# Supergene phases from ferruginous duricrusts: non-destructive microsampling and mineralogy prior to (U–Th)/He geochronological analysis

Karina P P Marques, Thierry Allard, Cécile Gautheron, Benoît Baptiste, Rosella Pinna-Jamme, Guillaume Morin, Ludovic Delbes, Pablo Vidal-Torrado

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## To cite this version:

Karina P P Marques, Thierry Allard, Cécile Gautheron, Benoît Baptiste, Rosella Pinna-Jamme, et al. "Supergene phases from ferruginous duricrusts: non-destructive microsampling and mineralogy prior to (U–Th)/He geochronological analysis." European Journal of Mineralogy, 2023, 35 (3), pp.383-395. 10.5194/ejm-35-383-2023. hal-04142575

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Submitted on 27 Jun 2023

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<figure><img src="image_2.png" /></figure>

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grain size, as a crystallite may potentially contain several MCDs separated by extended defects. In addition, Al content in goethite influences the He retention and the correction factor (Bassal et al., 2022). Second, using this methodology, the (U–Th)/He ages of the cortex and core of the pisolitic samples were determined and discussed as an example. Pisolites are frequently observed in lateritic ferruginous duricrusts, may exhibit contrasting mineralogy between the core and cortex, and are thus relevant for the two objectives of this study. This methodology is shown to be appropriate to identify and quantify different Fe-oxides and oxyhydroxides in dated grains and also potentially to reveal mineral contaminants.

# 2 Material and methods

## 2.1 Sampling site

A lateritic ferruginous duricrust profile (~1100 m elevation) developed on relict low-relief uplands in the tropical Brazilian continental interior, Alto Paranaíba region (Minas Gerais State, Brazil), was chosen for sampling (Fig. 1). It is formed at the expense of pyroclastic and epiclastic rocks from the Mata da Corda group (upper Cretaceous) (CPRM, 2014). Despite being an important tropical Brazilian area of occurrence of thick lateritic profiles including duricrusts (Marques et al., 2021), no weathering geochronological data are available yet. The climate of the study area is Aw (Köppen-Geiger's classification), characterized by warm and dry winters and moist and warm summers. The average annual rainfall is 1600 mm, and the mean annual temperature is 22 °C (Oliveira et al., 2020).

The morphological description of the profile was carried out based on McFarlane (1976), Tardy (1993), and Aleva (1994), and then non-disturbed samples were taken from all the described layers. Based on the morphological description at the fieldwork and laboratory, the pisolitic facies, which is often encountered in lateritic ferruginous duricrusts (Nahon, 1991; Tardy, 1993), was chosen due to being structurally suitable for dating, and also because it presented a usual complexity or heterogeneity allowing us to carry out our methodological approach.

## 2.2 Selection and preparation of samples

The morphological characteristics of a hand specimen from the pisolitic facies were observed and described using a binocular microscope. The populations of Fe-oxides were identified based on texture, color, and morphology (Tardy, 1993), and then the pisolites were isolated. The semi-quantitative composition of a complete pisolite was analyzed by a Zeiss Ultra55 SEM with a FEG-Schottky electronic source coupled with an EDS. The working distance was 19.6 mm, and the acceleration voltage was 15 kV.

Another pisolite with similar morphological characteristics and belonging to the same identified population was sampled using a diamond micro-drill to divide the cortex and core. Grains (ca. 500 µm large) were carefully selected from the cortex and core using an optical microscope. Some of the grains were prepared by hand grinding using a mortar and pestle and then homogenized and sieved at 100 µm in order to obtain powder samples (~0.50 mg). In the other selected grains (~0.10 mg), no preparation was carried out.

