Paul Jyotirmoy, Golabek Gregor J, Rozel Antoine B, Tackley Paul J, Katsura Tomoo, Fei Hongzhan
Bayerisches Geoinstitut, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany.
Centre for Planetary Habitability, University of Oslo, 0316 Oslo, Norway.
Prog Earth Planet Sci. 2024;11(1):64. doi: 10.1186/s40645-024-00658-3. Epub 2024 Dec 4.
Recent experimental investigations of grain size evolution in bridgmanite-ferropericlase assemblages have suggested very slow growth for these bimodal phases. Despite numerous speculations on grain size-dependent viscosity, a comprehensive test with realistic grain size evolution parameters compatible with the lower mantle has been lacking. In this study, we develop self-consistent 2-D spherical half-annulus geodynamic models of Earth's evolution using the finite volume code StagYY to assess the role of grain size on lower mantle viscosity. We explore several models with and without grain size evolution to compare their effects on mantle viscosity. In models with grain size evolution, we consider three scenarios: (1) uniform grain growth throughout the entire mantle with a composite rheology, (2) different grain growth in the upper and lower mantle with a composite rheology, and (3) different grain growth in the upper and lower mantle with purely diffusion creep rheology. In the case of different grain size evolution, the upper mantle's grain size evolution law is controlled by forsterite-enstatite grain growth, while the lower mantle's grain size evolution law is controlled by bridgmanite-ferropericlase grain growth. Our results suggest that mantle viscosity is primarily controlled by temperature, whereas grain size has a minor effect compared to the effect of temperature. We attribute two primary reasons for this: First, the bridgmanite-ferropericlase growth is very slow in the lower mantle and the grain size variation is too small to significantly alter the mantle viscosity. Secondly, if grains grow too fast, thus the mantle deforms in the dislocation creep regime, making viscosity grain size-independent. To establish the robustness of this finding we vary several other model parameters, such as surface yield strength, phase transition grain size reset, different transitional stresses for creep mechanisms, pressure dependence on grain growth, and different grain damage parameters. For all our models, we consistently find that grain size has a very limited effect on controlling lower mantle viscosity in the present-day Earth. However, large grain size may have affected the lower mantle viscosity in the early Earth as larger grains of single phase bridgmanite could increase the viscosity of the early mantle delaying the onset of global convection.
最近对布里奇曼石 - 铁方镁石组合中晶粒尺寸演化的实验研究表明，这些双峰相的生长非常缓慢。尽管对晶粒尺寸依赖性粘度有众多推测，但缺乏与下地幔兼容的实际晶粒尺寸演化参数的全面测试。在本研究中，我们使用有限体积代码StagYY开发了地球演化的自洽二维球形半环地球动力学模型，以评估晶粒尺寸对下地幔粘度的作用。我们探索了几个有和没有晶粒尺寸演化的模型，以比较它们对幔粘度的影响。在有晶粒尺寸演化的模型中，我们考虑三种情况：（1）整个地幔中具有复合流变学的均匀晶粒生长，（2）上地幔和下地幔中具有复合流变学的不同晶粒生长，以及（3）上地幔和下地幔中具有纯扩散蠕变流变学的不同晶粒生长。在不同晶粒尺寸演化的情况下，上地幔的晶粒尺寸演化规律由镁橄榄石 - 顽火辉石晶粒生长控制，而下地幔的晶粒尺寸演化规律由布里奇曼石 - 铁方镁石晶粒生长控制。我们的结果表明，地幔粘度主要由温度控制，而与温度的影响相比，晶粒尺寸的影响较小。我们将此归因于两个主要原因：第一，下地幔中布里奇曼石 - 铁方镁石的生长非常缓慢，晶粒尺寸变化太小，无法显著改变地幔粘度。其次，如果晶粒生长过快，地幔会在位错蠕变 regime 中变形，使粘度与晶粒尺寸无关。为了确定这一发现的稳健性，我们改变了其他几个模型参数，如表面屈服强度、相变晶粒尺寸重置、蠕变机制的不同过渡应力、压力对晶粒生长的依赖性以及不同的晶粒损伤参数。对于我们所有的模型，我们一致发现，在当今地球中，晶粒尺寸对控制下地幔粘度的影响非常有限。然而，大晶粒尺寸可能在早期地球中影响了下地幔粘度，因为单相布里奇曼石的较大晶粒可能会增加早期地幔的粘度，延迟全球对流的开始。
