Thermochemical regime of the early mantle inferred from numerical models of the coupled magmatism‐mantle convection system with the solid‐solid phase transitions at depths around 660 km
Thermochemical regime of the early mantle inferred from numerical models of the coupled magmatism‐mantle convection system with the solid‐solid phase transitions at depths around 660 km
复制标题
根据 660 km 深度处固-固相变耦合岩浆作用-地幔对流系统的数值模型推断出早期地幔的热化学状况
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发表时间：
1998
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通讯作者：
Hidenori Nakamura
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作者：
Masaki Ogawa;Hidenori Nakamura
提出了整个地幔中岩浆作用-地幔对流耦合系统的数值模型，研究耦合系统如何在660 km左右深度的固-固相变影响下根据内部加热速率控制地幔的热化学状态。地幔中的固态对流是通过内部热源均匀加热的二维方形盒子中具有恒定粘度的二元共晶材料的对流来模拟的。该物质的端元之一代表最上地幔中富含橄榄石的物质，并在深度大于 660 公里左右的阈值时转变为高压相。相界具有负克劳修斯-克拉佩龙斜率。另一个端元代表最上地幔中富含石榴石的物质，在660公里左右的深度范围内随着深度的增加逐渐转变为高压相。材料的密度取决于其成分、相、熔体含量和温度。岩浆作用是通过材料部分熔化的压力释放所产生的可渗透熔体流来模拟的。可渗透流动由熔体的浮力驱动。无论由于固-固相变而阻止上地幔和下地幔之间的质量交换的屏障的强度如何，地幔的热化学状态都存在两种状态。在一个称为TC分支的区域，岩浆活动最多只发生轻微，地幔整体上保持化学均匀，固态对流主要以热对流形式发生。对流循环是地幔范围内的或分层的，具体取决于固-固相变引起的屏障的强度。仅当内部加热速率低于阈值时，TC 分支才稳定。在阈值处的 TC 分支上发生分岔，并且热化学状态落在阈值以上的另一个称为 CS 分支的区域。幕式岩浆活动活跃发生，使地幔发生化学分层，上地幔大部分被富含橄榄石的残余物质占据，下地幔较深部分被CS分支玄武岩成分的岩浆产物占据。当屏障较弱时，固态对流作为整个地幔对流发生；而当由于富含石榴石物质的相变而导致的屏障足够强时，固态对流作为层状对流发生，并被冲刷事件所打断，从而诱发特别剧烈的岩浆活动。 CS分支上热化学状态的总体特征与太古宙和早元古代大陆的许多观测结果相符。
A numerical model is presented for the coupled magmatism-mantle convection system in the entire mantle to study how the coupled system controls the thermochemical state of the mantle under the influence of the solid-solid phase transitions at depths around 660 km depending on the internal heating rate. The solid-state convection in the mantle is modeled by a convection of a binary eutectic material with constant viscosity in a two-dimensional square box uniformly heated by an internal heat source. One of the end-members of the material stands for an olivine-rich material in the uppermost mantle and is transformed into its high-pressure phase at depths greater than a threshold around 660 km. The phase boundary has a negative Clausius-Clapeyron slope. Another end-member stands for a garnet-rich material in the uppermost mantle and is gradually transformed into its high-pressure phase with increasing depth in a depth range around 660 km. The density of the material depends on its composition, phase, melt content, and temperature. Magmatism is modeled by a permeable flow of melt generated by a pressure release partial melting of the material. The permeable flow is driven by the buoyancy of the melt. There are two regimes in the thermochemical state of the mantle regardless of the strength of the barrier against mass exchange between the upper mantle and the lower mantle due to the solid-solid phase transitions. On one regime called the TC branch, magmatism occurs only mildly at most, the mantle remains chemically homogeneous as a whole, and the solid-state convection occurs dominantly as a thermal convection. The convective circulation is mantle-wide or layered depending on the strength of the barrier due to the solid-solid phase transitions. The TC branch is stable only when the internal heating rate is lower than a threshold. A bifurcation occurs on the TC branch at the threshold, and the thermochemical state falls on another regime called the CS branch above the threshold. An episodic magmatism actively occurs to make the mantle chemically stratified with the upper mantle largely occupied by olivine-rich residual materials and the deeper part of the lower mantle occupied by magmatic products of basaltic composition on the CS branch. The solid-state convection occurs as a whole mantle convection when the barrier is weak, while it occurs as a layered convection punctuated by flushing events that induce particularly vigorous magmatic activities when the barrier due to the phase transition of the garnet-rich material is sufficiently strong. The overall features of the thermochemical state on the CS branch mesh in many observations from the Archean and early Proterozoic continents.