In the equatorial western Pacific, iron-manganese oxyhydroxide crusts(Fe-Mn crusts) and nodules form on basaltic seamounts and on the top of drowned carbonate platform guyots that have been swept free of pelagic sediments. To date, the Fe-Mn crusts have been considered to be almost exclusively of abiotic origin. However, it has recently been suggested that these crusts may be a result of biomineralization. Although the Fe-Mn crust textures in the equatorial western Pacific are similar to those constructed by bacteria and algae, and biomarkers also document the existence of bacteria and algae dispersed within the Fe-Mn crusts, the precipitation, accumulation and distribution of elements, such as Fe, Mn, Ni and Co in Fe-Mn crusts are not controlled by microbial activity. Bacteria and algae are only physically incorporated into the crusts when dead plankton settle on the ocean floor and are trapped on the crust surface. Geochemical evidence suggests a hydrogenous origin of Fe-Mn crusts in the equatorial western Pacific, thus verifying a process for Fe-Mn crusts that involves the precipitation of colloidal phases from seawater followed by extensive scavenging of dissolved trace metals into the mineral phase during crust formation.
WANG GuozhiJANSA LubaCHU FengyouZOU CanSUN Guosheng
长期以来,成矿时代的精确限定是困扰MVT铅锌矿床研究中最大难题之一。随着放射性同位素定年方法精度的提高和制样工艺的改进,闪锌矿等硫化物Rb-Sr同位素等时线法已成为可以直接测定MVT矿床成矿年龄的有效方法(Nakai and Halliday,1993)。前人采用这一方法对分布于四川盆地北缘米仓山一带的马元铅锌矿已进行过Rb-Sr同位素定年(李厚民等,2007)。该矿床与古油藏密切共生,
油气藏成藏时间的确定一直是石油地质研究领域的难点。该文利用磷灰石裂变径迹测龄及温度-时间热史模拟技术探索了如何精确限定油气藏的成藏时间。从渤海蓬莱9-1潜山花岗岩储层裂缝带和基岩带分别获取2件磷灰石样品,根据磷灰石裂变径迹的长度、年龄和径迹分布,利用AFT Solve软件模拟获得了它们的温度-时间热史轨迹。2件样品相似的温度-时间热史轨迹表明,在3.8-2.6 Ma B.P.期间,由于受石油流体充注的影响,样品发生异常热扰动和温度突变,在3.2 Ma B.P.时期受扰动的温度达到最大。根据受异常热扰动的起始时间,确定蓬莱9-1潜山花岗岩石油充注时间为3.8-2.6 Ma B.P.,油气藏的主成藏期为3.2Ma B.P.。
