Olivine Stability Research Articles

Shear wave anisotropy beneath the Andes from the BANJO, SEDA, and PISCO experiments

We present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western South America: the Broadband Andean Joint experiment (BANJO), a 1000‐km‐long east‐west line at 20°S, and the Projecto de Investigacion Sismologica de la Cordillera Occidental (PISCO) and Seismic Exploration of the Deep Altiplano (SEDA), deployed several hunderd kilometers north and south of this line. We determined the splitting parameters ϕ (fast polarization direction) and δt (splitting delay time) for waves that sample the above‐ and below‐slab regions: teleseismic *KS and S, ScS waves from local deep‐focus events, as well as S waves from intermediate‐focus events that sample only the above‐slab region. All but one of the *KS stacks for the BANJO stations show E‐W fast directions with δt varying between 0.4 and 1.5 s. However, for *KS recorded at most of the SEDA and PISCO stations, and for local deep‐focus S events north and south of BANJO, there is a rotation of ϕ to a more nearly trench parallel direction. The splitting parameters for above‐slab paths, determined from events around 200 km deep to western stations, yield small delay times (≤0.3 s) and N‐S fast polarization directions. Assuming the anisotropy is limited to the top 400 km of the mantle (olivine stability field), these data suggest the following spatial distribution of anisotropy. For the above‐slab component, as one goes from east (where *KS reflects the above‐slab component) to west, ϕ changes from E‐W to N‐S, and delay times are substantially reduced. This change may mark the transition from the Brazilian craton to actively deforming (E‐W shortening) Andean mantle. We see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion. The small delay times for above‐slab paths in the west require the existence of significant, spatially varying below‐slab anisotropy to explain the *KS results. The implied anisotropic pattern below the slab is not easily explained by a simple model of slab‐entrained shear flow beneath the plate. Instead, flow induced by the retrograde motion of the slab, in combination with local structural variations, may provide a better explanation.

Kinetics of diffusion-controlled evaporation of Fe-Mg olivine: experimental study and implication for stability of Fe-rich olivine in the solar nebula

Evaporation is a process that caused chemical fractionation in the solar nebula. The evaporation rates of meteorite-forming minerals and silicate melts are key parameters for constraining the timescale of high temperature processes in the early solar nebula and for understanding the mechanisms of cosmochemical fractionation. The kinetics of evaporation of olivine (initial Fo = 92), the most common silicate in meteorites, was experimentally investigated at 1414°C for various duration under continuous evacuation. The surface of olivine becomes more magnesian and the interior becomes zoned as evaporation proceeds, indicating preferential evaporation of the fayalite (Fa) component. The weight loss, surface composition, and interior zoning profile were analyzed using a semi-infinite one-dimensional diffusion model with boundary migration, and the controlling model parameters were optimized by fitting the experimental results. The evaporation rate of olivine shows notable composition and orientation dependence. As olivine becomes more magnesian, the evaporation rates of the (100), (010), and (001) surfaces decrease, among which the rate at the (001) surface is greatest (1.0 μm/h at Fo = 91.8), which is about 2 orders of magnitude faster than that of pure synthetic forsterite. The estimated Fe-Mg fractionation factor shows anisotropy ranging from 0.017 to 0.035, which is significantly greater than the equilibrium value of the Fe-Mg distribution coefficient between olivine and gas, suggesting kinetic suppression of evaporation of the Fa component. The Fe-Mg interdiffusion coefficient at 1414°C, which also shows notable composition and orientation dependence, is estimated to be 8.7 × 10 −15 m 2/sec along the c-axis. The results were applied to constrain the timescale of heating for a spherical Fo50 olivine grain to retain the Fa component. Olivine dust ∼1 μm in diameter can retain 80% of the initial Fa component for one minute in vacuum, and for ∼30 min at ∼10 Pa P H2 when heated at 1414°C. Therefore, Fo50 olivine in chondrite matrices could have survived an incipient heating event depending on the ambient hydrogen pressure, if they were originally present in the solar nebula.

