Mixed paragenesis diamond inclusions form during metasomatic fluid evolution
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Keywords: diamond formation, fluid-rock metasomatism, mixed paragenesis inclusions, lithospheric mantle, thermodynamic modelling
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Abstract
Figures
 Figure 1 Predicted mineral precipitation during reaction progress in two DEW fluid-rock interaction models. Log mineral abundance (mol) is plotted against log reaction progress (ξ), where ξ represents the extent of fluid-rock interaction simulated by irreversible mass transfer in EQ6. (a) Model I predicts progressive garnet growth (Py 0.46–0.83), followed by omphacite (Jd ∼0.67), diamond precipitation, and late olivine (Mg# ∼0.95), with minor orthopyroxene and kyanite at advanced reaction progress. (b) Model II shows a simpler sequence dominated by garnet and omphacite formation followed by diamond precipitation. Values in parentheses indicate representative mineral compositions predicted by the models. The simulations illustrate mineral assemblages produced during mantle fluid-rock interaction capable of forming diamond bearing eclogitic-peridotitic assemblages. |  Figure 2 Conceptual illustrations of metasomatic fluid-rock interaction represented by Models I and II (top and bottom). Fluids infiltrate mantle rocks along veins and react with the surrounding lithologies, precipitating minerals and diamond along the reaction path. Although the DEW models have no spatial dimension, the cartoons illustrate processes observed in diamondiferous mantle xenoliths (Howarth et al., 2025). Progressive diffusive equilibration may remove the original metasomatic veins, leaving inclusion-bearing and inclusion-free diamonds preserved within the mantle matrix. |  Figure 3 Ternary diagram comparing predicted model fluid compositions (curves and arrows) with high density fluid (HDF) compositions in fibrous diamonds (coloured fields from Weiss et al., 2022). Solid curves and arrows show fluid evolution paths predicted by DEW models from this study (Models I and II). Dashed curves and arrows show paths from Rinaldi et al. (2023), and Huang and Sverjensky (2020). Models I and II trajectories reproduce the transition from silicic to carbonatitic compositions and overlap natural HDF arrays. The simulations of Rinaldi et al. (2023) represent reactions between fluids of different initial compositions (eclogitic, silicic, transitional silicic-carbonatitic, and carbonatitic) and mantle lithologies including carbonated peridotite (Models 151 and 152) and eclogite or websterite (Models 154 and 158) and overlap numerous natural fluid inclusion trends. The model of Huang and Sverjensky (2020) describes the evolution of saline fluids as they mix and react with highly Mg-rich carbonatitic compositions. |
| Figure 1 | Figure 2 | Figure 3 |
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Introduction
Mineral inclusions in diamonds provide constraints on the geological environment of diamond formation. However, the relative abundance of minerals encapsulated in diamonds does not reflect the modal mineralogy of Earth’s mantle (Stachel and Harris, 2008
Stachel, T., Harris, J.W. (2008) The origin of cratonic diamonds: Constraints from mineral inclusions. Ore Geology Reviews 34, 5–32. https://doi.org/10.1016/j.oregeorev.2007.05.002
). This mismatch may reflect [1] preferential inclusion of minerals associated with lower surface energies (Meyer and Boyd, 1972Meyer, H.O.A., Boyd, F.R. (1972) Composition and origin of crystalline inclusions in natural diamonds. Geochimica et Cosmochimica Acta 36, 1255–1273. https://doi.org/10.1016/0016-7037(72)90048-8
), [2] a syngenetic process where sulphides, garnet, and Mg chromite may precipitate or recrystallise more readily during diamond growth driven by fluid (Mikhail et al., 2019Mikhail, S., McCubbin, F.M., Jenner, F.E., Shirey, S.B., Rumble, D., Bowden, R. (2019) Diamondites: Evidence for a distinct tectono-thermal diamond-forming event beneath the Kaapvaal craton. Contributions to Mineralogy and Petrology 174, 71. https://doi.org/10.1007/s00410-019-1608-0
; 2021Mikhail, S., Rinaldi, M., Mare, E.R., Sverjensky, D.A. (2021) A genetic metasomatic link between eclogitic and peridotitic diamond inclusions. Geochemical Perspectives Letters 17, 33–38. https://doi.org/10.7185/geochemlet.2111
; Rinaldi et al., 2023Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
) and/or melt metasomatism (Pintér et al., 2022Pintér, Z., Foley, S.F., Yaxley, G.M. (2022) Diamonds, dunites, and metasomatic rocks formed by melt/rock reaction in craton roots. Communications Earth & Environment 3, 263. https://doi.org/10.1038/s43247-022-00630-3
), or [3] a combination of both.Silicate inclusions in diamond are typically classified into three paragenetic groups: peridotitic (e.g., Cr-rich pyrope, diopside, enstatite, olivine), eclogitic (e.g., Cr-poor pyrope-almandine and omphacite), and websteritic (intermediate compositions; Gurney et al., 1984
Gurney, J.J., Harris, J.W., Rickard, R.S. (1984) Silicate and oxide inclusions in diamonds from the Orapa Mine, Botswana. In: Kornprobst, J. (Ed.) Developments in Petrology, Volume 11, Issue 2. Kimberlites II: the mantle and crust–mantle relationships. Elsevier, Amsterdam, 3–9. https://doi.org/10.1016/B978-0-444-42274-3.50007-X
). Rarely, diamonds contain disequilibrium assemblages termed mixed paragenesis, in which minerals from different paragenetic groups occur within a single crystal (Prinz et al., 1975Prinz, M., Vincent, M.D., Hlava, P.F., Keil, K. (1975) Inclusions in diamonds: Garnet lherzolite and eclogite assemblages. Physics and Chemistry of the Earth 9, 797–815. https://doi.org/10.1016/0079-1946(75)90052-X
; Mikhail et al., 2019Mikhail, S., McCubbin, F.M., Jenner, F.E., Shirey, S.B., Rumble, D., Bowden, R. (2019) Diamondites: Evidence for a distinct tectono-thermal diamond-forming event beneath the Kaapvaal craton. Contributions to Mineralogy and Petrology 174, 71. https://doi.org/10.1007/s00410-019-1608-0
; Lai et al., 2022Lai, M.Y., Stachel, T., Stern, R.A., Hardman, M.F., Pearson, D.G., Harris, J.W. (2022) Formation of mixed paragenesis diamonds during multistage growth: Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds. Geochimica et Cosmochimica Acta 323, 20–39. https://doi.org/10.1016/j.gca.2022.02.020
). For example, eclogitic minerals may occur in the core of a diamond (e.g., omphacite, eclogitic garnet, or coesite), whereas peridotitic minerals occur near the rim (e.g., forsteritic olivine or Mg chromite) (Lai et al., 2022Lai, M.Y., Stachel, T., Stern, R.A., Hardman, M.F., Pearson, D.G., Harris, J.W. (2022) Formation of mixed paragenesis diamonds during multistage growth: Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds. Geochimica et Cosmochimica Acta 323, 20–39. https://doi.org/10.1016/j.gca.2022.02.020
). Mixed paragenesis diamonds have been suggested to record multiple episodes of diamond growth in different host rocks which requires physical movement of the diamond between different mantle environments (e.g., Wang, 1998Wang, W. (1998) Formation of diamond with mineral inclusions of mixed eclogite and peridotite paragenesis. Earth and Planetary Science Letters 160, 831–843. https://doi.org/10.1016/S0012-821X(98)00131-9
; Lai et al., 2022Lai, M.Y., Stachel, T., Stern, R.A., Hardman, M.F., Pearson, D.G., Harris, J.W. (2022) Formation of mixed paragenesis diamonds during multistage growth: Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds. Geochimica et Cosmochimica Acta 323, 20–39. https://doi.org/10.1016/j.gca.2022.02.020
