Thermal annealing mechanisms of latent fission tracks: Apatite vs. zircon
Thermal annealing mechanisms of latent fission tracks: Apatite vs. zircon
复制标题
DOI：
10.1016/j.epsl.2010.12.016
复制
发表时间：
2011-02-01
影响因子：
5.3
通讯作者：
Ewing, Rodney C.
中科院分区：
文献类型：
--
作者：
Li, Weixing;Wang, Lumin;Ewing, Rodney C.
磷灰石和锆石中的裂变径迹是最常用于裂变径迹定年和热年代学的矿物，它们被认为是通过相同的机制退火的，基本上是外延再结晶。在这项研究中，直接的，原子尺度的观察未蚀刻的，潜在的轨道在高温下的热退火，透射电子显微镜（TEM）表明，磷灰石中的轨道的退火行为是完全不同的非晶轨道的锆石。在700 ℃下原位加热130分钟后，可以看到磷灰石中潜在径迹的残留，与在360 ℃下退火1小时后径迹完全消失，然后通过化学蚀刻放大形成鲜明对比。磷灰石中径迹的“褪色”或收缩是由于空位从多孔核向周围基质的热发射造成的，与锆石中无定形径迹内的晶体和空位的复合相反。高表面能和高扩散率的原子在表面上的磷灰石的基本上是多孔的轨道导致的不连续性的轨道由几个不同的机制：瑞利不稳定性，布朗运动，和优先运动的轨道段。原子沿着c轴的优先运动解释了垂直于磷灰石c轴的裂变径迹的更快退火。除了收缩之外，磷灰石中裂变径迹的不连续性防止溶液进入多孔径迹进行进一步蚀刻，从而减小蚀刻径迹长度。这解释了在低得多的温度下磷灰石中可蚀刻径迹的完全消失，与TEM观察到的潜在径迹相比。与此相反，在锆石的非晶轨道不段由于低表面能和低扩散率的原子在轨道矩阵边界。这解释了锆石在830 ℃下90分钟后潜在径迹完全消失与在800 ℃下1小时后可蚀刻径迹完全消失之间的相似性。锆石和磷灰石中裂变径迹的非常不同的行为是径迹内部结构差异的直接结果-锆石中的无定形域与磷灰石中的低原子密度空隙。(C)2010 Elsevier B. V.保留所有权利。
Fission tracks in apatite and zircon, the minerals most often used for fission track dating and thermochronology, have been considered to anneal by the same mechanism, essentially epitaxial recrystallization. In this study, direct, atomic-scale observations of thermal annealing of unetched, latent tracks at elevated temperatures by transmission electron microscopy (TEM) demonstrate that the annealing behavior of tracks in apatite is entirely different from that of the amorphous tracks in zircon. Remnants of the latent tracks in apatite can be seen after in situ heating at 700 degrees C for 130 min, in clear contrast to the complete disappearance of tracks after they have been annealed at 360 degrees C for 1 h and then enlarged by chemical etching. The "fading" or shrinkage of tracks in apatite results from thermo-emission of vacancies from the porous core to the surrounding matrix, in contrast to the recombination of interstitials and vacancies within the amorphous tracks in zircon. The high surface energy and high diffusivity of atoms at the surface of the essentially porous tracks in apatite cause the discontinuity in the tracks by several different mechanisms: Rayleigh instability, Brownian motion, and preferential motion of track segments. The preferential motion of atoms along the c-axis accounts for more rapid annealing of fission tracks perpendicular to the c-axis of apatite. In addition to the shrinkage, the discontinuity of fission tracks in apatite prevents solutions from entering into the porous tracks for further etching, thus reducing the etched track length. This accounts for the complete disappearance of etchable tracks in apatite at much lower temperatures, as compared to that of the latent tracks observed by TEM. In contrast, the amorphous tracks in zircon do not segment due to the low surface energy and low diffusivity of atoms at the track-matrix boundary. This explains the similarity between the complete disappearance of latent tracks in zircon at 830 degrees C after 90 min and that of etchable tracks at 800 degrees C after 1 h. The very different behavior of fission tracks in zircon and apatite is a direct result of differences in the internal structure of the track - the amorphous domain in zircon vs. the low atomic density void in apatite. (C) 2010 Elsevier B.V. All rights reserved.