In situ LA-ICPMS U-Pb dating of Sulfates: Applicability of carbonate reference materials as matrix-matched standards

. Recent developments in analytical capabilities in the field of in situ laser ablation mass spectrometry (LA-ICPMS) have expanded the applications of U-Pb geochronometers in low-U minerals such as carbonates or garnets. The rapid evolution of the technique relies on well-characterized matrix-matched reference materials. In this article, we explore the suitability of using carbonate as “almost-matrix-matched reference materials” for in situ U-Pb dating of sulfates. For such purpose, we have used the astrochronologically dated gypsum and anhydrite samples deposited during the Messinian Salinity Crisis (5.97 ‒ 5.33 15 Ma) and compared these dates with the U-Pb ages obtained by LA-ICPMS. Although the majority of the samples failed due to the elevated common Pb content and low 238 U/ 204 Pb ratios, five of the samples showed a higher dispersion on U/Pb ratios. The obtained dates in four of these samples are comparable with the expected ages while another gave an unexpected younger age, each of them with 6-11% of uncertainty. The pit depth of the spots showed that the sulfates ablate similar to carbonates, so the offset due to the crater geometry mismatch or downhole fractionation can be assumed to be negligible. To sum up, the 20 bias between the U-Pb and expected cyclostratigraphic ages, if any, is included in the uncertainty and thus, the results obtained here suggest that carbonate reference material is currently the best option for standardisation of in situ U-Pb sulfate analyses


Introduction
Latest years developments in instrumentation and analytical capabilities of LA-ICPMS techniques have widely opened the applicability of the U-Pb geochronometer. The high spatial resolution, low cost of analysis and high throughput with relatively 25 good precision (Schaltegger et al., 2015) achievable with the new generation of laser and mass spectrometers favour the study of minerals with low and heterogeneous U concentrations like carbonates or garnets (e.g. Roberts et al., 2020). In fact, carbonate geochronology has gone from scarce publications that involve tedious and long-lasting isotope dilution techniques (e.g., Brannon et al., 1996;Grandia et al., 2000;Woodhead et al. 2006Woodhead et al. , 2012Rasbury and Cole, 2009) to a bloom of dozens of publications per year (extensive review in Roberts et al., 2020). Likewise, garnet U-Pb dating is rapidly developing in skarn 30 or metamorphic garnets, with U contents even below 100 parts per billion (e.g., Burisch et al., 2019;Yan et al., 2020;Millonig et al., 2020). In addition, several laboratories have started to investigate the possibility of measuring other types of minerals: dolomites (Burisch et al., 2017), fluorite (Piccione et al., 2019;Lenoir et al., 2021), nacrite (Piccione et al., 2019) or anatase (Sindern et al., 2019), among others.
The rapid evolution of U-Pb dating in low-U phases is closely related to the availability of reference materials (WC-1 35 carbonate, Roberts et al., 2017;or Mali garnet, Seaman et al., 2017). Well-characterized matrix-matched reference material is essential for U-Pb analyses by ion probe or laser ablation as sample matrix affects the ablation, transport and ionisation (Sylvester, 2008;Yang et al., 2018). Indeed, LA-ICPMS dates could only be as good as the homogeneity of the reference materials, and the accuracy and precision to which such material is known (Schaltegger et al., 2015). Several authors, however, have appraised the suitability of using non-matrix-matched standardisation, with different levels of success. Deng et al. (2017) 40 and Wafforn et al. (2018) used 91500 respectively GJ1 zircon to correct U/Pb fractionation of garnet and assumed they obtained the correct ages, whereas Yang et al. (2018) got 11 % too old garnet ages using zircon standardisation. Similarly, Parrish et al. (2018) measured Mud Tank zircon within carbonate analyses and reported a bias between zircon and calcite of c. 4.7%. Piccione et al. (2019) used the WC-1 carbonate reference material for fluorite analysis assuming that the bias between calcite and fluorite may likely be less than the one between calcite and zircon. 45 This study aims to continue opening new possibilities in the field of in-situ U-Pb dating of low-U minerals, by (I) demonstrating that sulfates can be dated by U-Pb and (II) examining the suitability and reliability of using calcite as "almost matrix-matched reference material" for sulfates. Accurate U-Pb dating of sulfates could contribute to a better understanding of their formation and/or transformation (hydration-dehydration) with the potential of dating diagenetic, pedogenic or tectonic processes. Gypsum (CaSO4·2H2O) and Anhydrite (CaSO4) are the two most abundant sulfates of marine and non-marine evaporite 50 deposits (e.g. Murray, 1963;Babel and Schreiber, 2014). Sedimentary gypsum forms by direct precipitation out of water evaporation under arid climatic conditions in hydrologically restricted environments. Under terrestrial evaporitic conditions, gypsum is the dominant primary mineral and anhydrite forms through gypsum dehydration caused during diagenesis. In the presence of water at shallower levels, the anhydrite is rapidly converted back to gypsum (e.g. Conley and Bundy, 1958;Murray, 1964;Ossorio et al., 2014;Warren, 2016). Although less frequent, non-evaporitic gypsum formation can also take place (see 55 Van Driessche et al., 2019 and references therein).
In the absence of sulfate matrix-matched reference material, we have assumed that the bias between calcite and sulfate is smaller than with the other available reference materials. Both minerals behave very similar during ablation (e.g., drill speed, U/Pb downhole fractionation, etc.) and ionization in the plasma (Ca 2+ as the main cation). For evaluating the suitability of the calcite-based corrections, we have analysed gypsum and anhydrite samples from the Messinian Salinity Crisis (MSC) in the 60 Mediterranean Sea (Roveri et al., 2014a(Roveri et al., , 2014bVasiliev et al., 2017;Grothe et al., 2020;Andreetto et al., 2021) and compared them with their astrochronological data (calibrated with astronomically tuned timescales, such as Milankovic cycles, Laskar et al. 1999). Chronostratigraphy of Late Miocene to Early Pliocene within the MSC is well constrained (CIESM, 2008;Manzi et al., 2013, Roveri et al., 2014a and thus, makes those samples ideal for comparison purposes. The first sessions, between December 2019 and May 2020, were performed with the SC-ICPMS. Prior to the measurements, signal strength was tuned for maximum sensitivity while keeping oxide formation below ~ 0.5 % (UO/U) and element fraction low (e.g. Th/U ~ 0.9). This was done by ablating at 3 µm/s with a 60 µm spot at 6 Hz and 3.5 J/cm 2 fluence in the glass SRMNIST 612 (Jochum et al., 2011). The average sensitivity obtained for the line is ca. 100,000 cps per µg/g for 238 U. The detection limits (4 x background signal) of the instrument for 206 Pb and 238 U were c. 0.3 and 0.03 ng/g. Data were acquired in 105 fully-automated mode overnight. Each analysis consists of 18 s background acquisition followed by 18 s of sample ablation and 20 s washout. During 36 s data acquisition, the signal of 206 Pb, 207 Pb, 208 Pb, 232 Th and 238 U were detected by peak jumping in simultaneous analogue and pulse counting mode. Detailed data acquisition parameters are summarised in Table 1. Due to the low precision obtained in those sessions, where only two samples from a single session can be considered acceptable (see results and discussion), the use of the more sensitive MC-ICPMS (Craig et al., 2018(Craig et al., , 2020 was deemed necessary for 110 subsequent measurements. The sessions with the MC-ICPMS were carried out between July 2020 and December 2020. As for the single collector, signal strength was tuned for maximum sensitivity while keeping oxide formation below ~ 0.5 % (UO/U) and element fraction low (e.g. Th/U ~ 0.9). In that case, it was done with a 35 µm, 6 Hz, ca. 3.5 J/cm 2 fluence and a 3 µm/s line ablated in the glass SRMNIST 614 (Jochum et al., 2011). The average sensitivity obtained for the line is ca. 120,000 cps per µg/g for 238 U (note the smaller spot size compare to the SC-ICPMS). The detection limits in the multicollector ICPMS 115 were c. 0.3 and 0.01 ng/g for 206 Pb and 238 U, respectively. The analyses were done during 31 s (14 s background and 16 s of ablation) in static mode, measuring 206 Pb and 207 Pb with Secondary Electron Multipliers (SEMs), 202 Hg and 204 Pb with Multiple Ion Counters (MICs) and 232 Th and 238 U on Faraday cups with 10 13 Ω amplifiers. Faraday signals in V are converted into cps by using a factor of 62,400,000. Detailed data acquisition parameters are summarised in Table 2. In each analytical session, soda-lime glass SRMNIST614 was used as the primary reference material to correct for mass bias 120 ( 207 Pb/ 206 Pb) and the interelement fractionation and instrumental drift ( 206 Pb/ 238 U) throughout the entire analytical session.
Carbonate reference material WC-1 (254 Ma, Roberts et al., 2017) was used to determine the difference of the Pb/U fractionation between carbonate and synthetic glass matrix. Depending on the analytical conditions (i.e, spot size, laser fluence, torch position, sample gas flows, etc.) the matrix effect can vary up to 12 % (FIERCE laboratory observation, e.g. Cruset et al. 2021) and even at similar tuning parameters, two sessions separated by some weeks could result in different Pb/U correction 125 factors. So far, this behaviour is not very well understood and due to its unpredictability, the matrix correction is calculated for each session (see below). Secondary reference calcite materials, ASH-15D calcite (2.965 ± 0.011 Ma, Nuriel et al., 2021), B-6 (42.99 ± 0.99 Ma, only LA-ICPMS data, Pagel et al., 2018) and in-house calcite (reproducible age of ca. 36 Ma) were measured for quality control. Not all the secondary reference materials were used in each session (see information in Tables 1   and 2). 130 Calibration strategy SRMNIST614 as primary RM, WC-1 as offset RM, and ASH15D as validation RM.
Mass discrimination 207 Pb/ 206 Pb and 206 Pb/ 238 U normalised to primary standard (variable in each session)

Common-Pb correction
No common-Pb correction applied to the data.

Uncertainty level & propagation
Uncertainties are quoted at 2σ absolute and are propagated by quadratic addition of the within run precision (SD of the mean of ratios in log-ratio space), counting statistics, background, common Pb correction (if applicable) and the excess of scatter (calculated from the primary RM). In addition, an excess of variance calculated for each session from the offset RM, was added quadratically to the 206 Pb/ 238 U ratios. Systematic uncertainties are reported as an expanded uncertainty, considering long term reproducibility (1.5%, 2σ) and decay constant uncertainties.  Raw data were corrected offline using an in-house VBA spreadsheet program Zeh, 2006, 2009). Following 135 background and interferences corrections, outliers (±2σ) were rejected based on the time-resolved 207 Pb/ 206 Pb and 206 Pb/ 238 U ratios and the Pb and U signal. All in all, five sessions were performed, and the matrix Pb/U correction factors (carbonate vs SRMNIST glass) applied to each of them are as follows: 4.5 % for SC-ICPMS session, 8 % for MC-ICPMS session 1 (same spot size for both carbonate and SRMNIST glass, see Table 2), 0.5 % for session 2 (different spot size, Table 2), 0 % for session 3, and 0 % for session 4.. The 206 Pb/ 238 U downhole fractionation during 16/18 s depth profiling was estimated to be 140 3%, based on the common Pb-corrected WC-1 analyses, and was applied as an external correction to all sulfates analyses and secondary reference materials. Uncertainties for each isotopic ratio are the quadratic addition of the within-run precision, counting statistic uncertainties of each isotope, and the excess of scatter and variance (Horstwood et al., 2016) calculated from the SRMNIST 614 and the WC-1 after drift correction. To account for the long-term reproducibility of the method we added by quadratic addition an expanded uncertainty of 1.5% to the final age of all analysed samples (Montano et al., 2021). This 145 was deducted from repeated analyses of ASH-15D in the FIERCE laboratory between 2017 and 2019. Data were displayed in Tera-Wasserburg plots and ages were calculated as lower concordia-curve intercepts using the same algorithms as Isoplot 4.15 (Ludwig, 2012). All uncertainties are reported at the 2σ level. After the analysis, the depth of the ablation pit was measured in several spots per sample, including the WC-1 and SRMNIST 614 reference materials, using the Keyence VHX 6000 digital microscope. 150

U-Pb Dating
U-Pb dating was applied to 32 samples from the different locations and all available gypsum/anhydrite varieties (large selenite crystals, banded selenite, gypsum cumulates, anhydrite, halite with gypsum and anhydrite intercalation) across the Mediterranean Sea ( Fig. 1), which display variable contents of Pb and U. Only five of them were successfully dated (15 % of 155 success). The undatable samples are characterized by analyses that clustered near the common Pb intercept, disclosing a large amount of common Pb (Fig. 2). This low µ ( 238 U/ 204 Pb ratio) makes it impossible to draw any regression line. No link between successful/unsuccessful samples and their texture could have been established and both successful and unsuccessful samples have been found within the same type of gypsum. The successfully dated samples are described below, and their results are presented in Fig. 3 as well as in Tables 3 and 4. 160

