Kei Kurita

Unquenchable high-pressure perovskite polymorphs of MnSnO3 and FeTiO3

New high-pressure orthorhombic (GdFeO3-type) perovskite polymorphs of MnSnO3 and FeTiO3 have been observed using in situ powder X-ray diffraction in a diamond-anvil cell with synchrotron radiation. The materials are produced by the compression of the lithium niobate polymorphs of MnSnO3 and FeTiO3 at room temperature. The lithium niobate to perovskite transition occurs reversibly at 7 GPa in MnSnO3, with a volume change of -1.5%, and at 16 GPa in FeTiO3, with a volume change of -2.8%. Both transitions show hysteresis at room temperature. For MnSnO3 perovskite at 7.35 (8) GPa, the orthorhombic cell parameters are a=5.301 (2) A, b=5.445 (2) Å, c=7.690 (8) Å and V= 221.99 (15) Å3. Volume compression data were collected between 7 and 20 GPa. The bulk modulus calculated from the compression data is 257 (18) GPa in this pressure region. For FeTiO3 perovskite at 18.0 (5) GPa, cell parameters are a=5.022 (6) Å, b=5.169 (5) Å, c=7.239 (9) Å and V= 187.94 (36) Å3. Based on published data on the quench phases, the FeTiO3 perovskite breaks down to a rocksalt + baddelyite mixture of “FeO” and TiO2 at 23 GPa. This is the first experimental verification of the pressure-induced breakdown of a perovskite to simple oxides.

The effect of water on the subcritical crack growth in silicate rocks

Abstract We investigated experimentally the effect of water on the velocity of crack propagation in an andesite and a basalt. The relation between the stress intensity factor KI at the crack tip and the crack velocity V was measured using the double torsion method. Experiments were conducted with the rock specimens either water-saturated or air-dry at laboratory humidity. Stress-dependent, slow crack growth was observed in both rocks used. The velocity of crack growth in water-saturated rocks was shown to be 2 ~ 3 orders of magnitude greater than in the room-dry ones. Our experimental results agree well with a general equation for crack velocity developed by Wiederhorn (1969). The present results suggest that stress-corrosion cracking should be an important process for the mechanism of dilatancy, creep and other characteristic behavior in the polycrystalline silicate rocks.

Petrological constraints on the density of the Martian crust

New insights into the chemistry of the Martian crust have been made available since the derivation of crustal thickness maps from Mars Global Surveyor gravity and topography data that used a conservative range of density values (2700–3100 kg/m3). A new range of crustal density values is calculated from the major element chemistry of Martian meteorites (3100–3700 kg/m3), igneous rocks at Gusev crater (3100–3600 kg/m3) and from the surface concentration of Fe, Al, Ca, Si, and K measured by the Gamma-Ray Spectrometer on board Mars Odyssey (3250–3450 kg/m3). In addition, the density of mineral assemblages resulting from low-pressure crystallization of primary melts of the primitive mantle are estimated for plausible conditions of partial melting corresponding to the Noachian to Amazonian periods (3100–3300 kg/m3). Despite the differences between these approaches, the results are all consistent with an average density above 3100 kg/m3 for those materials that are close to the surface. The density may be compatible with the measured mass of Mars and the moment of inertia factor, but only if the average crustal thickness is thicker than previously thought (approaching 100 km). A thicker crust implies that crustal delamination and recycling could be possible and may even control its thickness, globally or locally. Alternatively, and considering that geoid-to-topography ratios argue against such a thick crust for the highlands, our results suggest the existence of a buried felsic or anorthositic component in the southern hemisphere of Mars.

The relationship between electrical conductivity and melt fraction in a partially molten simple system: Archie's law behavior

Abstract Electrical conductivity in the partially molten state was measured to clarify the relationship between conductivity and the melt fraction. The use of an H 2 O ice-KCl aqueous solution system as an analog of partially molten Earth materials enables us to control the melt fraction precisely. A steep increase in the conductivity was observed at the solidus even when the melt fraction was less than 1 vol.%. The increase in conductivity with the melt fraction follows Archies law: the bulk conductivity normalized by the conductivity of the melt is proportional to a power of the melt fraction. This behavior has been generally observed for sedimentary rocks saturated with water, and is predicted to occur in partially molten materials. Our study confirms this prediction in a simple analog material. Archies law is interpreted in terms of the connectivity of the melt, which is estimated based on a resistor network theory. The electrical conductivity in the partially molten upper mantle is also discussed. Archies law is suitable for use in interpreting geoelectrical data for that region.

