# H2O in nominally anhydrous mineral inclusions in diamonds and the volatile composition of diamond forming media

Andrea Curtolo, Maxwell C. Day, Francesca Innocenzi, Nathalie Bolfan-Casanova, Martha G. Pamato, Simon Falvard, Fabrizio Nestola, Jeff W. Harris, Davide Novella

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Andrea Curtolo, Maxwell C. Day, Francesca Innocenzi, Nathalie Bolfan-Casanova, Martha G. Pamato, et al.. H2O in nominally anhydrous mineral inclusions in diamonds and the volatile composition of diamond forming media. Earth and Planetary Science Letters, 2025, 658, 10.1016/j.epsl.2025.119311. insu-05099837

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Submitted on 5 Jun 2025

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# H₂O in nominally anhydrous mineral inclusions in diamonds and the volatile composition of diamond forming media

Andrea Curtolo<sup>a, *,</sup> Maxwell C. Day<sup>a, b</sup>, Francesca Innocenzi<sup>a, b</sup>, Nathalie Bolfan-Casanova<sup>b</sup>, Martha G. Pamato<sup>a, b</sup>, Simon Falvard<sup>b</sup>, Fabrizio Nestola<sup>a, b</sup>, Jeff W. Harris<sup>c</sup>, Davide Novella<sup>a, b</sup>

## ARTICLE INFO

**Keywords:**

Diamonds

Inclusions

H₂O

Cratons

FTIR

H diffusion

## ABSTRACT

Many studies have investigated the H₂O content of mantle minerals in xenoliths, which in various cases show evidence of metasomatism, implying that the measured H₂O contents may reflect these modification processes rather than the H₂O content in the environment where they were sourced. In this work, the H₂O content of 118 mineral inclusions in lithospheric diamonds, that are shielded from metasomatic and alteration events by their diamond host, was investigated.

Most of the inclusions studied here are clinopyroxene and garnet of eclogitic and peridotitic paragenesis and their H₂O content was determined by recording Fourier Transform Infrared (FTIR) spectra of each inclusion within their diamond host. Additional analyses conducted on seven inclusions that were extracted from their diamond host show good agreement with the analysis performed while still trapped inside the diamond, confirming the validity of the H₂O contents determined for the inclusions still trapped by their diamond host. The measured H₂O contents range from 0 to 550 ppm wt. with eclogitic inclusions showing a higher H₂O content with respect to their peridotitic counterparts. On average, clinopyroxene has the highest H₂O content compared to all other inclusion minerals (with average H₂O content of 110 ppm wt. and 61 ppm wt. for eclogitic and peridotitic samples, respectively). The H₂O contents of mineral inclusions in diamonds studied here and from the literature are much lower than those reported for cratonic mantle xenoliths. This difference may be due to (i) sampling bias, (ii) metasomatism subsequent to diamond formation, or (iii) lithospheric mantle involved in diamond formation not being representative of the average cratonic lithosphere (i.e., with a different H₂O activity). Modelled H diffusion in mineral inclusions in diamonds at the pressure and temperature conditions of diamond formation is fast, indicating complete H₂O re-equilibration with the diamond forming medium. It follows that the diamond forming medium must be a silicate melt with a low H₂O content, a carbonatitic melt and/or a fluid characterized by a low H₂O activity.

## 1. Introduction

Studies on mantle rocks have shown that considerable amounts of H₂O are present in the cratonic lithosphere (see e.g., Demouchy and Bolfan-Casanova, 2016; Novella et al., 2024, and references therein) as H atoms bound to oxygens in the structure of nominally anhydrous minerals (NAMs). Even if present in trace concentrations (ppm wt.), H₂O has critical effects on the properties of the mantle such as its melting temperature (Green, 1973) and rheology (Hirth and Kohlstedt, 2003; Peslier et al., 2010), and in turn, controls the mantle compositional

evolution and the stability of the lithosphere. Given the critical role played by H₂O in different geochemical and geodynamic processes, furthering our understanding of the distribution and behavior of H₂O (and volatiles in general) in the mantle remains imperative.

For these reasons, over the last 40 years, there has been an enormous amount of work (see e.g., Aines and Rossman, 1984; Moine et al., 2020; Aulbach et al., 2024) focused on investigating the H₂O content of NAMs from both cratonic and non-cratonic contexts (Skogby et al., 1990; Bell and Rossman, 1992a) to constrain where and how H₂O is stored in the mantle. These studies showed that NAMs can host large amounts of H₂O,

<footnote><sup>a</sup> Dipartimento di Geoscienze, Università degli Studi di Padova, Via Giovanni Gradenigo 6, Padova 35131, Italy</footnote>

<footnote><sup>b</sup> Laboratoire Magmas et Volcans, CNRS-IRD-OPGC, Université Clermont Auvergne, 6 Avenue Blaise Pascal, Clermont-Ferrand 63170, France</footnote>

<footnote><sup>c</sup> School of Geographical and Earth Sciences, University of Glasgow, Molema Building, Glasgow G12 8QQ, United Kingdom</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: andrea.curtolo.2@phd.unipd.it (A. Curtolo).</footnote>
even in old, depleted cratonic lithospheric settings, with H₂O contents reaching hundreds ppm wt. in olivine (Matsyuk and Langer, 2004; Jackson and Gibson, 2023), orthopyroxene (Bell and Rossman, 1992b; Chin et al., 2021) and garnet (Ragozin et al., 2014; Aulbach et al., 2023), and up to a striking 5065 ppm wt. in clinopyroxene (Moine et al., 2020). Careful examination of the geochemical signature of mantle xenoliths shows that their H₂O content reflects different degrees of metasomatism (see Doucet et al., 2014) combined with potential H₂O loss during kimberlite eruption. Assessment of the H₂O content of the cratonic lithosphere based on analyses of minerals in mantle xenoliths might therefore prove problematic as such minerals may not preserve the H₂O content of the mantle. Nevertheless, important information regarding the H₂O content of the mantle can still be retrieved from these samples.

Recently, attention has shifted to inclusions in diamonds as an alternative method to investigate the H₂O content of the deep mantle. During their formation, diamonds can encapsulate fragments of the ambient mantle as mineral inclusions and subsequently transport such inclusions to the surface while protecting them from physical breakdown and/or chemical alteration. Moreover, diamonds, being a product of metasomatic events themselves, could also provide important information regarding metasomatic fluids or melts in the mantle (Weiss et al., 2022). Mineral inclusions in diamonds have been studied extensively over the past decades providing fundamental information regarding the composition, petrology and chemistry of the mantle (see e.g., Aulbach et al., 2022a; Curtolo et al., 2023; Day et al., 2023). However, to date, only 12 studies reporting the H₂O content of inclusions in diamonds have been published (Kurosawa et al., 1997; Matsyuk et al., 1998; Koch-Müller et al., 2003; Matsyuk and Langer, 2004; Matveev and Stachel, 2009; Pearson et al., 2014; Novella et al., 2015; Jean et al., 2016; Taylor et al., 2016; Palot et al., 2016; Bassoo and Befus, 2021; Vangu et al., 2023). Notably, most of these studies focus on olivine and minimal work has been done on other mineral inclusions especially of the eclogitic paragenesis. Except for a ringwoodite inclusion found in a super-deep diamond from Brazil containing 1.4 wt% H₂O (Pearson et al., 2014), inclusions in lithospheric diamonds have lower H₂O contents compared to their counterparts in xenoliths. Clinopyroxenes, both in peridotitic and eclogitic substrates, are particularly important minerals, as they can host large amounts of H₂O and thus may comprise an important reservoir of H₂O in the mantle (Skogby, 2006). However, the H₂O content of only three clinopyroxene inclusions in diamond have been determined so far (Taylor et al., 2016). Clearly, more work investigating the H₂O content of NAM inclusions in diamonds is needed to improve our understanding of the H₂O content of the Earth's interior.

In this study, the H₂O content of 118 mineral inclusions in diamonds was determined. The samples studied here constitute a larger dataset than that currently available in the literature (101 inclusions). We focus primarily on clinopyroxene and garnet inclusions of both the eclogitic and peridotitic paragenesis as such samples are underrepresented in the available literature. The H₂O content of inclusions was determined using Fourier Transform Infrared (FTIR) spectroscopy. Single crystal X-ray diffraction and Raman spectroscopy were conducted to identify each inclusion and characterize their structure and composition. In addition, X-ray computed tomography (CT) analyses were conducted to obtain accurate thicknesses of each inclusion (ensuring accurate H₂O quantification) and to confirm that no fractures around the inclusions were present that could have allowed for alteration of the inclusion. The measured H₂O contents are compared with analogous data from inclusions in diamonds and minerals in xenoliths from the literature to discuss the presence and origin of H₂O in cratonic lithospheric mantle and the nature of volatiles in the diamond forming fluid/melt.

## 2. Materials and methods

### 2.1. Samples

In total, 114 inclusions were studied from 75 diamonds and four

garnet inclusions already extracted from their diamond hosts were investigated in this work (Table 1). Samples are from different cratonic areas including the Kaapvaal (46), Zimbabwe (8), Siberian (8), Tanzanian (4) and North Australian (4) cratons. Moreover, two diamonds were collected from an alluvial placer deposit in the Urals (Russia) and three diamonds are of unknown origin.

