Thermochemistry of the high structural state plagioclases

The enthalpies of solution of a suite of 19 high-structural state synthetic plagioclases were measured in a Pb₂B₂O₅ melt at 970 K. The samples were crystallized from analyzed glasses at 1200°C and 20 kbar pressure in a piston-cylinder apparatus. A number of runs were also made on Amelia albite and Amelia albite synthetically disordered at 1050–1080°C and one bar for one month and at 1200°C and 20 kbar for 10 hr. The component oxides of anorthite, CaO, Al₂O₃ and SiO₂, were remeasured.The ΔH of disorder of albite inferred in the present study from albite crystallized from glass is 3.23 kcal, which agrees with the 3.4 found by Holm and Kleppa (1968). It is not certain whether this value includes the ΔH of a reversible displacive transition to monoclinic symmetry, as suggested by Helgesonet al. (1978) for the Holm-Kleppa results. The enthalpy of solution value for albite accepted for the solid solution series is based on the heat-treated Amelia albite and is 2.86 kcal less than for untreated Amelia albite.The enthalpy of formation from the oxides at 970 K of synthetic anorthite is ?24.06 ± 0.31 kcal, significantly higher than the ?23.16 kcal found by Charluet al. (1978), and in good agreement with the value of ?23.89 ± 0.82 given by Robieet al. (1979), based on acid calorimetry.The excess enthalpy of mixing in high plagioclase can be represented by the expression, valid at 970 K: ΔH^ex(±0.16 kcal) = 6.7461 X_abX²_An + 2.0247 X_AnX²_Ab where X_Ab and X_An are, respectively, the mole fractions of NaAlSi₃O₈ and CaAl₂Si₂O₈. This ΔH^ex, together with the mixing entropy of Kerrick and Darken's (1975) Al-avoidance model, reproduces almost perfectly the free energy of mixing found by Orville (1972) in aqueous cation-exchange experiments at 700°C. It is likely that Al-avoidance is the significant stabilizing factor in the high plagioclase series, at least for X_An≥ 0.3. At high temperatures the plagioclases have nearly the free energies of ideal one-site solid solutions. The Al-avoidance model leads to the following Gibbs energy of mixing for the high plagioclase series: $\text{ΔG}{}^{\mathrm{mix}}\text{= ΔH}{}^{\mathrm{ex}}\text{+ RT X}{}_{\mathrm{Ab}}\text{ln[X}{}^2{}_{\mathrm{Ab}}\text{(2 ? X}{}_{\mathrm{Ab}}\text{)]+ X}{}_{\mathrm{An}}\text{ln}\text{[X}{}_{\mathrm{An}}\text{(1+X}{}_{\mathrm{An}}\text{)}{}^2\text{]}\text{4}$. The entropy and enthalpy of mixing should be very nearly independent of temperature because of the unlikelihood of excess heat capacity in the albite-anorthite join.     

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.     

Sodium and potassium coprecipitation in aragonite

The coprecipitation of Na and K was experimentally investigated in aragonite. The distribution functions were determined at pH 6.8 and 8.8 over aqueous Na and K concentrations of between 5 × 10^?4and 2.0 M and temperatures of between 25 and 75°C.The mole fractions of Na and K in aragonite are related to the aqueous ratios of Na and Ca by a function of the form

$\text{log}\text{X}{}_{\mathrm{Na}}{}_2\text{CO}{}_3\text{,K}{}_2\text{CO}{}_3\text{= C}{}_0\text{+ C}{}_1\text{log}\text{a}{}^2{}_{\mathrm{Na}}\text{? ,K}{}^{\mathrm{?}}\text{a}{}_{\mathrm{Ca}}{}^{\mathrm{2+}}$

where C₀ and C₁ are constants at a given temperature. This equation was derived by a statistical model assuming a heterogeneous energy distribution for the sites of incorporation. The independence of the coprecipitation process from aqueous anion activities suggests that carbonate is the only anionic species in the solid solution.     

