奥地利格拉茨大学Simon Schorn最新的研究利用放热反应和³⁹Ar-⁴⁰Ar热年代学方法，揭示了水合作用能导致更年轻的表观年龄。相关论文于2024年3月13日发表在《地质学》杂志上。

研究人员使用一个简单的一维数值模型（包括在不同温度下释放的热量），来模拟放热水合作业，研究跟踪了从深处挖出的岩石的冷却历史。计算的冷却路径的用于量化明显的白云母、黑云母和长石中的表观的³⁹Ar-⁴⁰Ar年龄，考虑了⁴⁰Ar的产生和扩散的Ar损失。对于相对于约10%的绿泥化的流体掺入，每1kg岩石释放约50kJ的潜热。在这种情况下，挖掘率在1mm·yr^-1和4mm·yr^-1之间，直径≤100 μm的白云母颗粒的表观年龄显示要年轻10%，但总是超过Ar测年的典型不确定性。黑云母和长石也表现出类似的变形，甚至对于约1mm大小的大颗粒也是如此。

随着焓的释放、挖掘速率和粒度的减小，相对幼化效应增加到30%以上，水合过程中，达到各个相应热循环仪的标称关闭温度时，水合作用达到最大值。利用已发表的数据集，研究人员认为，表观云母年龄的恢复与挖掘过程中放热水合导致的不同Ar损失是一致的。该方法适用于任何热活化过程，如矿物温度计中的元素交换或裂变径迹退火，前提是热量在接近特征闭合温度时释放。研究结果强调，这延伸到水合作用以外的过程，如剪切加热或局部岩浆侵位，研究结果适用于各种热年代计和各种条件下的温度敏感方法。

据悉，退化和水合作用通常影响大片的地球地壳，形成不同程度的绿泥石化、绢云母化和/或蛇纹石化。水合作用是一个强烈的放热过程，它部分地打开了同位素系统，从而扭曲了记录的改造过的地体的表观年龄和冷却历史。

附：英文原文

Title: Exothermic reactions and ³⁹Ar-⁴⁰Ar thermochronology: Hydration leads to younger apparent ages

Author: Simon Schorn, Evangelos Moulas, Kurt Stüwe

Issue&Volume: 2024-03-13

Abstract: Retrogression and hydration commonly affect large swaths of Earth’s crust, causing variable degrees of chloritization, sericitization, and/or serpentinization. Hydration is a strongly exothermic process that partially opens isotopic systems, thereby distorting the recorded apparent ages and cooling histories of reworked terranes. Using a simple one-dimensional numerical model involving heat released at variable temperatures to simulate exothermic hydration, we track the cooling histories for rocks that exhume from depth. The calculated cooling paths are used to quantify apparent ³⁹Ar⁴⁰Ar ages in muscovite, biotite, and feldspar considering ⁴⁰Ar production and diffusive Ar loss. For fluid incorporation relative to chloritization of ～10%, ～50 kJ of latent heat are released per kilogram of rock. For this scenario and exhumation rates between 1 mm·yr^-1 and 4 mm·yr^-1, muscovite grains ≤100 μm in diameter yield apparent ages that are younger by up to 10%, but always exceeding the typical uncertainty of Ar dating. Biotite and feldspar display a similar distortion, even for large grains of ～1 mm in size. The relative younging effect increases to >30% with enthalpy released, exhumation rate, and decreasing grain size, with younging reaching a maximum for hydration at approximately the nominal closure temperature of the respective thermochronometers. Using published data sets (from Sifnos, Greece, and Tian Shan, China), we suggest that rejuvenation of apparent mica ages is consistent with diffusive Ar loss due to exothermic hydration during exhumation. Our method applies to any thermally activated process, like element exchange in mineral thermometers or fission-track annealing, provided heat is released close to the characteristic closure temperature. This extends to processes beyond hydration, such as shear heating or localized magma emplacement, making our results pertinent for diverse thermochronometers and temperature-sensitive methods across a broad range of conditions.

DOI: 10.1130/G51776.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G51776.1/636307/Exothermic-reactions-and-39Ar-40ArredirectedFrom=fulltext