东太平洋水合物海岭钻井沉积物铁硫化物的磁学特征及其意义

The magnetic properties of iron sulfide minerals from Hydrate Ridge cores,East Pacific and their significance

对ODP(大洋钻探Ocean Drilling Program)204航次在水合物海岭南峰钻取的1245B、1249C、1251B和1252A孔岩心进行磁学分析,结合自生黄铁矿集合体结果探讨沉积物中的铁硫化物的磁学特征及其与甲烷渗漏、水合物赋存的关系。结果表明:4个钻孔岩心中磁性矿物含量总体不高,主要为中、低矫顽力的软磁性矿物,载磁矿物为亚铁磁性的磁铁矿、胶黄铁矿和磁黄铁矿。沉积物中自生黄铁矿是甲烷厌氧氧化(AOM)作用下硫酸盐还原的主要产物,减缓了磁性矿物含量、颗粒大小差异对χARM、HIRM和SIRM/χ的影响,加大了χ/χARM、ARM/SIRM对磁性矿物含量的敏感度。自生黄铁矿集合体含量随深度的变化可能反映古代AOM活动。在甲烷渗漏、水合物赋存背景下,胶黄铁矿和磁黄铁矿的赋存可能是局部与水合物相关的氧化还原作用产物,也可能是早更新世末、中更新世以来某一时期硫酸盐-甲烷转换带(SMTZ)快速上移的产物。早更新世沉积层内连续的胶黄铁矿沉积反映了孔隙水中HS-不足的还原成岩环境,与甲烷渗漏、水合物的赋存无关。

A study of magnetic properties in ODP （Ocean Drilling Program） Leg 204 1245B, 1249C, 1251B and 1252A cores from Hydrate Ridge, East Pacific, focusing on the iron sulfate minerals variation in depth and their response to the seep and gas hydrate bearing environment, was carried out on the basis of previous researches and other relative results. The sediments from these four cores contained rare magnetic minerals which mainly are middle to low coercivity soft minerals. The ferromagnetic minerals are magnetite, greigite and pyrrhotite. The authigenic pyrite is the main iron sulfide product in the methane anaerobic oxidation （AOM） process. Affected by the authigenic pyrite, the variation of the sediment magnetic parameters of χARM, χ/χARM and SIRM/χ caused by the content and sizes of magnetic particles were smoothed out. In contrast, theχ/ χARM and ARM/SIRM are more sensitive to the magnetic particles content. The content of authigenic pyrite aggregate changing in depth could be the records of ancient AOM processes. The greigite and pyrrhotite have been detected in several layers, which may be related to the methane drive local oxidation-reduction reaction; however, they also may be the products of the rapid shifts of the sulfide-methane transition zone （SMTZ） since the Pleistocene. The greigite in the early Pleistocene sediments was the product of diagenesis process in the situation of pore water lacking HS-, having no relevance to the methane seep and gas hydrate bearing environment.