The electrical conductivity of olivine under highly reducing conditions

The electrical conductivity of San Carlos olivine has been measured at 1100 °C under reducing conditions at controlled oxygen fugacity, inside and outside the olivine stability field, in order to study the kinetics of olivine destabilization. Electrical conductivity increases along the direction [010] and decreases along [001]. as oxygen fugacity decreases. To understand these \(\) dependences, electrical conductivity transitory regimes were studied. In response to decreases in oxygen fugacity, two transient regimes with different time scales have been observed. A fast (≈1–2 min) increase of electrical conductivity is first observed, followed by a slower decrease (1–10 h, depending on the crystal orientation). After a few hours of annealing, precipitation of metallic iron and nickel and formation of amorphous silica can be observed at the crystal surface. The fast conductivity increase in the first transient regime is ascribed to an increase in the population of electrons at the olivine surface. Two effects: (1) equilibration of surface defects with the bulk of the crystal, and (2) iron loss from the olivine due to metal precipitation, could explain the subsequent decrease of electrical conductivity. Anisotropic diffusion of surface defects to the bulk of the crystal, by a process faster than atomic diffusion is the most likely.

Evidence for an enriched mantle source for jøtunite (orthopyroxene monzodiorite) associated with the St. Urbain anorthosite, Quebec

Mafic orthopyroxene monzodiorite (jøtunite) lithologies are exposed in the St. Urbain plutonic suite as a marginal facies to quartz mangerite and massif anorthosite intrusive bodies and as dikes within a variety of host rocks. High concentrations of Ti, Fe, P, K, Ba, Nb, La, Ce, Zn, Ga, Zr and Y characterize these rocks and are distinctive of many mafic lithologies associated with anorthosite massifs worldwide. Characteristically low concentrations of Ni and Cr, in conjuction with low Mg numbers, have been used by previous investigators as evidence for either partial melting of mafic granulitic lower crust or extensive fractional crystallization of a mantle-derived magma. In an attempt to distinguish between these competing models, we note that jøtunite display many features that bear a strong resemblance to continental tholeiitic flood basalts, including chemical signatures on normalized multi-element (‘spider’) diagrams. Ratios of incompatible trace elements and patterns on rare earth and ‘spider’ diagrams collectively indicate that the jøtunite rocks were derived from an enriched, rather than depleted, mantle source. Enrichment may have occured by subduction-derived fluids or by mixture with a plume component prior to partial melting so that isotopic and trace-element compositions are decoupled. Small amounts of partial melting of mafic granulite has been advanced as an alternative model; we show, however, that the experimental data on which this model is built are not applicable. Our preferred model begins with partial melting of a trace-element enriched mantle source that fractionates olivine at high to moderate pressures. Increasing concentrations of P (and Ti) eventually caused a contraction of the olivine stability field in favor of orthopyroxene. Fractional crystallization may yield the series of rocks from anorthosite, leuconorite, oxide-apatite gabbronorite, to jøtunite. Mafic magmas emplaced into continental crust are typically attributed to incipient rifting or mantle upwelling, which are features common to many models for the genesis of anorthosite and related rocks.

Alpe Arami: a peridotite massif from the Mantle Transition Zone?

