The organic geochemistry of ancient sediments—Part II

Several shales and oils ranging in age from 3 million to 2·7 billion years have been investigated for their hydrocarbon content using gas chromatography and mass spectrometry as primary analytical tools. From the Soudan Shale from Minnesota (2·7 × 10⁹yr) the C₁₈, C₁₉, C₂₀ and C₂₁ isoprenoid-alkanes were obtained. The Antrim shale from Michigan (about 265 × 10⁶ yr) yielded the C₁₆, C₁₈, C₁₉, C₂₀ and C₂₁ isoprenoids, as well as a C₁₆ iso-alkane and the C₁₈ and C₁₉ cyclohexyl n-alkanes. The San Joaquin Oil (30 × 10⁶ yr) and the Abbott Rock Oil (3 × 10⁶ yr) contained the C₁₆, C₁₈, C₁₉, C₂₀ and the C₁₈, C₁₉, C₂₀ and C₂₁ isoprenoids respectively. In addition, a series of iso-alkanes (C₁₆−C₁₈), anteiso-alkanes (C₁₆−C₁₈) and n-alkylcyclohexanes (C₁₆−C₁₉) as well as a C₂₁ isoprenoid were obtained from the Nonesuch Seep Oil (1 × 10⁹ yr). This analysis provides a comprehensive picture of the types of biogenic hydrocarbons found in oils and shales of widely differing ages, and in particular, the finding of isoprenoid alkanes in the Soudan Shale furnishes evidence for life processes at that period of geological time.     

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.     

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。     

High-resolution methane profiles across anoxic brine–seawater boundaries in the Atlantis-II, Discovery, and Kebrit Deeps (Red Sea)

A decrease in temperature (ΔT up to 45.5 °C) and chloride concentration (ΔCl up to 4.65 mol/l) characterises the brine–seawater boundary in the Atlantis-II, Discovery, and Kebrit Deeps of the Red Sea, where redox conditions change from anoxic to oxic over a boundary layer several meters thick. High-resolution (100 cm) profiles of the methane concentration, stable carbon isotope ratio of methane, and redox-sensitive tracers (O₂, Mn⁴⁺/Mn²⁺, Fe³⁺/Fe²⁺, and SO₄²⁻) were measured across the brine–seawater boundary layer to investigate methane fluxes and secondary methane oxidation processes.

Substantial amounts of thermogenic hydrocarbons are found in the deep brines (mostly methane, with a maximum concentration up to 4.8×10⁵ nmol/l), and steep methane concentration gradients mainly controlled by diffusive flow characterize the brine–seawater boundary (maximum of 2×10⁵ nmol/l/m in Kebrit Deep). However, locally the actual methane concentration profiles deviate from theoretical diffusion-controlled concentration profiles and extremely positive δ¹³C–CH₄ values can be found (up to +49‰ PDB in the Discovery Deep). Both, the actual CH₄ concentration profiles and the carbon-13 enrichment in the residual CH₄ of the Atlantis-II and Discovery Deeps indicate consumption (oxidation) of ¹²C-rich CH₄ under suboxic conditions (probably utilizing readily available—up to 2000 μmol/l—Mn(IV)-oxihydroxides as electron acceptor). Thus, a combined diffusion–oxidation model was used to calculate methane fluxes of 0.3–393 kg/year across the brine–seawater boundary layer. Assuming steady-state conditions, this slow loss of methane from the brines into the Red Sea bottom water reflects a low thermogenic hydrocarbon input into the deep brines.     

Terrestrial Kr-Kr ages of Antarctic meteorites

The production rate of ³⁸Ar in meteorites—P(38)—has been determined, as a function of the sample's chemical composition, from ⁸¹Kr-Kr exposure ages of four eucrite falls. The cosmogenic ⁷⁸Kr/⁸³Kr ratio is used to estimate the shielding dependence of P(38).

From the “true” ³⁸Ar exposure ages and the apparent ⁸¹Kr-Kr exposure ages of nine Antarctic eucrite finds, terrestrial ages are calculated. They range from about 3 × 10⁵ a (Pecora Escarpment 82502) to very recent falls (Thiel Mountains 82502). Polymict eucrites from the Allan Hills (A78132, A79017 and A81009) have within the limits of error the same exposure age (15.2 × 10⁶ a) and the same terrestrial age (1.1 × 10⁵ a). This is taken as strong evidence that these meteorites are fragments of the same fall. A similar case are the Elephant Moraine polymict eucrites A79005, A79006 and 82600 with an exposure age of 26 × 10⁶ a and a terrestrial age of 1.8 × 10⁵ a. EETA79004 may be different from this group because its exposure age and terrestrial age are 21 × 10⁶ a and 2.5 × 10⁵ a, respectively.

The distribution of terrestrial ages of Allan Hills meteorites is discussed. Meteorites from this blue ice field have two sources: Directly deposited falls and meteorites transported to the Allan Hills inside the moving Antarctic ice sheet. During the surface residence time meteorites decompose due to weathering processes. The weathering “half-life” is about 1.6 × 10⁵ a. From the different age distributions of Allan Hills and Yamato meteorites, it is concluded that meteorite concentrations of different Antarctic ice fields need different explanations.     

Gum rosin (colophony): A suitable material for thermomechanical modelling of the lithosphere

We investigate the use of a ductile material with temperature-sensitive viscosity for thermomechanical modelling of the lithosphere. First, we consider the scaling of mechanical and thermal properties. For a normal field of gravity, the balance of stresses and body forces sets the stress scale, in proportion to the linear dimensions and the densities. The equation of thermal conduction sets the time scale. The activation enthalpy for creep sets the temperature scale; but the thermal expansivity provides an additional constraint on this temperature scale.

Gum rosin appears to be a suitable material for lithospheric modelling. We have measured its flow properties, at various temperatures, in a specially designed rotary viscometer with unusually low machine friction. The rosin is almost Newtonian. Strain rate depends upon stress to the power n, where 1.0 <n < 1.14. The viscosity varies over 5 orders of magnitude, from about 10² Pa s at 80°C, to about 10⁷ Pa s at 40°C. The activation enthalphy is thus about 250 kJ/mol. Measured with a needle probe, the thermal conductivity is 0.113 ± 0.001 W m⁻¹K⁻¹; the thermal diffusivity, (6±3) ×10⁻⁷ m² s⁻¹. Calculated from X-ray profiles, the thermal expansivity is about 3 × 10⁻⁴ K⁻¹. These thermal and mechanical properties make gum rosin suitable for thermomechanical models, where linear dimensions scale down by a factor of 10⁶; time, by 10¹¹; viscosity, by 10¹⁷; and temperature change, by 10¹.     

天津平原区浅层地下水水化学特征及碳酸盐风化碳汇研究

大气 CO₂浓度在控制全球气候变化方面具有至关重要的作用,研究碳循环、CO₂收支平衡和精确评估是制定区域CO₂减排策略和寻找新的碳汇途径最重要的组成部分。碳酸盐风化碳汇是全球碳循环研究的一个重要方向。为此,本研究以天津平原区浅层地下水为研究对象,通过对地下水调查及水样的采集与分析,运用水化学分析方法分析了地下水水化学特征,并估算了地下水总储存量、DIC储量和碳酸盐风化碳汇量。研究结果表明:浅层地下水化学场自北部山前平原向南部冲积平原和滨海平原,呈现出自北而南和由北西向南东的水平水化学分带规律,地下水由低浓度的淡水、微咸水变为高浓度咸水,沿此方向水化学类型由HCO₃-Ca·Na·Mg→Cl·SO₄-Na→Cl·HCO₃-Na→Cl-Na型转变;淡水区、微咸水区和咸水区面积分别为733、3 034和6 564 km²。地下水水化学组分中Ca²⁺、Mg²⁺和 {\mathrm{HCO}}_3^{-}主要来源于碳酸盐的溶解作用。研究区浅层地下水总储存量为2 241 640万m³,总DIC储量为8.13×10⁶ t,总碳汇量为4.11×10⁶ t。研究区浅层地下水淡水区、微咸水区和咸水区地下水储存量分别为157 799万、6 245 936万和1 459 247万 m³,DIC浓度分别为19200、19200和19342 mg/L,DIC储量分别为0.67×10⁶、1.65×10⁶和0.58×10⁶ t,碳汇量分别为0.22×10⁶、0.90×10⁶和2.98×10⁶ t。沿地下水流向,DIC、储量和碳汇量的空间分布均呈现出由低到高的趋势。     

Mantle helium in Sacramento basin natural gas wells

Helium isotope ratios in Sacramento basin natural gas wells show a strong mantle signal. The ³He/⁴He ratios range from 0.11 times the atmospheric ratio (0.11 R_A) in the Rio Vista field to 2.75 R_A in the Moon Bend field, indicating that 1% to 34% of the helium is mantle-derived. ³He/⁴He versus CH₄/₄He ratios provide evidence of two-component mixing between crustal and magmatic end-members. Extrapolation of the linear regression line to CH₄/⁴He = 0 gives a hypothetical magmatic end-member ³He/⁴He ratio of 3.84 R_A, half the typical mantle ratio. This indicates that the magmatic end-member may actually represent a mixture of mantle and crustal helium. Gases which deviate from the simple two-component mixture can be explained by addition of pure methane, radiogenic ⁴He, or a high N₂-He component with ³He/⁴He = 0.6 R_a to 1.0 R_A. The CH₄/³He ratio of the magmatic end-member remains poorly constrained (0 to 3 × 10⁹) and one cannot rule out the possibility that a significant proportion of the methane in some fields may be of deep-earth origin. However, fields with the highest ³He/⁴He ratios are associated with buried Plio-Pleistocene intrusives which have up-arched sediments to form hydrocarbon traps. The methane in these fields may have been produced by rapid thermal alteration of the intruded sediment. Elsewhere, the methane appears either to have migrated from deeply-buried sediments in the western basin or to have been produced by local microbial activity.     

Studying the migration behaviour of selenate in Boom Clay by electromigration

For the disposal of high-level waste (HLW) in a deep geological formation as Boom Clay, safety assessment studies have shown that long lived ⁷⁹Se is one of the more critical fission products. Therefore, the knowledge of its migration properties (diffusion, retention) through the geological barrier (Boom Clay) is of paramount importance. The migration behaviour of selenium strongly depends on its speciation. Under reducing conditions, selenide would be the dominant species and selenium migration would mainly be controlled by the low solubility of Se(−II)-bearing minerals. However Se species are often found in redox disequilibrium and more oxidized species might also coexist. Therefore, the study of selenate migration requires attention, as it might be the most mobile selenium species in the host rock. Electromigration experiments performed with a ⁷⁵Se-labeled selenate in Boom Clay indicate a high mobility for this species. The apparent diffusion coefficient (D_app) of selenate in Boom Clay is estimated from electromigration experiments performed under different electric fields. Using two independent approaches, the value of D_app for selenate is shown to fall in the range from 1.7×10⁻¹¹ to 6.2×10⁻¹¹ m² s⁻¹. Moreover, no reduction of selenate in Boom Clay was observed.     

Quaternary geochemical evolution of the salars of Uyuni and Coipasa, Central Altiplano, Bolivia

The central trough of the Bolivian Altiplano is occupied by two wide salt crusts: the salar of Uyuni, which is probably the largest salt pan in the world (10,000 km²) and the salar of Coipasa (2,500 km²). Both crusts are essentially made of porous halite filled with an interstitial brine very rich in Li, K, Mg, B (up to 4.7 g/l Li, 4.3 g/l B, 30 g/l K and 75 g/l Mg). Lithium reserves are the highest known in the world, around 9 × 10⁶ tons. Potassium, magnesium and boron reserves in brines are also important (around 194 × 10⁶ tons K, 8 × 10⁶ tons B and 211 × 10⁶ tons Mg).

The crusts are the remnant of saline Lake Tauca (13,000–10,000 yr BP). Its salinity was estimated approximately at 80 g/l. Its paleochemistry was derived in two ways: (1) by dissolving the present amounts of all chemical components in the former lake volume, and (2) by simulating the evaporation of the major inflows to the basin. The resulting chemical compositions are quite different. The dissolution-derived one is 5 to 50 times less concentrated in Li, K, Mg, B than the evaporation-simulated ones. However all compositions present the same Na and Cl contents. This suggests either a removal of bittern salts or an enrichment of the former lake water in Na and Cl.

The most probable interpretation is that Lake Tauca redissolved a salt crust akin to that existing today. Several older lakes have been detected on the Altiplano. Nevertheless, such an explanation only pushes the problem back. It is likely that the anomaly was transferred from one lake to an other. Three hypotheses may be put forward: (1) bittern seepage through bottom sediments, (2) uptake of the missing components by minerals, and (3) leaching of ancient evaporites from the catchment area at the beginning of the lacustrine history of the basin. The excess halite could have been recycled from lake to lake. This latter process seems to be the most effective to explain the large excess of Na and Cl over the bittern solutes — Li, K, Mg and B. The occurrence of almost pure Na/1bCl saline springs flowing out from a gypsum diapir in the northern Altiplano gives substantial support to this hypothesis.     

The Kostanjek landslide in Zagreb

The large landslide, Kostanjek, on the southern slopes of the mountain, Zagreba ka Gora, in the western suburb of Zagreb was activated in 1963 after some 2.1 × 10⁶ m³ of marl for the cement factory, Croatia, was excavated at the foot of the slope. The geological features (sinclinal structure, faults, hydrogeological conditions, extension of the “Tripoli” marl strata) which had an important role in the formation of the slide, as well as the means of marl exploitation by means of mass blasting, are presented in the paper. The Kostanjek slide involves an urbanized area of some 100 ha. It is estimated that a sliding mass of some 32 × 10⁶ m³ is involved, with a maximum depth of 90 m. Sliding occurs in three levels. The displacements of the surface are 3–6 m. The excavation of marl was stopped in 1988, when a total of 5.3 × 10⁶ m³ of material was excavated. Since then the magnitude of surface displacement per year has decreased. The Kostanjek slide is still active. It is estimated that natural stabilization will take a long time, probably decades. Analyses of possible measures for the stabilization of movements suggested that preventive drainage with continuous observation of the slide is most suitable.     

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.     

Effects of volcanic eruptions on the CO2 content of the atmosphere and the oceans: the 1996 eruption and flood within the Vatnajökull Glacier, Iceland

The October 1996 eruption within the Vatnajökull Glacier, Iceland, provides a unique opportunity to study the net effect of volcanic eruptions on atmospheric and oceanic CO₂. Volatile elements dissolved in the meltwater that enclosed the eruption site were eventually discharged into the ocean in a dramatic flood 35 days after the beginning of the eruption, enabling measurement of 50 dissolved element fluxes. The minimum concentration of exsolved CO₂ in the 1×10¹² kg of erupted magma was 516 mg/kg, S was 98 mg/kg, Cl was 14 mg/kg, and F was 2 mg/kg. The pH of the meltwater at the eruption site ranged from about 3 to 8. Volatile and dissolved element release to the meltwater in less than 35 days amounted to more than one million tonnes, equal to 0.1% of the mass of erupted magma. The total dissolved solid concentration in the floodwater was close to 500 mg/kg, pH ranged from 6.88 to 7.95, and suspended solid concentration ranged from 1% to 10%. According to H, O, C and S isotopes, most of the water was meteoric whereas the C and S were of magmatic origin. Both C and S went through isotopic fractionation due to precipitation at the eruption site, creating “short cuts” in their global cycles. The dissolved fluxes of C, Ca, Na, Si, S and Mg were greatest ranging from 1.4×10¹⁰ to 1.4×10⁹ mol. The dissolved C flux equaled 0.6 million tonnes of CO₂. The heavy metals Ni, Mn, Cu, Pb and Zn were relatively mobile during condensation and water–rock interactions at the eruption site. About half of the measured total carbon flood flux from the 1996 Vatnajökull eruption will be added to the long-term CO₂ budget of the oceans and the atmosphere. The other half will eventually precipitate with the Ca and Mg released. Thus, for eruptions on the ocean floor, one can expect a net long-term C release to the ocean of less than half that of the exsolved gas. This is a considerably higher net C release than suggested for the oceanic crust by Staudigel et al. [Geochim. Cosmochim. Acta, 53 (1989) 3091]. In fact, they suggested a net loss of C. Therefore, magma degassed at the ocean floor contributes more C to the oceans and the atmosphere than magma degassed deep in the oceanic crust. The results of this study show that subglacial eruptions affecting the surface layer of the ocean where either Mn, Fe, Si or Cu are rate-determining for the growth of oceanic biomass have a potential for a transient net CO₂ removal from the ocean and the atmosphere. For eruptions at high latitudes, timing is crucial for the effect of oceanic biota. Eruptions occurring in the wintertime when light is rate-determining for the growth of biota have much less potential for bringing about a transient net negative CO₂ flux from the ocean atmosphere reservoir.     

铷同位素分析方法及研究进展

Rb作为一个具有中度挥发性、流体活动性、在岩浆过程中呈不相容性的碱金属元素,能为各种地质过程和物质源区提供制约;同时,⁸⁷Rb是放射性母体,Rb-Sr定年体系在确定长时间尺度的地质体年龄方面也有广泛的应用。传统研究认为特定地质年代下Rb同位素比值(⁸⁷Rb/⁸⁵Rb)是一个定值,但随着分离纯化方法的改进和质谱分析精度的提高,高精度Rb同位素组成得以测定,其结果显示,不同地质样品存在明显的Rb同位素组成的差异,意味着地质过程中存在Rb同位素的分馏。Rb同位素分馏能否为示踪Rb的地质过程提供更多有用信息,是否会对传统Rb-Sr定年体系产生影响,这些基础性的问题目前仍然没有答案。要回答这些问题,首先需要了解不同地质储库的Rb同位素组成,发现不同地质过程中的Rb同位素分馏,探讨其发生的控制机制。然而,这方面的研究目前还非常欠缺。文章回顾了近20年来国际上地球科学领域中的Rb同位素已有的研究,包括技术方法、分馏机理等各个方面,在此基础上对其研究前景进行展望,主要包括:(1)总结了Rb同位素组成测定的化学纯化及仪器测量方法,并对其优缺点进行点评,同时指出谨慎的化学前处理方法及质谱测定流程是获得高精度Rb同位素组成结果的基本前提;(2)收集了现有的地外样品Rb同位素组成数据,简述了Rb同位素在宇宙化学中的应用研究成果,指出Rb作为一个中度挥发性元素,对太阳系行星的吸积和演化过程具有重要的指示意义;(3)对Rb同位素在地质过程中潜在广阔的应用前景进行展望,例如完善经典的Rb-Sr定年体系,限定壳幔及地壳内部的分异过程,制约大陆硅酸盐岩风化,以及揭示超大型Rb矿的形成机制。     

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.     

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.     

西北天山莱历斯高尔岩体年代学、地球化学及其成因

西北天山莱历斯高尔岩体主要由花岗闪长斑岩和二长花岗斑岩组成,二者具有相似的地球化学特征,可能为同源岩浆分异演化的产物。锆石SHRIMP U-Pb年龄显示岩体形成于晚泥盆世(374±4 Ma)。莱历斯高尔岩体具有较高的SiO₂(67.75% ～74.71%)含量和K₂O/Na₂O(1.24～2.20)比值,属于高钾钙碱性系列。稀土元素配分曲线为轻稀土富集型(La/Yb)_N(5.25～10.57),具有中等-弱的Eu负异常δEu(0.55～0.92)。微量元素特征显示富集大离子亲石元素(LILE),而亏损高场强元素(HFSE),具有较低的Sr(122×10^-6～356×10^-6)含量、较高的Y(17.19×10^-6～21.82×10^-6)和Yb(1.78×10^-6～2.57×10^-6)含量。岩体的Sr-Nd同位素特征为I_Sr=0.707 867～0.709 654,ε_Nd(t)=-2.79～-1.46。铅同位素特征为²⁰⁶Pb/²⁰⁴ Pb=18.423～19.915,²⁰⁷ Pb/²⁰⁴Pb=15.576～15.685,²⁰⁸ Pb/²⁰⁴Pb=38.344～39.305。元素和同位素地球化学特征表明,莱历斯高尔岩体岩浆源区主要为中元古代下地壳的部分熔融,并有部分地幔物质的加入。岩体形成于晚泥盆世准噶尔洋向伊犁-中天山微板块陡角度俯冲的大陆弧构造环境。     

Comparative study of the kinetics and mechanisms of dissolution of carbonate minerals

The influence of pH on the rate of dissolution of various carbonates (calcite, aragonite, witherite, magnesite and dolomite) has been investigated at 25°C using a continuous fluidized bed reactor. The general rate dependence on pH observed for the simple carbonates is very similar and is in agreement with the results observed for calcite and aragonite by L.N. Plummer and coworkers. However, the rate of dissolution of magnesite is approximately four orders of magnitude lower than calcite.

For simple carbonates, the elementary steps involved in the dissolution reaction are:

Full-size image

where M represents the metal ion which can be Ca, Mg and Ba. According to the stoichiometry of the three reaction steps and the thermodynamic constraints, the total forward and backward rates can be expressed as:

R_f=k₁a_H+k₂a_H2CO3^*+K₃

r_b=k_-1^aM²+^aHCO₃^-+k_-2^aHCO3^-+k_{-3^aM²+^aCO3^2-}

The rate constants (k₁, k₂, k₃ and k₋₃) determined with our experimental results for calcite, aragonite and witherite show that the dissolution rates are similar for these three minerals and that the nature of the cations does not play a significant role. The good agreement between the K_sp calculated from the measured k₃/k₋₃ ratio and the theromodynamic value suggests that our dissolution mechanism is coherent.

The rate dependence on pH of the dissolution of dolomite obeys a fractional order at low pH's and confirms previously published observations therein. However, the two-step reaction mechanism proposed does not explain the fractional reaction order observed, which is likely due to a more complex surface reaction.     

Temperature independent thermal expansivities of sodium aluminosilicate melts between 713 and 1835 K

The thermal expansivities of eight sodium aluminosilicate liquids were derived from the slope of new volume data at low temperatures (713−1072 K) combined with the high temperature (1300−1835 K) volume measurements of Stein et al. (1986) on the same liquids. Melt compositions range from 47−71 wt% SiO₂, 0−31 wt% A1203, and 17−33 wt% Na₂O; the volume of albite supercooled liquid at 1092 K was also determined. The low temperature volumes were derived from measurements of the glass density of each sample at 298 K, followed by measurements of the glass thermal expansion coefficient from 298 K to the respective glass transition interval. This technique takes advantage of the fact that the volume of a glass is equal to the volume of the corresponding liquid at the limiting fictive temperature (T′_f), and that T′_f can be approximated as the onset of the rapid rise in thermal expansion at the glass transition in a heating curve (Moynihan, 1995). No assumptions were made regarding the equivalence of enthalpy and volume relaxation through the glass transition. The propagated error on the volume of each supercooled liquid at T′_f is 0.25%. Combination of these low temperature data with the high temperature measurements of Stein et al. (1986) allowed a constant thermal expansivity of each liquid to be derived over a wide temperature interval (763−1001 degrees) with a fitted 1σ error of 0.6–4.6%; in every case, no temperature dependence to dV/dT^liq could be resolved. Calibration of a linear model equation leads to fitted values ± 1σ (units of cm³/mole) for (26.91 ± .04), (37.49 ± .12), (26.48 ± .06) at 1373 K, and (7.64 ± .08 × 10^-3 cm³/mole-K). The results indicate that neither Si02 nor Al₂O₃ contribute to the thermal expansivity of the liquids, and that dV/dT^liq is independent of temperature between 713–1835 K over a wide range of liquid composition. Calculated volumes based on this model recover both low and high temperature measurements with a standard deviation <0.25%, whereas values of dV/dT^liq can be predicted within 5.6%.     

阿尔金造山带南缘晚奥陶世赞岐质闪长岩的发现及其地质意义

本文以阿尔金造山带南缘出露的高镁赞岐质闪长岩为研究对象,通过系统的野外地质工作、显微岩相学、年代学和全岩地球化学分析,探讨其成因类型和源区性质,限定了阿尔金造山带构造演化格架。闪长岩LA-ICP-MS锆石U-Pb测年结果表明其形成年龄为(445±3) Ma,属晚奥陶世。地球化学数据显示:闪长岩Al₂O₃含量为15.11%~16.19%,且具有高MgO(3.93%~5.29%)和K₂O(1.68%~2.67%)含量的特征,属于高镁钙碱性准铝质岩石;闪长岩具有较高的Cr(29.3×10^-6~140×10^-6)、Ni(27.1×10^-6~43.5×10^-6)、Ba(606×10^-6~936×10^-6)、Sr(513×10^-6~734×10^-6)和Y(26.4×10^-6~31.1×10^-6)含量,与典型的高镁赞岐岩类地球化学特征一致;稀土配分图和微量元素蛛网图显示闪长岩样品富集轻稀土元素和大离子亲石元素(如Rb和K),而亏损高场强元素(如Nb、Ti和Zr),表现出与俯冲带环境岩浆岩相似的性质;高Ti/Zr比值(48~60)与Mg^#(46.74~53.06)、高Cr(29.3×10^-6~140×10^-6)与高Ni(27.1×10^-6~43.5×10^-6)含量等暗示闪长岩岩浆源区来自遭受俯冲组分交代过的富集岩石圈地幔。岩石成因分析表明,阿尔金造山带南缘晚奥陶世高镁赞岐质闪长岩是后碰撞构造环境下俯冲板片断离诱发地幔楔发生减压熔融而形成,代表了早古生代最强烈的区域性幔源基性岩浆侵入事件。