XENOCRYSTIC RICHTERITE IN AN OLIVINE NEPHELINITE - DESTABILIZATION AND DIFFUSION PHENOMENA

A partly destabilised Na-richterite has been found in an olivine-nephe linite from Morocco. The richterite crystal (600 x 420 mum) is surroun ded by a reaction zone (400-700 mum) of K- and Si-rich glass containin g small (< 50 mum) olivine (Fo80-83%) and endiopside crystals. Outward s, another zone is formed of normal magmatic minerals and circumscribe s the original crystal, indicating that the destabilisation event took place at the end of the crystallisation sequence. Estimated ascent ti me of about 100 hours would have completely decomposed an isolated ric hterite crystal, which suggests that the amphibole was originally incl uded in a xenolith. A mass-balance calculation shows that the richteri te isovolumic decomposition was accompanied by exchanges with the magm a. The loss of Na from the reaction zone and the gain of Al from the m agma allowed the precipitation of an analcime-rich zone observed aroun d the destabilised amphibole and the concentration of K in the reactio n 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-degrees-C, i.e. just below the solidus temperat ure. Diffusion coefficients for the amphibole are proposed: e.g 10(-9) cm2 s-1 for K2O and 16(-10) for FeO at 900-degrees-C.