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Single crystals of ferromagnesian olivine (San Carlos, AZ, peridot; ~Fo88-90) have been deformed in both uniaxial creep and load relaxation under conditions of ambient pressure, T = 1500 °C and pO2= 10–10 atm; creep stresses were in the range 40 ≤ σ1 (MPa) ≤ 220. The crystals were oriented such that the applied stress was parallel to [011]c, which promotes single slip on the slowest slip system in olivine, (010)[001]. The creep rates at steady state match well the results of earlier investigators, as does the stress sensitivity (a power-law exponent of n = 3.6). Dislocation microstructures, including spatial distribution of low-angle (subgrain) boundaries additionally confirm previous investigations. Inverted primary creep (an accelerating strain rate with an increase in stress) was observed. Load-relaxation, however, produced a singular response—a single hardness curve—regardless of the magnitude of creep stress or total accumulated strain preceding relaxation. The log-stress v. log-strain rate data from load-relaxation and creep experiments overlap to within experimental error. The load-relaxation behavior is distinctly different than that described for other crystalline solids, where the flow stress is affected strongly by work hardening such that a family of distinct hardness curves is generated, which are related by a scaling function. The response of olivine for the conditions studied, we argue, indicates flow that is rate-limited by dislocation glide, reflecting specifically a high intrinsic lattice resistance (Peierls stress).

Light noble gas (He–Ne–Ar) solubility has been experimentally determined in a range of materials with six-member, tetrahedral ring structures: beryl, cordierite, tourmaline, antigorite, muscovite, F-phlogopite, actinolite, and pargasite. Helium solubility in these materials is relatively high, 4 × 10 −10 to 3 × 10 −7 mol g −1 bar −1 , which is ∼100 to 100,000× greater than He solubility in olivine, pyroxene, or spinel. Helium solubility broadly correlates with the topology of ring structures within different minerals. Distinctive He–Ne–Ar solubility patterns are associated with the different ring structure topologies. Combined, these observations suggest ring structures have a strong influence on noble gas solubility in materials and could facilitate the recycling of noble gases, along with other volatiles (i.e., water, chlorine, and fluorine), into the mantle. Measurements of Ne and Ar solubility in antigorite, however, are highly variable and correlated with each other, suggesting multiple factors contribute the solubility of noble gases in serpentine-rich materials.

Remote sensing observations have identified aluminate spinel, in the absence of measureable olivine and pyroxene, as a globally distributed component of the lunar crust. Earlier remote sensing observations and returned samples did not indicate the presence of this component, leaving its geologic significance unclear. Here, we report visible to mid-infrared (V-IR) reflectance (300–25 000 nm) and Mossbauer spectra of aluminate spinels, synthesized at lunar-like oxygen fugacity (ƒO2), that vary systematically in Fe abundance. Reflectance spectra of particulate ( 6 Fe#. Although the 2000 and 2800 nm bands are assigned to Fe 2+ IV electronic transitions, spectra of aluminate spinels with excess Al2O3 demonstrate that the strengths of the 1000 nm bands are related to the abundance of Fe 2+ VI. The abundance of Fe 2+ VI depends on bulk Fe content as well as factors that control the degree of structural order-disorder, such as cooling rate. Consequently the strength of the 1000 nm bands are useful for constraining the Fe content and cooling rate of remotely sensed spinel. Controlling for cooling rate, particle size, and ƒO2, we conclude that spinels with >12 Fe# (

Helium partitioning between olivine, orthopyroxene, clinopyroxene, and spinel and basaltic melt has been experimentally determined under upper mantle melting conditions (up to 20 kbar and 1450 °C). Under the conditions explored, helium partition coefficients are similar in all minerals investigated ( K d He ∼ 10 − 4 ), suggesting He is evenly distributed between the minerals of spinel peridotite. This is in contrast to most incompatible elements, which are concentrated in clinopyroxene in spinel peridotite. The studied minerals have different concentrations of point defects, but similar He solubility, providing no evidence for He partitioning onto specific defects sites (e.g. cation vacancies). Upper limits on the partition coefficients for Ne and Ar have also been determined, constraining these elements to be moderately to highly incompatible in olivine at the conditions of spinel peridotite melting ( 10 − 2 and 10 − 3 , respectively). Helium partitioning in peridotite minerals varies little within the range of temperatures, pressures, and mineral compositions explored in this study. Reported partition coefficients, in combination with previous work, suggest that moderate to high degree mantle melting is not an efficient mechanism for increasing (U+Th)/He, (U+Th)/Ne, or K/Ar of the depleted mantle (DMM) through time, and consequently, supports the argument that recycling of oceanic crust is largely responsible for the relatively strong radiogenic noble gas signatures in the depleted mantle. Mantle residues with lowered (U+Th)/He, (U+Th)/Ne, and K/Ar may be produced through large extents of melting, but concentrations of noble gases will be low, unless noble gas solubility in solids deviate from Henryʼs Law at high fugacity.

