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首页> 外文期刊>Chemical geology >A helium, argon, and nitrogen record of the upper continental crust (KTB drill holes, Oberpfalz, Germany): implications for crustal degassing
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A helium, argon, and nitrogen record of the upper continental crust (KTB drill holes, Oberpfalz, Germany): implications for crustal degassing

机译：上大陆壳的氦，氩和氮记录（KTB钻孔，德国奥伯法茨）：对地壳脱气的影响

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An extensive set of bulk rock samples and mineral separates from the German Deep Drilling Project (KTB) has been analyzed for nitrogen, helium, and argon concentrations. The samples comprise metabasites, paragneisses, and hornblende-gneisses from a 7-km section in the crystalline basement metamorphosed during the Variscan orogeny. The data indicate a dependence of the N and Ar concentrations on the mica content of the rocks. The He-N_2-Ar composition of fluid inclusions in quartz from felsic veins suggests that the fluids are old basement brines with large proportions of radiogenic He and Ar. There is no systematic down-hole variation in N and Ar concentrations, but He concentrations appear to increase with depth. Calculated degassing rates suggest near complete degassing of He from the gneisses, but some He retention in the metabasites. Nonetheless, overall degassing fluxes of He (1.2-4.0 * 10 ~(10) atoms m~(-2) s~(-1)) are not in conflict with estimates of the He flux in continental crust based on the steady-state degassing model. In contrast, our estimates indicate that Ar fluxes are significantly lower than predicted by a steady-state degassing model which is thus probably inappropriate to describe Ar fluxes correctly. The down-hole trend of increasing He concentration is interpreted as a degassing profile. A model of simultaneous radiogenic ingrowth and diffusive loss of He has been applied to determine the bulk transport coefficient of He in crystalline basement, which was found to be between 10~(-10) and 10~(-7) m~2 s~(-1). This is much higher than reasonable diffusion rates, suggesting that episodes of enhanced degassing, probably related to hydrothermal or tectonic activities, played an important role in the degassing history of the crust. The relationship between the transport of heat and He in the crust in the KTB area is examined. Under the assumption that the ratio of heat flow and He flux from the mantle is the same as in ocean basins, the He isotopic composition of the basement fluids in the KTB boreholes is consistent with the inferred reduced heat flow at the KTB site. On a global scale, however, heat flow and flux of mantle He are probably not correlated.
机译：已对德国深度钻探项目（KTB）的大量散装岩石样品和矿物分离物进行了氮，氦和氩浓度分析。样品包括在瓦里斯卡造山运动过程中变质的结晶基底中7公里处的变质岩，石笋和片状片麻岩。数据表明，N和Ar浓度对岩石中云母含量的依赖性。细长脉石英中流体包裹体的He-N_2-Ar组成表明，这些流体是具有大量放射性He和Ar的旧地下卤水。 N和Ar浓度没有系统的井下变化，但He浓度似乎随深度增加。计算出的脱气速率表明，片麻岩中的He几乎已完全脱气，但某些He保留在代谢物中。尽管如此，He（1.2-4.0 * 10〜（10）原子m〜（-2）s〜（-1）的总脱气通量与基于稳态的大陆壳中He通量的估算值没有冲突除气模型。相反，我们的估计表明，氩气通量明显低于稳态脱气模型的预测值，因此可能不适合正确地描述氩气通量。 He浓度增加的井下趋势被解释为脱气曲线。利用He的放射源向内生长和扩散损失的模型来确定He在晶体基底中的体积输运系数，该系数在10〜（-10）m〜2 s〜2 s〜2 s〜之间。 （-1）。这远高于合理的扩散速率，这表明可能与热液活动或构造活动有关的脱气增强事件在地壳脱气历史中发挥了重要作用。研究了KTB地区地壳中热与He的传递之间的关系。假设来自地幔的热流和He通量之比与海盆中的流量相同，则KTB钻孔中地下流体的He同位素组成与KTB现场推断的热流减少一致。然而，在全球范围内，地幔He的热流和通量可能不相关。

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