A simple method for estimating excess pressure over horsts from seismic sections

Observations in the North Sea Basin max indicate significant overpressure in sediments over horst blocks but not over grabens at the same submudline depth. The purpose is to show that over a horst, of top width W, with grabens on either side of top widths G₁ and G₂, respectively, the equivalent mud density. _r can be estimated approximately from the simple equation.

A Coupled Riverine-Marine Fractionation Model for Dissolved Rare Earths and Yttrium

Fractionation of yttrium (Y) and the rare earth elements (REEs) begins in riverine systems and continues in estuaries and the ocean. Models of yttrium and rare earth (YREE) distributions in seawater must therefore consider the fractionation of these elements in both marine and riverine systems. In this work we develop a coupled riverine/marine fractionation model for dissolved rare earths and yttrium, and apply this model to calculations of marine YREE fractionation for a simple two-box (riverine/marine) geochemical system. Shale-normalized YREE concentrations in seawater can be expressed in terms of fractionation factors ( _ij ) appropriate to riverine environments ( ) and seawater ( ):

Manganese olivine I: Electrical conductivity

To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn₂SiO₄, electrical conductivity () of single crystals was measured along either the [100] or the [010] direction. The experiments were carried out at temperatures T=850–1200 °C and oxygen fugacities atm under both Mn oxide (MO) buffered and MnSiO₃ (MS) buffered conditions. Under the same thermodynamic conditions, charge transport along [100] is 2.5–3.0 times faster than along [010]. At high oxygen fugacities, the electrical conductivity of samples buffered against MS is 1.6 times larger than that of samples buffered against MO; while at low oxygen fugacities, the electrical conductivity is nearly identical for the two buffer cases. The dependencies of electrical conductivity on oxygen fugacity and temperature are essentially the same for conduction along the [100] and [010] directions, as well as for samples coexisting with a solid-state buffer of either MO or MS. Hence, it is proposed that the same conduction mechanisms operate for samples of either orientation in contact with either solid-state buffer.The electrical conductivity data lie on concave upward curves on a log-log plot of vs , giving rise to two regimes with different oxygen fugacity exponents. In the low- regime , the exponent, m, is 0, the MnSiO₃-activity exponent, q, is 0, and the activation energy, Q, is 45 kJ/mol. In the high regime 10^{ - 7} {\text{atm}}} \right)$$ " align="middle" border="0"> , m=1/6, q=1/4–1/3, and Q=45 and 200 kJ/mol for T<1100 °c=" and=">T>1100 °C, respectively.     

Spinel-pyroxene-garnet relationships and their dependence on Cr/Al ratio

The partitioning of Cr and Al between coexisting spinel and clinopyroxene and the dependence of spinel-cpxgarnet equilibria on Cr/Al ratio have been investigated by a combination of phase equilibrium experiments, high temperature solution calorimetry and thermodynamic calculations.The exchange equilibrium: has a measured enthalpy change for pure phases of –2,100±500 cal at 970 K and 1 atm. Experimental reversals of Cr-Al partitioning between the spinel and clinopyroxene phases yield the following partitioning relationship: where X _i ^j refers to atomic fraction of i in the octahedral sites of phase j. The compositional dependence of partitioning implies that Al-Cr mixing in spinel is nonideal with, on the symmetrical model, a W _Cr-Al ^Sp of 2,700±500 cal/gm. atom. In contrast, aluminum-chromium mixing in clinopyroxene is close to ideal.The measured stability field of knorringite (Mg₃Cr₂Si₂O₁₂) and mixing properties of garnet have been used in conjunction with our experimental data to calculate the influence of Cr/Al ratio on the important reaction: orthopyroxene+clinopyroxene+spinel=olivine+garnetThe stability field of spinel lherzolite increases by about 2.8 Kb for every increase of 0.1 in Cr/(Cr+Al) ratio up to Cr/(Cr+Al) of 0.7. The calculated stabilization is in very good agreement with the experimental results of O'Neill (1981). The partitioning relationships are such that, at the low ratios of Cr/Al (0.07) of primitive lherzolite, clinopyroxene buffers spinel composition and sharpens the spinelgarnet reaction interval from 10 Kb (little or no clinopyroxene) down to about 2 Kb in pyroxene-rich pyrolite.     

