南海东沙西南海域冷泉碳酸盐岩特征及其意义

Diagenetic Environment and Implication of Seep Carbonate Precipitations from the Southwestern Dongsha Area,South China Sea

海底冷泉流体或自生碳酸盐沉积可为碳氢化合物(主要指常规油气或天然气水合物)、冷泉生物群落的调查和研究提供有利线索。对东沙群岛海区、神狐海区和ODP184-1146钻孔碳酸盐沉积的特征进行了对比和研究,结果表明这些海区碳酸盐岩的岩性特征、碳氧同位素组成等存在明显差异。东沙群岛海区和神狐海区的碳酸盐岩的化学成分如TFe、A l2O3、TiO2、K2O、Na2O、MnO、P2O5等不同,表明它们可能分别受到了粘土矿物和铁锰氧化物影响。在神狐海区至少发生了一次冷泉流体活动,形成了早晚两期冷泉碳酸盐岩,而在东沙群岛海区发生了至少3次冷泉流体活动,形成了多期冷泉碳酸盐岩,反映了这两个海区碳酸盐沉积的甲烷成因不同,且形成环境也受不同的因素控制。研究揭示,在东沙群岛南部海区可能发育热成因甲烷汇聚的天然气水合物,其海底可能存在冷泉生物群落,这为东沙西南海域天然气水合物和冷泉生物群落的进一步寻找和研究提供了新思路。

Cold fluids and authigenic carbonate precipitations at the seafloor are closely associated with gas hydrate dissociation, thus the identification and study on authigenic carbonates at cold fluids can be a key in the investigation for gas hydrates and seep communities. In this paper a comprehensive comparison has been performed on the lithology, chemical composition, isotope geochemistry and evaluation of seep carbonate precipitations collected from the Dongsha Islands area, Shenhu area and deep-sea core of Site 1146, ODP Leg 184, South China Sea. Those seep precipitations are different in lithology, stable isotopes of carbon and oxygen, possibly reflecting their different diagenetic environments. Chemical compositions of TFe, Al2O3, TiO2, K2 O, Na2 O, MnO and P2O5 are different in seep precipitations from the Dongsha Islands area and Shenhu area, implying they were influenced possibly by clay minerals and was micro Fe-, Mn-nodules or crusts, respectively. Cold fluid activity occurred once at the Shenhu area which was formed in early and later stages of seep precipitations. At least three times of cold fluid activities and several stages of carbonate precipitations were identified at the Dongsha Islands area, reflecting the difference in carbon sources and tectonics systems in two areas. It is concluded that gas hydrates could exist in the area of southern Dongsha Island, which were associated with thermogenic methane from deep sediments, and seep communities could also occur at that seafloor. This is a new access to investigate for gas hydrates and seep communities at the Southwestern Dongsha area.