Distribution of hydrate on Europa: Further evidence for sulfuric acid hydrate

Sulfuric acid hydrate has been proposed as an important species on Europa's surface, the acid being produced by radiolysis of surficial sulfur compounds. We investigated the spectral properties of disordered and crystalline forms of sulfuric acid and suggest that the hydration properties of Europa's hypothesized sulfuric acid lie between two end members: liquid sulfuric acid and its higher crystalline hydrates. The spectra of these end members are similar except for spectral shifts at the band edges. We measured the optical constants of sulfuric acid octahydrate and used these with simple radiative transfer calculations to fit Europa spectra obtained by Galileo's Near Infrared Mapping Spectrometer (NIMS). The global distribution of the hydrate that we associate here with hydrated sulfuric acid shows a strong trailing-side enhancement with a maximum fractional hydrate abundance of 90% by volume, corresponding to a sulfur atom to water molecule ratio of 10%. The hydrate concentration spatially correlates with the ultraviolet and visible absorption of the surface and with the sulfur dioxide concentration. The asymmetric global distribution is consistent with Iogenic plasma ion implantation as the source of the sulfur, possibly modified by electron irradiation and sputtering effects. The variegated distribution also correlates with geologic forms. A high spatial resolution image shows resolved lineae with less hydrate appearing within the lineae than in nearby crustal material. The low concentration of hydrated material in these lineae argues against their conveying sulfurous material to the surface from the putative ocean.     

Performances of two instrumented laboratory models for the study of rainfall infiltration into unsaturated soils

Rainfall infiltration and suction variation in unsaturated soils must be taken into consideration in the analysis of most slope stability problems, particularly in the tropical regions where the annual precipitation is high. The process of rainfall infiltration into unsaturated soils is an extremely complex problem attributed to the non-linearity of the hydraulic property functions of the unsaturated soils. This paper describes in detail two instrumented laboratory models, i.e. one-dimensional soil column, and two-dimensional slope model used to provide experimental evidences for the transient suction variations in the unsaturated soils under certain rainfall conditions. The performances of the laboratory models were tested on four typical types of residual soils, i.e. sand-gravel, silty gravel, sandy silt, and silt (kaolin), and a two-layered soil system, i.e. sandy silt underlain by silty gravel. The results showed that the suction distributions for the single-layered homogeneous soils obtained from the simpler one-dimensional soil column were almost identical to that of two-dimensional slope model. However, the two-dimensional slope model should be employed for the two-layered soil system because of the dominant effect of the lateral flow mechanism. The capillary barrier effect was observed when a less permeable soil layer was underlain by a more permeable soil layer. The minimum suction value in soil is governed by the rainfall intensity, rainfall duration, and the saturated permeability of soil. The infiltration rate of the fine-grained soils that subjected to shrink and crack was independent of the soil permeability, but was significantly governed by the preferential flows developed in the soils.     

Linking gold mineralization to regional-scale drivers of mineral systems using in situ U–Pb geochronology and pyrite LA-ICP-MS element mapping

Proterozoic orogens commonly host a range of hydrothermal ores that form in diverse tectonic settings at different times. However, the link between mineralization and the regional-scale tectonothermal evolution of orogens is usually not well understood, especially in areas subject to multiple hydrothermal events.Regional-scale drivers for mineral systems vary between the different classes of hydrothermal ore, but all involve an energy source and a fluid pathway to focus mineralizing fluids into the upper crust. The Mount Olympus gold deposit in the Proterozoic Capricorn Orogen of Western Australia, was regarded as an orogenic gold deposit that formed at ca. 1738 Ma during the assembly of Proterozoic Australia. However,the trace element chemistry of the pyrite crystals closely resembles those of the Carlin deposits of Nevada,with rims that display solid solution gold accompanied by elevated As, Cu, Sb, Hg, and Tl, surrounding gold-poor cores. New SHRIMP UeP b dating of xenotime intergrown with auriferous pyrite and ore-stage alteration minerals provided a weighted mean~(207) Pb*/~(206) Pb* date of 1769 ± 5 Ma, interpreted as the age of gold mineralization. This was followed by two discrete episodes of hydrothermal alteration at 1727 ± 7 Ma and 1673 ± 8 Ma. The three ages are linked to multiple reactivation of the crustal-scale Nanjilgardy Fault during repeated episodes of intracratonic reworking. The regional-scale drivers for Carlin-like gold mineralization at Mount Olympus are related to a change in tectonic regime during the final stages of the intracratonic 1820 -1770 Ma Capricorn Orogeny. Our results suggest that substantial sized Carlin-like gold deposits can form in an intracratonic setting during regional-scale crustal reworking.     

TEM study of a special grain boundary in a synthetic K-feldspar bicrystal: Manebach Twin

 A monoclinic KAlSi₃O₈ feldspar Manebach twin boundary was synthesized by diffusion bonding and examined using high-resolution transmission electron microscopy. The sharp (001) twin boundary is straight and free of strain. The boundary width is smaller than d₀₀₁. There is no rigid body shift observed at the twin boundary, and the feldspar structure is arranged symmetrically across (001). The twin boundary structure consists of bridged tetrahedral crankshafts, which are characteristic of the feldspar lattice. The grain boundary structure is in good agreement with the geometrical model of Taylor et al. (1934). The grain boundary composition of K_1/2H_1/2AlSi₃O₈ differs from their model. Received: 13 February 2002 / Accepted: 24 December 2002 Acknowledgements We thank M. Rühle, S. Hutt, J. Mayer, A. Strecker and U. Salzberger at MPI, Stuttgart, for their support and valuable advice in preparing TEM sections of bicrystals.     

