THE INCONGRUENT MELTING AND CRYSTALLIZATION OF AMPHIBOLE: A SECOND PERITECTIC REACTION IN BOWEN’S REACTION SERIES AND ITS PETROLOGIC IMPLICATIONS (Invited Presentation)

The reaction of pyroxene plus melt to form amphibole was recognized by Bowen in his earliest depictions of the reaction series. This reaction was included primarily on the basis of observations of amphibole reaction rims about pyroxene. However, the nature of the amphibole-forming reaction was not known to Bowen. Experimental and geochemical work completed since the late 1980’s has now identified this reaction, known as dehydration melting, as a peritectic reaction whereby amphibole melts incongruently to form an anhydrous phase assemblage, mostly pyroxene, coexisting with a hydrous, but water-undersaturated melt. It is analogous to the incongruent melting of Mg-Fe pyroxene that was first documented by Bowen himself. Bowen’s reaction series is integral to most introductory geology (and virtually all introductory petrology) curricula. This latterday documentation of a second peritectic reaction in the series is a powerful proof-of-concept that further validates the importance and relevance of the reaction series as a pedagogic tool. The peritectic behavior of amphibole has been shown to have important implications for the genesis, crystallization, hybridization, and fractionation behavior of magmas. These include the genesis of silicic magmas, the behavior of water during crystallization and the chemistry of fractionating systems. Thus, the exploration of the petrological consequences of the incongruent melting behavior of amphibole also has a place for discussion in graduate level petrology classes.