Polycrystalline Olivine Research Articles

The onset of anelastic behavior in fine-grained synthetic dunite

Micromechanical models suggest that the onset of anelastic relaxation in polycrystalline olivine, critical to interpretation of the seismic wave attenuation and dispersion in the upper mantle, should be a mild dissipation peak caused by elastically accommodated grain-boundary sliding. Such behavior has been tentatively invoked to explain both a short-period shear modulus deficit and a dissipation plateau poorly resolved at 900–700 °C in previous forced-oscillation experiments on fine-grained dunite tested within mild-steel jackets. However, these observations may have been complicated by the austenite to ferrite plus cementite phase transition in the jacket material, compliance associated with interfacial Ni70Fe30 foils, and modeling of the mechanical properties of polycrystalline alumina as control specimen. To investigate the influence of these complications within the experimental setup and provide forced-oscillation data of better quality especially at moderate temperatures, we have conducted further forced-oscillation tests for which we removed the interfacial foils, employed single-crystal sapphire as reference sample, and used alternative jacket materials (stainless steel or copper) which experience no phase transition during the staged cooling. The newly acquired forced-oscillation data, although broadly consistent with the previous results, differ significantly especially in temperature sensitivity, and allow refinement of an appropriate Burgers creep-function model. A mild dissipation peak superimposed on monotonic dissipation background during the onset of anelastic relaxation in dry, melt-free and fine-grained dunite has now been consistently observed at temperatures of ∼950–1050 °C and seismic periods of 1–1000 s. Such a dissipation peak with relaxation strength 0.02 ± 0.01 is attributed to elastically accommodated grain-boundary sliding. The high activation energy (> 600 kJ/mol) of viscoelastic behavior involving both dissipation and related dispersion suggests that grain-boundary diffusion may be limited by interfacial reaction within grain boundaries. The reduced relaxation strength makes it difficult to attribute the oceanic lithosphere-asthenosphere boundary to water-mediated elastically accommodated grain-boundary sliding.

Dislocations in naturally deformed olivine: Example of a mylonitic peridotite

We have investigated the microstructure of naturally deformed olivine (chemically equilibrated at 1000 °C) by conventional transmission electron microscopy and electron tomography. The peridotite specimen, from Oman ophiolite, has a mylonitic microstructure with remnant, strongly deformed, millimetric porphyroclasts co-existing with small newly formed olivine grains generated by dynamic recrystallization. Imaging by transmission electron microscopy reveals that both newly formed grains and porphyroclasts display [100] and [001] dislocations activity. Subgrain boundaries are composed of either [100] or [001] dislocations. The characterization of this natural sample also permits to identify sporadic [100] dislocation loops, rare [010] dislocation, infrequent melt, and intragranular bubbles or along subgrain boundaries. Electron tomography permits to identify several glide planes, which are similar to previous observations acquired on experimentally deformed polycrystalline olivine, more importantly electron tomography also permits to evidence combination of glide, climb and mixed climb (dislocation moving in an intermediate plane between the plane of glide plane and the plane of pure climb). Our study further infers the diversity of linear defects responsible for plastic deformation of olivine at lithospheric conditions.

Electrical conductivity of anhydrous and hydrous gabbroic melt under high temperature and high pressure: implications for the high-conductivity anomalies in the mid-ocean ridge region

