Kinetics of the olivine→spinel transition: Implications to deep-focus earthquake genesis

The rate of the olivine→spinel transition at high overpressure increases with diminishing grain size, or increasing shear stress, temperature, and possibly pressure. The transition rate is higher in Fe-rich compositions than in Mg-rich compositions, and it can be greatly increased by adding water or other mineralizers. Of all variables controlling the kinetics of the olivine→spinel transition in the mantle, temperature is the most critical. The olivine→spinel transition can be suppressed below 500°C in Mg-rich compositions, even in geological period of time. Since the temperature within a downgoing slab varies greatly according to different models of calculation, it is not clear at this stage whether the temperature is low enough to suppress the olivine→spinel transition. If the olvine→spinel transition cannot be suppressed, it may not be responsible for the genesis of deep-focus earthquakes. However, the rise of the olivine-spinel boundary in the cold interior of downgoing slabs provides an additional driving force for the plunging of these slabs. The distortion of the olivine-spinel boundary may also control the stress distribution in downgoing slabs and may be responsible for the observed alignment of principal stress axes of deep-focus earthquakes.     

Hugoniot equation of state of olivine and its geodynamic implications

Large olivine samples were hot-pressed synthesized for shock wave experiments. The shock wave experiments were carried out at pressure range between 11 and 42 GPa. Shock data on olivine sample yielded a linear relationship between shock wave velocity D and particle velocity u described by D=3.56(?0.13)+2.57(?0.12)u. The shock temperature is determined by an energy relationship which is approximately 790°C at pressure 28 GPa. Due to low temperature and short experimental duration, we suggest that no phase change occurred in our sample below 30 GPa and olivine persisted well beyond its equilibrium boundary in metastable phase. The densities of metastable olivine are in agreement with the results of static compression. At the depth shallower than 410 km, the densities of metastable olivine are higher than those of the PREM model, facilitating cold slab to sink into the mantle transition zone. However, in entire mantle transition zone, the shock densities are lower than those of the PREM model, hampering cold slab to flow across the "660 km" phase boundary.     

Experimental investigation of phase transformations of olivine and enstatite at the lower part of the mantle transition zone: Implications for structure of the 660 km seismic discontinuity

High-pressure polymorphs of olivine and enstatite are major constituent minerals in the mantle transition zone(MTZ).The phase transformations of olivine and enstatite at pressure and temperature conditions corresponding to the lower part of the MTZ are import for understanding the nature of the 660 km seismic discontinuity.In this study,we determine phase transformations of olivine(MgSi2O4) and enstatite(MgSiO3) systematiclly at pressures between 21.3 and 24.4 GPa and at a constant temperature of 1600℃.The most profound discrepancy between olivine and enstatite phase transformation is the occurency of perovskite.In the olivine system,the post-spinel transformation occures at 23.8 GPa,corresponding to a depth of 660 km.In contrast,perovskite appears at 23 GPa(640 km) in the enstatite system.The ~1 GPa gap could explain the uplifting and/or splitting of the 660 km seismic discountinuity under eastern China.     

Hematite grains: Size and coercive force from AF demagnetization at high temperatures

The coercivity spectrum of low-field high-temperature partial thermoremanent magnetization (PTRM) of a synthetic hematite powder, extremely high at room temperature, decreases very slowly with increasing temperature up to 500°C then decreases rapidly, especially above 600°C. From the AF demagnetization curves at 600 and 650°C it is calculated, following the Néel's theory of single-domain particles that the grains carrying the PTRM have a mean coercive force of 23 ± 5 kOe and a mean grain size of 0.40 ± 0.15 μm, which is not significantly different from the mean grain size of 0.48 ± 0.03 μm from electron micrograph observations.     

Laboratory measurements of ‘porosity‐free’ intrinsic Vp and Vs in an olivine gabbro of the Oman ophiolite: Implication for interpretation of the seismic structure of lower oceanic crust

