# The McClure Mountain Syenite Apatite as a Potential Age Control Reference Material for LA-ICP-MS AFT and U-Pb Double Dating

Nathan Cogné, Alexis Derycke, Kerry Gallagher

► To cite this version:

Nathan Cogné, Alexis Derycke, Kerry Gallagher. The McClure Mountain Syenite Apatite as a Potential Age Control Reference Material for LA-ICP-MS AFT and U-Pb Double Dating. Geostandards and Geoanalytical Research, In press, 10.1111/ggr.12545. insu-04472127

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Submitted on 22 Feb 2024

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# The McClure Mountain Syenite Apatite as a Potential Age Control Reference Material for LA-ICP-MS AFT and U-Pb Double Dating

Nathan Cogné, Alexis Derycke, Kerry Gallagher

► To cite this version:

Nathan Cogné, Alexis Derycke, Kerry Gallagher. The McClure Mountain Syenite Apatite as a Potential Age Control Reference Material for LA-ICP-MS AFT and U-Pb Double Dating. Geostandards and Geoanalytical Research, 2024, 48 (2), pp.381-391. 10.1111/ggr.12545. insu-04472127

HAL Id: insu-04472127

https://insu.hal.science/insu-04472127v1

Submitted on 22 Feb 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

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# The McClure Mountain Syenite Apatite as a Potential Age Control Reference Material for LA-ICP-MS AFT and U-Pb Double Dating

Nathan Cogné (1)* ID, Alexis Derycke (2) and Kerry Gallagher (1)

Apatite fission track (AFT) dating is now routinely performed using LA-ICP-MS, and U-Pb age data are often acquired at the same time, allowing double dating of individual grains. Furthermore, additional geochemical data, such as REE mass fractions are readily acquired during the analysis. It is therefore of practical interest to identify a single material as a quality control on ages and composition. Here we present AFT and U-Pb data for the McClure Mountain Syenite (MMS) apatite, already widely used as an age reference material by the U-Pb community. We performed double dating on 238 grains, in thirteen measurement sessions over two years. The AFT data show remarkable reproducibility with an overall central age of 254.1 ± 5.1 Ma. The U-Pb results agree with the reference age for individual sessions (and for all data together). The results show that the McClure Mountain apatite is a valid quality control material for double dating, allowing the use of the same material for both methods. We also propose to extend the quality control using MMS to trace element measurements performed in conjunction AFT and U-Pb dating.

Keywords: AFT and U-Pb double dating, LA-ICP-MS, apatite, reference material, McClure Mountain syenite.

Received 01 Dec 23 - Accepted 25 Jan 24

The fission track dating method, based on the counting of the linear defects created by spontaneous fission of ²³⁸U in apatite, zircon and some other minerals, was developed during the 1960s and 1970s (e.g., Price and Walker 1962, Fleischer et al. 1975). Various techniques were considered for uranium content measurement before settling on the external detector method with a ζ empirical calibration (EDM, Hurford and Green 1983, Hurford et al. 1990). The method requires sample irradiation by thermal neutrons to produce artificial or induced ²³⁵U tracks (to estimate the ²³⁸U content via the known isotopic ratio). The ζ calibration relies on well-calibrated age reference materials that are widely available and homogeneous in age, such as Durango (e.g., McDowell et al. 2006) and Fish Canyon (e.g., Boehneke and Harrison 2014). The calibration factor is obtained for each analyst individually and is regularly monitored by recounting old and new reference materials. Therefore, using the EDM method, the FT community rarely

report secondary reference material calibrations alongside published datasets. We define a secondary reference material or quality control reference material as an internationally recognised sample of known age treated as an unknown, while a primary age reference material is a sample of known age against which all measurements can be calibrated.

For various reasons, not least the logistics involved in irradiation, the use of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for determination of U content has been developed over the last 15–20 years and is now widely used, mainly for apatite. Two different approaches are used for U measurement: (i) an absolute approach (e.g., Hasebe et al. 2004, De Grave et al. 2012, Soares et al. 2014), and (ii) a modified ζ method (e.g., Donelick et al. 2005, Chew and Donelick 2012, Cogné et al. 2020). In parallel, statistical tools for processing the

<footnote>(1) Géosciences Rennes, UMR6118, CNRS Université de Rennes, France</footnote>

<footnote>(2) Université de Lorraine, CNRS, CRPG, F-54000 Nancy, France</footnote>

<footnote>* Corresponding author. e-mail: nathan.cogne@univ-rennes.fr</footnote>
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

# The McClure Mountain Syenite Apatite as a Potential Age Control Reference Material for LA-ICP-MS AFT and U-Pb Double Dating

Nathan Cogné (1)* ID, Alexis Derycke (2) and Kerry Gallagher (1)

Apatite fission track (AFT) dating is now routinely performed using LA-ICP-MS, and U-Pb age data are often acquired at the same time, allowing double dating of individual grains. Furthermore, additional geochemical data, such as REE mass fractions are readily acquired during the analysis. It is therefore of practical interest to identify a single material as a quality control on ages and composition. Here we present AFT and U-Pb data for the McClure Mountain Syenite (MMS) apatite, already widely used as an age reference material by the U-Pb community. We performed double dating on 238 grains, in thirteen measurement sessions over two years. The AFT data show remarkable reproducibility with an overall central age of 254.1 ± 5.1 Ma. The U-Pb results agree with the reference age for individual sessions (and for all data together). The results show that the McClure Mountain apatite is a valid quality control material for double dating, allowing the use of the same material for both methods. We also propose to extend the quality control using MMS to trace element measurements performed in conjunction AFT and U-Pb dating.

Keywords: AFT and U-Pb double dating, LA-ICP-MS, apatite, reference material, McClure Mountain syenite.

Received 01 Dec 23 - Accepted 25 Jan 24

The fission track dating method, based on the counting of the linear defects created by spontaneous fission of ²³⁸U in apatite, zircon and some other minerals, was developed during the 1960s and 1970s (e.g., Price and Walker 1962, Fleischer et al. 1975). Various techniques were considered for uranium content measurement before settling on the external detector method with a ζ empirical calibration (EDM, Hurford and Green 1983, Hurford et al. 1990). The method requires sample irradiation by thermal neutrons to produce artificial or induced ²³⁵U tracks (to estimate the ²³⁸U content via the known isotopic ratio). The ζ calibration relies on well-calibrated age reference materials that are widely available and homogeneous in age, such as Durango (e.g., McDowell et al. 2006) and Fish Canyon (e.g., Boehneke and Harrison 2014). The calibration factor is obtained for each analyst individually and is regularly monitored by recounting old and new reference materials. Therefore, using the EDM method, the FT community rarely

report secondary reference material calibrations alongside published datasets. We define a secondary reference material or quality control reference material as an internationally recognised sample of known age treated as an unknown, while a primary age reference material is a sample of known age against which all measurements can be calibrated.

For various reasons, not least the logistics involved in irradiation, the use of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for determination of U content has been developed over the last 15–20 years and is now widely used, mainly for apatite. Two different approaches are used for U measurement: (i) an absolute approach (e.g., Hasebe et al. 2004, De Grave et al. 2012, Soares et al. 2014), and (ii) a modified ζ method (e.g., Donelick et al. 2005, Chew and Donelick 2012, Cogné et al. 2020). In parallel, statistical tools for processing the

<footnote>(1) Géosciences Rennes, UMR6118, CNRS Université de Rennes, France</footnote>

<footnote>(2) Université de Lorraine, CNRS, CRPG, F-54000 Nancy, France</footnote>

<footnote>* Corresponding author. e-mail: nathan.cogne@univ-rennes.fr</footnote>
<figure><img src="image_4.png" /></figure>

data (such as handling zero spontaneous track grains, mixture modelling, radial plots) have also been developed (Galbraith 2010, Vermeesch 2017, 2019). The use of a secondary reference material to test the accuracy of the data reduction scheme with 'standard' bracketing and the reporting of this secondary reference material data is common practice for other laser ablation geochronology communities using LA-ICP-MS (e.g., Horstwood et al. 2016 for U-Pb or Glorie et al. 2023 for Rb-Sr). Although the FT dating community often uses an in-house apatite age reference material, the data are rarely published.

Given this, we argue that using a secondary reference material is important for the FT community as it ensures that the quality of both counting and data processing can be assessed a posteriori. Furthermore, as the LA-ICP-MS method allows for acquisition of other data (often U-Pb dating and trace element mass fractions) on the same laser spot used for U determination (e.g., Chew and Donelick 2012, Chew et al. 2016), it is of practical interest that the secondary reference material used for FT dating should be used in similar way for U-Pb dating and/or trace element mass fractions. Quality control can then be performed on the same ablation spot for the different methods.

Currently, there are apatite reference materials distributed among the FT community, mainly Durango and Fish Canyon. However, these two reference materials are both relatively young, which creates two drawbacks. First, spontaneous tracks need to be counted over a large area to reach a sufficient number of tracks to constrain the age to an appropriate precision. This in turn implies the analysis of a large number of areas with LA-ICP-MS, increasing the analysis time. Second, young samples are not particularly useful as U-Pb dating reference materials. Iwano et al. (2019) proposed the Duluth Complex apatite as an FT reference material for LA-ICP-MS dating. However, while this reference material is well known for zircon U-Pb dating, to date it is not widely distributed, nor characterised for apatite U-Pb dating. LA-ICP-MS apatite U-Pb dating yields a slightly younger age than zircon U-Pb (1078 ± 12 Ma for apatite, Härtel et al. 2023, compared with 1099.0 ± 0.6 Ma for zircon, Paces and Miller 1993) but no TIMS characterisation exists. This characterisation is important to constrain both the reference age and the common Pb isotopic composition, the latter being used to anchor the discordia in Tera-Wasserburg diagrams, thereby increasing the precision of the estimated age. Also, the FT apatite age is relatively old (ca. 850–900 Ma, Iwano et al. 2019, Cogné and Gallagher 2021) compared with most published apatite FT ages, so is relatively atypical.

In contrast, the McClure Mountain Syenite Complex (MMS) apatite is often used as a primary or secondary reference material for U-Pb dating (e.g., Chew et al. 2014a) and is characterised by TIMS (Schoene and Bowring 2006, Krestianinov et al. 2021). Therefore, this presents a possible double dating reference material for both U-Pb and FT dating of apatite. Here, we present FT, U-Pb and trace element data acquired over the past two years on the MMS apatite and propose it as a secondary reference material for apatite FT and U-Pb double dating. Additionally, we compare our trace element data with a large LA-ICP-MS trace element dataset previously published (Chew et al. 2016) and suggest that the MMS could also be used as secondary reference material for trace element mass fractions. In that context, we do not address the geological/thermal history of the MMS here, our aim being only to assess its use as a new control reference material.

## McClure Mountain Syenite (MMS) Complex

The MMS complex outcrop spans an area of approximately 46 km² in the northern Wet Mountains (Colorado, USA) (Olson et al. 1977). The complex was intruded in Precambrian metamorphic basement and is composed of mafic-ultramafic layered intrusions that are themselves intruded by hornblende-biotite syenite and nepheline syenite (Shawe and Parker 1967). The major constituents of the hornblende-biotite syenite are K-feldspar, plagioclase, hornblende, biotite and pyroxene, while titanite and apatite make up several percent of the rock (Schoene and Bowring 2006). Zircon, baddeleyite, calcite, nepheline, magnetite, ilmenite, zirconolite and alteration products are minor constituents, making up < 2% of the rock (Schoene and Bowring 2006).