## 2.3 X-ray diffraction data collection and analysis

Both powder samples and single polycrystalline grain samples from the pisolitic cortex and core were analyzed for their mineralogical composition using XRD. Samples were loaded in borosilicate capillaries either as powder fillings or as inserted grains. Laboratory X-ray experiments were performed at the X-ray diffraction platform of the Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Université (Paris, France). The single polycrystalline grain samples from the pisolitic core were first analyzed using a Panalytical XpertPro MPD two-circle diffractometer (Co sealed tube in Debye–Scherrer transmission geometry – λ*K*α1 = 1.789010 Å, λ*K*α2 = 1.792900 Å – at 45 keV and 40 mA, with a range scan of 3–90° 2θ and a step size of 0.03° 2θ s⁻¹). The signal collected from the grain was too low, even after several hours of exposure, and even the powder in the capillary exhibited noise after 15 h counting (Fig. S1 in the Supplement). Therefore, more intense and collimated sources were required to analyze small grains in transmission geometry.

A Rigaku MM007HF diffractometer equipped with Varimax focusing optics, a RAXIS4++ image plate detector placed at a distance of 200 mm from the sample, and a Mo rotating anode (λ*K*α1 = 0.709319 Å and λ*K*α2 = 0.713609 Å) at 50 keV and 24 mA were used, with a range scan of 3 to 45° 2θ and the acquisition time of 60 min. The Fit2D program (Hammersley, 2016) was used for the integration of 2D images into 1D patterns after a calibration with LaB6 standard. Rotating anode generators are one of the most powerful X-ray sources available in the laboratory. In these experimental settings operating under a high vacuum, a cooled rotating surface is continuously irradiated by an intense electron beam (Fig. 2), resulting in an improved signal-to-noise ratio and in the possibility to analyze small samples.

Higher X-ray fluxes require the use of a synchrotron beamline. Therefore, XRD_synch patterns were collected on the same grains at the CRISTAL beamline of the SOLEIL Synchrotron (Saint-Aubin, France). We used the two-circle diffractometer of the CRISTAL beamline equipped with a MYTHEN2 X 9K detector (DECTRIS). The measurements were performed in continuous mode at 17 keV (0.72896 or 0.727913 Å) using 5 min exposure time, with a scan range of 3–65° 2θ and a scan speed of 0.04° 2θ s⁻¹.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Ferruginous duricrust profile and pisolitic facies. (a) Location of the Fe duricrust profile in a Brazilian landscape, (b) distinct horizons of the profile, (c) pisolitic facies, (d) SEM (backscattered electrons) picture of the internal structure of a pisolite, and (e) contrasting compositional variation from the core to cortex. The white line in the SEM image in (d) indicates the transect analyzed and shown in (e).</figcaption></figure>

Rietveld refinements of the multiphase XRD patterns were performed with the FullProf software (Rodriguez-Carvajal, 1993) and included hematite, goethite, kaolinite, and anatase as mineral phases. Starting crystal structure data were taken from Finger and Hazen (1980), Gualtieri and Venturelli (1999), Bish and Von Dreele (1989), and Howard et al. (1991), respectively. Scale factors, cell parameters,

