【摘要】 冷泉碳酸盐岩是指示南海北部可能发育天然气水合物的重要证据之一。对在西沙海槽采集到的冷泉碳酸盐结壳颗粒进行了形貌、稳定同位素等的研究,讨论和揭示了冷泉碳酸盐结壳的形成机理及其与甲烷冷泉活动的关系。结果表明,碳酸盐结壳具有多孔结构,冷泉矿物为文石和重晶石,文石呈针状,含量为39.5%—46.6%,重晶石呈矮柱状,含量为15.6%—21.2%,它们均是从流体中沉淀出来的。重晶石的δ34S值为23.47‰—26.32‰,表明硫同位素发生正漂移,属于与冷泉流体有关的成岩成因重晶石。文石的碳同位素轻度亏损,δ13C值为-13.30‰—-29.59‰,主要来源于热解成因气,但结合了少量正常海水的溶解碳,δ18O值为2.32‰—3.74‰,指示碳酸盐结壳形成时的温度为3.5—9.7℃。研究表明,碳酸盐结壳形成于流体活动缓慢的环境,是深部富含Ba2+的甲烷冷泉流体向上排放和喷溢,在近海底遇到向下渗漏的富含SO42-海水,发生缺氧甲烷氧化反应沉淀出文石,流体中剩余的重34S的SO42-与Ba2+形成重晶石,因此,碳酸盐结壳是西沙海槽存在甲烷冷泉活动的证据。研究结果为进一步开展该海域潜在天然气水合物的调查和研究提供了新证据。更多还原

【Abstract】 The Xisha Trough,located in the northern South China Sea(SCS),is a key promising area of gas hydrate resources in China.Seep carbonate crusts were newly collected in the seafloor of the Xisha Trough for structural,mineralogical and isotopic studies.The carbonates have travertine-like and porous fabrics.Authigenic seep minerals of carbonates consist of aragonite and barite,which were precipitated from cold seeping fluids.Aragonite is characterized by needle and cluster shapes,with a content ranging between 39.5% and 46.6%,and barite shows pillar shapes,with a percent of 15.6% to 21.2%.The δ13C values of carbonates vary from-13.30‰ to-29.59‰,and the δ18O values from 2.32‰ to 3.74‰.Lightly depleted 13C reflects that the carbonates mainly came from thermogenic methane,but mixed with a minor amount of dissolved carbon in the seawater;whereas light δ18O values imply the involvement of deep fluids with a temperature of 3.5—9.7℃.The δ34S values of 23.47‰—26.32‰ show a lightly positive excursion and support that barite in the carbonates were diagenetic one.The results demonstrate that the carbonate crusts formed at relatively gentle fluid expulsion regimes,where barium-enriched seeping fluids from deep thermogenic origins came into contact with downwards-diffusing seawater sulfate,and then anaerobic oxidation of methane took place.This process resulted in the formation of aragonite due to increase of alkalinity,and super-saturated Ba2+ and SO42-with heavy 34S were combined to form barite.Therefore the seep carbonate crusts in the Xisha Trough are evidence of seeping fluids in the seafloor,which would provide new clues for further exploration and detailed research of potential gas hydrates in this sea area.更多还原