Alteration of fresh rock surfaces proceeds very rapidly in most terrestrial environments so that initial stages of modification of newly exposed surfaces are quickly masked by subsequent aqueous weathering processes. The hyper-arid and hypo-thermal environment of Beacon Valley, Antarctica, is limited in terms of available liquid water and energy available for alteration, which severely slows weathering processes so that the initial stages of alteration can be studied in detail. We report on the nature of initial chemical alteration of the Ferrar Dolerite in Beacon Valley, Antarctica, using a multiplicity of approaches to characterize the process. We suggest that initial chemical alteration is primarily driven by cation migration in response to the oxidizing environment. Morphological studies of altered rock surfaces reveal evidence of small-scale leaching and dissolution patterns as well as physical erosion due to surface weakening. Within the alteration front, mineral structures are largely preserved and alteration is only indicated by discrete zones of discoloration. Mineralogical investigations expose the complexity of the alteration process; visible/near-infrared reflectance and mid-infrared emission spectroscopy reveal significant variations in mineralogical contributions that are consistent with the introduction of oxide and amorphous phases at the surfaces of the rocks, while X-ray diffraction analyses reveal no definitive changes in mineralogy or material properties. Chemical analyses reveal large-scale trends that are consistent with cation migration and leaching, while small-scale electron microprobe analyses indicate that chemical variations associated with magmatic processes are still largely preserved within the alteration rind. This work confirms the incomplete and immature chemical alteration processes at work in the McMurdo Dry Valleys. Liquid water is not a significant contributor to the alteration process at this early stage of rind development, but assists in the removal of alteration products and their local accumulation in the surrounding sediments. These results also suggest that the McMurdo Dry Valleys (and Beacon Valley, in particular) are relevant terrestrial analogs to hyper-arid and hypo-thermal alteration processes that may be dominant on the martian surface.

The formation of extended planar defects in minerals such as olivine is related to high point defect concentration and can be driven by large gradients in chemical potential, where the energy of the system is lowered by the ordering of defects along specific planes in the crystal. The presence of extended defects has the potential to create the (apparently) anomalous ionic diffusion in olivine as reported recently (Spandler and O’Neill in Contrib Mineral Petrol 159(6):791–818, 2010). High-resolution transmission electron microscopy and energy-filtered imaging were done using experimental samples designed to examine the impact of a TiO2 and f O2 on the potential to form such defects in ferromagnesian olivine. Doped basalt (5 wt% TiO2)–olivine reaction couple experiments were run at 1 atm and 1,310 and 1,410 °C for 50 h at various f O2, ranging from 102 below to 102 above the quartz–fayalite–magnetite buffer. Our results show that extended planar defects in olivine, parallel to {101}ol and occurring in ordered “clusters” with a prolate spheroid geometry ~5–25 nm across and extending up to 150 nm into the olivine, are present near the olivine–glass interfaces in all of our experimental high-TiO2 basalt–olivine samples. Increased Ti content in the olivine is associated with the defects; ordering of Ti4+ and octahedral site vacancies leads to a two- or three-layer superstructure in the olivine. Defect nucleation and growth is driven by the large TiO2 chemical potential gradient across the phase boundary at the start of the experiments, which provides access to microstructures not otherwise present.

A new viscoelastic creep function that incorporates both the effects of elastically-accommodated grain boundary sliding (GBS) and transient diffusion creep is proposed. It is demonstrated that this model can simultaneously describe both the transient microcreep curves and the shear attenuation/modulus dispersion in a fine-grained (d ∼ 5 µm) peridotite (olivine + 39 vol. % orthopyroxene) specimen. Low-frequency shear attenuation, , and modulus dispersion, G(ω), spectra were measured in a one-atmosphere reciprocating torsion apparatus at temperatures of 1200 ≤ T ≤ 1300°C and frequencies of 10−2.25 ≤ f ≤ 100 Hz. Reciprocating tests were complemented by a series of small stress (τ ∼ 90 kPa) microcreep experiments at the same temperatures. In contrast to previous models where the parameters of viscoelastic models are derived by fitting the Laplace transform of the creep function to measured attenuation spectra, the parameters are derived solely from the fit of the creep function to the experimental microcreep ...

