Masayasu Tokonami

Dupal anomaly of Brazilian carbonatites: Geochemical correlations with hotspots in the South Atlantic and implications for the mantle source

Abstract Geochemical and Sr, Pb, O and C isotopic data are reported for carbonatite samples from five locations in southeast Brazil. Elemental abundances and δ13CPDB data (between −5.8 and −7.2‰) prove that all the samples are derived from the mantle. Dupal isotopic characteristics in the all carbonatite samples from five locations in southeast Brazil are found in this study, characteristics that have not previously been recognized in carbonatites. Brazilian carbonatites possess average Δ8 4 values of between 101 and 145, average Δ7 4 values of between 5.2 and 10.3, and initial Sr isotopic compositions of between 0.7046 and 0.7062. The Brazilian carbonatites comprise two groups: The northern group is coincident with the passage of the Trindade non-Dupal hotspot at ca. 80 Ma, while the southern group mainly corresponds to the passage of the Tristan de Cunha Dupal hotspot at ca. 130 Ma. Although we expected a geochemical correlation between the Brazilian carbonatites and the South Atlantic hotspots, the enriched isotopic signature (EM1) of all the carbonatite samples is similar to that of alkali basalts on Tristan de Cunha. The combined OSr isotopic diagram indicates that the southern group carbonatites have negligible or only slight crustal contamination. The northern group samples show significantly higher δ18OSMOW values of 9–14‰, more radiogenic Pb isotopic ratios, and a 87 Sr 86 Sr value of 0.705. Even if these signatures are derived from the contamination of a lower crustal component with mantle sources, it is clear that the parental magma also has inherent EM1 isotopic characteristics. The interpretation of the origin of EM1 in the Brazilian carbonatites (subcontinental lithospheric mantle vs. asthenosphere) is dependent on the model of Parana volcanism at ca. 130 Ma, which remains controversial. One possibility is that both the northern and southern carbonatites came from enriched SCLM under a part of Gondwanaland. In this case, a hotspot would provide the thermal energy to melt the lithospheric source region for both the Parana flood basalts and the alkali carbonatitic volcanism. Another possibility is that the source of the northern carbonatites is also the Tristan plume carbonate-rich material which had once been trapped under the crust and reactivated by the Trindade hotspot, on the assumption that Parana volcanism at ca. 130 Ma was mainly triggered by the huge Tristan plume activity. If anything, we favour the latter and believe an asthenospheric mantle plume origin for both the ultimate carbonate source and the Dupal anomaly in the Brazilian carbonatites.

Structure and crystal chemistry of a dense polymorph of tricalcium phosphate Ca3 (PO4)2: A host to accommodate large lithophile elements in the earth's mantle

A phase of Ca3 (PO4)2, synthesized at 12GPa and 2300° C, is structurally analyzed by the single crystal X-ray diffraction method. This Ca3(PO4)2 is found to be a dense polymorph of tricalcium phosphate isostructural with Ba3 (PO4)2 and named γ-Ca3 (PO4)2. In the structure of Ca3 (PO4)2, a phosphorus atom is tetrahedrally coordinated by oxygen atoms and calcium atoms occupy two types of large metal sites. The Ca(1) site has twelve oxygen neighbours with the mean bond length of 2.739 Å while the other Ca(2) site is coordinated by ten oxygen atoms with the mean Ca-O distance of 2.588Å. The structure is characterized by the translationally interconnected polyhedral sequence PO4-Ca(2)O10-Ca (1)O12-Ca (2)O10-PO4 in the direction of the c axis. This dense phase of Ca3(PO4)2 with two large metal sites may be an important host for very large lithophile elements in the deep upper mantle of the earth.

