Coupled K–Ca and Rb–Sr dating by LA-ICP-MS/MS – reaction gas optimisation and geological applications

Both the Rb–Sr and K–Ca β-decay isotopic systems can be used to date a range of mica- and feldspar-group minerals and have the potential to unravel cooling and alteration processes across a wide range of geological settings. The development of LA-ICP-MS/MS has enabled direct in-situ analysis of these β-decay geochronometers via chemical separation with reactive gases within the mass-spectrometer. In addition to rapid analysis, a main advantage of in-situ K–Ca dating is the ability to avoid Ca- and Sr-bearing inclusions, which are a limiting factor for conventional bulk mineral dating via TIMS. Both Sr and Ca are highly reactive with both SF₆ and N₂O to form M-F, M-O or M-OH reaction products. In contrast, K and Rb are non-reactive with either gas enabling efficient separation of the parent-daughter isotope pairs, ⁴⁰K–⁴⁰Ca and ⁸⁷Rb–⁸⁷Sr. Additionally, mixing a small amount of H₂ with SF₆ or N₂O efficiently eliminates ⁴⁰Ar based interferences and reduces the background generated by the high ion load in the reaction cell when measuring mass to charge ratio ($m/z$) 40. This study compares the accuracy, precision, and product ion sensitivity of four reaction gas combinations: SF₆ only, SF₆ plus 2 mL min⁻¹ H₂, N₂O plus 7 mL min⁻¹ H₂, and N₂O plus 10 mL min⁻¹ H₂, by analysing a range of micas and feldspars with previously constrained dates: MDC and Kola phlogopites, Högsbo and Robins Folly muscovites, G71560 polylithionite, and F-KN and Bohus K-feldspars. Using these gas mixtures, coupled Rb–Sr and K–Ca dates were obtained from a single ablation spot in low Ca-bearing (5–300 ppm) micas and feldspars, with age uncertainties of 2 % and 5 %, respectively. The direct coupling of Rb–Sr and K–Ca dates within the same ablation volume allows assessment of isotopic disturbances at high spatial resolution. The gas combination of SF₆ plus 2 mL min⁻¹ H₂ was found to be the most effective for coupled K–Ca and Rb–Sr dating, producing the highest sensitivity for reacted species while minimising the background on reacted ⁴⁰Ca. Analysis of the FK-N feldspar from Madras, India shows the potential for the two isotopic systems to reveal decoupled dates, with the 515±22 Ma Rb–Sr date representing the crystallisation of the granite and the 437±38 Ma K–Ca date indicating late-stage hydrothermal activity.

1 Introduction

Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) is a widely used technique for element and isotopic analyses in the earth sciences and has been used for U–Pb geochronology for over 30 years. Recent technological advancements have led to the development of ICP-MS/MS systems that utilise a reaction cell between two mass selecting devices. This enables selective reactions and chemical separation of atomic and polyatomic interferences using a range of reactive gasses. Coupling this with in-situ sampling by laser ablation has led to an expansion of applications, especially in geochronology (e.g. Gilbert et al., 2024; Gorojovsky and Alard, 2020; Olierook et al., 2020; Tillberg et al., 2020; Laureijs et al., 2021a; Rösel and Zack, 2022; Simpson et al., 2023; Glorie et al., 2023; Tamblyn et al., 2022; Brown et al., 2022). Over the past decade, this technology has been applied to the analysis of β-decay geochronometers such as Rb–Sr, K–Ca, Lu–Hf and Re–Os (Redaa et al., 2021; Simpson et al., 2021; Tamblyn et al., 2024; Hogmalm et al., 2017, 2019; Zack and Hogmalm, 2016). In these systems, the parent and daughter isotopes have the same isotopic mass, creating an isobaric interference. These can be effectively separated using a range of reaction gases: N₂O, SF₆, HN₃ or CH₄.

Radioactive ⁴⁰K decays via branched pathways to ⁴⁰Ca (β⁻decay, ∼89 %) and ⁴⁰Ar (electron capture, ∼11 %), enabling both K–Ca and K–Ar (³⁹Ar–⁴⁰Ar) dating (Nägler and Villa, 2000). These decay systems can be used to date high K, low common Ca minerals such as biotite, muscovite and K-felspars. Due to similar chemical behaviour and ionic radii, Rb readily substitutes for K in these minerals; hence, minerals that are suitable for K–Ca dating are also suitable for Rb–Sr dating. Micas are readily susceptible to deformation and alteration with reactive fluids. Thus, both isotopic systems can be reset at low temperatures through element diffusion, crystal deformation, and dissolution/recrystallisation and have the potential to be used to determine cooling dates or to improve the understanding of low temperature diagenetic processes (Fletcher et al., 1997; Gopalan, 2008). Although closure temperatures in micas are poorly constrained, an apparent closure temperature for ⁴⁰Ca has been calculated at 300 °C (Harrison and Lovera, 2014), compared to 350–550 °C for the Rb–Sr system in micas (Jenkin, 1997; Nebel, 2014).

Analysis of highly precise Ca isotopes is routine via chemical dissolution and TIMS or MC-ICP-MS analysis for K-Ca dating of minerals (Farkaš, 2016; DePaolo, 2004; Antonelli and Simon, 2020; Gopalan, 2008; Heuser et al., 2002). These techniques can provide the highest level of analytical precision. However, the technique has some limitations: chemical dissolution and column chemistry can be time consuming and can easily incorporate common Ca-bearing inclusions within the target mineral; Ca has low ionisation efficiency by TIMS; and the precision of the ${}^{\mathrm{40}}\mathrm{Ca}{/}^{\mathrm{44}}$Ca ratios is compromised in high K/Ca minerals due to the low sensitivity of ⁴⁴Ca. Analysis by LA-ICP-MS/MS can overcome some of these limitations via rapid analysis at high spatial resolution and is ideally suited to highly radiogenic, low Ca-bearing minerals.

In-situ Rb–Sr and K–Ca dating via LA-ICP-MS/MS was first demonstrated using N₂O and SF₆ as reaction gasses (Hogmalm et al., 2017; Zack and Hogmalm, 2016). Uptake of in-situ Rb–Sr dating via LA-ICP-MS/MS has been rapid since then (Loyola et al., 2025; Wang et al., 2022; Subarkah et al., 2022; Liebmann et al., 2022; Jegal et al., 2022; Dauphas et al., 2022; Tillberg et al., 2021; Laureijs et al., 2021b; Olierook et al., 2020; Li et al., 2020). This is primarily due to the highly efficient and “simple” reaction chemistry of Sr with both N₂O and SF₆. However, the K–Ca analytical technique is currently underdeveloped and there are several isotopic and analytical challenges to be overcome for in-situ K–Ca dating.

The parent ⁴⁰K is a minor isotope of K (0.012 %), whereas the radiogenic daughter ⁴⁰Ca is also the most abundant common Ca isotope (96.9 %). The decay constant for ⁴⁰K (${\mathit{\lambda }}_{\mathit{\beta }}\mathrm{4.9548}\pm \mathrm{0.0134}\times {\mathrm{10}}^{-\mathrm{10}}$ a⁻¹; Renne et al., 2011) is larger than ⁸⁷Rb ($\mathrm{1.3972}\pm \mathrm{0.0045}\times {\mathrm{10}}^{-\mathrm{11}}$ a⁻¹; Villa et al., 2015), resulting in a shorter half-life and faster production of the daughter ⁴⁰Ca than ⁸⁷Sr: 1.25×10³ Ma (Marshall, 1998) versus 49.61×10³ Ma (Villa et al., 2015). Despite the faster radiogenic ingrowth of K–Ca, the low ⁴⁰K parent isotopic abundance requires ∼98 000 µg g⁻¹ of K to produce ∼4 µg g⁻¹ of ⁴⁰Ca, compared to ∼1000 µg g⁻¹ of Rb to produce the same amount of ⁸⁷Sr over 1 Ga (Zack and Gilbert, 2024). Therefore, the radiogenic ingrowth of ⁴⁰Ca can easily be overshadowed by the presence of common Ca in the mineral of interest or as inclusions. This study aims to provide a guide for the limitations of K–Ca dating in terms of the resolvable age versus the common Ca content.

