Origin of CO2 and carbonate veins in mantle-derived xenoliths in the Pannonian Basin

The origin and evolution of CO2 inclusions and calcite veins in peridotite xenoliths of the Pannonian Basin, Hungary, were investigated by means of petrographic investigation and stable isotope analyses. The fluid inclusions recovered in paragenetic olivine and clinopyroxene belong to distinct populations: type A (texturally early) inclusions with regular shapes (often with negative crystal forms) forming intragranular trails; type B (texturally late) inclusions defining randomly oriented trails that reach grain boundaries. Type B inclusions are often associated with silicate melt (type C) inclusions. Stable carbon isotope compositions in inclusion-hosted CO2 were obtained by vacuum crushing followed by conventional dual inlet as well as continuous flow mass spectrometry in order to eliminate possible lab artifacts. Olivines, clino- and orthopyroxenes of the host peridotite have oxygen isotope compositions from 5.3 to 6.0‰ (relative to V-SMOW), without any relationship with xenolith texture. Some of the xenoliths contained calcite in various forms: veins and infillings in silicate globules in veins, secondary carbonate veins filling cracks and metasomatic veins with diffuse margins. The former two carbonate types have δ13C values around –13‰ (relative to V-PDB) and low Sr contents (< 0.5 wt.%), whereas the third type,veins with high-temperature metasomatic features have a δ13C value of –5.0‰ and high Sr contents up to 3.4 wt.%. In spite of the mantle-like δ13C value and the unusually high Sr content typical for mantle-derived carbonate, trace element compositions have proven a crustal origin. This observation supports the conclusions of earlier studies that the carbonate melt droplets found on peridotite xenoliths in the alkaline basalts represent mobilized sedimentary carbonate. The large δ13C range and the 12C-enrichment in the carbonates can be attributed to devolatilization of the migrating carbonate or infiltration of surficial fluids containing 12C-rich dissolved carbon.Carbon isotope compositions of inclusion-hosted CO2 range from –17.8 to –4.8‰ (relative to V-PDB) with no relation to the amount of CO2 released by vacuum crushing. Low-δ13C values measured by stepwise heating under vacuum suggest that the carbon component is pristine and not related to surficial contamination, and that primary mantle fluids with δ13C values around –5‰ were at least partly preserved in the xenoliths. Tectonic reworking and heating by the basaltic magma resulted in partial CO2 release and local 13C-depletion.