Helium diffusion and low-temperature thermochronometry of apatite
Helium diffusion and low-temperature thermochronometry of apatite
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DOI：
10.1016/s0016-7037(96)00192-5
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发表时间：
1996-11
影响因子：
5
通讯作者：
R. A. Wolf;K. Farley;L. Silver
中科院分区：
文献类型：
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作者：
R. A. Wolf;K. Farley;L. Silver
为了探讨潜在的（U Th）/He系统的低温热年代学，我们已经研究了氦扩散，并已测量了氦年龄杜兰戈氟磷灰石和磷灰石辉长岩和两个英云闪长岩半岛山脉玄武岩。使用长持续时间增量除气实验测量中低温至极低温度（低至80°C）下的扩散率以达到高分析精度。所有四个磷灰石表现出非常相似的氦扩散行为。氦的损失显然是通过亚晶畴（<60 μm）的体积扩散发生的，在所有样品中，亚晶畴的尺寸几乎相同。在低于290°C的温度下，扩散率服从高度线性的Arrhenius关系，隐含的活化能约为36 kcal/mol。在此温度以上，扩散率从线性向较低的活化能偏离。这种转变并不是由多个扩散域引起的，而是由氦扩散的物理机制的可逆变化引起的。对于热计时目的，高温扩散行为在很大程度上是无关紧要的，因为在地质时期，温度高于290°C时，基本上没有氦被保留下来。使用低温状态的结果，所有样品的氦闭合温度均在75 ± 7°C范围内。该值与磷灰石的化学成分和粒度无关，表明单一闭合温度可适用于广泛的样品。这些磷灰石（17-120 Ma）的（U Th）/He年龄范围从一小部分到近100%的结晶年龄的寄主岩石，是一致的低温热年代学解释。这些结果有力地支持了以前的建议，磷灰石（U <$Th）/He定年可以提供高精度的超低温地质事件的计时。
To investigate the potential of the (UTh)/He system for low-temperature thermochronometry, we have studied helium diffusion and have measured helium ages on Durango fluorapatite and on apatites from a gabbro and two tonalites from the Peninsular Ranges Batholith. Diffusivity at moderate to very low temperatures (as low as 80°C) was measured to high analytical precision using long duration incremental outgassing experiments. All four apatites displayed remarkably similar helium diffusion behavior. Helium loss apparently occurs via volume diffusion from subgrain domains (<60 μm) which are nearly identical in size in all samples. At temperatures below 290°C, diffusivity obeys a highly linear Arrhenius relationship with an implied activation energy of about 36 kcal/mol. Above this temperature, diffusivity deviates from linearity toward lower activation energies. This transition does not arise from multiple diffusion domains, but rather from a reversible change in the physical mechanism of helium diffusion. For thermochronometric purposes the high-temperature diffusion behavior is largely irrelevant because essentially no helium is retained over geologic time at temperatures above 290°C. Using the results from the low-temperature regime, all samples yield helium closure temperatures in the range 75 ± 7°C. This value is independent of chemical composition and grain size of the apatites, suggesting that a single closure temperature may apply to a wide range of samples. The (UTh)/He ages of these apatites (17–120 Ma) range from a small fraction to nearly 100% of the crystallization age of their host rocks, and are consistent with a low-temperature thermochronometric interpretation. These results strongly support previous suggestions that (UTh)/He dating of apatite can provide high precision chronometry of very low temperature geological events.