Uncertainty exists over what environmental conditions and mineralogical/chemical properties are required to ensure retention of helium such that (UTh)/He dates record the time of goethite crystallization. We undertook vacuum step heating experiments to determine He diffusion parameters for extrapolation to Earth surface conditions on 10 goethite specimens in which we had created a uniform distribution of He. Arrhenius plots of apparent diffusion coefficients on all samples follow the same pattern. At temperatures <200 °C the data define arrays consistent with progressive degassing of increasingly large crystallites. However, above 200 °C the computed diffusivities increase dramatically until about 80% of the helium is extracted, after which they suddenly decline. The sudden increase in diffusivity at 200 °C coincides with the onset of dehydration of the goethite structure, a process which continues throughout the remainder of the step heat. There is a strong correlation between evolved water and He amounts. These observations likely reflect previously reported processes of formation, growth, and coalescence of pores as the phase transition to hematite proceeds. While significantly higher dehydration temperatures of ∼270 °C are observed using techniques such as thermogravimetric analysis in air, the long step durations, and vacuum conditions of our experiments destabilize goethite. Orders of magnitude differences in the computed diffusivities among our samples may reflect crystallite-size-distribution control on dehydration kinetics, and the qualitative size distributions we infer from the step heats are consistent with SEM observations of crystallite lengths. Vacuum step-heating experiments in which goethite is decomposing cannot be used to determine He diffusion behavior in nature, but the inferred crystallite size distribution provides some useful indirect evidence. A strong relationship exists between metrics of the crystallite size distribution from step heating results and the degree of He retention in nature inferred from the He/He method. This observation provides evidence supporting the validity of the He/He technique for estimating retention, and further suggests that the dominant control on He retention in nature is the crystallite size distribution. Experiments under hydrothermal conditions in which goethite remains stable may be an alternative approach to address the question of He diffusion behavior in nature.

中文翻译：

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针铁矿在真空步进加热过程中的脱水及其对保持力表征的影响


需要什么样的环境条件和矿物学/化学性质来确保氦气的保留，以便 (UTh)/He 日期记录针铁矿结晶的时间，存在不确定性。我们进行了真空步进加热实验，以确定氦扩散参数，以便外推到 10 个针铁矿样本的地球表面条件，在这些样本中我们创建了均匀的氦分布。所有样品的表观扩散系数阿伦尼乌斯图都遵循相同的模式。在温度 <200°C 时，数据定义的阵列与越来越大的微晶的渐进脱气一致。然而，在 200°C 以上，计算出的扩散率急剧增加，直到大约 80% 的氦气被提取出来，之后扩散率突然下降。 200°C 时扩散率的突然增加与针铁矿结构脱水的开始同时发生，该过程在整个步骤加热的剩余时间内持续进行。析出水和 He 量之间存在很强的相关性。这些观察结果可能反映了先前报道的随着赤铁矿相变的进行而形成、生长和合并孔隙的过程。虽然使用空气中热重分析等技术观察到脱水温度明显较高，约为 270 °C，但我们实验的长步骤持续时间和真空条件会破坏针铁矿的稳定性。我们的样品之间计算的扩散率存在数量级差异，这可能反映了脱水动力学上微晶尺寸分布的控制，并且我们从阶梯加热中推断出的定性尺寸分布与微晶长度的 SEM 观察结果一致。 针铁矿分解的真空逐步加热实验不能用于确定自然界中的 He 扩散行为，但推断的微晶尺寸分布提供了一些有用的间接证据。分步加热结果的微晶尺寸分布指标与 He/He 方法推断出的 He 在自然界中的保留程度之间存在密切关系。这一观察结果提供了支持 He/He 技术用于估计保留的有效性的证据，并进一步表明对自然界中 He 保留的主要控制是微晶尺寸分布。在针铁矿保持稳定的水热条件下进行实验可能是解决自然界氦扩散行为问题的另一种方法。