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     

Scaling properties of corner frequencies of Kamchatka earthquakes

Scaling properties of earthquake populations bear the major information on the physics of the source process of an earthquake. To determine scaling properties, source spectra of more than 400 earthquakes of Kamchatka were determined in a frequency range 0.1–30 Hz using materials of digital registration of PET station, and characteristic frequencies of earthquakes were estimated. The range of magnitudes is 4–6.5, the range of distances is 80–220 km. To enable reduction of a spectrum to the source, attenuation properties of the medium around PET were determined beforehand. It is revealed that source spectra show several corner (characteristic) frequencies: f _c1, f _c2 and f _c3; where the spectral trend changes: from f ⁰ to f ^?1, from f ^?1 to f ^?2, and from f ^?2 to f ^?3, respectively. Although in some cases f _c1 ≈ f _c2 in agreement with the usual ω^?2 spectral model, the main part of spectra has more complicated character. For a large part of the studied earthquakes a source-controlled upper cutoff of acceleration spectrum, or corner frequency f _c3, is observed. This is an important fact, as the existence of f _c3 (source-controlled f _max) is not recognized in the bulk of the seismological literature. For f _c1, the observed scaling agrees with the usual hypothesis of similarity of the earthquake sources of different size (magnitude); it is close to f _c1 ～ M ₀ ^?1/3 , where M ₀ is seismic moment. For f _c2, scaling is close to f _c2 ～ M ₀ ^?0.17 ～ f _c1 ^0.5 , that indicates an expressed violation of similarity. For f _c3, scaling is close to f _c2 ～ M ₀ ^?0.08 ～ f _c1 ^0.25 , so that similarity is broken even sharper in this case. Hypotheses about possible causes of the observed scaling are discussed.     

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     

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%.     

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.     

The isotopic composition and concentration of Ag in iron meteorites and the origin of exotic silver

The isotopic composition of Ag and the concentration of Ag and Pd have been determined in Canyon Diablo (IA), Grant (IIIB), Hoba, Santa Clara, Tlacotepec and Warburton Range (IVB), Piñon and Deep Springs (anom.). Troilite from Grant and Santa Clara have also been analyzed. All of these meteorites, with the exception of Canyon Diablo, give ${}^{107}\text{Ag}{}^{109}\text{Ag}$ in the metal phase that is greater than the terrestrial value with the enrichments of ¹⁰⁷Ag ranging from ~2% to 212%. These data show that Ag of anomalous isotopic composition is common to all IVB and anomalous meteorites. The results on Grant suggest that the anomalies may be widespread including more common meteorite groups. There is a general correlation of ${}^{107}\text{Ag}{}^{109}\text{Ag}$ with $\text{Pd}\text{Ag}$ except for the data from FeS of Santa Clara. It is concluded that the excess ¹⁰⁷Ag is the result of decay of ¹⁰⁷Pd, a nuclide that is extinct at present with an abundance of ${}^{107}\text{Pd}{}^{108}\text{Pd}$ of about 3 × 10^?5. The troilite in Grant exhibits normal ${}^{107}\text{Ag}{}^{109}\text{Ag}$ to within errors, a high Ag concentration and a low ratio of ${}^{108}\text{Pd}{}^{109}\text{Ag}$ ~0.17. Grant metal has ${}^{107}\text{Ag}{}^{109}\text{Ag}$ that is ~2% greater than normal and a high ratio of ${}^{108}\text{Pd}{}^{109}\text{Ag}$ ~ 10³. The data from Grant appear to represent a ¹⁰⁷Pd-¹⁰⁷Ag isochron and indicate that the cooling rate at elevated temperatures was sufficiently rapid to preserve substantial isotopic differences between metal and troilite. Troilite in Santa Clara was found to contain Ag with a very high ${}^{107}\text{Ag}{}^{109}\text{Ag}$ ratio (108% above normal), an Ag concentration only a factor of three above the metal and a high value of ${}^{108}\text{Pd}{}^{109}\text{Ag}$ ~1.3 × 10⁴. The troilite has a higher ${}^{107}\text{Ag}{}^{109}\text{Ag}$ than the metal. These data are not compatible with a simple model of in situ decay and subsequent local Ag redistribution between metal and troilite during cooling. These data suggest that Ag in Santa Clara and possibly other IVB meteorites is made up of almost pure ¹⁰⁷Ag produced from ¹⁰⁷Pd decay and ¹⁰⁹Ag produced by nuclear reactions with only a small amount of “normal” Ag. This indicates an intense energetic particle bombardment history in the early solar system (~10²⁰ p/m²) which occurred after the formation of small planetary bodies. We infer that a T-Tauri activity by the early sun contributed to some late stage “nucleosynthesis” and the heating of a dust cloud. In addition, implications on the early thermal evolution of iron meteorites are presented based on ¹⁰⁷Pd decay and models of the cooling history.     