Estimation of covariance matrix from geochemical data with observations below detection limits

Multivariate statistical analyses have been extensively applied to geochemical measurements to analyze and aid interpretation of the data. Estimation of the covariance matrix of multivariate observations is the first task in multivariate analysis. However, geochemical data for the rare elements, especially Ag, Au, and platinum-group elements, usually contain observations the below detection limits. In particular, Instrumental Neutron Activation Analysis (INAA) for the rare elements produces multilevel and possibly extremely high detection limits depending on the sample weight. Traditionally, in applying multivariate analysis to such incomplete data, the observations below detection limits are first substituted, for example, each observation below the detection limit is replaced by a certain percentage of that limit, and then the standard statistical computer packages or techniques are used to obtain the analysis of the data. If a number of samples with observations below detection limits is small, or the detection limits are relatively near zero, the results may be reasonable and most geological interpretations or conclusions are probably valid. In this paper, a new method is proposed to estimate the covariance matrix from a dataset containing observations below multilevel detection limits by using the marginal maximum likelihood estimation (MMLE) method. For each pair of variables, sayY andZ whose observations containing below detection limits, the proposed method consists of three steps: (i) for each variable separately obtaining the marginal MLE for the means and the variances, , , , and forY andZ: (ii) defining new variables by and and lettingA=C+D andB=C–D, and obtaining MLE for variances, and forA andB; (iii) estimating the correlation coefficient _YZ by and the covariance _YZ by . The procedure is illustrated by using a precious metal geochemical data set from the Fox River Sill, Manitoba, Canada.     

Impact of land use change on groundwater quality in a typical karst watershed of southwest China: a case study of the Xiaojiang watershed,Yunnan Province

The impact of land-use change on the quality of groundwater in the Xiaotjiang watershed, China was assessed for the period 1982–2004. Groundwater samples were collected from 30 monitoring points across the watershed, and were representative of the various changes, determined by remote sensing and geographical information systems. The results indicate that 610 km² (60% of the total watershed area) were subject to land-use change during the period. The most important changes were the conversion of 135 km² of forested land to cultivated land, and 211 km² of unused land to cultivated land. The main impact was ascribed to diffuse pollution from fertilizers applied to newly cultivated land, and from building development. Overall the groundwater pH value was significantly increased, as were the concentrations of ions , , , , and Cl⁻ in groundwater whilst the concentrations of Ca²⁺ and declined. More precisely, in the regions where forested land and unused land were converted into cultivated land, the pH value and the concentrations of Mg²⁺, , , , , Cl⁻ increased whilst the concentrations of Ca²⁺ and declined. However in the region where cultivated land was converted into construction land, the pH value and the concentrations of Ca²⁺, Mg²⁺, , , , , , Cl⁻ increased.

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Résumé  L’impact des changements de l’utilisation du territoire sur la qualité de l’eau souterraine dans le bassin versant de Xiaojiang, en Chine, a été évalué de 1982 à 2004. Des échantillons d’eau souterraine ont été récoltés à partir de 30 points d’observation éparpillés sur le bassin, représentant les divers changements déterminés par télédétection et système d’information géographique. Les résultats indiquent que 610 km² (soit 60% de la surface du bassin) ont été sujets à des modifications de l’utilisation du territoire sur cette période. Les changements les plus importants furent la conversion de 135 km² de forêt et 211 km² de terres inutilisées en terres cultivées. Le principal impact est attribué à la pollution diffuse des engrais utilisés en agriculture et pour les batiments. De manière générale le pH de l’eau souterraine a augmenté significativement, ainsi que les concentrations des ions , , , , et Cl⁻, tandis que les concentration en Ca²⁺ et ont diminué. Plus précisément dans les régions transformées en terres cultivées, la valeur du pH et les concentrations en Mg²⁺, , , , , Cl⁻ ont augmenté tandis que les concentrations en Ca²⁺ et ont diminué. Toutefois dans les régions cultivées converties en zones de construction, le pH et les concentrations en Ca²⁺, Mg²⁺, , , , , , Cl⁻ ont augmenté.