Diamonds in this study are mainly colorless octahedra and rounded dodecahedra, but some irregular crystals, aggregates and macles are present. A number of yellowish diamonds and one greenish diamond are also present in the sample suite, with two examples shown in Fig. 1.

The mineral inclusion suite comprises 48 clinopyroxenes, 61 garnets, six olivines, and three orthopyroxenes. The paragenesis of each diamond was defined by the identified mineral inclusion(s) (e.g., presence of olivine), the color of garnet inclusions (Stachel and Harris, 2008) and based on Raman and X-ray diffraction analysis of some clinopyroxene inclusions.

Seven diamonds were broken to extract their inclusions and to validate the H₂O quantifications performed by FTIR analyses of the inclusions measured through the diamond and after their release from the diamond. These seven inclusions should not be confused with the four garnets mentioned above, as those were already released from their diamond host.

### 2.2. Raman spectroscopy

Analyses were performed with a Witec alpha 300R confocal Raman micro-spectrometer at the Dipartimento di Geoscienze (Università degli Studi di Padova, Italy) using a laser wavelength of 532 nm and a nominal laser power of 5 to 20 mW. All the major upper mantle minerals (olivine, pyroxenes and garnet) hosted as inclusions in diamond were identified and representative spectra of these phases are reported in Fig. S1. Clinopyroxene inclusions were assigned a peridotitic (diopside) or eclogitic (omphacite) paragenesis based on the observation of specific features in the respective Raman spectra (Fig. S1). Diopside usually exhibits a ~1015 cm⁻¹ band with higher intensity than the ~665 cm⁻¹ band (Smith et al., 2022), but this is not always the case as the intensity of the ~1015 cm⁻¹ band in diopside is related to the orientation of the crystal, and in specific orientations, the ~665 cm⁻¹ band is more intense (Prencipe et al., 2012). This observation was combined with the presence of sharp, discrete bands between 300 and 400 cm⁻¹ to identify diopside, since omphacites display broad, overlapping bands in this region (Smith et al., 2022). Clinopyroxene spectra showing a more intense band at ~1015 cm⁻¹ and/or sharp bands in the 300–400 cm⁻¹ region were classified as diopside, and consequently assigned a peridotitic paragenesis. Clinopyroxenes showing a more intense ~665 cm⁻¹ band and broad overlapping bands in the 300–400 cm⁻¹ region were classified as omphacite and assigned an eclogitic paragenesis. This approach will assign pyroxenitic clinopyroxenes (which are diopsides) to the peridotitic suite. However, this has minimal effect on our interpretation since pyroxenitic inclusions comprise 2.3 % of the global database (Stachel and Harris, 2008).

Analyses of the interface of 73 (out of 114) mineral inclusions within the diamonds revealed two broad overlapping bands around ~620–670 and ~760–830 cm⁻¹ (representative spectrum reported in Fig. S1), regardless of the characteristics of the diamond, its paragenesis, or the identity of the mineral inclusion. These bands were first observed by Nimis et al. (2016) and were attributed to fluid rims that contain Si (OH)₄ monomers and Si₂O(OH)₆ dimers in an aqueous solution. Further work in the present study using X-Ray CT (Section 2.4), regardless of resolution, to try and identify and then determine the nature of any fluid, in all cases failed, indicating that these rims in the present diamond set are extremely thin with a thickness <1–2 µm.

### 2.3. Single crystal X-ray diffraction

When phase identification with Raman spectroscopy was not
<table><caption>**Table 1**