Abstract. The electrical conductivity of gabbroic melt with four different water contents (i.e., 0 %, 2.59 wt %, 5.92 wt %, and 8.32 wt %) was measured at temperatures of 873–1373 K and pressures of 1.0–3.0 GPa using a YJ-3000t multi-anvil high-pressure apparatus and Solartron-1260 impedance spectroscopy analyzer. At a fixed water content of 2.59 wt %, the electrical conductivity of the sample slightly decreased with increasing pressure in the temperature range of 873–1373 K, and its corresponding activation energy and activation volume were determined as 0.87 ± 0.04 eV and −1.98 ± 0.02 cm3 molec.−1, respectively. Under the certain conditions of 873–1373 K and 1.0 GPa, the electrical conductivity of the gabbroic melts tends to gradually increase with a rise in water content from 0 wt % to 8.32 wt %, and the activation enthalpy decreases from 0.93 to 0.63 eV accordingly. Furthermore, functional relation models for the electrical conductivity of gabbroic melts with variations of temperature, pressure, and water content were constructed at high-temperature and high-pressure conditions. In addition, the dependence relation of the electrical conductivity of melts with the degree of depolymerization was explored under conditions of four different water contents at 1373 K and 1.0 GPa, and three previously available reported results on those of representative calc-alkaline igneous rock melts (i.e., dacitic melt, basaltic melt, and andesitic melt) were compared in detail. In combination with our presently acquired electrical conductivity data on gabbroic melt with four different water contents and the available data on polycrystalline olivine, the electrical conductivity of a gabbroic melt–olivine system with variation of the volume percentage of anhydrous and hydrous melts was successfully constructed by using the typical Hashin–Shtrikman upper-bound model. In light of the electrical conductivity of the gabbroic melt–olivine system with previous magnetotelluric (MT) results, we find that anhydrous and hydrous gabbroic melts can be employed to reasonably interpret the high-conductivity anomalies in the Mohns Ridge of the Arctic Ocean.

Dislocation and disclination densities in experimentally deformed polycrystalline olivine

Abstract. We report a comprehensive data set characterizing and quantifying the geometrically necessary dislocation (GND) density in the crystallographic frame (ραc) and disclination density (ρθ) in fine-grained polycrystalline olivine deformed in uniaxial compression or torsion, at 1000 and 1200 ∘C, under a confining pressure of 300 MPa. Finite strains range from 0.11 up to 8.6 %, and stresses reach up to 1073 MPa. The data set is a selection of 19 electron backscatter diffraction maps acquired with conventional angular resolution (0.5∘) but at high spatial resolution (step size ranging between 0.05 and 0.1 µm). Thanks to analytical improvement for data acquisition and treatment, notably with the use of ATEX (Analysis Tools for Electron and X-ray diffraction) software, we report the spatial distribution of both GND and disclination densities. Areas with the highest GND densities define sub-grain boundaries. The type of GND densities involved also indicates that most olivine sub-grain boundaries have a mixed character. Moreover, the strategy for visualization also permits identifying minor GND that is not well organized as sub-grain boundaries yet. A low-temperature and high-stress sample displays a higher but less organized GND density than in a sample deformed at high temperature for a similar finite strain, grain size, and identical strain rate, confirming the action of dislocation creep in these samples, even for micrometric grains (2 µm). Furthermore, disclination dipoles along grain boundaries are identified in every undeformed and deformed electron backscatter diffraction (EBSD) map, mostly at the junction of a grain boundary with a sub-grain but also along sub-grain boundaries and at sub-grain boundary tips. Nevertheless, for the range of experimental parameters investigated, there is no notable correlation of the disclination density with stress, strain, or temperature. However, a broad positive correlation between average disclination density and average GND density per grain is found, confirming their similar role as defects producing intragranular misorientation. Furthermore, a broad negative correlation between the disclination density and the grain size or perimeter is found, providing a first rule of thumb on the distribution of disclinations. Field dislocation and disclination mechanics (FDDM) of the elastic fields due to experimentally measured dislocations and disclinations (e.g., strains/rotations and stresses) provides further evidence of the interplay between both types of defects. At last, our results also support that disclinations act as a plastic deformation mechanism, by allowing rotation of a very small crystal volume.