In order to determine ‘porosity‐free’ intrinsic ultrasonic compressional (Vp) and shear wave (Vs) velocities and Vp/Vs of an olivine gabbro from the Oman ophiolite, we developed a new experimental system using a piston‐cylinder type high‐pressure apparatus. The new system allowed us to measure velocities at pressures ranging from 0.20 to 1.00 GPa and at temperatures up to 300°C for Vp and 400°C for Vs. At room temperature, the Vp and Vp/Vs increase rapidly with pressure up to 0.40 GPa, while between 0.45 and 1.00 GPa the increase is more gradual. The change in increasing rate is attributed to closure of porosity at pressures above 0.45 GPa. Based on the linear regression of data obtained at higher pressures (0.45–1.00 GPa) and extrapolation to the lower pressures, combined with temperature derivatives of velocities of the sample measured at 1.00 GPa, we determined the intrinsic Vp and Vs of the sample as a function of pressure (P, in GPa) and temperature (T, in °C). The intrinsic velocities can be expressed as Vp (km/s) = 7.004 + 0.096 × P ? 0.00015 × T, and Vs (km/s) = 3.827 + 0.007 × P ? 0.00008 × T. We evaluated the intrinsic Vp and Vs of the olivine gabbro at oceanic crustal conditions and compared them with a velocity depth‐profile of the borehole seismic observatory WP‐2 area in the northwestern Pacific Basin. Although the intrinsic Vp (～7.0 km/s) and Vs (～3.8 km/s) for the olivine gabbro studied are comparable to those of seismic layer 3 in the WP‐2 area, the estimated vertical gradients of intrinsic velocities are significantly smaller than those reported from layer 3. These results suggest that velocity profiles of layer 3 in the WP‐2 area may reflect the presence of a minor porosity in lower oceanic crust, which closes with increasing depth and/or continuous changes in mineralogy of layer 3 rocks.     

The November 2002 eruption of Piton de la Fournaise,Réunion: tracking the pre-eruptive thermal evolution of magma using melt inclusions

The November 2002 eruption of Piton de la Fournaise in the Indian Ocean was typical of the activity of the volcano from 1999 to 2006 in terms of duration and volume of magma ejected. The first magma erupted was a basaltic liquid with a small proportion of olivine phenocrysts (Fo₈₁) that contain small numbers of melt inclusions. In subsequent flows, olivine crystals were more abundant and richer in Mg (Fo_83–84). These crystals contain numerous melt and fluid inclusions, healed fractures, and dislocation features such as kink bands. The major element composition of melt inclusions in this later olivine (Fo_83–84) is out of equilibrium with that of its host as a result of extensive post-entrapment crystallization and Fe²⁺ loss by diffusion during cooling. Melt inclusions in Fo₈₁ olivine are also chemically out of equilibrium with their hosts but to a lesser degree. Using olivine–melt geothermometry, we determined that melt inclusions in Fo₈₁ olivine were trapped at lower temperature (1,182 ± 1°C) than inclusions in Fo_83–84 olivine (1,199–1,227°C). This methodology was also used to estimate eruption temperatures. The November 2002 melt inclusion compositions suggest that they were at temperatures between 1,070°C and 1,133°C immediately before eruption and quenching. This relatively wide temperature range may reflect the fact that most of the melt inclusions were from olivine in lava samples and therefore likely underwent minor but variable amounts of post-eruptive crystallization and Fe²⁺ loss by diffusion due to their relatively slow cooling on the surface. In contrast, melt inclusions in tephra samples from past major eruptions yielded a narrower range of higher eruption temperatures (1,163–1,181°C). The melt inclusion data presented here and in earlier publications are consistent with a model of magma recharge from depth during major eruptions, followed by storage, cooling, and crystallization at shallow levels prior to expulsion during events similar in magnitude to the relatively small November 2002 eruption.     

Experimental investigation of CaMg exchange between olivine,orthopyroxene, and clinopyroxene: potential for geobarometry

The distribution of Ca and Mg among coexisting olivine, clinopyroxene and orthopyroxene has been studied in a piston-cylinder apparatus in the temperature range 1100–1300°C and pressure range 9–41 kbar. Ca in olivine decreases with increasing pressure and decreasing temperature. The pressure effect is the result of Ca going into the higher-coordination M2 site in clinopyroxene as pressure is increased. For the CaMg exchange reaction between olivine and clinopyroxene, ΔV°=0.249J bar^?1 mole^?1; this is sufficient for pressure estimates accurate to ±3kbar if temperatures of equilibration are independently known. CaMg exchange between olivine and orthopyroxene is not sufficiently pressure dependent to be used as a geobarometer.Application of the olivine-clinopyroxene geobarometer to coarse garnet lherzolites from southern Africa gives P-T results consistent with a continental geotherm. For spinel lherzolites from southwestern United States, the geotherm appears to be displaced to higher temperatures indicating oceanic affinities. Application of the geobarometer to natural systems requires assumptions about activity relationships in clinopyroxene which should be checked by experiment.     

Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks,Japan, a possible high-pressure dehydration product of antigorite serpentinite

Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks (HMR), west-central Kyushu, Japan, may be formed by high-pressure dehydration of antigorite, and may indicate deep subduction of serpentinite reaching a pressure–temperature condition of 1.6 GPa and 740–750 °C. Three rock types have been identified based on mineral assemblage and rock texture: Type I (L) consisting of medium-grained (1–5 cm long) olivine + enstatite + chromite ±tremolite with secondary talc and anthophyllite that occurs in low-grade metamorphic rocks of the biotite zone, Type I (H) of coarse-grained (up to 10 cm long) olivine + enstatite (with clinoenstatite lamella) + chromite ±tremolite with secondary talc that occurs in high-grade metamorphic rocks of the garnet-cordierite zone, and Type II composed of Al-spinel + chlorite + olivine + apatite + ilmenite with minor sodic gedrite in the garnet-cordierite zone together with Type I (H). Olivines in all rock types are mostly serpentinized during exhumation. The chromite-olivine thermometer gives 560–690 °C for Type I (L) rocks, and the spinel-olivine thermometer gives 610–740 °C for Type II rocks. The peak metamorphic pressure will be higher than 1.6 GPa based on the location of the experimentally determined invariant point (P = 1.6 GPa and T = 670 °C) of antigorite + forsterite + enstatite + talc + H₂O. This estimate is consistent with the occurrence of chlorite in Type II rocks, which is stable up to 890 °C at 2.0 GPa. The spinifex-like textured metaperidotites occur as small bodies in the low P/T type gneisses, implying tectonic juxtaposition of them probably during exhumation of the HMR. Recent findings of medium pressure (0.9–1.2 GPa) granulites and gneisses from the HMR may indicate that the HMR has a deep root into the wedge mantle from which the spinifex-like textured metaperidotites have derived.     

TEMPERATURE EFFECTS ON AIRGUN SIGNATURES1

Experiments in an 850 litre water tank were performed in order to study temperature effects on airgun signatures, and to achieve a better understanding of the physical processes that influence an airgun signature. The source was a bolt airgun with a chamber volume of 1.6 cu.in. The pressure used was 100 bar and the gun depth was 0.5 m. The water temperature in the tank was varied between 5°C and 45°C. Near-field signatures were recorded at different water temperatures. Typical signature characteristics such as the primary-to-bubble ratio and the bubble time period increased with increasing water temperature. For comparison and in order to check whether this is valid for larger guns, computer modelling of airguns with chamber volumes of 1.6 and 40 cu.in. was performed. In the modelling the same behaviour of the signatures with increasing water temperature can be observed. The increase in the primary-to-bubble ratio and the bubble time period with increasing water temperature can be explained by an increased mass transfer across the bubble wall.     

Inferences on the lunar temperature from gravity,stress state and flow laws

Inferences on the lunar temperature regime are made from the inversion of gravity for density anomalies and the stress-state of the Moon's interior, and by comparing these results with flow laws and estimates of likely strain-rates.The nature of the spectrum of the lunar gravitational potential indicates that the density anomalies giving rise to the potential are mainly of near-surface origon. The average stress-differences in the lunar mantle required to support these density anomalies are of the order of a few tens of bars and have persisted for more than 3 · 10⁹ years. If current flow laws for dry olivine can be extrapolated to the conditions of the lunar mantle, and the selenotherms based on electrical conductivity models are valid, the strain rates are too high to explain the preservation of the lateral near-surface density anomalies. We suggest that the present temperatures in the Moon are relatively low, of the order of 800°C or less, at a depth of about 300 km. This compares with 1100°C based on electrical conductivity models and is near the lower limit predicted by Keihm and Langseth (1977) from lunar heat-flow observations.     

Frequency dependence of elastic wave speeds at high temperature: a direct experimental demonstration

At sufficiently high temperatures and/or long periods, the elastic behaviour of crystalline material gives way progressively to viscoelastic behaviour associated with the stress-induced migration of crystal defects. This transition is marked by the onset of appreciable strain energy dissipation accompanied by frequency dependence (dispersion) of the shear modulus and elastic wave speeds. Ultrasonic interferometry and torsional forced-oscillation techniques can be used to probe the low-amplitude stress-strain behaviour of fine-grained polycrystalline material in two very different frequency ranges, respectively 10-100 MHz and 1 mHz-1 Hz. Here we demonstrate and apply these two complementary methods in a study of the high-temperature mechanical behaviour of a fine-grained synthetic olivine polycrystal. At the high frequencies of ultrasonic interferometry, the shear wave speed varies linearly with temperature between room temperature and the highest experimentally accessible temperature (1300 °C) in close accord with expectations based on similarly high-frequency studies of the elastic behaviour of single-crystal olivine. However, at teleseismic frequencies (<1 Hz) and temperatures >900 °C, the shear wave speed becomes much more strongly temperature-sensitive reflecting markedly viscoelastic behaviour. Newly emerging laboratory-derived constraints on this viscoelastic enhancement of the temperature sensitivity of seismic wave speeds and its grain-size dependence will provide a more robust interpretation of seismological models for the variation of wave speeds and attenuation within the Earth's interior.     

