Theoretical Prediction of Gibbs Free Energies of Formation for Crystalline α-MOOH and α-M2O3 Based on a Linear Free-Energy Relationship

In the present study, the modified Sverjensky–Molling equation, derived from a linear-free energy relationship, is used to predict the Gibbs free energies of formation of crystalline phases of α-MOOH (with a goethite structure) and α-M2O3 (with a hematite structure) from the known thermodynamic properties of the corresponding aqueous trivalent cations (M3+). The modified equation is expressed as ΔG0f,MVX=aMVXΔG0n,M3++bMVX+βMVXγM3+, where the coefficients aMVX, bMVX, and βMVX characterize a particular structural family of MvX (M is a trivalent cation [M3+] and X represents the remainder of the composition of solid); γ3+ is the ionic radius of trivalent cations (M3+); ΔG0f,MVX is the standard Gibbs free energy of formation of MvX; and ΔG0n,M3+ is the non-solvation energy of trivalent cations (M3+). By fitting the equation to the known experimental thermodynamic data, the coefficients for the goethite family (α-MOOH) are aMVX=0.8838, bMVX=?424.4431 (kcal/mol), and βMVX=115 (kcal/mol.?), while the coefficients for the hematite family (α-M2O3) are aMVX=1.7468, bMVX=?814.9573 (kcal/mol), and βMVX=278 (kcal/mol.?). The constrained relationship can be used to predict the standard Gibbs free energies of formation of crystalline phases and fictive phases (i.e. phases that are thermodynamically unstable and do not occur at standard conditions) within the isostructural families of goethite (α-MOOH) and hematite (α-M2O3) if the standard Gibbs free energies of formation of the trivalent cations are known.     

Adsorption of NO, SO2 and light hydrocarbons on activated Greek brown coals

Twenty-eight samples of peat, peaty lignites and lignites (of both matrix and xylite-rich lithotypes) and subbituminous coals have been physically activated by pyrolysis. The results show that the surface area of the activated coal samples increases substantially and the higher the carbon content of the samples the higher the surface area.The adsorption capacity of the activated coals for NO, SO₂, C₃H₆ and a mixture of light hydrocarbons (CH₄, C₂H₆, C₃H₈ and C₄H₁₀) at various temperatures was measured on selected samples. The result shows a positive correlation between the surface area and the gas adsorption. In contrast, the gas adsorption is inversely correlated with the temperature. The maximum recorded adsorption values are: NO = 8.22 × 10^− 5 mol/g at 35 °C; SO₂ = 38.65 × 10^− 5 mol/g at 60 °C; C₃H₆ = 38.9 × 10^− 5 mol/g at 35 °C; and light hydrocarbons = 19.24 × 10^− 5 mol/g at 35 °C. Adsorption of C₃H₆ cannot be correlated with either NO or SO₂. However, there is a significant positive correlation between NO and SO₂ adsorptions. The long chain hydrocarbons are preferentially adsorbed on activated lignites as compared to the short chain hydrocarbons.The results also suggest a positive correlation between surface area and the content of telohuminite maceral sub-group above the level of 45%.     

Forsterite-diopside-spinel-liquid equilibria in the system CaO−MgO−Al2O3−SiO2 at 20 kbar and petrological applications

