Chemical compositions and precipitation timing of basement calcium carbonate veins from the South China Sea

Sixteen calcium carbonate veins (CCVs) within the igneous basement recovered from both East and Southwest Sub-basin, close to the fossil spreading ridge of the South China Sea during the Integrated Ocean Drilling Program (IODP) Expedition 349 were investigated. The CCVs are composed primarily of either calcite or aragonite, and some of mixed aragonite and calcite. The δ18O values of CCVs range from 25.5 to 31.8‰ VSMOW, indicating these are typical low temperature basement carbonates precipitated at temperatures of 12-40 °C. The 87Sr/86Sr ratios of CCVs from Site U1431 show a strong negative correlation with δ18O-calculated temperatures, regardless of carbonate phases - calcite or aragonite, indicating CCVs with lower 87Sr/86Sr ratios have precipitated from moderately warmer and more geochemically evolved hydrothermal fluids, and reflecting that precipitation of CCVs might have occurred any time between 14.5 and 0 Ma at Site U1431. The formation timing of CCVs at Site U1431 is consistent with the ongoing hydrothermal flow and circulation led by recharging seawater into the volcanic basement through the nearby outcropped seamount. The oldest ages of CCVs from Site U1433 at the Southwestern sub-basin of SCS were determined to be ~ 18-11 Ma, based on basement age of 18.5 Ma and the well-established seawater 87Sr/86Sr ratio curve. It indicates that the hydrothermal circulation at Site U1433 which is more distal to a recharging/discharging site was only active until ~ 11 Ma. In consequence, the CCVs within basalts from Sites U1431 and U1433 provide more insights into the past hydrologic conditions and hydrothermal circulation along the fossil ridge flank in the SCS.