In addition, ⁴⁰Ar (99.6 % abundance) is also present at exceedingly high concentrations from the Ar burning ICP. It has been shown that the daughter ⁴⁰Ca can be efficiently separated from the non-reactive ⁴⁰K and ⁴⁰Ar using N₂O or SF₆ reaction gases (Hogmalm et al., 2017). However, the high ion load from ⁴⁰Ar⁺ passing through the first quadrupole into the reaction cell has the potential to compromise the analysis and reaction mechanisms. In this study, we investigate the effects of adding H₂ as a second reaction gas to mitigate the Ar⁺ via charge transfer reaction (Ar${}^{+}+$ H${}_{\mathrm{2}}\to \mathrm{Ar}+$ H${}_{\mathrm{2}}^{+}$; Eiden et al., 1996; Craig et al., 2021).

The ease of measuring multiple isotopes across the mass range is a distinct advantage for quadrupole LA-ICP-MS/MS analysis. This enables simultaneous measurement of two, or more, isotope systems from the same ablation volume. This study investigates the accuracy of simultaneous Rb–Sr and K–Ca isotopic measurements by LA-ICP-MS/MS in a range of micas and feldspars, and optimises the analytical procedure through comparison of N₂O, SF₆ and H₂ reaction gases.

2 Sample descriptions

2.1 Mica group minerals

The MDC phlogopite was cut from a $\mathrm{10}\times \mathrm{7}\times \mathrm{1.5}$ cm crystal, sourced from the Bekily region, Madagascar (Redaa et al., 2021). It is taken to be the equivalent in geological source and age as the Mica-Mg powdered reference material distributed by the Centre de Recherches Petrographiques et Geochimiques (CRPG; Govindaraju, 1979, 1995). The expected age of 519.4±6.5 Ma used in this study, is the average reported Rb–Sr, K–Ar and U–Pb age of the region (Hogmalm et al., 2017; and references therein). The initial ${}^{\mathrm{87}}\mathrm{Sr}{/}^{\mathrm{86}}$Sr ratio of 0.72607±0.00070 was measured from two diopside analyses from the Mica-Mg sample (Morteani et al., 2013). More recent ID-MC-ICP-MS results from the Mica-Mg powdered samples give a relatively imprecise date of 521±24 from 12 aliquots (Jegal et al., 2022) and 524.9±3.6 from 1 aliquot (Redaa et al., 2022), indicating some uncertainty in the batch-to-batch reproducibility of the Rb–Sr age from the Mica-Mg powdered sample.

The Kola phlogopite was sourced from the Kovdor Massif, located on the Kola Penisnsula, Kovdorsky District, Russia. The cm-scale phlogopite book was purchased from a mineral dealer and the exact location is unknown. Similar sized phlogopites characteristic of the deposit are formed from high temperature ultrabasic pegmatites within a carbonatite-bearing alkaline-ultrabasic complex (Amelin and Zaitsev, 2002; Krasnova, 2001). A narrow range of TIMS U-Th-Pb dates have been reported from the Kovdor Massif by Amelin and Zaitsev (2002): ca. 378 Ma (baddeleyite, ${}^{\mathrm{238}}\mathrm{U}{/}^{\mathrm{206}}$Pb lower intercept), 377.52±0.94 Ma (zircon ${}^{\mathrm{208}}\mathrm{Pb}{/}^{\mathrm{232}}$Th), 377.5±3.5 Ma (apatite, ${}^{\mathrm{238}}\mathrm{U}{/}^{\mathrm{206}}$Pb lower intercept) and 380.6±2.6 Ma (apatite and calcite, ${}^{\mathrm{238}}\mathrm{U}{/}^{\mathrm{206}}$Pb lower intercept). Baddeleyite dates of 377±4 and 383±3 have also been reported (Bayanova, 2006). Phlogopites have been dated by conventional Rb–Sr, giving a date of 378.3±1.5 Ma (Amelin and Zaitsev, 2002), recalculated using the decay constant of Villa et al. (2015) and initial ${}^{\mathrm{87}}\mathrm{Sr}{/}^{\mathrm{86}}$Sr of 0.70368±0.00055. A weighted mean date based on U–Pb and Rb–Sr analyses of 378.1±1.2 was used as the reference age in this study (Table 1).

Table 1 Reference ages for minerals used in this study, with additional details in text.

^* Approximate common (c) Ca and Sr concentrations, measured on ⁴³Ca and ⁸⁸Sr, respectively (Table S10 in the Supplement). ${}^{\mathrm{40}}\mathrm{Ca}{/}^{\mathrm{44}}$Ca initial assumed to be 47.15±0.02 (Russell et al., 1978).

The Robins Folly muscovite was sourced form undeformed pegmatites in the Hartz Range metaigneous province, Northern Territory, Australia, emplaced during the late stages of the Alice Springs Orogeny (450–300 Ma). The sample was collected near the Ruby Mines from the same location as described in Mortimer et al. (1987). This study reported a whole rock Rb–Sr date of 320±22 Ma (recalculated using the decay constant of Villa et al., 2015) with an initial ${}^{\mathrm{87}}\mathrm{Sr}{/}^{\mathrm{86}}$Sr ratio of 0.7445±0.0028. Additional model muscovite and biotite dates from the same area are reported as 327–337 Ma and 329±2 Ma respectively (Mortimer et al., 1987), suggesting up to 5 % age variability between samples.

The Högsbo muscovite was sampled from the Högsbo pegmatite quarry in Gothenburg, Sweden. The crystallisation age of the pegmatite, constrained by a columbite ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}$Pb TIMS date of 1029.7±2.0 Ma (Romer and Smeds, 1996), was used as the reference in this study, along with an initial ${}^{\mathrm{87}}\mathrm{Sr}{/}^{\mathrm{86}}$Sr ratio of 0.72000±0.00010 (Hogmalm et al., 2017). In-situ Rb–Sr dates have been reported as 1037±11 Ma from the Högsbo nano-powder pellet (Hogmalm et al., 2017), and 1035±7 and 1039±7 Ma from muscovite crystals (Rösel and Zack, 2022). More recently, an ⁴⁰Ar–³⁹Ar muscovite date of 1025.2±2.2 Ma, and in-situ Rb–Sr dates between 1023.0±2.4 and 1024±7 Ma, have been reported in Olierook et al. (2026). Significantly younger in situ Rb–Sr dates have also been reported within the same muscovite crystals: 1003±6 (Rösel and Zack, 2022) and 981±8 Ma (Olierook et al., 2026), suggesting some alteration of the pegmatites is recorded in the muscovite.