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Resumen  El impacto del cambio en uso de la tierra en la calidad del agua en la cuenca Xiaojiang, China fue evaluado para el periodo 1982–2004. Muestras de agua subterránea fueron tomadas de 30 puntos de monitoreo a través de la cuenca, y fueron representativas de los múltiples cambios, determinados por sensores remotos y sistemas de información geográfica. Los resultados indican que 610 km² (60% del área total de la cuenca) estaban sujetos a cambios de uso de la tierra durante el periodo estudiado. Los cambios más importantes fueron la conversión de 135 km² de bosques a tierra cultivada, y 211 km² de tierra sin uso (ociosa) a tierra cultivada. El impacto principal fue causado por contaminación difusa de fertilizantes aplicados a la tierra recientemente cultivada, y a desarrollo de construcciones. En general el pH en agua subterránea creció significantemente, al igual que las concentraciones de los iones , , , , y Cl⁻ en agua subterránea mientras que las concentraciones de Ca²⁺ y decrecieron. Mas precisamente, en las regiones donde bosque y tierra ociosa fueron convertidas en tierra cultivada, el valor de pH y las concentraciones de Mg²⁺, , , , , Cl⁻ crecieron mientras las concentraciones de Ca²⁺ y decrecieron. Sin embargo en la región donde tierra cultivada fue convertida en construcciones, el valor de pH y las concentraciones de Ca²⁺, Mg²⁺, , , , , , Cl⁻ crecieron.

     

Use of Pitzer Equations to Examine the Formation of Mercury(II) Hydroxide and Chloride Complexes in NaClO<Subscript>4</Subscript> Media

The thermodynamic stability constants for the hydrolysis and formation of mercury (Hg²⁺) chloride complexes

Titania precipitation in sapphire containing iron and titanium

Titania, TiO₂, precipitation in natural blue sapphire (Fe, Ti: -Al₂O₃) has been investigated using high resolution and analytical transmission electron microscopy. The structure and habit of the TiO₂ precipitate depends on both the Ti⁴⁺ concentration and the temperature at which the precipitate formed. Tetragonal TiO₂ (Rutile) grows at 1350° C but at 1150° C an orthorhombic non-equilibrium TiO₂ polymorph precipitates. Both TiO₂ polymorphs nucleate in the (0001)s plane as lens shaped discs twinned along their diameter. The crystallographic alignment of each type of TiO₂ precipitate with respect to the -Al₂O₃ host matrix provides a high degree of structural coherency with minimal lattice mismatch. Electron diffraction analysis established the following precipitate/host orientation relationships: tetragonal TiO₂: {011}r {11 07B;100}r(0001)s and 01 r10 0s twinned along the (011)r planeand orthorhombic TiO₂: {021}{11 0}s, {100}(0001)s and 0 2 10 0s twinned along the (021) plane.     