Petrologic discoveries made over the last ten years have shown that in regions of continent-continent collision, rocks of the continental crust can be subducted to much greater depths than previously considered reasonable. Documentation of such extreme subduction has come from discovery of diamond, coesite and other high pressure phases; in many cases these phases, metastable at the Earth's surface, are only preserved as inclusions in strong, refractory minerals that either have a broad pressure range of stability or exhibit very sluggish kinetics of reaction to low pressure forms. The Alpe Arami peridotite of the Swiss Alps displays extensive exsolution of FeTiO 3 rods in the oldest generation of olivine. The shape, orientation and abundance of the titanate rods provide strong indication that the phase originally exsolved was the orthorhombic pervoskite phase stable only at pressures greater than 10 GPa (300 km depth) at mantle temperatures. We show here that the dislocation substructure of the oldest generation of olivine is younger than the titanate rods and similar to that observed in peridotites the world over and in experiments; the slip systems represented are incapable of producing the unique and unexplained lattice preferred orientation (LPO) displayed by this generation of olivine. We also have conducted preliminary experiments to investigate the maximum solubility of FeTiO 3 in olivine. Our results suggest that the solubility of TiO 2 implied by the abundance of titanate precipitates may be impossible under any conditions of olivine stability. On the other hand, the measured solubility in wadsleyite (β-olivine) under the conditions of our experiments is comparable to that inferred for Alpe Arami olivine. This latter observation combined with the determination that the titanate rods and LPO of this generation of olivine are the oldest features yet identified in these rocks, leads us to speculate that this massif has been brought to the Earth's surface from within the mantle transition zone, at depths of 410–660 km. The only mechanism by which we can envision this to have been accomplished is for the Lepontine gneisses that now surround the massif to have been subducted to great depth following collision of Africa and Europe, and to have picked up the peridotite on their way back to the surface by buoyant upwelling.

Transient electrical response of San Quintin Dunite as a function of oxygen fugacity changes: Information about charge carriers

The electrical conductivity (σ) of San Quintin dunite (SQD) measured between 950 and 1150°C at controlled oxygen fugacity (fO2) within the olivine stability field shows transients in response to changes in fO2. Such behavior has not been reported previously for measurements made under similar conditions on either olivine single crystals or polycrystalline samples (dunites and lherzolites) in which olivine is the major phase. In general for olivine, an increase in fO2 results in an increase in σ. The transient is manifested as a change in σ from a stable equilibrium value at a specific fO2 to a quickly established subsequent value, the direction of which is opposite that of the final value that will be attained for the fO2 change. This transient may be caused by a changing population of electrons produced by oxygen vacancies, the grain boundaries, or a short‐lived defect. We postulate that the transient is observed in this particular dunite because of its large surface area to volume ratio. The transient is more pronounced at relatively high fO23 and temperatures between 950 and 1100°C. At relatively low fO2s and temperatures higher than 1100°C, the effect is diminished. The Seebeck coefficient (S), at 1200°C, is slightly smaller than that reported for single‐crystal olivine, and at 1100°C is similar to that reported for single crystal olivine.

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 α → β 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 β polymorph is the main constituent of other areas. Crystals of the β 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 β phase contain significantly higher concentrations of Fe and trivalent cations. All β 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 β phase. Coupled with these previously measured OH concentrations, our new measurements suggest that incorporation of hydrogen into the β 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 β structure and coexisting phases in order to better estimate the modal mineralogy of the transition zone.

Experimental study of a jotunite (hypersthene monzodiorite): constraints on the parent magma composition and crystallization conditions (P, T, f O 2) of the Bjerkreim-Sokndal layered intrusion (Norway)

