Melt inclusion record of immiscibility between silicate, hydrosaline, and carbonate melts: Applications to skarn genesis at Mount Vesuvius

摘要

Foid-bearing syenites and endoskarn xenoliths of the A.D. 472 Vesuvius eruption represent the magma chamber-carbonate wall-rock interface. Melt inclusions hosted in crystals from these rocks offer a rare opportunity to depict the formation and the composition of metasomatic skarn-forming fluids at the peripheral part of a growing K-alkaline magma chamber disrupted by an explosive eruption. Four principal types of melt inclusions represent highly differentiated phonolite (type 1), hydrosaline melt (type 3), unmixed silicate-salt melts (type 2), and a complex chloride-carbonate melt with minor sulfates (type 4). The high-temperature (700-800 ℃) magmatic-derived hydrosaline melt is considered to be the main metasomatic agent for the skarn-forming reactions. The interaction between this melt (fluid) and carbonate wall rocks produces a Na-K-Ca carbonate-chloride melt that shows immiscibility between carbonate and chloride constituents at ～700℃ in 1 atm experiments. This unmixing can be viewed as a possible mechanism for the origin of carbonatites associated with intrusion-related skarn systems.