experiments through dehydration of a talc sleeve talc enstatite coesite h 2 o around the sample while the differential stress and strain rate of the specimen were calculated from in situ x ray diffraction and time resolved imaging respectively run products were investigated using fourier transform infrared ftir spectroscopy and optical electron microscopy sem and tem structurally bound hydroxyl content c oh derived from absorbance in ftir spectra based on the paterson 1982 relation were typically between 290 and 720 ppm h si in the run products and the majority of these hydroxyls were observed as distinct peaks in the range of wavenumber from 3620 to 3500 cm 1 tem reveals that the 1 0 0 0 1 0 slip system was effectively activated and responsible for crystal deformation and that dislocation glide was significantly assisted by recovering processes such as dislocation climb and dipole annihilation combining the experimental data from this study and mackwell et al 1985 we estimate the activation volume v 110 c for san carlos olivine in order to produce a reasonable water fugacity exponent r in the flow law and obtain v 110 c 12 1 cm 3 mol for r 0 and v 110 c 17 3 cm 3 mol for r 1 2 comparisons of the strengths of the wet single crystals to the strengths of dry single crystals calculated from the rheological laws bai et al 1991 darot and gueguen 1981 raterron et al 2009 indicate that the wet single crystal cylinders were between 1 3 and 1 6 times weaker than their dry counterpart at same conditions which is consistent with previous studies of 1 0 0 0 1 0 olivine rheology performed at low confining pressure 300 mpa

circulation of modified circumpolar deep water and basal melt beneath the amery ice shelf east antarctica antarctic ice sheet mass loss has been linked to an increase in oceanic heat supply which enhances basal melt and thinning of ice shelves here we detail the interaction of modified circumpolar deep water mcdw with the amery ice shelf the largest ice shelf in east antarctica and provide the first estimates of basal melting due to mcdw we use subice shelf ocean observations from a borehole site am02 situated 70 km inshore of the ice shelf front together with open ocean observations in prydz bay we find that mcdw transport into the cavity is about 0 22 0 06 sv 1 sv 106 m3 s 1 the inflow of mcdw drives a net basal melt rate of up to 2 0 5 m yr 1 during 2001 23 9 6 52 gt yr 1 from under about 12 800 km2 of the north eastern flank of the ice shelf the heat content flux by mcdw at am02 shows high intra annual variability up to 40 our results suggest two main modes of subice shelf circulation and basal melt regimes 1 the ice pump high salinity shelf water circulation on the western flank and 2 the mcdw meltwater driven circulation in conjunction with the ice pump on the eastern flank these results highlight the sensitivity of the amery s basal melting to changes in mcdw inflow improved understanding of such ice shelf ocean interaction is crucial to refining projections of mass loss and associated sea level rise

relative strength of the pyrope majorite solid solution and the flow law of majorite containing garnets even though the garnet phase is the second most abundant phase in the upper mantle and transition zone no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions here we report the results of constant strain rate and stress relaxation experiments on garnets in the pyrope majorite solid solution which constrain the strength of majoritic containing garnets relative to pyrope as a function of majorite content and temperature we find that at temperatures below 650 degrees c both pure pyrope and majoritic garnets have the same strength conversely above 650 degrees c we find that majoritic garnets are initially stronger than pure pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than pyrope above approximately 800 degrees c we develop a flow law for the entire pyrope majorite solid solution as a function of temperature and majorite content c 2010 elsevier b v all rights reserved

phase transformations of subducted basaltic crust in the upmost lower mantle phase relations in natural mid oceanic ridge basalt morb were investigated to depths of 700 1400 km in the lower mantle using a laser heated diamond anvil cell and synchrotron x ray diffraction xrd the crystalline phases their unit cell volumes and proportions were determined from rietveld refinement of the in situ xrd spectra at the top of the lower mantle mg rich perovskite is the most abundant phase followed by ca rich perovskite and stishovite two al rich phases occur in smaller amounts the ca ferrite type and the new aluminum rich phase or nal this assemblage is stable down to 1150 km approximately at greater depths the new aluminum rich phase disappears and the assemblage of mg rich perovskite ca rich perovskite stishovite and ca ferrite phase is stable up to at least 1400 km in view of the present phase diagram we discuss the nature and relations of the lower mantle al phases in morb composition and their bearing on the density evolution of the oceanic crust

olivine flow mechanisms at 8 gpa abstract the mechanisms responsible for high temperature olivine deformation are investigated at a pressure of 8 gpa and temperatures up to 1780 k san carlos olivine specimens of different average grain sizes 0 5 and 5 m were deformed simultaneously between hard alumina pistons during relaxation experiments these experiments are carried out in a multi anvil high pressure apparatus coupled with synchrotron x ray radiation the different grain size specimens experienced identical p t stress condition at any given time a new method for measuring strains and strain rates 10 6 s 1 of specimens at high pressure is documented this method uses time resolved in situ x ray imaging and an image analysis computation the microstructures of run products recovered after being quenched at different temperatures were characterized by transmission electron microscopy tem we find that high temperature olivine flow is grain size insensitive at 8 gpa which suggests that dislocation creep dominates olivine deformation at high pressure this result is confirmed by the tem investigation of our deformed specimens in which we find evidences of the activation of olivine dislocation slip systems specimen microstructures are consistent with dynamic recrystallization as an assisting process in olivine deformation during the high pressure experiments extrapolation of our results to the low stress level and large grain size expected in the mantle suggests that dislocation creep assisted by dynamic recrystallization may also dominate natural olivine deformation in the upper mantle

