Argon Diffusion in Feldspars
Argon Diffusion in Feldspars
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DOI：
10.1007/978-94-011-1106-5_11
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发表时间：
1994
影响因子：
5.4
通讯作者：
K. A. Foland
中科院分区：
文献类型：
--
作者：
K. A. Foland
Ar在长石中的扩散很重要，因为它对K-Ar年代学具有重要意义，尤其是随着斜长石和碱性长石在40Arp9Ar技术测年中的广泛使用，这一点变得尤为重要。尽管经过了30多年的研究和争论，长石的基本输运现象仍然存在相当大的不确定性。许多研究报道了释放动力学，但结果表明，表观基本参数如活化能的范围很大。虽然这些测量记录了长石的反应性和结构复杂性，但很少有人能明确地解释。对Arrhenius参数的最明确描述是通过对Benson Mines斜长石的测量提供的，该斜长石缺乏通常提供快速Ar传输路径的内部特征，并且物理颗粒尺寸是有效的扩散维度。在500-800℃的长期等温实验表明，Ar的损失符合产生线性Arrhenius关系的理论行为。对于同位素Ar输运的球形模型，扩散系数用频率因子0.00982 cm~2/s和活化能43.8kcal/m o l来描述。Ar在这个样品中的行为导致了几个概括。Ar的扩散似乎遵循基本理论所预测的动力学，只有一个单一的机制，尽管在高温和放气程度下出现了一些陷阱。输送动力学不受水活度升高的外部大气或大气气体存在的影响。虽然中子辐照会对缺陷浓度有很大影响，并可能促进或减缓迁移，但在超过600℃的温度下，它不会明显改变扩散速率。然而，39Ar的反冲重新分配对许多长石来说将是重要的。有几个理由可以预期，上面给出的阿累尼乌斯参数描述了本征Ar体积扩散。它们可以用来建模，并在承认操作机制的不确定性的情况下，外推到较低的温度。同时，增量加热脱气过程中的行为表现出重要的差异。在较高温度(超过约925℃)和较大的分数损失时，与简单的线性Arrhenius关系严重背离；这种严重影响似乎是由于长时间高温加热下的Ar陷阱。此外，由于裂纹和剥落导致的晶粒尺寸减小，Ar的释放速率在阶梯加热实验中得到了提高。捕集和粒度减小都对碱性长石实验室增量加热释放动力学及其用于模拟自然界中的Ar损失具有深远的意义。
Diffusion of argon in feldspar is important because of its significance to K-Ar geochronology and becomes especially so with increasingly widespread use of both plagioclase and alkali feldspar for dating by the 40 Arp9 Ar technique. Despite study and debate for more than 30 years, the basic feldspar Ar transport phenomena are subject to considerable uncertainty. Many studies report release kinetics but the results show a very large range in apparent fundamental parameters such as activation energy. While these measurements document the reactivity and structural complications of feldspars, few can be interpreted unambiguously. The most definitive description of Arrhenius parameters is provided by measurements on the Benson Mines orthoclase which is devoid of the internal features that commonly provide pathways for rapid Ar transport and for which physical grain size is the effective diffusion dimension. Long-term isothermal experiments at 500 to 800 C show Ar loss that conforms to theoretical behavior producing a linear Arrhenius relation. For the spherical model of isotopic Ar transport, diffusion coefficients are described by a frequency factor of 0.00982 cm2/sec and an activation energy of 43.8 kcal/mol. The behavior of Ar in this specimen leads to several generalizations. Diffusion of Ar appears to follow kinetics predicted by basic theory with a single mechanism although some trapping appears at high temperatures and degrees of outgassing. Kinetics of transport are not affected by the presence of external atmospheres of elevated water activity or atmospheric gases. Although neutron irradiation will have a great effect on defect concentration and may enhance or retard migration, it does not apparently alter diffusion rates at temperatures in excess of about 600 C. However, recoil redistribution of 39 Ar will be important for many feldspars. There are several reasons to expect that Arrhenius parameters given above describe intrinsic Ar volume diffusion. They may be used for modeling and, acknowledging uncertainty about the operative mechanisms, extrapolated to lower temperatures. At the same time, behavior during incremental-heating degassing shows important differences. There are serious departures from a simple linear Arrhenius relation at higher temperatures (in excess of about 925 C) and large fractional losses; this serious effect appears to be due to Ar trapping with heating at high temperature for long times. In addition, rates of Ar release are enhanced during step-heating experiments due to reduction in grain size as a result of cracking and spalling. Both trapping and grain size reduction have profound implications for alkali feldspar laboratory incremental-heating release kinetics and their use for modeling of Ar loss in nature.