Biotite resorption in dacite lavas from northeastern Algeria
Fougnot, Jean; Pichavant, Michel; Barbey, Pierre
European Journal of Mineralogy Volume 8 Number 3 (1996), p. 625 - 638
published: Jun 17, 1996
manuscript accepted: Feb 5, 1996
manuscript received: Apr 24, 1995
ArtNo. ESP147050803013, Price: 29.00 €
Abstract Tertiarty dacite lavas from northeastern Algeria contain remarkable quenched microtextural relations involving biotite, orthopyroxene, ilmenite and sometimes K-feldspar, in a glassy matrix. Textural evidence suggests reaction relationships between biotite and glass. Reactants, reaction products and phenocrysts show no compositional difference. Phases coexisting in the biotite reaction rims yield temperature estimates in the 800-900 °C range. The magmatic phenocryst assemblage equilibrated at a maximum pressure of 4 kbar. Values of log/O2 calculated from the Fe2+/Fe3+ ratio of glass are above the NNO buffer (NNO + 1 to + 1.25) and are interpreted to reflect a late post-emplacement oxidation. Textural and chemical data, including mass-balance calculations, suggest that the reactions are a natural equivalent of the [Qz] reaction in the KMASH system. Application of experimental studies for natural Fe-Mg-Ti-bearing biotite compositions allows the reaction to be located around 850 °C, and at a maximum pressure of 1 kbar. Biotite resorption was caused by rapid decompression during magma ascent. Grain sizes of orthopyroxene in experimentally produced amphibole breakdown reactions are used to constrain durations of the biotite resorption reaction, between 30 and 90 days. The occurrences described constitute a reference example for biotite resorption in silicic to intermediate magmas.