Giovanni Ruggieri* and Giovanni Gianelli Pressure-temperature evaluation of a contact metamorphic mineral assemblage from the Larderello geothermal field, Italy

ABSTRACT - The emplacement of post-Miocene granitic intrusions below the Larderello-Travale geothermal area, generally at depths 3000 m below the present ground-level (b.g.l.) led to contact metamorphic phenomena in the sorrounding roks. In a gneiss cored at 3450m b.g.l. from geothermal well badia 1B (south-west of the town of Larderello), contact metamorphism is shown by the crystallization of post-tectonic biotite and the development of a high-temperature mineral assemblage (andalusite, cordierite, muscovite, tourmaline, apatite, titanite). Fluid inclusion and mineral chemistry data together with computed mineral equilibria, allowed evaluation of pressure-temperature conditions and fluid characteristics during contact metamorphism. Electron microprobe analyses showed that cordierite is Mg-rich (Mg-cordierite ca. 66%), whereas the annite and phlogopite end-members are present in biotite in similar proportions. Tourmaline compositions are dominated by the schorl-dravite solid solution. Tourmalines are often zoned, with cores enriched in Na and Fe and depleted in Al in comparison with the rims. Microthermometric and Raman analyses on the fluid inclusions in quartz and tourmaline crystals revealed that the fluid trapped during contact metamorphism was low-density (0.30-0.35 g/cm³) and water-rich (65.9-70.7 mol%), containing significant amounts of CO₂ (9.4-15.7 mol%) and CH₄ (14.9-19.4 mol%). The carbonic phases probably originated from interactions between fluid and graphite present in the Larderello-Travale basement rocks. Fluid inclusion isochores were constructed and an equilibrium curve was developed (calculated using thermodynamic data in the KFMASH system) for a mineral reaction involving quartz and contact phases (andalusite, muscovite and mineral solid solutions: biotite and cordierite). The intersections of the fluid inclusion isochores and this equilibrium curve provide the ranges of temperature (520-545°C) and pressure (940-1140 bar) of the contact metamorphism. Lastly, geothermometric measurements based on the Mg/Fe ratio in biotite and tourmaline indicated a temperature range (525-550°C) consistent with that evaluated for contact metamorphism.

* Corresponding author. E-mail: ruggieri@iirg.pi.cnr.it

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Andrea Orlando* and Daniele Borrini Solubility of Ti in andradite in upper mantle conditions: preliminary results

ABSTRACT - Piston cylinder experiments were carried out, aimed at determining Ti solubility in andradite in upper mantle conditions (P=1.5-2.0 GPa, T=1200-1300°C). Starting materials were Ti- undersaturated and Ti- saturated (with respect to 1 atm) synthetic andradites. From unreversed experiments at 1.5 and 2.0 GPa, the maximum Ti contents in andradites were determined as 1.0-1.4 atoms per formula unit (a.p.f.u.), compared with 1.5-1.9 a.p.f.u. at 1 atm. Oxygen fugacity was not controlled in experimental runs. The main mechanism controlling the entry of Ti in the garnet structure at high pressure appears to be "schorlomitic" substitution, whereas some "morimotoitic" substitution may occur in Ti- undersaturated andradites.

* Corresponding author. E-mail: aorlando@geo.unifi.it

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Giulio Morteani*, Dietrich Ackermand and Adolf Heinrich Horn Aluminium-phosphates and borosilicates in muscovite-kyanite metaquartzites near Diamantina (Minas Gerais, Brazil): Petrogenetic implications

ABSTRACT - Within the muscovite-dumortierite-kyanite metaquartzites of the Middle Proterozoic Banderinha formation near Diamantina (State of Minas Gerais, Brazil), five rock types, A1 through A4, and B are distinguished on the basis of whole-rock composition. In addition to quartz, muscovite and Ti-bearing hematite (0.2 to 5.0 wt.% TiO₂), the five rock types show the following typical mineral assemblages: A1: lazulite, kyanite, andalusite; A2: lazulite, kyanite, augelite; A3: lazulite, amblygonite, berlinite, tourmaline; A4: lazulite, goyazite, tourmaline, kyanite, and B: dumortierite, kyanite. From lithostratigraphical considerations, microscopic investigations and microprobe mineral analyses, as well as stable oxygen isotope thermometry, these parageneses formed during a multistage tectonothermal event at a maximum temperature of 440°C and pressure of at least 3.4 kbar during the Brasiliano tectonothermal episode between 680 and 450 Ma. Considerable solid solution series exist between lazulite and scorzalite (Mg § Fe), goyazite and svanbergite (P § S) and goyazite and crandallite (Sr § Ca). The MgO/FeO ratio of lazulite increases with the increasing MgO/FeO ratio of the bulk rock. A contact between lazulite and dumortierite was never observed. The protolith of the lazulite-bearing layer was probably a phosphorus- and locally also boron-rich clay-bearing sandstone deposited in a sabkha-like environment in originally highly alkaline conditions. In the study area, the lazulite-bearing muscovite-kyanite metaquartzites mark the strongly tectonised contact between the Middle Proterozoic Rio Paraúna and Espinhaço supergroups.