Experimental studies on elastic and rheological properties of amphibolites at high pressure and high temperature

The experimental studies done at high temperature and high pressure find that increased temperature can lead to dramatic velocity and strength reductions of most of rocks at high confining pressure[1,2]. What causes this phenomenon? Is it due to dehydrati…     

The importance of cross-calibration and protecting water temperature sensors against direct solar radiation heating in hydrological studies

ABSTRACT

Water temperature monitoring is important in many scientific studies. This study compares three models of water temperature sensors (Vemco Minilog II, HOBO TidbiT and HOBO Pendant) to test the importance of: (1) using cross-calibration to minimize relative errors, (2) using cross-calibration to improve on the accuracy of less accurate sensors, and (3) protecting sensors against direct solar radiation heating. The results show that when the same sensor models are cross-calibrated, the relative error can be reduced (Vemco: 0.01°C; HOBO TidbiT: 0.02°C; and HOBO Pendant: 0.07°C). Cross-calibration can also improve less accurate sensors (HOBO TidbiT and Pendant) to similar accuracies of Vemco (±0.1°C). Finally, no evidence of solar radiation heating was observed for Vemcos (unprotected); however, HOBO TidbiT and Pendant showed heating up to 2°C (maximum). When HOBO TidbiT and Pendant are shielded (flow-through system), heating is no longer an issue.     

Recently formed elastic anisotropy and petrological models for the continental subcrustal lithosphere in southern Germany

This paper advances new evidence for elastic anisotropy in the continental subcrustal lithosphere in southern Germany. The range of petrological models compatible with the observed azimuthal variation of seismic P-wave velocity is explored. The azimuthal distribution of amplitudes of mantle phases and the observed increase of P velocity with depth both indicate a continuation of anisotropy with depth together with an increase of preferred orientation. Even depletion of the upper mantle in basaltic components, as suggested by mantle xenoliths from various parts of Germany, cannot explain the velocity-depth and azimuthal amplitude observations without an increase of anisotropy with depth.Preferred orientation of olivine is the most likely mechanism for the observed phenomena. Its fast a-axis at the Moho level is directed towards N22.5°E. The b-axis is also required to be horizontal; i.e., the b-plane, one of the preferred glide planes of olivine, is vertical, with a strike of N22.5°E. Therefore, this preferred glide plane of olivine practically coincides with the plane of maximum horizontal shear stress deduced from fault-plane solutions of earthquakes in western Germany. This is a strong indication that the preferred orientation of olivine is formed in the recent West European crustal stress field leaking into the upper mantle. The distribution of velocities to a depth of at least 50 km requires slight horizontal rotation of the a-axis with depth by ～ 10° towards N32°E, and a change in the modal composition towards a depletion increasing with depth compatible with the composition of mantle xenoliths from western Germany. Further experiments are needed to substantiate this suggestion, which could lead to a better understanding of the interaction of crustal and upper-mantle stress-strain fields.     

Disproportionation of Ni2SiO4 to stishovite plus bunsenite at high pressures and temperatures

Samples of Ni₂SiO₄ in both olivine and spinel phases have been compressed to pressures above 140 kbar in a diamond-anvil cell and heated to temperatures of 1400–1800°C using a continuous YAG laser. After quenching and releasing pressure, X-ray diffraction examination indicates that the samples disproportionate to a mixture of stishovite (SiO₂) and bunsenite (NiO) at pressures between 140 and 190 kbar. The exact disproportionation pressure is not certain due to transient increases in pressure during the local and rapid heating. However, thermodynamic calculations suggest that the transition pressure is about 192 ± 4 kbar at 1545°C and that the equation of the spinel-mixed oxides phase boundary isP(kbar) = 121 + (0.046 ± 0.020) T (°C).     

