Xenocrystic Richterite in an Olivine-Nephelinite: Destabilisation and Diffusion Phenomena

Christiane Wagner1, Abdelkader Mokhtari1, 2 and Danielle Velde1
1 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
2 Laboratoire de Géologie, Université de Meknès, Morocco

Abstract: A partly destabilised Na-richterite has been found in an olivine-nephelinite from Morocco. The richterite 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 richterite 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: richterite • olivine-nephelinite • destabilisation • diffusion • Morocco

Mineralogical Magazine; September 1993 v. 57; no. 388; p. 515-525; DOI: 10.1180/minmag.1993.057.388.12
© 1993, The Mineralogical Society
Mineralogical Society (www.minersoc.org)