The G17560 polylithionite was sourced from the South Australian Museum collection from a non-specific location near Itinga, Minas Gerais, Brazil. This would place it within the Araçuaí District in the Eastern Brazilian Pegmatite Province. Similar gem-quality polylithonite specimens have been found in an artisanal mine within the Taquaral pegmatite field. Two different pegmatite suites have been identified in this region with zircon U–Pb dates of nearby granite bodies ranging from 520–480 and 530–500 Ma (G5 and G4 supersuites; Pedrosa-Soares et al., 2011). There are no published Rb–Sr dates for these pegmatites, nor measured ${}^{\mathrm{87}}\mathrm{Sr}{/}^{\mathrm{86}}$Sr initial ratios. For this study, the initial was assumed to be 0.710±0.015 (assumed for enriched magmatic rocks; Rösel and Zack, 2022). The polylithionite contains very high Rb (∼1.4 wt %) and negligible common Sr, making the Sr component highly radiogenic. Hence, the choice of initial Sr ratio has no significant effect on the calculated Rb–Sr date.

All micas were sub-sampled from large mica crystals and mounted in epoxy blocks perpendicular to cleavage. Mounting in this orientation produces the most consistent spot-to-spot ablation compared to mounting parallel to cleavage (Larson et al., 2025).

2.2 K-feldspars

The FK-N K-feldspar is used as an international trace element reference material, distributed by CRPG (Govindaraju, 1984), sourced from Tamil Nadu Province, India. Feldspar chips from the original source were obtained and mounted for analysis. The FK-N powdered sample has been dated by Jegal et al. (2022), giving an Rb–Sr date of 512±30 Ma. Co-genetic pegmatites and leucogranites from the region have also been dated giving a Rb–Sr date of 521±9 Ma and initial ${}^{\mathrm{87}}\mathrm{Sr}{/}^{\mathrm{86}}$Sr ratio of 0.7093 (Pandey et al., 1993).

The Bohus K-feldspars were separated from the Bohus granite, located in south-west Sweden. The granite massif compromises of several granitic intrusions, and has been commercially quarried since the 1840's. The granite intrudes into the Dal Group (1100 to 1050 Ma) at the end of the Sveconorwegian orogeny. It has a Rb–Sr date of 910±35 Ma and an initial Sr ratio of 0.711, and K–Ar model-ages range from 861 to 680 Ma (Skiöld, 2010). A higher precision zircon U–Pb date of 920±5 Ma is regarded as the crystallisation age of the Bohus granite (Eliasson and Schöberg, 1991).

3 Analytical Methods

3.1 Instrumentation

Analyses were conducted at Adelaide Microscopy, Adelaide University, using a RESOlution-LR 193 nm excimer laser system coupled to an Agilent 8900x ICP-MS/MS. The interface tubing between the laser sample chamber and the ICP torch was cleaned prior to each analytical session and was not used for the ablation of high Ca materials (e.g. NIST glasses, 8.15 wt % Ca) to minimise the potential for Ca-contamination. A pure SiO₂ glass was also ablated at the start of each session to mobilise any residual, previously ablated aerosol particles in the tubing (Gilbert et al., 2014). A Teflon bulb (∼20 mL) was used as the signal smoothing device rather than a squid, given the latter is difficult to clean and remain Ca free. Carrier gas flow rates were 350 mL min⁻¹ He, mixed with 890 mL min⁻¹ Ar, and the addition of 6 mL min⁻¹ N₂ to enhance sensitivity. The laser parameters for all sessions were: 3.5 J cm⁻² fluence, 5 Hz repetition rate, and 100 µm spot size. All instrument parameters, including ICP-MS/MS lens tuning, are provided in Table S1.

A range of reaction gas mixtures and flow rates were tested based on the previously published reaction products of Sr and Ca (Hogmalm et al., 2017; Redaa et al., 2021). The aim was to determine the optimal gas flow rates in terms of reaction product ion sensitivity and the signal to background ratio for ^40+XCa (where X represents the mass shift of the product ions, see reaction gas optimisation section below). The four gas combinations selected for detailed testing were: SF₆ only (0.77 mL min⁻¹), SF₆ and H₂ mix (0.57 mL min⁻¹ and 2 mL min⁻¹, respectively), and N₂O and H₂ mixed in two different proportions: 0.16 mL min⁻¹ N₂O and 7 mL min⁻¹ H₂; and 0.16 mL min⁻¹ N₂O and 10 mL min⁻¹ H₂. These setting are referred to as SF₆ only, SF₆ + H₂, N₂O + low H₂ and N₂O + high H₂, from here on. Using N₂O as the reaction gas without H₂ was not tested in detail due to the significant production of ⁴⁰Ar¹⁶O which causes an interference with the target ⁴⁰Ca¹⁶O reaction product. When H₂ is added, Ar is neutralised via a charge transfer reaction (Eiden et al., 1996). In our experiments, no Ar reaction products were detected (measured as ^36+XAr), indicating the charge transfer reaction is thermodynamically favourable in the reaction cell. Both SF₆ and N₂O gases were introduced into the reaction cell using the third mass flow controller (MFC3), N₂O as 99.999 % purity, and SF₆ as a mixture of 10 % SF₆ in He (referred to as SF₆ throughout). The 99.999 % purity H₂ was introduced via a dedicated MFC.

3.2 Reaction products

To determine the most abundant rection products for Ca and Sr, product ion scans (PIS) were conducted. The first quadrupole (Q1) was set to a mass-to-charge ratio ($m/z$) of 44 or 88, respectively, and Q2 scanned across the mass range with mass shifts of 0 to 150 amu. When using SF₆, both with and without the addition of H₂, the predominant reaction products were CaF and SrF (+19 amu mass shift; Fig. 1a). For reactions with N₂O plus H₂, hydroxide (OH) product ions were most abundant (+17 amu mass shift; Fig. 1b). Thus, based on the PIS results, the highest abundant reaction products of Ca¹⁹F, Sr¹⁹F, Ca¹⁷OH and Sr¹⁷OH were used in further analysis. For both Ca and Sr, measurable amounts of hydrides and oxides were also present. For Ca, heavier Ca(H₂O)OH and Ca(H₂O)F were also detected. These H₂O bearing reaction products may have been caused by a contaminant (e.g. air or moisture) in the H₂ gas lines, as these occurred without the deliberate addition of oxygen i.e. with SF₆. Additional gas line flushing is recommended to reduce the production of these molecules.

Figure 1 Product ion scans for ⁴⁴Ca (a) and ⁸⁸Sr (b) using SF₆ + H₂ and N₂O + H₂ reaction gases, measured on NIST SRM 610 glass and plotted to show reaction products greater than 100 cps only. Major reaction product ions that were chosen for Sr and Ca analysis in further experiments are highlighted in bold. Circled in orange are the SF₆ gas breakdown products (SF_x).

For SF₆, the PIS also showed the SF_x breakdown products at masses 51, 70, 89, 108 and 127 amu (Fig. 1). These reaction products occur across 3 masses in each experiment, reflecting the isotopic proportions of S. As these product ions are created within the reaction cell from the breakdown of the SF₆ reaction gas itself, their occurrence is independent of the ions that passes through Q1 into the reaction cell. For example, the SF₆ reaction products were also measurable at the same rates when Q1 was set to 3 amu, meaning that no ions were entering the reaction cell. For K-Ca and Rb-Sr isotopic analysis, these products do not affect the measurements; however, they can have implications for the range of trace elements monitored in the same analysis; e.g. ⁵¹V, ⁵³Cr, ⁸⁹Y, and ⁹⁰Zr will all have elevated backgrounds when SF₆ is used as the reaction gas.