). In contrast, our results show that, despite comments to the contrary (Lai et al., 2022Lai, M.Y., Stachel, T., Stern, R.A., Hardman, M.F., Pearson, D.G., Harris, J.W. (2022) Formation of mixed paragenesis diamonds during multistage growth: Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds. Geochimica et Cosmochimica Acta 323, 20–39. https://doi.org/10.1016/j.gca.2022.02.020
), the mixed parageneses in a single diamond may form from the progressive trapping of minerals formed during metasomatic reactions (e.g., Mikhail et al., 2021Mikhail, S., Rinaldi, M., Mare, E.R., Sverjensky, D.A. (2021) A genetic metasomatic link between eclogitic and peridotitic diamond inclusions. Geochemical Perspectives Letters 17, 33–38. https://doi.org/10.7185/geochemlet.2111
). We use predictive reaction path models of irreversible reactions between rocks and diamond forming aqueous fluids (Sverjensky and Huang, 2015Sverjensky, D.A., Huang, F. (2015) Diamond formation due to a pH drop during fluid–rock interactions. Nature Communications 6, 8702. https://doi.org/10.1038/ncomms9702
; Huang and Sverjensky, 2020Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
; Mikhail et al., 2021Mikhail, S., Rinaldi, M., Mare, E.R., Sverjensky, D.A. (2021) A genetic metasomatic link between eclogitic and peridotitic diamond inclusions. Geochemical Perspectives Letters 17, 33–38. https://doi.org/10.7185/geochemlet.2111
; Rinaldi et al., 2023Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
) with thermodynamic data from the DEW model (Huang and Sverjensky, 2019Huang, F., Sverjensky, D.A. (2019) Extended Deep Earth Water Model for predicting major element mantle metasomatism. Geochimica et Cosmochimica Acta 254, 192–230. https://doi.org/10.1016/j.gca.2019.03.027
; Sverjensky, 2019Sverjensky, D.A. (2019) Thermodynamic modelling of fluids from surficial to mantle conditions. Journal of the Geological Society 176, 348–374. https://doi.org/10.1144/jgs2018-105
) as a process based framework to explore the possibility that mixed paragenesis inclusions can form during a single episode of diamond growth via fluid-rock metasomatism. In addition to examining the mineral assemblages produced during metasomatism, we evaluate whether such reactions reproduce the compositional range of diamond forming fluids observed in fibrous diamonds.top
Methods
Modelling approach. We used the Extended Deep Earth Water (DEW) model to simulate fluid-rock metasomatism in the lithospheric mantle. DEW extends the EQ3/EQ6 geochemical modelling framework for aqueous speciation and irreversible mass transfer reactions to mantle pressures and temperatures (Huang and Sverjensky, 2019
Huang, F., Sverjensky, D.A. (2019) Extended Deep Earth Water Model for predicting major element mantle metasomatism. Geochimica et Cosmochimica Acta 254, 192–230. https://doi.org/10.1016/j.gca.2019.03.027
). EQ3 was used to calculate equilibrium fluid compositions, and EQ6 was used to simulate irreversible fluid-rock reactions during metasomatism. The thermodynamic data set is calibrated using experimentally determined solubilities at upper mantle pressures and temperatures (e.g., Kessel et al., 2015Kessel, R., Pettke, T., Fumagalli, P. (2015) Melting of metasomatized peridotite at 4–6 GPa and up to 1200 °C: An experimental approach. Contributions to Mineralogy and Petrology 169, 37. https://doi.org/10.1007/s00410-015-1132-9
) and has previously been applied to model diamond forming fluids and metasomatic reactions in the lithospheric mantle (Sverjensky and Huang, 2015Sverjensky, D.A., Huang, F. (2015) Diamond formation due to a pH drop during fluid–rock interactions. Nature Communications 6, 8702. https://doi.org/10.1038/ncomms9702
; Huang and Sverjensky, 2020Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
; Mikhail et al., 2021Mikhail, S., Rinaldi, M., Mare, E.R., Sverjensky, D.A. (2021) A genetic metasomatic link between eclogitic and peridotitic diamond inclusions. Geochemical Perspectives Letters 17, 33–38. https://doi.org/10.7185/geochemlet.2111
; Rinaldi et al., 2023Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
).Model parameterisation. Simulations were performed at 5 GPa, 1000 °C and logfO2 = -2 ΔFMQ, conditions consistent with lithospheric diamond formation (Stachel and Harris, 2008
Stachel, T., Harris, J.W. (2008) The origin of cratonic diamonds: Constraints from mineral inclusions. Ore Geology Reviews 34, 5–32. https://doi.org/10.1016/j.oregeorev.2007.05.002
). The initial fluid compositions were calculated from equilibrium between water and eclogite or water and carbonated dunite assemblages following Huang and Sverjensky (2020)Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
. Mineral solid solutions were represented using ideal site mixing between pyrope, almandine, and grossular in garnet, and non-ideal mixing between diopside, hedenbergite, and clinoenstatite in clinopyroxene following Huang and Sverjensky (2020)Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
. The chosen temperature of 1000 °C lies at the lower end of the estimated diamond inclusion entrapment temperature range (1155 ± 105 °C; Stachel and Luth, 2015Stachel, T., Luth, R.W. (2015) Diamond formation: Where, when and how? Lithos 220–223, 200–220. https://doi.org/10.1016/j.lithos.2015.01.028
). Two model scenarios were investigated, termed Model I and Model II, as described below.top
Results
Model I. Infiltration of eclogitic fluid into dry carbonated peridotite. This simulation models the infiltration of an externally derived fluid into a dry mantle rock, following an approach similar to that of Rinaldi et al. (2023)
Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
. An eclogitic fluid was generated by equilibrating water with an eclogite assemblage (clinopyroxene + garnet + coesite + diamond) using EQ3. The resulting fluid was then reacted with an anhydrous carbonated peridotite using EQ6 to simulate irreversible fluid-rock interaction. Because the infiltrating fluid and host rock are far from equilibrium, progressive reaction results in sequential mineral precipitation (Fig. 1a). At early reaction progress, the precipitating phases have an eclogitic character, dominated by omphacite (Jd67) and garnet (Py46). Diamond precipitation begins during these early stages and may encapsulate these minerals as inclusions in the cores of growing crystals. With increasing reaction progress, the mineral assemblage evolves toward more peridotitic compositions, including olivine (Fo95) and a more magnesian garnet (Py83). Continued diamond growth during these later stages allows these minerals to be preserved as inclusions near diamond rims. This sequence predicts that a single diamond growth episode may first encapsulate eclogitic and then later peridotitic inclusions as metasomatism progresses.Figure 1 Predicted mineral precipitation during reaction progress in two DEW fluid-rock interaction models. Log mineral abundance (mol) is plotted against log reaction progress (ξ), where ξ represents the extent of fluid-rock interaction simulated by irreversible mass transfer in EQ6. (a) Model I predicts progressive garnet growth (Py 0.46–0.83), followed by omphacite (Jd ∼0.67), diamond precipitation, and late olivine (Mg# ∼0.95), with minor orthopyroxene and kyanite at advanced reaction progress. (b) Model II shows a simpler sequence dominated by garnet and omphacite formation followed by diamond precipitation. Values in parentheses indicate representative mineral compositions predicted by the models. The simulations illustrate mineral assemblages produced during mantle fluid-rock interaction capable of forming diamond bearing eclogitic-peridotitic assemblages.
