Numerical models of P-T, time and grain-size controls on Ar diffusion in biotite: An aide to interpreting 40Ar/39Ar ages
Numerical models of P-T, time and grain-size controls on Ar diffusion in biotite: An aide to interpreting 40Ar/39Ar ages
复制标题
P-T、时间和粒度控制黑云母中 Ar 扩散的数值模型：解释 40Ar/39Ar 年龄的助手
DOI：
10.1016/j.chemgeo.2018.06.005
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发表时间：
2018
期刊：
影响因子：
3.9
通讯作者：
Skipton D
中科院分区：
文献类型：
--
作者：
Skipton D
用40Ar/39Ar方法测定黑云母的年龄被广泛用于确定变质岩中冷却和折返的时间。对40Ar/39Ar年龄的解释通常假设40Ar通过与温度有关的体积扩散从黑云母中扩散出来，因此该日期代表了黑云母通过封闭温度冷却的时间。然而，有几个过程可能会扰乱Ar系统，使得40Ar/39Ar日期不能唯一地表示通过关闭温度冷却的时间，包括Ar系统的不完全重置、过量Ar的加入、充当Ar陷阱或快速通道的晶体缺陷，或存在流体的再结晶/溶解。我们给出了一系列扩散模型的数值结果，这些结果表明，在一个仅通过体积扩散失去Ar的理想开放系统中，理论上应该保留在不同颗粒半径的黑云母中的放射性Ar的百分比在一定的P-T条件下停留不同的时间。第二组模型展示了在一个完美的开放系统中，不同的冷却速度对生物活性、40Ar/39Ar年龄和颗粒内Ar分布的影响。该模型的结果对于约束不同变质环境下40Ar/39Ar黑云母数据的冷却和折返历史是有用的。它们还提供了生物活性40Ar保留量、40Ar/39Ar年龄和颗粒内年龄分布的基线数据，这些数据理论上是由体积扩散单独作用产生的。因此，这些模型可以帮助评估可能影响40Ar/39Ar日期的替代情景的合理性，包括外来的Ar污染或通过扩散以外的过程造成的Ar损失。结合良好约束的成岩历史，数值扩散模型是解释40Ar/39Ar黑云母年龄的有力工具，特别是当与粒内40Ar/39Ar年龄剖面相联系时。
Dating of biotite using the40Ar/39Ar method is used extensively to determine the timing of cooling and exhumation in metamorphic rocks. Interpretations of40Ar/39Ar dates commonly assume that40Ar diffuses out of biotite through temperature-dependent volume diffusion, and therefore that the date represents the time at which biotite cooled through a closure temperature. Several processes, however, may perturb Ar systematics such that the40Ar/39Ar date does not uniquely represent the timing of cooling through a closure temperature, including incomplete re-setting of Ar systematics, incorporation of excess Ar, crystal defects acting as Ar traps or fast-pathways, or fluid-present recrystallization/dissolution. We present a series of numerical diffusion model results that show the percentage of radiogenic Ar that should theoretically be retained in biotite with different grain radii residing for various periods over a range of P–T conditions, in a perfect open system that loses Ar via volume diffusion alone. A second set of models demonstrates the effects of different cooling rates on biotite40Ar/39Ar dates and intra-grain Ar distributions in a perfect open system. The model results are useful for constraining cooling and exhumation histories from40Ar/39Ar biotite data in a variety of metamorphic settings. They also provide baseline data for biotite40Ar retention,40Ar/39Ar ages and intra-grain age distributions that would theoretically be produced from volume diffusion acting alone. Consequently, the models can help evaluate the plausibility of alternative scenarios that may have affected biotite40Ar/39Ar dates, including extraneous Ar contamination or Ar loss via processes other than diffusion. In conjunction with well-constrained petrogenetic histories, numerical diffusion models are a powerful tool for interpreting40Ar/39Ar biotite ages, especially when linked with intra-grain40Ar/39Ar age profiles.