Numerous geochronological studies have been published on the MMS (Table 1). The MMhb K/Ar hornblende reference material was extracted from MMS and has been dated at 519.5 ± 5.0 Ma (Alexander et al. 1978, here, and throughout this publication, uncertainties are quoted at the 2s level) and at 520.4 ± 3.4 Ma by interlaboratory cross calibration (Samson and Alexander 1987). Subsequently, Spell and McDougall (2003) provided a total fusion ⁴⁰Ar/³⁹Ar date of 523.3 ± 1.8 Ma. Schoene and Bowring (2006) published an isotope dilution, thermal ionisation mass spectrometry (ID-TIMS) ²⁰⁷Pb/²³⁵U zircon date of 523.98 ± 0.12 Ma interpreted as the crystallisation age. In the same publication they also provided titanite and apatite weighted mean ²⁰⁷Pb/²³⁵U ages at 523.26 ± 0.65 Ma and 523.51 ± 1.47 Ma respectively. Common lead compositions were also reported in the same contribution
<figure><img src="image_4.png" /></figure>

data (such as handling zero spontaneous track grains, mixture modelling, radial plots) have also been developed (Galbraith 2010, Vermeesch 2017, 2019). The use of a secondary reference material to test the accuracy of the data reduction scheme with 'standard' bracketing and the reporting of this secondary reference material data is common practice for other laser ablation geochronology communities using LA-ICP-MS (e.g., Horstwood et al. 2016 for U-Pb or Glorie et al. 2023 for Rb-Sr). Although the FT dating community often uses an in-house apatite age reference material, the data are rarely published.

Given this, we argue that using a secondary reference material is important for the FT community as it ensures that the quality of both counting and data processing can be assessed a posteriori. Furthermore, as the LA-ICP-MS method allows for acquisition of other data (often U-Pb dating and trace element mass fractions) on the same laser spot used for U determination (e.g., Chew and Donelick 2012, Chew et al. 2016), it is of practical interest that the secondary reference material used for FT dating should be used in similar way for U-Pb dating and/or trace element mass fractions. Quality control can then be performed on the same ablation spot for the different methods.

Currently, there are apatite reference materials distributed among the FT community, mainly Durango and Fish Canyon. However, these two reference materials are both relatively young, which creates two drawbacks. First, spontaneous tracks need to be counted over a large area to reach a sufficient number of tracks to constrain the age to an appropriate precision. This in turn implies the analysis of a large number of areas with LA-ICP-MS, increasing the analysis time. Second, young samples are not particularly useful as U-Pb dating reference materials. Iwano et al. (2019) proposed the Duluth Complex apatite as an FT reference material for LA-ICP-MS dating. However, while this reference material is well known for zircon U-Pb dating, to date it is not widely distributed, nor characterised for apatite U-Pb dating. LA-ICP-MS apatite U-Pb dating yields a slightly younger age than zircon U-Pb (1078 ± 12 Ma for apatite, Härtel et al. 2023, compared with 1099.0 ± 0.6 Ma for zircon, Paces and Miller 1993) but no TIMS characterisation exists. This characterisation is important to constrain both the reference age and the common Pb isotopic composition, the latter being used to anchor the discordia in Tera-Wasserburg diagrams, thereby increasing the precision of the estimated age. Also, the FT apatite age is relatively old (ca. 850–900 Ma, Iwano et al. 2019, Cogné and Gallagher 2021) compared with most published apatite FT ages, so is relatively atypical.

In contrast, the McClure Mountain Syenite Complex (MMS) apatite is often used as a primary or secondary reference material for U-Pb dating (e.g., Chew et al. 2014a) and is characterised by TIMS (Schoene and Bowring 2006, Krestianinov et al. 2021). Therefore, this presents a possible double dating reference material for both U-Pb and FT dating of apatite. Here, we present FT, U-Pb and trace element data acquired over the past two years on the MMS apatite and propose it as a secondary reference material for apatite FT and U-Pb double dating. Additionally, we compare our trace element data with a large LA-ICP-MS trace element dataset previously published (Chew et al. 2016) and suggest that the MMS could also be used as secondary reference material for trace element mass fractions. In that context, we do not address the geological/thermal history of the MMS here, our aim being only to assess its use as a new control reference material.

## McClure Mountain Syenite (MMS) Complex

The MMS complex outcrop spans an area of approximately 46 km² in the northern Wet Mountains (Colorado, USA) (Olson et al. 1977). The complex was intruded in Precambrian metamorphic basement and is composed of mafic-ultramafic layered intrusions that are themselves intruded by hornblende-biotite syenite and nepheline syenite (Shawe and Parker 1967). The major constituents of the hornblende-biotite syenite are K-feldspar, plagioclase, hornblende, biotite and pyroxene, while titanite and apatite make up several percent of the rock (Schoene and Bowring 2006). Zircon, baddeleyite, calcite, nepheline, magnetite, ilmenite, zirconolite and alteration products are minor constituents, making up < 2% of the rock (Schoene and Bowring 2006).

Numerous geochronological studies have been published on the MMS (Table 1). The MMhb K/Ar hornblende reference material was extracted from MMS and has been dated at 519.5 ± 5.0 Ma (Alexander et al. 1978, here, and throughout this publication, uncertainties are quoted at the 2s level) and at 520.4 ± 3.4 Ma by interlaboratory cross calibration (Samson and Alexander 1987). Subsequently, Spell and McDougall (2003) provided a total fusion ⁴⁰Ar/³⁹Ar date of 523.3 ± 1.8 Ma. Schoene and Bowring (2006) published an isotope dilution, thermal ionisation mass spectrometry (ID-TIMS) ²⁰⁷Pb/²³⁵U zircon date of 523.98 ± 0.12 Ma interpreted as the crystallisation age. In the same publication they also provided titanite and apatite weighted mean ²⁰⁷Pb/²³⁵U ages at 523.26 ± 0.65 Ma and 523.51 ± 1.47 Ma respectively. Common lead compositions were also reported in the same contribution
<table><caption>**Table 1.**
Summary table of previously published geochronology data on the McClure Mountain syenite (MMS)</caption><thead><tr><th>Reference</th><th>Method used</th><th>Mineral</th><th>Age range or age (Ma)</th></tr></thead><tbody><tr><td>Olson et al. (1977)</td><td>Fission Track</td><td>Apatite</td><td>239 ± 52</td></tr><tr><td>Olson et al. (1977)</td><td>Fission Track</td><td>Titanite</td><td>506 ± 43</td></tr><tr><td>Alexander et al. (1978)</td><td>K/Ar</td><td>Homblende</td><td>519.5 ± 5</td></tr><tr><td>Samson and Alexander (1987)</td><td>K/Ar</td><td>Homblende</td><td>520.4 ± 3.4</td></tr><tr><td>Spell and McDougall (2003)</td><td>Ar-Ar</td><td>Homblende</td><td>523.3 ± 1.8</td></tr><tr><td>Schoene and Bowring (2006)</td><td>U-Pb</td><td>Zircon</td><td>523.98 ± 0.12</td></tr><tr><td>Schoene and Bowring (2006)</td><td>U-Pb</td><td>Titanite</td><td>523.26 ± 0.65</td></tr><tr><td>Schoene and Bowring (2006)</td><td>U-Pb</td><td>Apatite</td><td>523.51 ± 1.47</td></tr><tr><td>Krestianinov et al. (2021)</td><td>U-Pb</td><td>Apatite</td><td>525.3 ± 1.7</td></tr><tr><td>Anderson et al. (2017)</td><td>U-Th-Sm/He</td><td>Zircon</td><td>474–230.3</td></tr><tr><td>Anderson et al. (2017)</td><td>U-Th-Sm/He</td><td>Titanite</td><td>523–447</td></tr><tr><td>Anderson et al. (2017)</td><td>U-Th-Sm/He</td><td>Apatite</td><td>138.9–88</td></tr><tr><td>Weisberg et al. (2018)</td><td>U-Th-Sm/He</td><td>Zircon</td><td>567–381</td></tr><tr><td>Weisberg et al. (2018)</td><td>U-Th-Sm/He</td><td>Titanite</td><td>501–488</td></tr><tr><td>Weisberg et al. (2018)</td><td>U-Th-Sm/He</td><td>Apatite</td><td>211–70</td></tr></tbody></table>

at $^{206}$Pb/$^{204}$Pb = 17.54 ± 0.27 and $^{207}$Pb/$^{204}$Pb = 15.47 ± 0.04 for apatite. More recently, Krestianinov et al. (2021) produced TIMS data on apatite, obtaining a 3D isochron age of 525.3 ± 1.7 Ma, which is in agreement with the age of Schoene and Bowring (2006). Using their data and those of Schoene and Bowring (2006) they proposed new common lead compositions of $^{206}$Pb/$^{204}$Pb = 17.78 ± 0.18 and $^{207}$Pb/$^{204}$Pb = 15.539 ± 0.038 for apatite, that are also in agreement with the previously reported values.

Olson et al. (1977) produced a titanite FT age of 506 ± 43 Ma. More recently a large dataset of (U-Th)/He dates was published on titanite, zircon and apatite (Anderson et al. 2017, Weisberg et al. 2018). Quoting only data that probably come from the same area as the original aliquot (MM1 sample) of Alexander et al. (1978), the ages range from 447 ± 12 to 523 ± 14 Ma for titanite, 230.3 ± 7.7 to 480 ± 9 Ma for zircons and 70 ± 2 to 150 ± 9 Ma for apatite (note the apatite and zircon ages are alpha ejection, or Ft, corrected).

Only one FT apatite age has been published on the MMS (Olsen et al. 1977) using the protocol outlined by Naeser (1967). Updating the spontaneous decay constant ($\lambda_t$) from the original value of 6.85 × 10⁻¹⁷ to the now accepted value of 8.45 × 10⁻¹⁷ (Holden and

Hoffman 2000 would change the originally published age of 293 ± 62 Ma to 239 ± 52 Ma, although it is likely that other unknown calibration errors were also involved, making this simple recalculation unreliable. In any case, this AFT age is largely uncertain. Kelley and Chaplin (2004) reported a large number of fission track ages in the Wet Mountains. Close to the vicinity of MMS, and at similar elevation (approximately 2300 m), the FT ages range from 141 ± 10 Ma to 299 ± 20 Ma. Overall, the available geochronological data imply an expected apatite FT age between 200 and 300 Ma.

## Sample and methods

To characterise MMS we used apatite separates from sample MM1, which was kindly provided by Ray Donelick. MM1 comes from "a boulder from the parking lot" (Donelick, pers. comm.) and aliquots from this same locality have been widely distributed to laboratories by Ray Donelick and Trinity College Dublin. It is likely to be from the same sample as sample MM1 of Weisberg et al. (2018) and from the same location than that described by Anderson et al. (2017). Apatites from this separate are euhedral and usually between 50 and 250 µm diameter. In cathodoluminescence (CL) they show a weak concentric to negligible zonation (Figure 1a), similar to that reported by Schoene and Bowring (2006).

Over two years thirteen measurement sessions were performed involving double FT and U-Pb dating of 240 crystals, on three different mounts (see Table 2). The first 130 grains, on the first mount, were also measured for trace elements (Mn, Sr, Y, REE). All mounts were 1.5 mm thick epoxy discs of 2 cm diameter. For FT dating, we followed the $\zeta$ protocol of Cogné et al. (2020). Firstly, large (a few thousands of mm²) areas were delimited for counting a large number of tracks. Then a first "primary" session was performed to characterise each area in terms of the U/Ca ratio and to calculate a precise $\zeta$ factor. Because Durango is quite homogeneous in U (apart from the outer rims), the analysed large area could subsequently be revisited during measurement sessions on unknowns to establish a fractionation factor, allowing the use of the primary $\zeta$ for age calculation.

Track etching was performed with 5.5 mol l⁻¹ HNO₃ at 21 °C for 20 s. The grain mounting and etching are similar to the protocol described by Donelick et al. (2005). Spontaneous fission track counting was carried out using a Zeiss Axio Imager M1 equipped with an automated stage
<table><caption>**Table 1.**
Summary table of previously published geochronology data on the McClure Mountain syenite (MMS)</caption><thead><tr><th>Reference</th><th>Method used</th><th>Mineral</th><th>Age range or age (Ma)</th></tr></thead><tbody><tr><td>Olson et al. (1977)</td><td>Fission Track</td><td>Apatite</td><td>239 ± 52</td></tr><tr><td>Olson et al. (1977)</td><td>Fission Track</td><td>Titanite</td><td>506 ± 43</td></tr><tr><td>Alexander et al. (1978)</td><td>K/Ar</td><td>Homblende</td><td>519.5 ± 5</td></tr><tr><td>Samson and Alexander (1987)</td><td>K/Ar</td><td>Homblende</td><td>520.4 ± 3.4</td></tr><tr><td>Spell and McDougall (2003)</td><td>Ar-Ar</td><td>Homblende</td><td>523.3 ± 1.8</td></tr><tr><td>Schoene and Bowring (2006)</td><td>U-Pb</td><td>Zircon</td><td>523.98 ± 0.12</td></tr><tr><td>Schoene and Bowring (2006)</td><td>U-Pb</td><td>Titanite</td><td>523.26 ± 0.65</td></tr><tr><td>Schoene and Bowring (2006)</td><td>U-Pb</td><td>Apatite</td><td>523.51 ± 1.47</td></tr><tr><td>Krestianinov et al. (2021)</td><td>U-Pb</td><td>Apatite</td><td>525.3 ± 1.7</td></tr><tr><td>Anderson et al. (2017)</td><td>U-Th-Sm/He</td><td>Zircon</td><td>474–230.3</td></tr><tr><td>Anderson et al. (2017)</td><td>U-Th-Sm/He</td><td>Titanite</td><td>523–447</td></tr><tr><td>Anderson et al. (2017)</td><td>U-Th-Sm/He</td><td>Apatite</td><td>138.9–88</td></tr><tr><td>Weisberg et al. (2018)</td><td>U-Th-Sm/He</td><td>Zircon</td><td>567–381</td></tr><tr><td>Weisberg et al. (2018)</td><td>U-Th-Sm/He</td><td>Titanite</td><td>501–488</td></tr><tr><td>Weisberg et al. (2018)</td><td>U-Th-Sm/He</td><td>Apatite</td><td>211–70</td></tr></tbody></table>

at $^{206}$Pb/$^{204}$Pb = 17.54 ± 0.27 and $^{207}$Pb/$^{204}$Pb = 15.47 ± 0.04 for apatite. More recently, Krestianinov et al. (2021) produced TIMS data on apatite, obtaining a 3D isochron age of 525.3 ± 1.7 Ma, which is in agreement with the age of Schoene and Bowring (2006). Using their data and those of Schoene and Bowring (2006) they proposed new common lead compositions of $^{206}$Pb/$^{204}$Pb = 17.78 ± 0.18 and $^{207}$Pb/$^{204}$Pb = 15.539 ± 0.038 for apatite, that are also in agreement with the previously reported values.

Olson et al. (1977) produced a titanite FT age of 506 ± 43 Ma. More recently a large dataset of (U-Th)/He dates was published on titanite, zircon and apatite (Anderson et al. 2017, Weisberg et al. 2018). Quoting only data that probably come from the same area as the original aliquot (MM1 sample) of Alexander et al. (1978), the ages range from 447 ± 12 to 523 ± 14 Ma for titanite, 230.3 ± 7.7 to 480 ± 9 Ma for zircons and 70 ± 2 to 150 ± 9 Ma for apatite (note the apatite and zircon ages are alpha ejection, or Ft, corrected).

Only one FT apatite age has been published on the MMS (Olsen et al. 1977) using the protocol outlined by Naeser (1967). Updating the spontaneous decay constant ($\lambda_t$) from the original value of 6.85 × 10⁻¹⁷ to the now accepted value of 8.45 × 10⁻¹⁷ (Holden and

Hoffman 2000 would change the originally published age of 293 ± 62 Ma to 239 ± 52 Ma, although it is likely that other unknown calibration errors were also involved, making this simple recalculation unreliable. In any case, this AFT age is largely uncertain. Kelley and Chaplin (2004) reported a large number of fission track ages in the Wet Mountains. Close to the vicinity of MMS, and at similar elevation (approximately 2300 m), the FT ages range from 141 ± 10 Ma to 299 ± 20 Ma. Overall, the available geochronological data imply an expected apatite FT age between 200 and 300 Ma.

## Sample and methods

To characterise MMS we used apatite separates from sample MM1, which was kindly provided by Ray Donelick. MM1 comes from "a boulder from the parking lot" (Donelick, pers. comm.) and aliquots from this same locality have been widely distributed to laboratories by Ray Donelick and Trinity College Dublin. It is likely to be from the same sample as sample MM1 of Weisberg et al. (2018) and from the same location than that described by Anderson et al. (2017). Apatites from this separate are euhedral and usually between 50 and 250 µm diameter. In cathodoluminescence (CL) they show a weak concentric to negligible zonation (Figure 1a), similar to that reported by Schoene and Bowring (2006).

Over two years thirteen measurement sessions were performed involving double FT and U-Pb dating of 240 crystals, on three different mounts (see Table 2). The first 130 grains, on the first mount, were also measured for trace elements (Mn, Sr, Y, REE). All mounts were 1.5 mm thick epoxy discs of 2 cm diameter. For FT dating, we followed the $\zeta$ protocol of Cogné et al. (2020). Firstly, large (a few thousands of mm²) areas were delimited for counting a large number of tracks. Then a first "primary" session was performed to characterise each area in terms of the U/Ca ratio and to calculate a precise $\zeta$ factor. Because Durango is quite homogeneous in U (apart from the outer rims), the analysed large area could subsequently be revisited during measurement sessions on unknowns to establish a fractionation factor, allowing the use of the primary $\zeta$ for age calculation.

Track etching was performed with 5.5 mol l⁻¹ HNO₃ at 21 °C for 20 s. The grain mounting and etching are similar to the protocol described by Donelick et al. (2005). Spontaneous fission track counting was carried out using a Zeiss Axio Imager M1 equipped with an automated stage
<figure><img src="image_5.png" /><figcaption>Figure 1. (a) CL image of MM-1 apatite separate. Very weak concentric zoning to no pattern is visible. (b) Typical MM-1 grain after etching. The track density allows potential U zoning to be seen while also allowing the grain to be easily counted.</figcaption></figure>

<table><caption>Table 2. Summary table of measurement results for FT and U-Pb dating</caption><thead><tr><th>Session</th><th>Mount / type of analyses</th><th>Ablation pattern</th><th>N grains</th><th>Ns</th><th>ρ·s (track cm<sup>-2</sup>)</th><th>U/Ca mean</th><th>AFT central age (Ma)</th><th>2s (Ma)</th><th>ρ(χ<sup>2</sup>)</th><th>Dispersion (%)</th><th>U-Pb age (Ma)</th><th>2s (Ma)</th><th>MSWD</th></tr></thead><tbody><tr><td>Session 1</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>40</td><td>1669</td><td>1.88E+06</td><td>5.32E-03</td><td>252.1</td><td>12.6</td><td>0.54</td><td>0.0</td><td>527.2</td><td>7.1</td><td>0.34</td></tr><tr><td>Session 2</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>20</td><td>664</td><td>1.71E+06</td><td>4.58E-03</td><td>269.2</td><td>22.1</td><td>0.35</td><td>4.8</td><td>530.0</td><td>9.6</td><td>0.45</td></tr><tr><td>Session 3</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>10</td><td>336</td><td>1.57E+06</td><td>4.27E-03</td><td>269.6</td><td>37.1</td><td>0.16</td><td>11.1</td><td>527.2</td><td>13.0</td><td>0.34</td></tr><tr><td>Session 4</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>8</td><td>266</td><td>2.02E+06</td><td>5.19E-03</td><td>281.6</td><td>35.2</td><td>0.87</td><td>0.0</td><td>531.6</td><td>13.8</td><td>1.33</td></tr><tr><td>Session 5</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>9</td><td>232</td><td>1.41E+06</td><td>3.61E-03</td><td>271.4</td><td>36.0</td><td>0.99</td><td>0.0</td><td>528.0</td><td>12.8</td><td>1.38</td></tr><tr><td>Session 6</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>13</td><td>420</td><td>2.05E+06</td><td>5.68E-03</td><td>253.0</td><td>25.4</td><td>0.76</td><td>0.0</td><td>522.8</td><td>7.9</td><td>0.98</td></tr><tr><td>Session 7</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>10</td><td>299</td><td>1.65E+06</td><td>4.52E-03</td><td>279.8</td><td>33.1</td><td>0.82</td><td>0.0</td><td>518.9</td><td>8.2</td><td>0.53</td></tr><tr><td>Session 8</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>20</td><td>626</td><td>1.86E+06</td><td>5.05E-03</td><td>255.1</td><td>20.9</td><td>0.51</td><td>0.0</td><td>522.5</td><td>6.8</td><td>0.46</td></tr><tr><td>Session 9</td><td>2 / AFT+UPb</td><td>10*10 µm raster 60 Hz 20 µm s<sup>-1</sup></td><td>20</td><td>3597</td><td>1.30E+06</td><td>3.61E-03</td><td>251.8</td><td>11.7</td><td>0.12</td><td>5.1</td><td>511.5</td><td>7.4</td><td>1.00</td></tr><tr><td>Session 10</td><td>2 / AFT+UPb</td><td>40*40 µm square; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>14</td><td>564</td><td>1.42E+06</td><td>4.22E-03</td><td>245.6</td><td>24.6</td><td>0.17</td><td>9.5</td><td>523.3</td><td>4.9</td><td>0.66</td></tr><tr><td>Session 11</td><td>2 / AFT+UPb</td><td>40*40 µm square; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>25</td><td>1139</td><td>1.36E+06</td><td>4.20E-03</td><td>238.4</td><td>15.1</td><td>0.32</td><td>4.8</td><td>525.1</td><td>4.3</td><td>1.04</td></tr><tr><td>Session 12</td><td>2 / AFT+UPb</td><td>40*40 µm square; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>19</td><td>1048</td><td>1.53E+06</td><td>4.46E-03</td><td>240.2</td><td>16.6</td><td>0.27</td><td>6.0</td><td>519.9</td><td>4.1</td><td>1.02</td></tr><tr><td>Session 13</td><td>3 / AFT+UPb</td><td>40*40 µm square; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>30</td><td>1728</td><td>2.02E+06</td><td>5.50E-03</td><td>261.1</td><td>12.9</td><td>0.33</td><td>0.0</td><td>527.3</td><td>4.2</td><td>0.79</td></tr></tbody></table>

Ns, Number of counted spontaneous tracks.

system using TrackWorks software at a magnification of 1000x. This software was used when measuring a total of 196 confined track lengths and their angle to the c axis, as

well as Dpar, on seventy grains (forty on mount 1, 30 on mount 3). Only sub-horizontal tracks in track (TINTS) were measured.
<figure><img src="image_5.png" /><figcaption>Figure 1. (a) CL image of MM-1 apatite separate. Very weak concentric zoning to no pattern is visible. (b) Typical MM-1 grain after etching. The track density allows potential U zoning to be seen while also allowing the grain to be easily counted.</figcaption></figure>

<table><caption>Table 2. Summary table of measurement results for FT and U-Pb dating</caption><thead><tr><th>Session</th><th>Mount / type of analyses</th><th>Ablation pattern</th><th>N grains</th><th>Ns</th><th>ρ·s (track cm<sup>-2</sup>)</th><th>U/Ca mean</th><th>AFT central age (Ma)</th><th>2s (Ma)</th><th>ρ(χ<sup>2</sup>)</th><th>Dispersion (%)</th><th>U-Pb age (Ma)</th><th>2s (Ma)</th><th>MSWD</th></tr></thead><tbody><tr><td>Session 1</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>40</td><td>1669</td><td>1.88E+06</td><td>5.32E-03</td><td>252.1</td><td>12.6</td><td>0.54</td><td>0.0</td><td>527.2</td><td>7.1</td><td>0.34</td></tr><tr><td>Session 2</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>20</td><td>664</td><td>1.71E+06</td><td>4.58E-03</td><td>269.2</td><td>22.1</td><td>0.35</td><td>4.8</td><td>530.0</td><td>9.6</td><td>0.45</td></tr><tr><td>Session 3</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>10</td><td>336</td><td>1.57E+06</td><td>4.27E-03</td><td>269.6</td><td>37.1</td><td>0.16</td><td>11.1</td><td>527.2</td><td>13.0</td><td>0.34</td></tr><tr><td>Session 4</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>8</td><td>266</td><td>2.02E+06</td><td>5.19E-03</td><td>281.6</td><td>35.2</td><td>0.87</td><td>0.0</td><td>531.6</td><td>13.8</td><td>1.33</td></tr><tr><td>Session 5</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>9</td><td>232</td><td>1.41E+06</td><td>3.61E-03</td><td>271.4</td><td>36.0</td><td>0.99</td><td>0.0</td><td>528.0</td><td>12.8</td><td>1.38</td></tr><tr><td>Session 6</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>13</td><td>420</td><td>2.05E+06</td><td>5.68E-03</td><td>253.0</td><td>25.4</td><td>0.76</td><td>0.0</td><td>522.8</td><td>7.9</td><td>0.98</td></tr><tr><td>Session 7</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>10</td><td>299</td><td>1.65E+06</td><td>4.52E-03</td><td>279.8</td><td>33.1</td><td>0.82</td><td>0.0</td><td>518.9</td><td>8.2</td><td>0.53</td></tr><tr><td>Session 8</td><td>1 / AFT+UPb+ REE</td><td>30 µm round; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>20</td><td>626</td><td>1.86E+06</td><td>5.05E-03</td><td>255.1</td><td>20.9</td><td>0.51</td><td>0.0</td><td>522.5</td><td>6.8</td><td>0.46</td></tr><tr><td>Session 9</td><td>2 / AFT +UPb</td><td>10*10 µm raster 60 Hz 20 µm s<sup>-1</sup></td><td>20</td><td>3597</td><td>1.30E+06</td><td>3.61E-03</td><td>251.8</td><td>11.7</td><td>0.12</td><td>5.1</td><td>511.5</td><td>7.4</td><td>1.00</td></tr><tr><td>Session 10</td><td>2 / AFT +UPb</td><td>40*40 µm square; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>14</td><td>564</td><td>1.42E+06</td><td>4.22E-03</td><td>245.6</td><td>24.6</td><td>0.17</td><td>9.5</td><td>523.3</td><td>4.9</td><td>0.66</td></tr><tr><td>Session 11</td><td>2 / AFT +UPb</td><td>40*40 µm square; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>25</td><td>1139</td><td>1.36E+06</td><td>4.20E-03</td><td>238.4</td><td>15.1</td><td>0.32</td><td>4.8</td><td>525.1</td><td>4.3</td><td>1.04</td></tr><tr><td>Session 12</td><td>2 / AFT +UPb</td><td>40*40 µm square; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>19</td><td>1048</td><td>1.53E+06</td><td>4.46E-03</td><td>240.2</td><td>16.6</td><td>0.27</td><td>6.0</td><td>519.9</td><td>4.1</td><td>1.02</td></tr><tr><td>Session 13</td><td>3 / AFT +UPb</td><td>40*40 µm square; 5 Hz; 3.9 J cm<sup>-2</sup></td><td>30</td><td>1728</td><td>2.02E+06</td><td>5.50E-03</td><td>261.1</td><td>12.9</td><td>0.33</td><td>0.0</td><td>527.3</td><td>4.2</td><td>0.79</td></tr></tbody></table>

Ns, Number of counted spontaneous tracks.

system using TrackWorks software at a magnification of 1000x. This software was used when measuring a total of 196 confined track lengths and their angle to the c axis, as

well as Dpar, on seventy grains (forty on mount 1, 30 on mount 3). Only sub-horizontal tracks in track (TINTS) were measured.
All laser ablations, apart for session 9 (see below), were performed at the University of Rennes GeOHeLiS analytical platform, using an ESI NWR193UC Excimer laser coupled to an Agilent 7700x Q-ICP-MS. We employed an energy of 3.9 J cm⁻² and a repetition rate of 5 Hz. Sessions 1 to 8 used a round 30 µm spot, while sessions 10 to 13 used a 40 × 40 µm square. One grain was destroyed during laser ablation in both sessions 10 and 11. Session 9 was performed in Trinity College Dublin, using a mapping approach (see Ansberque et al. 2020 for details) on a Photon Machines Iridia 193 nm ArF excimer laser-ablation system coupled to an Agilent 7900 Q-ICP-MS. The mapping was performed with raster of 10 µm × 10 µm squares over each grain. The scanning speed was set up at 20 µm s⁻¹, with energy of 2 J cm⁻² and a repetition rate of 60 Hz. As the mapping allows counting tracks over a larger area, because the whole grain is analysed by LA-ICP-MS, it produces more precise single grain and central ages.

For sessions 1 and 13 the MM1 grains were ablated as an unknown, i.e., fifteen to twenty grains in a row between two series of reference materials. A series of reference material is the suite of three analyses on NIST SRM 612 glass (Jochum et al. 2011), three analyses on Madagascar apatite (U-Pb primary reference material, Thomson et al. 2012), and three analyses on Durango Zeta apatite (see Cogné et al. 2020). One mapping session (session 9) was dedicated to MM1 dating and three grains were analysed between each batch of reference materials (same as above, except that instead of spots, the reference materials were ablated with a rastered mapping approach). For the remainder of the sessions, MM1 grains were positioned in the reference material batch, with two or three grains in a row either side of a batch of approximately twenty unknowns. For each session the number of analysed MMS grains was between eight and forty. LA-ICP-MS data were processed using lolite software v4 (Paton et al. 2011), using Trace Elements DRS (Woodhead et al. 2007) for trace element mass fraction, a modified version of it (Cogné et al. 2020) for FT dating and VizualAge_UComPbine DRS (Chew et al. 2014a) for U-Pb dating. Chlorine content was also measured using the protocol of Chew et al. (2014b) and a synthetic Cl-apatite (Klemme et al. 2013) as high-Cl reference material.

The FT dating used the ζ protocol of Cogné et al. (2020). Artificial glass NIST SRM 612 (Jochum et al. 2011) was used to correct machine drift and Durango apatite shards analysed during a primary session were revisited to calculate the fractionation factor. The age calculation was implemented in an in-house python script and produced single grain ages as well as both central and pooled ages, $p(\chi^2)$, and

dispersion for the dated sample. Trace element estimations were made using NIST SRM 612 as the calibration reference material and $^{43}$Ca as the internal standard element. The chondrite values used to normalise trace element mass fractions are from Barrat et al. (2012). The Madagascar apatite (Thomson et al. 2012) was used as the primary reference material to calculate U-Pb ratios. An in-house python script was used for U-Pb age calculation. The York et al. (2004) algorithm was used to calculate the best-fit regression line (and its uncertainty envelope) for the population of $^{207}$Pb/$^{206}$Pb and $^{238}$U/$^{206}$Pb ratios. The lower intercept of this line with the Concordia in the Tera-Wasserburg space (and its uncertainty) was calculated using the approach of Ludwig (1980). The y intercept of the best-fit line was anchored to a $^{207}$Pb/$^{206}$Pb value of 0.874 (Krestianinov et al. 2021).

## Results

All results are summarised in Table 2 and the single grain data are available in online supporting information files. Of the 240 counted grains, two were destroyed during laser analysis, leaving a total of 238 analysed grains. The fission track density ranged from 0.75 to $3.5 \times 10^{-6}$ tracks cm⁻². and often showed slight spatial variation on the scale of the grain, implying minor uranium zonation (Figure 1b). The mean track length (MTL – c axis projected) is 13.41 ± 0.16 µm (2s), with no significant difference between the two mounts (Table S1, Figure S1). The mean Dpar value is 1.58 ± 0.02 µm, and is similar between the two mounts (Table S1, Figure S1). Single grain ages (SGA) range from 162.0 ± 47.5 to 375.6 ± 117.8 Ma while session central ages (CA) range from 238.4 ± 15.1 to 281.6 ± 35.2 Ma (Figure 2). None of the sessions showed large SGA dispersion, with the $p(\chi^2)$ values ranging from 0.12 to 0.99. The most precise (i.e., with the lowest uncertainty) session (No. 9, mapping session) yielded a central age of 251.8 ± 11.7 Ma. Taken together, the 238 grains yield a central age of 254.1 ± 5.1 Ma, with a dispersion of 4.3% and a $p(\chi^2)$ of 0.26. All individual session central ages are consistent with this overall central age.

Single grain U-Pb 207-corrected ages (with an initial $^{207}$Pb/$^{206}$Pb ratio of 0.874 – Krestianinov et al. 2021) range from 459.4 ± 56.7 to 581.6 ± 40.3 Ma. Taken together the 238 grains yield a lower intercept (regression is anchored at an initial $^{207}$Pb/$^{206}$Pb ratio of 0.874) of 523.5 ± 1.7 Ma (MSWD = 0.79, Figure 3a). The lower intercept ages in the Tera-Wasserburg diagram range from 531.6 ± 13.8 to 511.5 ± 7.4 Ma, in accordance with the reference age of 523.51 ± 1.47 Ma (Schoene and
All laser ablations, apart for session 9 (see below), were performed at the University of Rennes GeOHeLiS analytical platform, using an ESI NWR193UC Excimer laser coupled to an Agilent 7700x Q-ICP-MS. We employed an energy of 3.9 J cm⁻² and a repetition rate of 5 Hz. Sessions 1 to 8 used a round 30 µm spot, while sessions 10 to 13 used a 40 × 40 µm square. One grain was destroyed during laser ablation in both sessions 10 and 11. Session 9 was performed in Trinity College Dublin, using a mapping approach (see Ansberque et al. 2020 for details) on a Photon Machines Iridia 193 nm ArF excimer laser-ablation system coupled to an Agilent 7900 Q-ICP-MS. The mapping was performed with raster of 10 µm × 10 µm squares over each grain. The scanning speed was set up at 20 µm s⁻¹, with energy of 2 J cm⁻² and a repetition rate of 60 Hz. As the mapping allows counting tracks over a larger area, because the whole grain is analysed by LA-ICP-MS, it produces more precise single grain and central ages.

For sessions 1 and 13 the MM1 grains were ablated as an unknown, i.e., fifteen to twenty grains in a row between two series of reference materials. A series of reference material is the suite of three analyses on NIST SRM 612 glass (Jochum et al. 2011), three analyses on Madagascar apatite (U-Pb primary reference material, Thomson et al. 2012), and three analyses on Durango Zeta apatite (see Cogné et al. 2020). One mapping session (session 9) was dedicated to MM1 dating and three grains were analysed between each batch of reference materials (same as above, except that instead of spots, the reference materials were ablated with a rastered mapping approach). For the remainder of the sessions, MM1 grains were positioned in the reference material batch, with two or three grains in a row either side of a batch of approximately twenty unknowns. For each session the number of analysed MMS grains was between eight and forty. LA-ICP-MS data were processed using lolite software v4 (Paton et al. 2011), using Trace Elements DRS (Woodhead et al. 2007) for trace element mass fraction, a modified version of it (Cogné et al. 2020) for FT dating and VizualAge_UComPbine DRS (Chew et al. 2014a) for U-Pb dating. Chlorine content was also measured using the protocol of Chew et al. (2014b) and a synthetic Cl-apatite (Klemme et al. 2013) as high-Cl reference material.