A TEM Study on a Polycrystalline Olivine Sample Deformed in a D-DIA under Mantle Conditions

We carried out an electron microscopy study on a polycrystalline olivine sample that was deformed with multiple deformation cycles under controlled differential stresses and strain rates at high pressures and high temperatures. Low-angle backscattered electron images thereof showed randomly oriented grains. Most of the grains were about 10–20 μm wide. The grains were irregular with wavy grain boundaries, indicating high grain boundary mobility during deformation. Transmission electron microscopy (TEM) images showed complex dislocation microstructure characteristics of high temperature, high pressure, and high strain. Free dislocations were predominantly either short and straight screw dislocations or curved dislocations with mixed screw and edge characters. Many of them split into partial dislocations. The differential stress estimated with the free dislocations was ~780 MPa, which was close to the value of differential stress attained in the final deformation cycle. We also observed dense dislocation tangles, which formed dislocation cell substructures under high strain. The existence of dislocation loops and jogs indicated significant climbing activity, providing evidence for high-temperature creep as the dominant deformation mechanism. All of the dislocations observed in this study were exclusively with a [001] Burgers vector. Dislocations with a [100] Burgers vector were absent, suggesting that the activity of the a-slip (i.e., (010)[100] and (001)[100] slip systems) was completely suppressed. These observations support a conclusion that was reported based on an X-ray texture analysis, which considered that a high pressure promotes the activities of the c-slip (i.e., (010)[001] and (100)[001] slip systems). It appears that the transition from the a-slip to the c-slip was complete with multiple deformation cycles at a relatively lower pressure of 5.1 GPa than previously thought, corresponding to a depth of 165 km in the mantle.

Grain-size-evolution controls on lithospheric weakening during continental rifting

Variation in the effective strength of the lithosphere allows for active plate tectonics and is permitted by different deformation mechanisms operating in the crust and upper mantle. The dominant mechanisms are debated, but geodynamic models often employ grain-size-independent mechanisms or evaluate a single grain size. However, observations from nature and rock deformation experiments suggest a transition to grain-size-dependent mechanisms due to a reduction in grain size can cause lithospheric weakening. Here, we employ a two-dimensional thermo-mechanical numerical model of the upper mantle to investigate the nature of deformation and grain-size evolution in a continental rift setting, on the basis of a recent growth law for polycrystalline olivine. We find that the average olivine grain size is greater in the asthenospheric mantle (centimetre-scale grains) than at the crust–mantle boundary (millimetre-scale grains). This grain-size distribution could result in dislocation creep being the dominant deformation mechanism in the upper mantle. However, we suggest that along lithospheric-scale shear zones, a reduction in grain sizes due to localized deformation causes a transition to diffusion creep as the dominant deformation mechanism, causing weakening of the lithosphere and facilitating the initiation of continental rifting.

A Tem Study on a Polycrystalline Olivine Sample Deformed in the D-Dia Under Mantle Conditions

A Tem Study on a Polycrystalline Olivine Sample Deformed in the D-Dia Under Mantle Conditions

Diamond formation during sulfidation of metal–carbon melts

Experimental studies modeling the mechanism of diamond crystallization during sulfidation of metal–carbon melts were performed on a multi-anvil high-pressure apparatus BARS at pressure of 6.3 GPa, temperature range of 1270–1470 °C, and run duration of 20 to 35 h. Sandwich experiments with layer filling were carried out using schemes FeS2-[Fe(Fe90Ni10) + C]-FeS2, FeS2-Ol-[Fe90Ni10 + C]-Ol-FeS2 and S-Ol-[Fe90Ni10 + C]-Ol-S. The experiments have revealed that crystallization of diamond and/or graphite occurs during sulfidation of a metal–carbon melt either due to mixing of metal–carbon and sulfide melts or due to influx of sulfur from external sources through a polycrystalline olivine layers. Transport of a sulfur fluid or a metal–sulfide–carbon melt through a silicate matrix occurs through the intergranular space and microcracks as well as by recrystallization of olivine. Sulfidation is accompanied by a decrease in the solubility of carbon and leads to supersaturation that is necessary for crystallization of diamond or graphite. The inhibitory effect of the sulfide component is the cause for changes in the diamond morphology and transition of carbon phase crystallization from diamond to metastable graphite. The synthesized diamond crystals are classified as low-nitrogen or nitrogen-free diamonds. The revealed regularities may be used to explain the genesis of diamonds with central inclusions and nitrogen-free CLIPPIR diamonds.

