Effect of pressure on closure temperature of a trace element in cooling petrological systems
Effect of pressure on closure temperature of a trace element in cooling petrological systems
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DOI：
10.1007/s00410-016-1327-8
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发表时间：
2017-01
影响因子：
3.5
通讯作者：
Yan Liang
中科院分区：
文献类型：
--
作者：
Yan Liang
闭合温度对许多涉及冷却的扩散相关问题很重要。在恒压条件下，建立了岩石学系统冷却的经典Dodson模型及其修正模型。许多岩石学过程涉及温度和压力的变化。Dodson的模型没有考虑压力变化对冷却岩石学系统中扩散损失的影响。在上升流期间，减压速率通过上升流路径的斜率与冷却速率有关。通过注意温度和压力都作为时间的函数沿着上升流路径降低，得到了冷却-上升流单矿物和双矿物系统的平均或平均闭合温度和闭合压力的简单解析表达式。这些压力-如果将等压公式中扩散的活化能和指前因子替换为路径依赖的活化能和指前因子，则调整后的方程与等压情况下的闭合温度方程几乎相同。后者也取决于上升流路径的斜率。上升流过程中压力和温度对扩散的竞争效应导致扩散的有效活化焓和分配的交换焓降低，这反过来又导致闭合温度与恒压情况下的系统偏差。对于扩散激活体积较大的体系，可以从选定元素的闭合温度和闭合压力推断出上升流路径和上升流速率。封闭温度和封闭压力的例子，稀土元素在石榴石和单斜辉石和石榴石-单斜辉石聚集体的扩散，并在未成年人的规则和稀土-石榴石-单斜辉石温压计的背景下进行了讨论。石榴石-单斜辉石聚集体中重稀土元素的封闭温度主要受单斜辉石中的扩散控制，除非石榴石的模态丰度很小或单斜辉石的有效粒度远小于石榴石。
Closure temperature is important to many diffusion-related problems involving cooling. The classic model of Dodson and its modifications for cooling petrological systems are formulated at constant pressure. Many petrologic processes involve changes in both temperature and pressure. The effect of changing pressure on diffusional loss in cooling petrological systems has not been considered in Dodson’s model. During upwelling, the decompression rate is related to the cooling rate through the slope of the upwelling path. Simple analytical expressions for the average or mean closure temperature and closure pressure in cooling-upwelling mono-mineralic and bi-mineralic systems are obtained by noting that both temperature and pressure decrease as a function of time along the upwelling path. These pressure-adjusted equations are nearly identical to closure temperature equations for isobaric cases if one replaces the activation energy and pre-exponential factor for diffusion in the isobaric formulations by the path-dependent activation energy and pre-exponential factor. The latter also depend on the slope of the upwelling path. The competing effects between pressure and temperature on diffusion during upwelling result in reductions in the effective activation enthalpy for diffusion and exchange enthalpy for partitioning, which in turn leads to systematic deviations in closure temperatures from cases of constant pressure. For systems with large activation volume for diffusion, it may be possible to deduce upwelling path and upwelling rate from closure temperatures and closure pressures of selected elements. Examples of closure temperature and closure pressure for REE diffusion in garnet and clinopyroxene and in garnet–clinopyroxene aggregates are presented and discussed in the context of the minor’s rule and the REE-in-garnet–clinopyroxene thermobarometer. Closure temperatures for middle-to-heavy REE in garnet–clinopyroxene aggregates are controlled primarily by diffusion in clinopyroxene unless the modal abundance of garnet is very small or the effective grain size of clinopyroxene is considerably smaller than that of garnet.