古代冰碛岩中钼元素赋存矿物及其古氧化还原环境意义

In recent years, it is well established that glacial diamictites can be used to trace the changing composition of upper continental crust through time and furtherly the information of paleoenvironment. Previous studies on glacial diamictites show that all glacial diamctites after the Paleoproterozoic and few paleoproterozic samples exhibit systematic depletion in bulk rock Mo concentration. Mo is a redox sensitive element used as a tracer in paleoenvironmental studies, with interpretation of Mo depletion in crustal rocks in oxygen-abundant atmosphere based on the assumption that crustal Mo resides primarily in sulfides where it can be easily dissolved and transported into marines by oxidative weathering. Systematic depletion of Mo observed in glacial diamictites has been interpreted to reflect the effects of oxidative weathering. However, the assumption that Mo resides primarily in sulfides hasn't been testified, thus it's very important to verify if Mo depletion found in younger glacial diamictites is controlled by oxidative weathering of sulfides. To test the assumption, explore Mo behavior and find out the host minerals of Mo in glacial diamictites and finally unveil the mechanism of Mo tracing paleo-redox conditions, we will systematically analyze Mo concentrations of minerals (e.g., sulfides, oxides, silicate minerals) and silicate matrix in glacial diamictites, collected from different areas, aging from Archean to Paleozoic. Based on above studies, combined with the bulk rock Mo contents in glacial diamictites, we will furtherly explore the past redox states of ancient atmospheres and marines, especially carefully trace the paleo-redox condition around and before Great Oxidation Event (GOE).

近年来，冰碛岩在示踪大陆地壳组成及演化、恢复地质历史时期古环境等方面取得创新性认识。前期研究发现，所有古生代、新元古代和部分古元古代冰碛岩系统性地亏损钼元素，而其他古元古代和所有中太古代冰碛岩不亏损钼。钼元素作为氧化还原敏感元素可以示踪古氧化还原环境：传统观点认为地壳钼元素主要赋存于硫化物中，并随赋存矿物（硫化物）被氧化风化进而运移至海洋，导致地壳岩石中钼元素亏损。前期研究中发现的系统性钼亏损反映了冰碛岩受氧化风化作用的影响。但钼元素赋存于硫化物的假说并未得到验证，故亟待证实的是：冰碛岩的钼亏损是否源于硫化物氧化风化作用。本项目拟选取不同地区、太古代到古生代的冰碛岩样品，系统分析样品中不同矿物（硫化物、氧化物和硅酸盐矿物）和基质中的钼元素含量，查明冰碛岩中钼元素赋存矿物，揭示钼元素示踪古氧化还原环境的机理，探索古海洋及古大气（尤其是大氧化事件附近及其之前）的氧化还原环境。

本项目选取了不同地区太古代到古生代的冰碛岩样品，系统测定了太古代-古生代冰碛岩中硫化物类型、含量及化学组成，分析了样品中不同矿物硫化物和基质中的钼元素含量，估算了样品中全岩钼在硫化物和基质中的分配比例，以此确定上地壳钼元素是否主要赋存于硫化物中。硫化物产在太古代和古元古代及一个年轻的显生宙冰碛岩样品中，该显生宙样品显示古老的源区特征。据冰碛岩中硫化物形态及组成特征，将其分为碎屑、沉积成岩及后生硫化物三类。碎屑硫化物多呈圆形，矿物种类多样（包括辉钼矿颗粒），以单矿物或矿物组合形式存在。沉积成岩硫化物主要为黄铁矿，包括草莓状黄铁矿及非草莓状-结构的自生黄铁矿。后生硫化物主要由不规则磁黄铁矿集合体、晚期自生黄铁矿和沿裂缝生长的黄铁矿集合体组成。高钼浓度（高达230ppm）出现在中太古代Coronation组和古元古代Makganyene组草莓状黄铁矿、古元古代Ramsay Lake组磁黄铁矿集合体、Timeball Hill组碎屑硫化物中。其他碎屑硫化物中钼浓度变化很大（0.5–36 ppm）。质量平衡计算表明，除Makganyene组外，所有样品中硫化物钼含量占全岩钼含量的比例均<8%。因此GOE前的硫化物（含辉钼矿）不可能是地壳钼元素的主要宿主矿物。Makganyene组冰碛岩，硫化物类型主要为沉积时生长在基质中的草莓状黄铁矿，约37%全岩钼赋存于硫化物。Makganyene组草莓状黄铁矿微晶大小与钼含量高低密切相关：微晶较小的草莓状黄铁矿明显含有高浓度钼元素，微晶较大的草莓状黄铁矿含有较低浓度钼元素。相关性表明，草莓状黄铁矿快速成核、生长可大量吸收孔隙水中存在的钼，或者草莓状黄铁矿重结晶时排出钼。GOE前Coronation组冰碛岩中的沉积成岩过程中形成的黄铁矿中大量钼的存在表明，GOE前海水及相关孔隙流体中存在Mo和S，表明在GOE前的古大气中已出现氧气上升-下降事件及大陆氧化风化作用。而GOE前冰碛岩缺乏钼亏损，又限制了GOE前氧化风化作用释放的钼元素含量。