慢速-超慢速扩张中心岩浆匮乏区热液活动的渗透机理

Numerical Modeling of the Permeation Mechanism of Hydrothermal Activities Along Slow-ultraslow Spreading Centers

慢速-超慢速扩张洋中脊作为全球洋中脊系统重要的组成部分,其岩浆补给、岩石圈热力学状态在沿轴/离轴方向表现出显著的差异,并广泛发育岩浆匮乏甚至缺失区,如拆离断层、非转换不连续带和非岩浆段等,在这些区域也发育大量的热液活动。为探究慢速-超慢速扩张洋中脊岩浆匮乏区的热液驱动机制及可能的热源,本文通过数值模拟方法,结合岩石圈冷却-破裂理论模型来定量解释洋中脊热液系统的产生条件、对流强度和维持时间与其深部潜在热源的关系。调整岩石圈矿物尺寸至5 mm×5 mm时,岩石圈最大破裂深度为~5 km。数值模拟结果表明,在这种情况下,产生并维持热液系统的洋壳渗透率阈值为~1.6×10^(-16)m^(2),且渗透率越大,热液活动越强烈。在相同的设置下,低渗模型(k_(1)=1.7×10^(-16)m^(2))维持热液系统持续活动~170 ka。提高模型对流层渗透率(k_(2)=1.7×10^(-15)m^(2))不仅有效增强了热液系统的热量输出,也将热液活动时间延长至~470ka。结合数值模拟结果和地球物理发现,慢速-超慢速扩张洋中脊岩浆匮乏区的热液活动或受控于深部多期次岩浆侵入体的侵入-加热-冷却-破裂过程。在这一过程中,若在岩浆体和围岩温度降至无法维持热液活动之前发生新一期岩浆侵入,上覆热液系统得以维持,并表现出脉冲状的热量变化;反之,若新一期岩浆侵入滞后,热液活动将最终衰亡。岩浆侵入与热液对流之间的时间差可能是导致慢速-超慢速扩张洋中脊热液系统下伏热源无法确定的因素之一。

Slow-ultraslow spreading ridges are important components of the global mid-ocean ridge system.The magma supply and thermodynamic states of the lithosphere below these ridges vary intensely along or off axis,where there are widespread zones of magma-deficient or even absent,including detachment faults,non-transform discontinuities,and amagmatic segments,and these areas also develop large amounts of hydrothermal vents.In order to explore their driving mechanism and possible heat sources,we used numerical simulations combined with the theoretical lithospheric cooling-cracking model to quantitatively explain the relationship between their conditions of occurrence,convecting intensity,lasting time,and potential heat sources at depth.Adjusting the size of the minerals to a 5 mm×5 mm resulted in a maximum cracking depth~5 km.The numerical simulation results show that the threshold of oceanic crustal permeability that generates and maintains hydrothermal systems is~1.6×10^(-16)m^(2),and the greater permeability,the stronger hydrothermal venting.Under the same settings,the low-permeability model(k_(1)=1.7×10^(-16)m^(2))can maintain the hydrothermal venting for ca.170 ka.Increasing the permeability of the cracking layer(k_(2)=1.7×10^(-15)m^(2))not only effectively increased the heat output but also extended the venting duration to ca.470 ka.Combining the simulations and geophysical discoveries,hydrothermal vents on magma-deficient zones of slow-and ultra-slow spreading ridges could be controlled by the process of intruding-heating-cooling-cracking of multiphase magma intrusions.During this process,if a new magma intrusion occurs before the temperature of the melt and surrounding rocks drop to the level impossible to maintain hydrothermal activity,the overlying hydrothermal system is maintained and exhibits pulse-like thermal changes.Conversely,if the new magmatic intrusion lags,the hydrothermal activity will eventually decline.The time difference between magmatic intrusion and hydrothermal convection may be one of the factors leading to the uncertain heat sources of the hydrothermal systems of the slow-and ultra-slow spreading ridges.