Contribution
Beryllium speciation in evolved granitic magmas: phosphates versus silicates
Charoy, Bernard
European Journal of Mineralogy Volume 11 Number 1 (1999), p. 135 - 148
66 références bibliographiques
publié: Feb 11, 1999
manuscrit accepté: Sep 9, 1998
manuscrit reçu: May 26, 1997
Abstract
Abstract High to extremely high Be contents > 200 ppm (the Clarke for 2 mica granites is between 5 and 10 ppm) are frequent in strongly fractionated granites or equivalents, but the mineral carrier, with the exception of beryl, is not always well identified. This study compares two small highly evolved, Be-rich granitic bodies and focuses on the contrasting speciation of Be in primary magmatic accessory phases: phosphate like beryllonite in the Beauvoir granite (France) or silicate like beryl in the Argemela microgranite (Portugal). Beryllium speciation is mainly controlled by the relative competition between Si and P as network-formers, with Al or alkalies as network-modifying species, during the crystallization course of these fluorine-rich, peraluminous and perphosphorous granites. The level of Si activity in these residual liquids monitors either an Al-dominant or an alkali (Na)-dominant speciation of P. Subsolidus alteration of the primary Be mineral at Beauvoir is complex: several Ca-Be-phosphates, up to a F-rich apatite, with a severe Be-leaching, whereas, at Argemela, beryl is devoid of any extensive alteration.
Mots-clefs
Be • phosphates • silicates • granites • differentiation