The isotopes measured with dwell times (ms) in brackets, were: ²⁷Al (5), ^36+XAr (20), ³⁹K (10), ^39+XK (20), ^40+XCa (50), ^42+XCa (50), ^43+XCa (50),^44+XCa (100), ⁸⁵Rb (20), ^86+XSr (50), and ^87+XSr (50). For the reacted isotopes X represents a mass shift of either 19 (e.g. ⁴⁰Ca¹⁹F) when using SF₆ reaction gas or 17 (e.g. ⁴⁰Ca¹⁷OH) when using the N₂O and H₂ mixture. The presence of unreacted ⁴⁰Ca and ⁸⁷Sr prevents the direct measurement of the parent isotopes, so ³⁹K and ⁸⁵Rb were measured as proxies for ⁴⁰K and ⁸⁷Rb respectively and calculated assuming natural isotopic abundance (Hogmalm et al., 2017; Redaa et al., 2021).

3.3 Analysis of mass 40 and reaction gas optimisation

Measurement of ⁴⁰Ca, the major Ca isotope (96.9 % abundance), is complicated by the presence of the major isotope of Ar (99.6 % abundance), as well as the parent ⁴⁰K. Since ⁴⁰Ar⁺ is the most abundant ion produced in the Ar burning ICP, the total number of ions passing through Q1 (set at 40 amu) and entering the reaction cell is very large, greater than the maximum detectable 8×10¹⁰ cps. The total ion load is thus likely to significantly exceed the maximum capacity of the reaction cell in terms of ensuring efficient and controlled reactions.

Product ion scans were recorded at $m/z$ 40, both in the gas background (⁴⁰Ar) and while ablating NIST SRM 610 glass (⁴⁰Ar and ⁴⁰Ca). Figure 2 illustrates the additional complexity of measuring ⁴⁰Ca compared to the measurement of ⁴⁴Ca. There are four major observations of note. Firstly, the enhancement of the reaction gas breakdown product ions e.g. 10⁶ cps for SF⁺ when Q1 = 40 (Fig. 2), compared to 2000 cps at Q1 = 44 (Fig. 1a).

Figure 2 Gas background product ion scans (no sample ablation) with Q1 = 40 amu using four different reaction gas mixtures: N₂O only (light blue), N₂O + H₂ (dark blue), SF₆ only (orange) and SF₆ + H₂ (red). The labelled high cps peaks represent the breakdown of the reaction gases themselves, and where Ca reaction products of interest would be measured at $m/z$ 57 and 59.

Secondly, N₂O product ions, such as N₂O-N₂O⁺ and N₂OH-N₂O⁺, were detected on $m/z$ 62 and 63 but were not detected at other Q1 masses (Fig. 1). This is likely due to the high density of Ar⁺, reacting with and breaking down the reaction gasses. Thirdly, N₂O and SF₆ can attenuate Ar⁺ ions via charge transfer reactions, although less effectively than H₂. The on-mass 40 amu measurement remains significant for reactions without H₂ (c. 10⁶ cps with SF₆ and c. 10⁸ cps with N₂O; Fig. 2); however, this is lower compared to when no reaction gasses are used (above the maximum 8×10¹⁰ cps). As predicted, the addition of H₂ to both SF₆ and N₂O reduced the detectable 40 amu to <1000 cps. Fourthly, a high gas background was observed on all Q2 masses when Q1 was set to 40 amu, indicated with the dashed line in Fig. 2. The addition of H₂ to SF₆ had little effect on this background (red and orange lines, Fig. 2), whereas its addition to N₂O significantly reduced the background (light and dark blue lines, Fig. 2). Therefore, the high background is likely to be caused by the presence of Ar⁺ ions, as opposed to the reactive H⁺ ions generated from the Ar-H₂ charge transfer reaction. This background can be significant at the CaF and CaOH reaction product masses, 57 and 59 amu (highest values > 10⁴ cps in the example below), and subsequent testing detailed below aimed to optimise the reaction gas flow rates to minimise this background. Ultimately, sensitivity is a function of signal to background ratio, thus background signals need to be carefully monitored prior to analysis.

The reaction gas flow rates and proportions between the gases (SF₆, N₂O and H₂) were systematically varied to determine their effect on ^40+XCa reaction products. Count rates on $m/z=\mathrm{57}$ (Q1 = 40, Q2 = 57) and 59 (Q1 = 40, Q2 = 59) were measured in the gas background and when ablating NIST610 to produce CaOH and CaF reaction products. The signal to background ratios were compared to determine the optimal flow rates for each reaction gas (Fig. 3).

Figure 3 Signal (SG) and background (BG) cps at $m/z$ 59 (CaF) for the optimisation of SF₆ gas flow rate. Units in % of total flow of the mass flow controller, where 30 % is a flow of 0.57 mL min⁻¹ and 40 % is 0.77 mL min⁻¹. Experiment 1 (blue symbols) is SF₆ gas only and experiment 2 (green symbols) is SF₆ with 2 mL min⁻¹ H₂.

As the SF₆ flow rate was increased from an initial rate of 20 % (0.38 mL min⁻¹), the sensitivity of CaF increased to a plateau at 30 % for SF₆ + H₂ (0.57 mL min⁻¹ SF₆, 2 mL min⁻¹ H₂) and 40 % for SF₆ only (0.77 mL min⁻¹), while the background decreased slightly or was stable (Fig. 3). These sensitivity plateaus occur at the flow rates that give the maximum reaction efficiency. Higher gas flow rates eventually attenuate the signal due to increased collisions in the cell. While the addition of SF₆ only can resolve the Ar interference on ⁴⁰Ca (no ⁴⁰ArF reaction), the SF₆ + H₂ mixture results in slightly higher sensitivity for ⁴⁰Ca¹⁹F and, more importantly, a significantly lower background (signal to background ratio increases from 3650 to 5770, Fig. 3).

Signal and background data was collected for a range of N₂O flow rates, from 0.16 to 0.32 mL min⁻¹ (15 %–30 %) using 7 mL min⁻¹ H₂ (Fig. 4a). The sensitivity dropped with increasing N₂O flow rate, resulting in a lower signal to background ratio. The same relationship was observed when using higher H₂ flow rates (e.g. 10 mL min⁻¹). Figure 4b presents cps data using 0.16 mL min⁻¹ N₂O (15 %) and varying the H₂ flow from 5–10 mL min⁻¹ (with 10 mL min⁻¹ being the highest flow rate achievable on the mass-flow controller). Signal sensitivity and background both decreased with increasing H₂ gas flow, however, the highest signal to background ratio was achieved using a maximum of 10 mL min⁻¹ H₂. The higher H₂ gas flow rates not only effect ⁴⁰Ca sensitivity, but they also effect the sensitivity of the minor ⁴⁴Ca, where higher sensitivity is beneficial to improve overall uncertainty of the measured ratios. The two conditions of 0.16 mL min⁻¹ N₂O and 7 mL min⁻¹ H₂ (N₂O + low H₂), and 0.16 mL min⁻¹ N₂O and 10 mL min⁻¹ H₂ (N₂O + high H₂), were used for further testing of mineral samples. The 7 mL min⁻¹ H₂ flow rate is a compromise in signal to background ratios, while maintaining higher signal sensitivity.

Figure 4 Signal (SG) and background (BG) cps at $m/z$ 57 (CaOH) for the optimisation of N₂O and H₂ gas flow rates. (a) Using 7 mL min⁻¹ H₂ and varying the N₂O flow from 15 % to 30 % (0.16 to 0.32 mL min⁻¹). (b) Using 15 % N₂O (0.16 mL min⁻¹) and varying the H₂ flow from 5–10 mL min⁻¹.

