| 地幔矿物中稀土元素扩散动力学的理论研究—以石榴子石为例 |
| 张航妥
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| 导师 | 苏根利
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| 2013
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| 学位授予单位 | 中国科学院研究生院
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| 学位授予地点 | 北京
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| 学位名称 | 硕士
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| 学位专业 | 矿物学、岩石学、矿床学
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| 关键词 | 石榴子石
稀土元素
扩散动力学
模拟计算
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| 摘要 | 地幔矿物中微量元素的浓度不仅受平衡分配的制约，还与元素的扩散特性有关。了解扩散行为对微量元素浓度的影响对认识幔源矿物及岩浆成分及其演化史有着重要的意义。本文通过建立模型采用球形扩散方程对地幔矿物石榴子石中稀土元素的浓度剖面及轻重稀土元素的分异情况进行了扩散动力学模拟计算。研究结果表明：1. 石榴子石中稀土元素扩散造成的浓度及分异受温度、压力、颗粒半径、时间、矿物组成及冷却速率的影响有不同程度的变化。2. 石榴子石中稀土元素受扩散作用影响时，短时间内只能影响到颗粒边部元素的浓度，而核心部的元素浓度变化需要的时间很长。这对判断矿物内部是否保留原始稀土元素组成是有意义的。3. 扩散影响下石榴子石与熔体之间化学相互作用能够形成稀土元素分带。对分带进行拟合，可以估算稀土元素在不同温度、压力等条件下的扩散距离和扩散时间。4. 扩散影响下石榴子石中稀土元素质量丢失分数受温度、颗粒半径、扩散时间及矿物组成的影响较大，而对压力的依赖较小。这可以评价稀土元素在不同矿物颗粒中保存需要的时间、温度等条件。5. 扩散影响下石榴子石中稀土元素浓度发生变化导致了元素之间的分异，这种分异是不可忽视的，例如在温度1300℃、压力2GPa、颗粒半径1mm、Mg含量在0.25~0.35之间时，石榴子石颗粒核心到边缘位置最大分异时?(CLa/CYb)约为14%，中心处为4%，而?(CNd/CSm)约为2%，中心处为0.8%。这对元素分异的成因及规模以及地质年代学、地质速率计的应用中都是有意义的。6. 在不同岩浆冷却速率下，冷却速率越快，颗粒边部稀土元素浓度丢失越快。而冷却速率减慢，随时间延长，颗粒内部元素浓度丢失的更多。7. 稀土元素扩散速率与离子半径有关，在仅考虑扩散作用影响下，稀土元素配分线近似一条直线，并且岩浆冷却速率越慢，扩散时间越长，直线更加倾斜，会造成轻稀土亏损、重稀土富集的现象。8. 扩散影响下稀土元素在石榴子石与熔体之间分配同时受分配系数制约时扩散丢失量会减慢。因此在推断地幔源区特征及稀土元素配分模式时应同时考虑分配系数和扩散共同的影响才能得出准确的判断。 总的来说，地幔矿物内稀土元素浓度随温度、压力、颗粒半径、矿物组成、冷却速率等因素在经历不同的地质时间尺度后有不同程度的变化。在漫长的地质时间内，扩散导致稀土元素浓度及质量的丢失是不容忽视的。此次模拟揭示了元素在矿物中的扩散行为是石榴子石中微量元素分布的一个重要成因，并且影响着稀土元素的配分模式。 |
| 其他摘要 | The concentration of elements in minerals not only depends on equilibrium partitioning, but is related to its diffusion behavior. It is important to understand influence of diffusion behavior on mantle derived magma composition and its evolution. In this paper, REE concentration profiles and fraction between LREE and HREE in the garnet were respectively calculated using spherical diffusion model. It can be drawn from this study as follows: (1) Diffusion of REE and fraction between LREE and HREE in garnet are constrained by temperature, pressure, grain radius, time, minerals compositions and cooling rate; (2) Rim components of garnet affected by diffusion are subject to change over a short timescale, whereas core compositions are preserved for a long timescale. It is important to determine whether initial compositions of mineral are preserved or not; (3) REE zonings are established by chemical interaction between garnet and melt affected by diffusion. Based on fitting of zoning, diffusion distance and diffusion time of REE at different temperature and pressure conditions can be estimated; (4) The mass loss fraction of REE in garnet affected by diffusion depends on temperature, grain radius, diffusion time and minerals composition larger than on pressure. It evaluates time and temperature condition at which REE are preserved among minerals; (5) Elements fractions are caused by REE concentration change as a result of diffusion and its fraction can not be neglected. Such as, the maximum fractional value of ?(CLa/CYb) and ?(CNd/CSm) reaches approximately 14% and 2% respectively from its core to rim, whereas its core is only 4% and 0.8% respectively at a temperature of 1300℃and pressure of 2 GPa with a 1mm radius crystal and Mg content in the range of 0.25~0.35; (6) Elements concentration from the crystal rim will be subject to very rapid loss with rate of cooling during different cooling rate of melt, whereas its internal composition will suffer from more loss with decreasing the cooling rate and increasing timescale; (7) Diffusion rate of REE is related to ion radius. Distribution curve of REE is approximately a linear only affected by diffusion, and this linear will be more steep with decreasing the cooling rate and increasing timescale, which result in depletion of LREE and enrichment of HREE; (8) REE distribution between garnet and melt is constrained by distribution coefficient and its concentration can be subject to a lower loss low. Therefore, feature of mantle source and pattern of REE can be inferred accurately by considering both influence of distribution coefficient and diffusion; Overall, REE concentration in mantle minerals depends on temperature, pressure, grain radius, mineral composition and cooling rate during different geological timescale. During a very long geological timescale, it can not be neglected that diffusion results in the loss of concentration and mass of REE. This simulation indicates diffusion behavior of element in minerals is an important controlling factor of zoning of trace elements in the garnet, and constrains on distribution pattern of REE in the garnet. |
| 学科领域 | 地球深部物质与流体作用地球化学
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| 语种 | 中文
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| 文献类型 | 学位论文
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| 条目标识符 | http://ir.gyig.ac.cn/handle/352002/5824
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| 专题 | 研究生_研究生_学位论文
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推荐引用方式
GB/T 7714 |
张航妥. 地幔矿物中稀土元素扩散动力学的理论研究—以石榴子石为例[D]. 北京. 中国科学院研究生院,2013.
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