A micro-scale investigation of melt production and extraction in the upper mantle based on silicate melt pockets in ultramafic xenoliths from the Bakony–Balaton Highland Volcanic Field (Western Hungary)

Abstract

Mantle xenoliths in Neogene alkali basalts of the Bakony–Balaton Highland Volcanic Field (Western Hungary) frequently have melt pockets that contain silicate minerals, glass, and often carbonate globules. Textural, geochemical and thermobarometric data indicate that the melt pockets formed at relatively high pressure through breakdown of mainly amphibole as a result of temperature increases accompanied, in most cases, by the influx of external metasomatic agents. New elemental and Sr–Nd–Pb isotope data show that in several xenoliths the external agent was either a LIL-enriched aqueous fluid or a CO2-rich fluid, whereas in other xenoliths the melt pockets were additionally enriched in LREE and sometimes HFSE, suggesting metasomatism by a silicate melt. The compositional character of the external agents might have been inherited by melting of a hydrated and probably carbonated deeper lithospheric component, which itself was metasomatized by melts with significant slab-derived components.