Model II. Infiltration of carbonatitic fluid into eclogite with mixing of eclogitic pore fluid. Model II simulates a more complex scenario involving simultaneous fluid mixing and reaction with the host rock, similar to the model of Huang and Sverjensky (2020)
Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
for fluid-rich diamonds from the Panda kimberlite. In the present model, a carbonatitic fluid infiltrates eclogite containing an eclogitic pore fluid. The assumption of localised pore or grain boundary fluids in eclogitic substrates is motivated by the broader evidence for fluid mediated metasomatism in mantle lithologies, including the transport of incompatible elements, carbonate species, and volatile-rich metasomatic agents through the lithospheric mantle (see Steele-MacInnis, 2025Steele-MacInnis, M. (2025) Physico-chemical properties of hydrothermal fluids. Treatise on Geochemistry. Third Edition, Elsevier, Oxford, 869–909. https://doi.org/10.1016/B978-0-323-99762-1.00089-9
for a review). As infiltration proceeds, the invading fluid progressively mixes with the resident pore fluid while reacting with the surrounding rock. The carbonatitic fluid was generated by reacting water with olivine + diamond and minor orthopyroxene and garnet using EQ6, producing a fluid in equilibrium with olivine (Fo97) and garnet (Py92). This fluid was then allowed to react with eclogite containing clinopyroxene, garnet and minor coesite while simultaneously mixing with the resident pore fluid. During early reaction progress, the composition of precipitating minerals is dominated by the eclogitic pore fluid, resulting in the formation of omphacite (Jd67) and garnet (Py45) (Fig. 1b). Diamond precipitation begins during these early stages and may encapsulate these minerals within the cores of growing diamonds. As the proportion of carbonatitic fluid increases with continued reaction, the mineral assemblage evolves toward more peridotitic compositions, including a more magnesian garnet (Py83). These later minerals may become trapped as inclusions near diamond rims.top
Discussion
The formation of mixed paragenesis diamond inclusions. To place the model results in a natural context, we compare them with mixed paragenesis diamonds from the Koidu kimberlite complex, Sierra Leone (Lai et al., 2022
Lai, M.Y., Stachel, T., Stern, R.A., Hardman, M.F., Pearson, D.G., Harris, J.W. (2022) Formation of mixed paragenesis diamonds during multistage growth: Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds. Geochimica et Cosmochimica Acta 323, 20–39. https://doi.org/10.1016/j.gca.2022.02.020
). The predicted mineral sequences closely reproduce the most common inclusion relationships documented in these diamonds. For example, their diamond #138-7 contains an eclogitic garnet inclusion in the core and olivine near the rim, a sequence consistent with Model I (Fig. 1a). Similarly, their diamond #133-6 contains omphacite in the core and Mg chromite near the rim, consistent with a broader evolution toward increasingly magnesian mineral assemblages during metasomatism. Our models did not produce coesite. However, variants of Model II involving higher initial coesite abundances could plausibly permit preservation and encapsulation of coesite during diamond growth, consistent with previous studies (Rinaldi et al., 2023Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
).Chromium is not included in our DEW models, and therefore Mg chromite is not predicted by these models. However, the Mg chromite reported by Lai et al. (2022)
Lai, M.Y., Stachel, T., Stern, R.A., Hardman, M.F., Pearson, D.G., Harris, J.W. (2022) Formation of mixed paragenesis diamonds during multistage growth: Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds. Geochimica et Cosmochimica Acta 323, 20–39. https://doi.org/10.1016/j.gca.2022.02.020
is an Mg-Fe-Al chromite ((Mg,Fe)O(Cr,Al,Fe)2O3), although the present simulations do not explicitly reproduce Mg chromite because chromium is not included within the current modelling framework. The appearance of these minerals during later stages of reaction reflects a shift toward more peridotitic mineral assemblages during progressive metasomatism (as predicted here; Fig. 1).Taken together, the models demonstrate that disequilibrium mineral assemblages can be produced and preserved during a single episode of diamond growth under constant pressure-temperature conditions. Mixed paragenesis inclusions may therefore form as direct products of diamond forming fluid-rock interaction, without requiring a single diamond to grow first in one mantle lithology and then again in a second mantle lithology.
Relationship between diamond inclusions and the diamond forming substrate. The Koidu kimberlite complex is unusual in that mantle xenoliths recovered from the pipes are exclusively eclogitic (Hills and Haggerty, 1989
Hills, D.V., Haggerty, S.E. (1989) Petrochemistry of eclogites from the Koidu kimberlite complex, Sierra Leone. Contributions to Mineralogy and Petrology 103, 397–422. https://doi.org/10.1007/BF01041749
). Nevertheless, diamonds from Koidu contain inclusions with clearly peridotitic compositions, including olivine and Mg-Fe-Al chromite (Lai et al., 2022Lai, M.Y., Stachel, T., Stern, R.A., Hardman, M.F., Pearson, D.G., Harris, J.W. (2022) Formation of mixed paragenesis diamonds during multistage growth: Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds. Geochimica et Cosmochimica Acta 323, 20–39. https://doi.org/10.1016/j.gca.2022.02.020
). This apparent mismatch between xenolith mineralogy and diamond inclusions has important implications for the nature of the diamond forming substrate. Our models predict that olivine precipitates from the fluid only when the reacting host rock is highly Mg-rich. This suggests that olivine bearing, mixed paragenesis diamonds likely formed within depleted peridotitic lithologies rather than within eclogite itself. Although such lithologies are not represented among xenoliths from the Koidu kimberlites, indicator mineral concentrates include both eclogitic and peridotitic compositions, with garnets characterised by high Cr and low Ca contents (Harder et al., 2013Harder, M., Nowicki, T.E., Hetman, C.M., Freeman, L., Abedu, B. (2013) Geology and evaluation of the K2 kimberlite, Koidu Mine, Sierra Leone. In: Pearson, D. et al. (Eds.) Proceedings of the 10th International Kimberlite Conference. Springer, New Delhi, 191–208. https://doi.org/10.1007/978-81-322-1173-0_13