The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events, during which most of the highly volatile elements were lost. Hydrogen, being the lightest element, is believed to have been completely lost during this period. Here we make use of considerable advances in secondary ion mass spectrometry to obtain improved limits on the indigenous volatile (CO(2), H(2)O, F, S and Cl) contents of the most primitive basalts in the Moon-the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon's formation and its thermal and chemical evolution.

Wavelength-dispersive X-ray spectroscopy (WDS) was used to characterize the morphology of the reactions between a liquid bronze alloy (Cu–36 at.%Sn) and two magnesium aluminosilicate, glass–ceramic-forming glassmelts, one of which was doped significantly with ZnO. Two suites of experiments were pursued for each glassmelt, an isochronal series (30-min reactions with temperatures ranging from ∼1300 to 1400 °C) and an isothermal series (1350 °C reactions with durations ranging from 5 to 60 min). The reactions are decidedly complex. Transient behavior sees initially rapid incorporation of Cu +,2+ into the glassmelts, effected primarily by a redox couple involving the SiO 2 component of the aluminosilicate. This behavior gives way to a more dominant kinetic response in which Sn 2+,4+ is incorporated into the glassmelt in a reaction and chemical diffusion process that, in part, pulls the early-incorporated ionic copper back out of the aluminosilicate. In the case of the ZnO-doped glassmelt, coupled redox and interdiffusion of ionic Sn and ionic Zn is important in the longer-time response, giving rise to a Liesegang-band morphology. The extent of metal–silicate reaction diminishes as the temperature is increased, a thermodynamic effect related to the solution thermodynamics of the liquid bronze alloy. The reaction kinetics are interpreted following the Wagner–Schmalzried formalism for diffusion-effected redox reactions.

The microstructures of two-phase binary aggregates of ice-I + salt-hydrate, prepared by eutectic solidification, have been characterized by cryogenic scanning electron microscopy (CSEM). The specific binary systems studied were H2O-Na2SO4, H2O-MgSO4, H2O-NaCl, and H2O-H2SO4; these were selected based on their potential application to the study of tectonics on the Jovian moon Europa. Homogeneous liquid solutions of eutectic compositions were undercooled modestly (Δ T ~ 1–5 °C); similarly cooled crystalline seeds of the same composition were added to circumvent the thermodynamic barrier to nucleation and to control eutectic growth under (approximately) isothermal conditions. CSEM revealed classic eutectic solidification microstructures with the hydrate phase forming continuous lamellae, discontinuous lamellae, or forming the matrix around rods of ice-I, depending on the volume fractions of the phases and their entropy of dissolving and forming a homogeneous aqueous solution. We quantify aspects of the solidification behavior and microstructures for each system and, with these data, articulate anticipated effects of the microstructure on the mechanical responses of the materials.

The volatilization kinetics of single crystal α-SiC, polycrystalline β-SiC, and SiO_2 (cristobalite or glass) were determined in H_2-CO_2, CO-CO_2, and H_2-CO-CO_2 gas mixtures at oxygen fugacities between 1 log unit above and 10 log units below the iron-wustite (IW) buffer and temperatures in the range 1151 to 1501°C. Detailed sets of experiments on SiC were conducted at 2.8 and 6.0 log units below IW (IW-2.8 and IW-6.0) at a variety of temperatures, and at 1300°C at a variety of oxygen fugacities. Transmission electron microscopic and Rutherford backscattering spectroscopic characterization of run products shows that the surface of SiC exposed to IW-2.8 is characterized by a thin ( 1100°C for P^(tot)= 10^(−6) atm and sticking coefficient = 0.01, then the calculated lifetimes would be about 10 yr for 10-μm-diameter grains, essentially independent of temperature. The results thus imply that presolar SiC grains would survive short heating events associated with formation of chondrules (minutes) and calcium-, aluminum-rich inclusions (days), but would have been destroyed by exposure to hot (≥900°C) nebular gases in less than several thousand years unless they were coated with minerals inert to reaction with a nebular gas.

