Dependence of Fission Track Annealing Kinetics on Crystallographic Orientation and Crystal Chemistry

PI已经开发和测试了一种方法，可以精确地确定磷灰石中平行和垂直于晶体c轴的平均可蚀刻裂变径迹(FT)长度。这样确定的长度对于单个FTs(垂直平行轨道)来说是不同的，但由于它们的热历史相同，所以它们被定义为“相同长度”。PIS平面使用这种新方法来确定磷灰石中FT退火的物理过程是否随着晶体取向的变化而变化，或者我们是否观察到由于各向异性腐蚀而产生的实验伪影。同样，他们将确定FT退火过程是否随晶体化学变化(例如富氯磁道长度和富F磁道长度)，或者这是否也是一种蚀刻现象。为了解决这些问题，并更好地校准这个重要的温度计时器，提出的方法是一系列等时实验室退火实验(从0.3-1000小时和23-400oC)。具体地说，他们将研究：1)观察到的FT长度各向异性与磷灰石中腐蚀各向异性的相关性，2)一系列磷灰石成分的平均可腐蚀FT长度随时间、温度和晶体取向的变化；(F-、Cl-、OH-和富Mn)在低温(23oC)和高温下，3)FT退火激活能如何随固定取向的平均FT长度变化，b)当量长度之间的变化，以及c)与主体晶体化学的变化，4)上述结果与最近发表的结果(Galbraith和Laslett，1988)将FT密度(磁道/单位面积)的依赖关系与平均可蚀刻FT长度相关联。然后，这些新数据将被用来推导出一个数学表达式，将“等效长度”与磷灰石的时间-温度(t-T)历史联系起来。这一新的表达式应该会大大改善地质样品经历的t-T路径的估计。

The PIs have developed and tested a method whereby the mean etchable fission track (FT) lengths parallel and perpendicular to the crystallographic c-axis in apatite may be precisely determined. The lengths so determined differ for a single population of FTs (parallel tracks perpendicular) but, because of their equivalent thermal histories, they are defined as "equivalent lengths". The PIs plane to use this new method to determine whether the physical process of FT annealing in apatite varies with crystallographic orientation or whether we are observing experimental artifacts due to anisotropic etching. Similarly, they will determine whether the FT annealing process varies with crystal chemistry (e.g. Cl-rich track lengths F- rich) or whether this is also an etching phenomenon. The method proposed, in order to address these questions and to better calibrate this important thermochronometer, is a series of isochronal laboratory annealing experiments (ranging from O.3- 1OOOO hours and 23-400oC). Specifically, they will investigate: 1) the correlation of observed FT length anisotropy with etching anisotropy in apatite, 2) the variation of mean etchable FT length with time, temperature and crystallographic orientation for a range of apatite compositions ;(F-, Cl-, OH-, and Mn-rich) at low temperature (23oC) and elevated temperatures, 3) how activation energy for FT annealing varies a) with mean FT length at fixed orientation, b) between equivalent lengths, and c) with host crystal chemistry, 4) the relationship of the above results with currently published results (Galbraith and Laslett, 1988) relating the dependence of FT density (tracks/unit area) to the mean etchable FT length. These new data will then be used to derive a mathematical expression relating the "equivalent length" to the time- temperature (t-T) history of the apatite. This new expression should result in a significant improvement in estimates of t-T paths experienced by geologic samples.