Subsolidus Phase Relationships in Carbonate Eclogites

J. F. Molina (j.f.m.palma@toyen.uio.no)¹, M Erambert (muriel.erambert@toyen.uio.no)¹, A Crottini², S Poli (stefano@biko.terra.unimi.it)² & H Austrheim (hakon.austrheim@toyen.uio.no)¹

¹ Minerelogisk-Geologisk Museum, Sarsgate 1, N-0562, Oslo, Norway

² Dipartimento Scienze della Terra, Università di Milano, Via Botticelli 23, 20133 Milano, Italy

Magnesite and dolomite are stable in mafic systems at high- and ultra high-pressure conditions, playing a key role in the global cycle of carbon. Carbonate-bearing assemblages and carbonate composition are strongly pressure-dependent, as demonstrated by Ca/Mg and Fe/Mg partitioning between carbonates and coexisting minerals in eclogite facies rocks from Bergen Arcs and Western Gneiss Region (Norway) as well as in high-pressure experiments. Carbonate eclogites from Bergen Arcs show the assemblage garnet + omphacite + amphibole + quartz + kyanite + dolomite + calcite (after aragonite), at ca. 700°C and 18-21 kbar. Eclogites from Western Gneiss Region contains the assemblage garnet + amphibole + omphacite + clinozoisite/epidote + quartz + phengite + paragonite + dolomite, at ca. 600°C and 16-17 kbar. High pressure experiments were performed on synthetic mixtures in the system Na-Ca-Fe-Mg-Al-Si-C-O-H, using a piston-cylinder apparatus at pressures ranging from 12 to 27 kbar and temperatures from 665 to 730°C, at fO₂ buffered by NNO. H₂O-CO₂ binary fluids were generated from oxalic acid dihydrate at low fH₂. Three mafic compositions ranging from Fe-basalt to Mg-basalt were employed in the experiments. In all three bulk compositions a large amphibole + carbonate phase field was found (Molina & Poli, 2000). A single carbonate (calcite or dolomite) occurs at low pressures, whereas at P > 16-18 kbar magnesite and dolomite coexist. Fe and Mg components in calcite and dolomite increase with pressure, causing an increase of Ca/Mg ratio in amphibole relative to calcite with increasing pressure. Fe/Mg partitioning data show that garnet has the highest affinity for Fe, whereas Fe and Mg are equally partitioned among coexisiting amphibole, clinopyroxene and carbonates at temperatures between 665°C and 730°C. Natural parageneses suggest that Fe is preferably partitioned in amphibole and garnet relative to carbonates with decreasing temperature.

Molina JF & Poli S, Earth Planet Sci. Lett, (2000 in press).

Sulphur in Silicate Melts at High Pressures: A Look at a Simple System

Duncan Hunter Sadleir Moncrieff (duncan@cnrs-orleans.fr)

ISTO, 1A rue de la Ferollerie, 45071 Orleans Cedex 2, France

The structural role of sulphur in silicate melts is effectively unknown. It is believed that sulphur dissolves principally in the melt as sulphide or sulphate with possibly minor quantities of other sulphur species. However, the manner in which sulphur reacts with the silicate melt structure is uncertain. The belief of early workers (e.g. Fincham and Richardson, 1954) was that sulphur reacted with free-oxygen anions in the melt. Our current understanding of silicate melts suggests that free-oxygen should not be present in more than trace quantities for silica rich melts. It seems more likely that sulphur reacts with non-bridging oxygens in the melt, which are present in much greater quantities. If sulphur does react with free-oxygen to dissolve in silicate melts, the presence of the necessary quantity of free-oxygen ions has, potentially, a large influence on the network speciation, and thus on several thermodynamic and transport properties (e.g. viscosity, activities of other melt components, in particular that of SiO₂). A barrier to structural studies of sulphur in silicate melts is the low solubility (normally lower than 1000 ppm) of sulphur in natural melts and synthetic analogues. The solubility of sulphur in system Na₂O-SiO₂ reaches wt% levels under high fS₂ at 1 atmosphere; the structures of Na₂O-SiO₂ glasses are comparatively well known, and so the system provides a realistic possibility of determining the role of sulphur. Experiments have been performed using piston-cylinder apparatus at 1 GPa and 1000°C in gold capsules on mixtures of sulphur with Na₂O.3SiO₂, and Na₂O.5SiO₂ glasses under dry and "wet" conditions. Sulphur solubility, preliminary spectroscopic data and interpretations are to be presented.

Fincham CJB and Richardson FD, Proc. Roy. Soc. London, 223A, 40-62, (1954).

An Experimental Study of Sulphur Solubility Behaviour in Hydrous Phonolitic and Rhyolitic Glasses

Duncan Hunter Sadleir Moncrieff (duncan @cnrs-orleans.fr)¹, Béatrice Clemente (b.clemente@opgc.univ-bpclermont.fr)², Michael R. Carroll (carroll@campus.unicam.it)³ & Bruno Scaillet (bscaille@cnrs-orleans.fr)¹

¹ ISTO, 1A rue de la Ferollerie, 45071 Orleans Cedex 2, France

² Département des Sciences de la Terre, 5 rue Kessler, 63038 Clermont-Ferrand, France

³ Università Degli Studi di Camerino, Dipartimento di Scienze della Terra, via Gentile III da Varano, 62032 Camerino, Italy

The behaviour of sulphur in magmatic systems remains poorly understood. The work that has been performed on the solubility of sulphur in simple binary and ternary oxide systems are not directly applicable to geological melts and glasses. Experiments that have been performed on geologically relevant compositions at pressures greater than 1 atmosphere in previous studies are generally poorly constrained. To help ameliorate this situation, we have performed a series of experiments using natural phonolitic and rhyolitic starting compositions. All the experiments were run in internally heated pressure vessels at 930°C and 0.5 to 4 kbar, under both oxidising and reducing conditions, fO₂ being buffered by hydrogen diffusion between the furnace atmosphere and the experimental samples, with fH₂ being measured by Shaw membrane or solid-sensors. Pyrrhotite is stable under reducing conditions in both melts, and immiscible FeS sulphide liquid is stable under certain conditions of pressure and fS₂ at 5 log units above the Ni-NiO buffer. Anhydrite and S-rich sodalite are the usual magmatic S-bearing phase under oxidising conditions in rhyolitic and phonolitic melts respectively. Melt sulphur content is positively correlated with fO₂ and fS₂; pressure has no significant effect for the conditions investigated. Using the model of Stolper (Silver et al., 1990) to provide an estimate of a(O₂)^melt, a thermodynamic model has been derived which relates melt sulphur solubility to fO₂, fS₂, a(O₂)^melt, and the equilibrium constants of sulphide and sulphate in the melt (Moncrieff, 1999). The model accurately reproduces the experimental results of a previous study on the solubility of sulphur in rhyolitic melts (Clemente, 1998).

Clemente B, Unpublished Ph. D. dissertation, Université d'Orléans, 227 pp, (1998).

Moncrieff DHS, Unpublished Ph. D. dissertation, University of Bristol, 220 pp, (1999).

Silver LA, Ihinger PD, and Stolper EM, Contrib. Mineral. Petrol, 104, 142-162, (1990).