MODELLING THE METALUMINOUS TRACHYTE TO PERALKALINE RHYOLITE TRANSITION USING THE RHYOLITE-MELTS ALGORITHM

Strongly peralkaline (mol Na+K>Al) rhyolite (viz., pantellerite) and metaluminous (mol Na+K<Al) trachyte comprise the dominant volume of rocks that crop out on the island of Pantelleria. The results of major and trace element modelling on natural samples as well as recent results from experimental studies (Romano et al. 2018; J Pet 59, 559-588) have provided evidence that pantellerites form as a result of fractional crystallization of an assemblage dominated (>90%) by alkali feldspar from metaluminous trachyte (via comenditic trachyte) along a path in Q-Ab-Or space termed the “thermal valley” (Carmichael and MacKenzie, 1963; AJS 261, 382-396). Rhyolite-MELTS (Gualda et al. 2012; J Pet 53, 875-890) is a recent calibration of the MELTS algorithm optimized for silicic systems that terminally crystallize at the feldspar-quartz cotectic. Although rhyolite-MELTS has been shown to effectively model metaluminous liquid lines of descent, it has not yet been well-tested for peralkaline systems (with the exception of Gleeson et al. 2017; JVGR 337, 44-61). In this study we evaluate the results of MELTS modelling of trachyte-pantellerite over a range of pressures (50-200 MPa), initial water concentrations (0 – 3 wt%), and oxygen fugacities (FMQ to FMQ-2).