When can muscovite 40Ar/39Ar dating constrain the timing of metamorphic exhumation?
When can muscovite 40Ar/39Ar dating constrain the timing of metamorphic exhumation?
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DOI：
10.1016/j.chemgeo.2011.09.017
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发表时间：
2012-01
期刊：
影响因子：
3.9
通讯作者：
C. Warren;F. Hanke;S. Kelley
中科院分区：
文献类型：
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作者：
C. Warren;F. Hanke;S. Kelley
用~(40)Ar/~(39)Ar年龄确定的变质冷却速率是许多构造模型的基础，但它们是基于几个简化的近似。我们证明，这些近似中的一些，特别是那些与指定单一“闭合温度”有关的近似，可能不适用于某些变质情景，特别是对于快速造山旋回的变质作用。利用数值扩散模型，包括最近报道的白云母中Ar扩散与压力的显著相关性，我们系统地询问了将40Ar/39Ar年龄与压力-温度历史联系起来的近似。模拟的结果以简单的图形形式给出，从而可以评估Ar扩散有效的压力-温度(PT)区，以及白云母可能产生真正的40Ar/39Ar冷却年龄的区域。我们认为，确定变质白云母40Ar/39Ar年龄是否与冷却时间有关的可靠方法包括：(1)确定变质旋回中白云母结晶的PT条件和相对时间；(2)收集白云母颗粒的高精度和高空间分辨率的40Ar/39Ar数据；(3)将分析数据与检验不同结晶后压力-温度历史的数值扩散模型进行比较。
Metamorphic cooling rates determined using muscovite40Ar/39Ar ages underpin many tectonic models, but are based on several simplifying approximations. We demonstrate that a number of these approximations, especially those relating to the assignment of a single “closure temperature”, might not hold for certain metamorphic scenarios, and particularly for metamorphism during rapid orogenic cycles. Using numerical diffusion models that include a recently reported significant pressure dependence of Ar diffusion in muscovite, we systematically interrogate the approximations associated with linking40Ar/39Ar dates to pressure–temperature histories. The results of the simulations are presented in a simple graphic form allowing evaluation of the pressure–temperature (PT) regions in which Ar diffusion is efficient, and hence those in which the muscovite may yield true40Ar/39Ar cooling ages. We suggest that a robust method for determining whether metamorphic muscovite40Ar/39Ar ages relate to the timing of cooling involves: (1) the determination of the PT conditions and relative timing of muscovite crystallisation during the metamorphic cycle, (2) the collection of high precision and high spatial resolution40Ar/39Ar data from muscovite grains and (3) the comparison between analytical data and numerical diffusion models which test different post-crystallisation pressure–temperature histories.