The FT dating used the ζ protocol of Cogné et al. (2020). Artificial glass NIST SRM 612 (Jochum et al. 2011) was used to correct machine drift and Durango apatite shards analysed during a primary session were revisited to calculate the fractionation factor. The age calculation was implemented in an in-house python script and produced single grain ages as well as both central and pooled ages, $p(\chi^2)$, and

dispersion for the dated sample. Trace element estimations were made using NIST SRM 612 as the calibration reference material and $^{43}$Ca as the internal standard element. The chondrite values used to normalise trace element mass fractions are from Barrat et al. (2012). The Madagascar apatite (Thomson et al. 2012) was used as the primary reference material to calculate U-Pb ratios. An in-house python script was used for U-Pb age calculation. The York et al. (2004) algorithm was used to calculate the best-fit regression line (and its uncertainty envelope) for the population of $^{207}$Pb/$^{206}$Pb and $^{238}$U/$^{206}$Pb ratios. The lower intercept of this line with the Concordia in the Tera-Wasserburg space (and its uncertainty) was calculated using the approach of Ludwig (1980). The y intercept of the best-fit line was anchored to a $^{207}$Pb/$^{206}$Pb value of 0.874 (Krestianinov et al. 2021).

## Results

All results are summarised in Table 2 and the single grain data are available in online supporting information files. Of the 240 counted grains, two were destroyed during laser analysis, leaving a total of 238 analysed grains. The fission track density ranged from 0.75 to $3.5 \times 10^{-6}$ tracks cm⁻². and often showed slight spatial variation on the scale of the grain, implying minor uranium zonation (Figure 1b). The mean track length (MTL – c axis projected) is 13.41 ± 0.16 µm (2s), with no significant difference between the two mounts (Table S1, Figure S1). The mean Dpar value is 1.58 ± 0.02 µm, and is similar between the two mounts (Table S1, Figure S1). Single grain ages (SGA) range from 162.0 ± 47.5 to 375.6 ± 117.8 Ma while session central ages (CA) range from 238.4 ± 15.1 to 281.6 ± 35.2 Ma (Figure 2). None of the sessions showed large SGA dispersion, with the $p(\chi^2)$ values ranging from 0.12 to 0.99. The most precise (i.e., with the lowest uncertainty) session (No. 9, mapping session) yielded a central age of 251.8 ± 11.7 Ma. Taken together, the 238 grains yield a central age of 254.1 ± 5.1 Ma, with a dispersion of 4.3% and a $p(\chi^2)$ of 0.26. All individual session central ages are consistent with this overall central age.

Single grain U-Pb 207-corrected ages (with an initial $^{207}$Pb/$^{206}$Pb ratio of 0.874 – Krestianinov et al. 2021) range from 459.4 ± 56.7 to 581.6 ± 40.3 Ma. Taken together the 238 grains yield a lower intercept (regression is anchored at an initial $^{207}$Pb/$^{206}$Pb ratio of 0.874) of 523.5 ± 1.7 Ma (MSWD = 0.79, Figure 3a). The lower intercept ages in the Tera-Wasserburg diagram range from 531.6 ± 13.8 to 511.5 ± 7.4 Ma, in accordance with the reference age of 523.51 ± 1.47 Ma (Schoene and
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Apatite fission track age plot. Single grain age and uncertainty (2s) are colour-coded following session number. For each session the central age and its uncertainty is plotted (thick bar and dashed thin bars, same colour code as single grain data) and are written below the data with $p(\chi^2)$. The number of grains used is indicated in the legend. The overall central age and statistics are also written as title. The running central age (black symbols) shows the variation of the overall central age along the two years of analysis.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 3.** U-Pb data. (a) Tera-Wasserburg diagram for all the analysed grains. The regression is anchored at an initial $^{207}$Pb/$^{206}$Pb of 0.88198. Ellipses represent 2s and colour coded following session number (see Figure 3b and Figure 2). (b) Concordia lower intercept age and uncertainty (2s) for each measurement session. The reference age of Schoene and Bowring (2006) is plotted as a thick black line.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Apatite fission track age plot. Single grain age and uncertainty (2s) are colour-coded following session number. For each session the central age and its uncertainty is plotted (thick bar and dashed thin bars, same colour code as single grain data) and are written below the data with $p(\chi^2)$. The number of grains used is indicated in the legend. The overall central age and statistics are also written as title. The running central age (black symbols) shows the variation of the overall central age along the two years of analysis.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 3.** U-Pb data. (a) Tera-Wasserburg diagram for all the analysed grains. The regression is anchored at an initial $^{207}$Pb/$^{206}$Pb of 0.88198. Ellipses represent 2s and colour coded following session number (see Figure 3b and Figure 2). (b) Concordia lower intercept age and uncertainty (2s) for each measurement session. The reference age of Schoene and Bowring (2006) is plotted as a thick black line.</figcaption></figure>
Bowring 2006, Figure 3b) apart for session 9 which is further discussed below.

Trace element mass fractions show some inter-grain variability (Table S1, Figure 4). However, a plot of data normalised to chondrite shows that the pattern of the multi-element spectrum (both abundances and standard deviations) are very similar to that published by Chew *et al.* (2016) (Figure 4a). Additionally our data and those of Chew *et al.* (2016) are similar in a LREE (La-Nd) versus Sr/Y plot (Figure 4b) that can be used for discrimination of apatite origin (O'Sullivan *et al.* 2020). The results from MMS apatite are clearly consistent with the overall mafic-ultramafic affinities of the McClure Mountain complex.

## Discussion

### MMS as a double FT and U-Pb dating validation material

The MMS fission track density is high enough to provide a well-constrained age on a small area and also to identify zoned areas on a grain. The first characteristic is advantageous as it is better to keep the counting area similar to the shape and size of the ablation spot (Cogné and Gallagher 2021). On average, our counted area was approximately 2.5 × 10⁻⁵ cm², which corresponds roughly

to a square of 50 µm × 50 µm. With this size and given the track density, about fifteen to twenty grains ensure a central age with less than 10% uncertainty (2s). On the other hand, the track density remains low enough to be easily, and thus rapidly, counted.

Our single grain FT age data show good reproducibility for the analyses carried out over two years and thirteen measurement sessions. The SGA dispersion of 4.3% seems reasonable for a natural sample and both individual sessions and all data together show a $p(\chi^2)$ well above 5%, which is consistent with a single age population. All session central ages are in agreement with each other and with the overall central age (254.1 ± 5.1 Ma). The mapping session, following Ansberque *et al.* (2020), was undertaken because of its ability to deal with U zonation on an X-Y plan. This allows us to count larger areas, and thus more tracks, which in turn permits a better constrained age. The mapping session yielded a central age of 251.8 ± 11.7 Ma, which is close to the overall central age. The latter is also in agreement with published ages in this area of the Wet Mountains (Kelley and Chapin 2004) and with the published apatite FT age on the McClure Syenite (Olson *et al.* 1977). Therefore, we suggest that a reference apatite FT age 254.1 ± 5.1 Ma can be used for MMS. This age is the result of our thirteen measurement sessions on MM1, but it is possible that different samples from the MMS complex could yield slightly different ages. Although the MMS outcrop is relatively small and the relief is limited to approximately

<figure><img src="image_8.png" /></figure>

**Figure 4.** Trace element data. (a) Chondrite-normalised trace element spectrum. Our data are plotted in red, solid line is the mean and dashed lines are the 1 standard deviation (1s). Chew *et al.* (2016) laser data are in blue and solution data in black for comparison. (b) LREE versus Sr/Y plot (O'Sullivan *et al.* 2020). Colours are the same as in (a), solid line are the 2s ellipses, dashed line the 1s ellipses, the white point being solution data of Chew *et al.* (2016). Background field limits are from O'Sullivan *et al.* (2020).
Bowring 2006, Figure 3b) apart for session 9 which is further discussed below.

Trace element mass fractions show some inter-grain variability (Table S1, Figure 4). However, a plot of data normalised to chondrite shows that the pattern of the multi-element spectrum (both abundances and standard deviations) are very similar to that published by Chew *et al.* (2016) (Figure 4a). Additionally our data and those of Chew *et al.* (2016) are similar in a LREE (La-Nd) versus Sr/Y plot (Figure 4b) that can be used for discrimination of apatite origin (O'Sullivan *et al.* 2020). The results from MMS apatite are clearly consistent with the overall mafic-ultramafic affinities of the McClure Mountain complex.

Discussion

MMS as a double FT and U-Pb dating validation
material

The MMS fission track density is high enough to provide
a well-constrained age on a small area and also to identify
zoned areas on a grain. The first characteristic is advanta-
geous as it is better to keep the counting area similar to
the shape and size of the ablation spot (Cogné and
Gallagher 2021). On average, our counted area was
approximately 2.5 × 10⁻⁵ cm², which corresponds roughly

to a square of 50 µm × 50 µm. With this size and given the
track density, about fifteen to twenty grains ensure a central
age with less than 10% uncertainty (2s). On the other hand,
the track density remains low enough to be easily, and thus
rapidly, counted.

Our single grain FT age data show good reproducibility
for the analyses carried out over two years and thirteen
measurement sessions. The SGA dispersion of 4.3% seems
reasonable for a natural sample and both individual
sessions and all data together show a $p(\chi^2)$ well above
5%, which is consistent with a single age population. All
session central ages are in agreement with each other and
with the overall central age (254.1 ± 5.1 Ma). The mapping
session, following Ansberque et al. (2020), was undertaken
because of its ability to deal with U zonation on an X-Y plan.
This allows us to count larger areas, and thus more tracks,
which in turn permits a better constrained age. The mapping
session yielded a central age of 251.8 ± 11.7 Ma, which is
close to the overall central age. The latter is also in
agreement with published ages in this area of the Wet
Mountains (Kelley and Chapin 2004) and with the
published apatite FT age on the McClure Syenite (Olson
et al. 1977). Therefore, we suggest that a reference apatite
FT age 254.1 ± 5.1 Ma can be used for MMS. This age is
the result of our thirteen measurement sessions on MM1, but
it is possible that different samples from the MMS complex
could yield slightly different ages. Although the MMS outcrop
is relatively small and the relief is limited to approximately

<figure><img src="image_8.png" /></figure>

Figure 4. Trace element data. (a) Chondrite-normalised trace element spectrum. Our data are plotted in red, solid line is the mean and dashed lines are the 1 standard deviation (1s). Chew et al. (2016) laser data are in blue and solution data in black for comparison. (b) LREE versus Sr/Y plot (O'Sullivan et al. 2020). Colours are the same as in (a), solid line are the 2s ellipses, dashed line the 1s ellipses, the white point being solution data of Chew et al. (2016). Background field limits are from O'Sullivan et al. (2020).
<figure><img src="image_9.png" /></figure>

300 m, we also recommend that this age be confirmed by
other laboratories and/or FT dating methods.

While MMS could be used for just FT dating, the goal of this
study was to propose a secondary reference material for
double FT and U-Pb apatite dating. As use of secondary
reference materials is common practice for U-Pb, the use of the
same material for the both methods is advantageous as it
requires only one ablation spot. The MMS also has the
advantage of being well characterised for apatite U-Pb age
and common lead composition (Schoene and Bowring 2006,
Krestianinov et al. 2021). Therefore, all our FT dated grains
were also dated for U-Pb. The U-Pb results for each session and
for all the data together (523.5 ± 1.7 Ma) are in agreement
with the reference age (523.51 ± 1.47 Ma, Schoene and
Bowring 2006). We therefore argue that MMS is a suitable
material for LA-ICP-MS apatite double dating validation. The
only supplementary time required for adding FT validation is
the track counting time, which thanks to a reasonable track
density, is relatively limited in the case of MMS.