isotropic pseudo-Voigt line-profile functions (Thompson-Cox-Hastings), and overall B factors were first refined for the four phases. The peak widths were significantly larger than the instrument resolution (~0.1° 2θ). The instrumental resolution function (IRF) was determined over the 2θ range measured from the LaB6 crystallographic standard Rietveld refinement. Assuming that the Lorentzian part of the peak
<table><caption>**Table 1.** Mineralogical parameters determined for the core and cortex samples by Rietveld refinement of XRD\_rotat (60 min exposure time) and XRD\_synch patterns: weight percentage (wt %) ratio of the main phases (numbers between parentheses are absolute errors), mean coherent domain sizes of the hematite and goethite, and Al³⁺ for Fe³⁺ substitution rate in goethite and hematite.</caption><thead><tr><th rowspan="3">Sample</th><th colspan="5">MCD<sup>a</sup></th><th colspan="2">Al<sup>b</sup></th></tr><tr><th colspan="2">wt %</th><th rowspan="2">wt %</th><th rowspan="2">wt %</th><th colspan="2">nm</th><th colspan="2">% mol</th></tr><tr><th>Hm</th><th>Gt</th><th>Gt</th><th>Hm</th><th>Gt</th><th>Hm</th></tr></thead><tbody><tr><td colspan="9" style="text-align: left; font-weight: bold;">XRD\_rotat</td></tr><tr><td>Core grain</td><td>41 (3.8)</td><td>59 (4.9)</td><td>0</td><td>0</td><td>15</td><td>12</td><td>9</td><td>1</td></tr><tr><td>Core powder</td><td>29 (4.7)</td><td>71 (7.1)</td><td>0</td><td>0</td><td>15</td><td>12</td><td>9</td><td>1</td></tr><tr><td>Cortex grain</td><td>0.3 (0.2)</td><td>95 (3.5)</td><td>2.7 (0.3)</td><td>2.5 (0.4)</td><td>23</td><td>c</td><td>4</td><td>c</td></tr><tr><td>Cortex powder</td><td>0.6 (0.5)</td><td>92 (4.6)</td><td>5.9 (1.3)</td><td>1.9 (0.5)</td><td>27</td><td>c</td><td>4</td><td>c</td></tr><tr><td colspan="9" style="text-align: left; font-weight: bold;">XRD\_synch</td></tr><tr><td>Core grain</td><td>32 (0.4)</td><td>68 (1.0)</td><td>0</td><td>nd</td><td>16</td><td>18</td><td>12</td><td>7</td></tr><tr><td>Cortex grain</td><td>0.7 (0.1)</td><td>94 (1.0)</td><td>5.3 (0.3)</td><td>nd</td><td>30</td><td>c</td><td>11</td><td>c</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Figure 4.** Synchrotron XRD patterns for grain samples from the pisolitic core and cortex. Gt: goethite; Hm: hematite; Kt: kaolinite; An: anatase. The red line represents the Rietveld fit, and the dashed black line is the experimental pattern.</figcaption></figure>

in laboratories, is powerful enough to analyze undisturbed grains that can be subsequently used for dating and thus appears much more appropriate than conventional XRD analysis that fails to analyze single grains (Fig. S1).

### 3.2.3 (U-Th)/He analysis

The effect of exposure time of XRD\_rotat recording on apparent age for pisolitic core and cortex is shown in Fig. 5, and

(U-Th)/He data are reported in Table 2. For each sample, there is no representative shift in age related to the acquisition time of mineralogical data: the results show a good reproducibility within the analytical uncertainty, i.e., 5%. Figure 5 clearly shows that all the samples exhibit apparent age clusters including the 0 exposure point, supporting the idea that performing XRD\_rotat mineralogical analysis prior to (U-Th)/He geochronological analysis is possible without impacting He content and age.