on the increase in thermal diffusivity caused by the perovskite to post perovskite phase transition and its implications for mantle dynamics the thermal diffusivity of perovskite and post perovskite cairo3 has been measured at elevated pressure and temperatures up to 600 c using the x radiographic angstrom method at high temperatures we find that the thermal diffusivity of post perovskite is slightly below twice that of isochemical perovskite over the temperature range investigated assuming a similar effect occurs in mgsio3 post perovskite the effect of the contrasting thermal transport properties between perovskite and post perovskite on mantle dynamics has been investigated using simple two dimensional convection models these show a reduction in extent and increase in depth of post perovskite lenses as well as increased core mantle boundary heat flux broader upwellings and more vigorous downwellings when compared to the reference constant a case

self diffusion of oxygen and silicon in mgsio3 perovskite abstract we present results of simultaneous measurements of self diffusion of oxygen and silicon in magnesium silicate perovskite oxygen diffusion is 2 orders of magnitude faster than si self diffusion and fe mg interdiffusion in perovskite with d0 9 16 10 4 0 16 9 2 10 6 m2 s and q 501 80 kj mol at 25 gpa despite this high o diffusivity however the electrical conductivity of the lower mantle will be dominated by electronic conduction and the present day core is likely to be out of redox equilibrium with the mantle

weakening of calcium iridate during its transformation from perovskite to post perovskite magnesium silicate perovskite the dominant mineral in the lower mantle is thought to transform into a post perovskite phase in the mantle s lowermost region laboratory experiments suggest substantial weakening could occur during the transformation from perovskite to post perovskite which could explain the anomalous physical properties of the lowermost mantle

grain boundary sliding in san carlos olivine flow law parameters and crystallographic preferred orientation 1 we performed triaxial compressive creep experiments on aggregates of san carlos olivine to develop a flow law and to examine microstructural development in the dislocation accommodated grain boundary sliding regime gbs each experiment included load and temperature steps to determine both the stress exponent and the activation energy grain boundary maps created with electron backscatter diffraction data were used to quantify grain size distributions for each sample inversion of the resulting data produced the following flow law for gbs gbs 104 8 0 8 2 9 0 3 d0 7 0 1 exp 445 20 kj mol 1 rt with d and gbs in units of mpa m and s 1 respectively although relatively weak crystallographic preferred orientations cpos have 010 maxima parallel to the compression direction along with 100 and 001 girdles perpendicular to the compression direction cpos and subgrain boundary misorientation axes suggest that the 010 100 slip system contributes significantly to deformation we propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations extrapolation of our flow laws to mantle conditions suggests that gbs is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep

upper carboniferous fine grained turbiditic sandstones from southwest england a model for growth in an ancient delta fed subsea fan abstract the upper carboniferous crackington and bude formations of southwest england comprise laminated black mudstones gray siltstones thin medium and thick bedded sandstones and contorted silty sandstones the thick bedded sandstones only occur in the bude formation which overlies the crackington the sandstones were deposited predominantly by turbidity currents in a relatively shallow basin they are unusual for two reasons 1 sediment coarser than fine sand is entirely absent 2 unlike many previously described ancient subsea fan deposits organized bundles of sandstones displaying asymmetric upward thickening or upward thinning sequences are no more common than symmetric sequences and both types of organized bundles are much less common than disorganized nonsequential bundle particularly in the bude formation sediment was supplied directly from a delta to the north by resedimentation of delta front sands and slumping of upper pro delta muds and silts to form a lower pro delta turbidite fan fan growth is not readily explained by current models for single sourced tripartite subsea fans displaying radial growth it developed instead a bipartite growth pattern which was strongly influenced in its upper part by external variables including grain size distributary channel switching and other deltaic processes active tectonism within the basin and possibly sea level fluctuations the fan grew by down slope construction of independent levee channel complexes each of which headed in the upper pro delta and even possibly the delta front slumping and mobilization of argillaceous facies resulted in the random occurrence of contorted beds in all parts of the fan regionally extensive mudstones known as key shales within the fan deposits probably reflect large scal upslope delta lobe abandonment the crati and ebro fans of the mediterranean sea would appear to be good modern analogs for the crackington bude fan system

