Evidence of an early alteration process driven by magmatic fluid in Mururoa volcano

Evidence for a deuteric alteration process induced by a magmatic fluid has been found in the feeder zone of the Mururoa volcano (French Polynesia). Within the dikes, where basaltic glass does not show any evidence of pervasive alteration, vesicles are filled with dioctahedral smectites and calcite, while olivine phenocrysts are replaced by dioctahedral smectites, ankerite and calcite.The ¹³C signature of carbonates, the carbon and H₂O content of the whole rocks and their impoverishment in deuterium are compatible with the presence of magmatic CO₂ during the crystallization of intruding lavas and exclude contamination by seawater. Mass balance calculations on selected thin sections photographs of partly filled up vesicles and replaced olivine crystals, constrain, assuming a closed system interaction, the chemical composition of the initial fluid and the respective amounts of the initial solid phases involved in the alteration process. Thermodynamic modelings using the EQ3/6 software package correctly predict the mineralogic, chemical and isotopic exchanges accompanying alteration, thus validating the closed system assumption. The model which allows prediction of the influence of CO₂ on the alteration products, shows that, above a 0.25 CO₂ mole fraction in the initial fluid, the alteration is entirely controlled by the chemical composition of the initial solid phases. The presence of CO₂ implies the precipitation of dioctahedral smectites and carbonates instead of the magnesian smectites commonly observed in CO₂-free systems.The Mururoa feeder zone shows alteration features typical of a closed system interaction between the basaltic rock and a magmatic fluid in which seawater did not take part.     

The junction of Fensch and Moselle rivers,France; mineralogy and composition of river materials

The Moselle river flows in the north-east of France, from Vosges Mountains to neighboring countries Luxembourg and Germany. One of its tributaries, the Fensch river, drains a highly industrialized watershed, strongly impacted by mining, smelting and surfacing activities. The objective of this work, part of a general research program on Moselle watershed (Zone Atelier Moselle) was to assess the impact of the polluted Fensch river on the global quality of the Moselle river. For that purpose, water, sediments and suspended particulate matter were sampled in both rivers, upstream and downstream the junction. Four main sampling campaigns were carried out, in winter during a flood event and in spring at low water level. On a first step, mineralogical analyses (XRD and FTIR) and chemical analyses (ICP-MS, ICP-AES), were performed on sediments, suspended particulate matters and filtered waters. Major and trace elements concentrations were obtained on two different granulometric fractions (0–2 mm and 0–50 μm) revealing the enrichment in heavy metals of fine particles. From one collecting campaign to another, seasonal variations could be evidenced on suspended matter composition even though major minerals (calcite, quartz and kaolinite) were always present. Furthermore, spatial variations were evidenced for Fensch and Moselle downstream sediments. Thus, very fine-grained sediments, poorly crystallized, displaying at the same time higher metal concentrations and higher organic matter content than in Fensch river material, were collected downstream, in a low hydrodynamic conditions zone, assumed as a preferential sedimentary zone or even as a placer. Strong correlations could be revealed between iron content and contaminant concentrations, confirming the origin of polluted material.     

我国东部花粉雨初探

通过对位于不同气候带的北京、禹城和桃源3地的全年空气孢粉采样以及表土尘孢粉采样的研究分析,初步了解代表暖温带、中亚热带的现代花粉雨的组成,其中空气孢粉组合较表土尘孢粉组合更能反映区域植被的成分;区域花粉雨特征可以作为气候带划分的标志;我国东部花粉雨分析还表明,花粉的远距离迁移与东亚季风气候关系密切,花粉的迁移路线反映了季风运行的轨迹。通过花粉雨的研究,进一步了解孢粉组合—植被—气候三者的联系,将有助于应用孢粉分析恢复植被、恢复气候的定量研究     

Fluid composition and evolution in coesite-bearing rocks (Dora-Maira massif,Western Alps): implications for element recycling during subduction

Fluid inclusions and F, Cl concentration of hydrous minerals were analysed in the coesite-pyrope quartzite, the interlayered jadeite quartzite and their country-rock gneiss from the Dora-Maira massif using a combination of microthermometry, Raman spectrometry, synchrotron X-ray microfiuorescence and electron microprobe analysis. Three populations of fluid inclusions were recognized texturally and can be related to distinct metamorphic stages. A low-salinity aqueous fluid occurs in the retrogressed country gneiss and as late secondary inclusions in jadeite quartzite and chloritized pyrope. An earlier secondary population is found in matrix quartz of the jadeite- and pyro-pe-quartzites. This population can be related to the early decompression and so to incipient breakdown of garnet into phlogopite-bearing assemblages. The inclusion fluid is highly saline (up to 84 wt% equivalent NaCl) and contains Na, Ca, Fe, Cu and Zn as major cations. In pyrope quartzite, additional K was found in these brines, which locally coexist with CO₂-rich inclusions. The oldest fluid inclusions are preserved in kyanite grains included in fresh pyrope and in pyrope itself. In pyrope, all inclusions have decrepitated and contain magnesite, an Mg-phosphate, sheet-silicate(s), a chloride and an opaque phase, with no fluid preser ved. In contrast, the kyanite inclusions in pyrope preserve primary H₂O-CO₂ low-salinity fluid inclusions, probably owing to the low compressibility of the kyanite inclusions and host garnet. In spite of in-situ re-equilibration, these inclusions can be interpreted as relics of the dehydration fluid that attended pyrope growth. These correlations between textural and chemical fluid inclusion data and metamorphic stages are consistent with the fluid composition calculated from the halogen content of different generations of phlogopite and biotite. The preservation of different fluid compositions, both in time and space, is evidence for local control and possibly origin of the fluids, in agreement with isotopic data. These results, in particular the absence of CO₂ in the jadeite quartzite, are best interpreted in terms of a fluid-melt system evolution. With increasing metamorphism, partitioning of H₂O, Na, Ca, Fe and heavy metals into melt (jadeite quartzite) and Mg, Na/K, F, CO₂ and P(?) into a residual aqueous fluid can account for depletion in Na, Ca and Fe of the pyrope quartzite. During the retrograde path, a _{H 2 O} rose as melt crystallized, generating the two populations of hypersaline and water-rich fluids that were highly reactive to pyrope. The process of fluid-melt interaction envisioned here coupled with models of melt extraction in subduction zones provides an attractive opportunity for the instantaneous ( < 1 Ma) and selective transport of elements between a downgoing slab and the overlying mantle wedge.