Experimental study of pressure dependence of electrical conductivity of olivine at high temperatures

Abstract The electrical conductivity of sintered polycrystalline olivines (Mg 1 − x Fe x ) 2 SiO 4 ( x = 10, 28, 52, 76 and 100%) has been measured at high pressures (2.9–7.0 GPa) and high temperatures (900–1900 K). Measurements at constant temperature and various pressures show that electrical conductivity increases with pressure in the case of high Fe content, whereas it decreases in the case of low Fe content. The results are interpreted as being due to two competing conduction mechanisms: ionic and electronic, which have different pressure dependences.

Experimental studies of He and Ar degassing during rock fracturing

Abstract We studied degassing of He and Ar from granite, basalt and volcanic tuff samples which were subjected to uniaxial compression. From the samples which were fractured in the dilatant region He was always degassed, while degassing of Ar dependent not only on the dilatancy, but also on the type of rock and the compressional conditions such as wet, room dry or vacuum conditions. It is concluded that degassing of rare gases from the compressed samples depends primarily on the generation of new surface area by dilatancy.

The influence of boundary heterogeneity in experimental models of mantle convection

Recent global seismological observations have revealed lateral variations in the thickness of the D 00 layer. We have performed laboratory experiments to explore how undulations of various sizes in the D 00 layer aect convection patterns. We nd that topography on the lower boundary can induce plumes and that there is a critical height above which topography controls convection patterns. Observed undulations in the D 00 layer exceed this critical height, sug- gesting that they may control mantle convection patterns.

On the fate of mantle plumes at density interfaces

Abstract The motion of plumes through compositionally layered systems is investigated by laboratory experiments in order to understand the ascent modes of deep mantle plumes. Experimental plumes entrain the ambient heavier fluid and the average density of the plume head ρP(t) increases with time. The degree of entrainment depends on the viscosity ratio of the matrix fluid μmatrix to the buoyant fluid μbuoyant, and the mass flux. Two types of plume penetration through the density interface are possible. The first, which can be characterized as a ‘pass-through mode’ (PTM), exhibits relatively little entrainment. Here the whole plume head passes directly through the interface because ρP(t) is smaller than the density of the upper layer ρu. In contrast, in the second type of interaction (named the ‘rebirth mode’, RBM), a large entrainment effect takes place. The plume is now greatly disturbed as it passes through the interface between the two fluid layers and entrained material remains at the density interface since ρP(t)>ρu. In addition, a new diapir emerges from the plume conduit and rises through the upper layer. The difference between these two types of interaction is reflected in the spatial and the temporal characteristics of geochemical data obtained from mantle plume products such as Ontong Java Plateau (OJP).

A shallow volatile layer at Chryse Planitia, Mars

We have investigated size distribution of rampart craters in the east edge of Chryse Planitia on Mars by Viking high resolution images. Clear existence of the onset diameter of rampart crater, which defines the minimum size of the rampart crater, has been recognized. If this diameter corresponds to the depth to the top of the volatile layer, the converted depth ranges from ∼20 m to 60 m. These values are systematically shallower than the previous estimates (Kuzmin, 1988). Martian volatile layer is thought as a main reservoir of the ancient fluvial processes and atmospheric water vapor. This shallow volatile layer gives us information of the inventory of Martian water and conditions of cryosphere.

Avalanche-like fluidization of a non-Brownian particle gel

We report on the fluidization dynamics of an attractive gel composed of non-Brownian particles made of fused silica colloids. Extensive rheology coupled to ultrasonic velocimetry allows us to characterize the global stress response together with the local dynamics of the gel during shear startup experiments. In practice, after being rejuvenated by a preshear, the gel is left to age for a time tw before being subjected to a constant shear rate [small gamma, Greek, dot above]. We investigate in detail the effects of both tw and [small gamma, Greek, dot above] on the fluidization dynamics and build a detailed state diagram of the gel response to shear startup flows. The gel may display either transient shear banding towards complete fluidization or steady-state shear banding. In the former case, we unravel that the progressive fluidization occurs by successive steps that appear as peaks on the global stress relaxation signal. Flow imaging reveals that the shear band grows until complete fluidization of the material by sudden avalanche-like events which are distributed heterogeneously along the vorticity direction and correlated to large peaks in the slip velocity at the moving wall. These features are robust over a wide range of tw and [small gamma, Greek, dot above] values, although the very details of the fluidization scenario vary with [small gamma, Greek, dot above]. Finally, the critical shear rate [small gamma, Greek, dot above]* that separates steady-state shear-banding from steady-state homogeneous flow depends on the width of the shear cell and exhibits a nonlinear dependence with tw. Our work brings about valuable experimental data on transient flows of attractive dispersions, highlighting the subtle interplay between shear, wall slip and aging whose modeling constitutes a major challenge that has not been met yet.