The Bjerkreim-Sokndal layered intrusion is part of the Rogaland anorthosite Province of southern Norway and is made of cumulates of the anorthositemangerite-charnockite suite. This study presents experimental phase equilibrium data for one of the finegrained jotunite (Tjorn locality) occurring along its northwestern lobe. These experimental data show that a jotunitic liquid similar in composition to the Tjorn jotunite, but slightly more magnesian and with a higher plagioclase component is the likely parent of macrocyclic units (MCU)III and IV of the intrusion. The limit of the olivine stability field in the experimentally determined phase diagram as well as comparison of the Al2O3 content of low-Ca pyroxenes from experiments and cumulates (≈1.5%) yields a pressure of emplacement ≤5 kbar. Experimentally determined Fe-Ti oxide equilibria compared to the order of cumulus arrival in the intrusion show that the oxygen fugacity was close to FMQ (fayalite-magnetite-quartz) during the early crystallization. It subsequently decreased relative to this buffer when magnetite disappeared from the cumulus assemblage and then increased until the reentry of this mineral. Calculated densities of experimental liquids show a density increase with fractionation at 7, 10 and 13 kbar due to the predominance of plagioclase in the crystallizing assemblage. At 5 kbar and 1 atm (FMQ-1), where plagioclase is the liquidus phase, density first increases and then drops when olivine (5 kbar) or olivine+ilmenite (1 atm: FMQ-1) precipitate. At 1 atm and NNO (nickel-nickel oxide), the presence of both magnetite and ilmenite as near liquidus phases induces a density decrease. In the Bjerkreim magma chamber, oxides are early cumulus phases and liquid density is then supposed to have decreased during fractionation. This density path implies that new influxes of magma emplaced in the chamber were both hotter and denser than the resident magma. The density contrast inferred between plagioclase and the parent magma shows that this mineral was not able to sink in the magma, suggesting anin situ crystallization process.

Dissolution of primary basaltic minerals in natural waters: saturation state and kinetics

The state of saturation of olivine, orthopyroxene and plagioclase of variable composition has been assessed in various types of natural waters in Iceland including river waters, groundwater and geothermal waters with temperatures up to 250°C. The stability of olivine and orthopyroxene decreases with increasing Mg content. Similarly, the stability of plagioclase decreases with increasing anorthite content. The river waters which are representative of the weathering environment are always undersaturated with both olivine and orthopyroxene, the degree of undersaturation being dominated by the water pH. All river waters tend to dissolve olivine of the composition encountered in basalts according to the linear rate law and orthopyroxene when the pH is < 8. Groundwaters, which have a higher pH, appear to dissolve orthopyroxene and Fc-rich olivine according to the slower near-equilibrium dissolution rate law. In river waters and groundwaters Na-rich plagioclase is stable but when Ca-rich it is close to equilibrium or slightly undersaturated. The state of saturation is independent of pH in the range occurring in these waters (pH 7–10). Geothermal waters of high temperature (> 200°C) are undersaturated with olivine, orthopyroxene and plagioclase, the first two particularly when Fe rich, and sufficient to cause olivine to dissolve according to the linear rate law. At intermediate temperatures (50–150°C) the geothermal waters are close to equilibrium with these minerals except for Mg-rich olivine. It seems likely that dissolution of glass from basalt, which is very reactive, will favor stability of the igneous minerals.

The electrical conductivity of an isotropic olivine mantle

In order to extend the useful temperature range of interpretation of olivine electrical conductivity σ we have used the nonlinear iterative Marquardt technique to fit experimental data over the range 720°–1500°C to the parametric form σ = σ1e−A1/kT + σ2e−A2/kT, where k is Boltzmann's constant and T is absolute temperature. The model describes conduction by migration of two different thermally activated defect populations with activation energies A1 and A2, and preexponential terms σ1 and σ2 that depend on number of charge carriers and their mobility and that may be different for each crystallographic direction. A combined interpretation of recent high (San Carlos olivine) and low (Jackson County dunite) temperature measurements has been made that demonstrates that a single activation energy A1 for all three crystallographic directions adequately fits the data. The parametric fits show that the high‐temperature conduction mechanism has far greater anisotropy than the low‐temperature mechanism, consistent with previous assignments to ionic and electronic conduction, respectively. The geometric mean of the conductivity in the three directions is approximately S/m and is presented as a model for isotropic olivine, SO2, appropriate from 720°C to above 1500°C, at oxygen fugacities near the center of the olivine stability field. It is observed that the magnitudes of σ1 for the three crystal directions are similar to the ratios of the inter‐ionic distances between the M1 magnesium sites in olivine, to within 5%, consistent with Fe3+ preferring the M1 site below 1200°C.

