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Xenocrystic richterite in an olivine-nephelinite: destabilisation and diffusion phenomena
Published online by Cambridge University Press: 05 July 2018
Abstract
A partly destabilised Na-richterite has been found in an olivine-nephelinite from Morocco. The riehterite crystal (600 × 420 μm) is surrounded by a reaction zone (400-700 μm) of K- and Si-rich glass containing small (<50 μm) olivine (Fo80-83%) and endiopside crystals. Outwards, another zone is formed of normal magmatic minerals and circumscribes the original crystal, indicating that the destabilisation event took place at the end of the crystallisation sequence. Estimated ascent time of about 100 hours would have completely decomposed an isolated richterite crystal, which suggests that the amphibole was originally included in a xenolith. A mass-balance calculation shows that the fichterite isovolumic decomposition was accompanied by exchanges with the magma. The loss of Na from the reaction zone and the gain of AI from the magma allowed the precipitation of an analcime-rich zone observed around the destabilised amphibole and the concentration of K in the reaction zone glass. Compositional variations, Fe and Ti increase and Mg, Ca and F decrease at the richterite edge are interpreted as the result of a diffusion process. No alkali gradients are observed. The diffusion phenomenon lasted less than 100 hours and ceased to be operative at a temperature of 900-950°C i.e. just below the solidus temperature. Diffusion coefficients for the amphibole are proposed: e.g 10−9 cm2 s−1 for K2O and 10−10 for FeO at 900°C
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- Mineralogy
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- Copyright © The Mineralogical Society of Great Britain and Ireland 1993
References
Brearley, M. and Scaffe, C. M. (1986) Dissolution rate of upper mantle minerals in an alkati basalt melt at high pressure: An experimental study and implications for ultramafic xenolith survival, J. Petrol.,
27, 1157-42.Google Scholar
Carslaw, H. S. and Jaeger, J. C. (1959) Conduction of heat in solids.
Oxford University Press, p. 510.Google Scholar
Charles, R. W. (1975) The phase equilibria of richterite and ferrorichterite. Am. Mineral.,
60, 367–74.Google Scholar
Charles, R. W. (1977) The phase equilibria of intermediate compositions on the pseudobinary NazCaMg5- SisO22(OH)2-Na2CaFesSisO22(OH)z. Am. J. Sci.,
277, 594–625.Google Scholar
Charlot, R., Choubert, G., Faure-Muret, A., and Hamel, C. (1964) Age des aïounites du Maroc NordOriental. C,R. Sommaire des Sdances de la Soc, Geol. France, 401-2.Google Scholar
Cooper, A. R. (1968) The use and limitations of the concept of an Effective Binary Diffusion Coefficient for multicomponent diffusion. In Mass transport in oxides (Watchman, J. B. and Franklin, A. D., eds,), NBS Spec, Publ., 196, 79-84.Google Scholar
Cotonian, C., Potdevin, J.-L., Bertrand, H., and Lombardo, N. (1988) Pseudomorphoses coronitiques d'amphibole dans le trachyte de Monac (Velay Oriental). Bilan de matirre et rraction magmatique. Bull Mindral.,
11, 89–95.Google Scholar
Dawson, J. B. (1987) The MARID suite of xenoliths in kimberlite: relationship to veined and metasomatised peridotite xenoliths. In Mantle xenoliths (Nixon, P,H.,ed.), 465-73.Google Scholar
Dawson, J. B. and Smith, J. V. (1982) Upper-mantle amphiboles: a review. Mineral. Mag.,
45, 35–46.Google Scholar
Dodson, M. H. (1973) Closure temperature in cooling geochronological and petrological systems. Contrib. Mineral. Petrol.,
40, 259–74.Google Scholar
Donaldson, C. H. (1985) The rates of dissolution of olivine, plagioclase and quartz in a basalt melt. Mineral. Mag.,
49, 683–93.Google Scholar
Farver, J. R. and Giletti, B. J. (t985) Oxygen diffusion in amphiboles. Geochim. Cosmochim. Acta,
49, 1403-11.Google Scholar
Forbes, W. C. (1971) Synthesis and stability relations of richterite Na2CaMgsSisO2z(OH)2. Am. Mineral.,
56, 997–1004.Google Scholar
Freer, R. (1981) Diffusion in silicate minerals and glasses: a data digest and guide to the literature. Contrib. Mineral. Petrol., 76, 440–56.Google Scholar
Gilbert, M. C. and Briggs, D. F. (1974) Comparison of the stabilities of OHand F-potassic richterite, a preliminary report (abstr.), Eos,
55, 480–1.Google Scholar
Gresens, R. L. (1967) Composition-volume relationships of metasomatism. Chemical. Geol,
2, 47–65.Google Scholar
Huebner, J. S. and Papike, J. J. (1970) Synthesis and sodium-potassium exchange in the richterite series (K,Na)NaCaMgsSi8Ozz(OH)2. Am. Mineral.,
55, 300.Google Scholar
Jambon, A. (t983) Diffusion darts tes silicates fondus: un bilan des connaissances actuelles. Bull; Mindral.,
106, 229–46.Google Scholar
Kushiro, I., Yoder, H. S. Jr., and Mysen, B. O. (1976) Viscosities of basalt and andesite melts at high pressures. J. Geophys. Res., 81, 6351–6.Google Scholar
Mokhtari, A. and Velde, D. (1988) Xenocrysts in Eocene camptonites from Taourirt. Mineral Mag.,
52, 587–601.Google Scholar
Potdevin, J.-L. and Marquer, D. (1987) Méthodes de quantification des transferts de matière par les fluides dans les roches métamorphiques déformées. Geodi-namica Acta,
1, 3, 193–206.Google Scholar
Rutherford, M. J. (1990) Experimental study of dehydration and crystallization produced by decompression of dacites; implications for magma ascent rates. Program and Abstracts, V.M. Golschmidt Conference, May 2-4, Baltimore, Maryland.Google Scholar
Rutherford, M. J. and Hill, P. M. (1993) Magma ascent rates from amphibole breakdown: An experimental study applied to the 1980-1986 Mount St. Helens eruptions. J. Geophys. Res. (in press).Google Scholar
Shaw, H. R., (1974) Diffusion of H2O in granitic liquids: Part II. Mass transfer in magma chambers. In Geochemical transport and kinetics (Hofmann, A. W., Giletti, B. J., Yodel H. S. Jr., and Yund, R. A., eds.), Carnegie InsL Washington, Publ. 634, 155-6.Google Scholar
Tilley, C. E. and Thompson, R. N. (1972) Melting relations of some ultra alkali volanics. J. Geol., 8, 65–70.Google Scholar