Crystallization of tholeiitic basalt in Alae Lava Lake,Hawaii

The eruption of Kilauea Volcano August 21–23, 1963, left 600,000 cubic meters of basaltic lava in a lava lake as much as 15 meters deep in Alae pit crater. Field studies of the lake began August 27 and include repeated core drilling, measurements of temperature in the crust and melt, and precise level surveys of the lake surface. The last interstitial melt in the lake solidified late in September 1964; by mid August 1965 the maximum temperature was 690°C at a depth of 11.5 meters. Pumice air-quenched from about 1140°C contains only 5 percent crystals — clinopyroxene, cuhedral olivine (Fo ₈₀), and a trace of plagioclase, (An ₇₀). Drill cores taken from the zone of crystallization in the lake show that olivine continued crystallizing to about 1070°C; below that it reacts with the melt, becoming corroded and mantled by pyroxene and plagioclase. Below 1070°C, pyroxene and plagioclase crystallized at a constant ratio. Ilmenite first appeared at about 1070°C and was joined by magnetite at about 1050°C; both increased rapidly in abundance to 1000°C. Apatite first appeared as minute needles in interstitial glass at 1000°C. Both the abundance and index of refraction of glass quenched from melt decreased nearly linearly with falling temperature. At 1070°C the quenched lava contains about 65 percent dark-brown glass with an index of 1.61; at 980°C it contains about 8 percent colorless glass with an index of 1.49. Below 980°C, the percentage of glass remained constant. Progressive crystallization forced exsolution of gases from the melt fraction; these formed vesicles and angular pores, causing expansion of the crystallizing lava and lifting the surface of the central part of the lake an average of 19.5 cm. The solidified basalt underwent pneumatolitic alteration, including deposition of cristobalite at 800°C, reddish alteration of olivine at 700°C, tarnishing of ilmenite at 550°C, deposition of anhydrite at 250°C, and deposition of native sulfur at 100°C. Ferric-ferrous ratios suggest that oxidation with maximum intensity between 550°C and 610°C moved downward in the crust as it cooled; this was followed by reduction at a temperature of about 100°C. The crystallized basalt is a homogeneous fine-grained rock containing on the average 48.3 percent by volume intergranular pyroxene (augite > pigeonite), 34.2 percent plagioclase laths (An_{60 70}), 7.9 percent interstitial glass, 6.9 percent opaques (ilmenite > magnetite), 2.7 percent olivine (Fo_{70 80}), and a trace of apatite. Chemical analyses of 18 samples, ranging from initially quenched pumice to lava cored more than a year after the eruption from the center and from near the base of the lake, show little variation from silica-saturated tholeiitic basalt containing 50.4 percent SiO₂, 2.4 percent Na₂O, and 0.54 percent K₂O. Apparently there was no significant crystal settling and no appreciable vapor-phase transport of these components during the year of crystallization. However, seven samples of interstitial liquid that had been filter-pressed into gash fractures and drill holes from partly crystalline mush near the base of the crust show large differences from the bulk composition of the solidified crust—lower MgO, CaO, and Al₂O₃; and higher total iron, TiO₂, Na₂O, K₂O, P₂O₅, and F, and, in most samples, SiO₂. The minor elements Ba, Ga, Li, Y, and Yb and possibly Cu tend to be enriched in the filter-pressed liquids, and Cr and possibly Ni tend to be depleted.     

Effects of shock pressure and temperature on titanomagnetite from ICDP cores and target rocks of the El’gygytgyn impact structure,Russia

The aim of this study was to investigate the effect of meteorite impacts on magnetic properties including magnetic susceptibility and the Verwey transition of Ti-poor titanomagnetite of volcanic rocks from the 3.6 Ma old El’gygytgyn impact structure located in the Okhotsk-Chukotka volcanic belt in north-eastern Russia. The target rocks consist mainly of rhyolite with some andesites, and is a rare example of impact structures within volcanic target rocks on Earth. 27 samples from outside the crater, the crater rim and from the depth interval 316 to 517 m below lake bottom (mblb) of the El’gygytgyn ICDP drilling were studied. A significant decrease of the average specific magnetic susceptibility by around 90% was observed between felsic volcanic rocks from the surface (18.1 × 10^-6 m³/kg) and the drill cores from near the crater central uplift (1.9 × 10^-6 m³/kg). Ferrimagnetic Fe-Ti oxide assemblages (Verwey transition temperature, T_V: -161 to -150°C, Curie temperature, T_C: 451 to 581°C), occurring in all studied samples, differ significantly. At the surface titanomaghemite is ubiquitously associated with titanomagnetite. The drill cores lack titanomaghemite, but either show a transformation into titanomagnetite and ilmenite or a strong fragmentation associated with a second T_V between -172 and -188°C. Reversible curves of temperature dependence of magnetic susceptibility in the suevite indicate high depositional temperatures of at least 500°C. In the polymict and monomict impact breccia mechanical deformation of titanomagnetite and temperatures of at least 200-350°C related to the shock are suggested from temperature dependent magnetic susceptibility cycling. Lowtemperature oxidation along strongly brecciated grain surfaces in titanomagnetite is suggested to cause the lower TV and we suggest that this phenomenon is related to postimpact hydrothermal activity. The strong magnetic susceptibility decrease at El’gygytgyn is mainly influenced by shock, and post-impact hydrothermalism causes a significant additional depletion. These observations explain why magnetic lows are a ubiquitous phenomenon over impact structures.     