Liquidus phase relationships in the CaAlAl–SiO₆–Mg₂SiO₄–CaMgSi₂O₆–CaAlSi₂O₈ portion of the simplified basalt tetrahedron in the CaO–MgO–SiO₂–Al₂O₃ system have been experimentally determined at 20 kbar pressure. The fo+di _ss+sp+li univariant curve, that pierces the fo-di-an join and meets the fo+di _ss+ en_ss+sp+li invariant point in the basalt tetrahedron, extends all the way to and pierces the di-fo-CaTs join, the limit of the simplified basalt tetrahedron toward the silica undersaturated portion.An algebraic method, relying on compositions of two successive liquids on a univariant curve and those of the crystalline phases in equilibrium with the respective liquids, is developed to identify the type of reaction that takes place along an isobarically univariant curve and to detect whether there is a temperature maximum on that curve. Use of this method for the di _ss+fo+sp+li univariant equilibria shows that a temperature maximum exists on this curve at the composition Fo₁₁Di₅₆An₃CaTs₃₀, very close to and slighthly to the SiO₂-rich side of the fo-di-CaTs join. The temperature along the univariant curve continuously decreases from the temperature maximum (1500°C) to the invariant point (1475°C) where the univariant curve is terminated by the appearance of e _ss as a member of the equilibrium assemblage. Along this part of the curve, a reaction relationship occurs according to the equation fo+li=di _ss+ sp. Compositions of di _ss in equilibrium with the liquids from the temperature maximum to the fo+di _ss+en_ss+ sp+li invariant point range from Di₆₆En₉CaTs₂₅ to Di₃₆En₄₀CaTs₂₄. Because of the reaction relationship of forsterite with liquid, fractional crystallization of a model alkalic basaltic liquid would cause liquids to move off the fo-di _ss-sp-li univariant curve onto the sp-di _ss divariant surface. Crystallization of di _ss and sp would then lead to silica enrichment of residual liquids. Thus at pressures below 30 kbar, at which pressure the Al₂O₃–CaSiO₃–MgSiO₃ plane becomes a new thermal divide cutting through both the tholeiitic and alkalic volumes, alkalic liquids will fractionate toward tholeiitic compositions without crossing a thermal divide. This relationship would be expected to persist at pressures down to about 4 kbar where a maximum on the fo-di-an-li boundary line causes a thermal divide near the fo-di-an plane. Strongly SiO₂-undersaturated liquids (e.g. nephelinites, basanites), on the other hand, cannot be derived from SiO₂-undersaturated basalts (e.g. alkali olivine basalt) by fractional crystallization at 20 kbar. We also found that no gt primary phase volume cuts the wo-en-Al₂O₃ join at 20 kbar pressure. The wehrlite, the olivine clinopyroxenite, and the Al-augite group lherzolite xenoliths, containing highly aluminous clinopyroxenes (enriched in Ca-Tschermak), can be interpreted as crystal cumulates from alkalic basalts in the light of this experimental study. This is consistent with the mode of origin of these xenoliths proposed from petrographic, mineralogic, and geochemical studies.Abbreviations and notations di CaMgSi₂O₆ - fo Mg₂SiO₄ - an CaAl₂Si₂O₈ - CaTs CaAlAlSiO₆ - sp MgAl₂O₄ - en MgSiO₃ - wo CaSiO₃ - gt Ca₃Al₂Si₃O₁₂–Mg₃Al₂Si₃O₁₂ - qz SiO₂ - li Liquid - gl glass - ss Solid Solution - A An mxn matrix - X A column vector - kbar kilobar     

Experimental study of the K2ZrSi3O9 (wadeite)–K2TiSi3O9 and K2(Zr,Ti)Si3O9–phlogopite systems at 2–3 GPa

Experiments ranging from 2 to 3 GPa and 800 to 1300 °C and at 0.15 GPa and 770 °C were performed to investigate the stability and mutual solubility of the K₂ZrSi₃O₉ (wadeite) and K₂TiSi₃O₉ cyclosilicates under upper mantle conditions. The K₂ZrSi₃O₉–K₂TiSi₃O₉ join exhibits complete miscibility in the P–T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating that K₂ZrSi₃O₉ is the more refractory end member. At 2 GPa, in the more complex K₂ZrSi₃O₉–K₂TiSi₃O₉–K₂Mg₆Al₂Si₆O₂₀(OH)₄ system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions [Zr/(Zr + Ti) > 0.85], comparable to wadeite found in nature, with TiO₂ partitioning strongly into the coexisting mica and/or liquid. However, at 1200 °C, with increasing pressure from 2 to 3 GPa, the partitioning behaviour of TiO₂ changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the K₂(Zr,Ti)Si₃O₉ phase decreasing from ∼0.9 to ∼0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree of melting to forsterite and liquid, following the major substitution ^VITi+^VI□=2^VIMg. Received: 26 January 1999 / Accepted: 10 January 2000     

3He4He ratio,noble gas abundance and K-Ar dating of diamonds—An attempt to search for the records of early terrestrial history

The ${}^3\text{He}{}^4\text{He}$ ratios measured in 27 Southern Africa diamond stones, four from Premier Mine and the rest of unidentified origin, range from 4.2 × 10^?8 to 3.2 × 10^?4, with three stones above 1 × 10^?4. We conclude that the initial helium isotopic ratio (${}^3\text{He}{}^4\text{He)}{}_0$ in the earth was significantly higher than that of the planetary helium-A (${}^3\text{He}{}^4\text{He = 1.42 × 10}{}^{\mathrm{?4}}$), but close to the solar helium (${}^3\text{He}{}^4\text{He ? 4 × 10}{}^{\mathrm{?4}}$).The apparent K-Ar ages for the twelve diamonds of unidentified origin show enormously old age, indicating excess argon-40. ${}^3\text{He}{}^4\text{He}$ evolution in diamonds suggests that the diamonds with the high ${}^3\text{He}{}^4\text{He}$ ratio (>2 × 10^?4) may be as old as the earth.Noble gas elemental abundance in the diamonds relative to the air noble gas abundance shows monotonie decrease with a decreasing mass number.This paper discusses the implications of these observations on the early solar system and the origin of diamonds.     