). These garnet compositions indicate the presence of highly depleted harzburgitic or dunitic mantle beneath the Koidu kimberlite field. Mixed paragenesis diamonds may therefore provide direct evidence of the lithologies involved in diamond formation in the absence of xenoliths. Where spatial relationships between inclusions are preserved, the sequence of inclusion entrapment may record metasomatic reaction pathways operating during diamond growth (i.e. progressive geochemical evolution; Figs. 1, 2).Figure 2 Conceptual illustrations of metasomatic fluid-rock interaction represented by Models I and II (top and bottom). Fluids infiltrate mantle rocks along veins and react with the surrounding lithologies, precipitating minerals and diamond along the reaction path. Although the DEW models have no spatial dimension, the cartoons illustrate processes observed in diamondiferous mantle xenoliths (Howarth et al., 2025
Howarth, G.H., Shaw Kahle, B., Janney, P.E., Gurney, J.J. (2025) Diamond–silicate–sulphide–oxide textural relationships in diamondiferous eclogites from the Kalahari craton revealed by X-ray computed tomography. Journal of Petrology 66, egaf062. https://doi.org/10.1093/petrology/egaf062
). Progressive diffusive equilibration may remove the original metasomatic veins, leaving inclusion-bearing and inclusion-free diamonds preserved within the mantle matrix.These data support a growing body of evidence that diamond and diamond inclusion formation is intimately linked to metasomatism of mantle rocks (illustrated in Fig. 2). For example, fluid-rock interaction can partially dissolve existing mantle minerals and precipitate new phases during diamond growth (e.g., Walter et al., 2011
Walter, M.J., Kohn, S.C., Araujo, D., Bulanova, G.P., Smith, C.B., Gaillou, E., Wang, J., Steele, A., Shirey, S.B. (2011) Deep mantle cycling of oceanic crust: Evidence from diamonds and their mineral inclusions. Science 334, 54–57. https://doi.org/10.1126/science.1209300
; Aulbach et al., 2013Aulbach, S., Griffin, W.L., Pearson N.J., O’Reilly S.J. (2013) Nature and timing of metasomatism in the stratified mantle lithosphere beneath the central Slave craton (Canada). Chemical Geology, 352, 153–169. https://doi.org/10.1016/j.chemgeo.2013.05.037
; Thomson et al., 2014Thomson, A.R., Kohn, S.C., Bulanova, G.P., Smith, C.B., Araujo, D., Walter, M.J. (2014) Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): Constraints from carbon isotopes and inclusion compositions. Contributions to Mineralogy and Petrology 168, 1081. https://doi.org/10.1007/s00410-014-1081-8
; Huang and Sverjensky, 2020Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
; Mikhail et al., 2021Mikhail, S., Rinaldi, M., Mare, E.R., Sverjensky, D.A. (2021) A genetic metasomatic link between eclogitic and peridotitic diamond inclusions. Geochemical Perspectives Letters 17, 33–38. https://doi.org/10.7185/geochemlet.2111
; Pasqualetto et al., 2022Pasqualetto, L., Nestola, F., Jacob, D.E., Pamato, M.G., Oliveira, B., Perritt, S., Chinn, I., Nimis, P., Milani, S., Harris, J.W. (2022) Protogenetic clinopyroxene inclusions in diamond and Nd diffusion modeling: Implications for diamond dating. Geology 50, 1038–1042. https://doi.org/10.1130/G50273.1
; Rinaldi et al., 2023Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
; Bruno et al., 2024Bruno, M., Ghignone, S., Aquilano, D., Nestola, F. (2024) A critique of using epitaxial criterion to discriminate between protogenetic and syngenetic mineral inclusions in diamond. Scientific Reports 14, 8674. https://doi.org/10.1038/s41598-024-59432-6
; Howarth et al., 2025Howarth, G.H., Shaw Kahle, B., Janney, P.E., Gurney, J.J. (2025) Diamond–silicate–sulphide–oxide textural relationships in diamondiferous eclogites from the Kalahari craton revealed by X-ray computed tomography. Journal of Petrology 66, egaf062. https://doi.org/10.1093/petrology/egaf062
). The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008Stachel, T., Harris, J.W. (2008) The origin of cratonic diamonds: Constraints from mineral inclusions. Ore Geology Reviews 34, 5–32. https://doi.org/10.1016/j.oregeorev.2007.05.002
; Stachel and Luth, 2015Stachel, T., Luth, R.W. (2015) Diamond formation: Where, when and how? Lithos 220–223, 200–220. https://doi.org/10.1016/j.lithos.2015.01.028
), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994Ireland, T.R., Rudnick, R.L., Spetsius, Z. (1994) Trace elements in diamond inclusions from eclogites reveal link to Archean granites. Earth and Planetary Science Letters 128, 199–213. https://doi.org/10.1016/0012-821X(94)90145-7
; Taylor et al., 1996Taylor, L.A., Snyder, G.A., Crozaz, G., Sobolev, V.N., Yefimova, E.S., Sobolev, N.V. (1996) Eclogitic inclusions in diamonds: Evidence of complex mantle processes over time. Earth and Planetary Science Letters 142, 535–551. https://doi.org/10.1016/0012-821X(96)00106-9
; Anand et al., 2004Anand, M., Taylor, L.A., Misra, K.C., Sobolev, N.V., Pokhilenko, N.P. (2004) Nature of diamonds in Yakutian eclogites: Views from eclogite tomography and mineral inclusions in diamond. Lithos 77, 333–348. https://doi.org/10.1016/j.lithos.2004.03.026
; Misra et al., 2004Misra, K.C., Anand, M., Taylor, L.A., Sobolev, N.V. (2004) Multi-stage metasomatism of diamondiferous eclogite xenoliths from the Udachnaya kimberlite pipe, Yakutia, Siberia. Contributions to Mineralogy and Petrology 146, 696–714. https://doi.org/10.1007/s00410-003-0529-z.
; Stepanov et al., 2008Stepanov, A.S., Shatsky, V.S., Zedgenizov, D.A., Ragozin, A.L. (2008) Chemical Heterogeneity in the Diamondiferous Eclogite Xenolith from the Udachnaya Kimberlite Pipe. Doklady Earth Sciences 419, 308–311.
