Seafloor Hydrothermal Activity Along The Mid-ocean Ridge With High Melt Supply:Study From Segment 27,southwest Indian Ridge

Basing on the"magmatic budget"hypothesis,the linear regression relationship between spreading rate and hydrothermal frequency indicates the distribution and control factors of hydrothermal activity along mid-ocean ridges and back-arc spreading ridges.Besides this,it can also provide detailed information for the seafloor sulfide deposit exploration.During the recent investigation in the different tectonic backgrounds like some fast-spreading ridges?East Pacific Rise 9-10°N,EPR?,intermediate-spreading ridges?Galapagos spreading center,GSC?and slow-spreading ridges?North Mid-Atlantic Ridge?,this linear regression hypothesis has been updated.But rare investigation along ultra-slow spreading ridges can support this hypothesis.China Ocean Mineral Resource R&D Association?COMRA?signed exploration contracts with International Seabed Authority?ISA?for polymetallic sulfides exploration and obtained the contact area which is mainly located between Indomed and Gallieni transform fault?46-52°E?on ultra-slow spreading Southwest Indian Ridge?SWIR?.Since 2010,several comprehensive investigations have been conducted on the contract area and a large amount of hydrothermal activity data have been obtained.In this thesis,the segment 27,SWIR was choosed as the main study area,data from deep hydrothermal detection system during Chinese Dayang 34^th,39^th,40^th,43^rdcruises were collected to analysis the hydrothermal activity.After calibrating and processing data,the criteria of the hydrothermal field along the ultra-slow spreading ridge was established.Using the data,the plume characters and hydrothermal frequency chould be concluded.And using the MTT model,plume rising height and heat flux has been calculated.At last,basing on those features,the hydrothermal activity mechanism is inferred.The thesis results are as follows:?1?Segment 27,SWIR develops 9 hydrothermal fields.Besides the confirmed Duanqiao hydrothermal field,there are 3 inferred hydrothermal fields and 5 suspected fields.3 inferred hydrothermal fields have been found on the west valley.And 3suspected diffused fields are on the neo-volcano area and the other 2 fields are found on the east valley.Hydrothermal frequency of segment 27 is 4.7/100km?10.7/100km,which is 3.8?8 times higher than global trend.ORP-only hydrothermal fields on segment 27 account for?50%of all fields,such diffuse fields are difficult for detection that may be the reason for low hydrothermal frequency along such ridges.?2?Using MTT model,the variation of the plume height,temperature,salinity during rising and the buoyancy flux and heat flux of segment 27,SWIR have been calculated.Using the background seawater and the modeling data to calculate the whole heat flux of segment 27 is?338MW and the average heat flux of each field is?112 MW,which is smaller than TAG vent heat flux and like fast-spreading East Pacific Rise's hydrothermal vent heat flux.That indicated the hydrothermal activity scale along segment 27,SWIR is not large and more like those on fast-spreading ridges.?3?The main control factors of the hydrothermal field along segment 27 are the axial magma chamber.Hydrothermal activities are influenced by the lateral magma redistribution on mid-crustal depth,leads to the asymmetric hydrothermal activity distribution along the ridge:high-temperature hydrothermal activities are concentrated on the west valley and the ridge center.And the heat source for off-axis hydrothermal circulation is magma away from the injection zone which is transported through the long-live boundary faults.?4?Combined the previous study and cruises data,particularly the CTD data from South Mid-Atlantic Ridge 9?10°S and the result on segment 27,SWIR,we found that segment like segment 27 which has high magma supply could have a higher hydrothermal frequency that normal ridge,often develops small-scale high-temperature fields and low-temperature diffuse vent fields,which is different from typical slow to ultraslow spreading ridges.The research of this paper may provide detailed information for hydrothermal activities along mid-ocean ridges and the exploration of deposits of China contract area on Southwest Indian ridge.And at the same time,more detailed multi-parameter surveys are needed along typical mid-ocean ridges for describing the distribution and mechanism of hydrothermal activities more clearly and exactly.