The high-temperature creep behavior of two fine-grained (∼3 μm) anorthite-rich glass-ceramics was characterized at ambient pressure and under a confining pressure of ∼300 MPa. Experiments were done at differential stresses of 15–200 MPa and temperatures of 1200°–1320°C. Of the two materials, one had a tabular (lathlike) grain structure with finely dispersed second phase of mullite, mostly in the form of ∼3–5 μm grains comparable to that of the primary anorthite phase, whereas the other had an equiaxed grain morphology with fine (∼400 nm) mullite precipitates concentrated at the anorthite grain boundaries. The results of creep experiments at ambient pressure showed that the material with the tabular grain structure had strain rates at least an order of magnitude faster than the equiaxed material. Creep in the tabular-grained material at ambient pressure was accompanied by a significant extent of intergranular cavitation: pore-volume analysis before and after creep in this material suggested that >75% of the bulk strain was due to growth of these voids. The equiaxed material, in contrast, showed a smooth transition from Newtonian (n= 1) creep at low stresses to non-Newtonian behavior at high stresses (n > 2). Under the high confining pressure, the microstructures of both materials underwent significant changes. Grain-boundary mullite precipitates in the undeformed, equiaxed-grain material were replaced by fine (∼100 nm), intragranular precipitates of silliminate and corundum because of a pressure-induced chemical reaction. This was accompanied by a significant reduction in grain size in both materials. The substantial microstructural changes at high confining pressure resulted in substantially lower viscosities for both materials. The absence of mullite precipitates at the grain boundaries changed the behavior of the equiaxed material to non-Newtonian (n= 2) at a pressure of ∼300 MPa, possibly because of a grain-boundary sliding mechanism; the tabular-grained material showed Newtonian diffusional creep under similar conditions.

Rutherford backscattering (RBS) and wavelength-dispersive X-ray (WDS) spectroscopies were employed to study the redox dynamics in reaction couples of liquid Au–28at.%Ge alloy and lightly iron-doped sodium aluminoborosilicate glasses. Glassmelts were floated in a reducing atmosphere for 30 min at temperatures from 1300°C to 1450°C. The depth of diffusive penetration of Ge 2+,4+ into the glassmelt was generally ∼20 μm, similar to Sn ions in conventionally processed float glass. Doping of the glassmelt with Fe 3+ affected significantly the penetration: with additional Fe 3+ , the internal redox reaction that forces germanium into the distinctly immobile Ge 4+ (network-former) state occurs closer to the metal–alloy/glassmelt interface. The observed chemical profiles in the glassmelt are consistent fully with a description of the float reaction as a cation-diffusion-controlled reduction of the glassmelt that involves multiple diffusion steps and internal structural reactions acting in series.

The impact of a chemically and texturally equilibrated melt phase on the shear creep and attenuation behaviors of polycrystalline olivine has been measured experimentally at seismic-to-subseismic frequencies. The experiments were performed on aggregates that had a particularly uniform and fine grain size (∼3 µm). The effect of the melt phase (∼5 vol%) is to decrease the (Newtonian) viscosity by a factor of ∼6; there is no dramatic disaggregation effect or melt-induced plummeting of the shear modulus. Both the melt-free and melt-bearing aggregates display an attenuation “band” of the form QG−1 ∝ f−1 4. This response cannot be attributed to a variation in microstructure; it is intrinsic to the diffusional creep behavior. There is no unique effect of the melt phase on the attenuation response: the slight increase in absorption of partial melt specimens is explained fully by the effect of the texturally equilibrated melt on aggregate viscosity.

The high-temperature (1200–1285°C) torsional dynamic attenuation (10−3–100 Hz) and unidirectional creep behavior of a fine, uniform grain sized (d ≈ 3 μm) olivine (∼Fo92) aggregate have been measured. In all cases, the material is found to be mechanically linear (i.e., γ(t), γ ∝ σxy1), indicating that diffusional processes dominate the deformation kinetics in these experiments. The creep response displays a large decelerating transient in the strain rate leading to a nominally constant “steady state.” The attenuation behavior displays a band in QG−1 that is moderately dependent on frequency (QG−1 ≈ f−0.35) and temperature with −1.5