List of all the diamonds analyzed in this work along with their provenance and geochemical data obtained from FTIR spectra.</caption><thead><tr><th>Sample</th><th>Craton</th><th>Mine</th><th>Paragenesis</th><th>N<sub>tot</sub> (at. ppm)</th><th>%B</th><th>Type</th><th>T<sub>N</sub> (1 byr)</th><th>T<sub>N</sub> (2 byr)</th><th>T<sub>N</sub>(3 byr)</th><th>H<sub>3107</sub>(cm<sup>-1</sup>)</th></tr></thead><tbody><tr><td>DIO_01</td><td>Siberian</td><td>Udachnaya</td><td>P</td><td>343</td><td>78</td><td>IaAB</td><td>1198</td><td>1180</td><td>1169</td><td>1.54</td></tr><tr><td>DIO_02</td><td>Siberian</td><td>Udachnaya</td><td>P</td><td>25</td><td>52</td><td>IaAB</td><td>1238</td><td>1219</td><td>1208</td><td>0.02</td></tr><tr><td>ECLO_2</td><td>Siberian</td><td>Udachnaya</td><td>E</td><td>775</td><td>41</td><td>IaAB</td><td>1136</td><td>1119</td><td>1109</td><td>3.51</td></tr><tr><td>Grt_E_2</td><td>Siberian</td><td>Udachnaya</td><td>E</td><td>875</td><td>73</td><td>IaAB</td><td>1166</td><td>1149</td><td>1139</td><td>8.52</td></tr><tr><td>Grt_E_3</td><td>Siberian</td><td>Udachnaya</td><td>E</td><td>638</td><td>65</td><td>IaAB</td><td>1165</td><td>1148</td><td>1138</td><td>1.15</td></tr><tr><td>Grt_E_4</td><td>Siberian</td><td>Udachnaya</td><td>E</td><td>922</td><td>67</td><td>IaAB</td><td>1158</td><td>1141</td><td>1131</td><td>1.70</td></tr><tr><td>Grt_P_3</td><td>Siberian</td><td>Udachnaya</td><td>P</td><td>193</td><td>53</td><td>IaAB</td><td>1184</td><td>1166</td><td>1155</td><td>0.72</td></tr><tr><td>Grt_P_4</td><td>Siberian</td><td>Udachnaya</td><td>P</td><td>113</td><td>13</td><td>IaAB</td><td>1146</td><td>1129</td><td>1120</td><td>0.42</td></tr><tr><td>KF2</td><td>Kaapvaal</td><td>Koffiefontein</td><td>P</td><td>0</td><td>NA</td><td>II</td><td>NA</td><td>NA</td><td>NA</td><td>0.16</td></tr><tr><td>KF4</td><td>Kaapvaal</td><td>Koffiefontein</td><td>P</td><td>56</td><td>40</td><td>IaAB</td><td>1202</td><td>1184</td><td>1173</td><td>0.23</td></tr><tr><td>PM2</td><td>Kaapvaal</td><td>Bultfontein</td><td>P</td><td>11</td><td>26</td><td>IaAB</td><td>1230</td><td>1211</td><td>1200</td><td>0.00</td></tr><tr><td>PM4</td><td>Kaapvaal</td><td>Bultfontein</td><td>P</td><td>65</td><td>29</td><td>IaAB</td><td>1186</td><td>1168</td><td>1157</td><td>0.36</td></tr><tr><td>PR4</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>155</td><td>16</td><td>IaAB</td><td>1145</td><td>1128</td><td>1118</td><td>0.85</td></tr><tr><td>PR5</td><td>Kaapvaal</td><td>Cullinan</td><td>P</td><td>76</td><td>40</td><td>IaAB</td><td>1194</td><td>1176</td><td>1166</td><td>1.05</td></tr><tr><td>V10_01</td><td>NA</td><td>NA</td><td>E</td><td>108</td><td>12</td><td>IaAB</td><td>1144</td><td>1128</td><td>1118</td><td>0.13</td></tr><tr><td>V10_02</td><td>NA</td><td>NA</td><td>E</td><td>907</td><td>32</td><td>IaAB</td><td>1123</td><td>1106</td><td>1097</td><td>1.53</td></tr><tr><td>V10_03</td><td>NA</td><td>NA</td><td>E</td><td>0</td><td>NA</td><td>II</td><td>NA</td><td>NA</td><td>NA</td><td>0.00</td></tr><tr><td>DBP1-1</td><td>Kaapvaal</td><td>De Beers Pool</td><td>P</td><td>73</td><td>38</td><td>IaAB</td><td>1193</td><td>1175</td><td>1164</td><td>0.01</td></tr><tr><td>DBP1-2</td><td>Kaapvaal</td><td>De Beers Pool</td><td>P</td><td>87</td><td>24</td><td>IaAB</td><td>1171</td><td>1153</td><td>1143</td><td>0.34</td></tr><tr><td>DBP2-1</td><td>Kaapvaal</td><td>De Beers Pool</td><td>E</td><td>>776</td><td>NA</td><td>IaAB</td><td>NA</td><td>NA</td><td>NA</td><td>2.83</td></tr><tr><td>Dut1</td><td>Kaapvaal</td><td>Dutoitspan</td><td>E</td><td>>849</td><td>NA</td><td>IaAB</td><td>NA</td><td>NA</td><td>NA</td><td>1.82</td></tr><tr><td>Wes1</td><td>Kaapvaal</td><td>Wesselton</td><td>P</td><td>>641</td><td>NA</td><td>IaAB</td><td>NA</td><td>NA</td><td>NA</td><td>0.11</td></tr><tr><td>FinB14</td><td>Kaapvaal</td><td>Finsch</td><td>E</td><td>>656</td><td>NA</td><td>IaAB</td><td>NA</td><td>NA</td><td>NA</td><td>0.04</td></tr><tr><td>FinB15-1</td><td>Kaapvaal</td><td>Finsch</td><td>E</td><td>100</td><td>37</td><td>IaAB</td><td>1184</td><td>1166</td><td>1156</td><td>0.47</td></tr><tr><td>FinB15-2</td><td>Kaapvaal</td><td>Finsch</td><td>E</td><td>>880</td><td>NA</td><td>IaAB</td><td>NA</td><td>NA</td><td>NA</td><td>9.64</td></tr><tr><td>FinB15P7</td><td>Kaapvaal</td><td>Finsch</td><td>E</td><td>813</td><td>79</td><td>IaAB</td><td>1178</td><td>1160</td><td>1150</td><td>5.31</td></tr><tr><td>FinB15P3</td><td>Kaapvaal</td><td>Finsch</td><td>E</td><td>812</td><td>80</td><td>IaAB</td><td>1179</td><td>1161</td><td>1151</td><td>1.25</td></tr><tr><td>KFB65P24</td><td>Kaapvaal</td><td>Koffiefontein</td><td>P</td><td>0</td><td>NA</td><td>II</td><td>NA</td><td>NA</td><td>NA</td><td>0.00</td></tr><tr><td>KFB65P25</td><td>Kaapvaal</td><td>Koffiefontein</td><td>E</td><td>174</td><td>6</td><td>IaA</td><td>1115</td><td>1099</td><td>1090</td><td>0.00</td></tr><tr><td>VeB126P4-1-1</td><td>Kaapvaal</td><td>Venetia</td><td>P</td><td>604</td><td>53</td><td>IaAB</td><td>1154</td><td>1137</td><td>1127</td><td>0.62</td></tr><tr><td>VeB126P4-1-3</td><td>Kaapvaal</td><td>Venetia</td><td>E</td><td>815</td><td>79</td><td>IaAB</td><td>1177</td><td>1159</td><td>1149</td><td>2.79</td></tr><tr><td>VeB126P4-2</td><td>Kaapvaal</td><td>Venetia</td><td>P</td><td>278</td><td>51</td><td>IaAB</td><td>1172</td><td>1155</td><td>1145</td><td>0.19</td></tr><tr><td>Pr210-1</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>241</td><td>83</td><td>IaAB</td><td>1216</td><td>1197</td><td>1186</td><td>1.49</td></tr><tr><td>Pr210-2</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>391</td><td>79</td><td>IaAB</td><td>1196</td><td>1178</td><td>1168</td><td>1.48</td></tr><tr><td>Pr210-3</td><td>Kaapvaal</td><td>Cullinan</td><td>P</td><td>187</td><td>82</td><td>IaAB</td><td>1222</td><td>1203</td><td>1193</td><td>1.17</td></tr><tr><td>Pr210-4</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>697</td><td>57</td><td>IaAB</td><td>1154</td><td>1137</td><td>1127</td><td>0.19</td></tr><tr><td>Pr210-5</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>132</td><td>64</td><td>IaAB</td><td>1206</td><td>1187</td><td>1177</td><td>0.40</td></tr><tr><td>Pr210-6</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>722</td><td>73</td><td>IaAB</td><td>1172</td><td>1155</td><td>1145</td><td>0.23</td></tr><tr><td>Pr210-8</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>114</td><td>68</td><td>IaAB</td><td>1215</td><td>1196</td><td>1185</td><td>0.98</td></tr><tr><td>Pr210-9</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>84</td><td>65</td><td>IaAB</td><td>1219</td><td>1200</td><td>1189</td><td>0.98</td></tr><tr><td>Pr210-10</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>444</td><td>54</td><td>IaAB</td><td>1163</td><td>1146</td><td>1136</td><td>0.46</td></tr><tr><td>Pr210-11</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>545</td><td>69</td><td>IaAB</td><td>1173</td><td>1156</td><td>1146</td><td>0.50</td></tr><tr><td>OrB18P40-1</td><td>Zimbabwe</td><td>Orapa</td><td>E</td><td>151</td><td>49</td><td>IaAB</td><td>1186</td><td>1168</td><td>1158</td><td>0.45</td></tr><tr><td>OrB18P40-2</td><td>Zimbabwe</td><td>Orapa</td><td>P</td><td>55</td><td>43</td><td>IaAB</td><td>1206</td><td>1187</td><td>1177</td><td>0.30</td></tr><tr><td>JwB33L6-2</td><td>Kaapvaal</td><td>Jwaneng</td><td>P</td><td>950</td><td>88</td><td>IaAB</td><td>1191</td><td>1173</td><td>1162</td><td>2.24</td></tr><tr><td>JwB33A-1</td><td>Kaapvaal</td><td>Jwaneng</td><td>E</td><td>95</td><td>52</td><td>IaAB</td><td>1201</td><td>1183</td><td>1172</td><td>1.07</td></tr><tr><td>JwB33A-2</td><td>Kaapvaal</td><td>Jwaneng</td><td>E</td><td>746</td><td>67</td><td>IaAB</td><td>1163</td><td>1146</td><td>1136</td><td>4.37</td></tr><tr><td>JwB33B-1</td><td>Kaapvaal</td><td>Jwaneng</td><td>E</td><td>103</td><td>34</td><td>IaAB</td><td>1180</td><td>1162</td><td>1151</td><td>1.84</td></tr><tr><td>JwB33B-2</td><td>Kaapvaal</td><td>Jwaneng</td><td>E</td><td>181</td><td>27</td><td>IaAB</td><td>1156</td><td>1139</td><td>1129</td><td>2.81</td></tr><tr><td>JwB34P</td><td>Kaapvaal</td><td>Jwaneng</td><td>E</td><td>30</td><td>92</td><td>IaB</td><td>1303</td><td>1282</td><td>1270</td><td>0.34</td></tr><tr><td>LB18P68-1</td><td>Zimbabwe</td><td>Letlhakane</td><td>E</td><td>174</td><td>33</td><td>IaAB</td><td>1165</td><td>1148</td><td>1138</td><td>0.30</td></tr><tr><td>LB18P68-2</td><td>Zimbabwe</td><td>Letlhakane</td><td>E</td><td>33</td><td>42</td><td>IaAB</td><td>1220</td><td>1201</td><td>1190</td><td>0.61</td></tr><tr><td>Mw-1</td><td>Tanzanian</td><td>Mwadui</td><td>E</td><td>398</td><td>27</td><td>IaAB</td><td>1136</td><td>1120</td><td>1110</td><td>2.14</td></tr><tr><td>Mw-4</td><td>Tanzanian</td><td>Mwadui</td><td>E</td><td>443</td><td>85</td><td>IaAB</td><td>1204</td><td>1186</td><td>1175</td><td>2.10</td></tr><tr><td>UrP26</td><td>NA</td><td>Urals</td><td>E</td><td>482</td><td>85</td><td>IaAB</td><td>1202</td><td>1184</td><td>1173</td><td>1.26</td></tr><tr><td>UrP43</td><td>NA</td><td>Urals</td><td>E</td><td>471</td><td>70</td><td>IaAB</td><td>1179</td><td>1161</td><td>1151</td><td>1.41</td></tr><tr><td>ArB58T3184(1)</td><td>North Australian</td><td>Argyle</td><td>P</td><td>17</td><td>88</td><td>IaAB</td><td>1305</td><td>1284</td><td>1272</td><td>0.45</td></tr><tr><td>MwP1</td><td>Tanzanian</td><td>Mwadui</td><td>P</td><td>92</td><td>36</td><td>IaAB</td><td>1184</td><td>1166</td><td>1156</td><td>0.54</td></tr><tr><td>MwP2</td><td>Tanzanian</td><td>Mwadui</td><td>P</td><td>102</td><td>47</td><td>IaAB</td><td>1194</td><td>1176</td><td>1166</td><td>0.73</td></tr><tr><td>ArP1</td><td>North Australian</td><td>Argyle</td><td>P</td><td>50</td><td>8</td><td>IaA</td><td>1153</td><td>1136</td><td>1126</td><td>0.00</td></tr><tr><td>ArP2</td><td>North Australian</td><td>Argyle</td><td>P</td><td>0</td><td>NA</td><td>II</td><td>NA</td><td>NA</td><td>NA</td><td>0.23</td></tr><tr><td>ArE1</td><td>North Australian</td><td>Argyle</td><td>E</td><td>53</td><td>83</td><td>IaAB</td><td>1259</td><td>1239</td><td>1228</td><td>1.08</td></tr><tr><td>KFP1</td><td>Kaapvaal</td><td>Koffiefontein</td><td>P</td><td>102</td><td>65</td><td>IaAB</td><td>1214</td><td>1195</td><td>1185</td><td>1.80</td></tr><tr><td>KFP2</td><td>Kaapvaal</td><td>Koffiefontein</td><td>P</td><td>174</td><td>29</td><td>IaAB</td><td>1160</td><td>1143</td><td>1133</td><td>1.18</td></tr><tr><td>KFE1</td><td>Kaapvaal</td><td>Koffiefontein</td><td>E</td><td>587</td><td>68</td><td>IaAB</td><td>1171</td><td>1153</td><td>1143</td><td>0.19</td></tr><tr><td>KFE2</td><td>Kaapvaal</td><td>Koffiefontein</td><td>E</td><td>310</td><td>50</td><td>IaAB</td><td>1168</td><td>1150</td><td>1140</td><td>1.47</td></tr><tr><td>PrP1</td><td>Kaapvaal</td><td>Cullinan</td><td>P</td><td>48</td><td>78</td><td>IaAB</td><td>1252</td><td>1233</td><td>1222</td><td>0.50</td></tr><tr><td>PrE1</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>225</td><td>79</td><td>IaAB</td><td>1212</td><td>1193</td><td>1183</td><td>0.76</td></tr><tr><td>PrE2</td><td>Kaapvaal</td><td>Cullinan</td><td>E</td><td>499</td><td>100</td><td>IaB</td><td>NA</td><td>NA</td><td>NA</td><td>2.55</td></tr><tr><td>VeP1</td><td>Kaapvaal</td><td>Venetia</td><td>P</td><td>86</td><td>49</td><td>IaAB</td><td>1200</td><td>1182</td><td>1171</td><td>0.67</td></tr><tr><td>VeE1</td><td>Kaapvaal</td><td>Venetia</td><td>E</td><td>519</td><td>0</td><td>IaA</td><td>NA</td><td>NA</td><td>NA</td><td>0.04</td></tr><tr><td>OrP1</td><td>Zimbabwe</td><td>Orapa</td><td>P</td><td>114</td><td>40</td><td>IaAB</td><td>1184</td><td>1166</td><td>1156</td><td>0.17</td></tr><tr><td>OrP2</td><td>Zimbabwe</td><td>Orapa</td><td>P</td><td>106</td><td>54</td><td>IaAB</td><td>1200</td><td>1182</td><td>1171</td><td>0.07</td></tr><tr><td>OrE1</td><td>Zimbabwe</td><td>Orapa</td><td>E</td><td>120</td><td>39</td><td>IaAB</td><td>1181</td><td>1164</td><td>1153</td><td>0.16</td></tr><tr><td>OrE2</td><td>Zimbabwe</td><td>Orapa</td><td>E</td><td>83</td><td>32</td><td>IaAB</td><td>1183</td><td>1165</td><td>1155</td><td>0.32</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Photos of two specimens analyzed in this study: (a) a broken macle twin with a green diopside inclusion (sample DBP1–1); (b) a greenish octahedral diamond with inclusions of purple peridotitic garnet (JwB33L6–2).</figcaption></figure>

possible, single crystal X-ray diffraction (SC-XRD) was conducted. Analyses were performed with a Rigaku Oxford Diffraction SuperNova diffractometer equipped with a Dectris Pilatus 200K areal detector at the Dipartimento di Geoscienze (Padova). Monochromatized Mo Kα radiation (λ = 0.7107 Å) at 50 kV and 0.12 mA was used to conduct the measurements (sample-to-detector distance = 69 mm). Data reduction was performed with the CrysAlis Pro software (Rigaku Oxford Diffraction).

Sample VeB126P4–2, which showed an ambiguous signal in the Raman spectrum, produced a diffraction pattern consistent with exsolutions of orthopyroxene in a crystal of clinopyroxene. Similar exsolution features in clinopyroxene were reported for the first time by Leost et al. (2003).

## 2.4. X-ray computed tomography

X-ray CT was performed on all samples where an OH signal was observed in the IR spectra to identify possible fractures connecting inclusions to the surface of the diamond. This instrument was also used to precisely measure the thickness of the inclusions. Analyses were performed with a RX Solutions EasyTom equipped with an X-ray tube with a LaB₆ cathode operating at a voltage of 70 kV and a Varian PaxScan 2520DX imager at Laboratoire Magmas et Volcans (LMV, Université Clermont-Auvergne, Clermont-Ferrand, France). A 3D reconstruction of a garnet inclusion and an image section of a diamond are shown in

<figure><img src="image_5.png" /><figcaption>Fig. 2. (a) A 3D model of the garnet inclusion in sample Grt_E_4 reconstructed from X-ray CT, the inclusion is approximately 400 µm long in the longest direction. (b) An almost empty cavity partially filled with a multiphase material, note how the cavity is connected by a fracture to the surface of the diamond (sample FinB14).</figcaption></figure>

Fig. 2.

The resolution of the X-ray CT was dependent on the size of the inclusions and diamonds, with voxel edge length of 1–2 µm, with only one sample having lower resolution data (2.4 µm for sample KFE2).

The thickness of mineral inclusions was calculated by computing the average of multiple measurements of the mineral in the propagation direction of the IR radiation to account for the irregular shape of some inclusions. X-ray CT also allowed recognition (and exclusion) of inclusions in contact with fractures in the diamond that could have allowed interaction with the surrounding mantle, thus modifying the composition and H₂O content of the inclusion after entrapment (Fig. 2b). Moreover, in some samples (e.g., DBP1–1), exsolutions of orthopyroxene in clinopyroxene inclusions were identified, although no exsolutions could be detected in sample VeB126P4–2 despite the opposite SC-XRD results described above.

## 2.5. Fourier-Transform infrared spectroscopy

FTIR spectra were recorded from diamonds to measure their N concentration and N-aggregation state, and from inclusions to measure their H₂O content. Analyses were performed using a Bruker Vertex 70 spectrometer with a globar IR source, a KBr beamsplitter and a liquid N₂ cooled MCT detector coupled with a Hyperion microscope with 15x objective at LMV (Clermont-Ferrand). Prior to analysis, diamonds were cleaned with ethanol in an ultrasonic bath for 15 min. Background spectra were collected through a CaF₂ window using 100 scans, sample spectra were collected using 300 scans at a spectral resolution of 2 cm⁻¹. Spectra of the diamond hosts were collected with unpolarized radiation and an aperture size that was adjusted to measure homogeneous and crack-free regions of each sample. Usually, five spectra were recorded from each diamond. Quantification of the N content and N-aggregation state was done by deconvolution by least-squares fitting of the N-region (i.e., ~1000 to 1400 cm⁻¹) of the diamond using the DiaMap Excel spreadsheet (Howell et al., 2012a; b). FTIR spectra were recorded where possible on mineral inclusions using an aperture size adjusted to enclose the entire inclusion whilst avoiding other inclusions and fractures. The H₂O content of the inclusions was determined by fitting the signal due to OH stretching bands between ~3000 and ~4000 cm⁻¹. The data processing consisted firstly of subtracting the spectrum of the diamond from that of the inclusion, because the signal due to diamond in the three-phonon region (Green et al., 2022) overlaps with the OH stretching region. After this step, the baseline of the resulting spectrum (of the inclusion only) was corrected using cubic spline fitting. Finally,
the integrated absorbance (area) of the OH stretching signal was calculated (Fig. 3).

The H₂O content of the mineral inclusions was calculated using absorption coefficients reported by Withers et al. (2012) for olivine, Bell et al. (1995) for orthopyroxene, peridotitic clinopyroxene, and garnet, and Koch-Müller et al. (2007) for eclogitic clinopyroxene.

Spectra were recorded from inclusions extracted from their diamond hosts using a Thermo Scientific Nicolet iN10 with an EverGlo air-cooled IR source, KBr beam splitter and liquid N₂ cooled MCT detector installed at the Dipartimento di Geoscienze (Padova). Spectra were collected with unpolarized radiation through a BaF₂ plate using 64 scans for both the background and spectrum at a spectral resolution of 4 cm⁻¹. Due to the small sizes, the extracted inclusions were not polished and spectra were recorded for different orientations of the same grain where possible and the resulting H₂O contents were averaged to calculate the final result. Baseline corrections were done using a cubic spline and the thickness was measured using an optical microscope with a precision of ±5 µm.

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Visual representation of the steps used to process spectra and to calculate the H₂O content of mineral inclusions (see text for more details). (a) Shows the FTIR spectra collected on a diamond and an inclusion within the diamond. (b) Shows an enlargement of (a) in the region between 3000 and 4000 cm⁻¹, the offset between the spectra in (b) was reduced to facilitate comparison of the spectra. (c) Shows the spectrum after subtraction of the contribution of the diamond (see text), the black dashed line represents the baseline used for that specific spectrum (sample JwB33B-2) such that the integrated area of the OH signal can be determined. The noise around ~3700 cm⁻¹ in (c) is due to atmospheric water.</figcaption></figure>

# 3. Results

## 3.1. Diamond N content and N-aggregation state

Nitrogen concentration in the diamonds studied here ranges from 0 (below FTIR detection which is approximately 10 at. ppm, see Day et al., 2023) to 950 at. ppm (Table 1, Fig. S2) with an average N content of 296 at. ppm. In five samples, the N signal in the one-phonon region was saturated and thus precise quantification of N content and N-aggregation state was not possible. However, fitting this saturated signal provided a lower estimate of N contents for these samples, which is always > 600 at. ppm.