We note, however, that session 9 (mapping session)
yielded lower U-Pb ratios than usual. There are different
possible explanations for this. Firstly, we may have extracted
data from the edge of the crystal and possibly included
epoxy. Examination of the ⁴³Ca maps show that the areas
used for data extraction (the same for FT and U-Pb,
mimicking the area of FT counting) were well inside the
apatite (a representative example is provided in Figure S2).
Secondly, it is possible that we obtained data on part of the
grain having lower U-Pb ratios. However, the maps of
²³⁸U/²⁰⁶Pb are relatively homogeneous, and subsequent
spot analyses on the same grains show a more usual
²³⁸U/²⁰⁶Pb value (i.e., between 8 and 11). We therefore
suggest that the difference may come from Pb contamination
on the surface. Raster mapping ablates only the very first
micrometre or so on the surface of the targeted grains. Then
Pb contamination on the surface could have an impact on
the measured values. For a Q-ICP-MS, the difference in
composition between this contaminant Pb and the common
Pb of the grain would remain undetectable. Then the effect
of this contamination would be a decrease of the U-Pb
ratios, driving the ellipses toward the common Pb end of the
Discordia. While this hypothesis could explain the observed
data, it would require more work to be tested, which is
beyond the scope of this contribution. The intercept age of
the session is also slightly younger (2%) than the reference
age (511.5 ± 7.4 compared with 523.51 ± 1.47). We do
not think that this is linked to the hypothesis of common Pb
contamination at the surface of the sample. In any case it
does not affect our main conclusion that MMS is suitable for
quality control of double AFT and U-Pb dating.

The potential use of MMS for trace element
analysis

In addition to obtaining a U-Pb date, the LA-ICP-MS FT
dating protocol can also be used for trace element mass
fractions (e.g., Cogné et al. 2020, Ansberque et al. 2021).
Data published by Chew et al. (2016) for MMS were
acquired over a period of 5 years of LA-Q-ICP-MS analysis
and more than 1500 individual laser spots. These authors
also used solution ICP-MS to assess the accuracy of the LA-
Q-ICP-MS element mass fractions, which used NIST SRM
612 as the primary reference material for trace elements.
They concluded that for a large series of elements (REE, Mn,
V, Y, Sr, Th and U) the LA-ICP-MS results were in agreement
with the solution values. However, they noticed that inter-
grain variation could be quite important for MMS and at
that stage precluded its use as primary reference material for
element mass fractions.

We performed determinations for Mn, Sr, Y, REE, Th and
U in 130 apatite grains from the MMS. Our measurement
results show a good similarity with those of Chew
et al. (2016), values and ranges being comparable for
all the elements (Figure 4). The shapes of the REE-
normalised spectra are also very similar. Thus, we argue
that despite the inter-grain variability, MMS could also be
used as a quality control material for trace element data
reduction schemes.

To assess the number of analysed grains required, we
merged the two datasets and calculated mean and
standard deviation for this whole dataset. We then used a
Python script to randomly draw a number (n) of analyses. The
mean mass fraction for each element was calculated and if
this was not in the ±1s range of the whole dataset, this draw
was defined as ‘failed’. This process was repeated 10,000
times with n ranging from 3 to 30. For each n, a percentage
of failed draws was calculated. The results show that for
more than five analyses, the fail percentage was < 10%
(see Figure S3), < 5% for more than ten analyses and finally
< 1% with or more twenty-one analyses. Therefore, we
assume that analysing ten grains and averaging them
should be enough for trace element measurement quality
control, but fifteen to twenty analyses would of course be
better.

Although most Durango crystals would show lower inter-
analysis dispersion, we do not know in advance what the
trace element abundances are for a given crystal. In contrast,
assuming the MMS sample originated from the car park
boulder, we do know the abundances of the various trace
elements and their dispersion. Therefore, using MMS has the
<figure><img src="image_9.png" /></figure>

300 m, we also recommend that this age be confirmed by
other laboratories and/or FT dating methods.

While MMS could be used for just FT dating, the goal of this
study was to propose a secondary reference material for
double FT and U-Pb apatite dating. As use of secondary
reference materials is common practice for U-Pb, the use of the
same material for the both methods is advantageous as it
requires only one ablation spot. The MMS also has the
advantage of being well characterised for apatite U-Pb age
and common lead composition (Schoene and Bowring 2006,
Krestianinov et al. 2021). Therefore, all our FT dated grains
were also dated for U-Pb. The U-Pb results for each session and
for all the data together (523.5 ± 1.7 Ma) are in agreement
with the reference age (523.51 ± 1.47 Ma, Schoene and
Bowring 2006). We therefore argue that MMS is a suitable
material for LA-ICP-MS apatite double dating validation. The
only supplementary time required for adding FT validation is
the track counting time, which thanks to a reasonable track
density, is relatively limited in the case of MMS.

We note, however, that session 9 (mapping session)
yielded lower U-Pb ratios than usual. There are different
possible explanations for this. Firstly, we may have extracted
data from the edge of the crystal and possibly included
epoxy. Examination of the ⁴³Ca maps show that the areas
used for data extraction (the same for FT and U-Pb,
mimicking the area of FT counting) were well inside the
apatite (a representative example is provided in Figure S2).
Secondly, it is possible that we obtained data on part of the
grain having lower U-Pb ratios. However, the maps of
²³⁸U/²⁰⁶Pb are relatively homogeneous, and subsequent
spot analyses on the same grains show a more usual
²³⁸U/²⁰⁶Pb value (i.e., between 8 and 11). We therefore
suggest that the difference may come from Pb contamination
on the surface. Raster mapping ablates only the very first
micrometre or so on the surface of the targeted grains. Then
Pb contamination on the surface could have an impact on
the measured values. For a Q-ICP-MS, the difference in
composition between this contaminant Pb and the common
Pb of the grain would remain undetectable. Then the effect
of this contamination would be a decrease of the U-Pb
ratios, driving the ellipses toward the common Pb end of the
Discordia. While this hypothesis could explain the observed
data, it would require more work to be tested, which is
beyond the scope of this contribution. The intercept age of
the session is also slightly younger (2%) than the reference
age (511.5 ± 7.4 compared with 523.51 ± 1.47). We do
not think that this is linked to the hypothesis of common Pb
contamination at the surface of the sample. In any case it
does not affect our main conclusion that MMS is suitable for
quality control of double AFT and U-Pb dating.

The potential use of MMS for trace element
analysis

In addition to obtaining a U-Pb date, the LA-ICP-MS FT
dating protocol can also be used for trace element mass
fractions (e.g., Cogné et al. 2020, Ansberque et al. 2021).
Data published by Chew et al. (2016) for MMS were
acquired over a period of 5 years of LA-Q-ICP-MS analysis
and more than 1500 individual laser spots. These authors
also used solution ICP-MS to assess the accuracy of the LA-
Q-ICP-MS element mass fractions, which used NIST SRM
612 as the primary reference material for trace elements.
They concluded that for a large series of elements (REE, Mn,
V, Y, Sr, Th and U) the LA-ICP-MS results were in agreement
with the solution values. However, they noticed that inter-
grain variation could be quite important for MMS and at
that stage precluded its use as primary reference material for
element mass fractions.

We performed determinations for Mn, Sr, Y, REE, Th and
U in 130 apatite grains from the MMS. Our measurement
results show a good similarity with those of Chew
et al. (2016), values and ranges being comparable for
all the elements (Figure 4). The shapes of the REE-
normalised spectra are also very similar. Thus, we argue
that despite the inter-grain variability, MMS could also be
used as a quality control material for trace element data
reduction schemes.

To assess the number of analysed grains required, we
merged the two datasets and calculated mean and
standard deviation for this whole dataset. We then used a
Python script to randomly draw a number (n) of analyses. The
mean mass fraction for each element was calculated and if
this was not in the ±1s range of the whole dataset, this draw
was defined as ‘failed’. This process was repeated 10,000
times with n ranging from 3 to 30. For each n, a percentage
of failed draws was calculated. The results show that for
more than five analyses, the fail percentage was < 10%
(see Figure S3), < 5% for more than ten analyses and finally
< 1% with or more twenty-one analyses. Therefore, we
assume that analysing ten grains and averaging them
should be enough for trace element measurement quality
control, but fifteen to twenty analyses would of course be
better.

Although most Durango crystals would show lower inter-
analysis dispersion, we do not know in advance what the
trace element abundances are for a given crystal. In contrast,
assuming the MMS sample originated from the car park
boulder, we do know the abundances of the various trace
elements and their dispersion. Therefore, using MMS has the
advantage of using only one ablation spot for quality control
in the case of FT, U-Pb and trace element determinations
being measured during the same session. The number of
required analysed grains is similar between AFT and trace
elements, that is fifteen to twenty.

Conclusions

Results of LA-ICP-MS fission track (FT) dating of MMS
apatite over two years of analysis are relatively homoge-
nous in terms of the estimated ages. The track density
ensures that the counting part of the dating can readily be
performed accurately and with minimal risk of undetected
U zonation. As only fifteen to twenty grains are necessary to
obtain a well-constrained FT age it can therefore be used
easily as a secondary reference material for FT dating. The
MMS apatite is already well characterised for U-Pb age
and common Pb content, and so it is widely used as
secondary reference material for this technique. Therefore,
we argue that the MMS is also a suitable material for
quality control of apatite double dating using LA-ICP-MS.
This would allow the FT community to follow the
recommended practice of other geochronological tech-
niques and we advocate that publishing secondary
reference material data for LA-ICP-MS FT ages should
become common practice.

The trace element data we acquired on the MMS are in
very close agreement with the large dataset published by
Chew *et al.* (2016). We accept that the inter-grain variation
is non-negligible but we think that MMS could also be used
as a secondary reference material if trace element
measurement is performed at the same time as fission track
and U-Pb dating.

Acknowledgements

NC thanks Raymond Donelick for providing MM1
apatite. Paul Sylvester is thanked for editorial handling.
Andrew Gleadow and an anonymous reviewer are thanked
for their comments that improved the manuscript. The authors
declare that they have no conflict of interest.
Scientific editing by Paul J. Sylvester.

**Data availability statement**

The data that support the findings of this study are
available in online supporting information files.

**References**

**Alexander E., Mickelson G. and Lanphere M. (1978)**
MMhb-1: A new $^{40}$Ar-$^{39}$Ar dating standard. In: Short
Papers of the Fourth International Conference,
Geochronology, Cosmochronology, and Isotope
Geology. U.S. Geological Survey, 78–701, 6–8.

**Anderson A.J., Hodges K.V. and van Soest M.C. (2017)**
Empirical constraints on the effects of radiation damage
on helium diffusion in zircon. *Geochimica et
Cosmochimica Acta*, 218, 308–322.

**Anderson A.J., Hodges K.V. and van Soest M.C. (2018)**
Comment on 'Distinguishing slow cooling versus
multiphase cooling and heating in zircon and apatite
(U-Th)/He datasets: The case of the McClure Mountain
syenite standard' by Weisberg, Metcalf and Flowers.
*Chemical Geology*, 498, 150–152.

**Ansberque C., Chew D.M. and Drost K. (2021)**
Apatite fission-track dating by LA-Q-ICP-MS mapping.
*Chemical Geology*, 560, 119977.

**Boehnke P. and Harrison M.T. (2014)**
A meta-analysis of geochronologically relevant half-lives:
What's the best decay constant? *International Geology
Review*, 56, 905–914.

**Chew D.M. and Donelick R.A. (2012)**
Combined apatite fission track and U-Pb dating by LA-ICP-
MS and its application in apatite provenance analysis.
*Mineralogical Association of Canada Short Course*, 42,
219–247.

**Chew D.M., Petrus J.A. and Kamber B.S. (2014a)**
U-Pb LA-ICP-MS dating using accessory mineral standards
with variable common Pb. *Chemical Geology*, 363, 185–
199.

**Chew D.M., Donelick R.A., Donelick M.B., Kamber B.S.
and Stock M. (2014b)**
Apatite chlorine concentration measurements by LA-ICP-
MS. *Geostandards and Geoanalytical Research*, 38, 23–
35.

**Chew D.M., Babechuk M.G., Cogné N., Mark C.,
O'Sullivan G.J., Henrichs I.A., Doepke D. and McKenna
C.A. (2016)**
(LA,Q)-ICP-MS trace-element analyses of Durango and
McClure Mountain apatite and implications for making
natural LA-ICP-MS mineral standards. *Chemical Geology*,
435, 35–48.