3.4 Calibration strategies

Isotopic ratios were calibrated using a glass reference material, manufactured at the Institute for Photonics and Advanced Sensing, Adelaide University. The glasses were made from SiO₂-Na₂O-(BaO)-MgO matrix doped with known quantities of K, Ca, Sr and Rb (referred to as SNBM-1 and SNM-1). The raw materials were ball-milled for 1 h to achieve thorough mixing and then loaded into a platinum crucible. The batch materials were melted at 1500 °C for 2 h and cast into a brass mould. On the following day, the sample was annealed at 520 °C for 2 h to relieve internal stress. To improve the homogeneity of the glass, the initial glass was powdered and re-melted at 1500 °C for 2 h, followed by the same casting and annealing steps as before.

The synthetic glasses were used in preference to the more commonly used high-Ca glasses: NIST SRM 610 (8.15 wt % Ca) and BCR-2G (5.05 wt % Ca), to avoid Ca contamination from remobilisation of previously ablated material in the interface tubing (i.e. false signal). The manufactured glasses also enabled customisation of the K, Rb, Sr and Ca concentrations to match the electron multiplier detector modes (pulse or analog counting) between reference materials and samples.

Initial testing used the SNBM-1 glass with a SiO₂-Na₂O-BaO-MgO matrix doped with 2000 ppm K and Rb, and 500 ppm Ca and Sr (data Tables S3, S4, S6 and S8). However, ablating this glass with 100 µm spot size and 5 Hz repetition rate resulted in ^40+XCa being measured in analog mode, while the samples were all in pulse mode. Note that ^44+XCa was always measured in pulse mode because the natural abundance ratio between ⁴⁴Ca and ⁴⁰Ca is c.47:1. Reducing the laser beam size or repetition rate to lower the ^40+XCa signal into pulse mode caused an unfavourable reduction in precision of the ratios for the mineral samples due to lower sensitivity of the minor ^44+XCa isotope. The MDC phlogopite (519±6.5 Ma, ${}^{\mathrm{87}}\mathrm{Sr}{/}^{\mathrm{86}}$Sr_i 0.72607; Hogmalm et al., 2017; Morteani et al., 2013; Redaa et al., 2021) was used as the mineral-matched reference material to correct for matrix-dependent mass bias on ${}^{\mathrm{87}}\mathrm{Rb}{/}^{\mathrm{87}}$Sr and ${}^{\mathrm{40}}\mathrm{K}{/}^{\mathrm{40}}$Ca (Glorie et al., 2024a) via Eqs. (1) and (2), respectively.

$$\begin{array}{}\text{(1)}& {\displaystyle }{\left({\displaystyle \frac{{}^{\mathrm{87}}\mathrm{Rb}}{{}^{\mathrm{87}}\mathrm{Sr}}}\right)}_{\mathrm{Corrected}}={\left({\displaystyle \frac{{}^{\mathrm{87}}\mathrm{Rb}}{{}^{\mathrm{87}}\mathrm{Sr}}}\right)}_{\mathrm{S}}\times {\displaystyle \frac{\left(\left(e^{\mathit{\lambda }{\mathrm{T}}_{\mathrm{M}}}\right)-\mathrm{1}\right)}{\left(\left(e^{\mathit{\lambda }{\mathrm{T}}_{\mathrm{R}}}\right)-\mathrm{1}\right)}}\text{(2)}& {\displaystyle }{\left({\displaystyle \frac{{}^{\mathrm{40}}\mathrm{K}}{{}^{\mathrm{40}}\mathrm{Ca}}}\right)}_{\mathrm{Corrected}}={\left({\displaystyle \frac{{}^{\mathrm{40}}\mathrm{K}}{{}^{\mathrm{40}}\mathrm{Ca}}}\right)}_{\mathrm{S}}\times {\displaystyle \frac{\left(\left(e^{\mathit{\lambda }{\mathrm{T}}_{\mathrm{M}}}\right)-\mathrm{1}\right)}{\left(\left(e^{\mathit{\lambda }{\mathrm{T}}_{\mathrm{R}}}\right)-\mathrm{1}\right)}}\end{array}$$

Where subscript S denoted the measured isotopic ratio of the sample corrected to SNBM-1 glass, T_R is the reference age of MDC and T_M is the raw measured date (Ma) calculated from the inverse isochron of the ratios corrected to SNBM-1 glass. Decay constants (λ) of $\mathrm{1.3972}\times {\mathrm{10}}^{-\mathrm{5}}$ and $\mathrm{5.5305}\times {\mathrm{10}}^{-\mathrm{4}}$ were used for the Rb-Sr and K-Ca systems, respectively (Renne et al., 2011; Villa et al., 2015). The correction to the matrix matched age reference material (MDC) was calculated in Excel.

The ^40+XCa signal was collected in pulse mode in MDC, compared to analog in the SNBM-1 glass. As the ^40+XCa in all other samples were measured in pulse mode, correcting the ${}^{\mathrm{40}}\mathrm{K}{/}^{\mathrm{40}}$Ca ratios to MDC also incorporated a pulse-analog component to the correction. The pulse and analog detector modes have been observed to drift relative to each other in some Rb-Sr sessions (Glorie et al., 2024a). To assess this, the SNBM-1 glass (analog) and MDC phlogopite (pulse) were both measured every 20–30 analyses throughout each analytical session. After correction to the SNBM-1 glass, no residual time-dependent drift was apparent in the MDC isochrons (see data tables in the Supplement).

The Kola phlogopite was used to calculate a pulse-analog correction for ${}^{\mathrm{44}}\mathrm{Ca}{/}^{\mathrm{40}}$Ca using the offset between the calculated, unanchored ${}^{\mathrm{44}}\mathrm{Ca}{/}^{\mathrm{40}}$Ca intercept and the expected ratio of 47.15±0.02 (Russell et al., 1978). Kola was used in preference to MDC for the pulse-analog correction for ${}^{\mathrm{44}}\mathrm{Ca}{/}^{\mathrm{40}}$Ca as it gave higher variability in measured ${}^{\mathrm{44}}\mathrm{Ca}{/}^{\mathrm{40}}$Ca ratios between analyses, leading to more robust isochron regression and calculated initial ${}^{\mathrm{44}}\mathrm{Ca}{/}^{\mathrm{40}}$Ca ratio. For the initial method validation, the pulse-analog correction was robust enough for comparative testing to optimise the reaction gas flows; however, it is not recommended as a routine protocol due to differences in mass-bias drift between pulse and analog modes (Glorie et al., 2024b).

A second glass, SNM-1, was made to address the pulse-analog issues identified in the initial testing. It had a SiO₂-Na₂O-MgO matrix doped with 2000 ppm K, 1000 ppm Rb, 100 ppm Ca and 500 ppm Sr. These concentrations were chosen such that ³⁹K and ⁸⁵Rb were measured in analog detector mode and ^40+XCa and ^87+XSr were measured in pulse mode at 100 µm spot size. This second glass better matched the detector mode in the samples to avoid the need for additional pulse-analog correction and is recommended for all future analyses. Analyses of the micas were repeated with the two most promising reaction gas mixtures (SF₆ + H₂ and N₂O + low H₂, data Tables S5 and S7) using the SNM-1 glass to validate the initial data sets without requiring an additional pulse-analog correction.

The synthetic glasses were used to correct for instrument drift, down-hole fractionation and matrix independent mass-bias using the LADR data processing software (Norris and Danyushevsky, 2018). The reference values used for normalisation of the measured isotopic ratios for both glasses were calculated from the nominal concentrations, assuming natural isotopic abundances and molar proportions.

