Cravero F. and Domínguez E. Origin of sedimentary kaolin in the Neuquén basin, Argentina as determined by oxygen isotopes 213-222

ABSTRACT - Two Gondwana granite-rhyolite provinces occur in southern Argentina. These are, Choiyoi, late Carboniferous-Early Permian to Triassic in age, in the Neuquén basin and Chon-Aike, Middle Jurassic, in Patagonia. In both areas, the clay facies of a fluvial sequence that overlay the complexes is composed of transported kaolinites. In Patagonia, a paleoweathered surface was formed on Chon-Aike rocks under humid temperature conditions indicated by the oxygen isotope composition of the kaolinites (d¹⁸O = 16-18). In this work it is demonstrated that isotopic composition of the kaolinites eroded from that paleosurface and deposited in a fluvial sequence has not changed. Taking this into consideration, the d¹⁸O values obtained in the sedimentary kaolinites of the Neuquén basin (15 to 17), can be considered as an indication of humid, temperature conditions as the primary origin of the kaolinite. The clasts composing the coarser facies (rhyolitic) indicate that the kaolinite has been eroded from a paleoweathered surface developed on Choiyoi rocks.
This work demonstrates that following a huge volcanic event in the southern hemisphere, paleoweathered surfaces developed in a humid and temperate climate, conditions already established for Chon-Aike kaolinites. Moreover all the continental sedimentary sequences that overlay Choiyoi and Chon-Aike are promising areas for prospecting for sedimentary kaolin deposits.
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ABSTRACT - The capacity of the demosponge Chondrosia reniformis to selectively incorporate and elaborate inorganic materials is described. The sponge actively engulfs sandy grains of various silica-bearing minerals, such as quartz, opale, chalcedony, feldspars, mica, chlorite, amphiboles, etc. Conversely, carbonates are rejected, at different stages during the sponge biological cycle. Different ectosome activity towards quartz and opale is evidenced by the sponge capacity to differentially react to different crystallographic forms of silica: while opale and chalcedony are stored in the collagenous fibres, quartz is dissolved and residual rounded micrograins (3-5 mm) are ejected. The ascorbic acid produced by the sponge is considered to be the agent for quartz dissolution. Finally, the production of calcium oxalates (whewellite and weddellite) and gypsum by the sponge, and their release in the surrounding sediment is evidenced.
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ABSTRACT - Late-Hercynian dyke swarms of Sarrabus (southern Sardinia) consists of metaluminous up to peraluminous felsic products, prevalently rhyolitic in composition (granite porphyries, microgranites and aplites) and basic and intermediate dykes comprising few tholeiitic basalts and a calc-alkaline suite dominated by basaltic andesites and andeites. New geochronological data contribute to define the time-span of dyke magmatism. Rb-Sr mica ages for rhyolitic rocks cluster around 290Ma and 270 Ma. Field data and one Rb-Sr biotite age (259±3 Ma) obtained on a basaltic andesite sample indicate that mafic dykes prevalently intruded later than felsic ones.
Mafic dyke rocks are mantle-derived, but both among tholeiitic dykes and calc-alkaline one, rocks representative of primary mantle melts are lacking. The least evolved calc-alkaline rocks are represented by rare basalts and by high-Mg basaltic andesites. Petrography, mineral and whole-rock chemistry, Sr and Nd isotopic features and mass balance calculations suggest that fractional crystallization associated with minor amount of crustal contamination controlled the evolution of the calc-alkaline suite. A model involving fractional crystallization and assimilation of metasedimentary materials (AFC) is proposed to account for the main geochemical and Sr and Nd isotopic variations from the high-Mg basaltic andesites to dacites. Tholeiitic basalts and the least evolved calc-alkaline rocks show an arc-type incompatible element signature (LILE and LREE enrichments and Ti, Nb, P depletions), positive e^tSr end negative e^tNd values suggesting derivation from a mantle source possibly enriched during a previous subduction event.
Differences in the Sr and Nd isotopic composition, in agreement with mineralogy and geochemistry, point to the occurrence of two different types of felsic dykes, i.e. the peraluminous (PR) and metaluminous to mildly peraluminous (MmPR) groups. It is proposed that PR dykes originated by partial melting from pelitic metasedimentary sources. The high (⁸⁷Sr/⁸⁶Sr)_t ratios (0.7154÷0.7173) and the negative e^tNd value (-7.4) show by this group of rhyolites match those of the metasediments from the Hercynian basement of Sardinia. Mass balance calculations indicate that only the least silicic MmPR rhyolites could be genetically related to the basic-intermediate dykes by fractional crystallization relationships. On the contrary, for most MmPR rocks, an origin by partial melting of crustal sources seems more likely. Their relatively low (⁸⁷Sr/⁸⁶Sr)_t and high e^tNd values (0.7076÷0.7089 and –5.7÷-6.3, respectively), partly overlapping those of the calc-alkaline basic-intermediate dykes, require source materials characterized by a less evolved Nd and Sr isotopic composition than those observed for the Sardinia metamorphic basement and imply the involvement of both crustal and juvenile mantle components.
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Ghiara M.R., Petti C. and Morbidelli P. Analcimization processes in the pyroclastic rocks from Phlegraen Field (southern Italy): compositional variations and geochemical balances 261-273