Transient creep and attenuation (internal friction) of a β-spodumene glass-ceramic has been examined. Four-point flexural creep tests have been performed at high tempera-ture(T = 925°-1000°C) and modest stress levels (σl,max= 7-25 MPa). The flexural creep response of the glass-ceramic is characterized by a large decelerating transient that precedes the establishment of a steady state. The creep response is transformed via a numerical algorithm to give the attenuation spectrum. In addition, low-stress (2.5-30 MPa), subresonant-frequency (10 −5 Hz ≤ f ≤ 1 Hz), com-pression-compression attenuation experiments have been performed. A first-order thermodynamic analysis of the effect of effective pressure on the volume fraction of re-sidual glass suggests that the dilatational process that is associated with the flexural deformation mode should be characterized by a single loss mechanism. The predictions that are available from the flexural creep tests, as well as those that have been directly measured in experiments, in-dicate that attenuation behavior is insensitive to both fre-quency and temperature and exhibits a characteristic band of absorption. The weak frequency dependence of this an-elastic response is well-characterized by a power law of the form Q−1χfα, where 0.15 ≥ a ≥ 0.3, which suggests a physical mechanism that possesses a distribution of relax-ation times. A plausible explanation exists in the fluid-mechanics description of deformation-induced melt migra-tion, in which the crystalline β-spodumene “matrix” undergoes compaction or dilatation while the residual glass phase, which forms an interpenetrative network along grain triple junctions of the matrix, flows: the evolution with time of the compacted (and/or dilated) layer thickness of the deforming matrix produces a distribution of compli-ances.

[1] Using uniaxial compression creep experiments, we characterized the transient and steady state deformation behaviors of eutectic aggregates of system ice I and MgSO4 • 11H2O (MS11; meridianiite), which has significance because of its likely presence on moons of the outer solar system. Synthetic samples of eutectic liquid bulk composition, which produce eutectic colonies containing 0.35–0.50 volume fraction MS11, were tested as functions of colony size and lamellar spacing, temperature (230–250 K), and confining pressure (0.1 and 50 MPa) to strains ≤ 0.2. Up to a differential stress of 6 MPa, the ice I-MS11 aggregates display an order of magnitude higher effective viscosity and higher stress sensitivity than do aggregates of pure polycrystalline ice at the same conditions. The creep data and associated microstructural observations demonstrate, however, that the aggregates are additionally more brittle than pure ice, approaching rate-independent plasticity that includes rupture of the hydrate phase at 6–8 MPa, depending on the scale of the microstructure. Microstructures of deformed samples reveal forms of semibrittle flow in which the hydrate phase fractures while the ice phase deforms plastically. Semibrittle flow in the icy shell of a planetary body would truncate the lithospheric strength envelope and thereby decrease the depth to the brittle-ductile transition by 55% and reduce the failure limit for compressional surface features from 10 to ∼6 MPa. A constitutive equation that includes eutectic colony boundary sliding and intracolony flow is used to describe the steady state rheology of the eutectic aggregates.

Absolute paleointensity estimates from submarine basaltic glass (SBG) typically are of high technical quality and accurately reflect the ambient field when known. SBG contains fine-grained, low-Ti magnetite, in contrast to the high-Ti magnetite in crystalline basalt, which has lead to uncertainty over the origin of the magnetite and its remanence in SBG. Because a thermal remanence is required for accurate paleointensity estimates, the timing and temperature of magnetite formation is crucial. To assess these factors, we generated a suite of synthetic glasses with variable oxygen fugacity, cooling rate, and FeO* content. Magnetic properties varied most strongly with crystallinity; less crystalline specimens are similar to natural SBG and have weaker magnetization, a greater superparamagnetic contribution, and higher unblocking temperatures than more crystalline specimens. Thellier-type paleointensity results recovered the correct field within 1σ error with 2 (out of 10) exceptions that likely result from an undetected change in the laboratory field. Unblocking and ordering temperature data demonstrate that low-Ti magnetite is a primary phase, formed when the glass initially quenched. Although prolonged heating at high temperatures (during paleointensity experiments) may result in minor alteration at temperatures < 580°C, this does not appear to impact the accuracy of the paleointensity estimate. Young SBG is therefore a suitable material for paleointensity studies.

