Ballentine C J, Schoell M, Coleman D, Cain B A
IGMR, Dept Erdwissenschaften, ETH Zentrum, Zürich, Switzerland.
Nature. 2001 Jan 18;409(6818):327-31. doi: 10.1038/35053046.
Except in regions of recent crustal extension, the dominant origin of carbon dioxide in fluids in sedimentary basins has been assumed to be from crustal organic matter or mineral reactions. Here we show, by contrast, that Rayleigh fractionation caused by partial degassing of a magma body can explain the CO2/3He ratios and delta13C(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 systems. 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 ago. Uplift to the north of the Val Verde basin between 310 and 280 Myr ago 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 approximately 300 Myr is far longer than the residence time predicted by diffusion models.
除了在近期地壳伸展的区域外，沉积盆地流体中二氧化碳的主要来源一直被认为是地壳有机质或矿物反应。相比之下，我们在此表明，岩浆体部分脱气引起的瑞利分馏可以解释在西德克萨斯瓦尔韦德盆地富含二氧化碳的天然气中观测到的CO₂/³He比值和δ¹³C(CO₂)值，以及在其他盆地系统中观测到的地幔³He/²²Ne比值。如果二氧化碳的输入早于盆地中甲烷的生成（约在2.8亿年前发生），那么CO₂/³He和CO₂/CH₄比值的区域变化就可以得到解释。在3.1亿年至2.8亿年前，瓦尔韦德盆地北部的隆升似乎是唯一在时间和位置上合适的构造事件，有可能是岩浆二氧化碳的来源。我们在前陆盆地中识别出岩浆二氧化碳，这表明其他中大陆盆地系统中二氧化碳的来源应该重新评估。此外，推断出的天然气在圈闭构造中约3亿年的封闭系统保存时间，远远长于扩散模型预测的停留时间。