; Liu et al., 2009Liu, Y., Taylor, L.A., Sarbadhikari, A.B., Valley, J.W., Ushikubo, T., Spicuzza, M., Kita, N., Sobolev, N.V. (2009) Metasomatic origin of diamonds in the world’s largest diamondiferous eclogite. Lithos 112, 1014–1024. https://doi.org/10.1016/j.lithos.2009.06.036
; Smart et al., 2012Smart, K.A., Chacko, T., Stachel, T., Tappe, S., Stern, R.A., Ickert, R.B. (2012) Eclogite formation beneath the northern Slave craton constrained by diamond inclusions. Earth and Planetary Science Letters 319–320, 165–177. https://doi.org/10.1016/j.epsl.2011.12.032
; Aulbach and Stachel, 2022Aulbach, S., Stachel, T. (2022) Evidence for oxygen-conserving diamond formation in redox-buffered subducted oceanic crust sampled as eclogite. Nature Communications 13, 1924. https://doi.org/10.1038/s41467-022-29567-z
).Evolution of diamond forming fluids. Irreversible reaction path models using the DEW model thermodynamic data predict that metasomatism should result in the precipitation of silicates which mirror those observed as diamond inclusions (Mikhail et al., 2021
Mikhail, S., Rinaldi, M., Mare, E.R., Sverjensky, D.A. (2021) A genetic metasomatic link between eclogitic and peridotitic diamond inclusions. Geochemical Perspectives Letters 17, 33–38. https://doi.org/10.7185/geochemlet.2111
; Rinaldi et al., 2023Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
; this study). Here we investigate whether our simulations reproduce natural diamond forming fluids. We compare modelled fluid compositions with the compositional range of high density fluids (HDFs) observed in fibrous diamonds (Fig. 3). In both Model I and Model II, progressive fluid-rock interaction drives fluid compositions away from silicic compositions toward increasingly carbonatitic compositions. The comparison also shows that different metasomatic scenarios generate different fluid evolution paths. In Model I, infiltration of an eclogitic fluid into peridotite produces fluid compositions that evolve along the silicic-carbonatitic transition. In Model II, mixing between carbonatitic and eclogitic fluids produces trajectories that intersect a similar region of HDF compositional space but follow a different path. The simulations of Rinaldi et al. (2023)Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
represent reactions between fluids of different initial compositions (eclogitic, silicic, transitional silicic-carbonatitic, and carbonatitic) and mantle lithologies including carbonated peridotite (Models 151 and 152 of Rinaldi et al., 2023Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
) and eclogite or websterite (Models 154 and 158 of Rinaldi et al., 2023Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
), and the model of Huang and Sverjensky (2020)Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
describes the evolution of saline fluids as they mix and react with highly Mg-rich carbonatitic compositions (Fig. 3; Table S-1). The DEW simulations reproduce the direction and extent of fluid evolution observed in natural systems, where the model fluid trajectories overlap the compositional arrays defined by natural HDF samples, including the silicic, silicic low-Mg carbonatitic, low-Mg carbonatitic, and saline fields (Fig. 3). These results suggest that multiple metasomatic pathways can generate fluids within the observed range of diamond forming compositions. Taken together, these trajectories follow the same general compositional gradients defined by natural HDF arrays (Weiss et al., 2022Weiss, Y., Czas, J., Navon, O. (2022) Fluid inclusions in fibrous diamonds. Reviews in Mineralogy and Geochemistry 88, 475–532. https://doi.org/10.2138/rmg.2022.88.09
), thus indicating that the diversity of diamond forming fluids can arise from progressive metasomatic reactions akin to those predicted by DEW simulations (Figs. 1–3).Figure 3 Ternary diagram comparing predicted model fluid compositions (curves and arrows) with high density fluid (HDF) compositions in fibrous diamonds (coloured fields from Weiss et al., 2022
Weiss, Y., Czas, J., Navon, O. (2022) Fluid inclusions in fibrous diamonds. Reviews in Mineralogy and Geochemistry 88, 475–532. https://doi.org/10.2138/rmg.2022.88.09
). Solid curves and arrows show fluid evolution paths predicted by DEW models from this study (Models I and II). Dashed curves and arrows show paths from Rinaldi et al. (2023)Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
, and Huang and Sverjensky (2020)Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
. Models I and II trajectories reproduce the transition from silicic to carbonatitic compositions and overlap natural HDF arrays. The simulations of Rinaldi et al. (2023)Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
represent reactions between fluids of different initial compositions (eclogitic, silicic, transitional silicic-carbonatitic, and carbonatitic) and mantle lithologies including carbonated peridotite (Models 151 and 152) and eclogite or websterite (Models 154 and 158) and overlap numerous natural fluid inclusion trends. The model of Huang and Sverjensky (2020)Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
describes the evolution of saline fluids as they mix and react with highly Mg-rich carbonatitic compositions.top
Conclusions
Thermodynamic models of fluid-rock metasomatism in the lithospheric mantle reproduce general metasomatic pathways consistent with observations of mixed paragenesis inclusions observed in diamonds from the Koidu kimberlite complex which evolve from Mg-poor to Mg-rich. The simulations demonstrate that disequilibrium inclusion assemblages can form during a single episode of diamond growth under constant pressure-temperature conditions as infiltrating fluids react with mantle rocks with and without pore fluids. Reaction pathways involving dunitic host rocks generate olivine during late stages of metasomatism, indicating that olivine-bearing mixed paragenesis diamonds formed within Mg-rich lithologies in the lithospheric mantle. Modelled fluid compositions evolve from silicic to carbonatitic and span the total range observed in fibrous diamond inclusions. These results show that both mixed paragenesis inclusions and the diversity of diamond forming fluids can arise from progressive metasomatic reactions, and that diamond inclusions do not necessarily represent samples of pristine mantle minerals.
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Acknowledgements
This work was supported by grants from the UKRI NERC (OPP341 and NE/V011383/1) to SM, the UK Space Agency (ST/T001763/1) to SM, and builds on the results of previous published work funded by the US Department of Energy and the National Science Foundation from 2019 to 2025 to DAS. The codes and thermodynamic data used were first presented at a workshop in Milan in 2025 hosted by Simone Tumiati at the Universita degli Studi di Milano and can be found on the Deep Earth Water Community site at http://www.dewcommunity.org/. DAS greatly appreciates the help in developing the EQ3/6 code applications for deep Earth conditions from Simon Matthews and Mark Ghiorso. This work benefited from thoughtful reviews by Thomas Stachel, one anonymous reviewer, and editorial handling of Ambre Luguet.
Editor: Ambre Luguet
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References
Anand, M., Taylor, L.A., Misra, K.C., Sobolev, N.V., Pokhilenko, N.P. (2004) Nature of diamonds in Yakutian eclogites: Views from eclogite tomography and mineral inclusions in diamond. Lithos 77, 333–348. https://doi.org/10.1016/j.lithos.2004.03.026
Show in context The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008; Stachel and Luth, 2015), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994; Taylor et al., 1996; Anand et al., 2004; Misra et al., 2004; Stepanov et al., 2008; Liu et al., 2009; Smart et al., 2012; Aulbach and Stachel, 2022).
View in article
Aulbach, S., Stachel, T. (2022) Evidence for oxygen-conserving diamond formation in redox-buffered subducted oceanic crust sampled as eclogite. Nature Communications 13, 1924. https://doi.org/10.1038/s41467-022-29567-z
Show in context The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008; Stachel and Luth, 2015), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994; Taylor et al., 1996; Anand et al., 2004; Misra et al., 2004; Stepanov et al., 2008; Liu et al., 2009; Smart et al., 2012; Aulbach and Stachel, 2022).
View in article
Aulbach, S., Griffin, W.L., Pearson N.J., O’Reilly S.J. (2013) Nature and timing of metasomatism in the stratified mantle lithosphere beneath the central Slave craton (Canada). Chemical Geology, 352, 153–169. https://doi.org/10.1016/j.chemgeo.2013.05.037
Show in context For example, fluid-rock interaction can partially dissolve existing mantle minerals and precipitate new phases during diamond growth (e.g., Walter et al., 2011; Aulbach et al., 2013; Thomson et al., 2014; Huang and Sverjensky, 2020; Mikhail et al., 2021; Pasqualetto et al., 2022; Rinaldi et al., 2023; Bruno et al., 2024; Howarth et al., 2025).
View in article
Bruno, M., Ghignone, S., Aquilano, D., Nestola, F. (2024) A critique of using epitaxial criterion to discriminate between protogenetic and syngenetic mineral inclusions in diamond. Scientific Reports 14, 8674. https://doi.org/10.1038/s41598-024-59432-6
Show in context For example, fluid-rock interaction can partially dissolve existing mantle minerals and precipitate new phases during diamond growth (e.g., Walter et al., 2011; Aulbach et al., 2013; Thomson et al., 2014; Huang and Sverjensky, 2020; Mikhail et al., 2021; Pasqualetto et al., 2022; Rinaldi et al., 2023; Bruno et al., 2024; Howarth et al., 2025).
View in article
Gurney, J.J., Harris, J.W., Rickard, R.S. (1984) Silicate and oxide inclusions in diamonds from the Orapa Mine, Botswana. In: Kornprobst, J. (Ed.) Developments in Petrology, Volume 11, Issue 2. Kimberlites II: the mantle and crust–mantle relationships. Elsevier, Amsterdam, 3–9. https://doi.org/10.1016/B978-0-444-42274-3.50007-X
Show in context Silicate inclusions in diamond are typically classified into three paragenetic groups: peridotitic (e.g., Cr-rich pyrope, diopside, enstatite, olivine), eclogitic (e.g., Cr-poor pyrope-almandine and omphacite), and websteritic (intermediate compositions; Gurney et al., 1984).