an experimental study into the rheology of synthetic polycrystalline coesite aggregates coesite has been found as a relic in ultrahigh pressure metamorphic uhpm crust worldwide and is expected to play a major role in the mechanical behavior of continental crust at uhpm conditions we performed triaxial compression tests on synthetic polycrystalline coesitite in a solid medium apparatus at confining pressures of 3 1 to 3 7 gpa temperatures of 700 to 1160 c and strain rates betweeen 6 10 7 and 1 10 3 s 1 the problem of the limited stress resolution of the solid medium apparatus was addressed by applying two extreme friction corrections that yield lower and upper bounds to the differential stress the correlation between the mechanical data and the microstructural record of the deformed samples as a function of temperature and imposed strain rate is consistent with deformation by dislocation creep we deduced parameters of a power law as n 3 1 and q 275 50 kj mol 1 extrapolation of the experimental data to natural conditions cannot be constrained by comparison with natural microstructures due to the lack of preserved coesite other than as single crystal inclusions nevertheless the extrapolation indicates a low strength of order 10 mpa for natural strain rates at typical uhpm conditions absent deformation of the uhpm brossasco granite dora maira massif western alps thus implies low stresses deformation must have been localized in very weak shear zones during burial and exhumation

experimental intracrystalline plastic flow in hot pressed synthetic quartzite prepared from brazilian quartz crystals samples of synthetic ultrafine grained quartzite were prepared by hot pressing aggregates of crushed clear brazilian quartz of mean grain size 0 4 m at 300 mpa and 1373 and 1473 k the samples displayed rapid grain growth to ca 12 20 m with 2 porosity at 1473 k facilitated by the 0 6 wt of water adsorbed onto the grain boundaries during sample preparation this water could be driven off by preheating thereby preventing grain growth sufficient water was incorporated during growth into the coarsened samples to render them weak and ductile these were deformed experimentally in the quartz field using an argon gas medium apparatus at 300 mpa confining pressure at temperatures mainly between t 1273 and 1473 k ductile flow was described by the flow law 10 4 9 3 2 9 7 f h 2 o exp 242000 rt with stress a in mpa and grain size d in microns and strain rate in s 1 r is the gas constant and f h 2 o is water fugacity based on observation of grain flattening optical strain features shapes and densities of dislocation arrays and flow law parameters samples deformed dominantly by dislocation creep with some contribution from grain boundary sliding bearing in mind possible changes in the flow law parameters over the extrapolation interval and possible effects of the phase transition extrapolation to natural strain rates and temperatures predicts plastic flow at higher flow stresses at the same water fugacity at greenschist facies temperatures than previously published flow laws

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

experimental constraints on the strength of the lithospheric mantle 1 to provide a better understanding of rheological properties of mantle rocks under lithospheric conditions we carried out a series of experiments on the creep behavior of polycrystalline olivine at high pressures 4 9 gpa relatively low temperatures 673 t 1273 k and anhydrous conditions using a deformation dia differential stress and sample displacement were monitored in situ using synchrotron x ray diffraction and radiography respectively experimental results were fit to the low temperature plasticity flow law on the basis of this analysis the low temperature plasticity of olivine deformed under anhydrous conditions is well constrained by our data with a peierls stress of p 5 9 0 2 gpa a zero stress activation energy of ek 0 320 50 kj mol 1 and ap 1 4 10 7 s 1 mpa 2 compared with published results for high temperature creep of olivine a transition from low temperature plasticity to high temperature creep occurs at 1300 k for a strain rate of 10 5 s 1 for a geological strain rate of 10 14 s 1 extrapolation of our low temperature flow law to 873 k the cutoff temperature for earthquakes in the mantle yields a strength of 600 mpa the low temperature high stress flow law for olivine in this study provides a solid basis for modeling tectonic processes occurring within earth s lithosphere

rheology of fine grained forsterite aggregate at deep upper mantle conditions high pressure and high temperature deformation experiments on fine grained synthetic dunite forsterite aggregate were conducted to determine the dominant deformation mechanism in the deep upper mantle the sintered starting material has 90 forsterite 10 enstatite and an average grain size of 1 m deformation experiments were performed using a deformation dia apparatus at pressures of 3 03 5 36 gpa temperatures of 1473 1573 k and uniaxial strain rates of 0 91 10 5 to 18 6 10 5 s 1 at dry circumstances 50 h 106si the steady state flow stress was determined at each deformation condition derived stress strain rate data is analyzed together with that reported from similar but low pressure deformation experiments using flow law equations for diffusion creep stress exponent of n 1 grain size exponent of p 2 and for dislocation accommodated grain boundary sliding gbs disl n 3 p 1 the activation volume for diffusion creep v dif and for gbs disl v gbs of dunite is determined to be 8 2 0 9 and 7 5 1 0 cm3 mol respectively calculations based on these results suggest that both diffusion creep and dislocation creep play an important role for material flow at typical deformation conditions in the earth s asthenospheric upper mantle whereas the contribution of gbs disl is very limited and dislocation creep is the dominant deformation mechanism during the deformation of olivine in sheared peridotite xenolith though these conclusions are not definitive these are the first results on potential deformation mechanisms of forsterite aggregate based on extrapolation in the pressure temperature stress and grain size space