Nitrogen isotopes in the solar system

Measurements of the isotopic composition of nitrogen in the solar system are summarized. We show that the 30% change, during the last 3 to 4 billion years, of ${}^{15}\text{N}{}^{14}\text{N}$ in solar-wind-bearing lunar soils and breccias probably does not reflect changes in this ratio at the solar surface. Such changes, whether by spallation or thermonuclear reactions are ruled out by comparing the yields of ¹⁵N with those of other rare isotopes such as ⁹Be, ¹¹B, ³He or ¹³C, even if an arbitrary degree of solar mixing is introduced. Moreover, we calculate that the solar activity required for producing significant amounts of ¹⁵N by spallation at the solar surface should have resulted in a particle bombardment of the Moon of an intensity that would have produced amounts of spallation isotopes (e.g.¹⁵N, ²¹Ne, ³⁸Ar, ¹³¹Xe) several orders of magnitude in excess of what is actually found in the whole regolith.We argue that accretion of interstellar matter also does not work as a cause for a significant change of the photospheric ${}^{15}\text{N}{}^{14}\text{N}$ ratio. Evidence is presented that the mixing depth at the solar surface on a time scale of ?10⁹ years is (10^?2 ?10^?1) M_⊙ Mixing to this depth renders accretion of interstellar matter as a source of compositional changes at the solar surface inefficient, even if allowance is made for the expected large difference in the accretion rates of condensed and gaseous matter. A quantitative treatment of several alternatives of solar accretion leads to serious contradictions (e.g. with the low Ne abundances in planetary atmospheres or with the amounts of nitrogen that should have been directly accreted by the Moon), and we conclude that accretion during the main sequence life of the Sun is an unlikely source of changes in ${}^{15}\text{N}{}^{14}\text{N}$ at the solar surface.A ratio of ${}^{15}\text{N}{}^{14}\text{N}\text{= (4.0 ± 0.3) × 10}{}^{\mathrm{?3}}$ is our best estimate for average solar system material and for the Sun. We propose that a rare, very light nitrogen component (called LPN) is admixed in varying amounts to planetary matter. Undiluted LPN has not been found in meteorites or planetary atmospheres, but we show that the combined effects of LPN admixture and isotope fractionation can in principle account for the variability of ${}^{15}\text{N}{}^{14}\text{N}$ observed in the planetary system. Determination of the Jovian ${}^{15}\text{N}{}^{14}\text{N}$ ratio with an accuracy of ~10% would crucially test our interpretation of the nitrogen isotope observations.     

Density calculations for silicate liquids. I. Revised method for aluminosilicate compositions

Equations are developed for calculating the density of aluminosilicate liquids as a function of composition and temperature. The mean molar volume at reference temperature T_r, is given by $\text{V}{}_{\mathrm{r}}\text{= ∑X}{}_{\mathrm{i}}\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{i}}\text{+ X}{}_{\mathrm{A}}\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{A}}$, where the summation is taken over all oxide components except A1₂O₃, X stands for mole fraction, terms are constants derived independently from an analysis of volume-composition relations in alumina-free silicate liquids, and $\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{A}}$ is the composition-dependent apparent partial molar volume of Al₂O₃. The thermal expansion coefficient of aluminosilicate liquids is given by $\text{α = ∑X}{}_{\mathrm{i}}\text{}\text{?}\text{ga}{}_{\mathrm{i}}{}^{\mathrm{o}}\text{+ X}{}_{\mathrm{A}}\text{}\text{?}\text{ga}{}_{\mathrm{A}}{}^{\mathrm{o}}$, where $\text{}\text{?}\text{ga}{}_{\mathrm{i}}{}^{\mathrm{o}}$ terms are constants independent of temperature and composition, and $\text{}\text{?}\text{ga}{}^{\mathrm{o}}{}_{\mathrm{A}}$ is a composition-dependent term representing the effect of Al₂O₃ on the thermal expansion. Parameters necessary to calculate the volume of silicate liquids at any temperature T according to V(T) = V_rexp[α(T-T_r)], where T_r = 1400°C have been evaluated by least-square analysis of selected density measurements in aluminosilicate melts. Mean molar volumes of aluminosilicate liquids calculated according to the model equation conform to experimentally measured volumes with a root mean square difference of 0.28 $\text{cc}\text{mole}$ and an average absolute difference of 0.90% for 248 experimental observations. The compositional dependence of $\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{A}}$ is discussed in terms of several possible interpretations of the structural role of Al³⁺ in aluminosilicate melts.     