Thermodynamic constraints on the geometry of melt/solid interfaces require that a texturally equilibrated partial melt respond to changes in mean normal stress through adjustment of the sizes of melt-bearing triple-grain junctions. This dilatational response is well-modeled as that of a standard anelastic solid provided that the response rate is limited by solid-state deformation processes attending the dilatation rather than by melt flow itself. Using this model, we have analyzed flexural creep data obtained for a fine-grained partial melt comprising a MgSiO3 polycrystalline solid phase in equilibrium with a sodium aluminosilicate melt. The anelastic dilatational (or “bulk”) viscosity for this two-phase material is nearly an order of magnitude less than its shear viscosity, the latter being determined by diffusional creep. The corresponding modulus that controls the extent of dilatation is several orders of magnitude smaller than the elastic bulk modulus of the two-phase aggregate. Applied to the case of harmonic loading, the model predicts a substantial band-limited dissipation spectrum for dilatation (Qk−1) tnat would be absent but for the presence of the melt. This creates a large and strong P wave absorption (Qp−1) band for the enstatite material, accompanied by substantial P wave velocity dispersion. Through this enhanced P wave attenuation, the presence of the melt phase suppresses P-to-S velocity ratios and produces equivalent Q values for the two modes.

As part of an experimental and observational study of the magnetic response of submarine basaltic glass (SBG), we have examined, using ion backscattering spectrometry (RBS), transmission and scanning electron microscopy, energy dispersive X-ray spectrometry, and surface X-ray diffraction, the textures wrought by the controlled, open and closed system oxidation of glasses prepared by the controlled environment remelting and quenching of natural SBG. Initial compositions with ∼9 wt % FeO* were melted at 1430°C with the oxygen fugacity buffered at fayalite-magnetite-quartz; melts were cooled at a rate of 200°C min−1 near the glass transition (Tg = 680°C). In open system experiments, where chemical exchange is allowed to occur with the surrounding atmosphere, polished pieces of glass were reheated to temperatures both below and above Tg for times 1–5000 h; undercooled melts were oxidized at 900°C and 1200°C for 18 and 20 h, respectively. RBS demonstrates unequivocally that the dynamics of open system oxidation involves the outward motion of network-modifying cations. Oxidation results in formation of a Fe-, Ca-, and Mg-enriched surface layer that consists in part of Ti-free nanometer-scale ferrites; a divalent-cation-depleted layer is observed at depths >1 μm. Specimens annealed/oxidized above Tg have magnetizations elevated by 1–2 orders of magnitude relative to the as-quenched material; this does not appear to be related to the surface oxidation. Quenched glass (closed system, i.e., no chemical exchange between sample and atmosphere) exhibits very fine scale chemical heterogeneities that coarsen with time under an electron beam; this metastable amorphous immiscibility is the potential source for the nucleation of ferrites with a wide range of Ti contents, ferrites not anticipated from an equilibrium analysis of the bulk basalt composition.

The flexural creep responses of a Co-Mg olivine-plus-basalt aggregate at temperatures near 1100°C have been transformed to give attenuation spectra (QE−1 vs. ω) as functions of temperature. The experimental creep response of this mantle-analog material is well-modeled as a Burgers solid, displaying a steady-state creep that is preceded by a transient deceleration. Calculated attenuation spectra are based on the Burgers solid viscosities and elastic constants obtained from the creep experiments. The attenuation becomes total and zero in the low- and high-frequency limits, respectively, as is appropriate for polycrystalline olivine having a diffusional creep rheology. At intermediate frequencies (10−4Hz to 10−2Hz), QE−1 experiences a local maximum. Both the magnitude (QE−1≈15) and central frequency of this loss maximum increase with temperature. The attenuation peak is an anelastic loss compatible with the transient creep response; it is inferred to represent a melt flow mechanism that adjusts the size of melt-bearing triple-grain junctions in response to changes in the stress state.

[1] This work explores the physical basis for Hart's mechanical equation of state in high-temperature plasticity. The experiments seek to identify a possible microstructural basis for the “hardness” parameters associated with load relaxation curves. The experiments also seek to examine the microstructural basis for scaling in load relaxation data and to explore the relationship between creep and load relaxation. Constant stress creep and load relaxation tests were conducted on [100] oriented single crystals of halite at 700°C and stresses between 0.6 and 3 MPa. Load relaxation tests were performed at 400°C up to a stress level of 13 MPa. After testing, specimens were sectioned, and dislocation densities and subgrain size distributions were measured. Results at 700°C reveal that distributions of subgrain size in crystals crept at different stress levels are similar to each other; that is, they have the same shape but different average subgrain sizes depending on stress level. Hardness curves obtained from load relaxation experiments at different levels of work hardening were found to correspond to different average subgrain size. Load relaxation data from 700°C and 400°C belong to a single-parameter family of curves, with hardness curves translating onto each other with a scaling slope m = 0.33 ± 0.05. Subgrain size distributions generated in creep are statistically identical to those from load relaxation. The hardness parameter, σ*, specified as the (apparent) high-strain rate limit of stress in the load relaxation data, is approximately 50Gb/DI, where G is the shear modulus, b is the Burgers vector, and DI is the mean intercept subgrain diameter. During creep under constant stress the subgrain size evolves until a steady value is approached. The experimental data lend credence to Hart's interpretation that load relaxation data represent (nearly) constant “structure” with subgrain size playing the role of the structural variable.

