Interpreting high-pressure phengite 40Ar/39Ar laserprobe ages: an example from Saih Hatat, NE Oman
Interpreting high-pressure phengite 40Ar/39Ar laserprobe ages: an example from Saih Hatat, NE Oman
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DOI：
10.1007/s00410-010-0576-1
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发表时间：
2011-06
影响因子：
3.5
通讯作者：
C. Warren;S. Sherlock;S. Kelley
中科院分区：
文献类型：
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作者：
C. Warren;S. Sherlock;S. Kelley
来自阿曼东北部Saih Hatat高压地层的新单粒融合和核心- rim40ar /39Ar激光探针云母岩数据表明，单个样品产生的表观年龄范围与之前报道的整个地层的表观年龄范围相似。大多数确定的年龄比先前报道的U-Pb锆石变质峰年龄更早。在单个颗粒中，岩心和边缘年龄的变化范围从没有可辨别的颗粒到具有较老岩心的颗粒，或者很少有较老的边缘；一些示例显示了所有三种模式。数值扩散模拟表明，由于峰值温度约为550℃，测量的表观年龄不能用简单的冷却或开放系统中结晶或碎屑年龄的部分保留来解释。在mm-cm尺度上，时空变化的开放或封闭系统行为加上普遍和非均匀分布的过量氩可以更好地解释年龄的变化。辉长岩40ar /39Ar异常老的年龄可能是由于从含钾前驱体继承的内部衍生的40ar，或者是由于晶界流体分布的外部衍生的40ar。榴辉岩中富含云母的片岩产生较年轻的辉云母年龄，表明过量的氩不存在或被稀释。承载榴辉岩的泥质体似乎在变形和绿片岩相叠印过程中受到后期流体进入的影响，并产生非常变化的年龄，通常在较年轻的岩心上明显有较老的边缘。在Saih Hatat测量的颗粒尺度和样品尺度的年龄变化表明，榴辉岩荚果中的晶界网络不是氩输运的有效途径，而周围泥质中的晶界网络在变质旋回的时间尺度上是一个更有效的途径。
New single grain fusion and core-rim40Ar/39Ar laserprobe phengite data from the Saih Hatat high-pressure terrane in NE Oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane. The majority of the determined ages are older than the previously reported U-Pb zircon peak metamorphic age. Core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores, or, rarely, older rims; some samples manifest all three patterns. Numerical diffusion modelling shows that due to the peak temperature of ca. 550°C, the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system. The age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm-cm scale coupled with pervasive and heterogeneously distributed excess argon. Anomalously old eclogite phengite40Ar/39Ar ages are due either to internally derived40Ar inherited from a K-bearing precursor, or externally derived40Ar distributed by grain boundary fluids. Mica-rich schists within the eclogite boudins yield younger phengite ages, suggesting excess argon was absent or diluted. Pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist-facies overprint and yield very variable ages commonly with apparently older rims on younger cores. The grain- and sample-scale age variations measured in Saih Hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport, whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle.