The fate of mantle-derived carbon in a continental sedimentary basin: Integration of C/He relationships and stable isotope signatures
The isotopic composition and abundances of the rare gases (He, Ne, and Ar) and active gases (CO 2, CH 4) 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% CO 2, to CH 4-dominated reservoirs with trace concentrations (ppm) of CO 2, 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 CO 2), all gas fields exhibit a trend whereby the most CO 2-rich samples approach CO2/3He mntl values similar to MORB (2 × 109 to 7 × 10 9), while CO 2-depleted samples extend to CO2/3He mntl values as low as 10 5. The lack of any correlation of CO2/3He mntl 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 CO 2 carrier phase during volatile transport and emplacement in the continental crust. Based on stable isotopic signatures from the Kismarja field, a crustal CO 2 endmember with an isotopic composition of -6.8‰ and a mantle CO 2 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 CO 2 can be reliably distinguished in a continental setting. Covariation in δ 13C CO2 and δ 13C CH4 values in the gas fields is used to place constraints on two alternative models whereby the trends in percent CO 2 and CO2/3He mntl ratios can be accounted for by (1) loss of the mantle-derived CO 2 carrier phase; (2) addition of crustal-derived CO 2; (3) addition of crustal-derived (thermogenic) CH 4; and potentially (4) conversion of the CO 2mntl carrier phase to mantle-derived CH 4. Based on an estimated total mantle 4He flux for the Pannonian Basin of 4.2 × 10 8 atoms m -2 s -1, mantle carbon flux estimates for this basin alone range from 3 × 10 8 g C/yr to 1 × 10 9 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.