A Disequilibrium Reactive Transport Model for Mantle Magmatism
A Disequilibrium Reactive Transport Model for Mantle Magmatism
复制标题
地幔岩浆作用的不平衡反应输运模型
作者：
B. Oliveira;J. Afonso;R. Tilhac
除了标准的热力学守恒定律外，地幔岩浆作用的一般描述还需要同时考虑相变（例如从固体到液体）、化学反应（即化学成分的交换）和多个动力学阶段（例如液体通过变形基质的膨胀）。通常，这些过程以不同的速率在多个空间尺度上演变，并表现出复杂的反馈回路和不平衡特征。部分由于这些复杂性，地幔岩浆活动的热，机械和化学演化的综合描述一直是具有挑战性的数值模型。在这里，我们提出了一个概念和数值模型，提供了一个多功能的平台，研究地幔岩浆活动中固有的动力学和非线性反馈，并在岩石学和地球化学数据集之间进行定量比较。我们的模型是基于三个主要模块的组合：（1）两相、多组分、反应输运模块，它描述了液体和固体在空间和时间中的演变;（2）熔化形式主义，称为动态不平衡熔化，基于热力学基础，能够描述不平衡状态下主要元素在相之间的化学交换;（3）扩散控制微量元素传质的颗粒尺度模型。我们通过分析大洋中脊状背景下地幔岩浆活动过程中的常量和微量元素来说明该模型的一些优点。本文系统地探讨了地幔位温、上涌速度、平衡程度和非均质源区对熔体和残余橄榄岩成分变化的影响。我们的模型不仅再现了减压熔融的主要热化学特征，而且还预测了由于不平衡发生的相变和运输而导致的反直觉的分化趋势。这些包括在相同的Tp和持续增加的熔体的CaO/Al 2 O3后，Cpx耗尽产生的熔体中的Na 2 O和FeO之间的负相关性。我们的模型结果还强调了微量元素签名的解释扩散所产生的不平衡的作用。后者被证明是能够调和的主要和微量元素组成的深海橄榄岩与现场证据表明橄榄岩和熔体之间的广泛反应。主要元素的化学不平衡和微量元素的缓慢扩散相结合，也可能导致削弱中稀土重稀土耗尽的洋中脊玄武岩中的残留石榴石的效果相媲美，尽管它没有在模拟熔体源。我们还发现，玄武岩的结晶上升，在不平衡的软流圈地幔可能是负责形成的橄榄石辉长岩和韦氏岩中观察到的蛇绿岩深部部分。所提出的框架是通用的，易于扩展，以适应地质相关的其他过程（如在挥发物和/或复杂的异质源的存在下熔融，岩石圈地幔的再施肥，岩浆通道化和浅过程）和其他地球化学和同位素代理的实施。在这里，我们说明了使用混合橄榄岩-辉石岩源的熔体和残留物的热-机械-化学演化的非均质源的效果。
Besides standard thermo-mechanical conservation laws, a general description of mantle magmatism requires the simultaneous consideration of phase changes (e.g. from solid to liquid), chemical reactions (i.e. exchange of chemical components) and multiple dynamic phases (e.g. liquid percolating through a deforming matrix). Typically, these processes evolve at different rates, over multiple spatial scales and exhibit complex feedback loops and disequilibrium features. Partially as a result of these complexities, integrated descriptions of the thermal, mechanical and chemical evolution of mantle magmatism have been challenging for numerical models. Here we present a conceptual and numerical model that provides a versatile platform to study the dynamics and nonlinear feedbacks inherent in mantle magmatism and to make quantitative comparisons between petrological and geochemical datasets. Our model is based on the combination of three main modules: (1) a Two-Phase, Multi-Component, Reactive Transport module that describes how liquids and solids evolve in space and time; (2) a melting formalism, called Dynamic Disequilibirum Melting, based on thermodynamic grounds and capable of describing the chemical exchange of major elements between phases in disequilibrium; (3) a grain-scale model for diffusion-controlled trace-element mass transfer. We illustrate some of the benefits of the model by analyzing both major and trace elements during mantle magmatism in a mid-ocean ridge-like context. We systematically explore the effects of mantle potential temperature, upwelling velocity, degree of equilibrium and hetererogeneous sources on the compositional variability of melts and residual peridotites. Our model not only reproduces the main thermo-chemical features of decompression melting but also predicts counter-intuitive differentiation trends as a consequence of phase changes and transport occurring in disequilibrium. These include a negative correlation between Na2O and FeO in melts generated at the same Tp and the continued increase of the melt’s CaO/Al2O3 after Cpx exhaustion. Our model results also emphasize the role of disequilibrium arising from diffusion for the interpretation of trace-element signatures. The latter is shown to be able to reconcile the major- and trace-element compositions of abyssal peridotites with field evidence indicating extensive reaction between peridotites and melts. The combination of chemical disequilibrium of major elements and sluggish diffusion of trace elements may also result in weakened middle rare earth to heavy rare earth depletion comparable with the effect of residual garnet in mid-ocean ridge basalt, despite its absence in the modelled melts source. We also find that the crystallization of basalts ascending in disequilibrium through the asthenospheric mantle could be responsible for the formation of olivine gabbros and wehrlites that are observed in the deep sections of ophiolites. The presented framework is general and readily extendable to accommodate additional processes of geological relevance (e.g. melting in the presence of volatiles and/or of complex heterogeneous sources, refertilization of the lithospheric mantle, magma channelization and shallow processes) and the implementation of other geochemical and isotopic proxies. Here we illustrate the effect of heterogeneous sources on the thermo-mechanical-chemical evolution of melts and residues using a mixed peridotite–pyroxenite source.
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DOI：
10.7185/geochemlet.1728
发表时间：
2017-07
期刊：
--
影响因子：
--
作者：
S. Lambart
通讯作者：
S. Lambart
化学不平衡、岩石圈厚度和洋岛玄武岩的来源
影响因子：
3.9
作者：
Grose, Christopher J;Afonso, Juan C
通讯作者：
Afonso, Juan C
影响因子：
3.5
作者：
J. Warren
通讯作者：
J. Warren
用于模拟适用于熔体迁移的热非平衡的多孔流方法
影响因子：
2.8
作者：
Schmeling;G. Marquart;M. Grebe
通讯作者：
M. Grebe