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Xenocrystic richterite in an olivine-nephelinite: destabilisation and diffusion phenomena

Published online by Cambridge University Press:  05 July 2018

Christiane Wagner ,
Abdelkader Mokhtari  and
Danielle Velde
Christiane Wagner
Affiliation:
Laboratoire de Pétrologie Minéralogique, (URA 736 du CNRS) U.P.M.C.—Paris 6, 4 Place Jussieu, Tour 26, E3, 75252 Paris Cedex 05, France
Abdelkader Mokhtari
Affiliation:
Laboratoire de Pétrologie Minéralogique, (URA 736 du CNRS) U.P.M.C.—Paris 6, 4 Place Jussieu, Tour 26, E3, 75252 Paris Cedex 05, France Laboratoire de Géologie, Université de Meknès, Morocco
Danielle Velde
Affiliation:
Laboratoire de Pétrologie Minéralogique, (URA 736 du CNRS) U.P.M.C.—Paris 6, 4 Place Jussieu, Tour 26, E3, 75252 Paris Cedex 05, France
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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

Keywords

richteriteolivine-nephelinitedestabilisationdiffusionMorocco
Type
Mineralogy
Information
Mineralogical Magazine , Volume 57 , Issue 388 , September 1993 , pp. 515 - 525
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1993

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