ABSTRACT - A series of scoriaceous fragments collected from pyroclastic fall deposits belonging to different volcanoes of the Phlegraean Fields (Neaples, Italy)shows progressive stages of analcimization. Particularly, major and trace elements and ⁸⁷Sr/⁸⁶Sr ratio in non-altered and altered samples from the Fondo Riccio volcano have been determined. With increasing analcimization the following geochemical balance appears: Si and K decrease, Al, Na and Fe increase, Ca and Mg remain constant. Pb is depleted whereas Rb is slightly enriched in the more analcimized samples. All rere-earth elements do not show any significant difference between non-altered and altered samples. ⁸⁷Sr/⁸⁶Sr ratio strongly decreases in the more altered samples. Chemical variations observed in the scoriaceous fragments from the Fondo Riccio volcano, suggest that the analcimization process occurs in an open system and that a fluid component enriched in Na is involved. Therefore in a petrogenetic modelling, Pb, Rb and ⁸⁷Sr/⁸⁶Sr must be used with very caution.
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Fumagalli P. and Poli S. Phase relationships in hydrous peridotites at high pressure: preliminary results of multianvil experiments 275-286

ABSTRACT - Phase relationships in hydrous peridotite at high pressure are investigated running experiments in a multianvil apparatus at 4.6-5.2 GPa pressures, 680-750°C temperatures and fluid saturated conditions in the peridotite model system Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O (NCFMASH). Pressure calibration was carried out both al room temperature using the transitions Bi I-II and Bi III-V (occurring at 2.55 GPa and 7.7 GPa, respectively), and at 1000°C using the phase transitions coesite-stishovite and garnet-perovskite structure in Ca-germanates (occurring at 8.7 and 6.1 GPa, respectively). Run duration were from 110 to 170 hours. Run products were identified by X-ray powder diffraction, back-scattered and secondary electron images and microprobe analysis.
In an Al-enriched peridotite, the assemblage olivine, clinopyroxene, garnet 10 phase was found at 5.2 GPa 680°C. Since the crystals structure of the 10 phase is broadly similar to micas, this phase can incorporate more than 0.8 atoms per formula unit (a.p.f.u.) of aluminium. However diffraction patterns do not exclude the occurrence of hydrous mixed-layered structures at these conditions. Although in the simple system MgO- SiO₂-H₂O the assemblage 10 phase + forsterite is incompatible with the join enstatite + H₂O, all these phases may coexist in Fe- and Al-bearing systems (lherzolites and pyroxenites). At 4.6 GPa 750°C the anhydrous assemblage olivine, clinopyroxene, orthopyroxene, garnet is stable. Phase abundances obtained by Rietveld refinement are in agreement with those obtained by mass balance and present evidence for the reaction 10 phase = enstatite + pyrope + H₂O.
The occurrences of the 10 phase at pressures above the chlorite stability field promotes H₂O transfer to Dense Hydrous Magnesium Silicates (DHMS) and H₂O transport into the deep mantle (> 200 km depth).
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Morbidelli P., Ghiara M.R., Lonis R. and Sau A. Zeolitic occurrences from Tertiary pyroclastic flows and related epiclastic deposits outcropping in northern Sardinia (Italy) 287-313

ABSTRACT -

This study is part of research work aimed at identifying zeolitic occurrences of economic interest in Sardinia.
The aim of the present work is to define genetic models for zeolite occurring in Tertiary pyroclastic flows outcropping in northern Sardinia (Logudoro region). The sample area, covering about 1000 km², includes the villages of Ozieri, Ploaghe, Ittireddu, Chilivani, Mores and Bonorva.
The most important volcanic formations, outcropping here, include the oldest manifestations (24-15 Ma) of Oligo-Miocene orogenic magmatism in Sardinia. Tertiary volcanics include poorly welded (ILP and IC) and welded (IS) ash and pumice flows; epiclastic (EV and EN) and base surge (BS) deposits are locally interbedded. Field research, devoted to lithostratigraphic reconstruction of outcropping suites, also showed numerous degassing pipes inside ignimbritic rocks.
Laboratory research identified various rock components (matrix, phenocrysts, pumice and xenoliths) and modal analysis were also carried out near Bonorva. The distributive and textural features of secondary minerals (zeolite, smectite, silica minerals and glauconite) were also examinated. Microscopic (optical and SEM) data defined the following crystallization order:
smectite => clinoptilolite => opal-CT => mordenite.
The most common zeolitic variety is clinoptilolite, which: 1)fills vugs and degassing pipe structures; 2) lines both tubular and sub-spherical vesicles in pumice; 3) grows on cuspate glassy fragments; 4) intergrows with silica minerals in patches randomly distributed in the matrix; 5) constitutes the felt-form intergrowths of well-developed crystals growing on the matrix; 6) lines veins which, in the central part, are filled with anhedral calcitic aggregates.
The overall data set suggests the following three genetic models for this zeolitic material: a) crystallization from fluids entrapped inside sub-spherical vesicles hosted in pumiceous fragments; b) crystallization linked to polyphase transformation of glassy components by juvenile fluids and c) crystallization linked to interaction processes on glassy components by external fluids.
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