Sample MTO 4-4
The MTO 4-4 sample was collected at the Monte Tondo gypsum quarry, located within the Vena del Gesso basin (along the western Romagna Apennines), and belongs to the PLG (Lugli et al., 2007(Lugli et al., , 2010Vasiliev et al., 2017). It is a banded selenite (type F4 of Lugli et al., 2010) and the cyclostratigraphic age is 5.920 Ma, close to the onset of the MSC. The sample was measured in three different sessions. The maximum U and Pb content on the analysed spots are 2.34 µg/g and 3.85 µg/g, 165 respectively, depicting a maximum U/Pb ratio of 98.4 in the best case. The first of the sessions was measured with the SC-ICPMS and the analyses define a regression line with a lower intercept at 6.01 ± 1.19 Ma (±2σ, MSWD = 1.07, Fig. 3

Sample MTO 11-3
This sample was also collected by Vasiliev et al., (2017) at the Monte Tondo gypsum quarry. It is a massive selenite (F3 of Lugli et al., 2010) and belongs to the younger cycles of the PLG. Its estimated cyclostratigraphic age is 5.701 Ma. MTO 11-3 175 was measured as well in three different sessions. The maximum U and Pb content on the analysed spots are 5.49 µg/g and 0.97 µg/g, respectively, depicting a maximum U/Pb ratio value of 155.2 in the best case. The first of the sessions was measured with the SC-ICPMS and the analyses define a regression line with a lower intercept at 5.40 ± 0.84 Ma (±2σ, MSWD = 1.13,

Sample BCR9644
The sample BCR9644 was collected from the cores of Deep Sea Drilling Program Site 42A hole 376 cored in 1975 West of Cyprus and stored at Bremen International Ocean Drilling Program repository. BCR9644 was collected from a Gypsum Breccia, at 170.28 m below sea level and has a stratigraphic age of ca. 5.55 -5.60 Ma. It was measured twice with the MC-ICPMS. The maximum U and Pb content on the analysed spots are 2.31 µg/g and 0.61 µg/g, respectively, although Pb rarely 200 exceeds 0.1 µg/g. The maximum U/Pb ratio obtained in that sample is 577.5 in the best case. The low Pb contents imply large error ellipses, but successful regression lines have been defined, with a lower intercept at 2.98 ± 0.34 Ma (±2σ, MSWD = 0.79,

Sample Pu 05
This sample was collected in the Ploutis region (Central Crete, Greece) and it is a gypsum breccia. The stratigraphic age of 205 these gypsum units is disputed between being part of the PLG (Zachariasse et al., 2008) but the texture is direct capping by Lago Mare deposits strongly suggest that Pu 05 belongs to the UG unit. Its Cyclostratigraphic age is ca. 5.40 Ma. Pu 05 was also measured twice with the MC-ICPMS. The maximum U and Pb content on the analysed spots are 1.44 µg/g and 0.16 µg/g, respectively, depicting a maximum U/Pb ratio value of 158.0 in the best case. Each session defines a regression line with a lower intercept at 5.15 ± 0.42 Ma (±2σ, MSWD = 0.68, Fig. 4) and 5.54 ± 0.61 Ma (±2σ, MSWD = 1.02, Fig. 4), respectively. 210

Pit depth measurements
After the analyses, pit depths were measured in all the samples as well as in the carbonate reference materials. The measured pit depth averages were used for calculating the U and Pb contents (Tables 3 and 4). The shape and depth of the craters in WC-1 primary carbonate are all similar and their average depth is 15.0 µm (SD = 1.34, n = 16). Few spots corresponding to the secondary reference materials were also checked and they are comparable to those of WC-1. The pits of the SRMNIST 614 215 are ca. 33% shallower than the ones in the calcite matrix, around 10 µm deep. Regarding the sulfate samples, the pit depth of samples MTO 4-4 and MTO 11-3 is rather homogeneous with mean values of 29.6 µm (SD = 6.2, n = 44) and 18.9 µm (SD = 5.9, n = 37, Fig. 4A), respectively. The samples BCR 9644 and BOX 108 display zones with different heights in some of the ablation holes (Fig. 4B). Although they are exceptional, two ca. 90 µm and two ca. 60 µm pits were measured in BOX 108.

