Disequilibrium degassing model determination of the 3He concentration and 3He/22Ne of the MORB and OIB mantle sources
Disequilibrium degassing model determination of the 3He concentration and 3He/22Ne of the MORB and OIB mantle sources
复制标题
MORB和OIB地幔源3He浓度和3He/22Ne的不平衡脱气模型测定
DOI：
10.1016/j.epsl.2014.11.021
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发表时间：
2015
影响因子：
5.3
通讯作者：
Weston B
中科院分区：
文献类型：
--
作者：
Weston B
喷发脱气动力学模型提供了对喷发前主要挥发物(CO2、H2O)、惰性气体和供应它们的地幔储藏的独特洞察：这是描述地球吸积和演化的基本组成部分。我们研究和发展了一个不平衡脱气模型，探索了在东太平洋隆起(EPR)、洛伊希海山和冰岛的两个洋岛玄武岩(OIB)和一个大洋中脊玄武岩(MORB)样品组中观察到的惰性气体成分所需的参数。最初的模型假设每个脱气步骤的主要挥发性成分的损失相同(Gonnermann和Mukhop adhyay，2007)。考虑到前几步的脱气历史，我们重新计算每个脱气步骤的主要挥发性气相成分。使用相同的喷发参数，最终的惰性气体元素比率可能与原始模型的计算结果相差数量级。我们进一步修改了我们的模型变量，以同时考虑岩浆上升过程中的减压驱动脱气和样品淬火过程中的恒压脱气。我们的结果表明，元素比值和惰性气体浓度可以通过喷发的不同脱气阶段有效地解耦。上升速率决定了岩浆上升过程中的脱气是以封闭系统脱气为主，还是以开放系统脱气为主。在缓慢上升的MORB中占主导地位的闭合系统条件导致脱气元素比率的下降没有OIB模型那么显著，这与观测数据一致。MORB模型还限制了初始MORB源熔体3He/22Ne的比率，允许MORB地幔具有低至OIB比率的比率，这是稳态地幔模型所要求的特征。尽管最终喷发压力有很大差异，但两个OIB样品套显示出非常相似的惰性气体比率和浓度。我们认为，如果淬火引起的气体损失的影响可以忽略不计，这可以与我们的模型相吻合。然后，我们的模型表明，在岩浆上升过程中，只有少量的脱气发生，这意味着初始OIB He熔体浓度必须与MORB源熔体浓度相似或低于MORB源熔体浓度，尽管需要更高的初始CO2浓度。这一结果与地幔模型完全一致，该模型认为OIB来源的地幔中有一个重要的再循环成分：这种物质预计会比枯竭的地幔具有更高的二氧化碳浓度但更低的3He浓度。
Models of the dynamics of eruptive degassing provide a unique insight into the pre-eruptive concentrations of the major volatiles (CO2, H2O), noble gases, and the mantle reservoirs supplying them: a fundamental component in describing the accretion and evolution of the Earth. We investigate and develop a disequilibrium degassing model, exploring the parameters required to reproduce noble gas compositions observed in two ocean island basalt (OIB) and one mid-ocean ridge basalt (MORB) sample suites from the East Pacific Rise (EPR), Loihi Seamount, and Iceland. The original model assumed an identical loss of major volatile components for each degassing step (Gonnermann and Mukhopadhyay, 2007). We recalculate the major volatile vapor phase composition for each degassing step, taking account of the degassing history over previous steps. Final noble gas elemental ratios, using the same eruption parameters, can differ by orders of magnitude from the original model's calculations. We further adapt our model variant to take into account both decompression-driven degassing during magma ascent and degassing at constant pressure during sample quenching. Our results show that elemental ratios and noble gas concentrations can be effectively decoupled by the different degassing stages of an eruption. Ascent rate determines whether degassing during magma ascent is modeled as predominantly closed or open system degassing. The closed system conditions that dominate the slowly ascending MORB result in a less dramatic decrease in degassed elemental ratios than for the OIB models, consistent with the observed data. The MORB model also constrains the initial MORB source melt3He/22Ne ratio, allowing that the MORB mantle could have a ratio as low as the OIB ratio, a feature required by steady state mantle models. The two OIB sample suites show very similar noble gas ratios and concentrations despite a large difference in final eruption pressures. We propose that this can fit our model if the effect of gas loss by quenching is negligible. Our model then shows that only a small amount of degassing takes place during magma ascent, meaning that initial OIB He melt concentrations must be similar to or lower than MORB source melt concentrations, although greater initial CO2concentrations are required. This result is entirely consistent with mantle models which see a significant recycled component in the OIB-source mantle: such material would be expected to have higher CO2but lower3He concentrations than the depleted mantle.
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DOI：
10.1016/j.epsl.2005.10.018
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期刊：
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DOI：
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发表时间：
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影响因子：
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DOI：
--
发表时间：
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期刊：
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