Stream temperature response to climate change and water diversion activities

Stream temperature is an important control of many in-stream processes. There is rising concern about increases in stream temperature with projected climate changes and human-related water activities. Here, we investigate the responses to climate change and water diversions in Eel River basin. The increase in stream temperatures is considered to be the result of changes in air temperature, the proportion of base flow and the amount of stream flow derived from historical and future simulations using the integrated VIC hydrologic model and ANN stream temperature model. The results show that stream temperature will increase throughout the basin in the future under two climate change representative concentration pathways (RCPs 4.5 and 8.5) and will also be influenced by the water diversion activities schedules. Specifically, the stream temperature increases, in the late twenty-first century under RCP8.5 scenarios, from 1.20 to 2.40 °C in summer and from 0.58–3.46 °C in winter respectively; Water diversion activities in Eel River Basin can increase nearly 1 °C in stream temperature. Therefore, both climate change and water diversion activities can substantially cause the rise of more than 2 °C in stream temperature. In conclusion, stream temperature is mainly sensitive to the proportion of base flow in summer, but also the change of the amount of stream flow in winter in our case study area. In addition, it should be noted that the low intensity irrigation schedule has lower impacts on increasing stream temperature, whereas the high intensity irrigation schedule will further exacerbate the rise of stream temperature. Understanding the different impacts of climate change scenarios and irrigation schedules on stream temperature can help identify climate-sensitive regions, climate-sensitive seasons and water diversion schedules as well as assist in planning for climate change and social adaptive management.     

Magnetic and crystallographic properties of synthetic titanomaghemite

Systematic magnetic and crystallographic studies were made on 57 titanomaghemite samples produced from sintered titanomagnetites with x values of 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0. Curie temperature and lattice parameter values are often significantly different from results of previous studies (Ozima and Sakamoto, 1971; Readman and O'Reilly, 1972; Nishitani, 1979), particularly for x values of 0.4 and 0.6. Other important results are: (1) Oxidation of titanomagnetite is mainly a function of temperature. Atmosphere and Ti content have little influence. (2) Progressive oxidation of homogeneous titanomaghemites can take place in the presence of a rhombohedral phase. The amount of rhombohedral phase produced during oxidation increases with increasing temperature and (less conclusively) with decreasing Ti content. (3) Saturation magnetization measurements at low temperatures show both P- and Q-type ferrimagnetic behavior in titanomaghemites, and also show the Verwey (?) transition in titanomaghemites with composition x=0.0. (4) The temperature of spinel inversion increases with degree of oxidation. Slightly oxidized samples invert near 300°C; for samples with z > 0.8 the inversion temperature is above 450°C. This last result, although neither expected nor understood, is supported by results of thermomagnetic studies on some oceanic basalts.     

The influence of oxidation state on the electrical conductivity of olivine

The electrical conductivity of a single crystal of San Carlos olivine (Fo₉₂, 0.16 wt.% Fe₂O₃) has been measured as a function of temperature and oxygen fugacity (fO₂). After heating to 1338°C at fO₂ = 10^?12 atm., the conductivity at 950°C was 10^?5 (ohm-m)^?1, almost 3 orders of magnitude less than that measured in air. This decrease is due to the reduction of Fe³⁺ to Fe²⁺. Further heating to 1500°C at fO₂ = 10^?14 atm., decreased the electrical conductivity at 950°C to 10^?6 (ohm-m)^?1. When recovered at room temperature, the speciment had Fo₉₆ composition and contained small, opaque blebs distributed throughout the crystal. Derivations of temperature profiles for the earth's mantle from conductivity-depth models must take account of the important role played by iron oxidation state in the electrical conductivity of olivine.