Persistence of mantle lithospheric Re–Os signature during asthenospherization of the subcontinental lithospheric mantle: insights from in situ isotopic analysis of sulfides from the Ronda peridotite (Southern Spain)

The western part of the Ronda peridotite massif (Southern Spain) consists mainly of highly foliated spinel-peridotite tectonites and undeformed granular peridotites that are separated by a recrystallization front. The spinel tectonites are interpreted as volumes of ancient subcontinental lithospheric mantle and the granular peridotites as a portion of subcontinental lithospheric mantle that underwent partial melting and pervasive percolation of basaltic melts induced by Cenozoic asthenospheric upwelling. The Re–Os isotopic signature of sulfides from the granular domain and the recrystallization front mostly coincides with that of grains in the spinel tectonites. This indicates that the Re–Os radiometric system in sulfides was highly resistant to partial melting and percolation of melts induced by Cenozoic lithospheric thermal erosion. The Re–Os isotopic systematics of sulfides in the Ronda peridotites thus mostly conserve the geochemical memory of ancient magmatic events in the subcontinental lithospheric mantle. Os model ages record two Proterozoic melting episodes at ~1.6 to 1.8 and 1.2–1.4 Ga, respectively. The emplacement of the massif into the subcontinental lithospheric mantle probably coincided with one of these depletion events. A later metasomatic episode caused the precipitation of a new generation of sulfides at ~0.7 to 0.9 Ga. These Proterozoic Os model ages are consistent with results obtained for several mantle suites in Central/Western Europe and Northern Africa as well as with the Nd model ages of the continental crust of these regions. This suggests that the events recorded in mantle sulfides of the Ronda peridotites reflect different stages of generation of the continental crust in the ancient Gondwana supercontinent.     

The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection

The LMDZ4 general circulation model is the atmospheric component of the IPSL–CM4 coupled model which has been used to perform climate change simulations for the 4th IPCC assessment report. The main aspects of the model climatology (forced by observed sea surface temperature) are documented here, as well as the major improvements with respect to the previous versions, which mainly come form the parametrization of tropical convection. A methodology is proposed to help analyse the sensitivity of the tropical Hadley–Walker circulation to the parametrization of cumulus convection and clouds. The tropical circulation is characterized using scalar potentials associated with the horizontal wind and horizontal transport of geopotential (the Laplacian of which is proportional to the total vertical momentum in the atmospheric column). The effect of parametrized physics is analysed in a regime sorted framework using the vertical velocity at 500 hPa as a proxy for large scale vertical motion. Compared to Tiedtke’s convection scheme, used in previous versions, the Emanuel’s scheme improves the representation of the Hadley–Walker circulation, with a relatively stronger and deeper large scale vertical ascent over tropical continents, and suppresses the marked patterns of concentrated rainfall over oceans. Thanks to the regime sorted analyses, these differences are attributed to intrinsic differences in the vertical distribution of convective heating, and to the lack of self-inhibition by precipitating downdraughts in Tiedtke’s parametrization. Both the convection and cloud schemes are shown to control the relative importance of large scale convection over land and ocean, an important point for the behaviour of the coupled model.     

Modeling of the transport and deposition of tungsten in the scheelite-bearing calc-silicate gneisses of the Montagne Noire,France

Scheelite-bearing calc-silicate gneisses (CSG) oceur in the Montagne Noire within a series of dominant micaschists. Detailed petrographical and mineralogical studies reveal three successive stages of metamorphism and hydrothermal alteration: (1) stage 1, a regional metamorphism at 550°C and 4.5 kb where no mineralization is formed; (2) stage 2a, a hydrothermal alteration at 500 to 450°C and 4 to 3 kb which is characterized by an intense sericitization of feldpars and deposition of Sn in Sn-bearing cale-silicates; and (3) stage 2b, a hydrothermal alteration characterized by the crystallization of idocrasegrossular in CSG with concomittant precipitation of scheelite. Tungsten was transported through the micaschist environment and deposited as scheelite only in the CSG of stage 2b at relatively low pressures. To characterize the mechanism of tungsten transport, tungsten speciation at high P-T and scheelite solubility in aqueous solations buffered by the CSG and by the micaschists assemblages were calculated. It was found that H₂WO ₀ ⁴ , HWO _- ⁴ and WO _2- ⁴ are the dominant tungsten aqueous species in H₂O–NaCl (one molal) solutions at 500°C and 2–4 kb. Calculations also indicate that scheelite deposition is controlled by decreasing pressure and increasing activity of aqueous calcium in this system. This is consistent with the petrographical and mineralogical observations. The consequences of the presence of volatiles (N₂, CH₄, CO₂) in the regional fluids were examined by determining the effect of N₂ on tungsten speciation and scheelite solubility. The addition of N₂ (up to 10 mol%) to the mineralizing fluids results in a marked increase in H₂WO ₀ ⁴ and HWO _- ⁴ concentrations relative to WO _2- ⁴ and in a large decrease of scheelite solubility. This mechanism favours scheelite precipitation and accounts for the commonly observed association of W (and Sn) deposits with graphitic series generating mixed volatiles fluids.     

Sedimentation, distribution and diagenesis of organic matter in a recent carbonate environment, Abu Dhabi, U.A.E.