The dependence of the Fe2+-Mg cation-partitioning between olivine and basaltic liquid on pressure,temperature and composition

The equilibrium partitioning of Fe²⁺ and Mg between olivine and liquid along a liquid line of descent has been determined for a calc-alkaline system, ranging in composition from picritic to andesitic. Experiments were conducted between 1000–1450° C and 1 bar to 30 kbar. Within the compositional range investigated and , the compositional dependence of the Fe²⁺ and Mg partitioning is a function of the Mg-content of the liquid. The Mg-content of the liquid correlates strongly with temperature. The variation of the Fe²⁺ and Mg partitioning were therefore evaluated individualy as functions of composition and temperature alone. The composition dependence of the cation-partitioning coefficients (Kd) is given by the following two equations:

The graphite-COH fluid equilibrium in P,T, f_{O_2 } space

The oxygen fugacity ( ) of a C-O-H fluid in equilibrium with graphite has been determined in the range 10–30 kbar by equilibrating solid -buffer assemblages in graphite capsules containing C-O-H fluid. By using different buffers (Fe_xO-Fe₃O₄, Ni-NiO, Co-CoO, Mo-MoO₂), the of the graphite-saturated fluid is bracketed within a narrow range. This technique produces a calibration for the imposed on a sample contained within a graphite capsule. To achieve a thermodynamically-invariant system at fixed P and T, the was imposed on the system with an external buffer and the double-capsule technique. The experiments were performed in solid-media, high pressure apparatus with 19 mm tale-pyrex assemblies. A series of experiments at 10, 15, 20, 25, and 30 kbar, 800–1600° C, with imposed by the Fe₂O₃-Fe₃O₄-H₂O equilibrium were conducted. The experimental results have been fitted to the following equation:

A regional gradient in the composition of metamorphic fluids in pelitic schist,Pecos Baldy,New Mexico

Two metamorphic isograds cut across graphitic schist near Pecos Baldy, New Mexico. The southern isograd marks the first coexistence of staurolite with biotite, whereas the northern isograd marks the first coexistence of andalusite with biotite. The isograds do not record changes in temperature or pressure. Instead, they record a regional gradient in the composition of the metamorphic fluid phase. Ortega Quartzite, which contains primary hematite, lies immediately north of the graphitic schist. Mineral compositions within the schist change gradually toward the quartzite, reflecting gradients in and . The chemical potential gradients, locally as high as 72 cal/m in and 9 cal/m in , controlled the positions of the two mapped isograds. The staurolite-biotite isograd records where fell below 0.80, at near 10^–23 bars; the andalusite-biotite isograd records where fell below 0.25, at near 10^–22 bars. Dehydration and oxidation were coupled by graphite-fluid equilibrium.The chemical potential gradients apparently formed during metamorphism, as graphite in schist reacted with hematite in quartzite. Local oxidation of graphite formed CO₂ which triggered dehydration reactions along the schistquartzite contact. This process created a C-O-H fluid which infiltrated into overlying rocks. Upward infiltration, local fluid-rock equilibration and additional infiltration proceeded until the composition of the infiltrating fluid evolved to that in equilibrium with the infiltrated rock. This point occurs very close to the staurolite-biotite isograd. Pelitic rocks structurally above this isograd show no petrographic evidence of infiltration, even though calculations indicate that volumetric fluid/rock ratios may have exceeded 15 and thin, rare calc-silicate beds show extensive K-metasomatism and quartz veining.     

Equations of state of Plagioclase Feldspars

The volume variation with pressure of seven intermediate plagioclase feldspars has been determined by high-pressure single-crystal X-ray diffraction. The bulk moduli of plagioclases for a 3rd-order Birch-Murnaghan EoS can be described by the following pair of equations:

High-temperature creep and kinetic decomposition of Ni2SiO4

To investigate high-temperature creep and kinetic decomposition of nickel orthosilicate (Ni₂SiO₄), aggregates containing 3 vol% amorphous SiO₂ have been deformed in uniaxial compression at a total pressure of one atomsphere. Twenty-three samples with grain sizes (d) from 9 to 30 m were deformed at temperatures (T) from 1573 to 1813 K, differential stresses () from 3 to 20 MPa, and oxygen fugacities (f o ₂) from 10^-1 to 10⁵ Pa. At temperatures up to 1773 K, the steady-state creep rate () can be described by the flow law

