Strontium diffusion in sanidine and albite, and general comments on strontium diffusion in alkali feldspars

Strontium chemical diffusion has been measured in albite and sanidine under dry, 1 atm, and QFM buffered conditions. Strontium oxide-aluminosilicate powdered sources were used to introduce the diffusant and Rutherford Backscattering Spectroscopy (RBS) used to measure diffusion profiles. For the 1 atm experiments, the following Arrhenius relations were obtained:

Sanidine (Or₆₁), temperature range 725–1075°C, diffusion normal to (001): D=8.4 exp(−450±13 kJ mol⁻¹/RT) m²s⁻¹. Albite (Or₁), temperature range 675–1025°C, diffusion normal to (001): D=2.9 × exp(−224±11 kJ mol⁻¹/RT) m²s⁻¹.

The alkali feldspars in this and earlier work display a broad range of activation energies for Sr diffusion, which may be a consequence of the thermodynamic non-ideality of the alkali feldspar system and/or the mixed alkali effect.     

Dissolution/precipitation kinetics of boehmite and gibbsite: Application of a pH-relaxation technique to study near-equilibrium rates

The dissolution and precipitation rates of boehmite, AlOOH, at 100.3 °C and limited precipitation kinetics of gibbsite, Al(OH)₃, at 50.0 °C were measured in neutral to basic solutions at 0.1 molal ionic strength (NaCl + NaOH + NaAl(OH)₄) near-equilibrium using a pH-jump technique with a hydrogen-electrode concentration cell. This approach allowed relatively rapid reactions to be studied from under- and over-saturation by continuous in situ pH monitoring after addition of basic or acidic titrant, respectively, to a pre-equilibrated, well-stirred suspension of the solid powder. The magnitude of each perturbation was kept small to maintain near-equilibrium conditions. For the case of boehmite, multiple pH-jumps at different starting pHs from over- and under-saturated solutions gave the same observed, first order rate constant consistent with the simple or elementary reaction: .

This relaxation technique allowed us to apply a steady-state approximation to the change in aluminum concentration within the overall principle of detailed balancing and gave a resulting mean rate constant, (2.2 ± 0.3) × 10⁻⁵ kg m⁻² s⁻¹, corresponding to a 1σ uncertainty of 15%, in good agreement with those obtained from the traditional approach of considering the rate of reaction as a function of saturation index. Using the more traditional treatment, all dissolution and precipitation data for boehmite at 100.3 °C were found to follow closely the simple rate expression:

R_net,boehmite=10^-5.485{m_OH^-}{1-exp(ΔG_r/RT)}, with R_net in units of mol m⁻² s⁻¹. This is consistent with Transition State Theory for a reversible elementary reaction that is first order in OH⁻ concentration involving a single critical activated complex. The relationship applies over the experimental ΔG_r range of 0.4–5.5 kJ mol⁻¹ for precipitation and −0.1 to −1.9 kJ mol⁻¹ for dissolution, and the pH_m ≡ −log(mH⁺) range of 6–9.6. The gibbsite precipitation data at 50 °C could also be treated adequately with the same model:R_net,gibbsite=10^-5.86{m_OH^-}{1-exp(ΔG_r/RT)}, over a more limited experimental range of ΔG_r (0.7–3.7 kJ mol⁻¹) and pH_m (8.2–9.7).     

Mg-Fe云母化学成分的解释和分类(Ⅰ)——基本置换和黑云母平面

中国东部花岗岩类141个Mg-Fe云母的化学成分将近90%的变化属于八面体层内的类质同象置换,置换矢量Mg ₁Fe⁺²和Fe_-3⁺²(R_Ⅵ⁺³)_-2□_Ⅵ组成了天然黑云母平面,大约80%的变化应当解释为基本置换8Mg ₁Fe⁺²+Fe_-3⁺²(R_Ⅵ⁺³)₂□_Ⅵ.这些是Mg-Fe云母在广泛的自然条件下表现出来的最主要的晶体化学关系。文中还提出了置换矢量的长度、分量和以及电价和三个参数,用以识别矿物化学成分变化的类质同象置换特征。     

Petrogenesis of ultramafic rocks and associated chromitites in the Nan Uttaradit ophiolite, Northern Thailand