Role of unbalanced slab resistive force in the 2004 off Sumatra mega-earthquake (<Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript> > 9.0) event

Present study addresses the role of major plate-driving forces, particularly the slab pull and slab resistive forces, for the generation of 26 December 2004 M _w > 9.0 off Sumatra megathrust earthquake. Major controls on the plate-driving forces are normally visualized through age, speed, and average dip of the slab during subduction. Wide variation in age, plate obliquity, stress obliquity, subduction rate, dip angle, and flexing depth of the subducting oceanic lithosphere between Andaman and Sumatra thus allowed us for quantitative evaluation of the slab pull (F _SP) and slab resistive (F _SR) forces in three well-defined sectors (I, II and III). Computed values of these forces in the three sectors: (1) F _SP = 1.29 × 10¹³ N/m, F _SR = 1.41 × 10¹³ N/m; sector I, (2) F _SP = 2.10 × 10¹³ N/m, F _SR = 1.13 × 10¹³ N/m; sector II, and (3) F _SP = 2.08 × 10¹³ N/m, F _SR = 2.72 × 10¹³ N/m; sector III clearly suggest a spatial variation of stress regime in the subducting oceanic lithosphere. Excess F _SR in sectors I and III are interpreted as the causative forces behind the triggering of major seismic energy bursts near Sumatra and Andaman on 26 December 2004. A gap of minimum seismic energy burst near Great Nicobar possibly was controlled by the excess of F _SP in sector II. This study further advocates that the cyclic stress, resulted from unbalanced component of slab resistive force, had a definite control on the occurrence of 2004 off Sumatra megathrust earthquake around the flexing zone of the subducting lithosphere.     

Effect of KCl on melting in the Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>–MgSiO<Subscript>3</Subscript>–H<Subscript>2</Subscript>O system at 5 GPa

To examine the effect of KCl-bearing fluids on the melting behavior of the Earth’s mantle, we conducted experiments in the Mg₂SiO₄–MgSiO₃–H₂O and Mg₂SiO₄–MgSiO₃–KCl–H₂O systems at 5 GPa. In the Mg₂SiO₄–MgSiO₃–H₂O system, the temperature of the fluid-saturated solidus is bracketed between 1,200–1,250°C, and both forsterite and enstatite coexist with the liquid under supersolidus conditions. In the Mg₂SiO₄–MgSiO₃–KCl–H₂O systems with molar Cl/(Cl + H₂O) ratios of 0.2, 0.4, and 0.6, the temperatures of the fluid-saturated solidus are bracketed between 1,400–1,450°C, 1,550–1,600°C, and 1,600–1,650°C, respectively, and only forsterite coexists with liquid under supersolidus conditions. This increase in the temperature of the solidus demonstrates the significant effect of KCl on reducing the activity of H₂O in the fluid in the Mg₂SiO₄–MgSiO₃–H₂O system. The change in the melting residues indicates that the incongruent melting of enstatite (enstatite = forsterite + silica-rich melt) could extend to pressures above 5 GPa in KCl-bearing systems, in contrast to the behavior in the KCl-free system.     

Some chemographic relationships in n-component systems

A petrologic problem of fundamental importance is to determine whether 2 or more mineral assemblages can be related to one another by continuous or discontinuous facies changes, or whether their bulk compositions occupy non-overlapping regions of composition space. A general method is developed by which 2 regions of n-dimensional space whose vertices are defined by the phases present are tested for compositional overlap. This is accomplished by generating mass balance equations of the type:

$\text{∑}\text{i = 1}\text{m}\text{a}{}_{\mathrm{i}}\text{A}{}_{\mathrm{i}}\text{=}\text{∑}\text{j = 1}\text{k}\text{b}{}_{\mathrm{j}}\text{B}{}_{\mathrm{j}}$

where A_i is the ith phase in one region and B_j is the th phase in the other. If any such equation satisfies the requirement that the sign of each a_i is the same, and that the sign of each b_j is the opposite for all i, j such that: k + m = n + 1 then the 2 regions overlap in phase space.By eliminating all overlapping assemblages in a given set, the bivariant fields bounded by univariant equilibria in n-dimensional systems are completely specified. All bulk compositions are considered within the space defined by the phases that participate in the bounding reactions. An extension of the method generates in sequence all bivariant fields and associated reactions about any invariant point. A further extension is applied to multi-system analysis.     

