Findings

Introduction

Recent development in mass spectrometry, such as the increase of sensitivity and the adoption of multiple collector system, enables to analyze the isotopic ratio for the single grain and spot analysis. Since the introduction of in-situ age dating for the small geological samples such as SHRIMP (sensitive high-resolution ion micro probe), the complex geological events can be revealed more precisely. The U-Pb spot age dating for zircon grains suggests the new concept of early Earth (e.g., Wilde et al. 2001) and provides the important keys to solve the juxtaposed geological events in a certain region (e.g., Cheong et al. 2014). However, to reveal the full history of a certain orogeny, it is indispensable to get the age information for the low- to mid-temperature geological activity. The Ar isotope system, whose closure temperatures range from ca. 130 to 690°C (e.g., Faure and Mensing 2005), can provide good clue to solve it. In addition, for the basaltic rocks with rare minerals adequate for U-Pb dating, the Ar ages have become the good proxy for the eruption age. For these purpose, K-Ar age dating method was introduced in early 50s and had provided age information related to various studies, such as the geomagnetic polarity timescale, one of the essential foundations of plate tectonic concepts (McDougall, 2014). However, the possible inhomogeneity due to the separate analyses of K and Ar and no information about the Ar-loss and/or Ar-gain after the closure in Ar isotope system should be considered for the proper interpretation of K-Ar ages. The 40Ar/39Ar technique, a variant of K-Ar dating system, has now commonly replaced the conventional K-Ar method to overcome these disadvantages. In addition, the introduction of laser heating/ablation technique enables to measure Ar age of single grains and monitor the variation in Ar isotope within minerals. In this technical note, we describe the 40Ar/39Ar age dating protocol using the newly installed multi-collector noble gas mass spectrometer in Korea Basic Science Institute (KBSI) and also report its performance for some geological samples.

Principle of 40Ar/39Ar age determination

40Ar/39Ar age dating is based on the same principle of K-Ar method, which uses the decay of 40K to 40Ar through the electron capture and positron emission. The concentration of parent 40K is measured indirectly the neutron irradiation process which converts a part of 39 K to 39Ar through the 39 K(n, p)39Ar reaction. The constant 40K/39K ratio in samples enables to estimate the content of 40K in samples through the measurement of 39Ar, if we exactly know how many portions of 39K in samples are converted to 39Ar during the neutron irradiation. The 40Ar/39Ar age is calculated using the following formula (McDougall and Harrison 1999):

$$ \mathrm{t}=\frac{1}{\lambda}\cdot \ln \left(1+J\cdot \frac{{}^{40}A{r}^{*}}{{}^{39}A{r}_{\mathrm{K}}}\right) $$

where

$$ J = \frac{{}^{39}K}{{}^{40}K}\cdot \frac{\lambda }{\lambda_{\mathrm{e}}+{\lambda}_{\mathrm{e}}\hbox{'}}\cdot \varDelta \mathrm{T}\cdot \int \phi \left(\mathrm{E}\right)\sigma \left(\mathrm{E}\right)\mathrm{d}\mathrm{E} $$
t :

age

λ :

total decay constant of 40K (5.534 × 10−10/year)

λ e :

decay constant of 40K to 40Ar by electron capture (0.572 × 10−10/year)

λ e′:

decay constant of 40K to 40Ar by emission of a positron (0.0088 × 10−10/year)

40Ar*:

radiogenic 40Ar

39ArK :

39Ar derived from 39K by the reaction with fast neutron

ΔT :

duration of irradiation

ϕ(E):

neutron flux density at energy E

σ(E):

neutron capture cross section at energy E

As the precise values of ϕ(E) and σ(E) are difficult to obtain, standard minerals of age-known are used to calculate J values using the following equation:

$$ J=\frac{ \exp \left(\lambda t\right)-1}{{}^{40}Ar*/{}^{39}A{r}_{\mathrm{K}}} $$

During neutron irradiations, the isotopes of Ca, K, and Cl in the samples commonly produce interfering Ar isotopes through interactions of neutron. Reactor-derived 40Ar from 40K, and 39Ar and 36Ar from 42Ca and 40Ca, respectively, are corrected by analyzing K- and Ca-salts irradiated together with samples. For the detailed calculation for these interfering isotopes, refer to McDougall and Harrison (1999).

Availability and requirements

Outline of the Ar age dating system

40Ar/39Ar age dating system in KBSI can be divided into the following three parts: (1) laser heating system, (2) gas preparation bench, and (3) high-sensitivity noble gas mass spectrometer (Figure 1).

