Extraction of 40Ar-39Ar ages from a multicomponent mixture: a case study from the Tatra Mountains, Poland

Extraction of meaningful information on the timing of fault activity from clay gouges using radiometric dating methods, such as those based on the K-Ar system, can be challenging. One of the factors complicating interpretation of the radiometric dating results is the presence of multiple K-bearing components in the gouge material. In the current study, an attempt was made to develop a new interpretative method for K-Ar and 40 Ar- 39 Ar dating, capable of handling a three-component mixture. In addition, the mineral composition of clay gouges from the Tatra Mountains (Poland), which has not been investigated before, is reported. The mineral compositions of the bulk clay gouge material and separated size fractions were determined by X-ray diffractometry and Fourier-transform infrared spectroscopy. The gouge samples were composed of quartz, dioctahedral mica (as a discrete phase and as a component of mixed-layered illite-smectite), and chlorite, commonly with plagioclase and more rarely with K-feldspar, dioctahedral smectite, calcite, anatase, or trace kaolinite. One feldspar-free sample containing three mica polytypes (1 M d , 1 M , and 2 M 1 ) was chosen for dating with the 40 Ar- 39 Ar method. The results of 40 Ar- 39 Ar dating were interpreted using three concepts: Illite Age Analysis (IAA), a method based on the MODELAGE software, and a newly developed three-component concept. The age values obtained with IAA were −14 Ma ± 31 Ma and 180 ± 91 Ma for authigenic (1 M d ) and inherited (1 M + 2 M 1 ) components, respectively. The MODELAGE -based approach returned –4 ± 40 Ma and 165 ± 62 Ma. The three-component approach returned age values of polytypes as follows: 1 M d , 15 ± 37 Ma; 1 M , 135 ± 57 Ma; 2 M 1 , 121 ± 56 Ma based on the medians and the interquartile ranges of non-normal distributions of Monte Carlo-simulated age values. The results obtained indicated that the 1 M d polytype was probably formed during the most recent stage of fault activity, while 1 M and 2 M 1 polytypes are of equal age, roughly.