Accumulation of heavy metals in wild commercial fish from the Baotou Urban Section of the Yellow River,China

In this work we studied the accumulation of heavy metals in nine species of fish with different life and feeding habitats which are native and major commercial fish in the Baotou Urban Section of the Yellow River. The results showed that the concentration of heavy metals was significantly dependent on fish species; the pollution index of heavy metals in different species were ranked as Hemiculter leucisclus > Carassius auratus auratus > Hemibarbus maculatus > Megalobrama amblycephala > Abbottina rivularis > Cyprinus carpio > Squaliobarbus curriculus > Perccottus glehni > Saurogobio dabryi. Product–moment correlation coefficients among the metal pairs Pb–Zn, Cu–Cd, Cu–Zn, Cu–Pb, Pb–Cd, and Zn–Cd revealed there was no competitions between metals in each tissue. Correlations between heavy metal concentrations and fish length or weight indicated that accumulation of the heavy metals by the different fish species was related to their surrounding environments and their life and feeding habitats. According to the mean bioconcentration factors (BCFs), the heavy metal concentrations in these nine species were ranked Zn ≫ Cu > Cd ≈ Pb. In this work, the bioaccumulation factors (BAFs) were developed by using the sum of exchangeable and bound-to-carbonate heavy metals as Cs values. It was found that BAFs better reveal the accumulation characteristics of the heavy metals in the fish, which might provide an effective method for assessing bioaccumulation of heavy metals.     

Redistribution of uranium and thorium series isotopes during isovolumetric weathering of granite

Previous studies of the distribution of U and Th in parent versus weathered granites have shown both depletion and enrichment of these elements during weathering. In this study, the distribution of U and Th decay series isotopes was determined in a weathering profile of a granitic saprolite, which showed textural preservation indicating isovolumetric weathering. Two types of dissolution methods were used: a whole-rock dissolution and a sodium-citrate dithionite leach to preferentially attack noncrystalline phases of weathering products. Using volume-based activities, 45–70 percent of the total ²³²Th was gradually removed during weathering. Although the whole-rock ${}^{228}\text{Th}{}^{232}\text{Th}$ activity ratios were in equilibrium, there were large excesses of ²²⁸Th in the leachable fraction of both parent rock (${}^{228}\text{Th}{}^{232}\text{Th}\text{= 2.06}$) and partially weathered saprolite (${}^{228}\text{Th}{}^{232}\text{Th}\text{= 3–6.5}$), due to alpha recoil and release of daughter ²²⁸Th to the weathering rind of the mineral grain. For the most weathered sample, 81 percent of the thorium was in the teachable fraction and ${}^{228}\text{Th}{}^{232}\text{Th}\text{= 1}$, indicating that even the more resistant minerals were attacked.The total U activities showed as much variation in the six parent rock samples as in the weathered profile, and ${}^{234}\text{U}{}^{238}\text{U}$ were in equilibrium in both the whole-rock and leachable fractions. ²³⁰Th was deficient relative to ²³⁴U and ²²⁶Ra in both fractions, suggesting recent addition of U and Ra to the entire profile. The large variation in U was not from absorption onto the intermediate weathering products, because only 11–23 percent of the U was in the leachable fraction.     

The variation of the marine 87Sr86Sr ratio during Phanerozonic time: interpretation using a flux model

The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in sea water has varied over geologic time due to the addition of strontium to the sea from rocks with a variety of ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. The measurements by Petermanet al. (Geochim. Cosmochim. Acta34, 105–120, 1970) of the value of the marine ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio have been confirmed by several other workers and by some new measurements on JOIDES samples. They form the basis of a model calculation of the relative proportions of ‘basaltic’ (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.704}$) and ‘granitic’ (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.718}$) strontium being supplied to the sea. For the last 200 million years, the proportions of these two sources appear to reflect the history of global tectonics; ‘basaltic’ during rifting and increasingly ‘granitic’ during the present episodes of uplift and continental collision     

Atmospheric mercury near Salmon Falls Creek Reservoir in southern Idaho

Gaseous elemental mercury (GEM) and reactive gaseous mercury (RGM) were measured over 2-week seasonal field campaigns near Salmon Falls Creek Reservoir in south-central Idaho from the summer of 2005 through the fall of 2006 and over the entire summer of 2006 using automated Tekran Hg analyzers. GEM, RGM, and particulate Hg (HgP) were also measured at a secondary site 90 km to the west in southwestern Idaho during the summer of 2006. The study was performed to characterize Hg air concentrations in the southern Idaho area for the first time, estimate Hg dry deposition rates, and investigate the source of observed elevated concentrations. High seasonal variability was observed with the highest GEM (1.91 ± 0.9 ng m⁻³) and RGM (8.1 ± 5.6 pg m⁻³) concentrations occurring in the summer and lower values in the winter (1.32 ± 0.3 ng m⁻³, 3.2 ± 2.9 pg m⁻³ for GEM, RGM, respectively). The summer-average HgP concentrations were generally below detection limit (0.6 ± 1 pg m⁻³). Seasonally averaged deposition velocities calculated using a resistance model were 0.034 ± 0.032, 0.043 ± 0.040, 0.00084 ± 0.0017 and 0.00036 ± 0.0011 cm s⁻¹ for GEM (spring, summer, fall and winter, respectively) and 0.50 ± 0.39, 0.40 ± 0.31, 0.51 ± 0.43 and 0.76 ± 0.57 cm s⁻¹ for RGM. The total annual RGM + GEM dry deposition estimate was calculated to be 11.9 ± 3.3 μg m⁻², or about 2/3 of the total (wet + dry) deposition estimate for the area. Periodic elevated short-term GEM (2.2–12 ng m⁻³) and RGM (50–150 pg m⁻³) events were observed primarily during the warm seasons. Back-trajectory modeling and PSCF analysis indicate predominant source directions to the SE (western Utah, northeastern Nevada) and SW (north-central Nevada) with fewer inputs from the NW (southeastern Oregon and southwestern Idaho).     