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.     

Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution

The ¹⁷⁶Lu-¹⁷⁶Hf isotope method and its applications in earth sciences are discussed. Greater fractionation of Lu/Hf than Sm/Nd in planetary magmatic processes makes ${}^{176}\text{Hf}{}^{177}\text{Hf}$ a powerful geochemical tracer. In general, proportional variations of ${}^{176}\text{Hf}{}^{177}\text{Hf}$ exceed those of ${}^{143}\text{Nd}{}^{\mathrm{l44}}\text{Nd}$ by factors of 1.5–3 in terrestrial and lunar materials. Lu-Hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time, and this is the principal importance of Lu-Hf. New data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in ${}^{176}\text{Hf}{}^{177}\text{Hf}\text{-}{}^{87}\text{Sr}{}^{86}\text{Sr}$ and ${}^{143}\text{Nd}{}^{\mathrm{l44}}\text{Nd}\text{-}{}^{87}\text{Sr}{}^{86}\text{Sr}$ diagrams, ${}^{176}\text{Hf}{}^{177}\text{Hf}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ display a single, linear isotopic variation in the suboceanic mantle. These discordant ${}^{87}\text{Sr}{}^{86}\text{Sr}$ relationships may allow, with the acquisition of further Hf-Nd-Sr isotopic data, a distinction between processes such as mantle metasomatism, influence of seawater-altered material in the magma source, or recycling of sediments into the mantle. In order to evaluate the Hf-Nd isotopic correlation in terms of mantle fractionation history, there is a need for measurements of Hf distribution coefficients between silicate minerals and liquids, and specifically for a knowledge of Hf behavior in relation to rareearth elements. For studying ancient terrestrial Hf isotopic variations, the best quality Hf isotope data are obtained from granitoid rocks or zircons. New data show that very U-Pb discordant zircons may have upwardly-biased ${}^{176}\text{Hf}{}^{177}\text{Hf}$, but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial ${}^{176}\text{Hf}{}^{177}\text{Hf}$. Until the controls on addition of radiogenic Hf to zircon are understood, combined zircon-whole rock studies are recommended. Lu-Hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples, but because it offers little advantage over existing methods, is unlikely to find wide application in pure chronological studies.     

Retrograde fluids in the Archean Shawmere anorthosite, Kapuskasing Structural Zone, Ontario, Canada

The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite ± plagioclase show that the vein minerals were stable at T < 600 °C, XCO₂ < 0.4 at P ≈ 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T < 600 °C and XCO₂ < 0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T < 500 °C with XCO₂ > 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H₂O-rich to CO₂-rich. The calcite in the retrograde veins has δ¹⁸O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ¹³C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO₂-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO₂ ≈ 1.13 to 1.05 g/cm³). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO₂-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO₂-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO₂-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path. Received: 2 April 1996 / Accepted: 15 November 1996     

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.     

Carbon isotope composition of organic seston and sediments in a Georgia salt marsh estuary

The distribution of δ¹³C values for organic seston and sediment was determined in three sounds in the Spartina marsh estuaries along the Georgia coast, which had high, moderate, and low inputs of freshwater. Organic matter in all three sounds had similar carbon isotope compositions, for the most part within the range of marine values (δ¹³C of ?18%. to ?24%.). It appears that river flow does not introduce significant quantities of particulate C₃ plant material (δ¹³C of ?25%. to ?28%.) to Georgia estuaries. Evaluation of δ¹³C values of estuarine seston and three size fractions of sediment indicated that while Spartina carbon (δ¹³C of ?13%.) can be an important component of organic matter in intertidal sediments (mean δ¹³C of ?14.3%. to ?20.0%.), it is less so in subtidal sediments (mean δ¹³C of ?18.8%. to ?21.2%.), and it is hardly present at all in the seston (mean δ¹³C of ?24.5%.). δ¹³C values of dissolved inorganic carbon (DIC) in several water samples ranged between ?2.5%. and ?5.6%., suggesting that the isotope composition of estuarine DIC is influenced by respiratory CO₂ derived from metabolism of ¹³C-depleted plant carbon. Phytoplankton production utilizing this comparatively light DIC could be a source of relatively negative δ¹³C carbon in the estuary. Additional origins of estuarine organic matter greatly depleted in ¹³C compared to Spartina carbon remain to be identified.     

