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 parts per thousand
(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 delta C-13 values around -13 parts per thousand
(relative to V-PDB) and low Sr contents (<05 wt %), whereas the third
type,veins with high-temperature metasomatic features have a delta C-13
value of -5 0 parts per thousand and high Sr contents up to 34 wt.% In
spite of the mantle-like delta C-13 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 delta C-13 range and the C-12-enrichment in the
carbonates can be attributed to devolanlization of the migrating
carbonate or infiltration of surficial fluids containing C-12-rich
dissolved carbon
Carbon isotope compositions of inclusion-hosted CO2 range from -17 8 to
-4.8 parts per thousand (relative to V-PDB) with no relation to the
amount of CO2 released by vacuum crushing. Low-delta C-13 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 delta C-13 values around -5 parts per
thousand were at least partly preserved in the xenoliths Tectonic
reworking and heating by the basaltic magma resulted in partial CO2
release and local C-13-depletion. (C) 2010 Elsevier B V All rights
reserved