Carbon stable isotope constraints on CO2 degassing models of ridge, hotspot, and arc magmas
Carbon stable isotope constraints on CO2 degassing models of ridge, hotspot, and arc magmas
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碳稳定同位素对山脊、热点和弧岩浆二氧化碳脱气模型的约束
DOI：
10.1016/j.chemgeo.2022.120962
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发表时间：
2022
期刊：
影响因子：
3.9
通讯作者：
Aubaud, C.
中科院分区：
文献类型：
--
作者：
Aubaud, C.
为了充分了解地球系统，特别是火山强迫对气候的影响，火山的二氧化碳排放是需要约束的重要参数。岩浆中碳浓度的表征已被用来约束火山通量。然而，由于co2在硅酸盐熔体中的溶解度较低，在向地表转移的过程中，由于减压，co2从岩浆中明显脱气。测定天然海底玻璃中碳稳定同位素比值（13C/12C， δ13C值）是研究岩浆脱气的一种有用的地球化学工具。岩浆温度下co2囊泡与熔体中溶解的碳酸盐离子之间的碳稳定同位素分馏仍然很大，足以引起δ 13c值的变化。这种变异性可以用来推断在给定的岩浆系统中操作的脱气模式（开放与封闭系统，平衡与动力学）。在这项研究中，我介绍了现有的三种不同背景（脊、热点和弧）岩浆的碳同位素数据。该审查允许(1)调查在给定设置内运行的脱气模式的多样性，(2)比较这三种设置之间的流行脱气模式。除了罕见的欠饱和样品和富含挥发物的泡状膨出岩外，中洋脊玄武岩（morb）主要是广泛脱气和co2过饱和，反映了其最后脱气步骤的不完全脱气。这种行为也反映在它们的囊泡溶解碳同位素分馏中，通常小于由动力学/扩散效应引起的平衡值。相比之下，热点岩浆和弧岩浆的CO2+ H2O气相主要与熔体处于化学平衡状态，这是因为富含挥发物的初始条件（因此具有较大的囊泡性）增强了汽融化学和同位素平衡。这种较大的初始挥发物含量是在热点和弧岩浆中普遍观察到的广泛的开系统（瑞利蒸馏）脱气的原因。这一综述强调了一个单一的脱气模型不能解释所有岩浆的地球化学演化。此外，没有单一的模型可以分配给一种特定类型的岩浆（即，在给定类型的岩浆中存在显著的脱气模式多样性，特别是在morb中）。在稀有气体（He/Ne或He/Ar）比的基础上应用脱气修正时，重要的是要考虑到这些观察结果。对气泡和溶解在玻璃中的碳浓度和稳定同位素比值进行表征有助于(1)确定给定岩浆系统的脱气操作模式，(2)对这些岩浆进行最合适的脱气校正。只有在这个关键的校正步骤完成之后，才能合理地评估岩浆的初始co2含量。
Carbon dioxide emissions from volcanoes are important parameters to constrain in order to fully understand the Earth system, especially the effect of volcanic forcing on climate. The characterization of carbon concentration in magmas has been used to constrain volcanic fluxes. Because of low CO2solubility in silicate melts, however, CO2is significantly degassed from magmas due to decompression during transfer to the surface. The measurement of the carbon stable isotope ratio (13C/12C expressed as δ13C-values) in natural submarine glasses has been a helpful geochemical tool to study magma degassing. Carbon stable isotope fractionation at magmatic temperature between CO2in vesicles and carbonate ions dissolved in the melt is still large enough to cause variations in δ13C-values. This variability can be used to deduce the mode of degassing (open vs. closed system, equilibrium vs. kinetic) operating in a given magmatic system.In this study, I present a review of the existing carbon isotope data for magmas of three different settings (ridge, hotspot and arc). This review allows to (1) investigate the diversity of degassing modes operating within a given setting and (2) compare the prevailing degassing mode between these three settings.Except for rare undersaturated samples and for volatile-rich, vesicular popping rocks, Mid-Ocean Ridge Basalts (MORBs) are predominantly extensively degassed and supersaturated in CO2reflecting incomplete degassing during their last degassing step. Such a behavior is also reflected in their vesicle-dissolved carbon isotopic fractionations that are generally smaller than equilibrium values stemming from kinetic/diffusive effects. By contrast, the CO2+ H2O gas phase in hotspot and arc magmas is predominantly in chemical equilibrium with the melt because of volatile-rich initial conditions (and thus larger vesicularity) enhancing vapor-melt chemical and isotopic equilibrium. This larger initial volatile content is responsible for the extensive open-system (Rayleigh distillation) degassing generally observed in hotspot and arc magmas.This review highlights the fact that one single degassing model cannot explain the geochemical evolution of all magmas. Also, no single model can be assigned to one specific type of magma (i.e., a significant diversity of degassing mode exists within a given type of magma, notably in MORBs). It is important to take into account these observations when degassing corrections are applied on the basis of noble gas (He/Ne or He/Ar) ratios. It appears helpful to characterize the carbon concentration and stable isotope ratios in vesicles and dissolved in the glass in order to (1) identify the mode of degassing operating in a given magmatic system and (2) apply the most appropriate degassing correction to these magmas. It is only after this critical correcting step has been achieved that an assessment of initial magmatic CO2contents can be reasonably undertaken.