Chemical anomalies of lunar plagioclase,described by substitution vectors and their relation to optical and structural properties

Lunar plagioclase posed an enigma because optical properties indicate slow cooling while structural properties suggest rapid quenching when compared directly to terrestrial plagioclase. If, however, the An-content is inferred from the Al/Si ratio rather than from the Ca/Na ratio, lunar plagioclase corresponds more closely to normal volcanic plagioclase. The differences between An=4(Al/(Al+Si))-1 and An=Ca/(Ca+Na+K) is caused by substitutions which appear so far to be unique to lunar feldspars. A vector representation of chemical composition makes it possible to single out several substitutions which account for the chemical peculiarities of lunar plagioclase. It permits to assign sites in the structure to various minor elements simply from the chemical analysis without having to rely on sophisticated spectroscopic techniques or an X-ray structure determination. 1. The deficiency in Al+Si (up to 0.06 per formula unit) is compensated for by Fe, Mg, and small amounts of Na or Ca occupying tetrahedral sites. The Ca/Na ratio is increased accordingly to balance the electric charge. This substitution accounts for most of the differences. 2. The vacancycoupled substitution Ca+□ → 2 Na (Weill et al., 1970) is another cause for the increased Ca/Na ratio. 3. The amount of alkali evaporation from plagioclase, if it occurred, is probably small. These unusual substitutions are an expression of special conditions on the moon during the crystallization of plagioclase. Crystallization at high temperatures, rapid cooling, and peculiar magma composition seem to be important factors, but experimental work and detailed study of terrestrial volcanic plagioclase are necessary to interpret the chemical composition quantitatively.     

Superstructures in ankerite and calcite

Two rhombohedral carbonates, ferroan dolomite (ankerite) and magnesian calcite from lower Jurassic ammonites, have been studied by transmission electron microscopy. The samples show small domains with the morphology of platelets parallel to (11 \(\bar 2\) 0) which have exceptionally well ordered superstructures and characteristic c-reflections. Diffraction patterns and contrast analysis of atomic resolution images indicate that the domains observed in ankerite are rich in Ca [Ca_0.75(Mg, Fe)_0.25CO₃] and those in calcite rich in Mg, and there is strong evidence that cation ordering is the cause for the observed superstructures which are proposed mainly on diffraction evidence. Rhombohedral carbonates have a close similarity to NaCl, with CO₃-groups substituting for Cl. Cation ordering patterns can therefore be discussed in analogy to f.c.c. alloys. We make use of ordering waves to describe superstructures and to qualitatively interpret kinetic conditions for their formation.     

Texture development and deformation mechanisms in ringwoodite

Ringwoodite Mg₂SiO₄ with spinel structure is an important phase in the earth's mantle transition zone. Controlled deformation experiments showed that ringwoodite underwent ductile deformation when compressed axially at 6-10 GPa and at room temperature in a multianvil D-DIA deformation apparatus. Texture evolution during cyclic compression has been recorded in situ using X-ray transparent anvils with monochromatic synchrotron X-ray diffraction and a two-dimensional detector. Quantitative analysis of the images with the Rietveld method revealed a 1 1 0 fiber texture. By comparing this texture pattern with polycrystal plasticity simulations, it is inferred that {1 1 1}〈 1 0〉 slip is the dominant deformation mechanism in ringwoodite, consistent with high temperature mechanisms observed in other spinel-structured materials. Although strong ringwoodite textures may develop in the transition zone, the contribution to bulk anisotropy is minimal due to the weak single-crystal anisotropy.     

Nanoscale porosity in SAFOD core samples (San Andreas Fault)