Nitrogen is incorporated in diamond as isolated substitutional N atoms (C-centers). With progressive mantle annealing, C-centers aggregate to form A-centers, consisting of two neighboring N atoms, and then into B-centers, consisting of four N atoms surrounding a C vacancy (Green et al., 2022). According to the concentration of N and its aggregation state, diamond is classified as Type I (if it contains detectable amounts of N) and Type II (if it does not contain detectable amounts of N). Type II samples can be classified as Ila (if B is not detected) and Iib (if B is detected). Type I specimens can be further classified as Type Ib if N defects occur mainly as C-centers, or as Type Ia if N occurs in an aggregated form. Type Ia specimens are classified as IaA (> 90 % A-centers), IaAB (between 10 and 90 % A-centers) or IaB (< 10 % A-centers). Specimens are mostly Type Ia, with only 4 Type Ila samples, and no Type Ib or Iib sample are present. Type Ia diamonds are mainly Type IaAB (66 samples) with a small amount of IaA (3 samples) and IaB (2 samples). As the five samples with saturated N signals show signals due to both A- and B-centers, they were classified as Type IaAB although the exact A- and B-center content could not be determined.

The mantle residence temperature of each diamond was calculated using N-aggregation thermochronometry, following Leahy and Taylor (1997). The residence temperatures were calculated assuming three different residence times (1, 2 and 3 byr) and the results are reported in Table 1 and illustrated in Fig. S2. The highest and lowest residence temperatures in this database are 1305 °C (for 1 byr residence time) and 1078 °C (for 3 byr residence time), respectively, with an average between 1159 and 1188 °C, in agreement with the global average residence temperature of 1141–1174 °C (for peridotitic diamonds) and 1141–1168 °C (for eclogitic diamonds) for residence times of 3 and 1 byr, respectively (Stachel and Harris, 2008).

Out of all samples, 70 diamonds showed a sharp peak at 3107 cm⁻¹ associated with H in the VN₃H�0 defect (Goss et al., 2014). The normalized linear absorbance (i.e., the peak height normalized to a sample thickness of 1 cm) of this peak was obtained using the DiaMap spreadsheet and is also reported in Table 1. Some samples with high N contents also showed a relatively weak peak at 3236 cm⁻¹, which has been tentatively attributed to H in the VN₄H⁰ defect (Day et al., 2024).

## 3.2. H₂O in mineral inclusions

Four different OH stretching signals were identified in the FTIR spectra of the inclusions, which are described as follows. The first group (Group 1 from here onwards) represents the most common case (107 samples) where, after subtraction of the diamond spectrum, a characteristic signal typical of OH stretching for silicate minerals, between 3000 and 4000 cm⁻¹, was observed (Table 2). In this case, the determination of the H₂O content of the mineral inclusion was straightforward (see Fig. 4). The second group (Group 2, 4 samples, see Table 2) consists of spectra that, after subtraction of the diamond spectrum, had a low signal/noise ratio and relatively weak signal due to OH stretching. This resulted in large uncertainties and prevented precise quantification of the low H₂O content of these minerals (Fig. S3a). The third group (Group 3, 3 samples, see Table 2) represents spectra where, after subtraction of the diamond spectrum, selecting a baseline was ambiguous and, consequently, only a range of H₂O contents could be determined
<caption>**Table 2**</caption>

Paragenesis, H₂O content (in ppm wt.) measured inside and outside the diamonds and abbreviated reference for the source of absorption coefficients used for H₂O quantifications for all mineral inclusions in this study.

<table><thead><tr><th>Sample</th><th>Para</th><th>Phase</th><th>Calib</th><th>H₂O in</th><th>H₂O out</th><th>Sample</th><th>Para</th><th>Phase</th><th>Calib</th><th>H₂O in</th><th>H₂O out</th></tr></thead><tbody><tr><td>Pr210-3</td><td>P</td><td>Cpx</td><td>B95</td><td>211</td><td>-</td><td>Grt P.3</td><td>P</td><td>Grt</td><td>B95</td><td>9<sup>2</sup></td><td>-</td></tr><tr><td>Pr210-3</td><td>P</td><td>Cpx</td><td>B95</td><td>126</td><td>-</td><td>Grt P.4</td><td>P</td><td>Grt</td><td>B95</td><td>0<sup>2</sup></td><td>-</td></tr><tr><td>DIO_01</td><td>P</td><td>Cpx</td><td>B95</td><td>0</td><td>-</td><td>Jwb33L6-2</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>DIO_02</td><td>P</td><td>Cpx</td><td>B95</td><td>12</td><td>-</td><td>Jwb33L6-2</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>DBP1-1</td><td>P</td><td>Cpx</td><td>B95</td><td>0-137<sup>3</sup></td><td>-</td><td>MwP1</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>DBP1-2</td><td>P</td><td>Cpx</td><td>B95</td><td>59</td><td>-</td><td>MwP2</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Wes1</td><td>P</td><td>Cpx</td><td>B95</td><td>0</td><td>24</td><td>MwP2</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>KFB65P24</td><td>P</td><td>Cpx</td><td>B95</td><td>96</td><td>-</td><td>ArP2</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>KFB65P24</td><td>P</td><td>Cpx</td><td>B95</td><td>46</td><td>-</td><td>KFP1</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>VeB126P4-1-1</td><td>P</td><td>Cpx</td><td>B95</td><td>0</td><td>-</td><td>KFP1</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>VeB126P4-2</td><td>P</td><td>Cpx</td><td>B95</td><td>61</td><td>168</td><td>KFP2</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>ArB58T3184(1)</td><td>P</td><td>Cpx</td><td>B95</td><td>42</td><td>-</td><td>PrP1</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>ECLO_2</td><td>E</td><td>Cpx</td><td>KM07</td><td>30</td><td>50</td><td>VeP1</td><td>P</td><td>Grt</td><td>B95</td><td>NA<sup>4</sup></td><td>-</td></tr><tr><td>ECLO_2</td><td>E</td><td>Cpx</td><td>KM07</td><td>46</td><td>-</td><td>VeP1</td><td>P</td><td>Grt</td><td>B95</td><td>NA<sup>4</sup></td><td>-</td></tr><tr><td>PR4</td><td>E</td><td>Cpx</td><td>KM07</td><td>451</td><td>-</td><td>OrP1</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>V10_01</td><td>E</td><td>Cpx</td><td>KM07</td><td>141</td><td>-</td><td>OrP2</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>V10_01</td><td>E</td><td>Cpx</td><td>KM07</td><td>54</td><td>-</td><td>Grt E.2</td><td>E</td><td>Grt</td><td>B95</td><td>22</td><td>-</td></tr><tr><td>V10_02</td><td>E</td><td>Cpx</td><td>KM07</td><td>17-141<sup>3</sup></td><td>-</td><td>Grt E.2</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>V10_03</td><td>E</td><td>Cpx</td><td>KM07</td><td>97</td><td>-</td><td>Grt E.3</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>DBP2-1</td><td>E</td><td>Cpx</td><td>KM07</td><td>55</td><td>-</td><td>Grt E.3</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Dut1</td><td>E</td><td>Cpx</td><td>KM07</td><td>90</td><td>105</td><td>Grt E.3</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>FinB14</td><td>E</td><td>Cpx</td><td>KM07</td><td>65</td><td>-</td><td>Grt E.3</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>FinB15-2</td><td>E</td><td>Cpx</td><td>KM07</td><td>65</td><td>-</td><td>Grt E.4</td><td>E</td><td>Grt</td><td>B95</td><td>550</td><td>471</td></tr><tr><td>KFB65P25</td><td>E</td><td>Cpx</td><td>KM07</td><td>241</td><td>223</td><td>FinB15-2</td><td>E</td><td>Grt</td><td>B95</td><td>NA<sup>4</sup></td><td>-</td></tr><tr><td>VeB126P4-1-3</td><td>E</td><td>Cpx</td><td>KM07</td><td>80</td><td>-</td><td>FinB15P7</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Pr210-1</td><td>E</td><td>Cpx</td><td>KM07</td><td>0</td><td>-</td><td>FinB15P3</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Pr210-2</td><td>E</td><td>Cpx</td><td>KM07</td><td>58</td><td>-</td><td>Pr210-1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Pr210-2</td><td>E</td><td>Cpx</td><td>KM07</td><td>170</td><td>-</td><td>Pr210-2</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Pr210-2</td><td>E</td><td>Cpx</td><td>KM07</td><td>170</td><td>-</td><td>Pr210-4</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Pr210-5</td><td>E</td><td>Cpx</td><td>KM07</td><td>0-195<sup>3</sup></td><td>-</td><td>Pr210-4</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Pr210-5</td><td>E</td><td>Cpx</td><td>KM07</td><td>40</td><td>-</td><td>Pr210-6</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Pr210-5</td><td>E</td><td>Cpx</td><td>KM07</td><td>0</td><td>-</td><td>Pr210-8</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Pr210-9</td><td>E</td><td>Cpx</td><td>KM07</td><td>174</td><td>-</td><td>Pr210-10</td><td>E</td><td>Grt</td><td>B95</td><td>0<sup>2</sup></td><td>-</td></tr><tr><td>Pr210-9</td><td>E</td><td>Cpx</td><td>KM07</td><td>0</td><td>-</td><td>Pr210-11</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Pr210-10</td><td>E</td><td>Cpx</td><td>KM07</td><td>285</td><td>-</td><td>Jwb33B-1</td><td>E</td><td>Grt</td><td>B95</td><td>353</td><td>259</td></tr><tr><td>Pr210-10</td><td>E</td><td>Cpx</td><td>KM07</td><td>201</td><td>-</td><td>Jwb33B-2</td><td>E</td><td>Grt</td><td>B95</td><td>0<sup>2</sup></td><td>-</td></tr><tr><td>Pr210-11</td><td>E</td><td>Cpx</td><td>KM07</td><td>113</td><td>-</td><td>Jwb34P</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>OrB18P40-1</td><td>E</td><td>Cpx</td><td>KM07</td><td>200</td><td>-</td><td>LB18P68-1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>OrB18P40-1</td><td>E</td><td>Cpx</td><td>KM07</td><td>212</td><td>-</td><td>LB18P68-1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>OrB18P40-2</td><td>E</td><td>Cpx</td><td>KM07</td><td>25</td><td>-</td><td>LB18P68-2</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Jwb33A-1</td><td>E</td><td>Cpx</td><td>KM07</td><td>0</td><td>-</td><td>LB18P68-2</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Jwb33A-2</td><td>E</td><td>Cpx</td><td>KM07</td><td>63</td><td>-</td><td>Mw-1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Jwb33B-2</td><td>E</td><td>Cpx</td><td>KM07</td><td>95</td><td>-</td><td>UrP26</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Jwb33B-2</td><td>E</td><td>Cpx</td><td>KM07</td><td>100</td><td>-</td><td>UrP26</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Mw-1</td><td>E</td><td>Cpx</td><td>KM07</td><td>105</td><td>-</td><td>UrP43</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Mw-4</td><td>E</td><td>Cpx</td><td>KM07</td><td>109</td><td>-</td><td>ArE1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>Mw-4</td><td>E</td><td>Cpx</td><td>KM07</td><td>112</td><td>-</td><td>ArE1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>KF2</td><td>P</td><td>Opx</td><td>B95</td><td>22</td><td>-</td><td>KFE1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>PR5</td><td>P</td><td>Opx</td><td>B95</td><td>10</td><td>-</td><td>KFE1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>PR5</td><td>P</td><td>Opx</td><td>B95</td><td>0</td><td>-</td><td>KFE1</td><td>E</td><td>Grt</td><td>B95</td><td>262</td><td>-</td></tr><tr><td>DIO_02</td><td>P</td><td>Ol</td><td>W12</td><td>4</td><td>-</td><td>KFE2</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>KF4</td><td>P</td><td>Ol</td><td>W12</td><td>7</td><td>-</td><td>PrE1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>PM2</td><td>P</td><td>Ol</td><td>W12</td><td>0</td><td>-</td><td>PrE2</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>PM2</td><td>P</td><td>Ol</td><td>W12</td><td>0</td><td>-</td><td>PrE2</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>PM4</td><td>P</td><td>Ol</td><td>W12</td><td>7</td><td>-</td><td>VeE1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td>ArP1</td><td>P</td><td>Ol</td><td>W12</td><td>0</td><td>-</td><td>OrE1</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>OrE2</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>VeP2</td><td>P</td><td>Grt</td><td>B95</td><td>NA<sup>4</sup></td><td>-</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>VeP3</td><td>P</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>ArE2</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>ArE3</td><td>E</td><td>Grt</td><td>B95</td><td>0</td><td>-</td></tr></tbody></table>