All uncertainties are reported at the 95 % confidence interval. The individual measurement uncertainties include the excess scatter of the ratios measured in the glass reference material as calculated in LADR. The MDC age correction factor was applied to the measured ${}^{\mathrm{87}}\mathrm{Rb}{/}^{\mathrm{87}}$Sr and ${}^{\mathrm{40}}\mathrm{K}{/}^{\mathrm{40}}$Ca ratios, and the calculated MDC age uncertainty (typically ∼2 % for K–Ca and ∼1 % for Rb–Sr) was propagated onto the uncertainty of the sample isochron dates. The long-term reproducibility of Rb–Sr dates at the Adelaide Microscopy isotope facility is ∼1.5 %, which is similar to 1.6 % reported in Olierook et al. (2026), and 1.4 % in Rösel and Zack (2022). This additional systematic uncertainty was propagated onto the final dates: 1.5 % for Rb-Sr and a more conservative 2 % for K–Ca as a realistic estimate, in the absence of long-term data.

Either the SNBM-1 or SNB-1 glass was used as the primary refence material for ${}^{\mathrm{86}}\mathrm{Sr}{/}^{\mathrm{87}}$Sr ratios without further correction, as isotopic ratios of the same element are not subject to matrix-dependent fractionation (Redaa et al., 2021; Gorojovsky and Alard, 2020) and all Sr isotope measurements were in pulse detector mode.

3.5 Data treatment

The low-Ca glasses (SNBM-1 and SNM-1) were used in preference to high-Ca NIST610, to minimise the potential for carry over of Ca between reference glasses and samples. However, contamination was still seen in some ablation signals either from the remobilisation of previously ablated material (predominantly affecting the lower sensitivity isotope ⁴⁴Ca) or surface contamination on the samples (predominantly affecting ⁴⁰Ca, the most abundant common Ca isotope). This was especially apparent in low-Ca micas and for the initial data sets. To remove surface contamination, the samples were repolished and cleaned using distilled water, and 5–10 pre-ablation shots were used to clean the surface prior to sample ablation. During data processing signals were cropped where possible to avoid Ca spikes and surface contamination. Outlier analyses with high ⁴⁴Ca giving a young age bias, were excluded from the isochrons. These contamination issues were most prevalent in the initial data sets (Tables S4 and S6) and minimised in the later after sample cleaning (Tables S5 and S7). Example ablation signals are provided in Supplement Figures.

A relatively long counting time was used for the low abundance ⁴⁴Ca, to reduce the impact of high uncertainty at low count rates (cps): 100 ms, where the minimum detectable 1 count equates to 10 cps. The signals were also cropped during data processing to avoid any zeros in the ⁴⁴Ca signals to ensure accurate uncertainty calculations. Average background subtracted signal sensitivities (cps) are provided in Supplement Data tables.

All data were plotted as inverse isochrons (plotting parent/daughter versus stable/daughter isotope ratios) to minimise uncertainty correlations and improve data visualisation (Li and Vermeesch, 2021), using isoplotR (Vermeesch, 2018). The ${}^{\mathrm{87}}\mathrm{Sr}{/}^{\mathrm{86}}$Sr initial ratios used for plotting sample isochrons were either obtained from the literature when known (Table 1) or an assumed value based on the recommendations by Rösel and Zack (2022). A ${}^{\mathrm{40}}\mathrm{Ca}{/}^{\mathrm{44}}$Ca initial value of 47.15±0.02 was used for plotting all K-Ca isochrons, as the range of naturally occurring Ca isotopic ratios can be considered invariant within the uncertainty of the LA-ICP-MS/MS measurements (Russell et al., 1978). This is a significant benefit of the K–Ca method because it allows direct calculation of dates for every analysis without a need to assume initial ratios.

4 Results and Discussion

4.1 Down Hole Fractionation

Both $\mathrm{Rb}/\mathrm{Sr}$ and $\mathrm{K}/\mathrm{Ca}$ isotope ratio pairs have contrasting element volatilities with Rb and K being the more volatile (Jackson, 2008). Hence, down-hole fractionation (DHF) between the elements during ablation is likely and has been described elsewhere for $\mathrm{Rb}/\mathrm{Sr}$ (Gorojovsky and Alard, 2020; Redaa et al., 2021; Glorie et al., 2024a). The DHF effects were minimised in this study by using a relatively large laser spot size (100 µm) and 5 Hz repetition rate to reduce the depth to diameter ratio of the crater. Nevertheless, DHF was still observed for both isotope pairs (Fig. 5).

Figure 5 Down hole fractionation curves for (a) ${}^{\mathrm{85}}\mathrm{Rb}{/}^{\mathrm{87}}$Sr and (b) ${}^{\mathrm{39}}\mathrm{K}{/}^{\mathrm{40}}$Ca ratios in the SNM-1 glass, micas and feldspars, starting 5 s after the start of ablation (signal has stabilised by this time). Each line represents an average of between 10–30 individual signals, excluding those displaying the most variable common Ca signals. Contributing individual signals are centred to a value of 1.0 so that the effects of mass bias and drift are removed as per Lloyd et al. (2025). Shaded regions are the model uncertainties at 95 % confidence intervals; however, they are typically less than the width of the plotted lines at this scale. Note the y-axis scales in (a) and (b) are different to exaggerate the curves and aid visual interpretation. Polylithionite (pln), phlogopite (phl), muscovite (msc), K-feldspar (ksp).

For $\mathrm{Rb}/\mathrm{Sr}$ there is very little difference in DHF between the materials after the first 10 s of ablation, with the exception of Bohus feldspar which shows a more variable average signal curve during the ablation (Fig. 5a, Table S9; Lloyd et al., 2025). The relative differences between materials are more pronounced for $\mathrm{K}/\mathrm{Ca}$ ratios, especially during the first 10 s of ablation (Fig. 5b), with up to 10 % DHF, compared to 5 % for $\mathrm{Rb}/\mathrm{Sr}$. The SNM glass, MDC, G17560 and Kola micas all have relatively flat $\mathrm{K}/\mathrm{Ca}$ ratios which is likely to be representative of laser induced DHF. The more variable profiles primarily reflect variations in common Ca rather than differences in ablation behaviour. This was most pronounced for feldspar, where the fewest number of signals were averaged due to naturally heterogeneous common Ca within the ablation volume of the feldspars, producing more variable average $\mathrm{K}/\mathrm{Ca}$. The main outlier is Högsbo, for which the steeper curve is attributed to minor common Ca contamination on the surface, which potentially penetrated along cleaves into the subsurface. Despite 7 pre-ablation shots, the common Ca signal was highest at the start of each ablation before decaying to a stable level. When pre-ablation shots were recorded, the Ca signal was significantly higher than during the main ablation, compared to major elements which had similar sensitivity, indicating surface contamination of Ca. For both causes of variable common Ca, the most stable part of the signal was selected to calculate dates, although the whole signals are used to generate the ratio curves in Fig. 5. Typically, the first 10 s of ablation was not included in the signal selection to avoid common Ca contamination and to minimise DHF differences between matrices (see Supplement Figures).

The SNM-01 glass was used to correct for DHF in the other materials. The average curve for SNM-01 is slightly flatter compared to the micas (Fig. 5), which is likely to under-correct the minerals. However, no residual slope was observed on individual analysis ratios for the minerals within the noise of the signals, and any residual bias is assumed to be corrected as a component of the matrix-match mineral age correction (to MDC phlogopite). In the current absence of data processing software which can use isochronous mineral-matched reference materials to correct DHF directly, the suitability of SNM-01 as a DHF correction material is evaluated in this study. Differences in ablation characteristics between minerals (especially between the MDC mineral-matched reference material and K-feldspars) are likely to become more apparent at smaller spot sizes due to increased differences in DHF curves between matrices.

