The isotopic composition and abundances of the rare gases (He, Ne, and Ar) and active gases (CO2, CH4) have been determined in a series of commercial gas reservoirs in the Pannonian and Vienna basins of Hungary and Austria, respectively. In these zones of continental extension, significant components of mantle-derived 4He (up to 39.8%) and 21Ne (up to 58%) are identified. The results of this study indicate a major component of mantle-derived carbon in these systems as well. Ranging in composition from close to 100% CO2, to CH4-dominated reservoirs with trace concentrations (ppm) of CO2, these gas reservoirs provide a unique opportunity to examine the relationship of the conservative rare gases to the active gas components and to examine the sources and sinks for mantle- and crustal-derived carbon phases. With the exception of Kismarja gas field (which shows evidence of addition of 3He-depleted crustal CO2), all gas fields exhibit a trend whereby the most CO2-rich samples approach CO2/3Hemntl values similar to MORB (2 × 109 to 7 × 109), while CO2-depleted samples extend to CO2/3Hemntl values as low as 105. The lack of any correlation of CO2/3Hemntl ratios with R/Ra values indicates the observed trends are not a function of mixing between crustal- and mantle-derived endmembers, but instead reflect progressive loss of the mantle-derived CO2 carrier phase during volatile transport and emplacement in the continental crust. Based on stable isotopic signatures from the Kismarja field, a crustal CO2 endmember with an isotopic composition of -6.8‰ and a mantle CO2 endmember with an isotopic composition no more depleted in 13C than -5.0‰ can be identified, one of the few instances where the isotopic signatures of mantle-and crustal-derived CO2 can be reliably distinguished in a continental setting. Covariation in δ 13CCO2, and δ 13CCH4 values in the gas fields is used to place constraints on two alternative models whereby the trends in percent CO2 and CO2/3Hemntl ratios can be accounted for by (1) loss of the mantle-derived CO2 carrier phase; (2) addition of crustal-derived CO2; (3) addition of crustal-derived (thermogenic) CH4; and potentially (4) conversion of the CO2mntl carrier phase to mantle-derived CH4. Based on an estimated total mantle 4He flux for the Pannonian Basin of 4.2 × 108 atoms m-2 s-1, mantle carbon flux estimates for this basin alone range from 3 × 108 g C/yr to 1 × 109 g C/yr, which over the lifetime of the basin is only 4-5 orders of magnitude less than flux estimates based on the total area of the spreading ridges. Clearly the addition of mantle-derived carbon to the crust in areas of continental extension is significant and may have been previously underestimated. We demonstrate here that integration of δ 13C data with rare gas isotopic tracers provides an important tool in constraining models of mantle carbon sources and sinks in continental settings. Copyright © 1997 Elsevier Science Ltd.
