Reconstructing mantle carbon and noble gas contents from degassed mid-ocean ridge basalts
Reconstructing mantle carbon and noble gas contents from degassed mid-ocean ridge basalts
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DOI：
10.1016/j.epsl.2018.05.024
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发表时间：
2018-08
影响因子：
5.3
通讯作者：
J. Tucker;S. Mukhopadhyay;H. Gonnermann
中科院分区：
文献类型：
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作者：
J. Tucker;S. Mukhopadhyay;H. Gonnermann
地幔中挥发性元素的通量长期以来一直被用来了解地幔结构和演化，并且是地球气候稳定性的关键控制因素。由于岩浆脱气的普遍性，从实测玄武岩中推断脱气前挥发分浓度需要应用脱气模型。这种模型，包括通常应用的平衡瑞利蒸馏，通常假设平衡或溶解度为基础的分配之间的熔体和蒸汽。在这里，我们证明了放射性同位素的比例的He，Ne，Ar，特别是，在全球洋中脊玄武岩（MORB）测量的氦是不一致的平衡脱气模型，即使不考虑他。我们的结论是动力学不平衡是一个至关重要的过程中影响挥发分丰度在脱气。我们提出了一个简单的非平衡Rayleigh蒸馏模型来重建脱气前MORB惰性气体和碳的浓度，该模型预测He和Ne在熔体和囊泡之间实现了接近平衡的分配，但扩散较慢的较重惰性气体受到非平衡的强烈影响，导致惰性气体元素比的非平衡分馏，我们将我们的模型应用于大量的MORB数据，发现脱气前3He，22Ne和36Ar的平均浓度分别为4.4 ± 0.9 × 10 − 10，6.6 ± 1.4 × 10 − 11，和6.8 ± 4.5 × 10 − 10ccSTP/g（2 σ），变化约为2个数量级，与其他高度不相容元素相似。脱气前的惰性气体浓度意味着大洋中脊3 He通量为800 ± 170 mol/yr，上地幔3 He/22 Ne和3 He/36 Ar比率为6.6 ± 2.0和0.64 ± 0.44，但样本之间的这些比率存在很大差异。应用我们的模型计算了地幔CO2/3He的平均摩尔比为1.67 ± 0.21 × 109，结合我们估算的~3He通量，得出上地幔CO2通量为5.9 ± 1.0 × 10~（13）g/yr，CO2浓度约为110ppm。因此表示时间积分通量基本上长于1000年。虽然我们的估计值处于之前估计值的高端，但并没有解决长期存在的热-氦悖论。此外，我们的要求不均匀的预脱气样品之间的3He/22 Ne和3He/36 Ar的比例是相反的不平衡脱气模型，主张统一的比例以前的应用程序的结论。此外，我们发现，CO2/Ba比率是高度可变的MORB样品，但仍然符合平均地幔质量比为1000。然而，估计前脱气CO2浓度和地幔CO2通量强烈依赖于约束较差的碳扩散率。因此，我们的示范流行的不平衡在洋中脊脱气，和潜在的不平衡在其他火山环境中，突出了需要更好地表征与火山脱气的物理参数。
The fluxes of volatile elements from the mantle have long been used to understand mantle structure and evolution, and are critical controls on Earth's climate stability. Because of the ubiquity of magmatic degassing, inferring pre-degassing volatile concentrations from measured basalts requires the application of a degassing model. Such models, including the commonly-applied equilibrium Rayleigh distillation, typically assume equilibrium or solubility-based partitioning between melt and vapor. Here, we demonstrate that ratios of radiogenic isotopes of He, Ne, Ar and, especially, Xe measured in global mid-ocean ridge basalts (MORBs) are inconsistent with equilibrium degassing models, even when not considering He. We conclude that kinetic disequilibrium is a crucial process affecting volatile abundances during degassing. We present a simple disequilibrium Rayleigh distillation model to reconstruct pre-degassing MORB noble gas and carbon concentrations, which predicts that He and Ne achieve nearly equilibrium partitioning between melt and vesicles, but the slower-diffusing heavier noble gases are strongly affected by disequilibrium, resulting in non-equilibrium fractionation of noble gas elemental ratios, and other volatile element ratios like CO2/He.We apply our model to a large set of MORB data, and find average pre-degassing3He,22Ne, and36Ar concentrations of 4.4 ± 0.9 × 10−10, 6.6 ± 1.4 × 10−11, and 6.8 ± 4.5 × 10−10ccSTP/g (2σ), with variations of approximately 2 orders of magnitude, similar to other highly incompatible elements. Pre-degassing noble gas concentrations imply a mid-ocean ridge3He flux of 800 ± 170 mol/yr and upper mantle3He/22Ne and3He/36Ar ratios of 6.6 ± 2.0 and 0.64 ± 0.44, but substantial variability in these ratios between samples. Applying our model to CO2, we calculate an average mantle CO2/3He molar ratio of 1.67 ± 0.21 × 109, which, when combined with our estimate of3He flux, implies an upper mantle CO2flux of 5.9 ± 1.0 × 1013g/yr and a CO2concentration of approximately 110 ppm.Our estimate of the mantle3He flux is the first determined independently of oceanographic3He measurements, and consequently represents a time-integrated flux substantially longer than ∼1000 years. And although at the high end of the range of previous estimates, our estimate does not resolve the long-standing heat-helium paradox. Additionally, our requirement for heterogeneous pre-degassing3He/22Ne and3He/36Ar ratios between samples is contrary to conclusions of previous applications of disequilibrium degassing models, which advocated uniform ratios. Furthermore, we find that CO2/Ba ratios are highly variable in MORB samples, but still consistent with an average mantle mass ratio of ∼100. However, estimating pre-degassing CO2concentrations and the mantle CO2flux depend strongly on the poorly-constrained carbon diffusivity. Consequently, our demonstration of the prevalence of disequilibrium during mid-ocean ridge degassing, and the potential for disequilibrium in other volcanic settings, highlights the need for better characterization of the physical parameters associated with volcanic degassing.