4.2 K–Ca and Rb–Sr dates of micas and feldspar

4.2.1 Reaction gas optimisation – micas

The MDC phlogopite was used as the matrix-matched reference material for all other samples analysed. Typical isochrons are shown in Fig. 6 for K–Ca and Rb–Sr (see Supplement Tables S3–8 for isochrons for all gas mixtures). There was a much larger spread in ${}^{\mathrm{40}}\mathrm{K}{/}^{\mathrm{40}}$Ca ratios compared to ${}^{\mathrm{87}}\mathrm{Rb}{/}^{\mathrm{87}}$Sr, despite the data points being collected from the same ablation volumes. The bimodal distribution of the ${}^{\mathrm{40}}\mathrm{K}{/}^{\mathrm{40}}$Ca ratios correlates with location on the MDC crystal. It reflects common-Ca heterogeneity within the mica, while the common-Sr is relatively homogeneous (Table 1). Therefore, the natural variation in minor common ⁴⁴Ca and ⁸⁶Sr is poorly correlated in this phlogopite which suggests that they can substitute differently into the mica structure in contrast to Rb which readily substitutes for K in the I-site in the general mineral formula for micas IM${}_{\mathrm{2}-\mathrm{3}}{\square }_{\mathrm{0}-\mathrm{1}}$T₄O₁₀A₂ (Rieder et al., 1998). The Ca will also substitute into the high coordination (often 12-fold) interlayer I-site along with K, Na and Rb. However, Sr has a preference for the 8-fold M-site (with Mg, Fe, Mn, Li etc.) but due to its large ionic radius will often be forced in the I-site (Harrison and Lovera, 2014; Tischendorf et al., 2007; Zhao et al., 2025). The MDC signals with the highest ${}^{\mathrm{44}}\mathrm{Ca}{/}^{\mathrm{40}}$Ca ratios had stable signals throughout the ablation, indicating ⁴⁴Ca is most likely incorporated within the mineral structure, rather than being a product from Ca-bearing mineral micro-inclusions. If Ca and Sr can be hosted differently in the mineral structure, this supports the theory that Ca and Sr can diffuse and be reset under different conditions, leading to potential de-coupling between the Rb–Sr and K–Ca apparent dates (Fletcher et al., 1997).

Figure 6 Representative K–Ca (a) and Rb–Sr (b) isochrons for MDC using SF₆ + H₂ reaction gasses. Dates are matrix-offset corrected to MDC itself; thus, they are equivalent to the reference age. Date uncertainties are analytical only, and this was propagated onto the date uncertainties of the other samples.

Robins Folly biotite and Högsbo muscovite give accurate dates within uncertainty for both Rb–Sr and K–Ca using three different reaction gas mixtures (Fig. 7, Table S2). The exception is N₂O + high H₂, which gave a K–Ca date for Robins Folly that is >5 % too old. The agreement between the K–Ca and Rb–Sr dates for these micas demonstrates that phlogopite (MDC) is a suitable matrix matched reference material for muscovite, within the uncertainty of the technique. It further demonstrates that the choice of reaction gas, SF₆ or N₂O, has little effect on the accuracy. The least consistency between isotope systems was obtained when using a N₂O + high H₂ flow. The highest sensitivity and highest signal to background ratio was achieved for reacted ⁴⁰Ca with SF₆ + H₂ (Figs. 3, 8). The highest Sr sensitivity was achieved using either SF₆ + H₂ and N₂O + low H₂. Using SF₆ only, or too high H₂ flow rate with N₂O, results in significantly lower sensitivity. The enhanced sensitivity with SF₆ + H₂ is considered advantageous for producing the best theoretical precision of individual isotope ratio measurements due to improved counting statistics on the lowest abundance isotope ⁴⁴Ca.

Figure 7 (a, b) Comparison in calculated K–Ca and Rb–Sr dates between the four reaction gas experiments for Robins Folly and Högsbo muscovites. Uncertainty bars represent 2SE propagated uncertainties. The solid horizontal line represents expected age, long and short dashed horizontal lines are ±2 % and 5 % uncertainty of the expected age, respectively. Indicative K–Ca (c, d) and Rb–Sr (e, f) isochrons using SF₆ + H₂ reaction gas, with matrix matched age correction to MDC phlogopite. Analytical and total systematic age uncertainties are both reported with the isochrons (x|y).

Figure 8 Sensitivity of ⁴⁰Ca¹⁹F or ⁴⁰Ca¹⁷OH, and ⁸⁸Sr¹⁹F or ⁸⁸Sr¹⁷OH, measured on SNMB-1 glass, plotted relative to the SF₆ + H₂ reaction gas mixture.

The Rb–Sr dates for Kola are slightly older than expected, but within 3 % when using SF₆ with or without H₂ (Fig. 9a). However, the K-Ca ages for Kola, corrected to MDC, are highly variable with poor reproducibility between repeat sessions with both SF₆ and N₂O reaction gases, and apparent dates tending up to ca. 6 % older than Rb–Sr dates (Fig. 9a). Likewise, G17560 K–Ca dates also show poor reproducibility (Fig. 10a), but with almost identical relative offsets as Kola. When G17560 is corrected against Kola, rather than MDC (Fig. 10b), the measured K–Ca dates are consistent within uncertainty for all reaction gas combinations tested, and K–Ca dates are slightly younger than the Rb–Sr dates. These two micas have the lowest common Ca of the micas analysed (<5 ppm), suggesting a potential bias or interference effecting the low abundance ⁴⁴Ca. Increasing the dwell time further for ⁴⁴Ca for future analyses may help to improve counting statistics and bias at low levels. This suggests that matching the Ca concentration between the matrix-matched correction material and the samples is important for low Ca micas.

Figure 9 (a) Comparison in calculated K–Ca and Rb–Sr dates between the four reaction gas experiments for Kola phlogopite. Uncertainty bars represent 2SE propagated uncertainties. Solid horizontal line represents expected age, long and short dashed horizontal lines are ±2 % and 5 % uncertainty of the expected age, respectively. Indicative K–Ca (b) and Rb–Sr (c) isochrons using SF₆ + H₂ reaction gas, with matrix matched age correction to MDC phlogopite. Analytical and total systematic age uncertainties are reported with the isochrons (x|y).

Figure 10 (a, b) Comparison in calculated K–Ca and Rb–Sr dates between the four reaction gas experiments for G17560 polylithionite normalised to MDC (a) and Kola (b). Uncertainty bars represent 2SE propagated uncertainties. Dashed horizontal lines indicate the expected age range from the region. Indicative K–Ca (c) and Rb–Sr (d) isochrons using SF₆ + H₂ reaction gas, with matrix matched age correction to Kola phlogopite. Analytical and total systematic age uncertainties are reported with the isochrons (x|y).