Lead isotope systematics of some igneous rocks from the Egyptian shield

Lead isotope data on late Precambrian igneous rocks from the eastern desert of Egypt are presented. Previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of ${}^{37}\text{Sr}{}^{86}\text{Sr}$ (< 0.7035). The Pb data define three groups, loosely corresponding to age. An older tonalite to granodiorite (OTG) suite, with ages in the range 610–710 Ma, has Pb isotope characteristics similar to modern, mantle-derived oceanic mafic rocks. The age-corrected initial Pb isotope ratios of the OTG group lie near the “ocean regression line” in correlation diagrams of ${}^{206}\text{Pb}{}^{204}\text{Pb}$ vs. ${}^{207}\text{Pb}{}^{204}\text{Pb}$ and ${}^{203}\text{Pb}{}^{204}\text{Pb}$. The isotope data imply an origin for the OTG group analogous to calc-alkalic igneous rocks in modern intra-oceanic island arcs.A younger suite of K-rich plutonic rocks (570–595 Ma) has similar ${}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ initial ratios relative to the OTG suite, but have higher ${}^{207}\text{Pb}{}^{204}\text{Pb}$ initial ratios when calculated from measured whole-rock U and Pb contents. However, K-feldspar and galena, associated with this suite, indicate initial Pb isotope ratios close to the OTG suite. We interpret this to indicate that the younger granites could have originated by anatexis of OTG-type material, but they probably experienced an episode of metasomatism after emplacement.A suite of volcanic rocks (the Dokhan Volcanics) from the central eastern desert are intermediate between the younger granites and OTG in both age (～ 610 Ma) and Pb isotope characteristics.Limited data on initial ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios suggest that all the Egyptian samples originated from a source with anomalously low $\text{Th}\text{Pb}$ ratios.Although the relative abundance of granitic rocks in the Egyptian Shield decreases to the south, no evidence of north-south heterogeneity in the Pb isotope composition of the respective source regions is apparent in the data. However, the westernmost sample studied, from the Aswan area, contains distinctly more radiogenic Pb relative to the mean. This location may mark the boundary between the Late Precambrian ensimatic orogen now outcropping in the Egyptian and Saudi Arabian Shields, and an older sialic craton to the west.     

Tourmaline from the rare-element Pinilla pegmatite, (Central Iberian Zone, Zamora, Spain): chemical variation and implications for pegmatitic evolution

Summary Tourmaline is an ubiquitous constituent in the Pinilla de Fermoselle rare-element pegmatite (Zamora, Spain), as well as in barren pegmatitic and quartz–tourmaline veins inside the associated leucogranite. The rare-element pegmatite shows internal zoning, evolving from a barren facies, in the lower border zone, in contact with the leucogranite, to a Li-rich facies in the upper border zone, close to the host-rocks.Tourmalines from the veins within the leucogranite have highest Mg contents, and belong to the schorl–dravite series. The tourmalines from the rare-element pegmatite mostly belong to the schorl–elbaite series, with chemical compositions within the range of the end-members, whereas the tourmalines associated with the most evolved zone in the pegmatite belong to the elbaite–rossmanite series. The broad compositional range shown by the tourmalines correlates quite well with the pegmatite zoning. The most plausible substitution mechanism for the chemical evolution of tourmalines during crystallization seems to be Mg_–1Fe²⁺₁, [X]_–1^YAl_–1^XNa_–1^YFe²⁺₁, for the foitite–schorl series; ^YFe²⁺_–3^YAl_1.5^YLi_1.5, for the schorl–elbaite vector; ^XNa_–1^YLi_–0.5[X]₁^YAl_0.5, for the elbaite–rossmanite series; and, (OH)₁F₁ for all the tourmalines except the pink elbaites. This chemical variation in tourmaline is consistent with a crystal fractionation model for the evolution of the Pinilla pegmatite.     