With transmission electron microscopy (TEM) we observed nanometer-sized pores in four ultracataclastic and fractured core samples recovered from different depths of the main bore hole of the San Andreas Fault Observatory at Depth (SAFOD). Cutting of foils with a focused ion beam technique (FIB) allowed identifying porosity down to the nm scale. Between 40 and 50% of all pores could be identified as in-situ pores without any damage related to sample preparation. The total porosity estimated from TEM micrographs (1–5%) is comparable to the connected fault rock porosity (2.8–6.7%) estimated by pressure-induced injection of mercury. Permeability estimates for cataclastic fault rocks are 10^? 21–10^? 19 m² and 10^? 17 m² for the fractured fault rock. Porosity and permeability are independent of sample depth. TEM images reveal that the porosity is intimately linked to fault rock composition and associated with deformation. The TEM-estimated porosity of the samples increases with increasing clay content. The highest porosity was estimated in the vicinity of an active fault trace. The largest pores with an equivalent radius > 200 nm occur around large quartz and feldspar grains or grain-fragments while the smallest pores (equivalent radius < 50 nm) are typically observed in the extremely fine-grained matrix (grain size < 1 μm). Based on pore morphology we distinguish different pore types varying with fault rock fabric and alteration. The pores were probably filled with formation water and/or hydrothermal fluids at elevated pore fluid pressure, preventing pore collapse. The pore geometry derived from TEM observations and BET (Brunauer, Emmett and Teller) gas adsorption/desorption hysteresis curves indicates pore blocking effects in the fine-grained matrix. Observations of isolated pores in TEM micrographs and high pore body to pore throat ratios inferred from mercury injection suggest elevated pore fluid pressure in the low permeability cataclasites, reducing shear strength of the fault.     

Texture and anisotropy analysis of Qusaiba shales

Scanning and transmission electron microscopy, synchrotron X‐ray diffraction, microtomography and ultrasonic velocity measurements were used to characterize microstructures and anisotropy of three deeply buried Qusaiba shales from the Rub’al‐Khali basin, Saudi Arabia. Kaolinite, illite‐smectite, illite‐mica and chlorite show strong preferred orientation with (001) pole figure maxima perpendicular to the bedding plane ranging from 2.4–6.8 multiples of a random distribution (m.r.d.). Quartz, feldspars and pyrite crystals have a random orientation distribution. Elastic properties of the polyphase aggregate are calculated by averaging the single crystal elastic properties over the orientation distribution, assuming a nonporous material. The average calculated bulk P‐wave velocities are 6.2 km/s (maximum) and 5.5 km/s (minimum), resulting in a P‐wave anisotropy of 12%. The calculated velocities are compared with those determined from ultrasonic velocity measurements on a similar sample. In the ultrasonic experiment, which measures the effects of the shale matrix as well as the effects of porosity, velocities are smaller (P‐wave maximum 5.3 km/s and minimum 4.1 km/s). The difference between calculated and measured velocities is attributed to the effects of anisotropic pore structure and to microfractures present in the sample, which have not been taken into account in the matrix averaging.     

The enigma of post-perovskite anisotropy: deformation versus transformation textures

The D′′ region that lies just above the core mantle boundary exhibits complex anisotropy that this is likely due to preferred orientation (texturing) of the constituent minerals. (Mg,Fe)SiO₃ post-perovskite is widely thought to be the major mineral phase of the D′′. Texture development has been studied in various post-perovskite phases (MgSiO₃, MgGeO₃, and CaIrO₃), and different results were obtained. To clarify this controversy, we report on transformation and deformation textures in MgGeO₃ post-perovskite synthesized and deformed at room temperature in the diamond anvil cell. Transformed from the enstatite phase, MgGeO₃ post-perovskite exhibits a transformation texture characterized by (100) planes at high angles to the direction of compression. Upon subsequent deformation, this texture changes and (001) lattice planes become oriented nearly perpendicular to compression, consistent with dominant (001)[100] slip. When MgGeO₃ post-perovskite is synthesized from the perovskite phase, a different transformation texture is observed. This texture has (001) planes at high angle to compression and becomes slightly stronger upon compression. We also find that the yield strength of MgGeO₃ post-perovskite is dependent on grain size and texture. Finer-grained samples exhibit higher yield strength and are harder to induce plastic deformation. Strong textures also affect the yield strength and can result in higher differential stresses. The inferred dominant (001) slip for pPv is significant for geophysics, because, when applied to geodynamic convection models, it predicts the observed anisotropies of S-waves as well as an anti-correlation between P- and S-waves.     

Tectonics of the Lepontine Alps: ductile thrusting and folding in the deepest tectonic levels of the Central Alps