Seismic wave dispersion and attenuation in Åheim dunite: an experimental study

A machine has been developed which provides for the study of both shear modulus dispersion and internal friction in geological materials through the observation of forced torsional oscillations of low frequency (10-1000 mHz) and strain amplitude (< 10-6) under conditions of high pressure (to 300 MPa) and temperature (to > 1000°C). The eventual goal is an understanding of the dispersion and attenuation of seismic shear waves in the Earth's upper mantle. Measurements have been made on cylindrical specimens of an olivine-rich rock from Åheim, Norway which } contains, in addition to olivine, about 10 per cent pyroxene and 5-10 per cent of hydrous silicate phases dominantly clinochlore, serpentine and talc. In order to separate aspects of the mechanical behaviour associated with the olivine aggregate from those attributable to the hydrous phases and/or their dehydration products, the specimens were either previously fired under controlled oxygen fugacity at 1200 °C for 24 hours in order to effect complete dehydration within the olivine stability field, or simply oven-dried at 110 °C. Linearity of the mechanical behaviour has been demonstrated by the amplitude independence of the results for the strain amplitude range 10-8-10-6, and by the quantitative consistency between the observed modulus dispersion and that calculated from the measured internal friction through the Kramers-Krönig relations of linear theory. Both the pressure dependence at room temperature, and the temperature dependence at 300 MPa, of the shear modulus and internal friction have been investigated. Much of the variation of the shear modulus is attributed to changes of crack porosity associated with changes in pressure and temperature and with in situ dehydration. Temporal evolution of Q-1 towards a lower asymptotic value over periods of hours of exposure to given conditions of high pressure and temperature is tentatively attributed to the gradual diminution of enhanced anelastic relaxation associated with regions of decaying stress concentration at asperities on cracks and/or grain boundaries. At the highest pressures and temperatures achieved in this study, 300 MPa and 1000 °C, marked dispersion of the shear modulus, amounting to 5 per cent between periods of 3 and 100 s, and concomitant strong internal friction, varying with oscillation period To approximately as 0.011To1/6, are observed. The energy dissipation appears to be concentrated within, rather than at the boundaries between, the olivine grains. These results provide the clearest indication yet that solid-state, probably intragranular, anelastic relaxation in ultramafic rocks gives rise to losses comparable with those observed seismologically in the Earth's upper mantle, although the mechanistic basis for the observed anelasticity remains to be established.

Kinetics measurement of the phase transformation in Ni2SiO4 under high pressure and temperature (abstract)

The kinetics of the pressure-induced phase transformations in minerals are of considerable importance in affecting the dynamics of convection in the Earth. Because of the long exposure time of the high pressure in situ x-ray observations, however, it was very difficult to study the kinetics of such transformations. Combination of the synchrotron radiation and a large-volume high-pressure and high-temperature apparatus made it possible to perform such a study. The kinetics of the olivine-spinel transformation in Ni2SiO4 have been studied at conditions of 3.6–4.8 GPa and 765–980 °C. X-ray diffraction experiments were carried out using the MAX-80 high-pressure, high-temperature cubic-anvil apparatus at the National Laboratory for High Energy Physics, Japan. Synchrotron radiation was supplied by an accumulator ring operating at 6.5 GeV and between 15 and 5 mA. Ni2SiO4 olivine powder was hot pressed in the olivine stability field and then the pressure-temperature conditions were changed to initiate the transformation to spinel. Reaction progress was monitored by collecting x-ray diffraction patterns at intervals of either 30 or 100 s. The extent of transformation was estimated as a function of time and the data were analyzed on the basis of a nucleation and growth model. Pressure and temperature dependence of the transformation rate were determined from the present study.

