古海洋氧化还原状态及其演化：来自霍邱BIF铁同位素研究的证据

As the direct metallogenic element, the isotope system of iron has recorded redox state of the ocean during the banded iron formation (BIF) deposition, and could trace the Fe sources. Our preliminary studies proposed that the Huoqiu BIF, located in the southeastern margin of the North China Craton (NCC), is characterized by no Ce negative anomalies and distinct positive Fe isotope compositions, implying that the ocean is in anoxic state during the iron ore deposition. In combination with Nd isotope and rare earth element (REE) systems, we attempt to provide further constraints on the Fe sources and metallogenic environments of the Huoqiu BIF. On the other hand, the Fe isotopes of the Fe-hosting minerals from BIF with dissimilar ages could reflect the redox state evolutional rule of the ocean, this subject plan to trace the early evolution of redox state of the Precambrian ocean by integrated Fe isotope study of single minerals of global BIF with different ages. The Fe isotopes in mineral scale are superior to those in whole-rock scale due to: (1) be easier achieve Fe isotope equilibrium with seawater; (2) no need to consider the Fe isotope fractionation between distinct mineral species. In the early stage, we collected abundant Fe isotope data of magnetite and hematite from global BIF with different ages, preliminary proposed that the oxygen fugacity of the ocean occurs a progressive increase during 2.7-2.2 Ga, very differs from atmosphere showing a huge jump in the oxygen at 2.3 Ga, yet this need further work to verify.

作为成矿元素，铁的同位素体系记录了BIF成矿时的海洋环境，且可以示踪铁物质来源。申请人前期研究表明，霍邱BIF具有缺乏Ce负异常和铁同位素成分为正值的特征，暗示霍邱铁矿沉淀时海水为缺氧环境。结合钕同位素和稀土元素体系，本项目拟对霍邱BIF的成矿物质来源和成矿海底环境作出进一步的约束。另一方面，不同时代BIF含铁单矿物中的铁同位素成分反映了古海水氧化还原状态的演化规律，本项目拟通过全球不同时代BIF中含铁单矿物铁同位素的全面研究，示踪前寒武纪海洋氧化还原状态的早期演化。单矿物铁同位素相对全岩具有如下优点：(1) 更易与海水达到平衡；(2) 不需要考虑不同矿物种间的铁同位素分馏。申请人在前期收集了大量不同时代BIF中的磁铁矿、赤铁矿铁同位素数据，初步认为2.7-2.2 Ga期间，古海水中的氧逸度是逐步增加的，不同于大气中的氧气在2.3 Ga时的突然跃升模式，这需要更多工作验证。

作为成矿元素，铁的同位素体系记录了BIF成矿时的海洋环境，且可以示踪铁物质来源。霍邱BIF具有缺乏Ce负异常和铁同位素成分为正值的特征，暗示霍邱铁矿沉淀时海水为缺氧环境。结合铁同位素和稀土元素体系，本项目对霍邱BIF的成矿物质来源和成矿海底环境作出了进一步的约束。不同时代BIF含铁单矿物中的铁同位素成分反映了古海水氧化还原状态的演化规律，本项目分析了20个霍邱BIF中含铁单矿物的铁同位素成分，发现：镜铁矿的铁同位素变化最小，但值较高（0.45-0.55 ‰,相对标准IRMM-014,后同），磁铁矿铁同位素变化较大0.08-0.77 ‰），偏轻的值多，大部分磁铁矿小于围岩中磁铁矿的铁同位素成分（0.29 ‰），有趣的是含碳或碳酸盐矿物的BIF中磁铁矿的铁同位素已经到负值（-0.46 to -0.09 ‰）。霍邱单矿物铁同位素的结果说明：（1）铁生物歧化还原反应（DIR）在霍邱的含碳或碳酸盐BIF中明显的降低了原始含铁矿物（本应偏重）的铁同位素；（2）苏必利尔湖型BIF的原始铁同位素值往往更能反映古海洋的铁同位素状态，而霍邱就是一例，从镜铁矿的铁同位素（更有可能是原始BIF的铁同位素）推测古海水的铁同位素。.对霍邱和舞阳地区的BIF铁矿及共生的岩石样品开展了全岩主微量元素分析，共获得数据30条。为了验证舞阳BIF与霍邱BIF是否属于同一成矿系统，对舞阳BIF开展了定年工作，获得有效碎屑锆石年龄204条。通过岩浆成因的最小碎屑锆石年龄峰限定了舞阳BIF沉积的时代上限2.60Ga,而最大变质成因的碎屑锆石年龄峰限定了舞阳BIF沉积的时代下限为2.45Ga。通过地球化学特征对比，初步认定舞阳BIF属于苏比利尔湖型BIF,以大陆滨海相沉积成矿为主，铁质来源有三个：热液，海水，河水。部分舞阳BIF样品中显示明显的Ce负异常，这一异常说明新太古代时部分近岸海表海水的氧逸度已经升高。结合霍邱，舞阳以及新蔡BIF的相似成矿时代和地球化学特征，本项目获得以下重要认识：华北克拉通南缘可能存在一条新太古代时期的苏必利尔湖BIF成矿带。