The Partitioning of Tungsten bwtween Aqueous Fluids and Silicate Melts

An experimental study has been carried out to determine the partition coefficients of tungsten between aqueous fluids and granitic melts at 800 °C and 1.5 kb with natural granite as the starting material. The effects of the solutions on the partition coefficients of tungsten show a sequence of P > CO ₃ ²⁻ > B > H₂O. The effects are limited (generallyK _D < 0.3) and the tungsten shows a preferential trend toward the melt over the aqueous fluid. The value ofK _D increases with increasing concentration of phosphorus; theK _D increases first and then reduces with the concentration of CO ₃ ²⁻ when temperature decreases, theK _D between the solution of CO ₃ ²⁻ and the silicate melt increases, and that between the solution of B₄O ₇ ²⁻ and the silicate melt decreases. The partition coefficients of phosphorus and sodium between fluids and silicate melts have been calculated from the concentrations of the elements in the melts. TheK _D value for phosphorus is 0.38 and that for sodium is 0.56. Evidence shows that the elements tend to become richer and richer in the melts.     

Spatial and multivariate analysis of geochemical data from metavolcanic rocks in the Ben Nevis area, Ontario

A study of the lithogeochemistry of metavolcanics in the Ben Nevis area of Ontario, Canada has shown that factor analysis methods can distinguish lithogeochemical trends related to different geological processes, most notably, the principal compositional variation related to the volcanic stratigraphy and zones of carbonate alteration associated with the presence of sulphides and gold. Auto- and cross-correlation functions have been estimated for the two-dimensional distribution of various elements in the area. These functions allow computation of spatial factors in which patterns of multivariate relationships are dependent upon the spatial auto- and cross-correlation of the components. Because of the anisotropy of primary compositions of the volcanics, some spatial factor patterns are difficult to interpret. Isotropically distributed variables such as CO ₂ are delineated clearly in spatial factor maps. For anisotropically distributed variables (SiO ₂ ), as the neighborhood becomes smaller, the spacial factor maps becomes better. Interpretation of spatial factors requires computation of the corresponding amplitude vectors from the eigenvalue solution. This vector reflects relative amplitudes by which the variables follow the spatial factors. Instability of some eigenvalue solutions requires that caution be used in interpreting the resulting factor patterns. A measure of the predictive power of the spatial factors can be determined from autocorrelation coefficients and squared multiple correlation coefficients that indicate which variables are significant in any given factor. The spatial factor approach utilizes spatial relationships of variables in conjunction with systematic variation of variables representing geological processes. This approach can yield potential exploration targets based on the spatial continuity of alteration haloes that reflect mineralization.List of symbols c _i Scalar factor that minimizes the discrepancy between andU _i - D Radius of circular neighborhood used for estimating auto- and cross-correlation coefficients - d Distance for which transition matrixU is estimated - d _ij Distance between observed valuesi andj - E Expected value - E _i Row vector of residuals in the standardized model - F(d _ij) Quadratic function of distanced _ij F(d _ij)=a+bd _ij+cd _ij ² - L Diagonal matrix of the eigenvalues ofU - _i Eigenvalue of the matrixU;ith diagonal element ofL - N Number of observations - p Number of variables - Q Total predictive power ofU - R Correlation matrix of the variables - R _0j Variance-covariance signal matrix of the standardized variables at origin;j is the index related tod andD (e.g.,j=1 ford=500 m,D=1000 m) - R _1j Matrix of auto- and cross-correlation coefficients evaluated at a given distance within the neighborhood - R _m ² Multiple correlation coefficient squared for themth variable - S _i Column vectori of the signal values - s _k ² Residual variance for variablek - T _i Amplitude vector corresponding toV _i;ith row ofT=V ^–1 - T Total variation in the system - U Nonsymmetric transition matrix formed by post-multiplyingR ₀₁ ^–1 byR _ij - U _i Componenti of the matrixU, corresponding to theith eigenvectorV _i;U _i= _iV_iT_i - U* _i ComponentU _i multiplied byc _i - U _ij Sum of componentsU _i+U _j - V _i Eigenvector of the matrixU;ith column ofV withUV=VL - w Weighting factor; equal to the ratio of two eigenvalues - X _i Random variable at pointi - x _i Value of random variable at pointi - y _i Residual ofx _i - Z _i Row vectori for the standardized variables - z _i Standardized value of variable     

Thermal expansion and high-temperature P21/c–C2/c phase transition in clinopyroxene-type LiFeGe2O6 and comparison to NaFe(Si,Ge)2O6