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Harder, M., Nowicki, T.E., Hetman, C.M., Freeman, L., Abedu, B. (2013) Geology and evaluation of the K2 kimberlite, Koidu Mine, Sierra Leone. In: Pearson, D. et al. (Eds.) Proceedings of the 10th International Kimberlite Conference. Springer, New Delhi, 191–208. https://doi.org/10.1007/978-81-322-1173-0_13
Show in context Although such lithologies are not represented among xenoliths from the Koidu kimberlites, indicator mineral concentrates include both eclogitic and peridotitic compositions, with garnets characterised by high Cr and low Ca contents (Harder et al., 2013).
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Hills, D.V., Haggerty, S.E. (1989) Petrochemistry of eclogites from the Koidu kimberlite complex, Sierra Leone. Contributions to Mineralogy and Petrology 103, 397–422. https://doi.org/10.1007/BF01041749
Show in context The Koidu kimberlite complex is unusual in that mantle xenoliths recovered from the pipes are exclusively eclogitic (Hills and Haggerty, 1989).
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Howarth, G.H., Shaw Kahle, B., Janney, P.E., Gurney, J.J. (2025) Diamond–silicate–sulphide–oxide textural relationships in diamondiferous eclogites from the Kalahari craton revealed by X-ray computed tomography. Journal of Petrology 66, egaf062. https://doi.org/10.1093/petrology/egaf062
Show in context Although the DEW models have no spatial dimension, the cartoons illustrate processes observed in diamondiferous mantle xenoliths (Howarth et al., 2025).
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For example, fluid-rock interaction can partially dissolve existing mantle minerals and precipitate new phases during diamond growth (e.g., Walter et al., 2011; Aulbach et al., 2013; Thomson et al., 2014; Huang and Sverjensky, 2020; Mikhail et al., 2021; Pasqualetto et al., 2022; Rinaldi et al., 2023; Bruno et al., 2024; Howarth et al., 2025).
View in article
Huang, F., Sverjensky, D.A. (2019) Extended Deep Earth Water Model for predicting major element mantle metasomatism. Geochimica et Cosmochimica Acta 254, 192–230. https://doi.org/10.1016/j.gca.2019.03.027
Show in context We use predictive reaction path models of irreversible reactions between rocks and diamond forming aqueous fluids (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023) with thermodynamic data from the DEW model (Huang and Sverjensky, 2019; Sverjensky, 2019) as a process based framework to explore the possibility that mixed paragenesis inclusions can form during a single episode of diamond growth via fluid-rock metasomatism.
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DEW extends the EQ3/EQ6 geochemical modelling framework for aqueous speciation and irreversible mass transfer reactions to mantle pressures and temperatures (Huang and Sverjensky, 2019).
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Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
Show in context We use predictive reaction path models of irreversible reactions between rocks and diamond forming aqueous fluids (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023) with thermodynamic data from the DEW model (Huang and Sverjensky, 2019; Sverjensky, 2019) as a process based framework to explore the possibility that mixed paragenesis inclusions can form during a single episode of diamond growth via fluid-rock metasomatism.
View in article
The thermodynamic data set is calibrated using experimentally determined solubilities at upper mantle pressures and temperatures (e.g., Kessel et al., 2015) and has previously been applied to model diamond forming fluids and metasomatic reactions in the lithospheric mantle (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023).
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The initial fluid compositions were calculated from equilibrium between water and eclogite or water and carbonated dunite assemblages following Huang and Sverjensky (2020).
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Mineral solid solutions were represented using ideal site mixing between pyrope, almandine, and grossular in garnet, and non-ideal mixing between diopside, hedenbergite, and clinoenstatite in clinopyroxene following Huang and Sverjensky (2020).
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Model II simulates a more complex scenario involving simultaneous fluid mixing and reaction with the host rock, similar to the model of Huang and Sverjensky (2020) for fluid-rich diamonds from the Panda kimberlite.
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For example, fluid-rock interaction can partially dissolve existing mantle minerals and precipitate new phases during diamond growth (e.g., Walter et al., 2011; Aulbach et al., 2013; Thomson et al., 2014; Huang and Sverjensky, 2020; Mikhail et al., 2021; Pasqualetto et al., 2022; Rinaldi et al., 2023; Bruno et al., 2024; Howarth et al., 2025).
View in article
The simulations of Rinaldi et al. (2023) represent reactions between fluids of different initial compositions (eclogitic, silicic, transitional silicic-carbonatitic, and carbonatitic) and mantle lithologies including carbonated peridotite (Models 151 and 152 of Rinaldi et al., 2023) and eclogite or websterite (Models 154 and 158 of Rinaldi et al., 2023), and the model of Huang and Sverjensky (2020) describes the evolution of saline fluids as they mix and react with highly Mg-rich carbonatitic compositions (Fig. 3; Table S-1).
View in article
Dashed curves and arrows show paths from Rinaldi et al. (2023), and Huang and Sverjensky (2020).
View in article
The model of Huang and Sverjensky (2020) describes the evolution of saline fluids as they mix and react with highly Mg-rich carbonatitic compositions.
View in article
Ireland, T.R., Rudnick, R.L., Spetsius, Z. (1994) Trace elements in diamond inclusions from eclogites reveal link to Archean granites. Earth and Planetary Science Letters 128, 199–213. https://doi.org/10.1016/0012-821X(94)90145-7
Show in context The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008; Stachel and Luth, 2015), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994; Taylor et al., 1996; Anand et al., 2004; Misra et al., 2004; Stepanov et al., 2008; Liu et al., 2009; Smart et al., 2012; Aulbach and Stachel, 2022).
View in article
Kessel, R., Pettke, T., Fumagalli, P. (2015) Melting of metasomatized peridotite at 4–6 GPa and up to 1200 °C: An experimental approach. Contributions to Mineralogy and Petrology 169, 37. https://doi.org/10.1007/s00410-015-1132-9
Show in context The thermodynamic data set is calibrated using experimentally determined solubilities at upper mantle pressures and temperatures (e.g., Kessel et al., 2015) and has previously been applied to model diamond forming fluids and metasomatic reactions in the lithospheric mantle (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023).
View in article
Lai, M.Y., Stachel, T., Stern, R.A., Hardman, M.F., Pearson, D.G., Harris, J.W. (2022) Formation of mixed paragenesis diamonds during multistage growth: Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds. Geochimica et Cosmochimica Acta 323, 20–39. https://doi.org/10.1016/j.gca.2022.02.020
Show in context Rarely, diamonds contain disequilibrium assemblages termed mixed paragenesis, in which minerals from different paragenetic groups occur within a single crystal (Prinz et al., 1975; Mikhail et al., 2019; Lai et al., 2022).
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For example, eclogitic minerals may occur in the core of a diamond (e.g., omphacite, eclogitic garnet, or coesite), whereas peridotitic minerals occur near the rim (e.g., forsteritic olivine or Mg chromite) (Lai et al., 2022).
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Mixed paragenesis diamonds have been suggested to record multiple episodes of diamond growth in different host rocks which requires physical movement of the diamond between different mantle environments (e.g., Wang, 1998; Lai et al., 2022).