Para = Paragenesis; Calib = Calibration. B95 = Bell et al. (1995), KM07 = Koch-Müller et al. (2007), W12 = Withers et al. (2012). “H2O in” refers to the H2O content (in ppm wt.) measured inside the diamond; “H2O out” refers to the H2O content measured after release of the inclusion from the diamond host. 2 = Group 2, 3 = Group 3, 4 = Group 4, all other samples not maked by an apex belong to Group 1.

(Fig S3b). Finally, in the last group (Group 4, 4 garnet samples, see Table 2) the OH stretching region contains a sharp, intense band at a high frequency (~3695 cm⁻¹) characteristic of hydrous minerals (see e. g., Heller-Kallai et al., 1975), which may be present as micro/nano inclusions (Fig. S3c). This type of signal prevents precise quantification of H₂O content, but equilibration of the mineral in a H₂O-rich environment can be inferred nonetheless.

Only H₂O contents obtained from group 1 spectra are considered in the following discussions as they provide the most reliable quantifications. The analyzed inclusions span a wide range of H₂O contents from 0 (below the FTIR detection limit, which depends on sample size and measurement conditions) to 550 ppm wt. H₂O (Table 2, Fig. 5). The six olivine inclusions analyzed have low H₂O contents, ranging from 0 to 7 ppm wt., with an average H₂O content of 3 ppm wt., on the low end of
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** FTIR spectra (after baseline correction and subtraction of the diamond spectrum) recorded from inclusions with the highest H₂O contents determined for each phase in this study. Absorbance was normalized to a thickness of 1 mm. The peak at 3107 cm⁻¹ is due to the fundamental C-H stretching mode of the VN₃H⁰ point defect in diamond (Goss et al., 2014). Cpx = clinopyroxene, Opx = orthopyroxenes, Ol = olivine, Grt = garnet. “P” and “E” refer to peridotitic and eclogitic paragenesis, respectively.</figcaption></figure>

the range reported previously for olivine inclusions in diamonds (Fig. 5, i.e., Kurosawa et al., 1997; Matsyuk and Langer, 2004; Matveev and Stachel, 2009; Novella et al., 2015; Jean et al., 2016; Taylor et al., 2016; Bassoo and Befus, 2021; Vangu et al., 2023).

Only three orthopyroxene inclusions were analyzed and their H₂O content ranges from 0 to 22 ppm wt. with the average at 10 ppm wt., in agreement with data reported previously for orthopyroxene (Bassoo and Befus, 2021). It is important to note however, that the spectra recorded from olivine and orthopyroxene inclusions hosted in diamonds from Guyana studied by Bassoo and Befus (2021) show severe atmospheric contamination and poorly-constrained baseline corrections in some of their samples, which may have led to overestimation of H₂O contents. Therefore, data from olivine and orthopyroxene inclusions from Bassoo and Befus (2021) were excluded from subsequent discussions in this work.

Clinopyroxenes have high H₂O contents, with an average content of 97 ppm wt., and ranging from 0 to 451 ppm wt. (the latter being the highest H₂O content reported for a clinopyroxene inclusion in a diamond), in agreement with data reported in the literature (7–260 ppm wt. H₂O) by Taylor et al. (2016). Garnets are mostly dry, as previously observed (Matsyuk et al., 1998; Novella et al., 2015; Taylor et al., 2016). Note however that five inclusions from the present dataset belong to Group 4, suggesting that these garnets contain detectable amounts of OH. Moreover, three garnet outliers of eclogitic paragenesis (KFE1, JwB33B-1, and Grt_E_4) exhibit notable H₂O contents (>100 ppm wt.). The most H₂O-rich inclusion studied here is an eclogitic garnet (Grt_E_4, which belongs to Group 1) from the Udachnaya mine (Siberian craton, Russia), which showed a broad absorption band from ~3000 to ~3750 cm⁻¹ with a superimposed band at ~3535 cm⁻¹. A similar spectrum was reported by Geiger et al. (1991) for a synthetic garnet hosting fluid inclusions. In fact, Grt_E_4 also exhibits a band around 1600 cm⁻¹, representative of the H₂O bending mode, which indicates the presence of fluid or melt nano inclusions that were not detectable with X-ray CT. Determination of H₂O content for this sample was performed after subtraction of the broad band between ~3000 and ~3700 cm⁻¹ attributed to OH stretching of the H₂O molecules associated with fluid inclusions, as done by Geiger et al. (1991). Following this approach, 550 ppm wt. H₂O were calculated for the structurally bound OH in the garnet, which is the highest H₂O content ever reported for a garnet inclusion in a diamond, and one of the highest for cratonic mantle garnets in general.

The inclusions extracted from their diamond hosts include three

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Distribution of H₂O content in peridotitic and eclogitic mineral inclusions from lithospheric diamonds. Hatched patterns represent H₂O contents from the literature. The bin size for the peridotitic histogram is 10 ppm wt., while for the eclogitic histogram is 20 ppm wt. Cpx = clinopyroxene, Opx = orthopyroxenes, Ol = olivine, Grt = garnet.</figcaption></figure>
omphacites, two diopsides and two eclogitic garnets. The H₂O contents of these minerals, measured inside and outside the diamond, agree within ±30 % (see Table 2 and discussion below), confirming the validity of H₂O quantifications of mineral inclusions determined while still inside the diamonds.

