Effect of solution saturation state and temperature on diopside dissolution

Steady-state dissolution rates of diopside are measured as a function of solution saturation state using a titanium flow-through reactor at pH 7.5 and temperature ranging from 125 to 175°C. Diopside dissolved stoichiometrically under all experimental conditions and rates were not dependent on sample history. At each temperature, rates continuously decreased by two orders of magnitude as equilibrium was approached and did not exhibit a dissolution plateau of constant rates at high degrees of undersaturation. The variation of diopside dissolution rates with solution saturation can be described equally well with a ion exchange model based on transition state theory or pit nucleation model based on crystal growth/dissolution theory from 125 to 175°C. At 175°C, both models over predict dissolution rates by two orders of magnitude indicating that a secondary phase precipitated in the experiments. The ion exchange model assumes the formation of a Si-rich, Mg-deficient precursor complex. Lack of dependence of rates on steady-state aqueous calcium concentration supports the formation of such a complex, which is formed by exchange of protons for magnesium ions at the surface. Fit to the experimental data yields [Formula: see text] where the Mg-H exchange coefficient, n = 1.39, the apparent activation energy, E(a )= 332 kJ mol(-1), and the apparent rate constant, k = 10(41.2 )mol diopside cm(-2 )s(-1). Fits to the data with the pit nucleation model suggest that diopside dissolution proceeds through retreat of steps developed by nucleation of pits created homogeneously at the mineral surface or at defect sites, where homogeneous nucleation occurs at lower degrees of saturation than defect-assisted nucleation. Rate expressions for each mechanism (i) were fit to [Formula: see text] where the step edge energy (α) for homogeneously nucleated pits were higher (275 to 65 mJ m(-2)) than the pits nucleated at defects (39 to 65 mJ m(-2)) and the activation energy associated with the temperature dependence of site density and the kinetic coefficient for homogeneously nucleated pits (E(b-homogeneous )= 2.59 × 10(-16 )mJ K(-1)) were lower than the pits nucleated at defects (E(b-defect assisted )= 8.44 × 10(-16 )mJ K(-1))

Mineral Associations In The Boussouma Post Birimian Mafic Dyke And Their Petrogenetic Significance (Burkina Faso, West-African Craton)

A systematic electron microprobe analysis, was carried out on constituent mineral phases (olivine, pyroxene, amphibole, plagioclase, micas, opaques) in dolerite, gabbro and pegmatitic suite associated with a mafic dyke of the Boussouma area in Burkina Faso, in order to characterize the magmatic and tectonic affinity of this intrusion. The compositions and microtextures of minerals and their paragenesis led the re- examination of the magmatic origin of this dyke. The mineralogical sequence made up of ferriferous olivine, calcic- plagioclase, orthopyroxene, augite, quartz/ micropegmatite and titano-magnetite, indicates a tholeiitic composition of the parent magma of the dyke. The composition of clinopyroxenes is in agreement with tholeiitic character of this mafic dyke and typical of rocks formed by anorogenic processes in a crustal distension zone. Enrichment in quartz/ micropegmatite and in ferro- titanic oxides from the edge to the centre of the dyke and the corresponding impoverishment of olivine, are compatible with a magmatic fractionation process. The temperature of the magma during crystallization using coexistent augite and orthopyroxene geothermometry, is estimated between 1000 and 1150°C and it progressively decreased to about 600 to 800°C, during the late crystallization stages as revealed by biotite thermometry. The post magmatic evolution of the dyke is marked by the neoformation hydrothermal minerals of dominantly low temperature hydrous phases Keywords: Dolerite, micro-analysis, minerals, tholeiite, distension, geothermometry.Global Journal of Geological Sciences Vol. 6 (2) 2008: pp. 157-17

Solid-liquid iron partitioning in Earth's deep mantle

International audienceMelting processes in the deep mantle have important implications for the origin of the deep-derived plumes believed to feed hotspot volcanoes such as those in Hawaii(1). They also provide insight into how the mantle has evolved, geochemically and dynamically, since the formation of Earth(2). Melt production in the shallow mantle is quite well understood, but deeper melting near the core-mantle boundary remains controversial. Modelling the dynamic behaviour of deep, partially molten mantle requires knowledge of the density contrast between solid and melt fractions. Although both positive and negative melt buoyancies can produce major chemical segregation between different geochemical reservoirs, each type of buoyancy yields drastically different geodynamical models. Ascent or descent of liquids in a partially molten deep mantle should contribute to surface volcanism or production of a deep magma ocean, respectively. We investigated phase relations in a partially molten chondritic-type material under deep-mantle conditions. Here we show that the iron partition coefficient between aluminium-bearing (Mg,Fe)SiO3 perovskite and liquid is between 0.45 and 0.6, so iron is not as incompatible with deep-mantle minerals as has been reported previously(3). Calculated solid and melt density contrasts suggest that melt generated at the core-mantle boundary should be buoyant, and hence should segregate upwards. In the framework of the magma oceans induced by large meteoritic impacts on early Earth, our results imply that the magma crystallization should push the liquids towards the surface and form a deep solid residue depleted in incompatible elements

Transport of dissolved Si from soil to river: a conceptual mechanistic model

This paper reviews the processes which determine the concentrations of dissolved silicon (DSi) in soil water and proposes a mechanistic model for understanding the transport of Si through a typical podzol soil to the river. DSi present in natural waters originates from the dissolution of mineral and amorphous Si sources in the soil. However, the DSi concentration in natural waters will be dependent on both dissolution and deposition/precipitation processes. The net DSi export is controlled by soil composition like (mineralogy and saturated porosity) as well as water composition (pH, concentrations of organic acids, CO2 and electrolytes). These state variables together with production, polymerization and adsorption equations constitute a mechanistic framework determining DSi concentrations. For a typical soil profile in a temperate climate, we discuss how the values of these key controls differ in each soil horizon and how it influences the DSi transport. Additionally, the impact of external forcings such as seasonal climatic variations and land use, is evaluated. This model is a first step to better understand Si transport processes in soils and should be further validated with field measurements