High common Pb content and potential applicability
The majority of the analysed samples, 27 out of 32, were unsuccessful due to the high common Pb content and hence, low or non-existent spread in the 238 U/ 206 Pb axis. Recent studies in the field of environmental hazards have shown that Pb tends to incorporate into sulfates, both gypsum and anhydrite (Astilleros et al., 2010;Morales et al., 2014;Kameda et al., 2017). In fact, in presence of high-Pb fluids, anglesite (PbSO4) is simultaneously intergrown with those sulfates. The behaviour of 230 Uranium remains unknown, although experiments carried out on phosphogypsum, a waste by-product generated from apatite in the production process of phosphoric acid and phosphate fertilizers, suggest that U uptake by gypsum is pH controlled (Lin et al., 2018). Thus, the more alkaline the environment is the higher U concentration could be expected in gypsum. However, the pH of evaporating seawater rarely reaches those values and tends to drop as the evaporation process goes on (Babel and Schreiber, 2014). Considering a low salinity, but high concentrations of Ca 2+ and SO2 −4 (Clauer et al., 2000) during the 235 formation of MSC evaporites, the alkalinity of the depositional environment might have increased. In any case, even the gypsum precipitated in U-rich environments like uranium mine tailings contain a high amount of Pb among other metals (Liu and Hendry, 2011).
The amount of common Pb is a challenge for dating young rocks, as their success strongly depends on the spread in the X-axis ( 238 U/ 206 Pb). In turn, given the same initial 238 U/ 206 Pb ratio, older samples would have produced sufficient radiogenic Pb, and 240 thus, a certain spread in the Y-axis ( 207 Pb/ 206 Pb) as to be projected in a more precise regression line. Indeed, older samples are more influenced by the 207 Pb/ 206 Pb ratio and therefore, it is highly likely that the success rate increases with the age of the sample.

SC-ICPMS vs. MC-ICPMS
The first set of samples was measured with the SC-ICPMS. The U and Pb contents in the samples were rather low and produce 245 large error ellipses in every single spot. This issue, together with low µ ratios (i.e., spread on 238 U/ 206 Pb), produces substantial uncertainties in the final ages (Fig. 3) and a comparison with the depositional ages is poor. In order to achieve better results, we decided to accomplish subsequent measurements with the MC-ICPMS, which provides about three times better sensitivity and simultaneous isotope detection (Craig et al., 2018;. The higher sensitivity implies smaller uncertainties in each spot and hence, more accurate and precise regression lines (i.e., ages) can be depicted. 250 Indeed, the improvement in age precision is clearly illustrated in Fig. 3. Although the results can be biased because fewer data were acquired during SC-ICPMS analyses, given a similar spread in the 238 U/ 206 Pb axis, the uncertainties of c. 15 % (MTO 11-3) or 20 % (MTO 4-4) obtained with the SC-ICPMS were reduced to 8 % (MTO 11-3, seq 2) and 11 % (MTO 4-4, seq 2) by using the MC-ICPMS (Fig. 3). Furthermore, the re-measurement of these two samples in another independent session in which higher 238 U/ 206 Pb ratios were found, reduced the uncertainties even more down to ca. 6%. 255