4.2.2 Feldspars

The Bohus and FK-N feldspars were analysed using SF₆ + H₂ and N₂O + low H₂ reaction gas mixtures (Fig. 11). The MDC-corrected Rb–Sr and K–Ca dates are accurate within uncertainty for Bohus feldspar (921±35 Ma and 924±66 Ma, respectively using SF₆ + H₂), indicating no significant matrix bias between mica and felspars under the laser conditions used. In contrast, the K–Ca dates for FK-N were systematically younger than the Rb–Sr dates despite the large uncertainties (437±38 Ma and 515±22 Ma, respectively using SF₆ + H₂), indicating decoupling between the isotope systems. This younger K–Ca date could represent hydrothermal events recorded in the region (458–422 Ma, rutile U-Pb ages; Amal Dev and Tomson, 2024) while the Rb–Sr date can be interpreted as a cooling age after the original emplacement of the granites during the late stages of the Pan African Orogeny (870–550 Ma; Kröner and Stern, 2005).

Figure 11 (a, b) Comparison in calculated K–Ca and Rb–Sr dates between the SF₆ + H₂ and N₂O + low H₂ reaction gas experiments for Bohus and FK-N feldspars. Uncertainty bars represent 2SE propagated uncertainties. Solid horizontal line represents expected age, dashed horizontal lines are ±5 % of expected age. Indicative K–Ca (c, d) and Rb–Sr (e, f) isochrons using SF₆ + H₂ reaction gas, with matrix matched age correction to MDC phlogopite. Analytical and total systematic age uncertainties are reported with the isochrons (x|y).

For both feldspars, the K–Ca date uncertainties are generally larger than the corresponding Rb–Sr date, due to the K–Ca system being less radiogenic (Fig. 11). Compared to the micas analysed, the feldspars are also significantly less radiogenic for both K–Ca and Rb–Sr. Multiple grains of FK-N and Bohus were mounted and tested for Ca concentration prior to analysis, and the lowest Ca grain was used for K-Ca measurements. These feldspars were selected due to their relatively low common Ca content (∼300 ppm). Feldspars with higher Ca would be more challenging to analyse within 5 % analytical precision on a quadrupole ICP-MS/MS, and a MC-ICP-MS/MS may be advantageous for more precise ${}^{\mathrm{40}}\mathrm{Ca}{/}^{\mathrm{44}}$Ca measurements.

4.3 Current Limitations and Future Direction

The accuracy of the current technique is limited by the reference K–Ca age of the matrix matched reference material. In this study, we assume MDC has the same K–Ca age as reported for Rb–Sr. This proves to be valid in accurately reproducing the ages of Robins Folly and Högsbo muscovites within 2 %–3 % uncertainty. However, independent validation of the K–Ca age would be required for the technique to develop further and to assess accuracy within 1 %–2 % uncertainty, comparable to the in-situ Rb–Sr method. Recent work by Olierook et al. (2026) has provided new Ar–Ar and Rb–Sr dates for a range of commonly used mica reference materials, and further characterisation of these micas for K–Ca would also be beneficial to the analytical community.

The low concentrations of common Ca, typical in micas, mean the analyses can be easily compromised by Ca contamination. This can occur via remobilisation of previously ablated particulates in the interface tubing, especially after ablation of high Ca glasses such as NIST SRM 610 and BCR-2G, or from surface contamination during sample preparation and polishing. The impact of contamination was minimised by using 5–10 pre-ablation shots prior to the main analysis, and by cleaning the samples with distilled water in an ultra-sonic bath to remove particulates inside past ablation craters and/or crystal defects. The effects of contamination can be seen as spiky signals, especially for the minor ⁴⁴Ca, which leads to a young age bias in contaminated data points (higher ${}^{\mathrm{44}}\mathrm{Ca}{/}^{\mathrm{40}}$Ca ratios). It may also, in part, explain the poor reproducibility of K–Ca dates between sessions (using the same reaction gas) for the highly radiogenic micas Kola and G17560 (<5 ppm common Ca). Hence, sample cleaning and pre-ablation are paramount for accurate K–Ca dates.

There is also an upper limit of allowable Ca concentrations to resolve the amount of radiogenic ⁴⁰Ca ingrowth from the common Ca. For FK-N feldspar, which is approximately 500 Ma and contains ∼300 ppm total K, the measured ⁴⁰Ca was only 1–5 % radiogenic (${}^{\mathrm{44}}\mathrm{Ca}{/}^{\mathrm{40}}$Ca = 0.02015–0.02099). Analysis using MC-ICP-MS/MS would improve the uncertainty on the ${}^{\mathrm{44}}\mathrm{Ca}{/}^{\mathrm{40}}$Ca ratio, enabling improved age precision and/or the ability to date younger Ca-bearing minerals such as feldspars. However, analysing both Rb–Sr and K–Ca simultaneously is currently only possible on a quadrupole ICP-MS/MS. An additional consideration is the pulse-analog transition of the electron multiplier detectors, which in this study was ∼2.7 Mcps, equivalent to ∼300 ppm total Ca.

One of the major advantages of quadrupole ICP-MS/MS is the ability to measure a wide range of isotopic masses in one analysis, enabling simultaneous Rb–Sr, K–Ca and trace element analyses. However, when using SF₆ as the reaction gas, there are some limitations as to the additional trace element isotopes than can be measured due to isobaric interferences from reaction gas breakdown products. Specifically causing high backgrounds of: SF on ⁵¹V, ⁵²Cr and ⁵³Cr, and SF₃ on ⁸⁹Y, ⁹⁰Zr and ⁹¹Zr. As these interferences are created within the reaction cell, they cannot be avoided. Thus, care is needed for isotope selection and interpretation of trace element data when using SF₆as reaction gas.

5 Conclusions

This study compared SF₆ and N₂O combined with H₂ as reaction gasses for simultaneous Rb–Sr and K–Ca analysis in micas and feldspars. Both Sr and Ca react efficiently to form either F or OH reaction products, enabling analysis free from parent isotope isobaric interference (⁴⁰K and ⁸⁷Rb). The addition of H₂ is highly efficient in minimising the contribution from ⁴⁰Ar and reducing the backgrounds created from the high ion load in the reaction cell when targeting $m/z$ 40. The gas combinations of SF₆ only, SF₆ plus H₂ or N₂O plus 7 mL min⁻¹ H₂ were found to produce accurate data for micas within 2 % and K-feldspars to within 5 %. However, the combination of SF₆ plus H₂ gave the highest sensitivity and best signal to noise ratio. This is advantageous for achieving the best precision, especially for young micas or those with low common-Ca concentration.

We show that the simultaneous dating using both K–Ca and Rb–Sr isotope systems is achievable from the same ablation volume for both mica and feldspars. This can be especially beneficial for small grains and/or detrital minerals where sample material is limited. Furthermore, in contrast to the initial ${}^{\mathrm{86}}\mathrm{Sr}{/}^{\mathrm{87}}$Sr ratio, the initial ${}^{\mathrm{44}}\mathrm{Ca}{/}^{\mathrm{40}}$Ca ratio is observed to be invariable over geological timescales. Thus, single spot K–Ca dates can be determined from anchored 2-point isochrons (similar to the Lu–Hf system), opening opportunities for detrital geochronology. Mineral dating via quadrupole LA-ICP-MS/MS is not only advantageous for simultaneous measurement of both isotope systems, but it is also best suited for low Ca-bearing phases (5–300 ppm). Analysing this concentration range compliments TIMS and MC-ICP-MS/MS techniques, which achieve higher precision for high Sr and Ca bearing, low radiogenic phases.

The potential for decoupling between the isotope systems has been demonstrated with the FK-N feldspar from the Madras region, where the younger K–Ca date is likely to represent regional hydrothermal alteration, compared to the Rb–Sr date which has retained the crystallisation age of the granite. This newly developed technique, where isotope decoupling can be identified, will have wide usage for understanding cooling and alteration processes in micas and feldspars, and increase knowledge of element diffusivity in these minerals.