Recycling Argon through Metamorphic Reactions: the Record in Symplectites
Recycling Argon through Metamorphic Reactions: the Record in Symplectites
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DOI：
10.1016/j.lithos.2017.11.028
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发表时间：
2018-02
期刊：
影响因子：
3.5
通讯作者：
C. McDonald;D. Regis;C. Warren;S. Kelley;S. Sherlock
中科院分区：
文献类型：
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作者：
C. McDonald;D. Regis;C. Warren;S. Kelley;S. Sherlock
在高温下结晶的变质云母的40 Ar/39 Ar年龄通常被解释为冷却年龄，晶粒被认为通过热驱动扩散进入晶界网络失去了40 Ar。最近报道的激光烧蚀数据表明，Ar在变质云母的空间分布并不总是符合扩散理论预测的模式，尽管变质温度高，氩没有有效地从本地系统在变质演化。在挪威西部片麻岩地区（WGR），长英质片麻岩保存了多硅白云母在c附近等温减压过程中分解为黑云母和斜长石合晶的显微结构证据。20-25到C。8-12 kbar，约700 °C。这些样品提供了一个理想的天然实验室，以评估是否一种含钾矿物在高温下被另一种矿物完全取代，完全“重置”Ar时钟，或者是否有一些新钾化阶段的40 Ar遗传。WGR变质旋回高温部分的时间在以前的研究中得到了很好的限制。然而，冷却后的时间叠印仍然有很大的争议。多硅白云母、黑云母-斜长石合晶岩和较粗的、质地较晚的黑云母中的原位激光烧蚀点测年产生了40 Ar/39 Ar年龄，这些年龄跨越了变质循环的大部分时间。这些数据共同表明，尽管在约700 °C的温度下停留，但Ar并没有通过扩散损失或在变质重结晶期间被完全去除。相反，Ar多硅白云母破裂过程中释放的似乎是部分重新纳入到新结晶的黑云母和斜长石（或被困在流体包裹体在这些阶段）在一个封闭的系统。我们的数据表明，显微结构和岩相演化的样品被定年提供了一个关键的框架中，本地40 Ar再循环可以跟踪，从而可能允许40 Ar/39 Ar日期更准确地联系到变质历史。
The40Ar/39Ar ages of metamorphic micas that crystallized at high temperatures are commonly interpreted as cooling ages, with grains considered to have lost40Ar via thermally-driven diffusion into the grain boundary network. Recently reported laser-ablation data suggest that the spatial distribution of Ar in metamorphic micas does not always conform to the patterns predicted by diffusion theory and that despite high metamorphic temperatures, argon was not removed efficiently from the local system during metamorphic evolution. In the Western Gneiss Region (WGR), Norway, felsic gneisses preserve microtextural evidence for the breakdown of phengite to biotite and plagioclase symplectites during near isothermal decompression from c. 20–25 to c. 8–12 kbar at ~ 700 °C. These samples provide an ideal natural laboratory to assess whether the complete replacement of one K-bearing mineral by another at high temperatures completely ‘resets’ the Ar clock, or whether there is some inheritance of40Ar in the neocrystallized phase. The timing of the high-temperature portion of the WGR metamorphic cycle has been well constrained in previous studies. However, the timing of cooling following the overprint is still much debated. In-situ laser ablation spot dating in phengite, biotite-plagioclase symplectites and coarser, texturally later biotite yielded40Ar/39Ar ages that span much of the metamorphic cycle. Together these data show that despite residence at temperatures of ~ 700 °C, Ar is not completely removed by diffusive loss or during metamorphic recrystallization. Instead, Ar released during phengite breakdown appears to be partially reincorporated into the newly crystallizing biotite and plagioclase (or is trapped in fluid inclusions in those phases) within a close system. Our data show that the microtextural and petrographic evolution of the sample being dated provides a critical framework in which local40Ar recycling can be tracked, thus potentially allowing40Ar/39Ar dates to be linked more accurately to metamorphic history.