“碳-水-稀土”组合深循环可能途径：羟基稀土碳酸盐高压实验模拟研究

Deep carbon/water cycles have long been attracting the attention of global geologists, but a possible deep cycling of rare earth elements (REE) has regrettably been neglected. The carbonatite REE deposits are probably of the product of deep REE cycle. The sedimentary carbonate rocks (including organic matters) and REE-rich nodules / crust / clay constitute a binary cover on the ocean basalt. When it is carried by ocean plate with sea-water into the subduction zone, under suitable conditions, these materials might be taken into the deep earth by minerals like hydroxyl rare earth element carbonate, and then through recombination, return to the superficial crust in the forms of carbonatite REE deposit / hydroxyl rare earth element carbonate. The present proposal suggests to carry out a series of high temperature high-ultrahigh pressure experiments with starting materials containing “C+H+O+REE”, synthesizing hydroxyl rare earth element carbonate crystals and investigating their structural stability, compressibility and phase variation process. On these bases, We try to explore the depth of “C+H+O+REE” cycling with hydroxyl rare earth element carbonate crystals as carriers and their returning mode to superficial crust.

地球深部碳/水循环是当前地球科学研究的热点课题，而稀土元素深循环迄今未引起应有的重视。许多碳酸岩型稀土矿床可能就是稀土元素深循环的产物。洋底沉积碳酸盐岩（包括有机质）和富稀土结核/结壳/黏土物质构成的“碳酸盐（有机质）+稀土元素”二元盖层和海水一起进入低温高压俯冲带后，在合适条件下，便能以羟基稀土碳酸盐为介质深入地幔，并通过重新组合后以碳酸岩型稀土矿床/羟基稀土碳酸岩形式返回地壳浅部。本项目拟以含“碳+氢+氧+稀土元素”的初始实验物料，在高温高压下合成羟基稀土碳酸盐，通过升温升压、冷却降压等过程，研究羟基稀土碳酸盐的结构稳定性及其相变过程；探讨俯冲板块以羟基稀土碳酸盐或其变化后的新物相为载体，可能将碳、稀土和水等物质带入地球的深度及碳、稀土和水返回地表的可能途径。

地球深部碳/水循环是当前地球科学研究的热点课题，稀土元素深循环迄今未引起应有的重视。许多碳酸岩型稀土矿床可能就是稀土元素深循环的产物。本项目以含“碳+氢+氧+稀土元素”的实验物料为初始物质，在高温高压下合成了羟基稀土碳酸钐、钕和镧，利用高温高压生成装置，通过升温升压分析并研究了合成物从“碳+氢+氧+稀土”至羟基稀土碳酸盐的生成过程，并探讨其在高温高压下的结构稳定性，用以探讨合成物在板块俯冲带环境稳定存在并携带碳、氢、稀土元素进行深循环的能力，进而探讨三种物质参与成矿的可能性。实验结果表明，羟基稀土碳酸镧在1GPa，400℃已经存在，直至5.0GPa，1000℃才开始出现部分分解产物。羟基稀土碳酸钐和羟基稀土碳酸钕都有证据可以在3.0GPa，600℃稳定存在并形成大颗粒单晶，并至少稳定存在于5.75GPa，800℃。实验数据证明，海底富碳酸盐和稀土元素及水的初始物质在随冷俯冲板块进入地球深部后，可以形成羟基稀土碳酸钐、钕和镧。羟基稀土碳酸钐和羟基稀土碳酸钕在高温高压下的结构变化表明，这些羟基稀土碳酸盐矿物可以携带水、稀土和碳进入至少地球软流圈附近，并进一步参入包括稀土元素等关键金属的成矿过程。。