In-situ UV-laser 40Ar/39Ar geochronology of a micaceous mylonite : an example of defect-enhanced argon loss
In-situ UV-laser 40Ar/39Ar geochronology of a micaceous mylonite : an example of defect-enhanced argon loss
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DOI：
10.1007/s00410-001-0325-6
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发表时间：
2002-03
影响因子：
3.5
通讯作者：
A. Mulch;M. Cosca;M. Handy
中科院分区：
文献类型：
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作者：
A. Mulch;M. Cosca;M. Handy
采用炉内分步加热和原位紫外激光烧蚀技术对南阿尔卑斯坡加洛剪切带地壳尺度糜伦岩带中的白云母和黑云母进行了年代学研究。未变形的白云母斑岩碎屑得到的40ar /39Ar平台年龄为182.0±1.6 Ma，而原位紫外激光烧蚀、40ar /39Ar定年和炉内阶梯加热的强变形白云母和黑云母颗粒的表观年龄范围更年轻。在变形的白云母和黑云母中测量的40ar /39Ar年龄范围与通过氩气封闭的长时间冷却相一致，这些矿物在粒内尺度上表现出不同的分段。这些微观结构控制的片段被一阶晶格不连续、亚微观结构缺陷和/或高缺陷密度区域所束缚，这为晶内氩扩散创造了可变的长度尺度。观察到的白云母和黑云母内部的变形微观结构在缓慢冷却的糜棱岩中充当了粒内快速扩散通道。因此，高空间分辨率的40ar /39Ar数据记录了大约60 Ma的时间跨度内氩扩散的开始和最终结束。
Muscovite and biotite from a crustal-scale mylonite zone (Pogallo Shear Zone, southern Alps) were investigated using furnace step-heating and in-situ UV-laser ablation40Ar/39Ar geochronology. Undeformed muscovite porphyroclasts yield40Ar/39Ar plateau ages of 182.0±1.6 Ma, whereas in-situ UV-laser ablation40Ar/39Ar dating and furnace step-heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger. The range of40Ar/39Ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra-grain scale. These microstructurally controlled segments are bound by either first-order lattice discontinuities, sub-microscopic structural defects and/or zones of high defect density, which create variable length-scales for intragranular argon diffusion. The observed deformational microstructures within muscovite and biotite acted as intra-grain fast diffusion pathways in the slowly cooled mylonitic rocks. Therefore, the high-spatial resolution40Ar/39Ar data record the initial and final closure to argon diffusion over a time span of about 60 Ma.