Oxygen diffusion in monazite
Oxygen diffusion in monazite
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DOI：
10.1016/j.epsl.2004.07.027
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发表时间：
2004-09-30
影响因子：
5.3
通讯作者：
Watson, EB
中科院分区：
文献类型：
--
作者：
Cherniak, DJ;Zhang, XY;Watson, EB
我们报道了在干燥、大气压和水热条件下天然独居石中氧扩散的测量结果。在干燥实验中，富含O-18的CePO4粉末和独居石晶体被密封在带有固体缓冲液的Ag-Pd胶囊中(在NNO下缓冲)，并在1-ATM炉中进行退火热处理。水热运行在冷封压力容器中进行，独居石颗粒被富含O-18的水包裹。在扩散退火后，利用O-18(p，α)N-15反应，用核反应分析(NRA)测量了氧浓度分布。在850-1100℃温度范围内，独居石的干扩散实验所确定的阿累尼乌斯关系式为：D-Dry=1.9×10(-6)exp(-356+/-26kJ mol(-1)/RT)m(2)S(-1)无扩散各向异性的证据。氧的扩散可用阿累尼乌斯关系式来描述：D-WEW=3.1×10(-17)exp(-100+/-20kJ mol(-1)/RT)m(2)S(-1)在800℃的水热条件下，氧的扩散在10-160 Mpa范围内几乎不依赖于P-H2O。鉴于活化能的这些差异，干扩散速率和湿扩散速率之间的差异随着温度的降低而增加；例如，在600℃时，干扩散将比水热条件下的扩散慢4个数量级以上。这些不同的扩散系数将导致氧同位素特征保留程度的显著差异。例如，在干燥条件下(可能在地壳中很罕见)和较高的下地壳温度(类似于800摄氏度)，半径为70微米的独居石岩芯将保持氧同位素比率约50万年；相比之下，它们将在这种温度下在潮湿条件下保持约15,000年。(C)2004爱思唯尔B.V.保留所有权利。
We report measurements of oxygen diffusion in natural monazites under both dry, 1-atm conditions and hydrothermal conditions. For dry experiments, O-18-enriched CePO4 powder and monazite crystals were sealed in Ag-Pd capsules with a solid buffer (to buffer at NNO) and annealed in 1-atm furnaces. Hydrothermal runs were conducted in cold-seal pressure vessels, where monazite grains were encapsulated with O-18-enriched water. Following the diffusion anneals, oxygen concentration profiles were measured with Nuclear Reaction Analysis (NRA) using the reaction O-18(p,alpha)N-15. Over the temperature range 850-1100 degreesC, the Arrhenius relation determined for dry diffusion experiments on monazite is given by:D-dry = 1.9 X 10(-6) exp(-356+/-26 kJ mol(-1)/RT) m(2) s(-1)There is no evidence of diffusional anisotropy.Under wet conditions at 100 MPa water pressure, over the temperature range 700-880 degreesC, oxygen diffusion can be described by the Arrhenius relationship:D-wet = 3.1 X 10(-17) exp(-100+/-20 kJ mol(-1)/RT) m(2) s(-1)Under hydrothermal conditions at 800 degreesC, oxygen diffusion shows little dependence upon P-H2O over the range 10-160 MPa.en diffusion under hydrothermal conditions has a significantly lower activation energy for diffusion than under dry conditions, as has been found the case for many other minerals, both silicate and nonsilicate. Given these differences in activation energies, the differences between dry and wet diffusion rates increase with lower temperatures; for example, at 600 degreesC, dry diffusion will be more than 4 orders of magnitude slower than diffusion under hydrothermal conditions. These disparate diffusivities will result in pronounced differences in the degree of retentivity of oxygen isotope signatures. For instance, under dry conditions (presumably rare in the crust) and high lower-crustal temperatures (similar to800 degreesC), monazite cores of 70-mum radii will preserve O isotope ratios for about 500,000 years; by comparison, they would be retained at this temperature under wet conditions for about 15,000 years. (C) 2004 Elsevier B.V. All rights reserved.