中挪威海Nyegga麻坑区冷泉碳酸盐岩的研究

发布时间：2018-01-02 16:21

本文关键词：中挪威海Nyegga麻坑区冷泉碳酸盐岩的研究　出处：《中国科学院研究生院(广州地球化学研究所)》2015年硕士论文　论文类型：学位论文

【摘要】：冷泉碳酸盐岩和化学自养贝壳忠实地记录了过去富甲烷冷泉流体渗漏活动的历史,同时也反映了冷泉活动区的古环境特征。本文以挪威海Storegga海底滑坡北缘的Nyegga麻坑区CN03、Tobic、Do Do和G11四个麻坑的冷泉碳酸盐岩和生物双壳为研究对象,开展了系统的岩石学、矿物学、同位素地球化学、U-Th研究,来认识该麻坑区冷泉碳酸盐岩的成岩环境、过去富甲烷渗漏流体的组成、甲烷来源以及演化特征、确定甲烷释放的年代,并探水合物分解与Storegga海底滑坡的关系。研究结果显示,Nyegga麻坑区碳酸盐岩孔洞和裂隙非常发育,部分碳酸盐岩胶结有大量的化学自养贝壳。碳酸盐岩中石英、长石、粘土等碎屑含量较高,表明冷泉碳酸盐岩最初形成于海底浅层沉积物孔隙中。显微镜下,碳酸盐矿物以泥晶高镁方解石和针状文石为主导,伴有极少量的白云石。草莓状黄铁矿和球粒分布广泛,揭示了碳酸盐岩形成时的还原环境及微生物的参与作用。碳酸盐岩的δ13C值为-58.67~-47.46‰PDB,清楚地表明冷泉碳酸盐岩经甲烷厌氧氧化过程而形成,且微生物成因的甲烷为其主导碳源。生物壳的δ13C值为-3.8~-2.7‰PDB,指示其碳源为正常海水DIC。冷泉碳酸盐岩的87Sr/86Sr比值为0.70913~0.70918,指示碳酸盐岩成岩流体主要受底层海水影响,而非深层流体,这也从另一方面证实了形成冷泉碳酸盐岩的甲烷为微生物成因的。通过铀系不平衡U-Th测年法测定了部分冷泉碳酸盐岩的年龄。定年初步结果显示,该麻坑区冷泉碳酸盐岩形成于18.5-12.3 ka,即末次冰盛期以来的不同年代,反映了Nyegga麻坑区的甲烷渗漏活动为间歇式的,经历了多次活跃和休眠。通过碳酸盐岩和生物壳的氧同位素重建的古流体δ18O值显示,形成于末次冰盛期的冷泉碳酸盐岩与古海水的氧同位素处于平衡态,渗漏流体的甲烷很可能来源于较深部的水合物分解释放的甲烷,水合物分解释放的重氧同位素对在浅层生成的冷泉碳酸盐岩的氧同位素组成影响不大。而形成于全新世的显示有稍偏重(~+0.7‰)的18O来源,推测其可能主要是由构造活动等其它因素导致的浅层水合物分解而来。综合研究结果,认为Nyegga麻坑区的甲烷渗漏活动自18 ka以来都很活跃,比最近的滑坡事件8.1 ka要早很多,因此初步推测该区水合物分解导致的甲烷渗漏可能是该区海底滑坡的诱导因素之一。
[Abstract]:Cold seep carbonate and chemoautotrophy shells to record the past methane rich cold fluid seepage activity history, but also reflects the ancient environment of the active region. Based on the cold spring and Weihai Storegga submarine landslides along the northern margin of the Nyegga pit area CN03, Tobic, Do, Do and biological seep carbonates and G11 four pockmark double shell as the research object, carry out the system of petrology, mineralogy, isotope geochemistry, U-Th, diagenetic environment of the pit area of cold seep carbonates, composed of methane rich fluid leakage past the methane source and evolution characteristics, determine the age of methane emissions, and to explore the relationship between hydrate decomposition and Storegga submarine landslide the results show that the Nyegga carbonate pit area porosity and crack development, part of carbonate cementation has a large number of chemoautotrophy shells. Quartz carbonate, feldspar, clay, etc. Detrital content is high, that cold seep carbonates originally formed in sediments of shallow pore. Under the microscope, micritic carbonate minerals with high magnesium calcite and aragonite needle is dominant, with very few dolomite. Framboid pyrite and chondrules are widely distributed, reveals a reductive environment and microbial carbonate formation in the the role of 13C in carbonate rocks. The delta value of -58.67~-47.46% PDB, clearly shows that the anaerobic oxidation of methane seep carbonates in the process of forming, and biogenic methane as the main carbon source. 8 13C biological shell value is -3.8~ -2.7% PDB, indicating the carbon source for the 87Sr/86Sr ratio of normal seawater DIC. cold seep carbonates for 0.70913~0.70918, indicating the carbonate diagenetic fluid is mainly affected by the impact of the bottom water, instead of deep fluid, which is on the other hand confirmed the formation of cold seep carbonates for micro bio methane Causes. By U-series part seep carbonate age determination of U-Th dating dating. Preliminary results show that the pit area of cold seep carbonates formed in 18.5-12.3 Ka, the last glacial maximum in different period, reflecting the methane venting Nyegga pit area is intermittent, experienced a number of active and dormant. Through the ancient fluid Delta 18O reconstruction oxygen isotope of carbonate rocks and biological value of the shell, the formation of cold seep carbonates in the last glacial maximum and ancient sea oxygen isotope in equilibrium, methane seep fluids probably derived from the deep release of methane hydrate decomposition little effect, heavy oxygen isotope decomposition on the release of oxygen isotope in seep carbonates in the shallow formation composition. Formed in the Holocene showed slightly biased (~+0.7%) of the 18O source, probably mainly by Shallow hydrate other factors that cause the tectonic activity of the decomposition. Based on the research results, that methane venting Nyegga pit area since 18 Ka are very active, much earlier than the last 8.1 landslide events Ka, therefore speculated that the methane seep in this area may lead to hydrate decomposition is one of the inducing factors in this area submarine landslide.

【学位授予单位】：中国科学院研究生院(广州地球化学研究所)
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P736;P588.245

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