He, Ne and Ar 'snapshot' of the subcontinental lithospheric mantle from CO2 well gases
He, Ne and Ar 'snapshot' of the subcontinental lithospheric mantle from CO2 well gases
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DOI：
10.1016/j.chemgeo.2017.09.028
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发表时间：
2018-03-05
期刊：
影响因子：
3.9
通讯作者：
Ballentine, Chris J.
中科院分区：
文献类型：
--
作者：
Gilfillan, Stuart M. V.;Ballentine, Chris J.
次大陆岩石圈地幔(SCLM)构成了上地幔向大陆提供岩浆挥发分的重要组成部分，但其地球化学特征和演化仍缺乏约束。本文对美国西南部两个岩浆CO2气田(布拉沃穹顶和羊山)的惰性气体数据进行了新的解释，从而对该地区新生代火山活动来源的SCLM的挥发性特征提供了独特的见解。我们认为，羊山CO2气田中He-3/He-4(地幔)比值的降低可以归因于SCLM中的放射性生产。利用减少的卡方极小值法，结合放射成因抬升的Ne-21/Ne-22(地幔)端元，对羊山地区样品中He-4/Ne-21(地壳)比值的变化进行了计算，得到了2.59+/-0.15到3.00+/-0.18R-a的He-3/He-4(地幔)比值。这些值对应于Ne-21/Ne-22(地幔)值0.136。利用这些He-3/He-4(地幔)端元值和Ne-21(地幔)的Ne-21(地幔)，我们得到元素He-3/Ne-22(地幔)为2.80+/-0.16，放射性成因的He-4/Ne-21*(地幔)范围为1.11+/-0.11到1.30+/-0.14。对Ne-21/Ar-40(地壳)比值的变化进行了第二次简化的卡方最小化，使我们能够同时确定场内He-4/Ar-40(地幔)0.78至1.21的范围和Ne-21/Ar-40(地幔)7.66+/-1.62至7.70+/-1.54的范围。将这些比值与已知的He-4/Ne-21和He-4/Ar-40的地幔生产范围相结合，可以分辨出与放射性成因较低的He-3/He-4(地幔)比值相对应的放射性成因的He/Ne和He/Ar比值。将这些值与Bravo Dome场解析的值进行比较，可以在两个数据集中明确和一致地识别He-Ne和He-Ar的耗竭。这种损耗只能通过软流圈熔体的小部分熔体的部分脱气来解释，这些熔体已经进入了SCLM。这是第一次有可能同时分辨和解释SCLm源内的地幔He/Ne和He/Ar比值。这些数据还排除了两个气田的地幔来源中的烟柱成分，从而排除了烟柱对科罗拉多高原隆升事件的影响
The subcontinental lithospheric mantle (SCLM) constitutes a significant portion of the upper mantle sourcing magmatic volatiles to the continents above, yet its geochemical signature and evolution remain poorly constrained. Here we present new interpretation of noble gas datasets from two magmatic CO2 fields in the SW US, namely Bravo Dome and Sheep Mountain, which provide a unique insight into the volatile character of the SCLM sourcing the Cenozoic volcanism in the region. We identify that reduction of He-3/He-4(mantle) ratio within the Sheep Mountain CO2 field can be attributed to radiogenic production within the SCLM. Using a Reduced Chi-Squared minimisation on the variation of derived He-4/Ne-21(crust) ratios within samples from the Sheep Mountain field, combined with a radiogenically raised Ne-21/Ne-22(mantle) end member, we resolve He-3/He-4(mantle) ratios of 2.59 +/- 0.15 to 3.00 +/- 0.18 R-a. These values correspond with a Ne-21/Ne-22(mantle) value of 0.136. Using these He-3/He-4(mantle) end member values with Ne-21(mantle) resolved from Ne three component analysis, we derive the elemental He-3/Ne-22(mantle) of 2.80 +/- 0.16 and radiogenic He-4/Ne-21*(mantle) range of 1.11 +/- 0.11 to 1.30 +/- 0.14. A second Reduced Chi-Squared minimisation performed on the variation of Ne-21/Ar-40(crust) ratios has allowed us to also determine both the He-4/Ar-40(mantle) range of 0.78 to 1.21 and Ne-21/Ar-40(mantle) of 7.66 +/- 1.62 to 7.70 +/- 1.54 within the field. Combining these ratios with the known mantle production ranges for He-4/Ne-21 and He-4/Ar-40 allows resolution of the radiogenic He/Ne and He/Ar ratios corresponding to the radiogenically lowered He-3/He-4(mantle) ratios. Comparing these values with those resolved from the Bravo Dome field allows identification of a clear and coherent depletion of He to Ne and He to Ar in both datasets. This depletion can only be explained by partial degassing of small melt fractions of asthenospheric melts that have been emplaced into the SCLM. This is the first time that it has been possible to resolve and account for both the mantle He/Ne and He/Ar ratios within a SCLM source. The data additionally rule out the involvement of a plume component in the mantle source of the two gas fields and hence any plume influence on the Colorado Plateau Uplift event