A synthetic clinopyroxene with composition LiFe³⁺Ge₂O₆, monoclinic s.g. P2₁/c, a = 9.8792(7), b = 8.8095(5), c = 5.3754(3) Å, β = 108.844(6)°, V = 442.75(16) ų, has been studied by in situ low- and high-temperature single-crystal X-ray diffraction. The variation of lattice parameters and the intensity of the b-type reflections (h + k = 2n + 1, only present in the P-symmetry) with increasing temperature showed a displacive phase transition from space group P2₁/c to C2/c at a transition temperature T _tr = 789 K, first order in character, with a sudden volume increase of 1.6% and a decrease of β by 1° at the transition. This spontaneous dilatation is reversible, shows a limited hysteresis of ±10°C, and corresponds to the vanishing of the b-type reflections, thus indicating a symmetry increase to space group C2/c. Below T _tr an expansion is observed for all the cell parameters, while the β angle remained almost constant; at T > T _tr the thermal volume expansion is due to dilatation of the structure in the $(\bar{1}\,0\,1) A synthetic clinopyroxene with composition LiFe³⁺Ge₂O₆, monoclinic s.g. P2₁/c, a = 9.8792(7), b = 8.8095(5), c = 5.3754(3) ?, β = 108.844(6)°, V = 442.75(16) ?³, has been studied by in situ low- and high-temperature single-crystal X-ray diffraction. The variation of lattice parameters and the intensity of the b-type reflections (h + k = 2n + 1, only present in the P-symmetry) with increasing temperature showed a displacive phase transition from space group P2₁/c to C2/c at a transition temperature T _tr = 789 K, first order in character, with a sudden volume increase of 1.6% and a decrease of β by 1° at the transition. This spontaneous dilatation is reversible, shows a limited hysteresis of ±10°C, and corresponds to the vanishing of the b-type reflections, thus indicating a symmetry increase to space group C2/c. Below T _tr an expansion is observed for all the cell parameters, while the β angle remained almost constant; at T > T _tr the thermal volume expansion is due to dilatation of the structure in the ([`1] 0 1)(\bar{1}\,0\,1) plane, mostly along [0 1 0], and pure shear in the (0 1 0) plane due to the decrease of β. From comparison with silicate analogues, the germanate clinopyroxenes are more expansible, while the P2₁/c expands more than the C2/c phase. The evolution of Q ² (calculated as the normalized intensity of b-type reflections) with T in the framework of the Landau theory has been done using a standard expression for a first order phase transition. We observe a jump of Q ₀² = 0.538(2) at T _tr, with T _c of 481(7) K, b/a = −2,290 K, and c/a = 3,192 K, and thus far from being tri-critical point. A closely related composition (LiFe³⁺Si₂O₆) shows an equivalent phase transition at 228 K, which is very close to the tri-critical point and 561 K cooler. This result indicates that a change in the composition of tetrahedral sites can have dramatic effects on the P2₁/c ↔ C2/c displacive phase transition in clinopyroxenes. The major changes observed in the evolution of the crystal structure with T are observed in the M2 polyhedron, with a volume decrease by ca. 13.3%, compared to ca. 1.3% observed in the M1 polyhedron. The tetrahedra behave as rigid units with neither a significant change of volume at T > T _tr (<1‰), nor a change of tilting of the basal plane. No change in coordination is observed at T > T _tr in the M2 polyhedron, which remains sixfold coordinated although a strong deformation of this polyhedron is observed. This deformation is related to a strong change by 51.4° at T _tr of the kinking angle (O3–O3–O3 angle) of the B-chain of tetrahedra, which switches from O-rotated to S-rotated [from 143.3(5)° to 194.7(6)°]. The A-chain is S-rotated at T < T _tr [206.8(5)° at 703 K] and extends by 12° at the transition.     

Lead isotopic compositions of South Sandwich Island volcanic rocks and their bearing on magmagenesis in intra-oceanic island arcs

Pb isotope ratios have been measured in 12 volcanic rocks from the South Sandwich Islands. The ranges are ${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 18.51–18.66;}{}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.55–15.64;}{}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 38.42–38.64}$. In ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{-}{}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{-}{}^{206}\text{Pb}{}^{204}\text{Pb}$ correlation diagrams, the South Sandwich data plot distinctly above the fields for ocean ridge basalts, and yield trends showing apparent mixing with a sedimentary end member similar to South Atlantic pelagic sediments as reported by Chow and Patterson (1962) and this study. Armstrong and Cooper (1971) have likewise shown that volcanics from the Lesser Antilles show mixing trends with North Atlantic sediments in Pb isotope correlation diagrams. The North Atlantic sediments have distinctly higher ${}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios compared to the South Atlantic sediments. The parallel relationships between sediments and volcanic island arc rocks of the North and South Atlantic provide strong evidence for a component of Pb from subducted sediments in the lavas of the west Atlantic basin. In contrast to these data, lavas from the Mariana Arc in the western Pacific show little or no component of Pb from pelagic sediments. The reason for the different behaviors in the two settings is speculative.     