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In contrast, our results show that, despite comments to the contrary (Lai et al., 2022), the mixed parageneses in a single diamond may form from the progressive trapping of minerals formed during metasomatic reactions (e.g., Mikhail et al., 2021).
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To place the model results in a natural context, we compare them with mixed paragenesis diamonds from the Koidu kimberlite complex, Sierra Leone (Lai et al., 2022).
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Chromium is not included in our DEW models, and therefore Mg chromite is not predicted by these models. However, the Mg chromite reported by Lai et al. (2022) is an Mg-Fe-Al chromite ((Mg,Fe)O(Cr,Al,Fe)2O3), although the present simulations do not explicitly reproduce Mg chromite because chromium is not included within the current modelling framework.
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Nevertheless, diamonds from Koidu contain inclusions with clearly peridotitic compositions, including olivine and Mg-Fe-Al chromite (Lai et al., 2022).
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Liu, Y., Taylor, L.A., Sarbadhikari, A.B., Valley, J.W., Ushikubo, T., Spicuzza, M., Kita, N., Sobolev, N.V. (2009) Metasomatic origin of diamonds in the world’s largest diamondiferous eclogite. Lithos 112, 1014–1024. https://doi.org/10.1016/j.lithos.2009.06.036
Show in context The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008; Stachel and Luth, 2015), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994; Taylor et al., 1996; Anand et al., 2004; Misra et al., 2004; Stepanov et al., 2008; Liu et al., 2009; Smart et al., 2012; Aulbach and Stachel, 2022).
View in article
Meyer, H.O.A., Boyd, F.R. (1972) Composition and origin of crystalline inclusions in natural diamonds. Geochimica et Cosmochimica Acta 36, 1255–1273. https://doi.org/10.1016/0016-7037(72)90048-8
Show in context This mismatch may reflect [1] preferential inclusion of minerals associated with lower surface energies (Meyer and Boyd, 1972), [2] a syngenetic process where sulphides, garnet, and Mg chromite may precipitate or recrystallise more readily during diamond growth driven by fluid (Mikhail et al., 2019; 2021; Rinaldi et al., 2023) and/or melt metasomatism (Pintér et al., 2022), or [3] a combination of both.
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Mikhail, S., McCubbin, F.M., Jenner, F.E., Shirey, S.B., Rumble, D., Bowden, R. (2019) Diamondites: Evidence for a distinct tectono-thermal diamond-forming event beneath the Kaapvaal craton. Contributions to Mineralogy and Petrology 174, 71. https://doi.org/10.1007/s00410-019-1608-0
Show in context This mismatch may reflect [1] preferential inclusion of minerals associated with lower surface energies (Meyer and Boyd, 1972), [2] a syngenetic process where sulphides, garnet, and Mg chromite may precipitate or recrystallise more readily during diamond growth driven by fluid (Mikhail et al., 2019; 2021; Rinaldi et al., 2023) and/or melt metasomatism (Pintér et al., 2022), or [3] a combination of both.
View in article
Rarely, diamonds contain disequilibrium assemblages termed mixed paragenesis, in which minerals from different paragenetic groups occur within a single crystal (Prinz et al., 1975; Mikhail et al., 2019; Lai et al., 2022).
View in article
Mikhail, S., Rinaldi, M., Mare, E.R., Sverjensky, D.A. (2021) A genetic metasomatic link between eclogitic and peridotitic diamond inclusions. Geochemical Perspectives Letters 17, 33–38. https://doi.org/10.7185/geochemlet.2111
Show in context This mismatch may reflect [1] preferential inclusion of minerals associated with lower surface energies (Meyer and Boyd, 1972), [2] a syngenetic process where sulphides, garnet, and Mg chromite may precipitate or recrystallise more readily during diamond growth driven by fluid (Mikhail et al., 2019; 2021; Rinaldi et al., 2023) and/or melt metasomatism (Pintér et al., 2022), or [3] a combination of both.
View in article
In contrast, our results show that, despite comments to the contrary (Lai et al., 2022), the mixed parageneses in a single diamond may form from the progressive trapping of minerals formed during metasomatic reactions (e.g., Mikhail et al., 2021).
View in article
We use predictive reaction path models of irreversible reactions between rocks and diamond forming aqueous fluids (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023) with thermodynamic data from the DEW model (Huang and Sverjensky, 2019; Sverjensky, 2019) as a process based framework to explore the possibility that mixed paragenesis inclusions can form during a single episode of diamond growth via fluid-rock metasomatism.
View in article
The thermodynamic data set is calibrated using experimentally determined solubilities at upper mantle pressures and temperatures (e.g., Kessel et al., 2015) and has previously been applied to model diamond forming fluids and metasomatic reactions in the lithospheric mantle (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023).
View in article
For example, fluid-rock interaction can partially dissolve existing mantle minerals and precipitate new phases during diamond growth (e.g., Walter et al., 2011; Aulbach et al., 2013; Thomson et al., 2014; Huang and Sverjensky, 2020; Mikhail et al., 2021; Pasqualetto et al., 2022; Rinaldi et al., 2023; Bruno et al., 2024; Howarth et al., 2025).
View in article
Irreversible reaction path models using the DEW model thermodynamic data predict that metasomatism should result in the precipitation of silicates which mirror those observed as diamond inclusions (Mikhail et al., 2021; Rinaldi et al., 2023; this study).
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Misra, K.C., Anand, M., Taylor, L.A., Sobolev, N.V. (2004) Multi-stage metasomatism of diamondiferous eclogite xenoliths from the Udachnaya kimberlite pipe, Yakutia, Siberia. Contributions to Mineralogy and Petrology 146, 696–714. https://doi.org/10.1007/s00410-003-0529-z.
Show in context The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008; Stachel and Luth, 2015), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994; Taylor et al., 1996; Anand et al., 2004; Misra et al., 2004; Stepanov et al., 2008; Liu et al., 2009; Smart et al., 2012; Aulbach and Stachel, 2022).
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Pasqualetto, L., Nestola, F., Jacob, D.E., Pamato, M.G., Oliveira, B., Perritt, S., Chinn, I., Nimis, P., Milani, S., Harris, J.W. (2022) Protogenetic clinopyroxene inclusions in diamond and Nd diffusion modeling: Implications for diamond dating. Geology 50, 1038–1042. https://doi.org/10.1130/G50273.1
Show in context For example, fluid-rock interaction can partially dissolve existing mantle minerals and precipitate new phases during diamond growth (e.g., Walter et al., 2011; Aulbach et al., 2013; Thomson et al., 2014; Huang and Sverjensky, 2020; Mikhail et al., 2021; Pasqualetto et al., 2022; Rinaldi et al., 2023; Bruno et al., 2024; Howarth et al., 2025).
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Pintér, Z., Foley, S.F., Yaxley, G.M. (2022) Diamonds, dunites, and metasomatic rocks formed by melt/rock reaction in craton roots. Communications Earth & Environment 3, 263. https://doi.org/10.1038/s43247-022-00630-3
Show in context This mismatch may reflect [1] preferential inclusion of minerals associated with lower surface energies (Meyer and Boyd, 1972), [2] a syngenetic process where sulphides, garnet, and Mg chromite may precipitate or recrystallise more readily during diamond growth driven by fluid (Mikhail et al., 2019; 2021; Rinaldi et al., 2023) and/or melt metasomatism (Pintér et al., 2022), or [3] a combination of both.