<footnote><sup>a</sup> Mean coherent domain sizes according to Scherrer's formula; <sup>b</sup> isomorphic substitution of Fe³⁺ by Al³⁺ according to Schulze (1984) for goethite and Schwertmann et al. (1979) for hematite. Hm: hematite; Gt: goethite; Kt: kaolinite; An: anatase; nd: not determined; c: absent.</footnote>
<table><caption>Table 2. Raw and corrected (U–Th)/He age for the investigated iron oxides and oxyhydroxides and Durango standard apatite.</caption><thead><tr><th>Sample</th><th>Exposure time</th><th>Weight</th><th>4He</th><th>Error</th><th>238U</th><th>232Th</th><th>147Sm</th><th>4He</th><th>Error</th><th>238U</th><th>232Th</th><th>147Sm</th><th>eU</th><th>Th/U</th><th>Raw ages</th><th>Correction factor ± error</th><th>Corrected age</th><th>Error corrected age</th></tr><tr><th></th><th>min</th><th>µg</th><th>mol</th><th>ng</th><th>ng</th><th>ng</th><th>ng</th><th>nmol g<sup>-1</sup></th><th>ppm</th><th>ppm</th><th>ppm</th><th>ppm</th><th>ppm</th><th>ppm</th><th>Ma</th><th>%</th><th>Ma</th><th>Ma</th></tr></thead><tbody><tr><td>Durango</td><td>0</td><td>29</td><td>6.70 × 10<sup>-13</sup></td><td>1.30 × 10<sup>-14</sup></td><td>0.69</td><td>14.12</td><td>1.27</td><td>22.69</td><td>0.05</td><td>23.4</td><td>480.2</td><td>43.2</td><td>139</td><td>20.6</td><td>30.7</td><td>0</td><td></td><td></td></tr><tr><td>Durango</td><td>0</td><td>13</td><td>2.20 × 10<sup>-13</sup></td><td>4.40 × 10<sup>-15</sup></td><td>0.22</td><td>4.99</td><td>0.52</td><td>16.64</td><td>0.05</td><td>16.7</td><td>375.1</td><td>38.8</td><td>107</td><td>22.5</td><td>29.3</td><td>0</td><td></td><td></td></tr><tr><td>Durango</td><td>0</td><td>29</td><td>5.30 × 10<sup>-13</sup></td><td>1.10 × 10<sup>-14</sup></td><td>0.53</td><td>11.44</td><td>1.12</td><td>17.98</td><td>0.02</td><td>17.9</td><td>388.0</td><td>38.1</td><td>111</td><td>21.6</td><td>30.4</td><td>0</td><td></td><td></td></tr><tr><td>Durango</td><td>0</td><td>46</td><td>1.00 × 10<sup>-12</sup></td><td>2.10 × 10<sup>-14</sup></td><td>1.04</td><td>22.56</td><td>1.75</td><td>22.65</td><td>0.02</td><td>22.7</td><td>494.7</td><td>38.5</td><td>142</td><td>21.8</td><td>30.1</td><td>0</td><td></td><td></td></tr><tr><td>Durango</td><td>0</td><td>64</td><td>1.90 × 10<sup>-12</sup></td><td>3.80 × 10<sup>-14</sup></td><td>2.03</td><td>39.66</td><td>3.82</td><td>29.68</td><td>0.05</td><td>31.6</td><td>618.0</td><td>59.5</td><td>180</td><td>19.6</td><td>31.0</td><td>0</td><td></td><td></td></tr><tr><td>Durango</td><td>0</td><td>55</td><td>1.20 × 10<sup>-12</sup></td><td>2.30 × 10<sup>-14</sup></td><td>1.21</td><td>23.9</td><td>2.77</td><td>21.15</td><td>0.03</td><td>22.0</td><td>433.9</td><td>50.3</td><td>126</td><td>19.7</td><td>31.5</td><td>0</td><td></td><td></td></tr><tr><td>Core</td><td>Hm+Gt</td><td>0</td><td>81</td><td>1.30 × 10<sup>-13</sup></td><td>2.60 × 10<sup>-15</sup></td><td>0.31</td><td>3.43</td><td>0.04</td><td>1.61</td><td>0</td><td>3.8</td><td>42.3</td><td>0.5</td><td>14</td><td>11.2</td><td>21.6</td><td>10 ± 10</td><td>23.8</td><td>2.2</td></tr><tr><td>Core</td><td>Hm+Gt</td><td>0</td><td>147</td><td>2.70 × 10<sup>-13</sup></td><td>5.40 × 10<sup>-15</sup></td><td>0.59</td><td>5.69</td><td>0.09</td><td>1.83</td><td>0</td><td>4.0</td><td>38.7</td><td>0.6</td><td>13</td><td>9.6</td><td>25.7</td><td>10 ± 10</td><td>28.3</td><td>2.6</td></tr><tr><td>Core</td><td>Hm+Gt</td><td>0</td><td>148</td><td>3.30 × 10<sup>-13</sup></td><td>6.60 × 10<sup>-15</sup></td><td>0.62</td><td>7.8</td><td>0.21</td><td>2.21</td><td>0</td><td>4.2</td><td>52.7</td><td>1.4</td><td>17</td><td>12.5</td><td>24.6</td><td>10 ± 10</td><td>27.1</td><td>2.5</td></tr><tr><td>Core</td><td>Hm+Gt</td><td>0</td><td>149</td><td>3.10 × 10<sup>-13</sup></td><td>6.10 × 10<sup>-15</sup></td><td>0.54</td><td>6.44</td><td>0.12</td><td>2.05</td><td>0</td><td>3.6</td><td>43.2</td><td>0.8</td><td>14</td><td>12.0</td><td>27.5</td><td>10 ± 10</td><td>30.3</td><td>2.8</td></tr><tr><td>Core</td><td>Hm+Gt</td><td>30</td><td>279</td><td>5.00 × 10<sup>-13</sup></td><td>1.00 × 10<sup>-14</sup></td><td>1.08</td><td>14.98</td><td>0.13</td><td>1.8</td><td>0</td><td>3.9</td><td>53.7</td><td>0.5</td><td>17</td><td>13.9</td><td>20.2</td><td>10 ± 10</td><td>22.2</td><td>2.0</td></tr><tr><td>Core</td><td>Hm+Gt</td><td>90</td><td>114</td><td>2.00 × 10<sup>-13</sup></td><td>3.90 × 10<sup>-15</sup></td><td>0.36</td><td>4.89</td><td>0.05</td><td>1.71</td><td>0</td><td>3.1</td><td>42.9</td><td>0.4</td><td>13</td><td>13.8</td><td>24.0</td><td>10 ± 10</td><td>26.4</td><td>2.4</td></tr><tr><td>Cortex</td><td>Gt</td><td>0</td><td>177</td><td>2.70 × 10<sup>-13</sup></td><td>5.40 × 10<sup>-15</sup></td><td>1.72</td><td>10.31</td><td>0.29</td><td>1.55</td><td>0.05</td><td>9.7</td><td>58.3</td><td>1.7</td><td>24</td><td>6.0</td><td>12.2</td><td>15 ± 10</td><td>14.0</td><td>1.2</td></tr><tr><td>Cortex</td><td>Gt</td><td>0</td><td>138</td><td>2.30 × 10<sup>-13</sup></td><td>4.60 × 10<sup>-15</sup></td><td>1.47</td><td>9.79</td><td>0.42</td><td>1.68</td><td>0.01</td><td>10.7</td><td>70.9</td><td>3.1</td><td>28</td><td>6.6</td><td>11.4</td><td>15 ± 10</td><td>13.1</td><td>1.1</td></tr><tr><td>Cortex</td><td>Gt</td><td>0</td><td>136</td><td>2.00 × 10<sup>-13</sup></td><td>4.00 × 10<sup>-15</sup></td><td>1.41</td><td>8.6</td><td>0.28</td><td>1.48</td><td>0.00</td><td>10.4</td><td>63.2</td><td>2.1</td><td>26</td><td>6.1</td><td>10.8</td><td>15 ± 10</td><td>12.4</td><td>1.1</td></tr><tr><td>Cortex</td><td>Gt</td><td>0</td><td>145</td><td>2.60 × 10<sup>-13</sup></td><td>5.20 × 10<sup>-15</sup></td><td>1.76</td><td>10.64</td><td>0.33</td><td>1.79</td><td>0.00</td><td>12.2</td><td>73.4</td><td>2.3</td><td>30</td><td>6.0</td><td>11.2</td><td>15 ± 10</td><td>12.9</td><td>1.1</td></tr><tr><td>Cortex</td><td>Gt</td><td>30</td><td>36</td><td>1.60 × 10<sup>-14</sup></td><td>3.10 × 10<sup>-16</sup></td><td>0.12</td><td>0.58</td><td>0.02</td><td>0.44</td><td>0.00</td><td>3.3</td><td>16.2</td><td>0.6</td><td>7</td><td>4.9</td><td>11.3</td><td>15 ± 10</td><td>13.0</td><td>1.1</td></tr><tr><td>Cortex</td><td>Gt</td><td>60</td><td>48</td><td>6.80 × 10<sup>-14</sup></td><td>1.40 × 10<sup>-15</sup></td><td>0.58</td><td>3.12</td><td>0.08</td><td>1.42</td><td>0.01</td><td>12.2</td><td>64.9</td><td>1.8</td><td>28</td><td>5.3</td><td>9.5</td><td>15 ± 10</td><td>10.9</td><td>1.0</td></tr><tr><td>Cortex</td><td>Gt</td><td>90</td><td>197</td><td>3.40 × 10<sup>-14</sup></td><td>6.80 × 10<sup>-16</sup></td><td>2.05</td><td>12.1</td><td>0.34</td><td>1.77</td><td>0.04</td><td>10.4</td><td>61.4</td><td>1.7</td><td>25</td><td>5.9</td><td>13.2</td><td>15 ± 10</td><td>15.2</td><td>1.3</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>**Figure 5.** (U–Th)/He data. (a) Ternary diagram indicating the He–Th–U concentrations and the uncorrected (U–Th)/He ages (Ma) for pisolitic core and cortex obtained in the R environment using the *Helioplot* function (Vermeesch, 2010); and (b) evolution of the corrected (U–Th)/He age as a function of the effective uranium content (eU = U + 0.238 × Th + 0.0012 × Sm: Gastil et al., 1967; Cooperdock et al., 2019).</figcaption></figure>

