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

Geology Pub Date : 2024-03-13 DOI:10.1130/g51776.1
Simon Schorn, E. Moulas, Kurt Stüwe
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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 39Ar−40Ar 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.
放热反应与 39Ar-40Ar 热年代学:水合作用导致表观年龄变小
逆回归和水合作用通常会影响大片地壳,造成不同程度的绿泥石化、绢云母化和/或蛇绿岩化。水化是一个强烈的放热过程,会部分打开同位素系统,从而扭曲记录的表观年龄和再造岩体的冷却历史。我们利用一个简单的一维数值模型,通过在不同温度下释放热量来模拟放热的水合作用,跟踪从深部喷出的岩石的冷却历史。考虑到 40Ar 的产生和扩散性 Ar 损失,计算出的冷却路径用于量化蕈云母、黑云母和长石的表观 39Ar-40Ar 年龄。相对于10%的绿泥石化,每公斤岩石释放出50千焦的潜热。在这种情况下,掘出率介于1毫米/年-1和4毫米/年-1之间,直径≤100微米的麝香石颗粒产生的表观年龄要年轻10%,但总是超过氩测年的典型不确定性。黑云母和长石也显示出类似的变形,即使是大小为 1 毫米的大颗粒也是如此。相对年轻化效应随着释放的焓、排空率和晶粒尺寸的减小而增加到大于30%,在水合作用下,年轻化达到最大值,大约是各热时器的名义闭合温度。利用已发表的数据集(来自希腊西弗诺斯和中国天山),我们认为表观云母年龄的年轻化与吐出过程中放热水化导致的扩散性氩损失是一致的。我们的方法适用于任何热激活过程,如矿物温度计中的元素交换或裂变轨道退火,只要释放的热量接近特征闭合温度即可。这也适用于水合以外的过程,如剪切加热或局部岩浆喷出,从而使我们的研究结果适用于各种不同条件下的不同热时计和温度敏感方法。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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