Petrography And Geochemistry Studies Of Calcium Carbonate Veins In Basement Basalts From IODP Exp349 Cored At The South China Sea

The basaltic basement was cored for the first time in the South China Sea during IODP Exp 349.The basalt was collected from two Sites U1431 and U1433 close to the fossil spreading ridge in East and Southwest Sub-basin respectively.Calcium carbonate veins(CCVs)(n=24)within the basalt were investigated under microscope and by the laser Raman spectra and electron probe microanalysis.Based on the analyses,the depth distribution of the carbonate vein was estimated through the high-resolution core scanning photographs.CCVs fill in the cracks or fractures within the basement basalt and were produced due to low-temperature hydrothermal circulation within the oceanic crust.CCVs occur throughout the entire cores.The abundance of carbonate veins in the basement show that the average abundance of the site U1431 is obviously higher than that at the site U1433,which indicates that the low-temperature hydrothermal circulation at Site U1431 is stronger.The carbonate minerals are calcite and aragonite in both sites.The morphology of aragonite is the same at both Sites U1431 and U1433-blocky,fibrous and radiating fibers.However it is not the case for calcite,at Site U1431 calcite is clotted blocky,blocky,granule,and fibrous,while at Site U1433 calcite only occur as blocks,suggesting that the hydrothermal alteration at Site U1433 is less active than that at site U1431.Low Sr contents in calcite and low Mg contents in aragonite indicate that the carbonate minerals in the veins are all primary minerals.At Site U1431,the carbonate veins are consisted of calcite veins or aragonite veins,while at Site U1433 carbonate veins are composed of either calcite veins,or aragonite veins,and calcite-aragonite mixed veins.At the bottom of the both sites,only calcite veins occur,and aragonite veins only occur at some shallow depths indicating that hydrothermal alteration increases with depth,accordingly calcium carbonate supersaturation level increases.The range of Mg/Ca ratios of calcites in site U1431 is obviously larger than that site U1433.Meanwhile,at ~42.1 m below basement two calcite veins and one aragonite vein occurring parallel,indicates that the hydrothermal circulation is active,and likely multiple hydrothermal fluids of different sources have circulated.The different hydrological conditions triggered by local geology mostly lead the differences in carbonate veins and associated hydrothermal activity,as Site U1431 is close to a giant seamount which serves as a recharge site,while Site U1433 is far away from any recharge or discharge site.