* Corresponding author. E-mail: gmorteani@gmx.de

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Paola Morbidelli, Maria Rosaria Ghiara*, Roberto Lonis and Carmela Petti Quantitative distribution and chemical composition of authigenic minerals in clinoptilolite-bearing ignimbrites from northern Sardinia (Italy): inferences for minerogenetic models

ABSTRACT - This paper describes the quantitative distribution and chemical compositions of the main authigenic phases occurring in Tertiary zeolite-bearing ignimbrites from northern Sardinia. Previous studies had indicated clinoptilolite as the most widespread authigenic mineral, with minor amounts of smectite, mordenite and silica minerals. Quantitative analyses using the combined Rietveld-RIR (Reference Intensity Ratio) method showed that, on a regional scale, clinoptilolite amounts ranging from 20 wt% to 30 wt% may be considered realistic; however, several levels of ignimbrites and related deposits revealed relative abundances for large-scale mining (>50 wt%).
The chemical composition of the clinoptilolites, ranging from K to Ca varieties, and distribution within the volcanic piles strongly support changes in the compositions of the interacting fluids.
The overall quantitative and chemical data confirm and define better the previous minerogenetic models carried out for the poorly welded ignimbritic deposits, which are mainly based on alteration processes of glassy components dominated by entrapped cognate fluids. On the other hand, external fluids are required for epiclastic and base surges deposits.

* Corresponding author. E-mail: mghiara@unina.it

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Bruno Messiga* and M.Pia Riccardi A Petrological Approach to the Study of Ancient Glass

ABSTRACT - In glass archaeology, "production indicators" are those remains which testify to specific operations carried out during the productive cycle; they allow us to reconstruct the processes and technological expedients used in the past to produce glass. The intermediate products of glass melting reveal textural and chemical inhomogeneities which may be used to infer parts of the production history. Samples of glass masses belonging to an early stage of glass making were unearthed in the archaeological sites of Lomello (province of Pavia, Italy) (I-IV centuries A.D.) and Val Gargassa (Genova) (late XII Century A.D.). In these materials, unmelted mineral phases, banded textures, phase separation, and crystallisation all produce different textures which supply information on: provenance of raw materials; components added and the efficiency of the melting process; liquid immiscibility and refining processes; cooling rate of vitreous products.
The composition of glass influences the different arrangements of tetrahedral-fold cations (SiO^4-, AlO^3-) and determines the various properties and features of the glass itself. The low effective diffusivity prevents the composition from becoming homogeneous during glass melting, and consequently evidence of early production steps is retained.
Mineral relics present in the micro-textures of glassy materials (i.e., fining slags) have great archaeometric value, because they may indicate the provenance of raw materials. The composition of relic mineral phases also supplies important analytical grounds on which to define the recipes followed during glass making, in terms of vitrifying, stabilising, flushing and additive components.

* Corresponding author. E-mail: messiga@crystal.unipv.it

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Michele Lustrino Debated topics of modern igneous petrology

ABSTRACT - Progress made thanks to the great amount of high-quality geochemical and isotopic data gathered over the last twenty years has allowed important insights into igneous petrology (e.g., Continental Flood Basalt petrogenesis, mantle source characterization, geophysical models of mantle plume systems, primary melt compositions, isotopic systematics of crustal and mantle domains, etc.). This large dataset has also been used to relate the compositional characteristics of igneous rocks with specific tectonic settings and to infer the geodynamic processes involved. However, inferring a tectonic setting mainly on the basis of geochemical constraints may fail, because partial melts with substantial compositional differences can originate from the same source, and the same melts may have been generated in different tectonic settings. Moreover, geochemical characterization of the main mantle components is still hotly debated, even in terms of concepts such as asthenosphere and lithosphere. Asthenospheric mantle is quite often believed to be a geochemically homogeneous convecting domain, whereas the lithospheric one is thought to be a variably enriched, heterogeneous, non-convecting reservoir, and capable of retaining geochemical and isotopic gradients for periods of time exceeding 2 Ga. These assumptions are clearly over-simplifications, particularly when relationships between physical and geochemical mantle characteristics are not properly constrained.
What emerges from recent literature is the "misuse" of petrological concepts tending towards the most convenient explanations and "effectively shutting out the entire creative thought process of the human mind" (Sheth, 1999). More appropriate use of petrological data is necessary to stimulate true scientific growth, especially as regards our knowledge of mantle-crust dynamics.

E-mail: michele.lustrino@uniroma1.it

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Alessandro F. Gualtieri and S. Bagni Crystal structure chemistry and color of doped-forsterites revisited
by X-ray powder diffraction and UV/Vis/NIR spectroscopy

ABSTRACT - The goal of this work is to revisit the crystal chemistry of synthetic forsterite containing doping elements in the octahedral site. A number of doping elements was systematically investigated: Li⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺, Fe²⁺, Cr³⁺, Cu²⁺, Ca²⁺, Cd²⁺, Al³⁺, V⁵⁺. The synthetic phases were obtained in air at high temperature (1350 °C for 3h) and environmental pressure. Results permit to revise and improve the knowledge on the crystal chemistry of forsterite and clarify the role of the doping elements in the structure modification and physical properties. The structures of the synthetic samples were analyzed either on a local and on a long range by the combination of UV/Vis/NIR spectroscopy data and Rietveld refinements.
It was possible to assess the factors that rule the solubility of the doping elements in olivine, mainly their charge and ionic radius for Co²⁺, Ni²⁺, Ca²⁺, Cd²⁺, V⁵⁺, crystal field for Cu²⁺ and Zn²⁺, kinetics and oxygen fugacity for Fe²⁺ and Mn²⁺.
The modifications of the structure were described only with respect to the distortion in the octahedral site induced by the doping element because there are no modifications of the tetrahedral polyhedra. Cation partitioning was also described and depends upon the ionic radius of the doping element. Exceptions are due to CFSE and differences in the cation-anion electronegativity.
The color of the synthesized powders was predicted using the crystal field theory and corroborated by the results of the spectroscopy experiments which clearly indicate a relationship between color and structure changes.

* Corresponding author. E-mail: alex@unimo.it

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