# 4. Discussion

## 4.1. Validation of H₂O contents of mineral inclusions

There are several errors associated with collecting the FTIR spectra of mineral inclusions inside diamonds that need to be considered to accurately derive H₂O contents from such spectra. (i) Firstly, recording FTIR spectra of inclusions through the diamond host decreases the ratio of absorption signal from OH in the inclusion to noise (i.e., the inclusion signal-to-noise ratio), especially in large diamonds. This makes H₂O quantification particularly challenging for inclusions with low H₂O contents that show relatively weak signal due to OH stretching modes which is hard to differentiate from noise (i.e., Group 2 spectra, see Section 3.4). (ii) Another important source of error is related to the determination of the thickness of the inclusions. In this study, thickness was calculated assuming that the IR radiation passed through the whole volume of the inclusion as the size of the IR aperture was set, where possible, such that it enclosed the entire inclusion. However, the high refractive index of diamond (Edwards and Ochoa, 1981) prevents precise determination of the optical path and of the volume traversed by IR radiation, introducing errors in the quantification of the thickness of inclusions. (iii) The procedure used to process each spectrum (see Section 2.5) may also result in errors on the measured H₂O contents. Variability in the position and intensity of broad-peaks in the diamond three-phonon region is common when comparing the spectra of different diamonds and is even observed when comparing spectra recorded from different regions of the same diamond. During subtraction of the diamond spectrum from each IR spectra (recorded through the inclusion and diamond), such variability in the three-phonon region between 3000 and 4000 cm⁻¹ can produce artefacts that may be mistaken for OH stretching signal (Group 3 signals). Baseline subtraction can also introduce errors, especially for spectra recorded from samples with relatively low H₂O contents where the distinction between signal and baseline is not clear. (iv) Moreover, the use of unpolarized radiation to measure a single unoriented section of an anisotropic mineral inclusion also contributes to the error, however, this is unavoidable as reliable orientation of inclusions inside the diamond is extremely challenging. (v) Lastly, inclusions often break in the process of release from their host, and only part of them can be recovered and analyzed, and this could also introduce some uncertainty in analyses outside the diamonds as only a fraction of inclusion (which may not be representative of the whole sample) is analyzed.

As all sources of error described above are associated with recording FTIR spectra of the inclusions through diamond, the error was determined by comparing H₂O contents obtained from spectra recorded from inclusions inside their diamond host and then from the same inclusions once they were extracted. The H₂O contents measured from inclusions inside and outside the diamond agree within ±30 % (Fig. 6), and a similar error is reported in other studies (e.g., 30–50 % by Novella et al. (2015). Two samples show an error larger than ±30 %, VeB126P4–2 and Wes1. VeB126P4–2 is a combination of clinopyroxene/orthopyroxene exsolutions which broke during extraction. This difference is likely due to sampling different of ortho- and clinopyroxene by the IR beam. No H₂O was detected in sample Wes1 measured inside the diamond, and this is likely due to relatively low H₂O content measured outside.

## 4.2. H₂O in inclusions in diamonds vs xenoliths

Inclusions in diamonds in this study show a relationship between

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Comparison of H₂O contents obtained for the same inclusions inside and outside of their diamond host. The colors correspond to the spectra of different inclusion phases as shown in Fig. 4. The error bars represent the standard deviation calculated from different H₂O quantifications on different pieces of the inclusions, or on different orientations of the same crystal.</figcaption></figure>

their H₂O content and paragenesis. Eclogitic clinopyroxenes show maximum and average H₂O contents of 451 and 110 ppm wt., respectively, while peridotitic clinopyroxenes show maximum and average H₂O contents of 211 and 61 ppm wt., respectively (Fig. 7).

The same general trend is observed for garnets, although it is more complex. Peridotitic garnets are always dry, except for the samples with Group 4 spectra (3 in total). These samples likely host some amount of H₂O, and the nano inclusions of hydrous phases may have formed from exsolution caused by H₂O saturation during decompression and cooling of the sample. It follows that at high P and T, these garnets could have hosted significant amounts of structurally bound H₂O. Eclogitic garnets are also mostly dry, but three outliers with H₂O contents >100 ppm wt., as well as one sample from Group 4, are present (Fig. 7), suggesting that on average eclogitic garnet contains more H₂O than peridotitic garnet, in agreement with observations from xenoliths (Fig. 7).

The more hydrated nature of the eclogitic mantle, compared to its peridotitic counterpart, is shown here for the first time based on data from mineral inclusions in diamonds, confirming prior observations made for (possibly) metasomatized xenoliths. This H₂O enrichment in eclogites is not surprising as they are derived from subducted oceanic crust, which is a more H₂O-rich environment compared to the peridotitic ambient mantle. The preferential incorporation of H₂O in eclogites is also related to mineral composition, where the presence of Al in clinopyroxene promotes H incorporation (O'Leary et al., 2010), causing eclogitic (Al-rich) clinopyroxenes to have a higher H₂O mineral/melt partition coefficient compared to their peridotitic (Al-poor) counterparts (see e.g., Aubaud et al., 2008; Novella et al., 2014).

Inclusions in diamonds have lower H₂O contents than their xenolithic equivalents. This discrepancy may be due to of the following: (i) sampling bias as most diamonds in this study and the literature come from the Kaapvaal and Siberian cratons and are compared to data from a worldwide suite of xenoliths. Overrepresentation of these two diamond localities could skew the distribution to lower average H₂O contents. However, this does not seem to be the case as xenoliths from the Kaapvaal and Siberian cratons exclusively, still show a higher average H₂O content (Fig. S4). (ii) Metasomatism subsequent to diamond formation, as previously described in the literature (Doucet et al., 2014; Demouchy and Bolfan-Casanova, 2016). However, some studies report high H₂O contents for minerals in pristine xenoliths (see e.g., Moine et al., 2020; Aulbach et al., 2023). Moreover, it was recently shown for eclogitic xenoliths from Africa that metasomatic alteration does not necessarily result in H₂O-enrichment (Aulbach et al., 2024). It follows
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Boxplot of the H₂O content measured in minerals in xenoliths (> 1700 samples from a worldwide database) and in inclusions in diamonds (left and right, respectively). For clarity, the vertical axis was cut leaving five samples outside of the plot. The box represents the data interval between the first (Q1) and third (Q3) quartiles of the distribution, the horizontal line in the box represents the median. The whiskers represent the tails of the distribution, and the dots are outliers defined as falling outside Q1–1.5\*IQR and Q3+1.5\*IQR (IQR = interquartile range). For details regarding the source of H₂O data from xenoliths see Appendix I.</figcaption></figure>

that metasomatism might play a role in enriching xenoliths in H₂O compared to inclusions in diamond, but it is unlikely to be the main/- only factor causing the difference in H₂O content. (iii) The H₂O content of mineral inclusions does not reflect that of the average cratonic lithosphere. Diamonds are the result of localized metasomatic events. Therefore, the inclusions they host may be different from the same minerals in different areas of the cratonic lithosphere that are not affected by the type of metasomatism required for diamond formation. If the diamond forming medium is characterized by a low H₂O activity (aH₂O) the diamond-hosted mineral inclusions will show a lower H₂O content than the same minerals in area of the lithosphere not interested by diamond growth.

### 4.3. Source of H₂O in mineral inclusions in diamonds

Mineral inclusions in diamonds may either be protogenetic (i.e., formed before their diamond host) or syngenetic (i.e., formed at the same time as their diamond host). Arguments in favor of both syngenesis (Bulanova, 1995; Wiggers de Vries et al., 2011) and protogenesis

(Thomassot et al., 2009; Nestola et al., 2017) have been proposed, however, there is growing body of evidence supporting the latter (see Bruno et al., 2024). Consequently, we can initially assume the inclusions studied here are protogenetic and therefore their H₂O content is either primary, representative of the H₂O content of the minerals in the mantle before diamond formation, or reflects equilibration of the mineral with the diamond forming medium during diamond growth. The latter scenario is possible due to the high diffusivity of H in silicates (see Ingrin and Blanchard, 2006, and references therein). To investigate this possibility by quantifying the diffusivity of H, a diffusion model was constructed for olivine, orthopyroxene, garnet and clinopyroxene at P/T conditions conducive to diamond formation by solving Fick's second law of diffusion (details regarding the boundary conditions, parameters and solution used for the modelling are given in Appendix II). The resultant diffusion profiles (Figs. 8, S5 and S6) confirm that H equilibration in NAMs at temperatures characteristic of diamond formation (i.e., 1000–1200 °C) is extremely fast. For example, the H₂O content of a 500 µm long garnet (the mineral with the lowest diffusivity among those considered and with a size significantly larger than the average size of

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Diffusion profiles of H₂O at different times and temperatures for olivine (left) and garnet (right). For details regarding the diffusion profiles calculations and the parameters used see Appendix II.</figcaption></figure>
inclusions observed here) will completely re-equilibrate in about seven days, with other minerals generally requiring less than 24 h. Although the time required for a diamond to form in the mantle remains uncertain, diamond growth is unlikely to occur in the timespan of minutes or hours (Gress et al., 2018), implying that the H₂O content of mineral inclusions in diamonds most likely reflects the H₂O content of the diamond forming medium. Similar metasomatic re-equilibration between mantle substrate and metasomatic fluids was already described for major elements by Mikhail et al. (2021).

In this regard, the initial assumption of protogenesis becomes irrelevant, as the H₂O content of inclusions is related to that of the diamond forming medium both in the case of syngeneesis (i.e. simultaneous formation of the inclusion and diamond from the same medium) and protogenesis (i.e. re-equilibration of H₂O between the inclusion and the diamond forming medium).