An experimental study of the diffusion of oxygen in quartz and albite using an overgrowth technique

Diffusion rates of¹⁸O tracer in quartz ( c, 1 Kb H₂O) and Amelia albite ( 001, 2 Kb H₂O) have been measured, using Secondary Ion Mass Spectrometry (SIMS). A new technique involving hydrothermal deposition of labelled materials has removed the possibility of pressure solution-reprecipitation processes adversely affecting the experiments. Reported diffusion constants are:-quartz ( c), ,Q=98±7 KJ mol^–1 (600–825° C, 1 Kb); Amelia albite ( 001), ,Q=85±7 KJ mol^–1, (400–600° C, 2 Kb). Measured quartz¹⁸O diffusivities decrease discontinuously at the- transition, reflecting strong structural influences. The reported albite data agree with previously recorded studies, but-quartz data indicate significantly lower activation energies. Possible causes of this discrepancy, and some geological consequences, are noted.     

The crystal structure of a triclinic roscherite

Summary The crystallography of roscherite is more complicated than previously thought. Single crystal X-ray work on material from Foote Mine (California) gave triclinic symmetry. The unit cell corresponding to the one adopted for monoclinic roscherite hasa=15.921,b=11.965,c=6.741 Å, =91°04, =94°21, =89°59 1/2, space group . The least-squares refinement of the structure using 2380 non zero reflections with anisotropic temperature factors resulted in a conventional reliability factorR=0.060.The X-ray study indicates the formula while that proposed for monoclinic roscherite is The atomic arrangements of both varieties of roscherite are very similar. The lowering of symmetry is caused by the segregation of the trivalent cations into only half of the sites of a monoclinic point position. Crystallochemical considerations suggest that the symmetry of roscherite does not depend on the kind of trivalent cations occupying the 6-coordinated position, but rather by the ratio between trivalent and divalent metal ions.

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Die Kristallstruktur eines triklinen Roscherites

Zusammenfassung Die Kristallographie des Roscherites ist komplizierter als man bisher annahm. Einkristall-Röntgenuntersuchungen an Material von Foote Mine (Kalifornien) ergaben trikline Symmetrie. Die Elementarzelle, welche der für monoklinen Roscherit angenommenen entspricht, hata=15,921,b=11,965,c=6,741 Å, =91°04, =94°21, =89°59 1/2, Raumgruppe . Die Verfeinerung der Struktur mit der Methode der kleinsten Quadrate ergab unter Verwendung anisotroper Temperaturfaktoren für 2380 beobachtete Reflexe einen konventionellen ZuverlässigkeitsindexR=0,060.Die Röntgenuntersuchung weist auf die Formel , während für monoklinen Roscherit vorgeschlagen wurde. Die Atomanordnungen beider Abarten des Roscherites sind sehr ähnlich. die Symmetrieerniedrigung wird dadurch hervorgerufen, daß die dreiwertigen Kationen nur die Hälfte der Positionen einer monoklinen Punktlage besetzen. Kristallchemische Überlegungen weisen darauf hin, daß die Symmetrie nicht von der Art der dreiwertigen Kationen, welche eine 6-koordinierte Punktlage besetzen, abhängt, sondern vielmehr von dem Mengenverhältnis zwischen 3-wertigen und 2-wertigen Metallionen.

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With 1 Figure     

H<Subscript>2</Subscript>O diffusion in peralkaline to peraluminous rhyolitic melts