Low‐Frequency Seismic Properties of Olivine‐Orthopyroxene Mixtures

AbstractAlthough the seismic properties of polycrystalline olivine have been the subject of systematic and comprehensive study at seismic frequencies, the role of orthopyroxene as the major secondary phase in the shallow parts of the Earth's upper mantle has so far received little attention. Accordingly, we have newly prepared three synthetic melt‐free polycrystalline specimens containing different proportions of olivine (Ol, Fo90) and orthopyroxene (Px, En90) by hot pressing precursor powders produced with the solution‐gelation method at 1,200°C and 300 MPa. The resulting specimens (of Ol95Px5, Ol70Px30, and Ol5Px95 phase proportions) were mechanically tested by torsional forced‐oscillation from 1,300 or 1,200–400°C during staged cooling under a confining pressure of 200 MPa. Within the observational window (1–1,000 s), the shear modulus and dissipation vary monotonically with period and temperature for each of the tested specimens. There is no evidence of the superimposed high‐temperature dissipation peak reported by Sundberg and Cooper (2010, https://doi.org/10.1080/14786431003746656) for an Ol60Px40 specimen derived from natural precursor material and containing ∼1.5% melt. The forced‐oscillation data for each specimen are well‐described by a model based on an extended Burgers‐type creep function. The findings suggest that an olivine‐based model for high‐temperature viscoelasticity in upper mantle olivine requires only modest adjustment of the unrelaxed shear modulus to accommodate the role of orthopyroxene.

Rheology of amorphous olivine thin films characterized by nanoindentation

The rheological properties of amorphous olivine thin films deposited by pulsed laser deposition have been studied based on ambient temperature nanoindentation under constant strain-rate as well as relaxation conditions. The amorphous olivine films exhibit a viscoelastic-viscoplastic behavior with a significant rate dependency. The strain-rate sensitivity m is equal to ∼0.05 which is very high for silicates, indicating a complex out-of-equilibrium structure. The minimum apparent activation volume determined from nanoindentation experiments corresponds to Mg and Fe atomic metallic sites in the (Mg,Fe)2SiO4 crystalline lattice. The ambient temperature creep behavior of the amorphous olivine films differs very much from the one of single crystal olivine. This behavior directly connects to the recent demonstration of the activation of grain boundary sliding in polycrystalline olivine following grain boundary amorphization under high-stress.

Electrical properties of dry polycrystalline olivine mixed with various chromite contents: Implications for the high conductivity anomalies in subduction zones

Chromite, a crucial high-conductivity mineral phase of peridotite in ophiolite suites, has a significant effect on the electrical structure of subduction zones. The electrical conductivities of sintered polycrystalline olivine containing various volume percents of chromite (0, 4, 7, 10, 13, 16, 18, 21, 23, 100 vol.%) were measured using a complex impedance spectroscopic technique in the frequency range of 10−1–106 Hz under the conditions of 1.0–3.0 GPa and 873–1223 K. The relationship between the conductivities of the chromite-bearing olivine aggregates and temperatures conformed to the Arrhenius equation. The positive effect of pressure on the conductivities of the olivine–chromite systems was much weaker than that of temperature. The chromite content had an important effect on the conductivities of the olivine–chromite systems, and the bulk conductivities increased with increasing volume fraction of chromite to a certain extent. The inclusion of 16 vol.% chromites dramatically enhanced the bulk conductivity, implying that the percolation threshold of interconnectivity of chromite in the olivine–chromite systems is ~16 vol.%. The fitted activation enthalpies for pure polycrystalline olivine, polycrystalline olivine with isolated chromite, polycrystalline olivine with interconnected chromites, and pure polycrystalline chromite were 1.25, 0.78–0.87, 0.48–0.54, and 0.47 eV, respectively. Based on the chemical compositions and activation enthalpies, small polaron conduction was proposed to be the dominant conduction mechanism for polycrystalline olivine with various chromite contents. Furthermore, the conductivities of polycrystalline olivine with interconnected chromite (10–1.5–100.5 S/m) provides a reasonable explanation for the high conductivity anomalies in subduction-related tectonic environments.

