Intercrystalline stable isotope diffusion: a fast grain boundary model
Intercrystalline stable isotope diffusion: a fast grain boundary model
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DOI：
10.1007/bf00310783
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发表时间：
1992-12
影响因子：
3.5
通讯作者：
J. Eiler;L. Baumgartner;J. Valley
中科院分区：
文献类型：
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作者：
J. Eiler;L. Baumgartner;J. Valley
我们制定了一个稳定同位素相互扩散的数值模型，预测两种或两种以上的矿物之间记录的温度，以及在每种矿物中的稳定同位素的颗粒内分布，作为矿物粒度和形状，扩散率，模式，平衡同位素分馏，和岩石的冷却速率的函数。该模型的主要假设之一是晶界是稳定同位素快速传输的区域。该快速晶界（FGB）模型描述了任意数量的矿物颗粒之间的相互扩散，假设局部平衡和质量平衡限制适用于整个体积建模的晶界。该模型可用于包含任何数量的矿物的岩石，以及每种矿物的颗粒尺寸的数量，几种颗粒形状，以及任何热历史或所需的域尺寸。先前描述冷却时稳定同位素相互扩散的模型是基于Dodson方程或等效的数值模拟。Dodson的闭合温度是矿物和无限储层之间记录的平均整体温度。通过使用Dodson方程，这些模型将闭合温度视为给定矿物的固有特性，与其他矿物的数量和扩散速率无关。这样的模型不能准确地描述许多稳定同位素互扩散问题的质量平衡。现有的阳离子相互扩散模型可以应用于稳定同位素，但仅描述特定条件下两种矿物之间的交换。FGB计算的结果在许多感兴趣的岩石类型中与Dodson方程的预测有很大差异。使用FGB模型的实际计算表明，闭合温度和扩散曲线是模态丰度和扩散系数相对差异的强函数，因为它们是晶粒尺寸和冷却速率的函数。通过扩散交换稳定同位素的两种矿物之间记录的闭合温度是模态丰度和扩散系数差异的函数，并且可能与Dodson方程预测的温度相差数百摄氏度。两种矿物中的一种或两种都可以保持可检测的环带，在某些情况下，在扩散速度较快的矿物中环带可能更大。含有三种或三种以上矿物的岩石可以记录单独由封闭系统过程产生的大跨度分馏。FGB扩散模型的结果表明，扩散交换的影响必须进行评估之前，解释矿物分馏，一致或不一致，记录在任何岩石中，扩散可以在可观察到的尺度。该模型的预测适用于温度测量、开放或封闭系统回归的评估以及冷却速率或扩散系数的确定。
We formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals, and the intragranular distribution of stable isotopes in each mineral, as functions of mineral grain sizes and shapes, diffusivities, modes, equilibrium isotopic fractionations, and the cooling rate of a rock. One of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes. This Fast Grain Boundary (FGB) model describes interdiffusion between any number of mineral grains, assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled. The model can be used for a rock containing any number of minerals, and number of grain sizes of each mineral, several grain shapes, and any thermal history or domain size desired. Previous models describing stable isotope interdiffusion upon cooling have been based on Dodson's equation or an equivalent numerical analogue. The closure temperature of Dodson is the average, bulk temperature recorded between a mineral and an infinite reservoir. By using Dodson's equation, these models have treated the closure temperature as an innate characteristic of a given mineral, independent of the amounts and diffusion rates of other minerals. Such models do not accurately describe the mass balance of many stable isotope interdiffusion problems. Existing models for cation interdiffusion could be applied to stable isotopes with some modifications, but only describe exchange between two minerals under specific conditions. The results of FGB calculations differ considerably from the predictions of Dodson's equation in many rock types of interest. Actual calculations using the FGB model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate. Closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient, and may differ from that predicted by Dodson's equation by hundreds of degrees C. Either or both of two minerals may preserve detectable zonation, which may in some instances be larger in the faster diffusing mineral. Rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone. The results of FGB diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations, concordant or discordant, recorded within any rock in which diffusion could have acted over observable scales. The predictions of this model are applicable to thermometry, evaluation of open or closed system retrogression, and determination of cooling rates or diffusion coefficients.