The ultramafic sequence and associated chromitites of the Nan-Uttaradit ophiolite in the northeastern part of Thailand have been studied in the field and by applying petrography and geochemistry to whole rock samples and minerals. The ultramafic rocks comprise irregulary shaped bodies of dunite, harzburgite, orthopyroxene-rich lherzolite and orthopyroxene-rich harzburgite, clinopyroxene-rich dunite and intrusive clinopyroxenite-websterite bodies. Three types of chromitite were distinguished. Type I chromitite lenses and type II layers which are hosted in orthopyroxenite in the northern part and in dunite in the central part of the ophiolite. Type III chromitite forms lenses or layers in clinopyroxenites in the central and southern parts of the belt. According to the modal and chemical composition the peridotites and orthopyroxenites are strongly refractory. They originated during different stages of interaction between percolating melts and peridotite. The chromitites of types I and II, which are very rich in Cr (up to 68 wt.% Cr₂0₃), crystallized from a boninitic parental magma under highly reducing conditions in the northern part and moderate oxygen fugacities (FMQ) in the central part of the ophiolite. The chromitite of type III which are characterized by the highest Fe³⁺/(Fe³⁺ + Cr + Al) -ratios, and hosted in intrusive clinopyroxenite-websterite-rocks, cumulated from a CaO-rich transitional boninitic melt under fO₂ conditions around FMQ.     

Status report on stability of K-rich phases at mantle conditions

Experimental research on K-rich phases and observations from diamond inclusions, UHP metamorphic rocks, and xenoliths provide insights about the hosts for potassium at mantle conditions. K-rich clinopyroxene (Kcpx–KM³⁺Si₂O₆) can be an important component in clinopyroxenes at P>4 GPa, dependent upon coexisting K-bearing phases (solid or liquid) but not, apparently, upon temperature. Maximum Kcpx content can reach 25 mol%, with 17 mol% the highest reported in nature. Partitioning ^(K)D_(cpx/liquid) above 7 GPa=0.1–0.2 require ultrapotassic liquids to form highly potassic cpx or critical solid reactions, e.g., between Kspar and Di. Phlogopite can be stable to about 8 GPa at 1250 °C where either amphibole or liquid forms. When fluorine is present, it generally increases in Phl upon increasing P (and probably T) to about 6 GPa, but reactions forming amphibole and/or KMgF₃ limit F content between 6 and 8 GPa. The perovskite KMgF₃ is stable up to 10 GPa and 1400 °C as subsolidus breakdown products of phlogopite upon increasing P. ^(M4)K-substituted potassic richterite (ideally K(KCa)Mg₅Si₈O₂₂(OH,F)₂) is produced in K-rich peridotites above 6 GPa and in Di+Phl from 6 to 13 GPa. K content of amphibole is positively correlated with P; Al and F content decrease with P. In the system 1Kspar+1H₂O K-cymrite (hydrous hexasanidine–KAlSi₃O₈·nH₂O–Kcym) is stable from 2.5 GPa at 400 to 1200 °C and 9 GPa; Kcym can be a supersolidus phase. Formation of Kcym is sensitive to water content, not forming within experiments with H₂O2O>Kspar. Phase X, a potassium di-magnesium acid disilicate ((K_1−x−n)₂(Mg_1−nM_n³⁺)₂Si₂O₇H_2x), forms in mafic compositions at T=1150–1400 °C and P=9–17 GPa and is a potential host for K and H₂O at mantle conditions with a low-T geotherm or in subducting slabs. The composition of phase-X is not fixed but actually represents a solid solution in the stoichiometries □₂Mg₂Si₂O₇H₂–(K□)Mg₂Si₂O₇H–K₂Mg₂Si₂O₇ (□=vacancy), apparently stable only near the central composition. K-hollandite, KAlSi₃O₈, is possibly the most important K-rich phase at very high pressure, as it appears to be stable to conditions near the core–mantle boundary, 95 GPa and 2300 °C. Other K-rich phases are considered.     

A palaeomagnetic study of some basaltoids and ores from the Pontic Ranges, Northern Turkey: Palaeogeographic implications

The palaeomagnetism of basic rocks and sulphide ores has been studied in the Küre area, Pontic Ranges, Turkey. Progressive alternating-field demagnetization revealed a characteristic remanent magnetization in all investigated rock types except a dacite. The following virtual geomagnetic poles were obtained:

Basalt and quartz diabase (oldest): D = 59°, I = +66°, ₉₅ = 4.8, pole 49°N, 93°E. Diabase: D = 210°, I = −15°, ₉₅ = 15.0, pole 47°N, 167°E. Massive sulphide ores: D = 107°, I = +63°, ₉₅ = 8.7, pole 18°N, 80°E. Peridotite: D = 131°, I = +54°, ₉₅ = 10.9, pole 2°S, 72°E. Amphibolitized diabase (youngest): D = 293°, I = +59°, ₉₅ = 12.6, pole 40°S, 145°E.

The longidutinal difference in pole positions between the oldest and the youngest rocks is interpreted as being due to a post-Permian counterclockwise rotation of the studied region in relation to the African continent. In addition, there are indications of local rotational movements within the Küre area.     