The Lepontine dome represents a unique region in the arc of the Central and Western Alps, where complex fold structures of upper amphibolite facies grade of the deepest stage of the orogenic belt are exposed in a tectonic half-window. The NW-verging Mont Blanc, Aar und Gotthard basement folds and the Lower Penninic gneiss nappes of the Central Alps were formed by ductile detachment of the upper European crust during its Late Eocene–Early Oligocene SE-directed underthrust below the upper Penninic and Austroalpine thrusts and the Adriatic plate. Four underthrust zones are distinguished in the NW-verging stack of Alpine fold nappes and thrusts: the Canavese, Piemont, Valais and Adula zones. Up to three schistosities S1–S3, folds F1–F3 and a stretching lineation XI with top-to-NW shear indicators were developed in the F1–F3 fold nappes. Spectacular F4 transverse folds, the SW-verging Verzasca, Maggia, Ziccher, Alpe Bosa and Wandfluhhorn anticlines and synclines overprint the Alpine nappe stack. Their formation under amphibolite facies grade was related to late ductile folding of the southern nappe roots during dextral displacement of the Adriatic indenter. The transverse folding F4 was followed since 30 Ma by the pull-apart exhumation and erosion of the Lepontine dome. This occurred coevally with the formation of the dextral ductile Simplon shear zone, the S-verging backfolding F5 and the formation of the southern steep belt. Exhumation continued after 18 Ma with movement on the brittle Rhone-Simplon detachment, accompanied by the N-, NW- and W-directed Helvetic and Dauphiné thrusts. The dextral shear is dated by the 29–25 Ma crustal-derived aplite and pegmatite intrusions in the southern steep belt. The cooling by uplift and erosion of the Tertiary migmatites of the Bellinzona region occurred between 22 and 18 Ma followed by the exhumation of the Toce dome on the brittle Rhone–Simplon fault since 18 Ma.     

Deformation of lower-mantle ferropericlase (Mg,Fe)O across the electronic spin transition

Recent high-pressure studies have shown that an electronic spin transition of iron in ferropericlase, an expected major phase of Earth’s lower mantle, results in changes in its properties, including density, incompressibility, radiative thermal conductivity, electrical conductivity, and sound velocities. To understand the rheology of ferropericlase across the spin transition, we have used in situ radial X-ray diffraction techniques to examine ferropericlase, (Mg_0.83,Fe_0.17)O, deformed non-hydrostatically in a diamond cell up to 81 GPa at room temperature. Compared with recent quasi-hydrostatic studies, the range of the spin transition is shifted by approximately 20 GPa as a result of the presence of large differential stress in the sample. We also observed a reduction in incompressibility and in the unit cell volume of 3% across the spin transition. Our radial X-ray diffraction results show that the {0 0 1} texture is the dominant lattice preferred orientation in ferropericlase across the spin transition and in the low-spin state. Viscoplastic self-consistent polycrystal plasticity simulations suggest that this preferred orientation pattern is produced by {1 1 0}<1–10> slip. Analyzing our radial X-ray diffraction patterns using lattice strain theory, we evaluated the lattice d-spacings of ferropericlase and Mo as a function of the ψ angle between the compression direction and the diffracting plane normal. These analyses give the ratio between the uniaxial stress component (t) and the shear modulus (G) under constant stress condition, which represents a proxy for the supported differential stress and elastic strength. This ratio in the mixed-spin and low-spin states is lower than what is expected from previous studies of high-spin ferropericlase, indicating that the spin transition results in a reduced differential stress and elastic strength along with the volume reduction. The influence of the spin transition on the differential stress and strength of ferropericlase is expected to be less dominant across the wide spin transition zone at high pressure–temperature conditions relevant to the lower mantle.     

Rezente Sedimente im Küstenbereich der Halbinsel Kaloum,Guinea

Zusammenfassung Es wurden rezente Sedimente aus dem unmittelbaren Küstenraum Guineas untersucht, in dessen Hinterland ausgeprägte lateritische Verwitterung herrscht. Von den erbohrten Tonen und Sanden wurden die Korngrößenverteilung, die chemische und mineralogische Zusammensetzung sowie die Kationsorption bestimmt.Neben Quarz, Kaolinit und Illit enthalten die Tone, die ein ungewöhnlich hohes Sorptionsvermögen haben, deutliche Anteile an Hydrargillit. Die Sande enthalten einen hohen Anteil Fe-reicher Körner, die auf eine Neubildung von Chamosit aus Brauneisen nach der Ablagerung hinweisen.

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Summary Recent marine sediments from the coastal area of Guinea, a country with intensive lateritic weathering, have been investigated. The grainsize distribution, chemical and mineralogical composition and the cation-exchange of the drilled clay-and sand-samples were determined. The clays have an anomally high cation-sorption and contain noticeable amounts of gibbsite besides quartz, kaolinite and illite. In the sands a new formation of chamosite from goethite after deposition is probable.

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Mit 2 Textabbildungen

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Herrn Professor Dr.F. Machatschki zum 70. Geburtstag gewidmet.