An entropy method for geodynamic modelling of phase transitions: capturing sharp and broad transitions in a multiphase assemblage
An entropy method for geodynamic modelling of phase transitions: capturing sharp and broad transitions in a multiphase assemblage
复制标题
用于相变地球动力学建模的熵方法：捕获多相组合中的急剧和广泛的转变
DOI：
10.1093/gji/ggac293
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发表时间：
2022
影响因子：
2.8
通讯作者：
Stixrude, Lars
中科院分区：
文献类型：
--
作者：
Dannberg, Juliane;Gassmöller, Rene;Li, Ranpeng;Lithgow-Bertelloni, Carolina;Stixrude, Lars
相变对地幔对流的类型起着重要的作用。虽然观测和理论都认为，在目前的条件下，俯冲板块和上升羽流的很大一部分可以穿过整个地幔，但这种行为在整个地球历史上可能是不同的。更高的温度，例如在早期的地球，导致不同的相变是占主导地位的，也降低地幔的粘度，有利于一个更分层的对流风格诱导相变。在地球的过去，一个时期的地幔对流层将有重要意义的地幔温度的长期演化和地幔非均匀性的混合。从层状到全地幔对流的过渡可能导致一段时期的地幔雪崩与岩浆活动的急剧增加。因此，重要的是要准确地模拟相变对地幔对流的影响。然而，现有的数值方法通常排除了仅存在于特定压力、温度或组成范围内的相变的建模，并且它们对相变的厚度施加了人为的下限。为了克服这些局限性，我们开发了一种新的数值方法，该方法求解熵而不是温度的能量方程。这种技术允许强大的热力学和地球动力学模型之间的耦合，使人们有可能模拟现实的急剧相变与广泛的属性和地幔过程的动态影响。我们证明了我们的方法的实用性，通过将其应用于区域和全球对流模型，调查在地球地幔中的个别相变的影响，其潜在的分层流。我们发现，相变的厚度对流的风格有一个更大的影响比以前认为的：与所有其他参数相同，薄相变可以诱导完全分层对流的广泛的相变将导致整个地幔对流。我们的应用程序的方法在早期地球对流表明，吸热相变可能会导致分层在地球的过去更高的地幔温度。
Phase transitions play an important role for the style of mantle convection. While observations and theory agree that a substantial fraction of subducted slabs and rising plumes can move through the whole mantle at present day conditions, this behaviour may have been different throughout Earth’s history. Higher temperatures, such as in the early Earth, cause different phase transitions to be dominant, and also reduce mantle viscosity, favouring a more layered style of convection induced by phase transitions. A period of layered mantle convection in Earth’s past would have significant implications for the secular evolution of the mantle temperature and the mixing of mantle heterogeneities. The transition from layered to whole mantle convection could lead to a period of mantle avalanches associated with a dramatic increase in magmatic activity. Consequently, it is important to accurately model the influence of phase transitions on mantle convection. However, existing numerical methods generally preclude modelling phase transitions that are only present in a particular range of pressures, temperatures or compositions, and they impose an artificial lower limit on the thickness of phase transitions. To overcome these limitations, we have developed a new numerical method that solves the energy equation for entropy instead of temperature. This technique allows for robust coupling between thermodynamic and geodynamic models and makes it possible to model realistically sharp phase transitions with a wide range of properties and dynamic effects on mantle processes. We demonstrate the utility of our method by applying it in regional and global convection models, investigating the effect of individual phase transitions in the Earth’s mantle with regard to their potential for layering flow. We find that the thickness of the phase transition has a bigger influence on the style of convection than previously thought: with all other parameters being the same, a thin phase transition can induce fully layered convection where a broad phase transition would lead to whole-mantle convection. Our application of the method to convection in the early Earth illustrates that endothermic phase transitions may have induced layering for higher mantle temperatures in the Earth’s past.