**Cogné N., Chew D.M., Donelick R.A. and Ansberque C.
(2020)**
LA-ICP-MS apatite fission track dating: A practical zeta-
based approach. *Chemical Geology*, 531, 119302.

**Cogné N. and Gallagher K. (2021)**
Some comments on the effect of uranium zonation on
fission track dating by LA-ICP-MS. *Chemical Geology*, 573,
120226.
advantage of using only one ablation spot for quality control in the case of FT, U-Pb and trace element determinations being measured during the same session. The number of required analysed grains is similar between AFT and trace elements, that is fifteen to twenty.

## Conclusions

Results of LA-ICP-MS fission track (FT) dating of MMS apatite over two years of analysis are relatively homogeneous in terms of the estimated ages. The track density ensures that the counting part of the dating can readily be performed accurately and with minimal risk of undetected U zonation. As only fifteen to twenty grains are necessary to obtain a well-constrained FT age it can therefore be used easily as a secondary reference material for FT dating. The MMS apatite is already well characterised for U-Pb age and common Pb content, and so it is widely used as secondary reference material for this technique. Therefore, we argue that the MMS is also a suitable material for quality control of apatite double dating using LA-ICP-MS. This would allow the FT community to follow the recommended practice of other geochronological techniques and we advocate that publishing secondary reference material data for LA-ICP-MS FT ages should become common practice.

The trace element data we acquired on the MMS are in very close agreement with the large dataset published by Chew *et al.* (2016). We accept that the inter-grain variation is non-negligible but we think that MMS could also be used as a secondary reference material if trace element measurement is performed at the same time as fission track and U-Pb dating.

## Acknowledgements

NC thanks Raymond Donelick for providing MM1 apatite. Paul Sylvester is thanked for editorial handling. Andrew Gleadow and an anonymous reviewer are thanked for their comments that improved the manuscript. The authors declare that they have no conflict of interest. Scientific editing by Paul J. Sylvester.

## Data availability statement

The data that support the findings of this study are available in online supporting information files.

## References

**Alexander E., Mickelson G. and Lanphere M. (1978)**
MMhb-1: A new $^{40}$Ar-$^{39}$Ar dating standard. In: Short Papers of the Fourth International Conference, Geochronology, Cosmochronology, and Isotope Geology. U.S. Geological Survey, 78-701, 6-8.

**Anderson A.J., Hodges K.V. and van Soest M.C. (2017)**
Empirical constraints on the effects of radiation damage on helium diffusion in zircon. *Geochimica et Cosmochimica Acta*, 218, 308-322.

**Anderson A.J., Hodges K.V. and van Soest M.C. (2018)**
Comment on 'Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard' by Weisberg, Metcalf and Flowers. *Chemical Geology*, 498, 150-152.

**Ansberque C., Chew D.M. and Drost K. (2021)**
Apatite fission-track dating by LA-Q-ICP-MS mapping. *Chemical Geology*, 560, 119977.

**Boehnke P. and Harrison M.T. (2014)**
A meta-analysis of geochronologically relevant half-lives: What's the best decay constant? *International Geology Review*, 56, 905-914.

**Chew D.M. and Donelick R.A. (2012)**
Combined apatite fission track and U-Pb dating by LA-ICP-MS and its application in apatite provenance analysis. *Mineralogical Association of Canada Short Course*, 42, 219-247.

**Chew D.M., Petrus J.A. and Kamber B.S. (2014a)**
U-Pb LA-ICP-MS dating using accessory mineral standards with variable common Pb. *Chemical Geology*, 363, 185-199.

**Chew D.M., Donelick R.A., Donelick M.B., Kamber B.S. and Stock M. (2014b)**
Apatite chlorine concentration measurements by LA-ICP-MS. *Geostandards and Geoanalytical Research*, 38, 23-35.

**Chew D.M., Babechuk M.G., Cogné N., Mark C., O'Sullivan G.J., Henrichs I.A., Doepke D. and McKenna C.A. (2016)**
(LA,Q)-ICP-MS trace-element analyses of Durango and McClure Mountain apatite and implications for making natural LA-ICP-MS mineral standards. *Chemical Geology*, 435, 35-48.

**Cogné N., Chew D.M., Donelick R.A. and Ansberque C. (2020)**
LA-ICP-MS apatite fission track dating: A practical zeta-based approach. *Chemical Geology*, 531, 119302.

**Cogné N. and Gallagher K. (2021)**
Some comments on the effect of uranium zonation on fission track dating by LA-ICP-MS. *Chemical Geology*, 573, 120226.
<figure><img src="image_10.png" /></figure>

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Nuclear tracks in solids: Principles and applications. University of California Press (Berkeley), 605pp.

**Flowers R.M., Ketcham R.A., Shuster D.L. and Farley K.A. (2009)**

Apatite (U-Th)/He thermochronometry using a radiation damage accumulation and annealing model. Geochimica et Cosmochimica Acta, 73, 2347–2365.

**Galbraith R. (2010)**

Statistics for LA-ICP-MS fission track dating. Thermo 2010 – 12th International Conference on thermochronology (Glasgow), 175.

**Galbraith R.F. and Laslett G.M. (1993)**

Statistical models for mixed fission track ages. Nuclear Tracks and Radiation Measurements, 21, 459–470.

**Gallagher K. (2012)**

Transdimensional inverse thermal history modeling for quantitative thermochronology. Journal of Geophysical Research, 117, B02408.

**Gautheron C., Tassan-Got L., Barbarand J. and Pagel M. (2009)**

Effect of alpha-damage annealing on apatite (U-Th)/He thermochronology. Chemical Geology, 266, 157–170.

**Gautheron C., Pinna-Jamme R., Derycke A., Ahadi F., Sanchez C., Haurine F., Monvoisin G., Barbosa D., Delpech G., Maltese J., Sarda P. and Tassan-Got L. (2021)**

Technical note: Analytical protocols and performance for apatite and zircon (U-Th)/He analysis on quadrupole and magnetic sector mass spectrometer systems between 2007 and 2020. Geochronology, 3, 351–370.

**Glorie S., Gilbert S.E., Hand M. and Lloyd J.C. (2023)**

Calibration methods for laser ablation Rb-Sr geochronology: Comparisons and recommendation based on NIST glass and natural reference materials. EGUSphere preprint repository.

**Härtel B., Matthews W.A. and Enckelmann E. (2023)**

Duluth Complex FC1 apatite and zircon: Reference materials for (U-Th)/He dating? Geostandards and Geoanalytical Research, 47, 669–681.

**Hasebe N., Barbarand J., Jarvis K., Carter A. and Hurford A.J. (2004)**

Apatite fission-track chronometry using laser ablation ICP-MS. Chemical Geology, 207, 135–145.

**Holden N.E. and Hoffman D.C. (2000)**

Spontaneous fission half-lives for ground-state nuclide (technical report). Pure and Applied Chemistry, 72, 1525–1562.

**Horstwood M.S.A., Kosler J., Gehrels G., Jackson S.E., McLean N.M., Paton C., Pearson N.J., Sircombe K., Sylvester P., Vermeesch P., Bowring J.F., Condon D.J. and Schoene B. (2016)**

Community-derived standards for LA-ICP-MS U-(Th-) Pb geochronology – Uncertainty propagation, age interpretation and data reporting. Geostandards and Geoanalytical Research, 40, 311–332.

**Hurford A.J. (1990)**

Standardization of fission track dating calibration: Recommendation by the Fission Track Working Group of the I.U.G.S. Subcommission on Geochronology. Chemical Geology, 80, 171–178.

**Hurford A.J. and Green P.F. (1983)**

The zeta age calibration of fission-track dating. Chemical Geology, 41, 285–317.

**Iwano H., Danhara T., Yuguchi T., Hirata T. and Ogasawara M. (2019)**

Duluth complex apatites: Age reference material for LA-ICP-MS-based fission-track dating. Terra Nova, 31, 247–256.

**Jochum K.P., Weis U., Stoll B., Kuzmin D., Yang Q., Raczek I., Jacob D.E., Stracke A., Birbaum K., Frick D.A., Günther D. and Enzweiler J. (2011)**

Determination of reference values for NIST SRM 610–617 glasses following ISO guidelines. Geostandards and Geoanalytical Research, 35, 397–429.

**Kelley S. and Chapin C. (2004)**

Denudation history and internal structure of the Front Range and Wet Mountains, Colorado, based on apatite fission track thermochronology. New Mexico Bureau of Geology and Mineral Resources Bulletin, 160, 41–78.

**Klemme S., John T., Wessels M., Kusebauch C., Berndt J., Rohrbach A. and Schmid-Beurmann P. (2013)**

Synthesis of trace element bearing single crystals of chlorapatite (Ca₅(PO₄)₃Cl) using the flux growth method. Chemistry Central Journal, 7, 56.

**Krestianinov E., Amelin Y., Neymark L.A. and Aleinikoff J.N. (2021)**

U-Pb systematics of uranium-rich apatite from Adirondacks: Inferences about regional geological and geochemical evolution, and evaluation of apatite reference materials for *in situ* dating. Chemical Geology, 581, 120417.

**Ludwig K.R. (1980)**

Calculation of uncertainties of U-Pb isotope data. Earth and Planetary Science Letters, 46, 212–220.

**McDowell F.W., McIntosh W.C. and Farley K.A. (2005)**

A precise ⁴⁰Ar-³⁹Ar reference age for the Durango apatite (U-Th)/He and fission-track dating standard. Chemical Geology, 214, 249–263.

**Naeser C.W. (1967)**

The use of apatite and sphene for fission track age determinations. Geological Society of America Bulletin, 78, 1523–1526.
<figure><img src="image_10.png" /></figure>

GEOSTANDARDS and
GEOANALYTICAL
RESEARCH

references

**De Grave J., Glorie S., Ryabinin A., Zhimulev F., Buslov M.M., Izmer A., Elburg M., Vanhaecke F. and Van den Haute P. (2012)**

Late Palaeozoic and Meso-Cenozoic tectonic evolution of the southern Kyrgyz Tien Shan: Constraints from multi-method thermochronology in the Trans-Alai, Turkestan-Alai segment and the southeastern Ferghana Basin. Journal of Asian Earth Sciences, 44, 149–168.

**Donelick R.A., O'Sullivan P.B. and Ketcham R.A. (2005)**

Apatite fission-track analysis. Reviews in Mineralogy and Geochemistry, 58, 49–94.

**Fleischer R.L., Price P.B. and Walker R.M. (1975)**

Nuclear tracks in solids: Principles and applications. University of California Press (Berkeley), 605pp.

**Flowers R.M., Ketcham R.A., Shuster D.L. and Farley K.A. (2009)**

Apatite (U-Th)/He thermochronometry using a radiation damage accumulation and annealing model. Geochimica et Cosmochimica Acta, 73, 2347–2365.

**Galbraith R. (2010)**

Statistics for LA-ICP-MS fission track dating. Thermo 2010 – 12th International Conference on thermochronology (Glasgow), 175.

**Galbraith R.F. and Laslett G.M. (1993)**

Statistical models for mixed fission track ages. Nuclear Tracks and Radiation Measurements, 21, 459–470.

**Gallagher K. (2012)**

Transdimensional inverse thermal history modeling for quantitative thermochronology. Journal of Geophysical Research, 117, B02408.

**Gautheron C., Tassan-Got L., Barbarand J. and Pagel M. (2009)**

Effect of alpha-damage annealing on apatite (U-Th)/He thermochronology. Chemical Geology, 266, 157–170.

**Gautheron C., Pinna-Jamme R., Derycke A., Ahadi F., Sanchez C., Haurine F., Monvoisin G., Barbosa D., Delpech G., Maltese J., Sarda P. and Tassan-Got L. (2021)**

Technical note: Analytical protocols and performance for apatite and zircon (U-Th)/He analysis on quadrupole and magnetic sector mass spectrometer systems between 2007 and 2020. Geochronology, 3, 351–370.