U and Th concentrations clearly differ between the pisolitic core and cortex, with higher values of both elements occurring in the pisolitic cortex. This suggests a relative U enrichment during the weathering of the Fe-oxides and oxyhydroxides from the core (Fig. 5). A grain from the pisolitic cortex (with 30 min exposure) contains lower U and Th concentrations than other grains (Fig. 5b), which can be explained by small differences between grains, as often occurs for grains of supergene minerals belonging to the same population (e.g., Vasconcelos et al., 2013; Monteiro et al., 2014; Riffel et al., 2016; Heller et al., 2022). Nevertheless, such a difference does not influence the age when compared to the other grains of the same facies at the millimeter scale (Fig. 5b).

Corrected (U–Th)/He ages for the pisolitic core range from 30 ± 2.8 to 22 ± 2.0 Ma and pisolitic cortex from 15 ± 1.3 to 11 ± 1.0 Ma (Table 2). The apparent age reveals that the pisolitic core is older than the pisolitic cortex, which

is in full agreement with the model that takes into account the development of pisolites coming from different localities (e.g., Didier et al., 1983, 1985; Tardy and Nahon, 1985; Nahon and Tardy, 1992). It must be pointed out that the core age may be considered an average age of the hematite and goethite, which raises the question of the relative age of hematite and goethite in the core. This remains undetermined. Indeed, although MCD sizes and Al³⁺ for Fe³⁺ substitution rate in goethite from the core and cortex indicate that they formed in different conditions (Table 1), we have no constraint on the age of the core goethite. According to the (U–Th)/He age of the pisolitic core, a first formation episode of ferruginous duricrusts at the Alto Paranaíba region (Minas Gerais State, Brazil) occurred at, or before, the Oligocene, which indicates a period of optimal climate conditions in the study area, as already reported in a surrounding area (Monteiro et al., 2014, 2018). Our results indicate, in addition, that a secondary weathering process took place from the middle Miocene (~15 to 11 Ma) (Table 2). More specifically, the Monteiro et al. (2014, 2018) studies provide age histograms covering a large period within the Cenozoic and exhibit a continuous occurrence of data in the short time interval revealed in our study. Other studies also identified distinct weathering episodes in Brazilian territory (e.g., Riffel et al., 2016; Allard et al., 2018; Vasconcelos et al., 2019). However, additional data are needed to further discuss the evolution of the studied duricrust and to relate its evolution to paleoclimates.