Figure 1
figure 1

ARGUS VI system in KBSI. (a) Photo of whole 40Ar/39Ar age dating system and (b) schematic illustrations for the gas preparation system.

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Laser heating system

Laser heating system (Fusions 10.6, Photon Machines) consists of an integrated CO2 laser source and beam-delivery system with CCD camera and sample chamber. The laser source generates the continuous CO2 laser with the wavelength of 10.5 μm. Gantry type beam delivery system consists of three mirrors, one iris and one focusing lens. Whole laser delivery system also navigates 3-dimensionally over sample chamber. The intensity and beam size of laser including the movement of beam delivery system are controlled by the Chromium II software. The sample chamber is made up with a 114 mm diameter Conflat flange and differentially pumped ZnS viewport by turbomolecular pump. A copper sample holder of 43 mm diameter and 5 mm in thickness with 133 holes is loaded into sample chamber for single grain analysis. KBr glass covers the sample holder to prevent the spillover and scattering of sample from the hole during heating. After sample loading, sample chambers are heated by infrared lamp during several hours to achieve good vacuum level. Released gases from sample by laser beam diffuse from sample chamber into gas preparation system.

Gas preparation system

The outline of the gas preparation system is shown in Figure 1b. All pipe line is made from internally polished stainless steel. In order to purify argon from the extracted gases, three SORB-AC getter pumps (NP10) are used. They are constructed from a cartridge of getter material (ST101 alloy of zirconium with 16% aluminum) placed around an axial heater. At room temperature, these getters pump out hydrogen and carbon monoxide which are major background gases in the mass spectrometer. The getter can be run at 400°C to enhance the pumping of less reactive gases such as hydrocarbons. The vacuum level of gas preparation system reaches approximately 2 × 10−9 mbar by the turbomolecular pump and ion pump. The air of 0.1 cm3 from the automatic pipette system consisting of standard volume and two pneumatic valves is routinely measured to derive the discrimination factor.

Mass spectrometer

The purified argon gas through gas preparation system is directly introduced into the mass spectrometer (ARGUS VI). ARGUS VI is an all-metal single focusing, 13 cm radius, 90° extended geometry magnetic sector mass spectrometer designed for operation in static mode (Mark et al. 2009). A Nier-type electron bombardment source is equipped with y- and z-focusing. All source parameters, including the 5 kV acceleration potential, are computer-controlled and monitored by electronic read-backs. This system is equipped with five Faraday detectors and one compact discrete dynode (CDD) detector for the simultaneous measurement of five isotopes of argon (36Ar, 37Ar, 38Ar, 39Ar, and 40Ar). The 40Ar beam is measured in the high-mass (H1) detector fitted with a 1011 ohm resistor. The 39Ar to 37Ar beams are collected in other Faraday detectors fitted with 1012 ohm resistor. The 36Ar beam is measure by CDD detector. The amplifiers for Faraday detectors are kept in housing evacuated to 1–2 mbar in order to reduce electronic noise. Sensitivity at 200 μA trap current for Ar reaches to 1.15 Amps/Torr and the mass resolution (M/ΔM) is ca. 250. The background value during sample analysis is <1.0 × 10−16 mols 40Ar and <1.8 × 10−18 mols 36Ar in static mode.

System integration and data reduction

Laser heating system, gas preparation system, and mass spectrometer are integrated and controlled by the MassSpec software (Alan Deino Software). It communicates with Chromium II software in laser heating system and Qtegra software in ARGUS VI and controls each parameter in source electronics, magnet control, and steering plate in front of multiple collectors as well as the manipulation of gas preparation system. In addition, it enables the scheduled automatic analysis for tens of analysis. Fitting of raw data, age calculation, and data presentation are also performed using MassSpec software.

Experimental procedures for 40Ar/39Ar age

Neutron irradiation

As suggested in the above section, neutron irradiation process is indispensable for the 40Ar/39Ar dating. For irradiation, several grains of sample were dropped in each hole on aluminum sample disk with 10 holes of 5-mm diameter along with K2SO4 and CaF2 monitors. Age-known standard minerals were placed in the center or margin of sample disk as the neutron flux monitor. Fisher Canyon sanidine (FCS, 28.294 ± 0.036 Ma: Renne et al. 2010) and Alder Creek sanidine (ACS, 1.193 ± 0.001 Ma: Nomade et al. 2005) were used as flux monitor minerals. Two or three sample disks were stacked and wrapped by 0.25-mm-thick cadmium foil in order to reduce the effect of thermal neutrons. Samples were irradiated for 100 h at IP-4(c) position of hydraulic rabbit irradiation facility in HANARO research reactor with the total neutron flux of ca. 3.7 × 1013n/cm2s in Korea Atomic Energy Research Institute (KAERI).

