Phase Relationships and Differentiation of MORB up to 0.2 GPa: Experimental Outline and First Results

Jasper Berndt (j.berndt @mineralogie.uni-hannover.de), Juergen Koepke & Francois Holtz

Institut für Mineralogie, Welfengarten 1, 30167 Hannover, Germany

Evolution and emplacement of SiO₂-rich melts from tholeiitic magmas represents an important stage in the development of the oceanic crust. In this study, the mechanism of differentiation of SiO₂-rich melts in the oceanic crust is investigated by systematic experiments at 0.1 to 0.2 GPa as a function of T and water content using MORB compositions as starting material. Experiments were performed in internally-heated-pressure-vessels (IHPV) equipped with a rapid-quench system used to prevent the formation of quench crystals. A specific sample holder was developed to fix the capsules to a Pt-wire in the hot zone. After completion of the experiment the capsule falls onto a copper block lying outside of the hot zone. The cooling rates completely avoid the formation of quench-crystals in low-viscosity basaltic melts containing up to 9.5 wt% H₂O at 1300°C. A problem is the Fe-loss by diffusion into the capsule material. Ag₇₅Pd₂₅ is a suitable capsule material for temperatures up to 1050°C. At higher temperatures Au75Pd25 alloy [1] was used to minimise the Fe-loss. No Fe was detected in an Au₇₅Pd₂₅-capsule of 0.2 mm thickness after an experiment at 1300°C, 24 h duration, 0.2 GPa, except for the first two µm. Phase relationships at 0.2 GPa for fO₂-conditions close to HM-buffer and water solubility determinations in the pressure range 0.05 - 0.5 GPa at 1200°C are presented. Following phases crystallised: ol, opx, cpx, plag, amph and titano-hematite. With decreasing total water content from 4.8 to 0.5 wt% H₂O Fo-content of olivine and An-content of plagioclase decreases too. The melts that could be analysed (melt fraction>20vol%) contained always < 60 wt% SiO₂. Water solubility is 1.7 wt% at 0.05 GPa and increases linearly from 3.4 wt% at 0.1 GPa to 9.5 wt% at 0.5 GPa.

Kawamoto T, Hirose K, Eur. J. Mineral, 6, 381-385, (1994).

Pressure Effect on Graphitization: Experimental Constraints

Olivier Beyssac (beyssac@geologie.ens.fr)¹, Jean Noël Rouzaud (rouzaud@cnrs-orleans.fr)², Fabrice Brunet (brunet@geologie.ens.fr)¹, Jean Pierre Petitet (petitet@limhp.univ-paris13.fr)³ & Bruno Goffé (goffe@geologie.ens.fr)¹

¹ Laboratoire de Géologie (CNRS UMR 8538), Ecole Normale Supérieure, 24 rue Lhomond, 75005 Paris, France

² CNRS, Centre de Recherches sur la Matière Divisée, 1b rue de la Férollerie, 45000 Orléans, France

³ CNRS, Laboratoire d'Ingénérie des Matériaux et des Hautes Pressions, Avenue Jean Baptiste Clément, 93430 Villetaneuse, France

The respective effects of temperature, pressure and strains are hardly discussed to explain the formation of graphite in nature and during laboratory experiments (Bustin et al, 1995). In the present work, reference cokes obtained from various synthetic molecules (a non-graphitizable microporous saccharose-based coke and a graphitizable lamellar anthracene-based coke) and a microporous natural anthracite were heated up to 1000°C under quasi-hydrostatic pressure up to 4 GPa in a piston-cylinder apparatus. Their structural evolution was characterized by coupling Raman microspectroscopy and High Resolution Transmission Electron Microscopy (HRTEM). With all precursors, graphitization seems to be strongly enhanced by hydrostatic pressure.

At the micrometric scale (Raman), the release of structural defects was quantified using the relative intensity of the defect band. This intensity is considerably lower in high-pressure samples compared to 1-bar experiments performed at the same temperature (Bény-Bassez & Rouzaud, 1985). At the nanometric scale (HRTEM), triperiodic graphite was detected in all samples synthetized at 1000°C and 2.0 GPa, whereas a temperature of 2800°C and a graphitizable precursor are required at ambient pressure to reach the same structural order (Rouzaud & Oberlin, 1989). In the case of the anthracene-based coke, the graphitization mechanism remains the same whatever the pressure (reorientation and in-plane growth of the basic structural unit). By contrast, for microporous precursors, graphitization under pressure is made possible by microtextural changes: graphitization mainly occurs on pore walls as a result of pore growing.

Using the same analytical approach, we have studied natural samples (coals and kerogens) metamorphosed under various geothermal gradient (Western Alps (HP-LT), Central Alps (LP-HT) and Japan (HP-LT)). In these natural samples, graphitization seems mainly controlled by temperature rather than pressure and/or strain. This result leads us to the question of a possible kinetics control on our experiments, especially with graphitizable precursors.

Bustin RM, Rouzaud JN & Ross JV, Carbon, 33, 679-692, (1995).

Bény-Bassez C & Rouzaud JN, Scanning Electron Microscopy, 1, 119-132, (1985).

Rouzaud JN & Oberlin A, Carbon, 27, 517-529, (1989).