Dissolved carbon in pore waters from the carbonate barrier reef of Tahiti (French Polynesia) and its basalt basement

This paper deals with dissolved inorganic carbon (DIC) and organic carbon (DOC) in pore waters from a 150 m deep hole drilled through the carbonate barrier reef of Tahiti and its underlying basalt basement. Alkalinity-pH measurements were used to calculate the DIC species concentration, and DOC was analysed according to the high temperature catalytic oxidation technique. Salinity was used as a conservative tracer to help identify water origin and mixing within the hole. Water mixing, calcium carbonate dissolution and mineralization of organic carbon combined to form three distinct groups of pore water. In the deeper basalt layers, pore water with alkalinity of 1.4 meq kg^–1 pH of 7.6 and p(CO₂) of 1.2 mAtm was undersaturated with respect to both aragonite and calcite. In the intermediate carbonate layer, pore water with alkalinity of more than 2.0 meq kg^–1, pH of 7.70 and p(CO₂) of 1.4 mAtm was supersaturated with respect to both aragonite and calcite. The transition zone between those two groups extended between 80 and 100 m depth. The shift from aragonite undersaturation to supersaturation was mainly attributed to the mixing of undersaturated pore waters from the basalt basement with supersaturated pore waters from the overlaying limestone. In the top of the reef, inputs from a brackish water lens further increased p(CO₂) up to 5.6 times the atmospheric P(CO₂).     

Models of rapidly rotating neutron stars: remnants of accretion-induced collapse

Equilibrium models of differentially rotating nascent neutron stars are constructed, which represent the result of the accretion-induced collapse of rapidly rotating white dwarfs. The models are built in a two-step procedure: (1) a rapidly rotating pre-collapse white dwarf model is constructed; (2) a stationary axisymmetric neutron star having the same total mass and angular momentum distribution as the white dwarf is constructed. The resulting collapsed objects consist of a high-density central core of size roughly 20 km, surrounded by a massive accretion torus extending over 1000 km from the rotation axis. The ratio of the rotational kinetic energy to the gravitational potential energy of these neutron stars ranges from 0.13 to 0.26, suggesting that some of these objects may have a non-axisymmetric dynamical instability that could emit a significant amount of gravitational radiation.     

Comparison of the high-pressure schist belts of New Caledonia and Sanbagawa,Japan

The high-pressure schist terranes of New Caledonia and Sanbagawa were developed along the oceanic sides of sialic forelands by tectonic burial metamorphism. The parent rocks were chemically similar, as volcanic-sedimentary trough or trench sequences, and metamorphic temperatures in both belts were 250° to 600° C. From phase equilibria curves, total pressures were higher for New Caledonia (6–15 kb) than for Sanbagawa (5–11 kb) and the estimated thermal gradients were 7–10° C/km and 15° C/km respectively.PT paths identify the higher pressure in New Caledonia (P differences 2 kb at 300° C and 4 kb at 550° C) with consequent contrast in progressive regional metamorphic zonation for pelites in the two areas: lawsonite-epidote-omphacite (New Caledonia) and chlorite-garnet-biotite (Sanbagawa). In New Caledonia the Na-amphibole is dominantly glaucophane and Na-pyroxenes associated with quartz are Jadeite (Jd_95–100) and omphacite; in Sanbagawa the amphibole is crossite or riebeckite and the pyroxene is omphacite (Jd₅₀). For both areas, garnet rims show increase in pyrope content with advancing grade, but Sanbagawa garnets are richer in almandine. Progressive assemblages within the two belts can be equated by such reactions as:New Caledonia Sanbagawa glaucophane+paragonite+H₂Oalbite+chlorite+quartz glaucophane+epidote+H₂Oalbite+chlorite+actinolite and the lower pressure Japanese associations appear as retrogressive phases in the New Caledonia epidote and omphacite zones.The contrasts inPT gradient, regional zonation and mineralogy are believed due to differences in the tectonic control of metamorphic burial: for New Caledonia, rapid obduction of an upper sialic plate over an inert oceanic plate and sedimentary trough; and for Sanbagawa, slower subduction of trench sediments beneath a relatively immobile upper plate.     

Thermal expansion and spontaneous strain associated with the normal-incommensurate phase transition in melilites

The linear thermal expansions of åkermanite (Ca₂MgSi₂O₇) and hardystonite (Ca₂ZnSi₂O₇) have been measured across the normal-incommensurate phase transition for both materials. Least-squares fitting of the high temperature (normal phase) data yields expressions linear in T for the coefficients of instantaneous linear thermal expansion, $$\alpha _1 = \frac{1}{l}\frac{{dl}}{{dT}}$$ for åkermanite: $$\begin{gathered} \alpha _{[100]} = 6.901(2) \times 10^{ - 6} + 1.834(2) \times 10^{ - 8} T \hfill \\ \alpha _{[100]} = - 2.856(1) \times 10^{ - 6} + 11.280(1) \times 10^{ - 8} T \hfill \\ \end{gathered} $$ for hardystonite: $$\begin{gathered} \alpha _{[100]} = 15.562(5) \times 10^{ - 6} - 1.478(3) \times 10^{ - 8} T \hfill \\ \alpha _{[100]} = - 11.115(5) \times 10^{ - 6} + 11.326(3) \times 10^{ - 8} T \hfill \\ \end{gathered} $$ Although there is considerable strain for temperatures within 10° C of the phase transition, suggestive of a high-order phase transition, there appears to be a finite ΔV of transition, and the phase transition is classed as “weakly first order”.