Temperature dependence of intersite distribution of Mg and Fe in olivine and the associated change of lattice parameters

The cation distribution of natural and heated ferromagnesian olivine with chemical composition, Fo67Fa33, from metagabbro was examined by X-ray diffraction. Heating and quenching experiments were made by a newly devised apparatus which enables us to obtain very fast quenching speed in comparison with the usual technique. The distribution constants, KD=(Fe+2/Mg)M1/(Fe+2/Mg)M2, of the natural samples were less than 1.07, and those of heat-treated samples were more than 1.15, indicating that cation ordering takes place with temperature. The distribution of Fe+2 and Mg is nearly random at low temperatures, whereas Fe+2 shows a slight but significant preference for a smaller M1 site at high temperatures. The change of the distribution constant was observed on specimens which were heated for a short period of time (6–1,060 s) and quenched within 10 ms. Thus the rate of the cation reordering reaction is a very fast process. The lattice parameters b and c decrease whereas a increases with the increase of distribution constant. The overall effect on unit cell volume is a decrease with the increasing distribution constant, suggesting the presence of significant pressure dependence of the cation distribution towards the ordering of Fe at M1 site in ferromagnesian olivine.

Crystal structures of high pressure modifications of Mn2GeO4 and Co2SiO4

Abstract The stability field and the crystal structure of β-Mn2GeO4 and the crystal structure of the β-Co2SiO4 have been determined. The β phase of Mn2GeO4 is stable over a wide range of temperature at pressure range intermediate between the fields of α-Mn2GeO4 (olivine-type) and δ-Mn2GeO4(Sr2PbO4-type), another high pressure polymorph. The space group and cell dimensions of β-Mn2GeO4 are Imma and a = 6.025, b = 12.095 and c = 8.752 A . The structure was determined by using the three-dimensional data to R = 0.061. Oxygen atoms are in a cubic close packing as in the spinel structure. Mn and Ge atoms are within octahedra and tetrahedra of oxygen atoms respectively. GeO4 tetrahedra share one of their oxygen atoms resulting in a Ge2O7 group and an oxygen atom which is not bounded to any Ge atom. The structure of β-Co2SiO4 is isostructural with that of β-Mn2GeO4. The structure of the β phase is compared with that of the γ phase (spinel-type) and their stability is discussed.

Anharmonic thermal vibrations of atoms in MgAl2O4 spinel at temperatures up to 1933 K

Anharmonic thermal vibrations of atoms in MgA120 4 spinel (Fd3m, Z = 4) have been studied using sets of accurate X-ray diffraction intensities of Bragg reflections at temperatures up to 1933 K. The study was initiated by refining the temperature factor T(Q) of each atom in the form of the second-rank tensor/3~j, the ellipsoidal model of harmonic vibration being assumed. Difference Fourier maps based on the structure factors obtained from the refinements at 293 K (R=0.0197), 1503 K (R=0.0413), 1663 K (g = 0-0432) and 1933 K (R =0.0431) revealed residual electron densities around the atoms at the tetrahedral site A (point symmetry 43m) and octahedral site B (3m) and also around the O atoms (3m), the models of occurrence suggesting anharmonic atomic thermal vibration. The observed intensities lob s (=IBragg+ ITOS) were corrected for thermal diffuse scattering to extract the real intensities of the Bragg reflections. The correction was based on the one-photon and first-order acoustic vibration model. For the anharmonic refinement, a cumulant expansion of T(Q) in

High temperature X-ray study of single crystal stishovite synthesized with Li2WO4 as flux

Single crystal stishovite with a square prismatic habit and maximum length 0.8 mm was grown from α-quartz at 120 kbar and ∼1,300° C. Li2WO4, chosen as a result of a previous experiment in growing coesite, was also successful as flux for stishovite. Single crystal X-ray structure analysis of the crystals thus obtained has been carried out at high temperatures under ambient pressure. Lattice constant measurements give a larger thermal expansion coefficient along the a-axis than along the c-axis. The bond distances and bond angles show a decreasing distortion of the SiO6 octahedron with increasing temperature. The increasing amplitude of thermal vibrations of oxygen atoms with increasing temperature results in increasing O-O repulsion in the basal plane, which explains the observed crystallographic changes.