## 4 Conclusions

Mineralogy of supergene phases from core and cortex of a pisolitic facies can be explored on submillimetric (≤ 500 µm) single grains by XRD\_rotat, which is a non-destructive method, yielding high-quality patterns, while standard XRD is not relevant. Rietveld refinement outputs such as phase concentrations and mean coherent domain sizes showed results consistent with XRD\_synch. The grains analyzed by XRD\_rotat do not exhibit X-ray exposure effects, and they are then suitable for further analysis such as (U–Th)/He dating. This approach could potentially be applied to other mineral parageneses (e.g., Mn-oxides, jarosites) prior to dating.

Although investigated over a small series of grains, corrected (U–Th)/He ages of pisolitic core ranging from ~30 to 22 Ma revealed that a first formation episode of ferruginous duricrusts at the Alto Paranaíba region (Minas Gerais State, Brazil) occurred at or before the Oligocene. A subsequent phase associated with a secondary weathering formation process took place from the middle Miocene, which is highlighted by the formation of the goethitic pisolitic cortex. Therefore, the apparent (U–Th)/He age reveals that the pisolitic core is older than the pisolitic cortex, agreeing with
the model previously proposed for the development of pisolites in duricrusts.

*Data availability.* All data generated or analyzed during this study are included in this paper or its Supplement.

*Supplement.* The supplement related to this article is available online at: https://doi.org/10.5194/ejm-35-383-2023-supplement.

*Author contributions.* KPPM, TA, CG, and GM conceptualized the research and designed the methodology. KPPM carried out laboratory analysis and data interpretation. BB supported laboratory analysis including Rietveld refinement. LD and RPJ participated in mineralogical and (U–Th)/He analysis, respectively. PVT acquired the financial support for the project. KPPM, TA, and CG wrote the manuscript, integrated contributions from all co-authors, and made corrections to the revised version of the manuscript.

*Competing interests.* The contact author has declared that none of the authors has any competing interests.

*Disclaimer.* Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

*Acknowledgements.* This research was funded by the São Paulo Research Foundation – FAPESP (grant nos. 2017/20788-2; 2017/22292-4; 2019/10708-7). This study was also financed in part by the Coordination for the Improvement of Higher Education Personnel (CAPES) – Finance Code 001. Thanks to the National Council for Scientific and Technological Development (CNPq) for the research grant to Pablo Vidal-Torrado (301818/2017-7). We thank Jessica Brest for help with sample preparation and Thierry Roisnel for fruitful discussions about Rietveld refinements. In the framework of the CNRS AÉCIPROCS network, this work has been accepted for synchrotron beamtime by the SOLEIL scientific proposal committee (BAG proposal 20191509). The authors would like to thank Erik Elkaïm for his help on CRISTAL beamline. We also thank Imène Esteve, Stéphanie Delbrel, and Béatrice Doisneau, who are working at the scanning electron microscope (SEM) of the Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC – Sorbonne Université). It is supported by Région Ile de France grant SESAME 2006 no. I-07-593/R, INSU-CNRS, INPCNRS, University Pierre et Marie Curie – Paris 6, and by the French National Research Agency (ANR) grant no. ANR-07-BLAN-0124-01. (U–Th)/He analyses were funded by the RECA ANR-17-CE01-0012-01 project. We thank the editor Elisabetta Rampone, Klaus Mezger, Florian Hofmann, and one anonymous review for the thoughtful comments that helped improve the manuscript.

*Financial support.* This research has been supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (grant nos. 2017/20788-2, 2017/22292-4, and 2019/10708-7) and the Agence Nationale de la Recherche (grant no. RECA ANR-17-CE01-0012-01).

*Review statement.* This paper was edited by Klaus Mezger and reviewed by Florian Hofmann and one anonymous referee.

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