The effect of ionic interactions on the oxidation of metals in natural waters

The effect of ionic interactions of the major components of natural waters on the oxidation of Cu(I) and Fe(II) has been examined. The various ion pairs of these metals have been shown to have different rates of oxidation. For Fe(II), the chloride and sulfate ion pairs are not easily oxidized. The measured decrease in the rate constant at a fixed pH in chloride and sulfate solutions agrees very well with the values predicted. The effect of pH (6 to 8) on the oxidation of Fe(II) in water and seawater have been shown to follow the rate equation

$\text{-d in [Fe(II)]/dt = k}{}_1\text{β}{}_1\text{α}{}_{\mathrm{Fe}}\text{/[H}{}^{\mathrm{+}}\text{] + k}{}_2\text{β}{}_2\text{α}{}_{\mathrm{Fe}}\text{/[H}{}^{\mathrm{+}}\text{]}{}^2$

where k₁ and k₂ are the pseudo first order rate constants, β₁ and β₂ are the hydrolysis constants for Fe(OH)⁺ and Fe(OH)⁰. The value of α_FE is the fraction of free Fe²⁺. The value of k₁ (2.0 ±0.5 min^?1) in water and seawater are similar within experimental error. The value of k₂ (1.2 × 10⁵ min^?1) in seawater is 28% of its value in water in reasonable agreement with predictions using an ion pairing model.For the oxidation of Cu(I) a rate equation of the form

$\text{?d ln [Cu(I)]/dt = k}{}_0\text{α}{}_{\mathrm{Cu}}\text{+ k}{}_1\text{β}{}_1\text{α}{}_{\mathrm{Cu}}\text{[Cl]}$

was found where k₀ (14.1 sec^?1) and k₁ (3.9 sec^?1) are the pseudo first order rate constants for the oxidation of Cu⁺ and CuCl⁰, β₁ is the formation constant for CuCl⁰ and α_Cu is the fraction of free Cu⁺. Thus, unlike the results for Fe(II), Cu(I) chloride complexes have measurable rates of oxidation.     

The coprecipitation of Sr2+ with calcite at 25°C and 1 atm

The incorporation of Sr²⁺ into calcite at earth surface aqueous conditions is affected by the absolute concentration of Sr²⁺, the presence of Ba²⁺ and NaCl in the solution, and the rate of precipitation. At solution ratios (molar) of Sr²⁺ to Ca²⁺ in the low 10⁻³ range, which yield calcites with several hundred ppm Sr²⁺, K^sr_calcite typically assumes a value between 0.10 and 0.20. Above these concentrations the value of k^Sr_calcite drops to approximately 0.06. Furthermore, if minor amounts of Ba²⁺ or large amounts of Na⁺ (0.48 M) are added to a dilute Sr²⁺ solution, a value around 0.06 for k^Sr_calciteis found. This “strontium concentration effect” and the associated “competitive cation effect” suggest that small amounts of Sr²⁺ may be incorporated into a limited number of nonlattice sites in calcite. Incorporation of Sr²⁺ into these sites, presumably defects, noticeably affects k^Sr_calcite only at low Sr²⁺ concentrations and in the absence of competition from other large cations.An increase in k^Sr_calcite with rate of precipitation, qualitatively similar to that found in other studies, was observed only when precipitation times were decreased from days to hours.For many geologic settings a partition coefficient for Sr²⁺ into calcite of 0.06 appears appropriate, but there are situations—very low Sr²⁺ concentrations, the presence of Mg²⁺, and fast precipitation rates—in which a larger value might better approximate natural partitioning.     

Conceptual problems with remote element synthesis

The notion of remote element synthesis has recently been modified to explain the presence of nucleogenetic isotopic anomalies and decay products of short-lived nuclides by injection of a small amount ofexotic nucleogenetic material. Even with this modification, remote element synthesis seems inconsistent with the following observations: