Stability and dynamics of the continental tectosphere

Continental cratons overlie thick, high-viscosity, thermal and chemical boundary layers, where the chemical boundary layers are less dense than they would be due to thermal effects alone, perhaps because they are depleted in basaltic constituents. If the continental tectosphere is the same age as the overlying Archaean crust, then the continental tectosphere must be able to survive for several billion years without undergoing a convective instability, despite being both cold and thick. Since platforms and shields correlate only weakly with Earth's gravity and geoid anomalies, acceptable models of the continental tectosphere must also satisfy this gravity constraint. We investigate the long-term stability of the continental tectosphere by carrying out a number of numerical convection experiments within a two-dimensional Cartesian domain. We initiate our experiments with a tectosphere (thermal and chemical boundary layers) immersed in a region of uniform composition, temperature, and viscosity, and consider the effects on the stability of the tectosphere of (1) activation energy (used to define the temperature dependence of viscosity), (2) compositional buoyancy, and (3) linear or non-linear rheology. The large lateral thermal gradients required to match oceanic and tectosphere structures initiate the dominant instability, a “drip” which develops at the side of the tectosphere and moves to beneath its center. High activation energies and high background viscosities restrict the amount and rate of entrainment. Compositional buoyancy does not significantly change the flow pattern. Rather, compositional buoyancy slows the destruction process somewhat and reduces the stress within the tectosphere. With a non-Newtonian rheology, this reduction in stress helps to stiffen the tectosphere. In these experiments, dynamical systems that adequately model the present ocean-continent structures have activation energy E*≥180 kJ mole⁻¹ — a value about one third the estimate of activation energy for olivine, E*≈520 kJ mole⁻¹. Although for E*≈520 kJ mole⁻¹, compositional buoyancy is not required for the tectosphere to survive, the joint application of longevity and gravity constraints allows us to reject all models not containing compositional buoyancy, and to predict that the ratio of compositional to thermal buoyancy within the continental tectosphere is approximately unity.     

A review and analysis of silicate mineral dissolution experiments in natural silicate melts

We present a database and a graphical analysis of published experimental results for dissolution rates of olivine, quartz plagioclase, clinopyroxene, orthopyroxene, spinel, and garnet in basaltic and andesitic melts covering a range of experimental temperatures (1100–1500°C) and pressures (10⁵ Pa-3.0 GPa). The published datasets of Donaldson (1985, 1990) and Brearly and Scarfe (1986) are the most complete. Experimental dissolution rates from all datasets are recalculated and normalized to a constant oxygen basis to allow for direct comparison of dissolution rates between different minerals. Dissolution rates (ν) range from 5·10⁻¹⁰ oxygen equivalent moles (o.e.m.) cm⁻² s⁻¹ for olivine in a basaltic melt to 1.3·10⁻⁵ o.e.m. cm⁻² s⁻¹ for garnet in a basaltic melt. Values of ln ν are Arthenian for the experiments examined and activation energies range from 118 to 1800 kJ/o.e.m. for quartz and clinopyroxene, respectively.

The relationship between calculated A/RT for the dissolution reactions, where A is the thermodynamic potential affinity, and values of ν is linear for olivine, plagioclase, and quartz. We interpret this as strong evidence in support of using calculated A as a predictor of ν for, at least, superliquidus melt conditions.     

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.     

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.     

Oxygen and carbon isotope composition of cold-water Berriedale Limestone (Lower Permian), Tasmania, Australia

The Berriedale Limestone formed at about 80°S paleolatitude and contains many glacial dropstones. It formed during a period of major Gondwana deglaciation.

The Berriedale Limestone contains mostly bryozoans, brachiopods and bivalves, with some intraclasts and rare pellets. The faunal diversity is low and the fauna are similar to the modern cold-water foramol faunal assemblage. Micrite, microspar and spar occur as equant to well developed rhombs of calcite. The coarse spar cements are bored and are ruptured by dropstones, indicating submarine origin of low-Mg calcite at water-temperatures of around 3°C. The mixing zone cementation was preceded by erosion of early formed crystals. The eroded crystals occur as inclusions in mixing zone cements.

The fauna are characterized by heavy δ¹³C and light δ¹⁸O. The whole-rock field of δ¹⁸O-δ¹³C falls at the edge of “Normal Marine Limestone” and deviates to lighter δ¹⁸O values (down to −16.7‰ PDB). Lightest δ¹⁸O values ( −22‰ PDB) of fresh-water sparry calcite cement are similar to those in the Early Permian continental tillites, suggesting that the Permian sea was diluted by isotopically light melt waters. Micrite δ¹⁸O values (−9.2 to −12.6‰ PDB) are within the range of whole-rock values. The δ¹⁸O values of calcite in shales are lighter than limestone values.

The δ¹⁸O values of the fauna give an unrealistic range of sea-water temperatures because the fauna have equilibrated with variable amounts of melt waters. However, calculated original δ¹⁸O values of the fauna indicate temperatures < 4°C. The heaviest δ¹⁸O of fauna gives cold temperatures of 9°C (with δ_w −2.8‰) and −3°C (with δ_w −6‰). The lightest values of sparry calcite cements (−22‰ PDB) indicate that the limestone reacted with cold melt waters.

The δ¹⁸O of Permian sea is estimated to be about +1.2‰ and was diluted by melt waters as light as −27‰ SMOW.     

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%.     

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).     

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.     

Fluid inclusions in Scourian granulites from the Lewisian complex of NW Scotland: evidence for CO2-rich fluid in Late Archaean high-grade metamorphism

In this paper the first fluid-inclusion data are presented from Late Archaean Scourian granulites of the Lewisian complex of mainland northwest Scotland. Pure CO₂ or CO₂-dominated fluid inclusions are moderately abundant in pristine granulites. These inclusions show homogenization temperatures ranging from − 54 to + 10 °C with a very prominent histogram peak at − 16 to − 32 °C. Isochores corresponding to this main histogram peak agree with P-T estimates for granulite-facies recrystallization during the Badcallian (750–800 °C, 7–8 kbar) as well as with Inverian P-T conditions (550–600 °C, 5 kbar). The maximum densities encountered could correspond to fluids trapped during an early, higher P-T phase of the Badcallian metamorphism (900–1000 °C, 11–12 kbar). Homogenization temperatures substantially higher than the main histogram peak may represent Laxfordian reworking (≤ 500 °C, < 4 kbar). In the pristine granulites, aqueous fluid inclusions are of very subordinate importance and occur only along late secondary healed fractures. In rocks which have been retrograded to amphibolite facies from Inverian and/or Laxfordian shear zones, CO₂ inclusions are conspicuously absent; only secondary aqueous inclusions are present, presumably related to post-granulite hydration processes. These data illustrate the importance of CO₂-rich fluids for the petrogenesis of Late Archaean granulites, and demonstrate that early fluid inclusions may survive subsequent metamorphic processes as long as no new fluid is introduced into the system.     

Compositional differences in enstatite chondrites based on carbon and nitrogen stable isotope measurements

Carbon and nitrogen abundance and isotopic compositions, from four EH4, one EH5, five EL6 chondrites and one aubrite, were determined by using stepped pyrolysis (N only) and combustion (N and C) extractions in attempts to distinguish the components present. Carbon contents range from 0.15 to 0.70 wt%, with no systematic relationship between carbon content and meteorite group or petrologic type. Whole-rock δ¹³C values range from −28.5 to −4.1 %., Most C occurs as graphite and when temperature steps above 700°C are considered, there is a difference between EH4,5 (δ¹³C = −9.1 to -5.8%.) and EL6 chondrites (δ¹³C = −6.7 to +4.2%.). Carbon in Bustee aubrite is isotopically lighter (δ¹³C = −24%.) than in any enstatite chondrite.

Nitrogen occurs as osbornite, sinoite and in isostructural substitution for oxygen in silicate lattice sites. Nitrogen abundances and isotopic compositions are more variable than C, due to the heterogenous distribution of N-bearing minerals. Three EL6's containing osbornite have higher N concentrations than other type 6 enstatite chondrites. Sinoite, where present, is depleted in ¹⁵N relative to osbornite. Nitrogen in the Bustee aubrite has a similar abundance and δ¹⁵N value to those of EL6's, again dominated by the presence of osbornite.

In addition to the refractory C-and N-bearing minerals there is also organic material (largely terrestrial contamination) and evidence for at least two “exotic” components. The first is a host for Xe (HL) and is characterized by δ¹³C <-−47%. and δ¹⁵N ≤−73%., whereas the second is less well-defined, but is marked by δ¹⁵N = +269%.     

Petrogenesis of the Maowu pyroxenite–eclogite body from the UHP metamorphic terrane of Dabieshan: chemical and isotopic constraints

The Maowu eclogite–pyroxenite body is a small (250×50 m) layered intrusion that occurs in the ultra-high-pressure (UHP) metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial ⁸⁷Sr/⁸⁶Sr ratios (0.707–0.708) and inhomogeneous negative _Nd(T) values of −3 to −10 at 500 Ma (probable protolith age). Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock (WR) Sm–Nd isochronal relationship, low δ¹⁸O values and an extreme enrichment of Th and REE in a clinopyroxenite. Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial ⁸⁷Sr/⁸⁶Sr ratios unsupported by in situ Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as monazite and apatite, is required to account for the REE budget.

Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221±5 Ma and (T)=−5.4 for an eclogite, 231±16 Ma and (T)=−6.2 for a garnet websterite, and 236±19 Ma and (T)=−6.9 for a garnet clinopyroxenite. The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths, UHP metamorphism and return to the crustal level.     

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.     

Neo-Proterozoic rift-related syenites (Northern Damara Belt, Namibia): Geochemical and Nd–Sr–Pb–O isotope constraints for mantle sources and petrogenesis

Major element, trace element and Nd–Sr–Pb–O isotope data for a suite of Neo-Proterozic, pre-orogenic, rift-related syenites from the Northern Damara orogen (Namibia) constrain their sources and petrogenesis. New U–Pb ages obtained on euhdreal titanite of inferred magmatic origin constrain the age of intrusion of the Lofdal and Oas syenites to ca. 750 Ma compatible with previous high-precision zircon analyses from the Oas complex. Major rock types from Lofdal and Oas are mildly sodic nepheline-normative and quartz-normative syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Primitive samples from Lofdal and Oas show depletion of Rb, K and Th relative to Ba and Nb together with variable negative anomalies of P and Ti on a primitive mantle-normalized diagram. Evolved samples from Oas develop significant negative Ba, Sr, P and Ti anomalies and positive U and Th anomalies mainly as a function of crystal fractionation processes. The lack of a pronounced negative Nb anomaly in samples from Lofdal suggests that involvement of a crustal component is negligible. For the nepheline-normative samples from Lofdal, the unradiogenic Sr and radiogenic Nd isotope composition and low δ¹⁸O values suggest derivation of these samples from a moderately depleted lithospheric upper mantle with crustal-like U/Pb ratios (⁸⁷Sr/⁸⁶Sr: 0.7031–0.7035, ε Nd: ca. + 1, δ¹⁸O: 7‰, ²⁰⁶Pb/²⁰⁴Pb: ca.18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.58–15.60). Primitive samples of the Oas quartz-normative syenites have identical isotope characteristics (⁸⁷Sr/⁸⁶Sr: 0.7034, ε Nd: ca. + 1, δ¹⁸O: 6.5‰, ²⁰⁶Pb/²⁰⁴Pb: ca.18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.59) whereas more differentiated samples have higher ⁸⁷Sr/⁸⁶Sr ratios (0.709–0.714), slightly higher δ¹⁸O values (7.0–7.1‰), less radiogenic ε Nd values (− 1.1 to − 1.4) and more radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.27. These features together with model calculations using Sr–Nd–Pb isotopes suggest modification of a primary syenite magma by combined AFC processes involving ancient continental crust. In this case, high Nb abundances of the parental syenite liquid prevent the development of significant negative Nb anomalies that may be expected due to interaction with continental crust.     

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).     

Mantle heterogeneity and crustal recycling in Archean granite-greenstone belts: Evidence from Nd isotopes and trace elements in the Rainy Lake area, Superior Province, Ontario, Canada

The 2685–2752 Ma old granite-greenstone crust in the Rainy Lake area, Ontario, consists of metaigneous and metasedimentary rocks that range in composition from tholeiite to monzogranite and include anorthosite, trachyandesite, monzodiorite and high-silica rhyodacite. Major element, rare earth and other trace element data are the basis for modelling the formation of the crust by melting of large-ionlithophile element enriched and unenriched mantle, by melting of basalt at mantle to crustal levels and by melting of monzodiorite and tonalite at crustal levels.

All metaigneous rocks lie on a ¹⁴³Nd/¹⁴⁴Nd vs. ¹⁴⁷Sm/¹⁴⁴Nd isochron with an age of 2737 ±42 Ma and an initial ¹⁴³Nd/¹⁴⁴Nd of 0.509178 ±33 (ε_Nd = +1.9). This age is consistent with U-Pb zircon ages, which suggests the Nd isotopic system has been unaffected since the crust-forming events. The positive initial ε_Nd's are further evidence for time-averaged depletion in Sm/Nd relative to CHUR for the Archean mantle. The similarity of the initial Nd isotopic composition for both mantle-derived and crustally-derived rocks suggests rapid recycling of crustal components, which were previously derived from depleted mantle sources.

Initial ¹⁴³Nd/¹⁴⁴Nd ratios on individual rocks range from ε_Nd = +3.3 to ε_Nd = −0.4. Younger granitoids have lower ε_Nd values (+1.5 to −0.1) relative to tholeiites and monzodiorites crystallized from mantle-derived melts (+3.3 to +1.0). Thus, incorporation of slightly older crust (ca. 100–200 Ma) in some of the granitoid source areas is possible. Mantle-derived rocks form an isochron of 2764 ±58 Ma that represents a minimum age for enrichment processes in the mantle sources for the Rainy Lake area. Consideration of data from the Abitibi belt suggests such enrichment processes in the mantle may have preceded crust-forming events in a wide area of the Superior Province, perhaps by as much as 50–70 Ma.     

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.     

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.     

Evolution of mineral–fluid interfaces studied at pressure with synchrotron X-ray techniques

In situ measurements of mineral surface evolution during the process of pressure solution are possible with the high brightness of synchrotron X-ray sources. This capability has been explored through the use of newly developed reaction vessels that allow transmission of the incident and scattered X-ray beam through a low atomic weight piston. Several new vessels are described, along with details of computational algorithms that are used to simulate X-ray scattering in this unconventional geometry. Results using calcite (CaCO₃) and halite (NaCl) as reactant crystals are presented and compared to other atomic-scale measurements of surface dissolution processes. Calcite was reacted with an unsaturated fluid at 30 bars of pressure for approximately 24 h. During reaction the root mean square surface roughness (σ) evolved from 13.7 Å (± 0.5 Å) to 19.5 Å (± 1.0 Å), giving a roughening rate of: dσ/dt = +6.3 × 10^− 5 Å s^− 1. This is consistent with other measurements made with free calcite surfaces and is driven almost entirely by chemical disequilibrium. Analysis of the surface ex situ post-reaction gives an identical σ value, showing that the in situ measurements are well-constrained. Experiments also at 30 bars but in a saturated solution indicate that the calcite surface does not significantly roughen, giving the result that pressure solution of calcite at this pressure cannot be monitored in experiments of several days duration. Experiments with halite, a much more reactive phase, in saturated solutions showed the reflectivity profile to be dynamic on a time scale of hours. This experiment was left to reach equilibrium over 108 days and then re-analyzed, showing that σ had increased from 34 Å (± 2 Å) to 41 Å (± 2 Å), giving a roughening rate of: dσ/dt ≤ +6.4 × 10^− 7 Å s^− 1. This is two orders of magnitude smaller than the calcite roughening rate caused by chemical disequilibrium and provides the first direct in situ atomic-scale measurement of the rate of surface roughening due to pressure solution.     

The crust-mantle boundary beneath cratons and craton margins: a transect across the south-west margin of the Kaapvaal craton

“Lower-crustal suite” xenoliths occur in “on-craton” and “off-craton” kimberlites located across the south-western margin of the Kaapvaal craton, southern Africa.

Rock types include mafic granulite (plagioclase-bearing assemblages), eclogite (plagioclase-absent assemblages with omphacitic clinopyroxene) and garnet pyroxenite (“orthopyroxene-bearing eclogite”). The mafic granulites are subdivided into three groups: garnet granulites (cpx + grt + plag + qtz); two pyroxene garnet granulites (cpx + opx + grt + plag); kyanite granulites (cpx + grt + ky + plag + qtz). Reaction microstructures preserved in many of the granulite xenoliths involve the breakdown of plagioclase by a combination of reactions: (1) cpx + plag → grt + qtz; (2) plag → grt + ky + qtz; (3) plag → cpx (jd-rich) + qtz. Compositional zoning in minerals associated with these reactions records the continuous transition from granulite facies mineral assemblages and pressure (P) — temperature (T) conditions to those of eclogite facies.

Two distinct P-T arrays are produced: (1) “off-craton” granulites away from the craton margin define a trend from 680 °C, 7.5 kbar to 850 °C, 12 kbar; (2) granulite xenoliths from kimberlites near the craton margin and “on-craton” granulites produce a trend with similar geothermal gradient but displaced to lower T by ˜ 100 °C. Both P-T fields define higher geothermal gradients than the model steady state conductive continental geotherm (40 mWm²) and are not consistent with the paleogeotherm constructed from mantle-derived garnet peridotite xenoliths.

A model involving intrusion of basic magmas around the crust/mantle boundary followed by isobaric cooling is proposed to explain the thermal history of the lower crust beneath the craton margin. The model is consistent with the thermal evolution of the exposed Namaqua-Natal mobile belt low-pressure granulites and the addition of material from the mantle during the Namaqua thermal event (c. 1150 Ma). The xenolith P-T arrays are not interpreted as representing paleogeotherms at the time of entrainment in the host kimberlite. They most likely record P-T conditions “frozen-in” during various stages of the tectonic juxtaposition of the Namaqua Mobile Belt with the Kaapvaal craton.     

Crustal structure beneath the Faroe Islands and the Faroe–Iceland Ridge

We have mapped the transition from the continental Faroe block (the Faroe Islands and surrounding shelf) to the thickened oceanic crust of the Faroe–Iceland Ridge in the North Atlantic using the results of a detailed sea-to-land seismic profile with wide-angle to normal-incidence recordings of explosive and airgun shots fired at sea along the Faroe–Iceland Ridge. Interpretation of all available seismic and gravity data indicates that this aseismic ridge is composed of 30±3-km-thick oceanic crust, with a gradual transition to ancient continental crust from 100 to 40 km northwest of the Faroe Islands, close to the shelf edge. This confirms that the crust beneath the Faroe Islands, which may be up to 46 km thick, comprises continental material in agreement with previous seismic and geochemical results. Results suggest that the upper 5.2±0.7 km of the Faroe crust consists of Tertiary basalts generated during continental breakup, overlying the continental crust beneath. The lower crust, where seismic constraint is poor, may exhibit high seismic velocities (7.1–7.6 km s⁻¹) which we attribute to underplating or intrusion by mafic melts during continental breakup in the early Tertiary.