Redox-controlled dissolution of monazite in fluids and implications for phase stability in the lithosphere
Redox-controlled dissolution of monazite in fluids and implications for phase stability in the lithosphere
复制标题
流体中独居石的氧化还原控制溶解及其对岩石圈相稳定性的影响
DOI：
10.2138/am-2018-6296
复制
发表时间：
2018
影响因子：
3.1
通讯作者：
D. Trail
中科院分区：
文献类型：
--
作者：
D. Trail
摘要独居石是岩石圈中稀土元素的重要寄主，包括氧化还原敏感型Ce，在陆相环境中可能以三价和四价形式存在。本文通过在925℃、1.5 GPa的富碱和H2O流体中进行一系列溶解实验，探讨了独居石溶解度与氧逸度的关系。氧逸度是由七种不同的固体缓冲液控制的，范围从铁-钨钛矿到磁铁矿-赤铁矿平衡反应以上。天然独居石的溶解度在氧逸度等于或高于费雅石-磁铁矿-石英平衡时单调增加。电镜显示Ni-NiO及以上的不一致溶解，其中ce -氧化物与独居石作为稳定相。还对合成晶体（CePO4、LaPO4和Th+ si掺杂的独居石）进行了溶解度实验。端元CePO4在氧化条件下晶体表面发生了深刻的变化，晶体表面的侵蚀深度达到~100 μm或更大，并伴有Ce-oxide的沉淀。相反，LaPO4的溶解度对实验的氧化还原状态不敏感。在氧化条件下，添加Th （~ 3wt %）和Si （~0.3 wt%）促进了单独居石的晶体稳定性，尽管这些晶体表面存在较小的ThO2-CeO2 （5-10 μm）晶体，其丰度随着氧逸度的增加而增加。综上所述，这些实验表明，单氮石的稳定性和溶解度受氧逸度的影响，流体的氧化还原状态可能是地壳中稀土元素和磷重新分布的部分原因。岩石圈流体的氧逸度处于或高于滑石-磁铁矿-石英平衡，这可能有助于在天然独居石中观察到一些复杂的结构、可变的化学成分和年龄关系。
Abstract Monazite is an important host of rare earth elements in the lithosphere including redox-sensitive Ce, which may occur as trivalent and tetravalent in terrestrial environments. Here, monazite solubility is explored as a function of oxygen fugacity through a series of dissolution experiments in alkali-rich and H2O fluids at 925 °C and 1.5 GPa. The oxygen fugacity was controlled with seven different solid-state buffers and ranged from about the iron-wüstite to above the magnetite-hematite equilibrium reactions. The solubility of natural monazite increases monotonically at oxygen fugacities equal to or higher than the fayalite-magnetite-quartz equilibrium. Electron microscopy reveals incongruent dissolution at Ni-NiO and above, where Ce-oxide is observed with monazite as a stable phase. Solubility experiments were also conducted with synthetic crystals (CePO4, LaPO4, and Th+Si-doped monazite). End-member CePO4 exhibits profound changes to the surface of the crystal under oxidized conditions, with erosion of the crystal surface to depths of ~100 μm or greater, coupled with precipitation of Ce-oxide. In contrast, the solubility of LaPO4 shows no sensitivity to the redox state of the experiment. The addition of Th (~3 wt%) and Si (~0.3 wt%) to monazite promotes crystal stability under oxidizing conditions, though small ThO2-CeO2 (5–10 μm) crystals are present on the surfaces of these crystals, whose abundance increases at higher oxygen fugacities. In aggregate, these experiments show that the stability and solubility of monazite is affected by oxygen fugacity, and that the redox state of a fluid may be partially responsible for redistribution of rare earth elements and phosphorus in the crust. Lithospheric fluids with oxygen fugacities at or above the fayalite-magnetite-quartz equilibrium may contribute to some of the complex textures, variable chemistry, and age relationships observed in natural monazite.
影响因子：
5.3
作者：
S. Skora;J. Blundy
通讯作者：
S. Skora;J. Blundy