In the modern hypersaline carbonate lagoon and sabkha sedimentary environments of Abu Dhabi (United Arab Emirates), three types of organic matter originate respectively from microbial mat, Avicennia mangrove, and Halodule lagoonal seagrass. The study of recent sedimentary processes and cross sections through the sabkha sediments lead to the definition of organo-sedimentary facies based on geochemical and sedimentological criteria. This permits the construction of an organo-sedimentary sequence which expresses the Holocene sedimentary record involving a transgressive and a regressive sequence. The various organic facies occur in both sequences.Heterogeneity within the individual organic facies reflects several factors, including sedimentation dynamics, mineral matrix, oxidation and reduction, and selective organic and mineral diagenesis. These parameters are discussed in terms of depositional environment and location within the organo-sedimentary sequence. Changes in distribution, quantity, and preservation potential of the buried organic matter are discussed in terms of sea level changes and sedimentary accretion rates.     

Cadmium bioconcentration in the echinoid Paracentrotus lividus: Influence of the cadmium concentration in seawater

Uptake and loss kinetics of cadmium were studied in Paracentrotus lividus exposed for 24 d to different stable Cd concentrations (0 to 1.14 μg Cd l⁻¹) and ¹⁰⁹Cd tracer added to seawater. The whole-body uptake kinetics were linear over the time course of the experiment. The whole-body ¹⁰⁹Cd concentration factor was independent of the stable Cd concentration in ambient seawater. Bioconcentration of Cd in whole individuals and their body compartments was directly proportional to the Cd concentration added to the seawater. The echinoid digestive tract wall showed the highest degree of Cd uptake. Whole-body loss kinetics were described by a 2-component exponential equation. The loss kinetics were similar for each of the treatments examined. Cadmium was found to display a long biological half-life in echinoid tissues; the major fraction (73 to 85%) of the Cd taken up by echinoids was eliminated following a loss component whose biological half-life was of the order of 1 year, regardless of the absolute Cd concentration accumulated by the echinoid. Paracentrotus lividus is suggested as a valuable biomonitor of Cd, particularly where knowledge of Cd concentration variations over long-term is of interest.     

Mineralogical and geochemical evolution of the 1982–1983 Galunggung eruption (Indonesia)

Pyroclastic deposits from the 1982–1983 eruption of Galunggung volcano (Java, Indonesia) reflect preeruptive magmatic evolution which is of interest because of: (1) its duration of nine months, compared to a few hours or days for most historical eruptions; (2) the diversity of eruptive styles, from ash and scoria flows to phreatomagmatic explosions, and to the strombolian activity that marked the end of the eruption; and (3) the progressive variation in chemical composition with time, from andesite (58 wt.% SiO₂) to high-Mg basalt (47 wt.% SiO₂). The 1982–1983 Galunggung basalts are rather primitive: 10 to 12 wt% MgO, 180 to 200 ppm Ni and 550 to 700 ppm Cr. Despite the presence of about 40% phenocrysts, they may represent the most primitive basalts recognized in western Java. Basalts contain phenocrysts of olivine (Fo_90-80), diopside-salite, and plagioclase (An_95-75). Andesites contain plagioclase (An_80–60), augite, hypersthene (En_67-64), and titanomagnetite. The distribution of mineral compositions in each petrographic type is nearly unimodal, although scarce plagioclase and olivine xenocrysts have been observed. Abundance of gabbroic cumulates associated with the pyroclastic flows and evolution of mineral compositions from high-Mg basalts to andesites support crystal fractionation as the main differentiation mechanism, although magma mixing of basaltic andesite and andesite cannot be excluded. Major and trace element trends, which display rough decreases of MgO, CaO, Ni, Cr with increasing degree of differentiation and also linear positive correlations of hygromagmaphile elements, are compatible with both processes. However, some discrepancies are observed between major and trace element modelling, which may be explained to some extent by the influence of in situ crystallization and/or magma mixing. The constancy of ¹⁴³Nd/¹⁴⁴Nd (0.51286±3), ²³⁰Th/²³²Th (0.65±0.02), Th/U (4.08±0.07) ratios, and to a lesser extent ¹⁸O values (+5.8 to +6.4 % SMOW) and ⁸⁷Sr/⁸⁶Sr ratios (0.70440 to 0.70468) is compatible with a magmatic evolution through fractional crystallization without significant crustal contamination. Nevertheless low-¹⁸O and high ⁸⁷Sr/⁸⁶Sr values in basaltic andesites may be due to the introduction of meteoric fluids into the Galunggung magma.     

Continuous vs. discontinuous melt segregation in migmatites: insights from a cellular automaton model

A cellular automaton model is presented that simulates melt extraction from migmatites. The varying parameters are the relative amount of melt, thresholds for melt connection leading to movement and melt escape, and type of deformation (pure and/or simple shear). It is shown that when the melt percentage is about half that of the melt escape threshold, the competition between production and deformation-assisted extraction results in a discontinuous rate of melt extraction. The process can be compared to stick-slip motion during frictional experiments. At higher melt fraction, it is continuously extracted. Deformation assists significantly in segregating melt, particularly when a pure shear (compaction) component is added to simple shear.     

Practical versus absolute salinity measurements: New advances in high performance seawater salinity sensors

Optical salinity sensors described here measure directly the seawater refractive index and thus enable a measurement of the seawater density and composition variation. We detail the measurement dependence to environmental parameters (in particular temperature and pressure) compared to conductivity sensors, and demonstrate that it may be advantageous to directly measure refractive index rather than electrical conductivity and so obtain a more direct route to density and absolute salinity.