洋壳渗透率结构对其内部热液对流的影响研究——基于热力学数值模拟

洋底特殊环境的限制使数值模拟成为研究海底热液对流与成矿机制的有效方法.本文通过数值模拟的方法,研究洋壳渗透率单因素变化对热液对流系统的形态和输出参数(热液喷发温度和热流值)进行研究.模拟结果表明,洋壳平均渗透率分别与热液喷发温度和热量输出呈反相关和正相关关系,符合达西流体热对流的基本解析规律.另外,洋壳渗透率的垂向变化不会使洋壳内部的对流系统产生明显的横向偏移,只使热液羽规模在垂向上产生规律性变化,且渗透率越大,热液羽规模越小.渗透率在水平方向的变化则是引发热液羽和热液喷发位置横向偏移的主要因素,且只有在特定的渗透率水平变化率范围内,上涌热液羽才会发生向高渗区的明显偏移,这也从一定程度上解释了现今拆离断层相关的热液系统均未发育于断层终止线上的事实.以大西洋Trans-Atlantic Geotraverse(TAG)热液区为例,断裂带高渗区必然影响相邻洋壳内部热液的上涌路径,但受区内拆离断裂带周缘的渗透率水平变化规律的影响,上涌热液羽不至于完全偏移至断裂带,从而产生拆离断层上盘的热液活动.

Study on the influence of the structure of oceanic crust permeability on the hydrothermal convection——Based on thermodynamic modelling

Extreme environment of the ocean floor makes numerical modelling be an effective way to study the convection and mineralization of the hydrothermal deposits. In this paper, thermodynamic modelling is used to study the influence of single factor (permeability) on the geometry of hydrothermal convection and output parameters (venting temperature and heat flux). Simulation results show that the average permeability of oceanic crust is inversely and positively correlated with venting temperature and output heat flux, which is consistent with the analytical solution of Darcy fluid convection. In the case with localized heat source, the vertical increase of permeability reduces the size of hydrothermal plumes, without any horizontal deviation; by contrast, the horizontal variation of permeability within oceanic crust does deviate the geometry of hydrothermal circulation laterally, and only in certain ranges of horizontal permeability variation the significant deviation of hydrothermal plumes toward high permeability areas will occur, which is consistent with the non-existence of hydrothermal venting on the termination of detachment faults on slow-ultraslow spreading ridges. Taking the TAG hydrothermal field on 26°N, Mid-Atlantic Ridge as an example, hydrothermal upwelling in the oceanic crust will be unavoidably affected by adjacent high permeability fault zone, but it will not cause a significant deviation, so that hydrothermal vents finally develop on the hanging wall of the detachment.