Back‐transformation mechanisms of ringwoodite and majorite in an ordinary chondrite

AbstractWe investigated the back‐transformation mechanisms of ringwoodite and majorite occurring in a shock‐melt vein (SMV) of the Yamato 75267 H6 ordinary chondrite during atmospheric entry heating. Ringwoodite and majorite in the shock melt near the fusion crust have back‐transformed into olivine and enstatite, respectively. Ringwoodite (Fa~18) occurs in the SMV as a fine‐grained polycrystalline assemblage. Approaching the fusion crust, fine‐grained polycrystalline olivine becomes dominant instead of ringwoodite. The back‐transformation from ringwoodite to olivine proceeds by incoherent nucleation and by an interface‐controlled growth mechanism: nucleation occurs on the grain boundaries of ringwoodite, and subsequently olivine grains grow. Majorite (Fs16–17En82–83Wo1) occurs in the SMV as a fine‐grained polycrystalline assemblage. Approaching the fusion crust, the majorite grains become vitrified. Approaching the fusion crust even more, clino/orthoenstatite grains occur in the vitrified majorite. The back‐transformation from majorite to enstatite is initiated by the vitrification, and growth continues by the subsequent nucleation in the vitrified majorite.

Characterization of recovery onset by subgrain and grain boundary migration in experimentally deformed polycrystalline olivine

Abstract. To apprehend plate tectonics and the dynamics of the lithosphere–asthenosphere boundary, composed principally of olivine, we need to understand the mechanisms that control plastic deformation of olivine in the relevant temperature domain. After more than 50 years of laboratory studies and investigations on natural rocks, the interplay of several key parameters (e.g. temperature, pressure, vacancy concentration, dislocation densities, grain size, strain rate) controlling polycrystalline olivine plasticity remains difficult to assess. Here, we study four olivine polycrystals, which have been deformed in axial compression under a confining pressure of 300 MPa, at 1273 or 1473 K. Despite significant differences in mechanical properties (stress–strain curves), previous characterization by scanning (SEM) and transmission electron microscopy (TEM) did not reveal significant differences in dislocation microstructures which could explain these contrasted behaviours. We have undertaken automatic crystallographic orientation mapping (ACOM) analyses in TEM to increase the spatial resolution of characterization compared to previously obtained electron backscatter diffraction maps to further decipher the microstructures at nanoscale. With this novel technique applied to olivine, a noticeable difference in the onset of microstructural recovery has been identified between specimens deformed at 1273 and 1473 K. The microstructures of the olivine polycrystals deformed at 1473 K exhibit numerous curved grain and subgrain boundaries, advocating for recovery by boundary migration. In contrast, the microstructures of the olivine polycrystals deformed at 1273 K have significantly fewer subgrain boundaries and show more straight boundaries (i.e. closer to an equilibrium microstructure) than in the specimen deformed at 1473 K. Characterization by ACOM-TEM has permitted the identification of the onset of recovery, which is led by boundary migration even for very low macroscopic finite strains.

