菱镁矿-铁体系生成超深金刚石的实验研究

Super-deep diamonds, specifically referred to as diamonds with a sublithospheric origin (>300 km), are of geological significance because they play a key role in kinetic processes of Deep Earth Carbon Cycle as well as a direct “window” of petrology, mineralogy and geochemistry of the deep Earth. Several mechanisms have been proposed to interpret super-deep diamond genesis but they are far too limited to constrain the nucleation and geodynamic specifics. In the present proposal we focus on formation mechanism of super-deep diamonds under the Earth’s mantle conditions by a multi-anvil apparatus and diamond anvil cells. Special attention will be paid on carbon isotope fractionation, nitrogen concentration and its aggregation pattern and composition of (Mg,Fe)O dependent on various pressure-temperature conditions. Besides, the retention time and subduction depth of crust-carbon will be estimated through dynamic calculations. Our results will provide experimental constraints for carbon mobilization and freezing during slab-mantle interactions and shed new light on mantle heterogeneity.

超深金刚石特指形成于地幔300km以下的金刚石，这些金刚石及其携带的矿物包裹体不仅是解析深部碳循环动力学过程的关键，也是揭示地球深部物质组成及状态的直接窗口。然而，超深金刚石的成因机制仍存在争议，其根源是对其结晶环境和俯冲带碳的反应动力学过程缺乏深入认知。本项目拟采用多面顶压机与金刚石压腔装置模拟过渡带-下地幔环境、以深俯冲带壳幔反应为研究对象开展相关实验研究，为探索超深金刚石的成因环境、解释其在化学成分（异常轻的13C值和极低的氮含量）和包裹体（如方镁铁矿(Mg,Fe)O）等方面的特征提供约束依据。此外，结合动力学计算获得俯冲带碳的存留时间和俯冲深度。结合前期实验探索研究结果，本项目的研究有望获得深俯冲带碳的迁移和演变行为，揭示地幔的物质组成和环境条件，还能够帮助理解洋壳深俯冲对深部地幔不均一性的影响。

超深金刚石特指形成于地幔300 km以下的金刚石，这些金刚石及其携带的矿物包裹体不仅是解析深部碳循环动力学过程的关键，也是揭示地球深部物质组成及状态的直接窗口。根据超深金刚石的化学成分、内部结构和包裹体特征等推知：超深金刚石结晶于洋壳深俯冲过程中形成的含铁碳酸盐熔体中。本项目采用多面顶压机与金刚石压腔装置模拟上地幔底部-地幔过渡带-下地幔顶部的极端温度-压力环境、以深俯冲带与碳相关的物质循环为研究对象，获得了多种含碳矿物的结构稳定性和弹性性质以及碳酸盐-铁体系和碳酸盐-铁-硫体系的氧化还原反应过程和产物特征，确定了深部碳的迁移演变行为和超深金刚石的形成机制，建立了超深金刚石的氮含量与成因环境的耦合关系以及产物方镁铁矿的成分对温度的依赖关系，为理解俯冲作用对地幔不均一性的贡献提供了关键理论支撑，最后估算了在全球俯冲背景下超深金刚石的产量，提出地幔过渡带是一个巨大的金刚石储库。