Oxygen diffusion in San Carlos olivine

The self‐diffusion of oxygen in San Carlos olivine single crystals has been measured along the [001] direction. The diffusion experiments were performed using a gas mixture Ar + H2/Ar + H218O flowing around the olivine specimens. This gas mixture acts as the isotopic reservoir and is also used to establish and adjust the desired oxygen fugacity, allowing us to determine the oxygen diffusion coefficient D as a function of pO2. Thirty runs have been performed at 1 atm total pressure within the (T,pO2) olivine stability field, T ranging from 1363 to 1773 K and pO2 from 7×10−7 to 5×10−2 Pa. The annealing duration t was chosen such that the characteristic isotope penetration depth ranges between 600 and 11,000Å. The diffusion profiles of 18O were determined using the 18O(p, α)15N nuclear microanalysis technique. The diffusion coefficient can be written D* = D0* exp(−E/RT)(pO2/p0)m, where E is the activation energy, R is the gas constant, T is the temperature in K, pO2 is the oxygen partial pressure, p0 is the room pressure, m is a numerical coefficient, and D0* is the preexponential term. A data inversion method has been used to determine the three parameters and yields log(D0*) = −5.2(±3.6) with D0* in m2/s, E = 318(±17) kJ/mol, m = 0.34(±0.02); D* = 6.7 × 10−6exp(−38,000/T)(pO2/p0)0.34 m2/s; the number in parentheses is the standard deviation. D* is related to the self‐diffusion coefficient D by the relation D* = ƒD, with ƒ close to 1.0. An attempt to show pipe diffusion in predeformed samples, with a dislocation density of about 2×1011 m−2, along dislocation cores perpendicular to the specimen surface, showed no contribution to the main bulk diffusion. We conclude that in olivine, Fo≃90, within the temperature range investigated, DFe, DMg ≫ D0x ≫ DSi as has already been demonstrated in FO100. E is of the same order of magnitude as the Si ‐ O bond energy and as E in Fo100. We infer that the oxygen diffusion occurs via an interstitial mode and that these defects carry one negative charge. This interstitial mechanism is rather surprising in view of the compact oxygen sublattice of olivine. However, interstitial transport of oxygen has already been identified in other oxygen compact compounds.

Surface destabilization and laboratory-induced non-stoichiometry in San Carlos olivine

Annealing experiments on natural olivine (Mg1-x Fe x )2SiO4 (with x≈0.11) crystals (San Carlos, Arizona, spinel-lherzolite context) have been performed between T=1,100° C and 1,500° C for oxygen partial pressures pO2=10−3 to 10−13 bar and times of 1 to 140 h in CO/CO2 or H2/H2O gas mixtures. Even specimens annealed within the T-pO2 theoretical stability field (TSF) calculated for stoichiometric olivine (Nitsan 1974) show systematic alterations developed within the first few microns of the surface of the crystals. Pyroxene crystals or melt form on the original olivine surface even at T=1,100° C, with preference of pyroxene when T 1,350° C. This glass (rhyolite-like) can concentrate calcium from the starting olivine, and aluminum when Cr-Al spinels are present as inclusions. These observations are in contradiction with the TSF. They are obviously due to the presence of platinum used as a container of our samples, even if the contact between olivine and platinum is very weak. Rapid surficial diffusion of iron toward platinum (or via the gas phase) induces a Fe-depleted surface. According to the TSF, this more forsteritic surface should have a wider pO2 range of stability. This is not the case, just because this situation is largely out of equilibrium. This iron loss induces a departure from cationic stoichiometry: (Mg, Fe)2(1−δ), SiO4 with δ small and positive. We extended the model that Nakamura and Schmalzried (1983) (N.S.) developed for fayalite (x=1) to our natural olivine composition, under the assumption that the majority defects are magnesium vacancies, Fe3+ occupying octahedral and tetrahedral sites, and the more complex neutral defect corresponding to Coulombic attraction between neighboring Fe3+ ions. We have recalculated the olivine stability field in pO2 vs. δ space at T=1,300° C using this model for x≈0.1 (its extreme limit of validity) and conclude that olivine is stable only in a very narrow range in pO2 which depends on δ. The calculation shows also that when olivine has nearly cationic stoichiometry (δ=0) as we believe for our starting material, the pO2 range of stability is narrower than indicated by the TSF. In particular, it explains why Fe precipitates from the olivine (δ=0) (in absence of any other precipitation of SiO2-rich phases) between 10−11 and 10−13 bar at 1,300° C; this was not predicted by the TSF. Magnetite or iron precipitates also coexist with SiO2-rich exsolutions or pyroxene when pO2 is close to the upper or lower boundaries of the TSF, respectively. The N.S. model may have important implications for the interpretation of the existence of partial melting and/or the low-viscosity/low velocity zone in the upper mantle.

