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首页> 外文期刊>Chemical geology >The K-Ar and ~(40)Ar/~(39)Ar methods revisited for dating fine-grained K-bearing clay minerals
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The K-Ar and ~(40)Ar/~(39)Ar methods revisited for dating fine-grained K-bearing clay minerals

机译：再次探讨了K-Ar和〜（40）Ar /〜（39）Ar方法对细粒含K粘土矿物进行测年的方法

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摘要

The strengths and weaknesses of the two Ar isotopic methods (K-Ar and ~(40)Ar/~(39)Ar) were evaluated on the basis of respective recent applications mainly on low-temperature K-bearing illite-type clay minerals. This review includes a presentation of basic, analytical and technical aspects for both methods, as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization. Whenever possible, the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates. The comparative review examines stratigraphic dating of glauconites, indirect dating of low-temperature ore deposits, dating of burial-related illitization, and dating of polyphased tectono-thermal activity, more specifically of fault gouges. Some pending questions such as the necessary encapsulation due to ~(39)Ar recoil and its restoration into step-heating patterns are also raised, together with the new potential of Ar-dating of nanometric illite crystals.Weakness of the K-Ar method is in its pioneering status that makes many believe that it is no longer accurate, because of its traditional analytical aspects, and of the K determinations leading to somewhat large uncertainties. However, precise evaluation of varied applications points to a K-Ar method having probably larger applicability in sedimentary to diagenetic environments than the ~(40)Ar/~(39)Ar method. The drawbacks become less important if the method is applied to nanometer-sized clay minerals in diagenetic to low-grade metamorphic environments. In this instance, the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations, if mathematically justified, can be reduced by duplicate analyses. Weakness of the ~(40)Ar/~(39)Ar method is in its basics such as the ~(39)Ar recoil, the necessary encapsulation, the reintegration of the ~(39)Ar into the step-heating patterns, and the meaning of the step-heating patterns that are more suggestive of variable ~(39)Ar "reservoirs" created among the clay particles by irradiation than of meaningful geologic ages. If the K-Ar method is the preferred method for dating diagenetic clay processes such as glauconitization, illite crystal nucleation and growth, or low-temperature hydrothermal activities, then the ~(40)Ar/~(39)Ar method has more potential in dating low-temperature tectono-thermal activities, and in detailing mixtures of multi-generation illite.
机译：在主要针对低温含钾伊利石型黏土矿物的最新应用基础上，评估了两种Ar同位素方法（K-Ar和〜（40）Ar /〜（39）Ar）的优缺点。这篇综述包括两种方法的基本，分析和技术方面的介绍，以及对这两种方法的各种主张以及有关样品制备和表征要求的讨论。在可能的情况下，将两种方法在相同的矿物分离物中获得的同时期结果进行比较，比较每种方法的优缺点。比较审查检查了青石岩的地层测年，低温矿床的间接测年，与埋葬有关的未熟化的测年以及多相构造热活动的测年，尤其是断层泥。还提出了一些悬而未决的问题，例如〜（39）Ar反冲导致的必要封装以及将其还原成阶梯加热模式的问题，以及纳米伊利石晶体Ar定年的新潜力.K-Ar方法的缺点是由于其传统的分析方面以及K的测定导致较大的不确定性，因此它以其开创性的地位使许多人认为它不再准确。但是，对各种应用的精确评估指出，与〜（40）Ar /〜（39）Ar方法相比，K-Ar方法在沉积环境到成岩环境中的适用性可能更大。如果将该方法应用于成岩至低等级变质环境中的纳米级粘土矿物，则缺点变得不太重要。在这种情况下，提取的尺寸分数通常是均匀的，并且如果在数学上是合理的话，则可以通过重复分析来减少由年龄计算得出的相对不确定性。 〜（40）Ar /〜（39）Ar方法的弱点在于它的基础知识，例如〜（39）Ar反冲，必要的封装，〜（39）Ar重新整合到步进加热模式中，以及阶梯加热模式的含义比有意义的地质年代更能暗示由辐射在粘土颗粒之间形成的〜39 Ar储层。如果K-Ar方法是确定成岩粘土过程（如青光岩化，伊利石晶体成核和生长或低温水热活动）的首选方法，则〜（40）Ar /〜（39）Ar方法具有更大的潜力记录了低温构造热活动，并详细介绍了多代伊利石的混合物。

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