Computer Science
Scientific paper
Jun 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.2295s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 61, Issue 11, pp.2295-2307
Computer Science
6
Scientific paper
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 4 He (up to 39.8%) and 21 Ne (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 3 He-depleted crustal CO 2 ), all gas fields exhibit a trend whereby the most CO 2 -rich samples approach CO 2 / 3 He mntl values similar to MORB (2 × 10 9 to 7 × 10 9 ), while CO 2 -depleted samples extend to CO 2 / 3 He mntl values as low as 10 5 . The lack of any correlation of CO 2 / 3 He 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 13 C 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 13 C CO 2 and 13 C CH 4 values in the gas fields is used to place constraints on two alternative models whereby the trends in percent CO 2 and CO 2 / 3 He 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 4 He 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 13 C data with rare gas isotopic tracers provides an important tool in constraining models of mantle carbon sources and sinks in continental settings.
Ballentine Chris J.
Keith O'Nions R.
Sherwood Lollar Barbara
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