Vickers indentation tests on olivine: size effects

We conducted Vickers indentation tests on Fe-free (Mg2SiO4) and Fe-bearing (Mg1.8Fe0.2SiO4) olivine single crystals and high-density polycrystalline material with average grain sizes ranging from 170 to 890 nm. The Vickers microhardness ($$H_{{\text{v}}}$$) of the Fe-free polycrystalline material with the finest grain size is ~ 17 GPa at a load of 0.1 N, while that of the Fe-bearing single crystal is ~ 8 GPa at the largest load applied. Overall, $$H_{{\text{v}}}$$ decreases with increasing grain size, load (indentation depth), and the presence of Fe. For each grain size, $$H_{{\text{v}}}$$ is well characterized by a power law of the form $$H_{{\text{v}}} /H_{{\text{v}}}^{0} \propto l^{ - x}$$, where $$H_{{\text{v}}}^{0}$$ is the depth-independent value of $$H_{{\text{v}}}$$, $$l$$ represents either grain size or indentation depth, and x is 0.09. Despite the small exponent value for each size effect, the nonlinear interaction of the two size effects results in large variations of $$H_{{\text{v}}}$$ in our samples. We show that our semi-empirically derived relationship as a function of grain size and indentation depth explains the $$H_{{\text{v}}}$$ of both polycrystalline and single-crystal olivine at any indentation conditions. Indentation fracture toughness of the finest-grained material is 0.8 $${\text{MPa}}\;{\text{m}}^{1/2}$$, which increases slightly to 1.1 $${\text{MPa}}\;{\text{m}}^{1/2}$$ with increasing grain size, while the toughness of the single crystals varies from 0.5 to 0.8 $${\text{MPa}}\;{\text{m}}^{1/2}$$ depending on the crystallographic orientation of the fracture planes.

Effect of iron content on thermal conductivity of olivine with implications for cooling history of rocky planets

The influence of Fe concentration on heat transport properties of olivine was investigated to understand the cooling history of rocky planets such as Mercury, Mars and asteroids. Thermal conductivity (λ) and thermal diffusivity (κ) were measured simultaneously for olivine polycrystal with different Fe contents (Fo, Fo90, Fo70, Fo50, Fo31 and Fo0) up to 10 GPa and 1100 K by a pulse heating method. With increasing Fe in olivine, thermal conductivity of olivine first decreases and then slightly increases. The minimum λ was found to be at composition near Fo31; the absolute λ value of Fo31 is about 65% lower than that of Fo. Small amounts of Fe in olivine can strongly reduce the thermal conductivity at low temperature; λ value of Fo90 is about 50% of Fo at room temperature. Thermal conductivities of polycrystalline olivine have smaller absolute values and weaker pressure and temperature dependences, compared with those of natural single crystal olivine determined by previous studies. Heat capacity of Fo70 and Fo50 calculated from λ and κ is independent of pressure and is controlled by nearly constant thermal expansion coefficient with increasing temperature. Smaller λ of olivine aggregate with high Fe content would produce a warmer mantle and, in turn, possibly a thicker crust in the Fe-rich Mars, while heat in the Fe-poor Mercury can escape faster than the other terrestrial planets. Olivine-dominant asteroids with high Fe concentration could have longer cooling history and lower thermal inertia on the surface.