Rutherford backscattering spectroscopy (RBS) and microscopy demonstrate that the approximately 1400deg;C oxidation of levitated droplets of a natural Fe2+-bearing aluminosilicate (basalt) melt occurs by chemical diffusion of Fe2+ and Ca2+ to the free surface of the droplet; internal oxidation of the melt results from the required counterflux of electron holes. Diffusion of an oxygen species is not required. Oxidation causes the droplets to go subsolidus; magnetite (Fe3O4) forms at the oxidation-solidification front with a morphology suggestive of a Liesegang-band nucleation process.

The majority of the residual glass in a silicate glass-ceramic or the melt phase in a silicate partial melt resides in the prismatic triple junction channels that are interconnected throughout the two-phase body. This morphology allows the aggregate to have a dilatational viscosity. Application of a differential stress requires that the local melt fraction reestablish structural equilibrium with the hydrostatic (mean-normal) component of the stress state. In the case of compressive σ1, the melt is driven to a free surface or to portions of the specimen having lower compressive potential. The melt flow can be rate limited by either the viscosity of the melt phase, or by the ability of the solid residuum to accommodate the triple junctions as they shrink or grow. Removal of the stress requires the “back flow” of the melt phase; the phenomenon is clearly anelastic. This coupled fluid mechanics problem, which has applications ranging from melt migration and seismic wave attenuation in Earth’s mantle to the transient creep response of ceramic composites, has been characterized in creep and fatigue experiments in both compressional and flexural loading configurations. The results of experiments on, and the development of numerical models for an enstatite glass-ceramic (EGC) undergoing four-point flexure are presented and interpreted.

Publisher Summary This chapter discusses melt migration and related attenuation in equilibrated partial melts. Anelasticity associated with the migration of the melt phase in synthetic olivine–basalt partial melts is examined experimentally in this chapter. Newtonian-viscous, texturally (quasi)equilibrated partial-melt aggregates were subjected to four-point flexural loading at elevated temperature (1070–1200°C); the creep response was characterized by a substantial, decelerating transient. The fact that melt-free grain boundaries characterize the steady-state microstructure of partial melts of upper-mantle composition supports the idea that dilatational anelasticity may be the unique dynamic signature of partial melting under conditions that produce a low melt fraction and that disallow the fracture of the crystalline residuum. The melt-migration attenuation effect noted in the experiments is sensitive to the magnitude of applied strain because of the coupling of the melt migration to the deformation kinetics of the crystalline residuum. Thus, further experimentation into the attenuation behavior of partial melts at seismic/subseismic frequencies requires working at low-strain amplitudes.

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.

Hot-pressing experiments have been used to study the effect of a chemically and texturally equilibrated liquid phase on the diffusional creep properties of polycrystalline olivine. Densification (creep) rate data on polycrystalline olivine both with and without the liquid basalt were collected as functions of applied stress/pressure (5–30 MPa), temperature (1300°–1400°C) and grain size (3–13 μm). Olivine-liquid basalt aggregates creep at rates which are persistently a factor of 2–5 greater than similar, melt-free specimens. Microstructural observations of dense olivine-basalt specimens indicate that the creep rate is enhanced by a solution-precipitation (pressure solution) mechanism during the densification of these specimens. Yet, the stress exponent (n = 1), grain-size exponent (m = 3) and average thermal activation energy (Q ≅ 380 kJ/mole) determined for the densification experiments are the same for olivine and olivine-basalt specimens. These observations suggest that because melt does not wet grain boundaries the kinetics of deformation of partially molten olivine-basalt aggregates are rate-limited by diffusive matter transport through melt-free olivine grain boundaries. The creep rate enhancement measured for olivine-basalt specimens occurs because the diffusive path length is effectively reduced by the presence of melt at triple junctions.