Argon isotope measurement

One or several grains of irradiated samples are placed in a hole on the 133-hole sample holder and then evacuated to ultra high vacuum of ~10−9 mbar. Multiple run sequences for sample analysis including air and blank measurement are executed on the MassSpec software. The heating of samples and subsequent cleaning of extracted gas and the measurement of isotopes are sequentially performed by previously defined procedure file. After heating samples for 6 seconds and purifying gases for another 4 minutes with 3 SAES getters, the argon gas is introduced to the mass spectrometer. Each signal of argon isotopes is measured by the multiple data collection using five detectors using H1 to CDD detector or by the peak jumping method using CDD for 36Ar to 39Ar and H1 detector for 40Ar, respectively. For the baseline, the signals at mass 35.7 and 40.3 are measured at the start and end of the analysis. Blank analyses are made every three unknown sample analysis with the identical condition to the actual samples except for laser heating. For the mass discrimination, standard air is measured after every 10 analysis. Variation of blank and mass discrimination factor during analysis are corrected by parabolic regression. Net intensity of each Ar peaks is extrapolated to the zero time when the argon gas was introduced into the mass spectrometer. After correcting the blank and the mass discrimination, J values and ages are calculated for the standard and unknown samples.

Determination of J value

As neutron flux changes considerably across the sample disk, monitor minerals such as FCS and ACS sanidine are placed in three positions to correct the horizontal gradient in neutron flux. For real samples, the corrected J value along the different position of samples in the sample disk is applied to the age calculation according to the 3-dimensional modeling for the horizontal flux gradient (Figure 2).

Figure 2
figure 2

3-Dimensional variation in J values across one irradiation disk.

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Results and discussion

Single-grain total fusion 40Ar/39Ar analyses were repeatedly performed on two standard materials in order to evaluate the performance of ARGUS VI system (Table 1). Figure 3 shows the age probability diagrams for the total fusion ages from single grains of FCS and ACS. The measured value of FCS (Figure 3a) is 28.4 ± 0.5 Ma (1σ, MSWD = 138, n = 24) which is consistent with recommended value (28.294 ± 0.036 Ma: Renne et al. 2010) within uncertainty. Figure 3b shows the single-grain total fusion ages of ACS sanidine (1.185 ± 0.004 Ma, 1σ, MSWD = 6.05, n = 26), which also coincide with recommended age value (1.193 ± 0.001 Ma, Nomade et al. 2005). Such performance shows that the abovementioned 40Ar/39Ar age protocol can be successfully applied to the Quaternary rocks such as ACS.

Table 1 Representative analysis of 40 Ar/ 39 Ar age for Fisher Canyon sanidine (FCS) and Alder Creek sanidine (ACS)
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Figure 3
figure 3

Age probability diagrams for the 40 Ar/ 39 Ar age of standard materials. (a) Fisher Canyon Sanidine. (b) Alder Creek Sanidine.

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For age dating of young rocks (<1 Ma), the exact derivation of radiogenic 40Ar (40Ar*) is indispensable to get meaningful 40Ar/39Ar ages, so that it needs to minimize the reactor-derived 40Ar from 40K. In this study, the (40Ar/39Ar)K with the 0.25-mm-thick cadmium shielding reaches approximately 0.08, which is higher than the majority of reactor facilities used for 40Ar/39Ar age dating (McDougall and Harrison 1999) owing to high slow/fast neutron flux of >780 in Hanaro (Nam S, personal communication). To reduce the incidence of slow neutrons and, thus, the production rate of 40Ar, thicker cadmium shielding should be highly considered.

Conclusions

40Ar/39Ar dating system with multi-collector mass spectrometer and CO2 laser heating device has been established at KBSI in order to date young and very small amounts of minerals and volcanic rocks. Preliminary experiments on sanidine standards show the reliable age results of total-fusion dating for single grains, i.e., 28.4 ± 0.5 Ma (1σ, MSWD = 138, n = 24) for FCS sanidine, and 1.185 ± 0.004 Ma (1σ, MSWD = 6.05, n = 26) for ACS sanidine as Quaternary age standard, within recommended values (28.294 ± 0.036 Ma and 1.193 ± 0.001 Ma, respectively).