Numerical Simulation Of Circulation Mechanism Of Hydrothermal System On Ultraslow Spreading Ridges

The modern seafloor hydrothermal system is an expression of mass and heat trans-portation in lithosphere,hydrosphere and biosphere.Ultra-slow spreading ridge,with half-spreading rate less than 10 mm/s,as a significant end-member of global mid-ocean ridge system has been a hot research topic.Discovering of the first active high tempera-ture hydrothermal vent on the ultra-slow spreading ridge is a revolutionary event for our knowledge of seafloor hydrothermal activity and earth science.It has been confirmed that ultra-slow spreading ridge has huge potential to form sulphide deposit with economical value.Since 2001,the geological and geophysical data surveyed on ultra-slow spreading ridges are increasing every year,especially the non-negligible contribution of Chinese scientists on Southwest Indian Ridge.Limited by deep sea survey technology,however,deep processes and flow patterns of seafloor hydrothermal system can not be observed directly.Seismic study,deep sea drilling and high-T high-P experiment study can provide some information of the deep system,but it is difficult to get data in a large spacial and time scale,and explore the continuous behavior of hydrothermal flow.Numerical simulation is an indispensable approach to understand the dynamics,circulation mechanism,fluid flow,heat/mass transportant,and reactions of hydrothermal circulation system.The thesis aims to study fluid dynamics,heat and metal transport,and mineral reactions of hydrothermal circulation system on ultra-slow spreading ridges by using finite-element method.Finally,based on geophysical and geological observation datasets,e.g.,seismic data,vent fluids data,rock samples,applying the numerical approaches to Longqi Hydrothermal system to study the mechanism of high temperature,high heat output system formation.The modeling will be of great significance to understand the process ofhydrothermal circulation,metal transport and mineral formation.We developed reactive-transport model of hydrothermal circulation system,and addressed three main questions.They are,(1)Can high-temperature hydrothermal system form without shallow magma chamber underneath ultra-slow spreading ridge?The effect of crustal permeability and fault on hydrothermal vent temperature.(2)The effect of mineral reaction on permeability structure,and thus on hydrothermal circulation pattern and vent temperature.How does anhydrite chimney build up in the subseafloor?(3)How is Longqi high-temperature hydrothermal system circulating under ultra-slow spreading Southwest Indian ridge?The effect of detachment fault and mineral precipitation on vent temperature and position.The two main necessary condition for a high-temperature hydrothermal system are heat source and fluid pathway,which are related to magmatism and tectonism.The ultra-slow spreading ridges present a focused magma supply pattern,which means some ridge segments are with pool magma supply or even without magma chamber.In this scenario,as a main focused flow pathway,detachment fault plays a significant role in hydrothermal circulation system.Based on finite element method and thermaldynamic properties of wa-ter,we modeled simplified models with a series of parameters under tectonic background of ultra-slow spreading ridges,to study the basic dynamic characteristics of hydrothermal fluid flow.High-temperature hydrothermal venting has been discovered on all of the modern mid-ocean ridges at all spreading rates.Although significant strides have been made in understanding the underlying processes that shape such systems,a number of first order discrepancies between model predictions and observations remain.One key paradox is that numerical experiments consistently show entrainment of cold ambient seawater in shallow,high permeability ocean crust causing a temperature drop that is difficult to reconcile with high vent temperatures.We investigate this conundrum using a hydro-thermo-chemical model that couples hydrothermal fluid flow with anhydrite and pyrite reactions in the shallow sub-seafloor.The models show that precipitation of anhydrite in warming seawater and in cooling hydrothermal fluids during mixing results in the formation of a chimney-like sub-seafloor structure around the upwelling,high-temperature plume.The establishment of such anhydrite-sealed zones reduces mixing between the hydrothermal fluid and seawater and results in an increase in vent temperature.Pyrite subsequently precipitates close to the seafloor within the anhydrite chimney,providing long-term stability of the upflow zone.Although anhydrite thus formed may be dissolved as colder seawater circulates through the crust away from the spreading axis,the inside pyrite walls would be preserved as veins in present-day metal deposits,thereby preserving the history of hydrothermal circulation through shallow oceanic crust.To reveal the mechanism of high-temperature hydrothermal circulation,we applied the above modeling approach to Longqi hydrothermal system on ultra-slow spreading Southwest Indian ridge.The 2D model of Longqi system is meshed by unstructured triangular elements,the geometry is constrained by observed bathymetry data.And the temperature boundary condition and detachment fault are constrained by micro-earthquake data.The layered structure of crust is estimated by seismic velocity data and seafloor samples.Adjusting the permeability of detachment fault kdf,permeability of basaltic layer k1,input mass flux to fit the vent data,e.g.,vent temperature,heat output and vent salinity.The models suggest that hydrothermal fluid can extract heat from brittle-ductile transition zone and transport mental from deep to seafloor,then form hydrothermal field and black smoker at seafloor.The position of hydrothermal field could be related to detachment fault,anhydrite precipitation and permeability structure of layered crust.And the serpentinization of perioditie alone detachment fault could cause the high salinity anomaly in ventfluid.We here explored permeabilities in a limited ranges,yet our results point in the same direction with the rate of anhydrite precipitation scaling directly with permeability.It is also possible that mechanical feedbacks,such as hydrofracturing,and or fracture flow instead of pervasive Darcy flow help to focus fluid flow on short time scales.