The dislocation structure, recrystallization, and elongation of quartz grains in tectonites from three localities along the Moine thrust fault have been analyzed by transmission electron and optical microscopy. The dislocation density, 5×108 cm−2, and the grain size after recrystallization, 15 μ, in the basal quartzite unit at the Stack of Glencoul are independent of distance from the fault, to the maximum sampling distance of ∼100 m. The differential stress level determined from the deformation-induced microstructures is on the order of 100 MPa. The magnitude of the differential stress at the fault decreases by a factor of 2 from Knockan Crag to Loch Eriboll, a distance of 50 km. At the Stack of Glencoul, grains in the basal quartzite are progressively elongated and recrystallized approaching the fault, corresponding to a progressive increase in strain. At 100 m from the fault, the aspect ratio of relict quartz grains is ∼1:1; at 0.01 m it is 85:1. A quantitative estimate of the strain, based on the aspect ratios of the relict quartz grains, as a function of distance from the fault has been used to calculate strain rate and temperature profiles. For a creep activation energy of 0.19 MJ mol−1 the temperature decreases away from the fault with a gradient of 7.5×10−3 °C cm−1. For a differential stress of 100 MPa, a steady state thrust sheet velocity of 11 cm yr−1 would be required to produce this gradient. At 100 m from the fault, 5% of the quartzite is recrystallized from 1-mm to 15-μm grains; at 0.01 m it is 100% recrystallized to 15-μm grains. This small grain size probably favored grain boundary deformation mechanisms over grain matrix mechanisms and thus helped to concentrate the strain at the fault. Systematic changes in the elongation and recrystallization of quartz grains in Lewisian Gneiss and Moine Schist are obscured by the constraints imposed by other mineral phases. The small size of the grains after recrystallization, highly serrated grain boundaries, and large densities of free dislocations suggest that very little static recovery followed the plastic deformation event.

Experimental results and a corresponding model for solution-precipitation enhanced diffusional creep in solid-liquid aggregates which display a dihedral angle in the range 0°

Recent experiments demonstrate that solution-precipitation (pressure-solution) processes in the liquid basalt-olivine system are exceedingly rapid. The effect of liquid basalt on the diffusional (Newtonian) creep rate of polycrystalline olivine is significant; fine-grained, chemically and texturally equilibrated, partially molten assemblages of olivine plus basalt deform at rates which are a factor of 2–5 faster than polycrystalline olivine specimens without the liquid basalt second phase. Measured values of the activation energy for creep suggest that while the kinetics of deformation in this partially molten system are affected by short-circuit diffusion through melt-filled triple junctions, the deformation process is rate-limited by matter transport through melt-free grain boundaries. A simple geometrical model, based on a dynamic equilibrium partial-melt morphology which consists of melt-free olivine grain boundaries coupled with an interconnected melt network along triple junction channels, adequately describes our deformation results. The extent to which the liquid phase penetrates from the triple junctions into the grain boundaries of texturally equilibrated partial melts determines the contribution of solution-precipitation mechanisms to the creep rate; the critical physical parameters, therefore, are those which affect the solid-liquid and solid-solid interfacial energies, such as various composition-related chemical activities in the liquid phase and the composition and spatial distribution of mineral phases in the crystal residuum.

On Venus, understanding of surface-atmosphere interactions resulting from chemical weathering is both critically important for constraining atmospheric chemistry and relative ages of surface features and multifaceted, requiring integration of diverse perspectives and disciplines of study. This paper evaluates the issue of surface alteration on Venus using multiple lines of evidence. Surface chemistry from Venera and Vega landers is inconsistent with significant breakdown from atmospheric interactions, with Fe > Mg toward the oxidizing Venus atmosphere, favoring creation of anhydrite and carbonate-rich surfaces on basalts with minor addition of hematite. When related to Venus-analog experiments, the kinetic calculations suggest a maximum coating of ~30 μm over 500,000 years. These changes would result in a slight overall volume increase in the outermost surface materials, which in turn decreases surface rock FeO contents. Those variations can be detected from orbit because emissivity is correlated with total FeO, and the predicted magnitudes are consistent with Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) observations. Models of anhydrite and hematite coatings on basalt mixtures suggest that changes in emissivity (e) spectra due to chemical weathering can result in ca.