High temperature a.c. electrical properties of olivine single crystal with varying oxygen partial pressure: implications for the point defect chemistry

The electrical conductivity and dielectric constant of a natural olivine single crystal were measured as a function of frequency over 0.1–1000 Hz. The measurements were made at temperatures of 1000–1400°C under a controlled oxygen partial pressure that was varied within the range of the olivine stability field. The observed dielectric constant of olivine increased with increasing oxygen pressure, and the change was greater at higher temperature and lower frequency. When the oxygen pressure Po 2 was relatively high, the dielectric constants measured at 0.1 Hz were proportional to P o 2 1 6 . This observation can be interpreted as a polarization effect of magnesium vacancies that are partially blocked at the specimen electrodes. The conductivity value and the frequency dependence of the dielectric constant indicate that holes as well as magnesium vacancies contribute to electrical properties of olivine. At lower oxygen pressure, the conductivity and dielectric constant are less dependent on oxygen pressure, suggesting that magnesium interstitial ions and/or electrons also contribute to the electrical conduction in olivine.

Sintering of olivine and olivine-basalt aggregates

The sintering behavior of olivine and olivine-basalt aggregates has been examined at temperatures near 1,300° C. Experimental factors contributing to rapid sintering kinetics and high-density, fine-grained specimens include: (i) the uniform dispersion of basalt throughout the specimen, (ii) a very fine, uniform particle size for the olivine powder, (iii) oxygen fugacities near the high PO2 end of the olivine stability field, and (iv) rapid heating to the sintering temperature. Olivine-basalt specimens prepared from olivine particles coated with a synthetic basalt achieve chemical and microstructural equilibrium more rapidly, as well as produce higher density and finer grain-sized aggregates, than do specimens prepared by mechanical mixing of olivine and natural basalt powders. The grain boundary mobility for olivine, measured for olivine-basalt aggregates which have undergone secondary recrystallization, is on the order of 2×10−15 (m/s)/(N/m2) in the temperature range 1,300–1,400° C. Solution-precipitation (pressure-solution) processes make an important contribution to the development of the microstructure in olivine-basalt aggregates.

The effect of water on high‐temperature deformation in olivine

Numerous natural single crystal specimens of olivine were examined to assess the levels of intrinsic water (hydrogen) concentration and to measure their plastic deformation characteristics within controlled environments in constant strain‐rate experiments. Our efforts to characterize the "water" content of the natural specimens, as well as to artificially "wet" single crystals, have indicated that the olivine structure can retain "water" in very small amounts only (a maximum of 55 H/106 Si was observed). Moreover, compressional creep deformation experiments at differential stresses between 10 and 450 bars, with strain‐rates from 10−6 to 10−5 sec−1 under zero confining pressure, and at a temperature of 1575°C, demonstrate no loss in strength for single crystal specimens deformed in "wet", relative to "dry", experimental environments. Our "wet" environments were obtained from an H2O‐H2 gas mixture with controlled oxygen fugacity within the olivine stability region, whereas "dry" conditions were obtained by using a CO2‐CO gas flow. The results of this study, as well as those from other recent investigations, suggest strongly that the "water weakening" effect, which has been observed in natural olivine and dunite, is most likely due to the increase of oxygen partial pressure resulting from the high temperature decomposition of H2O, rather than the hydrolysis of crystal bonds.

