Prediction of plagioclase-melt equilibria in anhydrous silicate melts at 1-atm

Many models for plagioclase-melt equilibria have been proposed over the past 30 years, but the focus is increasingly on the effects of water content and pressure. However, many geological and petrological applications concern low pressure and low water systems, such as the differentiation of large terrestrial basaltic magma chambers, and lunar and asteroidal magmatism. There is, therefore, a justified need to quantify the influence of anhydrous liquid composition on the composition of equilibrium plagioclase at 1-atm. With this in mind, a database of over 500 experimentally determined plagioclase-liquid pairs has been created. The selected low pressure, anhydrous, experiments include both natural and synthetic liquids, whose compositions range from basalt to rhyolite. Four equations are proposed, derived from this data. The first is based on a thermodynamically inspired formalism, explicitly integrating the effect of temperature. This equation uses free energies and activities of crystalline anorthite available from the literature. For the activity of anorthite in the liquid phase, it is found that current models of the activity of individual oxides are insufficient to account for the experimental results. We have therefore derived an empirical expression for the variation of anorthite activity in the liquid as a function of melt composition, based upon inversion of the experimental data. Using this expression allows the calculation of plagioclase composition with a relative error less than 10%. However, in light of the fact that temperature is not necessarily known for many petrological applications, an alternative set of T-independent equations is also proposed. For this entirely empirical approach, the database has been divided into three compositional groups, treated independently for regression purposes: mafic–ultramafic, alkali-rich mafic–ultramafic, and intermediate-felsic. This separation into distinct subgroups was found to be necessary to maintain errors below acceptable limits, but results across group boundaries were found to be comparable. Overall, 50% of plagioclase compositions are predicted to within 2% of the experimentally derived value, and 90% to within 5%, representing a significant improvement over existing models.     

Residence times for protons bound to three oxygen sites in the AlO4Al12(OH)24(H2O)12 polyoxocation

Although, the kinetic reactivity of a mineral surface is determined, in part, by the rates of exchange of surface-bound oxygens and protons with bulk solution, there are no elementary rate data for minerals. However, such kinetic measurements can be made on dissolved polynuclear clusters, and here we report lifetimes for protons bound to three oxygen sites on the AlO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺ (Al₁₃) molecule, which is a model for aluminum-hydroxide solids in water. Proton lifetimes were measured using ¹H NMR at pH ∼ 5 in both aqueous and mixed solvents. The ¹H NMR peak for protons on bound waters (η-H₂O) lies near 8 ppm in a 2.5:1 mixture of H₂O/acetone-d₆ and broadens over the temperature range −20 to −5 °C. Extrapolated to 298 K, the lifetime of a proton on a η-H₂O is τ²⁹⁸ ∼ 0.0002 s, which is surprisingly close to the lifetime of an oxygen in the η-H₂O (∼0.0009 s), but in the same general range as lifetimes for protons on fully protonated monomer ions of trivalent metals (e.g., Al(H₂O)₆³⁺). The lifetime is reduced somewhat by acid addition, indicating that there is a contribution from the partly deprotonated Al₁₃ molecule in addition to the fully protonated Al₁₃ at self-buffered pH conditions. Proton lifetimes on the two distinct sets of hydroxyls bridging two Al(III) (μ₂-OH) differ substantially and are much shorter than the lifetime of an oxygen at these sites. The average lifetimes for hydroxyl protons were measured in a 2:1 mixture of H₂O/dmso-d₆ over the temperature range 3.7-95.2 °C. The lifetime of a hydrogen on one of the μ₂-OH was also measured in D₂O. The τ²⁹⁸ values are ∼0.013 and ∼0.2 s in the H₂O/dmso-d₆ solution and the τ²⁹⁸ value for the μ₂-OH detectable in D₂O is τ²⁹⁸ ∼ 0.013 s. The ¹H NMR peak for the more reactive μ₂-OH broadens slightly with acid addition, indicating a contribution from an exchange pathway that involves a proton or hydronium ion. These data indicate that surface protons on minerals will equilibrate with near-surface waters on the diffusional time scale.     

Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust

The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100-500 m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe-Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An₆₈-An₆₀), trace elements (Sr: 1023-1071 ppm; Ba: 132-172 ppm) and Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr_i: 0.70356-0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An₆₀-An₃₈). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm³ during early fractionation of troctolites and then decreased continuously to 2.16 g/cm³ with fractionation of Fe-Ti oxide-bearing gabbros. Plagioclase (An₆₉-An₆₀) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (<An₆₀) became negatively buoyant and anorthosite stopped forming. Blocks of anorthosite (autoliths) even fell downward to the basal cumulate pile. The presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and/or minor plagioclase nucleation within the main magma body. Dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid.The processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust, with (gabbro-)noritic rocks at the base and anorthositic rocks at the top. We propose that the lunar crust mainly crystallized bottom-up. This basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian-suite without KREEP contamination, while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust.     

Persistence of Artemisia steppe in the Tangra Yumco Basin,west-central Tibet,China: despite or in consequence of Holocene lake-level changes?

