Confined fission tracks in ion-irradiated and step-etched prismatic sections of Durango apatite
Confined fission tracks in ion-irradiated and step-etched prismatic sections of Durango apatite
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DOI：
10.1016/j.chemgeo.2007.03.015
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发表时间：
2007-07
期刊：
影响因子：
3.9
通讯作者：
R. Jonckheere;E. Enkelmann;M. Min;C. Trautmann;L. Ratschbacher
中科院分区：
文献类型：
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作者：
R. Jonckheere;E. Enkelmann;M. Min;C. Trautmann;L. Ratschbacher
磷灰石裂变径迹分析回溯了地质样品在~ 120°C下的热历史，但限制径迹数量少，往往导致对建模结果的约束较弱。离子辐照和增加刻蚀时间是增加刻蚀约束径迹数量的有效手段。我们的结果支持交叉径迹的数量与离子和裂变径迹的长度和密度呈线性关系。离子辐照的有效性还取决于蚀刻轨道的几何形状，即光束方向和蚀刻条件。平行于磷灰石c轴的刀形离子轨迹比垂直于c轴的漏斗形离子轨迹更有效，但会增加有利于高角度c的角偏置。照射条件不影响给定角度c的平均轨迹长度。在一定阈值以上，受限轨迹的数量与蚀刻时间成线性比例增加，我们将其解释为蚀刻轨迹所需的时间。超过这个阈值，封闭轨道的暴露速度很快。这种高增长率本身就是这样一个事实的直接后果，即如果蚀刻时间少于某一最小长度，则封闭轨道是无法测量的。蚀刻时间不影响偏角，但增加了各角度的平均轨迹长度。化石径迹长度的平均增长速度在2σ水平上超过了诱导径迹长度的平均增长速度。在某些条件下，离子辐照导致裂变径迹被阻断，形成“半径迹”。在棱柱状磷灰石剖面中，平均水平受限轨迹长度与c轴角度的椭圆模型相对于已知的对称性过于受限，并且不能提供与数据集的最佳拟合。一些观察表明，化石足迹和人工足迹之间存在着根本的区别。
Apatite fission-track analysis retraces the thermal histories of geological samples under ∼120 °C but low numbers of confined tracks often lead to weak constraints on the modelling results. Ion irradiation and increasing etching time are effective means to increase the number of etched confined tracks. Our results support a linear dependence of the number of intersected tracks on the lengths and densities of both the ion and fission tracks. The effectiveness of ion irradiation also depends on the etched-track geometries, i.e. on the beam orientation and etching conditions. Knife-blade shaped ion tracks parallel to the apatite c-axis are more efficient than funnel-shaped tracks perpendicular to c but add to the angular bias in favour of high angles to c. The irradiation conditions do not affect the mean track length at a given angle to c. The number of confined tracks increases in linear proportion to etching time above a certain threshold, which we interpret as the time required to etch a track to measurable dimensions. Past this threshold, the rate of confined track revelation is rapid. The high rate of increase is itself a direct consequence of the fact that confined tracks are not measurable if etched for less than a certain minimum length of time. Etching time does not affect the angular bias but it does increase the mean track lengths at all angles. The average rate of increase of the fossil track lengths exceeds that of the induced tracks at the 2σ level. Under some conditions, ion irradiation leads to blocking of fission tracks and the development of “semi-tracks”. The elliptical model of mean horizontal confined track length vs. angle to the c-axis in prismatic apatite sections is over-constrained with respect to the known symmetries and does not provide the best possible fit to the dataset. Some observations suggest a fundamental difference between fossil and induced tracks.