Lithium in the interlayer space of synthetic trioctahedral micas

This paper gives several new and strong arguments in favour of the possibility of fixation of anhydrous lithium in the interlayer space of trioctahedral potassium micas. From the chemical viewpoint Li⁺ can replace K⁺, but is located out of the alkaline cation site; it enters pseudo-octahedral cavities limited by the triangular bases of two aluminous tetrahedra of two consecutive sheets. The solubility limit of Li⁺ in the interlayer is a function of the Al^IV content of the mica. It is given by the relation (Li/Li + K)_max = 2[(Al/Si + Al)^IV]². In both micas investigated — phlogopite, KMg₃(Si₃Al)O₁₀(OH)₂, and eastonite, K(Mg_2.5Al_1.5)(Si_2.5Al_1.5)O₁₀(OH)₂ — there is a remarkable agreement between the calculated values of the solubility limits and those measured by exchange reactions with hydrothermal solutions, at 600°C, 2 kbar. In high-Al micas, the interlayer Li content can be very important, with about one-third of K⁺ replaced by Li⁺.

The fixation of Li⁺ according to this model provokes a strong flattening of the interlayer (strong decrease of the reticular distance d₀₀₅) and a slight increase of the reticular distance d₀₆₀. Infra-red (IR) absorption spectrometry shows that vacant K⁺ sites are created when Li⁺ enters the interlayer; one observes low-frequency OH stretching bands attributed to OH dipoles lying towards these empty sites. Fixation of Li⁺ does not provoke any modification of the IR spectra in the region 1200-300 cm⁻¹, indicating that Li⁺ is really out of the sheet. For both cell dimensions and IR spectra, a comparison is made with “ordinary” lepidolites, having Li⁺ in the octahedral sheet; it provides a guide for the distinction between the two species of Li-bearing micas.     

Geochemistry of microgranular enclaves and host granite from Telões (Vila Pouca de Aguiar), Northern Portugal

At Telões, a subaluminous medium- to coarse-grained porphyritic biotite granite, crops out along the Vila Real NNE–SSW fault. It is a post-tectonic granite of 299±3 Ma old given by U–Pb isotopic data on zircon. It contains metaluminous to subaluminous tonalitic, granodioritic and monzogranitic enclaves. All granitoids have Fe²⁺-biotite and some enclaves contain magnesiohornblende and subsolidus actinolite. Monzogranitic enclaves show obvious similarities to the host granite. Linear array between enclaves and host granite is observed in Rb–Sr typical isochron diagram and gives the age of 286±11 Ma and (⁸⁷Sr/⁸⁶Sr)₀=0.7063±0.0011. Microgranular enclaves have δ¹⁸O values similar to those of the hosting granite. Microgranular enclaves are hybrid rocks probably formed by mixing between a tonalitic enclave magma and a host granite magma as supported by the modelling of major and trace elements. The similar isotopic signatures suggest a subsequent partial equilibration of the enclaves and granite magmas.     

Mineral chemistry and thermobarometry of inclusions from De Beers Pool diamonds, Kimberley, South Africa

Silicate and oxide mineral inclusions in diamonds from the geologically and historically important De Beers Pool kimberlites in Kimberley, South Africa, are characterised by harzburgitic compositions (>90%), with lesser abundances from eclogitic and websteritic parageneses. The De Beers Pool diamonds contain unusually high numbers of inclusion intergrowths, with garnet+orthopyroxene±chromite±olivine and chromite+olivine assemblages dominant. More unusual intergrowths include garnet+olivine+magnesite and an eclogitic assemblage comprising garnet+clinopyroxene+rutile. The mineral chemistry of the De Beers Pool inclusions overlaps that of most worldwide localities. Peridotitic garnet inclusions exhibit variable CaO (<5.8 wt.%) and Cr₂O₃ contents (3.0–15.0 wt.%), although the majority are harzburgitic with very low calcium concentrations (<2 wt.% CaO). Eclogitic garnet inclusions are characterised by a wide range in CaO (3.3–21.1 wt.%) with low Cr₂O₃ (<1 wt.%). Websteritic garnets exhibit intermediate compositions. Most chromite inclusions contain 63–67 wt.% Cr₂O₃ and <0.5 wt.% TiO₂. Olivine and orthopyroxene inclusions are magnesium-rich with Mg-numbers of 93–97. Olivine inclusions in chromite exhibit the highest Mg-numbers and also contain elevated Cr₂O₃ contents up to 1.0 wt.%. Peridotitic clinopyroxene inclusions are Cr-diopsides with up to 0.8 wt.% K₂O. Eclogitic and websteritic clinopyroxene inclusions exhibit overlapping compositions with a wide range in Mg-numbers (66–86).

Calculated temperatures for non-touching inclusion pairs from individual diamonds range from 1082 to 1320 °C (average=1197 °C), whereas pressures vary from 4.6 to 7.7 GPa (average=6.3 GPa). Touching inclusion assemblages are characterised by equilibration temperatures of 995 to 1182 °C (average=1079 °C) and pressures of 4.2–6.8 GPa (average=5.4 GPa). Provided that the non-touching inclusions represent equilibrium assemblages, it is suggested that these inclusions record the conditions at the time of diamond crystallisation (1200 °C; 3.0 Ga). The lower average temperatures for touching inclusions are attributed to re-equilibration in a cooling mantle (1050 °C) prior to kimberlite eruption at 85 Ma. Pressure estimates for touching garnet–orthopyroxene inclusions are also skewed towards lower values than most non-touching inclusions. This apparent difference may be an artefact of the Al-exchange geobarometer and/or the result of sampling bias, due to limited numbers of non-touching garnet–orthopyroxene inclusions. Alternatively pressure differences could be caused by differential uplift in the mantle or possibly variations in thermal compressibility between diamond and silicate inclusions. However, thermodynamic modelling suggests that thermal compressibility differences would cause only minor changes in internal inclusion pressures (<0.2 GPa/100 °C).     

Thermodynamics of U(VI) sorption onto Shewanella putrefaciens

We have conducted acid–base potentiometric titrations and U(VI) sorption experiments using the Gram negative, facultatively anaerobic bacterium Shewanella putrefaciens. Results of reversed titration studies on live, inactive bacteria indicate that their pH-buffering properties result from the equilibrium ionization of three discrete populations of functional groups. Carboxyl (pK_a=5.16±0.04), phosphoryl (pK_a=7.22±0.15) and amine (pK_a=10.04±0.67) groups most likely represent these three resolvable functionalities, based on their pK_a values. Site densities for carboxyl, phosphoryl and amine groups on the bacterial surface were approximately 31.7 μmol sites/g bacteria (0.35±0.02 sites/nm²), 8.95 μmol/g (0.11±0.007 sites/nm²) and 38.0 μmol/g (0.42±0.008 sites/nm²), respectively, based on an estimated bacterial specific surface area of 55 m²/g. Sorption experiments showed that U(VI) can reversibly complex with the bacterial surface in the pH 2–8 interval, with maximum adsorption occurring at a pH of 5. Sorption is not strongly sensitive to ionic strength (NaCl) in the range 0.02–0.10 M. The pH and ionic strength dependence of U(VI) sorption onto S. putrefaciens is similar to that measured for metal-oxide surfaces and Gram positive bacteria, and appears to be similarly governed by competitive speciation constraints. Measured U(VI) sorption is accounted for by using two separate adsorption reactions forming the surface complexes >COO–UO₂⁺ and >PO₄H–UO₂(OH)₂. Using S. putrefaciens as a model organism for dissimilatory metal-reducing Gram negative anaerobes, our results extend the applicability of geochemical speciation models to include bacteria that are capable of reductively solubilizing or precipitating a wide variety of environmentally and geologically important metals and metallic species.     

Beitrag zur Geochemie des Chroms

Chromium has been determined in 227 rocks and minerals by means of the diphenylcarbazido colorimetric method, which was modified for this purpose. The mean error of the method, expressed as the difference between duplicate determinations, is ±2%, and chromium contents down to 1 p.p.m. can be measured.

The analyses show that chromium is concentrated in the basic and ultrabasic rocks, while the granites investigated generally contained only about 1 p.p.m. Cr. In the basic rocks, the chromium is present in specific chromium minerals such as chromite and picotite, and also in magnetites and pyroxenes. The felspar contains almost no chromium.

The chromium content of volcanic rocks decreases steadily with increasing SiO₂ content. Thus, the melilite basalt of Gotzenbruhl with 34·7% SiO₂ contains 1380 p.p.m. Cr, while the Aar granite with 77% SiO₂ contains but 1 p.p.m. Cr.

Among the sedimentary rocks, high chromium contents are found in bauxites and in sedimentary iron ore deposits. In the bauxites this is due to relative (secondary) enrichment of chromium, while in the iron ores chromium may be associated with the iron in a colloidal state.

Pelitic sediments show a very uniform chromium content. Of 40 samples investigated, 24 contained between 70 and 110 p.p.m. Cr., while the remainder were also fairly close to these values.

For the most part, the chromium in sediments is concentrated in the micas and clay minerals, particularly in illite.

Zusammenfassung

Nach der kolorimetrischen Methode mit Diphenylcarbazid, die für diesen Zweck modifiziert wurde, sind an 227 Gesteinen und Mineralen Chrombestimmungen durchgeführt worden. Der mittlere Fehler der Methode, mit der man Chrommengen bis zu 1 p.p.m. nachweisen kann, beträgt bei je zwei Bestimmungen ±2%.

Die Analysen zeigen, dass das Chrom in den basischen und ultrabasischen Gesteinen angereichert ist, während die untersuchten Granite gewöhnlich nur 1 p.p.m. Cr enthielten. In den basischen Gesteinen steckt das Chrom in eigenen Chrommineralen, wie Chromit und Picotit, in den Magnetiten und in den Pyroxenen. Der Feldspat enthält fast kein Chrom.

Der Chromgehalt der Eruptivgesteine nimmt mit steigendem SiO₂-Gehalt kontinuierlich ab. So enthält der Melilithbasalt von Götzenbrühl 34,7% SiO₂ und 1380 p.p.m. Cr, der Aaregranit 77% Si0₂ und dementsprechend nur 1 p.p.m. Cr.

Bei den Sedimenten findet man hohe Chromgehalte in den Bauxiten und auch in manchen sedimentären Eisenerzlagerstätten. In den Bauxiten handelt es sich um eine relative Anreicherung des Chroms, in die Eisenerzlagerstätten kann es kolloidal mit dem Eisen gelangen.

Einen sehr gleichmässigen Chromgehalt weisen die Pelite auf. Von 40 untersuchten Proben liegen 24 zwischen 70 und 110 p.p.m. Die restlichen Proben liegen nahe um diese Werte herum.

Das Chrom der Sedimente steckt zum allergrössten Teil in den Glimmern und Tonmineralen und da vor allem im Illit. Die Mobilität des Chroms bei der Metamorphose ist sehr gering. Deshalb kann man mit Hilfe des Chromgehalts in manchen Fällen entscheiden, ob ein bestimmter Amphibolit ein Orthooder ein Paragestein ist.

Der durchschnittliche Chromgehalt für die Sedimente wurde zu 92 p.p.m. Cr berechnet. Der Chromgehalt der Eruptivgesteine konnte nicht genau berechnet werden. Es wurde angenommen, dass er nicht allzusehr vom mittleren Chromgehalt der Sedimente abweicht, so dass sich ein mittlere Chromgehalt der Erdruste von etwa 90 p.p.m. Cr ergibt.     

Solubility of silica polymorphs in electrolyte solutions, I. Activity coefficient of aqueous silica from 25° to 250°C, Pitzer''s parameterisation

Within the framework of Pitzer's specific interaction model, interaction parameters for aqueous silica in concentrated electrolyte solutions have been derived from Marshall and co-authors amorphous silica solubility measurements. The values, at 25°C, of the Pitzer interaction parameter (λ_SiO2(aq)−i) determined in this study are the following: 0.092 (i = Na⁺), 0.032 (K⁺), 0.165 (Li⁺), 0.292 (Ca²⁺, Mg²⁺), −0.139 (SO₄²⁻), and −0.009 (NO₃⁻). A set of polynomial equations has been derived which can be used to calculate λ_SiO2(aq)−i for these ions at any temperature up to 250°C. A linear relationship between the aqueous silica-ion interaction parameters (λ_SiO2(aq)−i) and the surface electrostatic field (Z_i/r_e,i) of ions was obtained. This empirical equation can be used to estimate, in first approximation, λ_SiO2(aq)−i if no measurements are available. From this parameterisation, the calculated activity coefficient of aqueous silica is 2.52 at 25°C and 1.45 at 250°C in 5 m NaCl solution. At lower concentrations, e.g. 2 m NaCl, the activity coefficient of silica is 1.45 at 25°C and 1.2 at 250°C. Hence, in practice, it is necessary to take into account the activity coefficient of aqueous silica (λ_SiO2(aq)≠1) in hydrothermal solutions and basinal brines where the ionic strength exceeds 1. A comparison of measured [Marshall, W.L., Chen, C.-T.A., 1982. Amorphous silica solubilities, V. Prediction of solubility behaviour in aqueous mixed electrolyte solutions to 300°C. Geochim. Cosmochim. Acta 46, 289–291.] and computed amorphous silica solubility, using this parameterisation, shows a good agreement. Because the effect of individual ions on silicate and silica polymorph solubilities are additive, the present study has permitted to derive Pitzer interaction parameters that allow a precise computation of γ_SiO2(aq) in the Na---K---Ca---Mg---Cl---SO₄---HCO₃---SiO₂---H₂O system, over a large range of salt concentrations and up to temperatures of 250°C.     

Influence of volcanic activity on spring water chemistry at Popocatepetl Volcano, Mexico

The results of the 7 years (1994–2000) of monthly monitoring of spring water before and during eruptions show response to volcanic activity. Low salinity and temperature characterize most of the springs, which are located on the flanks of Popocatepetl Volcano. The pH ranges from 5.8 to 7.8 and temperature from 3 to 36 °C. Oxygen and hydrogen isotopic data show that the water is of meteoric origin, but SO₄²⁻, Cl⁻, F⁻, HCO₃⁻, B, and SO₄²⁻/Cl⁻ variations precede main eruptive activity, which is considered linked to influx of magmatic gases and acid fluids that react with sublimates and host rock and mix with the large water system. Na⁺, Ca²⁺, SiO₂ and Mg²⁺ concentrations in the water also increased before eruptive activity. The computed partial pressure of CO₂ in equilibrium with spring waters shows values higher than air-saturated water (ASW), with the highest values up to 0.73 bar of pCO₂. Boron is detected in the water only preceding the larger eruptions. When present, boron concentration is normally under health standard limits, but in two cases the concentration was slightly above. Other components are within health standard limits, except for F⁻ in one spring.     

松辽盆地北部中二叠统碳酸盐岩元素和稳定同位素地球化学特征与古环境

松辽盆地中二叠统哲斯组发育一套暗色泥岩、灰色砂砾岩与灰岩沉积组合,灰岩中产海相腕足类和双壳类化石。由于受钻井进尺和取心资料限制,前人对盆地覆盖区晚古生代地层及其沉积环境研究较少。对采自松辽盆地杜101井、杜103井剖面哲斯组的白云质灰岩进行了系统的稳定同位素和微量元素、稀土元素分析,首次获取了盆地覆盖区中二叠统灰岩元素地球化学资料。测试结果显示碳同位素δ¹³C_PDB值一般>0(均值为1.68‰),氧同位素δ¹⁸O_PDB值全部<-15‰(均值为-19.98‰),碳、氧稳定同位素关系图解显示二者呈正相关关系,表明白云岩化作用对灰岩同位素改造作用明显。稀土元素(REE+Y)总量为56.88×10^-6~143.72×10^-6,均值为108.92×10^-6,PAAS标准化后显示具有轻稀土相对于中稀土和重稀土亏损、δEu(均值0.94,最大值1.57)负异常、δCe(均值0.87,最大值1.06)普遍负异常和相对高的Y/Ho均值(均值34.59)等特点,与正常海相具有大体相同的稀土元素配分模式;而微量元素Ba(均值281.55×10^-6)、Zn(均值71.91×10^-6)、Y(均值17.44×10^-6)、Zr(均值117.75×10^-6)和Rb(均值66.84×10^-6)等元素含量整体较高,Sr/Ba(均值2.91)、Th/U(均值2.75)、V/Cr(均值1.45)和V/(V+Ni)(均值0.66)等比值适中,指示研究区哲斯组沉积时期主体为一种频繁受陆源碎屑混染作用影响的碳酸盐台地或滨海岸环境。     

Deuterium abundance of natural waters

The deuterium abundance of natural water of various sources has been determined by the use of a deuterium mass spectrometer. The absolute deuterium contents of the reference sample nos. 1 and 1a of the U.S. National Bureau of Standards were found to be 0·0149 and 0·0128 mole % D₂O, respectively, by the comparison of the samples with the standard samples of known deuterium content. Relative enrichment of no. la to no. 1 (R_1a−R₁)/R₁×100 was 14·13 ± 0·23, which agrees perfectly to the value reported by . The variation of deuterium contents of tap waters of Tokyo and Osaka during last 2 years were within ± 1 per cent and any seasonal regularities could not be detected. Marine waters of the West Pacific have almost the same deuterium content as those of the Pacific coast of the United States. Marine waters at various depths of the Antarctic showed relatively large variation of deuterium content (from +0·84 to −1·16 per cent on the SMOW-scale), whereas the samples of Black Current of the Pacific showed little variation. It was found that the deuterium content of spring water suggests the origin of springs in some cases.     

塔里木北缘震旦纪—寒武纪转折期碳同位素漂移事件及成因机制

震旦纪—寒武纪转折期是地球演化的关键节点,这一时期的碳-氧同位素记录在塔里木保留完整却关注较少。选取苏盖特布拉克露头震旦系与寒武系交界地层实测采样,并开展了古生物、镜下鉴定、碳同位素漂移事件的综合分析,探讨了N1(BACE)、P1(ZHUCE)等碳同位素漂移事件的成因机制。样品的δ¹³C_carb和δ¹⁸O_carb相关系数(R²=0.05)、δ¹⁸O_carb和Mn/Sr相关系数(R²=0.09)及岩石学特征表明,后期成岩改造并未导致碳-氧同位素的显著分馏,原始碳同位素特征得以基本保留。由下至上,在该剖面识别出P-1事件(δ¹³C_carb峰值1.9‰2.4‰)、N1事件(-6.8‰-10.3‰)、P1事件(1.4‰4.1‰)、N2a-c事件(-0.4‰-2.8‰)、P2a-c事件(0.2‰0.6‰)和N3事件(-3.4‰)。综合碳同位素演化特征与古生物、年代学数据,确认了玉尔吐斯组底部硅质页岩与奇格布拉克组顶部藻云岩的分界面为塔里木震旦系与寒武系的分界线,并实现了该露头与老林、肖滩、三峡、西伯利亚、阿曼和摩洛哥剖面的地层对比。分析认为,塔里木北缘震旦纪—寒武纪转折期的碳漂移事件,更多受控于古海洋氧气含量变化引起的固碳率f_org的波动。玉尔吐斯组早期海侵背景下的大规模缺氧事件导致的初级生产力和固碳率降低,是N1负漂移事件(BACE)的主因;随后海退中氧气含量的增加引起生产力重建和固碳率增加,形成了P1正漂移事件(ZHUCE)。这一成果性认识有助于塔里木盆地寒武纪古环境研究与深层超深层油气远景资源评价。     

稀有金属矿物溶解度对花岗伟晶岩成矿作用的制约

花岗伟晶岩型矿床是稀有金属矿床重要的类型之一。在花岗伟晶岩中,稀有金属元素Li、Be、Nb和Ta主要以独立矿物的形式存在,前人对稀有金属独立矿物在硅酸盐熔体中的溶解度及其影响因素展开了系统研究。本文综合分析了已有的实验数据,其结果表明,影响稀有金属独立矿物溶解度最为重要的2个参数是温度(T)和铝饱和指数(ASI)。因此本文建立了稀有金属独立矿物,尤其是铌锰矿和钽锰矿溶解度,与温度(T)和铝饱和指数(ASI)之间的定量关系: lg [w(Li)/10^-6]=-0.37×[1 000/(T/K)]+4.56,R²=0.44 lg [w(BeO)/10^-6]=-4.21×[1 000/(T/K)]+6.86,R²=0.91 lg [K_sp(Nb)/(mg²·kg^-2)]=-(2.86±0.14)×ASI_(Mn+Li)-(4.95±0.31)×[1 000/(T/K)]+(4.20+0.28),R²=0.86 lg [K_sp(Ta)/(mg²·kg^-2)]=-(2.46±0.11)×ASI_(Mn+Li)-(4.86±0.30)×[1 000/(T/K)]+(4.00+0.30),R²=0.80 式中,温度T为热力学温度,ASI_(Mn+Li)(ASI=Al₂O₃/(CaO+Na₂O+K₂O+Li₂O+MnO),摩尔分数比)和T的适用范围分别为0.6~1.2和1 073~1 373 K的范围内。上述公式为估算硅酸盐熔体中稀有金属含量提供了便利,为量化花岗伟晶岩成矿模型提供了基础。 稀有金属独立矿物溶解度随温度降低和铝饱和指数的增加而急剧降低,因此,在岩浆演化过程中,由岩浆侵位、分离结晶以及流体作用等因素引起的岩浆温度降低和铝饱和指数的增加,是导致稀有金属独立矿物结晶的主要机制。     

Effect of Fe on garnet–melt trace element partitioning: experiments in FCMAS and quantification of crystal-chemical controls in natural systems

Garnet–melt trace element partitioning experiments were performed in the system FeO–CaO–MgO–Al₂O₃–SiO₂ (FCMAS) at 3 GPa and 1540°C, aimed specifically at studying the effect of garnet Fe²⁺ content on partition coefficients (D^Grt/Melt). D^Grt/Melt, measured by SIMS, for trivalent elements entering the garnet X-site show a small but significant dependence on garnet almandine content. This dependence is rationalised using the lattice strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454], which describes partitioning of an element i with radius r_i and valency Z in terms of three parameters: the effective radius of the site r₀(Z), the strain-free partition coefficient D₀(Z) for a cation with radius r₀(Z), and the apparent compressibility of the garnet X-site given by its Young's modulus E_X(Z). Combination of these results with data in Fe-free systems [Van Westrenen, W., Blundy, J.D., Wood, B.J., 1999. Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt. Am. Mineral. 84, 838–847] and crystal structure data for spessartine, andradite, and uvarovite, leads to the following equations for r₀(3+) and E_X(3+) as a function of garnet composition (X) and pressure (P):

r₀(3+) [Å]=0.930X_Py+0.993X_Gr+0.916X_Alm+0.946X_Spes+1.05(X_And+X_Uv)−0.005(P [GPa]−3.0)(±0.005 Å)

E_X(3+) [GPa]=3.5×10¹²(1.38+r₀(3+) [Å])^−26.7(±30 GPa)

Accuracy of these equations is shown by application to the existing garnet–melt partitioning database, covering a wide range of P and T conditions (1.8 GPa<P<5.0 GPa; 975°C<T<1640°C). D^Grt/Melt for all 3+ elements entering the X-site (REE, Sc and Y) are predicted to within 10–40% at given P, T, and X, when D^Grt/Melt for just one of these elements is known. In the absence of such knowledge, relative element fractionation (e.g. D_Sm^Grt/Melt/D_Nd^Grt/Melt) can be predicted. As an example, we predict that during partial melting of garnet peridotite, group A eclogite, and garnet pyroxenite, r₀(3+) for garnets ranges from 0.939±0.005 to 0.953±0.009 Å. These values are consistently smaller than the ionic radius of the heaviest REE, Lu. The above equations quantify the crystal-chemical controls on garnet–melt partitioning for the REE, Y and Sc. As such, they represent a major advance en route to predicting D^Grt/Melt for these elements as a function of P, T and X.     

Oxidative dissolution of pyritic sludge from the Aznalcóllar mine (SW Spain)

As a result of the collapse of a mine tailing dam, a large extension of the Guadiamar valley was covered with a layer of pyritic sludge. Despite the removal of most of the sludge, a small amount remained in the soil, constituting a potential risk of water contamination. The kinetics of the sludge oxidation was studied by means of laboratory flow-through experiments at different pH and oxygen pressures. The sludge is composed mainly of pyrite (76%), together with quartz, gypsum, clays, and sulphides of zinc, copper, and lead. Trace elements, such as arsenic and cadmium, also constitute a potential source of pollution. The sludge is fine grained (median of 12 μm) and exhibits a large surface (BET area of 1.4±0.2 m² g⁻¹).

The dissolution rate law of sludge obtained is r=10^{−6.1(±0.3)} [O₂(aq)]^0.41(±0.04) a_H+^0.09(±0.06) g_sludge m⁻² s⁻¹ (22 °C, pH=2.5–4.7). The dissolution rate law of pyrite obtained is r=10^{−7.8(±0.3)} [O₂(aq)]^0.50(±0.04) a_H+^0.10(±0.08) mol m⁻² s⁻¹ (22 °C, pH=2.5–4.7). Under the same experimental conditions, sphalerite dissolved faster than pyrite but chalcopyrite dissolves at a rate similar to that of pyrite. No clear dependence on pH or oxygen pressure was observed. Only galena dissolution seemed to be promoted by proton activity. Arsenic and antimony were released consistently with sulphate, except at low pH conditions under which they were released faster, suggesting that additional sources other than pyrite such as arsenopyrite could be present in the sludge. Cobalt dissolved congruently with pyrite, but Tl and Cd seemed to be related to galena and sphalerite, respectively.

A mechanism for pyrite dissolution where the rate-limiting step is the surface oxidation of sulphide to sulphate after the adsorption of O₂ onto pyrite surface is proposed.     

Marcasite precipitation from hydrothermal solutions

Pyrite and marcasite were precipitated by both slow addition of aqueous Fe²⁺ and SiO₃²⁻ to an H₂S solution and by mixing aqueous Fe²⁺ and Na₂S₄ solutions at 75°C. H₂S₂ or HS₂⁻ and H₂S₄ or HS₄⁻ were formed in the S₂O₃²⁻ and Na₂S₄ experiments, respectively. Marcasite formed at pH < pK₁ of the polysulfide species present (for H₂S₂, pK₁ = 5.0; for H₂S₄, pK₁ = 3.8 at 25°C). Marcasite forms when the neutral sulfane is the dominant polysulfide, whereas pyrite forms when mono-or divalent polysulfides are dominant. In natural solutions where H₂S₂ and HS₂ are likely to be the dominant polysulfides, marcasite will form only below pH 5 at all temperatures.

The pH-dependent precipitation of pyrite and marcasite may be caused by electrostatic interactions between polysulfide species and pyrite or marcasite growth surfaces: the protonated ends of H₂S₂ and HS₂ are repelled from pyrite growth sites but not from marcasite growth sites. The negative ions HS₂ and S₂²⁻ are strongly attracted to the positive pyrite growth sites. Masking of 1π_g^* electrons in the S₂ group by the protons makes HS₂ and H₂S₂ isoelectronic with AsS²⁻ and As₂²⁻, respectively ( et al., 1981). Thus, the loellingitederivative structure (marcasite) results when both ends of the polysulfide are protonated.

Marcasite occurs abundantly only for conditions below pH 5 and where H₂S₂ was formed near the site of deposition by either partial oxidation of aqueous H₂S by O₂ or by the reaction of higher oxidation state sulfur species that are reactive with H₂S at the conditions of formation e.g., S₂O₃²⁻ but not SO₄²⁻. The temperature of formation of natural marcasite may be as high as 240°C ( and , 1985), but preservation on a multimillion-year scale seems to require post-depositional temperatures of below about 160°C ( , 1973; and , 1985).