Occurrence of Excess 40Ar in Amphibole: Implications of 40Ar/39Ar Dating by Laser Stepwise Heating and in vacuo Crushing
Occurrence of Excess 40Ar in Amphibole: Implications of 40Ar/39Ar Dating by Laser Stepwise Heating and in vacuo Crushing
复制标题
角闪石中出现过量 40Ar：通过激光逐步加热和真空破碎进行 40Ar/39Ar 测年的意义
DOI：
10.1007/s12583-017-0947-x
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发表时间：
2018-04-01
影响因子：
3.3
通讯作者：
Qiu, Hua-Ning
中科院分区：
文献类型：
--
作者：
Hu, Rong-Guo;Bai, Xiu-Juan;Qiu, Hua-Ning
采用40Ar/39Ar激光逐级加热和真空破碎联合方法对柴达木北缘超高压变质角闪岩中的角闪岩进行了测年。两个角闪洞样品经激光加热后得到鞍形年龄谱，总气体年龄分别为574.5 +/- 2.5 Ma和562.5 +/- 2.5 Ma。这些年龄远高于已有报道的Yuka榴辉岩锆石U-Pb年龄（相似于495 Ma），表明存在过量的40Ar。为了解释过量40Ar的赋存状态和限制角闪岩相退退的年龄，进一步采用两个重复的角闪岩样品进行40Ar/39Ar真空破碎分析。两种样品表现出相似的单调下降的释放谱，其特征是在早期阶段异常年老的表观年龄迅速下降。晚期台阶的表观年龄与高原年龄一致，分别为460.9 +/- 1.2和459.6 +/- 1.8 Ma。我们认为，早期阶段释放的气体来自含40Ar过量的次生流体包裹体（sfi），它们沿裂缝分布，容易被提取，而后期阶段释放的气体来自较小的原生流体包裹体（pfi）。
The joint methods of 40Ar/39Ar laser stepwise heating and in vacuo crushing have been applied to date amphiboles from the North Qaidam ultra-high pressure metamorphic amphibolites. Two amphibole samples analyzed by laser heating yielded saddle-shaped age spectra with total gas ages of 574.5 +/- 2.5 and 562.5 +/- 2.5 Ma. These ages are much older than the reported zircon U-Pb ages (similar to 495 Ma) from Yuka eclogite, indicating the presence of excess 40Ar. In order to decipher the occurrence of excess 40Ar and constrain the age of amphibolite-facies retrogression, two duplicate amphibole samples were further employed for 40Ar/39Ar in vacuo crushing analyses. Both samples exhibit similar monotonically declining release spectra, which are characterized by rapid decline of anomalously old apparent ages in the early steps. The data of the late steps yielded concordant apparent ages with plateau ages of 460.9 +/- 1.2 and 459.6 +/- 1.8 Ma. We interpret that gases released in the early steps derive from the significant excess 40Ar containing secondary fluid inclusions (SFIs) due to their distribution characteristics along cracks leading to be easily extracted, whereas those released in the later steps represent the contribution of the small primary fluid inclusions (PFIs).