The role of CO2-rich fluids in trace element transport and metasomatism in the lithospheric mantle beneath the Central Pannonian Basin, Hungary, based on fluid inclusions in mantle xenoliths

Upper mantle peridotite xenoliths from the Tihany Maar Volcanic Complex, Bakony–Balaton Highland Volcanic Field (Central Pannonian Basin, Hungary) contain abundant pyroxene-hosted negative crystal shaped CO2-rich fluid inclusions. The good correlation between enrichment of the clinopyroxenes in Al2O3, TiO2, Na2O, MREE and Zr, and the presence of fluid inclusions in the xenoliths provide strong evidence for fluid-related cryptic metasomatism of the studied xenoliths. The FIB-SEM (focused ion beam-scanning electron microscopy) exposure technique revealed a thin glass film, covering the wall of the fluid inclusions, which provides direct evidence that the silicate components were formerly dissolved in the CO2-rich fluid phase. This means that at upper mantle conditions CO2-rich fluids are capable of transporting trace and major elements, and are the agents responsible for cryptic metasomatism of the peridotite wall rock. Several daughter phases, including magnesite, quartz and sulfide, were identified in the fluid inclusions. Magnesite and quartz are the products of a post entrapment carbonation reaction, whereby the reactants are the CO2-rich fluid and the host orthopyroxene. It is likely that the thin glass film prevented or arrested further growth of the magnesite and quartz by isolating the fluid from the host orthopyroxene, resulting in the preservation of residual CO2 in the fluid inclusions. ► We studied CO2-rich fluid inclusions and their host mantle xenoliths. ► We found evidence of fluid-related cryptic metasomatism in the studied rocks. ► A thin glass film identified on inclusion walls was formerly dissolved in the fluid. ► Glass may have armored the Opx and prevented post-entrapment carbonation reactions. ► Trace elements in mantle fluids are controlled by the dissolved silicate component.