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Real-time observation of crystallization and textural evolution in magmatic systems with the moissanite cell

Abstract : We have developed a new experimental technique that allows real-time in-situ investigation of crystallization and melting processes up to 1250°C. The method uses a moissanite cell specifically designed for long-term direct observation under an optical microscope. The method was tested at moderate temperatures (300-850°C) by observing the crystallization and textural coarsening of polycrystalline salt aggregates that are analogues for silicate systems with a low to moderate surface energy anisotropy (i.e. olivine-melt, quartz-melt). The experiments show that the technique is particularly suitable for the direct investigation of processes occurring along grain boundaries on the micro-scale. A major result is the first detailed in-situ observation of coalescence between two chloride grains during static grain growth in the presence of melt. The moissanite cell also allowed for the first time the real-time observation of crystallization of a melt with basaltic-andesitic composition at both temperatures near the liquidus (ca. 1250°C) and just above the glass-transition (900°C). In these experiments the growth history of individual crystals was observed, and direct measurements of crystal growth and dissolution rates were performed. The formation of crystal clusters, and the interaction between crystals, magmatic flow and bubbles during nucleation, growth and dissolution processes were studied. Detailed texture analysis and crystal size distribution (CSD) measurements on the basaltic andesite crystallizing at 900°C provided insight on the effect of texture-modifying processes on the evolution of the CSD. In particular overgrowth and coalescence of plagioclase crystals at 900°C were observed and appear to be important in controlling the evolution of CSD. Our results show that optical in-situ observations reveal insights that are not accessible by conventional methods; thus it emphasizes the importance of in-situ experimental investigations for a correct interpretation of rock textures, and for better understanding growth and dissolution kinetics. The final textures of igneous rocks do not necessarily record the early stages of texture evolution and current models for the interpretation of CSD curves require revision.
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Submitted on : Wednesday, December 11, 2019 - 3:10:17 PM
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Federica Schiavi, N. Walte, H. Keppler. Real-time observation of crystallization and textural evolution in magmatic systems with the moissanite cell. American Geophysical Union, Fall Meeting 2009, Dec 2009, San Francisco, United States. ⟨hal-02404796⟩



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