南海北部冷泉碳酸盐岩中钙质生物壳体和自生碳酸盐矿物的Mg/Ca、Sr/Ca及其环境指示

南海北部冷泉碳酸盐岩具有包裹结构和孔洞结构,并且含有丰富的生物遗迹。利用电子探针对位于不同结构单元(包裹结构和基质)的自生碳酸盐矿物和钙质生物壳体进行了Mg/Ca和Sr/Ca分析,结果显示:自生碳酸盐矿物Mg/Ca变化范围为0.50~39.19mmol/mol,平均为12.50mmol/mol,Sr/Ca变化范围为0.06~2.90mmol/mol,平均为0.53mmol/mol;钙质生物壳体Mg/Ca变化范围为1.14~84.57mmol/mol,平均为24.57mmol/mol,Sr/Ca变化范围为0.77~1.84mmol/mol,平均为1.08mmol/mol。Mg/Ca和Sr/Ca的关系显示自生碳酸盐矿物均具有低到中等的Mg/Ca、低Sr/Ca的特征,没有明显分组现象;但是钙质生物壳体却根据其在岩石中的结构位置呈现明显的分组现象,基质钙质生物壳体具有高Mg/Ca、中等Sr/Ca的特征,包裹的钙质生物壳体具有低Mg/Ca、中等Sr/Ca的特征。根据碳酸盐矿物和钙质生物壳体的Mg/Ca特征,认为基质钙质生物壳体是在甲烷渗漏环境下钙化的,无机碳酸盐矿物的出现影响了其Mg/Ca;而包裹的钙质生物壳体是在非甲烷渗漏环境下钙化的,由于生物扰动、灌洗等作用以包裹团块的形式保存在碳酸盐岩中。依据Sr/Ca在成岩过程中的变化特点推测最初形成的基质钙质生物壳体可能具有较现在高的Sr/Ca。研究结果在甲烷渗漏信息地球化学指标提取方面具有一定意义。

Mg/Ca and Sr/Ca ratios of authigenic carbonate minerals and calcareous biogenic shells in the cold-seep carbonates,north of the South China Sea and their environmental implication

There are inclusion and pore space structures in the cold-seep carbonates with plenty of biological marks in the north of the South China Sea. The Mg/Ca and Sr/Ca ratios of the carbonate minerals and the calcareous biogenic shells in different structures are analyzed by the electron microprobe.The results show that the Mg/Ca and Sr/Ca ratios of the authigenic carbonate minerals have a range from 0.50 mmol/mol to 39.19 mmol/mol with the mean 12.50 mmol/mol and from 0.06 mmol/mol to 2.90 mmol/mol with the means 0.53 mmol/mol,individually.The Mg/Ca and Sr/Ca ratios of the calcareous biogenic shells have a range from 1.14 mmol/mol to 84.57 mmol/mol with the mean 24.57 mmol/mol and from 0.77 mmol/mol to 1.84 mmol/mol with the means 1.08 mmol/mol,individually. According to the relationship between Mg/Ca and Sr/Ca ratios,the authigenic carbonate minerals have no groups which are all low to middle Mg/Ca ratio and low Sr/Ca ratio,but the calcareous biogenic shells can be divided into two groups,and the one on the groundmass has high Mg/Ca ratio and middle Sr/Ca ratio,and the other one in the inclusions has low Mg/Ca ratio and middle Sr/Ca ratio. Based on the Mg/Ca characteristic,the sedimental environment is discussed with a result that the calcareous biogenic shells on the ground mass grew up in the methane seep environment and the inorganic carbonate minerals influenced their Mg/Ca ratio,but the calcareous biogenic shells in the inclusions grew up in normal environment without methane seep influences,and they entred into the carbonates by bioturbation and/or bioirrigation.Moreover,according to Sr/Ca ratios varies during diagenis,the Sr/Ca ratios of the calcareous biogenic shells on the ground mass may be higher when they formed initiall.The results can give some directions to find chemical proxies of methane seeps.