The diffusion of water in a peralkaline and a peraluminous rhyolitic melt was investigated at temperatures of 714–1,493 K and pressures of 100 and 500 MPa. At temperatures below 923 K dehydration experiments were performed on glasses containing about 2 wt% H₂O _t in cold seal pressure vessels. At high temperatures diffusion couples of water-poor (<0.5 wt% H₂O _t ) and water-rich (~2 wt% H₂O _t ) melts were run in an internally heated gas pressure vessel. Argon was the pressure medium in both cases. Concentration profiles of hydrous species (OH groups and H₂O molecules) were measured along the diffusion direction using near-infrared (NIR) microspectroscopy. The bulk water diffusivity () was derived from profiles of total water () using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between and Both methods consistently indicate that is proportional to in this range of water contents for both bulk compositions, in agreement with previous work on metaluminous rhyolite. The water diffusivity in the peraluminous melts agrees very well with data for metaluminous rhyolites implying that an excess of Al₂O₃ with respect to alkalis does not affect water diffusion. On the other hand, water diffusion is faster by roughly a factor of two in the peralkaline melt compared to the metaluminous melt. The following expression for the water diffusivity in the peralkaline rhyolite as a function of temperature and pressure was obtained by least-squares fitting:

Spatial relationships of multivariate data

Spatial factor analysis (SFA) is a multivariate method that determines linear combinations of variables with maximum autocorrelation at a given lag. This is achieved by deriving estimates of auto-/cross-correlations of the variables and calculating the corresponding eigenvectors of the covariance quotient matrix. A two-point spatial factor analysis model derives factors by the formation of transition matrixU comparing auto-/cross-correlations at lag 0,R ₀, with those at a specified lag d,R _d, expressed asU _d=R ₀ ^–1 R_d. The matrixU _d can be decomposed into its spectral components which represent the spatial factors. The technique has been extended to include three points of reference. Spatial factors can be derived from the relationship:

Dissolution Kinetics of Dolomite in Water at Elevated Temperatures

Kinetic experiments of dolomite dissolution in water over a temperature range from 25 to 250°C were performed using a flow through packed bed reactor. Authors chose three different size fractions of dolomite samples: 18–35 mesh, 35–60 mesh, and 60–80 mesh. The dissolution rates of the three particle size samples of dolomite were measured. The dissolution rate values are changed with the variation of grain size of the sample. For the sample through 20–40 mesh, both the release rate of Ca and the release rate of Mg increase with increasing temperature until 200°C, then decrease with continued increasing temperature. Its maximum dissolution rate occurs at 200°C. The maximum dissolution rates for the sample through 40–60 mesh and 60–80 mesh happen at 100°C. Experimental results indicate that the dissolution of dolomite is incongruent in most cases. Dissolution of fresh dolomite was non-stoichiometric, the Ca/Mg ratio released to solution was greater than in the bulk solid, and the ratio increases with rising temperatures from 25 to 250°C. Observations on dolomite dissolution in water are presented as three parallel reactions, and each reaction occurs in consecutive steps as

An almandine garnet isograd in the Rogers Pass area,British Columbia: The nature of the reaction and an estimation of the physical conditions during its formation

In the Rogers Pass area of British Columbia the almandine garnet isograd results from a reaction of the form: 5.31 ferroan-dolomite+8.75 paragonite+4.80 pyrrhotite+3.57 albite+16.83 quartz+1.97 O₂=1.00 garnet+16.44 andesine+1.53 chlorite+2.40 S₂+1.90 H₂O+10.62 CO₂. The coefficients of this reaction are quite sensitive to the Mn content of ferroan-dolomite.Experimental data applied to mineral compositions present at the isograd, permits calculation of two intersecting P, T equilibrium curves. P=29088–39.583 T is obtained for the sub-system paragonite-margarite (solid-solution), plagioclase, quartz, ferroan-dolomite, and P=28.247 T–14126 is obtained for the sub-system epidote, quartz, garnet, plagioclase. These equations yield P=3898 bars and T=638° K (365° C). These values are consistent with the FeS content of sphalerite in the assemblage pyrite, pyrrhotite, sphalerite and with other estimates for the area.At these values of P and T the composition of the fluid phase in equilibrium with graphite in the system C-O-H-S during the formation of garnet is estimated as: bars, bars, bars, bars, bars, bars, bars, bars, , bars, bars.