Exploring different K-Ar methodologies in the NG-MS Noblesse

K-Ar was one of the first wide-range techniques to dating minerals rich in potassium, mainly in the second half of the last century. Subsequently, ⁴⁰Ar/³⁹Ar was developed, giving more robust data results since the step of potassium determination became unnecessary in the current methodology. Now, all required elements for dating have been determined in the same experiment routine. Although ⁴⁰Ar/³⁹Ar delivers accurate data, it becomes a challenge for many laboratories since the samples need to be exposed to a neutron reactor to convert potassium to argon ³⁹K(n,p)³⁹Ar. Due to this constraint, the K-Ar has been developing in particular branches of study, such as geological dating of younger tectonothermal events using the same instrumentation.

The Noble Gas Mass Spectrometer (NG-MS) Noblesse installed in the laboratory is assembled with a sample preparation system specially designed for argon gas, which contains two methods for the entrance of samples (laser fusion and resistive furnace). After the gas extraction of the grains, the gases are exposed to a SAES PG-50 getter working at 400°C to clean active gases (N₂, CO, and CO₂). Then, gases are expanded to an NP-10 getter at room temperature to remove H₂ and to a cold finger (PolyCold P102) to trap residual CO₂ and interstitial moisture.

This study deals with two sets of standard samples (AG1550 – 98.5 Ma (N=13) and KA86 – 97.9 Ma (N=10)). The goals involve three methodological tests for K-Ar dating, which take one of the standards to work as input parameters and then adopt the methodological approaches to obtain the ages of the unknown samples. Briefly speaking, the first method considers a calibration curve for the equipment, building from one of the standards (input), then takes it to calculate all the output parameters required from the unknown samples; the second method explores the direct inter-calibration of data with standard to calculate parameters of unknown samples, in the last one, the data are displayed as isochron, and its ages are obtained by conventional linear fitting. To quantify the amount of potassium in the samples, each sample were weighted in a micro-balance to determine the total mass; in addition, the oxide potassium rate content was achieved using a micro-probe analysis in the WDS acquisition mode. As usual, before starting the routines of analysis, it was certificated that all mass spectrometer parameters, such as background, signal stability, and sensitivity, were optimized. In complement, all parameters were well monitored using aliquots of atmospheric air highly diluted during the experiments.

The partial results based on the standards (GA1550 and KA86) infer a background signal for ⁴⁰Ar = 5.3x10^-15 mol, ³⁸Ar = 4.7x10^-16 mol, and ³⁶Ar =5.0x10^-16 mol. The regular atmospheric air aliquot counts for ⁴⁰Ar_air = 1.99x10^-13 mol, which are compatible with the argon range extracted in the bench of samples. Also, the results have returned stability signal for air aliquot shots (N=25), giving values for ⁴⁰Ar/³⁶Ar = 296.2±2.1 and ³⁸Ar/³⁶Ar = 0.1883±0.0011, all of them obtained during the experiments routines. The age calculated for the supposed unknown (standard KA86 of the 97.9Ma) by the first method (calibration curve) was 97.2±1.5Ma; when considered the second method (inter-calibration of data), the age was 97.6±1.9Ma, and considering the third method (isochron ⁴⁰Ar/³⁶Ar vs ⁴⁰K/³⁶Ar) the age calculated was 97.0Ma and the curve intercept for ⁴⁰Ar/³⁶Ar variable returned the value 299, the last one, as expected for atmospheric inclusion of argon. The results around the standards are consistent, and they also show that the NG-MS is well-prepared to date minerals using the K-Ar methodologies. Likewise, implementing these methodologies in diverse K-Ar-bearing minerals contributes to overcoming the timing constraint of younger geological events.