**Glorie S., Gilbert S.E., Hand M. and Lloyd J.C. (2023)**

Calibration methods for laser ablation Rb-Sr geochronology: Comparisons and recommendation based on NIST glass and natural reference materials. EGUSphere preprint repository.

**Härtel B., Matthews W.A. and Enckelmann E. (2023)**

Duluth Complex FC1 apatite and zircon: Reference materials for (U-Th)/He dating? Geostandards and Geoanalytical Research, 47, 669–681.

**Hasebe N., Barbarand J., Jarvis K., Carter A. and Hurford A.J. (2004)**

Apatite fission-track chronometry using laser ablation ICP-MS. Chemical Geology, 207, 135–145.

**Holden N.E. and Hoffman D.C. (2000)**

Spontaneous fission half-lives for ground-state nuclide (technical report). Pure and Applied Chemistry, 72, 1525–1562.

**Horstwood M.S.A., Kosler J., Gehrels G., Jackson S.E., McLean N.M., Paton C., Pearson N.J., Sircombe K., Sylvester P., Vermeesch P., Bowring J.F., Condon D.J. and Schoene B. (2016)**

Community-derived standards for LA-ICP-MS U-(Th-) Pb geochronology – Uncertainty propagation, age interpretation and data reporting. Geostandards and Geoanalytical Research, 40, 311–332.

**Hurford A.J. (1990)**

Standardization of fission track dating calibration: Recommendation by the Fission Track Working Group of the I.U.G.S. Subcommission on Geochronology. Chemical Geology, 80, 171–178.

**Hurford A.J. and Green P.F. (1983)**

The zeta age calibration of fission-track dating. Chemical Geology, 41, 285–317.

**Iwano H., Danhara T., Yuguchi T., Hirata T. and Ogasawara M. (2019)**

Duluth complex apatites: Age reference material for LA-ICP-MS-based fission-track dating. Terra Nova, 31, 247–256.

**Jochum K.P., Weis U., Stoll B., Kuzmin D., Yang Q., Raczek I., Jacob D.E., Stracke A., Birbaum K., Frick D.A., Günther D. and Enzweiler J. (2011)**

Determination of reference values for NIST SRM 610–617 glasses following ISO guidelines. Geostandards and Geoanalytical Research, 35, 397–429.

**Kelley S. and Chapin C. (2004)**

Denudation history and internal structure of the Front Range and Wet Mountains, Colorado, based on apatite fission track thermochronology. New Mexico Bureau of Geology and Mineral Resources Bulletin, 160, 41–78.

**Klemme S., John T., Wessels M., Kusebauch C., Berndt J., Rohrbach A. and Schmid-Beurmann P. (2013)**

Synthesis of trace element bearing single crystals of chlorapatite (Ca₅(PO₄)₃Cl) using the flux growth method. Chemistry Central Journal, 7, 56.

**Krestianinov E., Amelin Y., Neymark L.A. and Aleinikoff J.N. (2021)**

U-Pb systematics of uranium-rich apatite from Adirondacks: Inferences about regional geological and geochemical evolution, and evaluation of apatite reference materials for *in situ* dating. Chemical Geology, 581, 120417.

**Ludwig K.R. (1980)**

Calculation of uncertainties of U-Pb isotope data. Earth and Planetary Science Letters, 46, 212–220.

**McDowell F.W., McIntosh W.C. and Farley K.A. (2005)**

A precise ⁴⁰Ar-³⁹Ar reference age for the Durango apatite (U-Th)/He and fission-track dating standard. Chemical Geology, 214, 249–263.

**Naeser C.W. (1967)**

The use of apatite and sphene for fission track age determinations. Geological Society of America Bulletin, 78, 1523–1526.
## references

**Olson J., Marvin R., Parker R. and Mehnert H. (1977)**
Age and tectonic setting of Lower Paleozoic alkalic and mafic rocks, carbonatites, and thorium veins in south-central Colorado. *Journal of Research of the U.S. Geological Survey*, 5, 673–687.

**O'Sullivan G.O., Chew D., Kenny G., Henrichs I. and Mulligan D. (2020)**
The trace element composition of apatite and its application to detrital provenance studies. *Earth-Science Reviews*, 201, 103044.

**Paces J.B. and Miller J.D. (1993)**
Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift system. *Journal of Geophysical Research*, 98, 13997–14013.

**Paton C., Hellstrom J., Paul B., Woodhead J. and Hergt J. (2011)**
Iolite: Freeware for the visualisation and processing of mass spectrometric data. *Journal of Analytical Atomic Spectrometry*, 26, 2508–2518.

**Price P. and Walker R. (1962)**
Observation of fossil particle tracks in natural micas. *Nature*, 196, 732–734.

**Samson S. and Alexander Jr. E.C. (1987)**
Calibration of the interlaboratory $^{40}$Ar-$^{39}$Ar dating standard, MMhb-1. *Chemical Geology* (Isotope Geoscience Section), 66, 27–34.

**Schoene B. and Bowring S.A. (2006)**
U–Pb systematics of the McClure Mountain syenite: Thermochronological constraints on the age of the $^{40}$Ar/$^{39}$Ar standard MMhb. *Contributions to Mineralogy and Petrology*, 151, 615–630.

**Soares C., Guedes S., Hadler J., Mertz-Kraus R., Zack T. and Iunes P. (2014)**
Novel calibration for LA-ICP-MS-based fission-track thermochronology. *Physics and Chemistry of Minerals*, 41, 65–73.

**Shawe D.R. and Parker R.L. (1967)**
Mafic-ultramafic layered intrusion at Iron Mountain, Fremont County, Colorado. *United States Geological Survey Bulletin*, 1251-A, A1–A20.

**Spell T. and McDougall I. (2003)**
Characterization and calibration of $^{40}$Ar/$^{39}$Ar dating standards. *Chemical Geology*, 198, 189–211.

**Thomson S.N., Gehrels G.E., Ruiz J. and Buchwaldt R. (2012)**
Routine low-damage apatite U-Pb dating using laser ablation-multicollector-ICP-MS. *Geochemistry, Geophysics, Geosystems*, 13.

**Vermeesch P. (2017)**
Statistics for LA-ICP-MS based fission track dating. *Chemical Geology*, 456, 19–27.

**Vermeesch P. (2019)**
Statistics for fission-track thermochronology. In: Malusa M.G. and Fitzgerald P.G. (eds), Fission-track thermochronology and its application to geology. Springer (Berlin), 109–122.

**Weisberg W.R., Metcalf J.R. and Flowers R.M. (2018)**
Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard. *Chemical Geology*, 485, 90–99.

**Woodhead J., Hellstrom J., Hergt J., Greig A. and Maas R. (2007)**
Isotopic and elemental imaging of geological materials by laser ablation-inductively coupled plasma-mass spectrometry. *Geostandards and Geoanalytical Research*, 31, 331–343.

**York D., Evensen N.M., Lopez Martinez M. and De Basabe Delgado J. (2004)**
Unified equations for the slope, intercept, and standard errors of the best straight line. *American Journal of Physics*, 72, 367–375.

## Supporting information

The following supporting information may be found in the online version of this article:

Figure S1. Dpar and track length histograms for the two sessions in which they were measured.

Figure S2. Isotopic maps of grain 10 of the McClure mount 2. The black line represents the area of data extraction that mimics the area of track counting. (a) $^{43}$Ca map, showing that the area is well inside the grain and (b) Final $^{206}$Pb/$^{238}$U showing a good homogeneity at the scale of the grain. See text for explanation.

Figure S3. Plot of results of a random draw of trace element abundances. The y-axis indicates the percentage of the random draw that failed over 10,000 iterations for a number of analyses randomly picked indicated on the x-axis. See text for explanation.

Table S1. Individual grain measurement results. Track length and Dpar data are also reported.

This material is available from: http://onlinelibrary.wiley.com/doi/10.1111/ggr.12545/abstract (This link will take you to the article abstract).
## references

**Olson J., Marvin R., Parker R. and Mehnert H. (1977)**

Age and tectonic setting of Lower Paleozoic alkalic and mafic rocks, carbonatites, and thorium veins in south-central Colorado. Journal of Research of the U.S. Geological Survey, 5, 673-687.

**O'Sullivan G.O., Chew D., Kenny G., Henrichs I. and Mulligan D. (2020)**

The trace element composition of apatite and its application to detrital provenance studies. Earth-Science Reviews, 201, 103044.

**Paces J.B. and Miller J.D. (1993)**

Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift system. Journal of Geophysical Research, 98, 13997-14013.

**Paton C., Hellstrom J., Paul B., Woodhead J. and Hergt J. (2011)**

Iolite: Freeware for the visualisation and processing of mass spectrometric data. Journal of Analytical Atomic Spectrometry, 26, 2508-2518.

**Price P. and Walker R. (1962)**

Observation of fossil particle tracks in natural micas. Nature, 196, 732-734.

**Samson S. and Alexander Jr. E.C. (1987)**

Calibration of the interlaboratory ⁴⁰Ar-³⁹Ar dating standard, MMhb-1. Chemical Geology (Isotope Geoscience Section), 66, 27-34.

**Schoene B. and Bowring S.A. (2006)**

U-Pb systematics of the McClure Mountain syenite: Thermochronological constraints on the age of the ⁴⁰Ar/³⁹Ar standard MMhb. Contributions to Mineralogy and Petrology, 151, 615-630.

**Soares C., Guedes S., Hadler J., Mertz-Kraus R., Zack T. and Iunes P. (2014)**

Novel calibration for LA-ICP-MS-based fission-track thermochronology. Physics and Chemistry of Minerals, 41, 65-73.

**Shawe D.R. and Parker R.L. (1967)**

Mafic-ultramafic layered intrusion at Iron Mountain, Fremont County, Colorado. United States Geological Survey Bulletin, 1251-A, A1-A20.

**Spell T. and McDougall I. (2003)**

Characterization and calibration of ⁴⁰Ar/³⁹Ar dating standards. Chemical Geology, 198, 189-211.

**Thomson S.N., Gehrels G.E., Ruiz J. and Buchwaldt R. (2012)**

Routine low-damage apatite U-Pb dating using laser ablation-multicollector-ICP-MS. Geochemistry, Geophysics, Geosystems, 13.

**Vermeesch P. (2017)**

Statistics for LA-ICP-MS based fission track dating. Chemical Geology, 456, 19-27.

**Vermeesch P. (2019)**

Statistics for fission-track thermochronology. In: Malusa M.G. and Fitzgerald P.G. (eds), Fission-track thermochronology and its application to geology. Springer (Berlin), 109-122.

**Weisberg W.R., Metcalf J.R. and Flowers R.M. (2018)**

Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard. Chemical Geology, 485, 90-99.

**Woodhead J., Hellstrom J., Hergt J., Greig A. and Maas R. (2007)**

Isotopic and elemental imaging of geological materials by laser ablation-inductively coupled plasma-mass spectrometry. Geostandards and Geoanalytical Research, 31, 331-343.

**York D., Evensen N.M., Lopez Martinez M. and De Basabe Delgado J. (2004)**

Unified equations for the slope, intercept, and standard errors of the best straight line. American Journal of Physics, 72, 367-375.

## Supporting information

The following supporting information may be found in the online version of this article:

Figure S1. Dpar and track length histograms for the two sessions in which they were measured.

Figure S2. Isotopic maps of grain 10 of the McClure mount 2. The black line represents the area of data extraction that mimics the area of track counting. (a) ⁴³Ca map, showing that the area is well inside the grain and (b) Final ²⁰⁶Pb/²³⁸U showing a good homogeneity at the scale of the grain. See text for explanation.

Figure S3. Plot of results of a random draw of trace element abundances. The y-axis indicates the percentage of the random draw that failed over 10,000 iterations for a number of analyses randomly picked indicated on the x-axis. See text for explanation.

Table S1. Individual grain measurement results. Track length and Dpar data are also reported.

This material is available from: http://onlinelibrary.wiley.com/doi/10.1111/ggr.12545/abstract (This link will take you to the article abstract).