Ca-enrichment in olivines from volcanic rocks

Data are presented on the Ca-content of olivines in a range of volcanic rocks from a variety of suites. These include olivine basalts through to trachytes from Gough, St. Helena and Tristan da Cunha Islands; trachytes from central Victoria, Australia, and a leucitite and phonclitic-tephrite series from Bufumbira, Uganda. In olivine crystals from basaltic lavas the Ca- and Mn-contents are low and Fe shows the most significant zoning from core to rim. In Fe-rich olivines from trachytic differentiates Ca and Mn frequently show more significant variation than Fe. While the Mn-content is proportional to the Fe-content of these olivines, Ca-zoning, in many cases, is unrelated to Fe-content. The marked Ca-enrichment in olivines occurs with the absence of plagioclase in the host lavas. The Ca(Al  Na  K) ratio is shown to be related not only to the Ca-content of the olivines in the lava, but may be used to predict olivine stability in evolved compositions.

Systematic study of liquidas phase relations in olivine melilitite +H2O +CO2 at high pressures and petrogenesis of an olivine melilitite magma

Near-liquidus phase relationships of a spinel lherzolite-bearing olivine melilitite from Tasmania were investigated over a P, T range with varying $${\text{x}}_{{\text{H}}_{\text{2}} {\text{O}}} $$ , $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ , and $${\text{f}}_{{\text{O}}_{\text{2}} } $$ . At 30 kb under MH-buffered conditions, systematic changes of liquidus phases occur with increasing $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ ( $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ = CO2/CO2 +H2O+olivine melilitite). Olivine is the liquidus phase in the presence of H2O alone and is joined by clinopyroxene at low $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ . Increasing $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ eliminates olivine and clinopyroxene becomes the only liquidus phase. Further addition of CO2 brings garnet+orthopyroxene onto the liquidus together with clinopyroxene, which disappears with even higher CO2. The same systematic changes appear to hold at higher and lower pressures also, only that the phase boundaries are shifted to different $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ . The field with olivine- +clinopyroxene becomes stable to higher $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ with lower pressure and approaches most closely the field with garnet+orthopyroxene+clinopyroxene at about 27 kb, 1160 °C, $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ ∼ 0.08 and $${\text{x}}_{{\text{H}}_{\text{2}} {\text{O}}} $$ ∼ 0.2 (i.e., 6–7% CO2+ 7–8% H2O). Olivine does not coexist with garnet+orthopyroxene+clinopyroxene under these MH-buffered conditions. Lower oxygen fugacities do not increase the stability of olivine to higher $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ and do not change the phase relationships and liquidus temperatures drastically. Thus, it is inferred that olivine melilitite 2927 originates as a ∼ 5% melt (inferred from K2 O and P2O5 content) from a pyrolite source at about 27kb, 1160 dg with about 6–7% CO2 and 7–8% H2O dissolved in the melt. The highly undersaturated character of the melt and the inability to find olivine together with garnet and orthopyroxene on the liquidus (in spite of the close approach of the respective liquidus fields) can be explained by reaction relationships of olivine and clinopyroxene with orthopyroxene, garnet and melt in the presence of CO2.

Thermodynamic relations among olivine, spinel, and phenacite structures in silicates and germanates. III. The system CuO [sbnd]MgO [sbnd]GeO 2

Phase relations in the system CuO MgO GeO 2 in air at 1000°C were determined. Activity-composition relations in the system CuO MgO at 1000°C were measured: the guggenite phase was found to exist over the composition range 0.695 < X CuO < 0.805. The enthalpy and free energy of formation of CuGeO 3 were determined by high temperature solution calorimetry and reduction equilibria, respectively. For the reaction: CuO + GeO 2 = CuGeO 3, ΔG° 1273 = −4.0kcal/mole, ΔH° 988 = −5.2kcal/mole, ΔS° = −0.9cal/deg mole. The stability of rocksalt, tenorite, olivine, enstatite, and CuGeO 2 structures in this system is discussed.