As it is likely that the H₂O in NAMs completely re-equilibrates with the diamond forming medium, diffusive loss of H from the inclusion after diamond formation could in principle occur. Matveev & Stachel (2009) explained the dry nature of olivine inclusions in diamonds from Ghana and Canada by escape of H from the inclusion to the mantle, following experiments suggesting high H diffusion in diamond by Popovici et al. (1995). However, it is important to carefully assess this possibility since high H diffusivities in diamonds do not necessarily imply that H can leave the inclusion, effectively cross the inclusion-diamond interface, and then pass through the diamond and enter the surrounding mantle. In fact, Vangu et al. (2023) measured different H contents in cuboid and octahedral sectors of an asteriated diamond and attribute this to a lack of H diffusion between sectors. For diffusion of H from the inclusion to the mantle to occur, a chemical gradient would be required. To remove H from the inclusions, a low $a_{H_2O}$ would be required in the surrounding mantle, and if this was the case, then H would escape from all diamonds (if not trapped by N-related defects) and all inclusions, assuming equilibration. This is, however, not the case, as H₂O-bearing inclusions exist (as shown) and H commonly occurs as an impurity in diamond (e.g., Day et al., 2024). Moreover, since minerals in xenoliths are generally more H₂O-rich than inclusions in diamond, even for xenoliths that have not been metasomatized, an imposed chemical gradient would be opposite of that required for diffusive loss of H from the inclusion. Instead, this gradient would promote diffusion of H into the inclusion from the mantle, increasing H₂O content of the inclusion after diamond formation. However, this scenario also appears to be inoperative given the fact that dry inclusions are observed. Another important observation is that in N-bearing diamonds (most diamonds reported in this study and in general, Cartigny et al., 2022b), H tends to be associated with N and vacancy (V)-related defects, as evidenced by IR peaks observed at 3107 (VN₃H⁰) and 3236 cm⁻¹ (VN₄H⁰). In fact, any point defect that involves substitutional N results in uncoupled electrons on adjacent C atom(s) that act to trap H via passivation of dangling C bonds. As the major N defects (i.e., C-, A-, or B- centers) involve substitutional N it is likely that H will be trapped by N, thus minimizing diffusive loss of H from diamonds of any aggregation state. The situation is different for Type IIa diamonds that contain N in concentrations below the FTIR detection limit, however, four inclusions contain H₂O in these diamonds, implying that significant loss of H is unlikely.

Combining all the above observations, it seems reasonable to assume that H will not escape the inclusion after diamond formation, and instead the H₂O content of the inclusion will be preserved after re-equilibration with the diamond forming medium.

## 4.4. H₂O and the diamond forming medium

In the previous section, it is argued that the H₂O content of NAM inclusions in diamonds reflects re-equilibration of pre-existing mantle minerals with the diamond forming medium during diamond formation. If equilibrium is achieved, through fast H diffusion, the H₂O content in

the mineral can be related to the H₂O content of the diamond forming medium. The diamond forming medium may be a melt or a fluid (i.e., high-density supercritical fluids and low-density fluids, Weiss et al., 2022).

i) *Melts*: the H₂O content of the inclusions will be related to that of the melt according to H₂O mineral/melt partition coefficients ($Kd_{H_2O}^{min/melt}$). By using the average H₂O content of olivine, pyroxene and garnet inclusions in diamonds (from this study and the literature), together with $Kd_{H_2O}^{min/melt}$ measured in high-pressure high-temperature experiments (Aubaud et al., 2008; Novella et al., 2014), a range of H₂O contents expected for diamond forming melts is calculated. For peridotitic diamond forming melts, H₂O contents of 0.07 to 0.5 wt% in the melt are estimated, similar to the H₂O content proposed by Novella et al. (2015) based on olivine and garnet inclusions in diamonds from the Udachnaya kimberlite (0.09 wt% H₂O). For eclogitic diamond forming melts, on the other hand, H₂O contents of 0.5 to 1.5 wt% are expected. To precipitate diamond, the melt must be C-rich, and the presence of C has been shown to lower $Kd_{H_2O}^{min/melt}$ (Sokol et al., 2013). Consequently, in a C-rich melt, H₂O will be more incompatible in NAMs, and a higher H₂O content in the melt would be required to equilibrate inclusions with H₂O contents like those measured here. Experimental work shows that, at least for olivine, $Kd_{H_2O}^{min/melt}$ is halved in systems with very high C contents (Sokol et al., 2013), implying that the H₂O content of diamond forming media could be approximately twice that reported above (between 0.14 and 3 wt%). The H₂O contents reported here were calculated using the average H₂O contents of the inclusions but almost identical values are produced even when the highest reported H₂O contents are considered. The only exceptions are the two H₂O-rich eclogitic garnets reported in this study, specifically Grt_E_4 (471 ppm wt. H₂O), which requires equilibration with a melt with ~26 wt% H₂O (if no CO₂ is present), suggesting diamond-formation in a different, extremely H₂O-rich, environment compared to what is observed from other inclusions. The H₂O content of diamond forming melts calculated here is in agreement with modelling of hydrous low-degree melts that can form in the convective upper mantle (see Appendix III and Fig. S7).

This model is valid for hydrous-carbonated silicate melts, however, carbonatitic melts may also be involved in diamond formation (e.g., Weiss et al., 2022). The H₂O mineral/melt partition coefficients nor the H₂O activity-composition relationship for carbonatitic melts has been determined and therefore modelling them based on the H₂O content of the mineral inclusions is not possible. These melts could still be compatible with the H₂O contents measured in the mineral inclusions because of the low $a_{H_2O}$ in carbonatites suggested by the markedly higher H₂O solubility in carbonatites compared to silicate melts (Keppler, 2003).

ii) *Fluids*: diamond forming fluids can either be supercritical high-density fluids described as brines (see Weiss et al., 2022) or low-density C-O-H fluids (Stachel and Harris, 2009). Both types of fluids may be compatible with the low H₂O contents of mineral inclusions if $a_{H_2O}$ is low. This can occur due to interaction of H₂O with C (e.g., Yang et al., 2014) or other chemical species dissolved in the fluids that can lower the activity coefficient of H₂O (to values lower than those relevant for silicate melts), thus resulting in equilibration of inclusions with low H₂O contents. Alternatively, highly reducing conditions (such that a CH₄-rich fluid would be stable) would drastically decrease $a_{H_2O}$ in the fluids by stabilizing CH₄ and H₂ (Frost and McCammon, 2008) and would also result in low H₂O contents in minerals equilibrated with such fluids. Unfortunately, no data are available regarding the activity coefficients and activity-composition relationships of these fluids, and therefore modelling their composition and H₂O content based on data from diamond-hosted mineral inclusions is not possible.
# 5. Conclusions

In this study the H₂O content of 118 NAM inclusions hosted in monocrystalline diamonds was measured with FTIR spectroscopy. The results show that:

* Eclogitic mineral inclusions contain more H₂O than peridotitic inclusions, implying a higher H₂O content in eclogitic environments related to diamond formation.

* NAM inclusions in diamonds have a lower H₂O content than the same minerals in mantle xenoliths, this is attributed to (i) a sampling bias, (ii) metasomatism subsequent to diamond formation, (iii) low aH₂O in the diamond-forming environment.

* The H₂O content of NAM inclusions reflects equilibration with the diamond forming medium which, at equilibrium, is controlled by the H₂O partition coefficient.

* Significant H loss from the inclusion is unlikely due to unrealistic chemical gradients between the mantle and the inclusion and the presence of several barriers mitigating diffusive loss of H from the inclusion to the mantle (e.g., H trapped by vacancy and N-related defects).

* Diamond forming media associated with the formation of lithospheric monocrystalline diamond may be described as silicate melts with a relatively low H₂O content (≤ 3 wt%) or carbonatitic melts or fluids with a low H₂O activity.

# Data availability

Data can be accessed from the University of Padova online repository at https://researchdata.cab.unipd.it/id/eprint/1517.

# Funding

This is contribution no. 690 of the ClerVolc program of the International Research Center for Disaster Sciences and Sustainable Development of the University of Clermont Auvergne, supported by the Agence Nationale de la Recherche. The Graduate Track of Involc is also thanked for funding visiting of A.C. at LMV. D.N. thanks the Rita Levi Montalcini program for support. This work was supported by the European Union (ERC, INHERIT, Strating Grant No. 101041620). Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them.

# CRediT authorship contribution statement

**Andrea Curtolo:** Writing – original draft, Visualization, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Maxwell C. Day:** Writing – review & editing, Validation. **Francesca Innocenzi:** Writing – review & editing. **Nathalie Bolfan-Casanova:** Writing – review & editing, Validation, Supervision, Funding acquisition. **Martha G. Pamato:** Writing – review & editing, Funding acquisition. **Simon Falvard:** Writing – review & editing, Investigation. **Fabrizio Nestola:** Writing – review & editing, Investigation. **Jeff W. Harris:** Writing – review & editing, Resources. **Davide Novella:** Writing – review & editing, Supervision, Conceptualization.

# Declaration of competing interest

The authors confirm that this work is original and has not been published elsewhere, nor is it currently under consideration for publication elsewhere. Use of generative AI. No artificial intelligence tools or services were used to write, construct or organize any part of this manuscript.

# Acknowledgments

The authors thank James Badro for editorial handling as well as Thomas Stachel and Hans Keppler for providing helpful reviews that greatly improved the quality of this manuscript.

# Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2025.119311.

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