U-Pb ages vs cyclostratigraphic ages 260
Well-characterized matrix-matched reference material is essential for U-Pb analytical techniques using laser probes as matrix differences between sample and reference standard can cause a significant offset in the obtained ages (Yang et al., 2018;Guillong et al., 2020). However, in the absence of sulfate reference materials, an attempt to use calcite reference materials was carried out, expecting that the offset between both materials was going to be low or negligible. The light absorption observed in calcite and gypsum is similar and they are easily ablated even at low fluence (less than 2 J/cm 2 ). As a comparison, Piccione 265 et al. (2019) obtained analogous ages on contemporary fluorite and nacrite, both corrected to the same calcite reference material, even when the fluorite has different light absorption and higher energy is needed for its ablation (5-6 J/cm 2 ). Due to those reasons, we expected a significantly lower matrix-induced offset than the one observed between calcite and zircon (4.7 %, Parrish et al., 2018).
The cyclostratigraphic ages of the MSC samples are well-known (e.g. Vasiliev et al. 2017) and we have used them for testing 270 the suitability of the corrections with respect to carbonate matrix. As pointed out above, the majority of the samples contain a significant amount of common Pb and only five ages were obtained. Although the µ of those samples was not extreme, the uncertainties range between 6 to 11 %. The ages obtained for the samples MTO 4-4, MTO 11-3, Pu 05 and BOX 108 are fully in accordance with the cyclostratigraphic ages (e.g. Lugli et al., 2007;Vasiliev et al., 2017). Unfortunately, the level of precision makes it impossible to discern whether the ages correspond to depositional or diagenetic/dehydration stages of the 275 evaporite formation. Likewise, it is not possible to distinguish the three different stages of evaporite deposits of the MSC.
Regardless, no matrix offset between sulfate and carbonate can be observed, and if any, this is included in the uncertainty.
On the other hand, the sample BCR 9644 resulted in an unexpected younger age of ca. 3 Ma. The brecciated nature of the sample, together with its extremely low Pb content (0.03 µg/g on average) in comparison with surrounding samples suggest a subsequent (re)crystallization and remobilization of U and Pb that could be related to the breccia formation. Warthman et al. 280 (2000) proposed an important bacterial activity after the evaporite formation. For the equivalent in time Site 374, located South-East Sicily, an ~3 m thick dolomitization front in Pliocene hemipelagic succession overlying the UG was identified.
Here, a hypothesized role of the deep biosphere, sulfate-reducing bacteria thriving on the dissolution of sulphate-bearing minerals (Warthman et al., 2000;Petrash et al., 2017) was suggested. Montano et al. (2019Montano et al. ( , 2021 showed that biological activity may control the U-Pb partitioning on carbonates, so the connection between the bacterial activity and the 3 Ma age 285 could not be discarded. Although gypsum to anhydrite to gypsum (two-step) transformation can be considered as another possible scenario, there is no observation neither in the literature, that supports this hypothesis. Guillong et al. (2020) showed that different ablation parameters produce distinctive pit profiles (the so-called "aspect ratio" or depth/diameter ratio) and it could result in a noticeable bias in the data. The carbonate reference materials analysed here with 290 a 130 µm spot size, resulted in a depth of ca. 15 µm (aspect ratio of 0.12), whereas the sulfates vary between 16 µm and 63 µm (aspect ratio between 0.12 and 0.48, Fig. 4). The ablation on NIST glass resulted in shallower ca. 10 µm deep holes and an aspect ratio of 0.13, similar to the carbonates. This divergence between the sulfates could be devoted to various nonexcluding features such as different textures, particle size, porosity or compaction (Elisha et al., 2021). However, in the cases with an aspect ratio mismatch relative to the primary standard of less than 2, a deviation lower than 5% is anticipated (Guillong 295 et al., 2020), which lies in the final result uncertainty of the majority of the samples analysed here. The larger discrepancy observed in the sample Pu 05 (relative mismatch of 4) could result in age offsets up to 10 % (Guillong et al., 2020, their Figure   4). However, Fig. 4b reveals an important heterogeneity in the pit profile in some samples, with a silhouette that resembles pores. Whether they correspond to porosity or chunks released due to a badly coupled laser beam, the signal remained stable.

Pit depth profiles
These pit depth issues are also related to the downhole fractionation corrections. Mangenot et al. (2018) claimed that shallow 300 pit depth compared to the spot size could minimize the downhole fractionation. That argument could apply to our reference materials and sulfates with shallower pit depth, but how it affects depths beyond 50-60 µm can be arguable. Lenoir et al., (2021) obtained coherent regression lines in fluorites even with pit depths (up to 50 % variable) larger than spot sizes.
Notwithstanding, the lack of bias between our U-Pb ages and cyclostratigraphic ages suggests that the different downhole fractionation is not noticeable or remains within the uncertainties. 305

Conclusions
In this contribution, we have evaluated the applicability of carbonates as "almost-matrix-matched reference materials" for U-Pb dating of sulfates and for that purpose, gypsum and anhydrite samples from the Messinian Salinity Crisis were analysed.
The known cyclostratigraphic ages of these evaporites were compared with the in-situ U-Pb ages obtained. The samples showed a high amount of common Pb and low spread in the U/Pb axis and therefore, only 15 % of the samples were successful. 310 In fact, due to the large uncertainties obtained at the beginning, we were forced to switch from the SC-ICPMS to MC-ICPMS in order to improve the precision of the measurements. Four of the five successfully dated samples were indistinguishable within error from the expected ages, while the other was considerably younger. We assume that all the factors that could produce a bias in the final age, if any, are contained in the uncertainty and therefore, the use of carbonate reference materials could be a trustworthy approach for in situ U-Pb dating of sulfates. We acknowledge that the availability of sulfate reference 315 material in the future will result in an improvement in both reliability and precision. 320