Argon storage and diffusion in Earth’s upper mantle

In this study, fine-grained polycrystalline olivine was doped with argon at static high pressure (0.30 ± 0.01 GPa) and high temperature (1050 ± 25 °C) conditions during 24 h in a Paterson press, and analysed using a step-heating extraction protocol coupled with noble gas mass spectrometry to investigate argon storage and diffusivity in Earth’s upper mantle. Our results show that a single diffusion mechanism controlled argon diffusion in our samples during the step heating experiments. Effective Ar diffusion in olivine has a low activation energy, implying that argon diffusivity is governed by both grain boundary and lattice diffusion. Mean values of lattice diffusion parameters obtained from our results and by reprocessing literature data are Ea = 166 ± 44 kJ mol−1 and D0 = 10−7.04 ± 1.13 m2·s−1, and grain boundary diffusion parameters determined from our data are Ea = 22 ± 5 kJ·mol−1 and D0 = 10−12.87 ± 0.3 m2·s−1. Isotopic diffusivity ratios were constant and close to the values determined by Graham’s law in the C-regime (i.e., bulk diffusion dominated by grain boundary diffusion) and A-regime (i.e., bulk diffusion controlled by grain boundary and lattice diffusion in proportion to the segregation of Ar between those sites), but varied in the B-regime (i.e., bulk diffusion controlled by both grain boundary and lattice diffusion in a complex manner), implying a higher isotopic fractionation in the kinetic B-regime. Extrapolation to typical mantle grain sizes implies that around 22% of the argon in the upper mantle can be stored at grain boundaries and that effective diffusion is mostly in the A-regime, suggesting a low isotopic fractionation and diffusivities faster than lattice diffusivities alone. The consideration of grain boundaries as a potential Ar storage site can modify equilibrium during partial melting and significantly enrich a liquid in Ar during fluid percolation. The grain size dependence of Ar storage and diffusivity highlights the underestimated role of grain boundaries in the upper mantle, especially in zones of reduced grain size (via dynamic recrystallization) possibly followed by fluid percolation and/or partial melting, such as in subduction zones or below oceanic ridges

Olivine melting at high pressure condition in the chassignite Northwest Africa 2737

Shock-induced melting textures and assemblages in the chassignite Northwest Africa (NWA) 2737 were investigated. NWA 2737 consists mainly of coarse-grained olivine (Fa19.6–20.5) and minor amounts of low-Ca pyroxene, plagioclase (now maskelynite), and chromite. Several melt inclusions formed through pre-igneous activity are enclosed in the olivine grains, and some melt inclusions were melted again by the impact event. Several olivine fragments are entrained in the melt inclusion and dissociated into spherulitic ferroan–periclase and interstitial fine-grained clinopyroxene assemblages. The olivine fragments were melted once by the shock metamorphism, and ferroan–periclase crystallized from the olivine melt. We expect that bridgmanite crystallized from the residual melt and inverted to clinopyroxene through the subsequent high-temperature event, although we cannot rule out the possibility that the ferroan–periclase and clinopyroxene assemblage is the partial melting product of olivine. Parts of the melted olivine in the melt inclusion are mixed with the glass (having low-Ca pyroxene + a small amount of silica components) filling the matrix portion of the melt inclusion. Then, olivine, clinopyroxene, and tridymite crystallized from the mixed melts. We infer that the olivine–clinopyroxene–tridymite assemblage is the crystallization product from the mixed melts during rapid cooling at low- or ambient-pressure conditions. Olivine grains adjacent to the melt inclusion dissociated to clinopyroxene + ferroan–periclase assemblage. As the distance from the melt inclusion increased, polycrystalline olivine crystal assemblage formed. We expect that the polycrystalline olivine assemblage is the remnant of ringwoodite: metastable ringwoodite formed in the olivine grain by the shock metamorphism, and back transformed into olivine again through high temperature and low pressure conditions. Mineral assemblages in and adjacent to the melt inclusion show evidence for high pressure and high temperature conditions and for low pressure and high temperature conditions. The pressure–temperature–time path recorded in NWA 2737 is not simple, i.e., the ejection path from the impact site is complex.

Solid-state redistribution of mineral particles in the upwelling mantle flow as a mechanism of chromite concentration in the ophiolite ultramafic rocks (on the example of Kraka ophiolite, the Southern Urals)

