1. IGMR, Dept Erdwissenschaften, ETH Zentrum NO CO 61.7, 8092 Zürich, Switzerland
2. Chevron Research and Technology Company , 6001 Bollinger Canyon, San Ramon, California 94583, USA
3. Isotech Laboratories Inc., 1308 Parkland Court, Champaign, Illinois 61821-1826, USA
4. Altura Energy LLP, Houston , Texas 77210-4294, USA

Correspondence to: Chris J. Ballentine¹ Correspondence and requests for materials should be addressed to C.J.B.
(e-mail: Email: ballentine@erdw.ethz.ch).

Abstract

Except in regions of recent crustal extension¹, the dominant origin of carbon dioxide in fluids in sedimentary basins has been assumed to be from crustal organic matter² or mineral reactions^{3, 4}. Here we show, by contrast, that Rayleigh fractionation caused by partial degassing of a magma body can explain the CO₂/ ³He ratios and ¹³C(CO₂) values observed in CO₂-rich natural gases in the west Texas Val Verde basin and also the mantle ³He/²²Ne ratios observed in other basin systems⁵. Regional changes in CO₂/ ³He and CO₂/CH₄ ratios can be explained if the CO₂ input pre-dates methane generation in the basin, which occurred about 280 Myr ago⁶. Uplift to the north of the Val Verde basin between 310 and 280 Myr ago⁶ appears to be the only tectonic event with appropriate timing and location to be the source of the magmatic CO₂. Our identification of magmatic CO₂ in a foreland basin indicates that the origin of CO₂ in other mid-continent basin systems should be re-evaluated. Also, the inferred closed-system preservation of natural gas in a trapping structure for 300 Myr is far longer than the residence time predicted by diffusion models^{7, 8}.