The anharmonic thermal vibration in ZnX (X = S, Se, Te) and its dependence on the chemical-bond characters

The X-ray diffraction study of anharmonic thermal vibration was carded out with respect to ZnX (X = S, Se, Te), having a zinc-blende structure, whose latticedynamical properties such as force constant and effective charge were determined by Raman spectroscopy. Difference Fourier syntheses based on harmonic refinements after correction of the diffraction intensities for thermal diffuse scattering exhibit a tetrapod shape of residual electron density around the Zn atoms, implying an anisotropy of the anhar

Coexisting sodic augite and omphacite in a Sanbagawa metamorphic rock, Japan

Coexisting sodic augite and omphacite were found in a zoisite amphibolite from the Iratsu epidote amphibolite mass in the Sanbagawa metamorphic terrain of central Shikoku, Japan. The occurrences of the sodic augite-omphacite pairs are classified into four types by texture: independent, composite, intergrowth and exsolution types. Sodic augite and omphacite of the independent and composite types (pair A) have XNa (=Na/(Na + Ca)) = 0.15 and 0.35, respectively, and were stable in the epidote amphibolite facies during the Sanbagawa progressive metamorphism. On the other hand, XNa values of sodic augite and omphacite of the intergrowth and exsolution types (pair B) are 0.10 and 0.44, respectively. The Na-poor augite and Na-rich omphacite of the pair B were formed by re-equilibration of the pair A at lower temperature. The pair A of the Iratsu sample suggests that a compositional gap lies between sodic augite and C2/c omphacite under epidote amphibolite facies conditions, and is in marked contrast to the coexistence of sodic augite and P2/n omphacite reported from some low-grade, high-pressure metamorphic terrains. A possible phase diagram to explain the chemistry and mode of occurrence of the coexisting sodic pyroxenes is proposed.

Interfacial layers of high-barrier Schottky diode of Al/n-type (100)Si exposed to H2 plasma

Al/Si Schottky diodes were fabricated from n-type silicon wafers which were exposed to several kinds of plasma in a magnetron using gases of hydrogen, deuterium, helium, nitrogen, oxygen and argon. The influence of plasma exposure on the Schottky barrier height is examined. The Schottky barrier height increases for only H2-containing plasma exposure. Hydrogen plays an important role in the increase of the Schottky barrier height. The plasma-exposed silicon surfaces are characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and secondary ion mass spectrometry. H2 plasma exposure produces both a surface hydrogen-absorbing zone about 10 nm thick and a zone including planar defects under the hydrogen-absorbing zone. In Ar plasma exposure, the only zone including planar defects is formed from the surface to 30 nm in Si. The increase in the Schottky barrier height can be attributed to the formation of the hydrogen-absorbing zone in Si.

On "Lattice Image" in Electron Micrographs

Assuming a plate-shaped crystal in which a lattice plane with the spacing d is perpendicular to the crystal surfaces and the potential varies according to the harmonic form V(x)=2V1cos (2πx/d) in direction x perpendicular to the lattice plane, the lattice image of electrons on the exit surface was calculated by using the exact solution of Schrodinger equation for electron waves in the crystal. The values of d and V1 were assumed to be 13.0 A and 2.14 volts, respectively. These values are those approximately appropriate to (001) of Pt-Phthalocyanine. Calculation was made both for the case of normal incidence of primary electrons and the case in which Bragg condition is satisfied. Wavelength of electrons was assumed to be 0.0603 A corresponding to the accelerating voltage 41.5 kV. A number of aspects of the lattice image which take place by the participation of higher order reflections were revealed. By the effect of higher order reflections, the peaks in the lattice image become much pronounced for the crystal thicker than about 300 A, while, on the contrary, the intensity profile turns out to be rather flat for the crystal thinner than about 150 A, in accordance with the trend expected from the phase grating theory.