Chemical compositions and precipitation timing of basement calcium carbonate veins from the South China Sea
Chemical compositions and precipitation timing of basement calcium carbonate veins from the South China Sea
复制标题
南海基底碳酸钙脉的化学成分及沉淀时代
DOI：
10.1016/j.margeo.2017.11.012
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发表时间：
2017-10
期刊：
影响因子：
2.9
通讯作者：
Cheng Z.H.
中科院分区：
文献类型：
--
作者：
Ding W.W.;Chen Y.F.;Sun Z;Cheng Z.H.
对综合大洋钻探（IODP）第349次考察期间在中国南海东部和西南次海盆中发现的16条碳酸钙脉（CCV）进行了研究。CCV主要由方解石或文石组成，以及一些文石和方解石的混合物。CCV的δ 18 O值为25.5 ~ 31.8‰ VSMOW，表明它们是典型的低温基底碳酸盐岩，沉积温度为12-40 °C。无论碳酸盐相-方解石还是文石，U1431站点CCV的87 Sr/86 Sr比值与δ 18 O计算的温度显示出强烈的负相关性，表明具有较低87 Sr/86 Sr比值的CCV来自适度温暖和地球化学演化程度更高的热液流体。反映了在U1431站14.5 ~ 0 Ma之间的任何时间都可能发生CCV的沉淀。在站点U1431的CCV的形成时间是一致的持续热液流和循环导致通过附近的出露海山注入火山基底的海水。根据18.5Ma的基底年龄和已建立的海水87 Sr/86 Sr比值曲线，确定南海西南次海盆U1433站CCV的最老年龄为18-11 Ma。这表明，在站点U1433，这是一个更远的充电/放电网站的热液循环只活跃，直到~ 11 Ma。因此，U1431和U1433站玄武岩中的CCV提供了对南海化石脊侧翼沿着过去水文条件和热液循环的更多了解。
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. The87Sr/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 lower87Sr/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 seawater87Sr/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.
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