The closed Tangra Yumco Basin underwent the strongest Quaternary lake-level changes so far recorded on the Tibetan Plateau. It was hitherto unknown what effect this had on local Holocene vegetation development. A 3.6-m sediment core from a recessional lake terrace at 4,700 m a.s.l., 160 m above the present lake level of Tangra Yumco, was studied to reconstruct Holocene flooding phases (sedimentology and ostracod analyses), vegetation dynamics and human influence (palynology, charcoal and coprophilous fungi analyses). Peat at the base of the profile proves lake level was below 4,700 m a.s.l. during the Pleistocene/Holocene transition. A deep-lake phase started after 11 cal ka BP, but the ostracod record indicates the level was not higher than ~4,720 m a.s.l. (180 m above present) and decreased gradually after the early Holocene maximum. Additional sediment ages from the basin suggest recession of Tangra Yumco from the coring site after 2.6 cal ka BP, with a shallow local lake persisting at the site until ~1 cal ka BP. The final peat formation indicates drier conditions thereafter. Persistence of Artemisia steppe during the Holocene lake high-stand resembles palynological records from west Tibet that indicate early Holocene aridity, in spite of high lake levels that may have resulted from meltwater input. Yet pollen assemblages indicate humidity closer to that of present potential forest areas near Lhasa, with 500–600 mm annual precipitation. Thus, the early mid-Holocene humidity was sufficient to sustain at least juniper forest, but Artemisia dominance persisted as a consequence of a combination of environmental disturbances such as (1) strong early Holocene climate fluctuations, (2) inundation of habitats suitable for forest, (3) extensive water surfaces that served as barriers to terrestrial diaspore transport from refuge areas, (4) strong erosion that denuded the non-flooded upper slopes and (5) increasing human influence since the late glacial.     

Land-use and climate change within assessments of biodiversity change: A review

Projected changes in biodiversity are likely inadequately estimated when climate and land-use change effects are examined in isolation. A review of studies of the effects of these drivers singly and in combination highlights little discussed complexities in revising these estimates. In addition to considering interactions, different characterisations of climate change, land-use change and biodiversity greatly influence estimates. Habitat loss leading to decreased species richness is the most common land-use change and biodiversity relationship considered with less attention being given to other land-use changes (e.g. other conversions, fragmentation, different management intensities) and biodiversity characterisations and responses (e.g. selected groups of species, increased species richness). Characterisations of more complex relationships between climate change, land-use change and biodiversity however are currently limited by a lack of process understanding, data availability and inherent scenarios uncertainties.     

Structural and mineralogical study of active and inactive hydrothermal fluid discharges in Panarea,Italy

The isle of Panarea (Aeolian Arc, Italy) is an active hydrothermal system, characterized by different types of fluid emissions, which cause various types of unique hydrothermal structures that are typical for hydrothermal fluid discharges. Such fluid discharge structures at shallow depths of less than 30 m have not been described in marine hydrothermal systems until now to the authors’ knowledge. The focus of the investigations was set on morphological, mineralogical, geochemical, and structural analyses. A broad variety of analytical methods was applied to gain as much information as possible. In addition to geological and sedimentological documentation and standard methods like making and analyzing thin and polished sections, optical microscopy or X-ray fluorescence analysis, the samples were investigated by X-ray computer tomography and scanning electron microscopy, including energy-dispersive X-ray spectroscopy. Because discharge features occur in various settings, a general classification of the features in the hydrothermal system was performed with special focus on their preservability in geological archives. In total, nine different facies types were distinguished based on genetic and/or morphological differences. All samples show laminated structures from µm to cm scale. The single laminae vary in shape and material witnessing various chemical compositions.     

Hydrogeologic Behavior of an Alluvial Aquifer,Salta Province,Argentina: Simulations of Hydraulic Conductivity Field,Groundwater Flow,and Chloride Migration

This work was designed to analyze the hydrogeological behavior of an alluvial aquifer in the River Mojotoro basin site in the Province of Salta, Argentina. The study area presents coarse-grained sediments with high infiltration capacity. The hydraulic conductivity field is affected by the physical heterogeneity of the medium and a geostatistical method, kriging, was used to construct this field from local measurements. The groundwater flow velocity field is computed using a hybrid mixed finite-element method and small variations in the flow directions were determined. Given the existence of a point source of chloride in the region under study, the spatial distribution of the generated plume in the saturated zone is analyzed. The advection-diffusion equation describing the transport of chloride in vertical and horizontal cross-sectional planes of the aquifer is discretized employing a method of characteristics combined with a mixed finite-element procedure, which provides an efficient method to eliminate spurious numerical oscillations and handle the convective term in the equation. The migration of chloride shows patterns, which are affected by the litologic heterogeneities of the aquifer and the regional groundwater flow.     

Oxygen and hydrogen isotope compositions of eclogites and associated rocks from the Eastern Sesia zone (Western Alps,Italy)

Oxygen and hydrogen isotope analyses have been made of mineral separates from eclogites, glaucophanites and glaucophane schists from the eastern Sesia zone (Italian Western Alps). Regularities in (1) hydrogen isotope compositions, (2) order of ¹⁸O enrichment among coexisting minerals, and (3) ¹⁸O (quartz-rutile) and ¹⁸O (quartz-phengite) imply attainment of a high degree of isotopic equilibrium. However, some scattering of ¹⁸O values of individual minerals indicates that the eclogitic assemblage did not form in the presence of a thoroughly pervasive fluid. Minerals from an eclogitic lens enclosed in marble have ¹⁸O values distinctly different from those measured in the other rocks. The ¹⁸O values are high in comparison with other type C eclogites of the world, and it is proposed that the fluid present during the high pressure metamorphism has to a large extent been inherited from the precursor rocks of amphibolite facies.An average formation temperature of 540 ° C is inferred from the oxygen isotope fractionations between quartz and rutile and between quartz and white mica. This temperature is in accordance with petrologic considerations and implies subduction of the precursor rocks into the upper mantle to achieve the high pressures required.     

Diffusion processes in resorbed garnets

Mn- (and Ca-) enriched border fringes around resorbed garnets, and Mn-enriched zones along cracks in resorbed garnets and along intergranular boundaries in resorbed sievegarnets, indicate a limited diffusion of Mn (and some Ca) into the garnet structure.