Carbon dioxide degassing and thermal energy release in the Monte Amiata volcanic-geothermal area (Italy)

The quaternary volcanic complex of Mount Amiata is located in southern Tuscany (Italy) and represents the most recent manifestation of the Tuscan Magmatic Province. The region is characterised by a large thermal anomaly and by the presence of numerous CO₂-rich gas emissions and geothermal features, mainly located at the periphery of the volcanic complex. Two geothermal systems are located, at increasing depths, in the carbonate and metamorphic formations beneath the volcanic complex. The shallow volcanic aquifer is separated from the deep geothermal systems by a low permeability unit (Ligurian Unit). A measured CO₂ discharge through soils of 1.8 × 10⁹ mol a⁻¹ shows that large amounts of CO₂ move from the deep reservoir to the surface. A large range in δ¹³C_TDIC (−21.07 to +3.65) characterises the waters circulating in the aquifers of the region and the mass and isotopic balance of TDIC allows distinguishing a discharge of 0.3 × 10⁹ mol a⁻¹ of deeply sourced CO₂ in spring waters. The total natural CO₂ discharge (2.1 × 10⁹ mol a⁻¹) is slightly less than minimum CO₂ output estimated by an indirect method (2.8 × 10⁹ mol a⁻¹), but present-day release of 5.8 × 10⁹ mol a⁻¹ CO₂ from deep geothermal wells may have reduced natural CO₂ discharge. The heat transported by groundwater, computed considering the increase in temperature from the infiltration area to the discharge from springs, is of the same order of magnitude, or higher, than the regional conductive heat flow (>200 mW m⁻²) and reaches extremely high values (up to 2700 mW m⁻²) in the north-eastern part of the study area. Heat transfer occurs mainly by conductive heating in the volcanic aquifer and by uprising gas and vapor along fault zones and in those areas where low permeability cover is lacking. The comparison of CO₂ flux, heat flow and geological setting shows that near surface geology and hydrogeological setting play a central role in determining CO₂ degassing and heat transfer patterns.     

Simulation of the molecular mass distributions of Si anions on the liquidus of the Na<Subscript>2</Subscript>O-SiO<Subscript>2</Subscript> system with regard for the disproportionation of <Emphasis Type="Italic">Q</Emphasis> structons

A new version of the STRUCTON (2009) computer model is proposed for the simulation of the molecular mass distributions (MMD) characterizing the diversity of anions in silicate melts depending on their polymerization and temperature. In contrast to earlier versions, the new version of the model accounts for disproportionation reactions of Q ⁿ species and makes use of their proportions in the statistical simulations of the origin of real Si-O complexes. The new potentialities of the STRUCTON program package are illustrated by its application to studying the structural-chemical characteristics of melts in the Na₂O-SiO₂ system along its liquidus line, including the points of eutectics and phase transitions at 0.333 ≤ $ N_{SiO_2 } $ N_{SiO_2 } < 0.500. This problem is solved with the use of a temperature-composition dependence of polymerization constants K _p ^Na in the Toop-Samis approximation. The variations in K _p ^Na were proved to be as large as three orders of magnitude due to both the temperature effect at a constant composition and the composition effect at a constant temperature. The results of the MMD simulations on the liquidus show that the concentration of the SiO₄⁴⁻ ion strongly decreases, and the proportion of chain species increases compared to those at a stochastic distribution. The concentration of the Si₂O₇⁶⁻ anion reaches its maximum (∼42%) at 40 mol % in the liquid, i.e., the composition of Na₆Si₂O₇. At $ N_{SiO_2 } $ N_{SiO_2 } > 0.40, this ion dominates over the SiO₄⁴⁻ monomer. More silicic melts with $ N_{SiO_2 } $ N_{SiO_2 } ≥ 0.45, are dominated by (Si _n O_3n )³ⁿ⁻ ring species, and the concentrations of these species are related as (Si₃O₉)⁶⁻ > (Si₄O₁₂)⁸⁻ > (Si₅O₁₅)¹⁰⁻. The maximum concentration of these flat rings also occurs near the composition of stoichiometric metasilicate with Si/O = 0.333. The comparison of the dependence of the average size of anions i _av and the average number of their species on depolymerization indicates that a change in the proportion of Q ⁿ species in melt at decreasing temperature results in structural restyling and an increase in the average size of Si-O complexes. The average number of anion species thereby decreases compared to that in a stochastic MMD. The results presented in this publication direct the progress in the thermodynamic theory of silicate melts to a new avenue that makes use of the capabilities and advantages of the ion-polymer model, the theory of associated solutions, spectroscopic data, and the experimental study of variations in oxide activities depending on composition and temperature.