The main regularities of the structure of chromitite-bearing zones of ultramafic rock of the ophiolitic association are considered on the example of Kraka massifs. In all studied chromitite-bearing zones, olivine demonstrates a strong preferably crystallographic orientation, indicating that plastic flow was one of the main factors of petrogenesis and ore formation. A critical review of existing ideas about the origin of ophiolitic chromitites has been carried out. It is shown that for models involving the reaction and magmatic formation of dunites and chromitites, there are a number of difficulties. In particular, the application of the magma mixing model to the mantle ultramafiс rocks for the formation of chrome ores is faced with the problem of “free space”. Free space is necessary for the deposition of large volumes of ores, which is absent in a very low-porous crystalline upper mantle. In the “melt-mantle” interaction model, it is difficult to explain the often observed abrupt contacts of dunites and harzburgites, as well as an increase in the content of orthopyroxene in the near-contact parts of harzburgites, which is very often observed in ophiolite massifs. In addition, there is no mechanism for the formation of chromitites as geological bodies in this model. We have shown that the main trend in the composition and structure of the mantle section of ophiolites is stratification, accompanied by the separation of the rheologically most “weak” aggregates of polycrystalline olivine (dunites), which are host rocks for chrome ores. The stratification of the mantle material occurred during the solid-phase redistribution of minerals in the rocks, which are a dispersion system. In this work, a thermodynamic model is substantiated, which demonstrates the possibility of the emergence of solid-state flows in the conditions of the upper mantle and which makes it possible to eliminate some of the difficulties and contradictions characteristic of the magmatic and reaction-magmatic hypotheses.

Measurement of Activation Volume for Creep of Dry Olivine at Upper‐Mantle Conditions

AbstractOlivine is the most abundant and among the weakest phases in Earth's upper mantle, and thus, its rheological properties play a critical role in governing thermal structure and convective flow in the upper mantle. A persistent obstacle to constraining the in situ flow properties of olivine by laboratory experiment has been the difficulty in resolving the effect of pressure, which is weak within the 0‐ to ~2‐GPa pressure range of conventional laboratory deformation instruments but potentially strong over the 1‐ to ~14‐GPa range of the upper mantle. Using a deformation‐DIA, one of a new generation of bonafide deformation devices designed for operation to ≥10 GPa, we have deformed dry, polycrystalline San Carlos olivine in high‐temperature creep with the singular intent of providing the best achievable measurement of activation volume V* and a comprehensive statement of uncertainty. Under strictly dry conditions, at constant temperature (1,373 K) and strain rate (1 × 10−5 s−1), varying only pressure (1.8 to 8.8 GPa), we measure V* = 15 ± 5 cm3/mol. We have reproduced the well‐known mechanism change from [100]‐slip to [001]‐slip near 5 GPa and determined that, whatever the change in V* associated with the change in slip system, the effective value of 15 ± 5 cm3/mol is still accurate for modeling purposes in the 2‐ to 9‐GPa pressure range. This is a substantial pressure effect, which in the absence of a temperature gradient would represent a viscosity increase from the top to bottom of the upper mantle of 5 ± 2 orders of magnitude.

Transport properties of olivine grain boundaries from electrical conductivity experiments

Grain boundary processes contribute significantly to electronic and ionic transports in materials within Earth’s interior. We report a novel experimental study of grain boundary conductivity in highly strained olivine aggregates that demonstrates the importance of misorientation angle between adjacent grains on aggregate transport properties. We performed electrical conductivity measurements of melt-free polycrystalline olivine (Fo90) samples that had been previously deformed at 1200 °C and 0.3 GPa to shear strains up to γ = 7.3. The electrical conductivity and anisotropy were measured at 2.8 GPa over the temperature range 700–1400 °C. We observed that (1) the electrical conductivity of samples with a small grain size (3–6 µm) and strong crystallographic preferred orientation produced by dynamic recrystallization during large-strain shear deformation is a factor of 10 or more larger than that measured on coarse-grained samples, (2) the sample deformed to the highest strain is the most conductive even though it does not have the smallest grain size, and (3) conductivity is up to a factor of ~ 4 larger in the direction of shear than normal to the shear plane. Based on these results combined with electrical conductivity data for coarse-grained, polycrystalline olivine and for single crystals, we propose that the electrical conductivity of our fine-grained samples is dominated by grain boundary paths. In addition, the electrical anisotropy results from preferential alignment of higher-conductivity grain boundaries associated with the development of a strong crystallographic preferred orientation of the grains.