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Prinz, M., Vincent, M.D., Hlava, P.F., Keil, K. (1975) Inclusions in diamonds: Garnet lherzolite and eclogite assemblages. Physics and Chemistry of the Earth 9, 797–815. https://doi.org/10.1016/0079-1946(75)90052-X
Show in context Rarely, diamonds contain disequilibrium assemblages termed mixed paragenesis, in which minerals from different paragenetic groups occur within a single crystal (Prinz et al., 1975; Mikhail et al., 2019; Lai et al., 2022).
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Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
Show in context This mismatch may reflect [1] preferential inclusion of minerals associated with lower surface energies (Meyer and Boyd, 1972), [2] a syngenetic process where sulphides, garnet, and Mg chromite may precipitate or recrystallise more readily during diamond growth driven by fluid (Mikhail et al., 2019; 2021; Rinaldi et al., 2023) and/or melt metasomatism (Pintér et al., 2022), or [3] a combination of both.
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We use predictive reaction path models of irreversible reactions between rocks and diamond forming aqueous fluids (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023) with thermodynamic data from the DEW model (Huang and Sverjensky, 2019; Sverjensky, 2019) as a process based framework to explore the possibility that mixed paragenesis inclusions can form during a single episode of diamond growth via fluid-rock metasomatism.
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The thermodynamic data set is calibrated using experimentally determined solubilities at upper mantle pressures and temperatures (e.g., Kessel et al., 2015) and has previously been applied to model diamond forming fluids and metasomatic reactions in the lithospheric mantle (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023).
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This simulation models the infiltration of an externally derived fluid into a dry mantle rock, following an approach similar to that of Rinaldi et al. (2023).
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Our models did not produce coesite. However, variants of Model II involving higher initial coesite abundances could plausibly permit preservation and encapsulation of coesite during diamond growth, consistent with previous studies (Rinaldi et al., 2023).
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For example, fluid-rock interaction can partially dissolve existing mantle minerals and precipitate new phases during diamond growth (e.g., Walter et al., 2011; Aulbach et al., 2013; Thomson et al., 2014; Huang and Sverjensky, 2020; Mikhail et al., 2021; Pasqualetto et al., 2022; Rinaldi et al., 2023; Bruno et al., 2024; Howarth et al., 2025).
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Irreversible reaction path models using the DEW model thermodynamic data predict that metasomatism should result in the precipitation of silicates which mirror those observed as diamond inclusions (Mikhail et al., 2021; Rinaldi et al., 2023; this study).
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The simulations of Rinaldi et al. (2023) represent reactions between fluids of different initial compositions (eclogitic, silicic, transitional silicic-carbonatitic, and carbonatitic) and mantle lithologies including carbonated peridotite (Models 151 and 152 of Rinaldi et al., 2023) and eclogite or websterite (Models 154 and 158 of Rinaldi et al., 2023), and the model of Huang and Sverjensky (2020) describes the evolution of saline fluids as they mix and react with highly Mg-rich carbonatitic compositions (Fig. 3; Table S-1).
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Dashed curves and arrows show paths from Rinaldi et al. (2023), and Huang and Sverjensky (2020).
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Models I and II trajectories reproduce the transition from silicic to carbonatitic compositions and overlap natural HDF arrays. The simulations of Rinaldi et al. (2023) represent reactions between fluids of different initial compositions (eclogitic, silicic, transitional silicic-carbonatitic, and carbonatitic) and mantle lithologies including carbonated peridotite (Models 151 and 152) and eclogite or websterite (Models 154 and 158) and overlap numerous natural fluid inclusion trends.
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Smart, K.A., Chacko, T., Stachel, T., Tappe, S., Stern, R.A., Ickert, R.B. (2012) Eclogite formation beneath the northern Slave craton constrained by diamond inclusions. Earth and Planetary Science Letters 319–320, 165–177. https://doi.org/10.1016/j.epsl.2011.12.032
Show in context The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008; Stachel and Luth, 2015), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994; Taylor et al., 1996; Anand et al., 2004; Misra et al., 2004; Stepanov et al., 2008; Liu et al., 2009; Smart et al., 2012; Aulbach and Stachel, 2022).
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Stachel, T., Harris, J.W. (2008) The origin of cratonic diamonds: Constraints from mineral inclusions. Ore Geology Reviews 34, 5–32. https://doi.org/10.1016/j.oregeorev.2007.05.002
Show in context Mineral inclusions in diamonds provide constraints on the geological environment of diamond formation. However, the relative abundance of minerals encapsulated in diamonds does not reflect the modal mineralogy of Earth’s mantle (Stachel and Harris, 2008).
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Simulations were performed at 5 GPa, 1000 °C and logfO2 = -2 ΔFMQ, conditions consistent with lithospheric diamond formation (Stachel and Harris, 2008).
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The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008; Stachel and Luth, 2015), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994; Taylor et al., 1996; Anand et al., 2004; Misra et al., 2004; Stepanov et al., 2008; Liu et al., 2009; Smart et al., 2012; Aulbach and Stachel, 2022).
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Stachel, T., Luth, R.W. (2015) Diamond formation: Where, when and how? Lithos 220–223, 200–220. https://doi.org/10.1016/j.lithos.2015.01.028
Show in context The chosen temperature of 1000 °C lies at the lower end of the estimated diamond inclusion entrapment temperature range (1155 ± 105 °C; Stachel and Luth, 2015). Two model scenarios were investigated, termed Model I and Model II, as described below.
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The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008; Stachel and Luth, 2015), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994; Taylor et al., 1996; Anand et al., 2004; Misra et al., 2004; Stepanov et al., 2008; Liu et al., 2009; Smart et al., 2012; Aulbach and Stachel, 2022).
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Steele-MacInnis, M. (2025) Physico-chemical properties of hydrothermal fluids. Treatise on Geochemistry. Third Edition, Elsevier, Oxford, 869–909. https://doi.org/10.1016/B978-0-323-99762-1.00089-9
Show in context The assumption of localised pore or grain boundary fluids in eclogitic substrates is motivated by the broader evidence for fluid mediated metasomatism in mantle lithologies, including the transport of incompatible elements, carbonate species, and volatile-rich metasomatic agents through the lithospheric mantle (see Steele-MacInnis, 2025 for a review).
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Stepanov, A.S., Shatsky, V.S., Zedgenizov, D.A., Ragozin, A.L. (2008) Chemical Heterogeneity in the Diamondiferous Eclogite Xenolith from the Udachnaya Kimberlite Pipe. Doklady Earth Sciences 419, 308–311.
Show in context The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008; Stachel and Luth, 2015), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994; Taylor et al., 1996; Anand et al., 2004; Misra et al., 2004; Stepanov et al., 2008; Liu et al., 2009; Smart et al., 2012; Aulbach and Stachel, 2022).
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Sverjensky, D.A. (2019) Thermodynamic modelling of fluids from surficial to mantle conditions. Journal of the Geological Society 176, 348–374. https://doi.org/10.1144/jgs2018-105
Show in context We use predictive reaction path models of irreversible reactions between rocks and diamond forming aqueous fluids (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023) with thermodynamic data from the DEW model (Huang and Sverjensky, 2019; Sverjensky, 2019) as a process based framework to explore the possibility that mixed paragenesis inclusions can form during a single episode of diamond growth via fluid-rock metasomatism.
View in article
Sverjensky, D.A., Huang, F. (2015) Diamond formation due to a pH drop during fluid–rock interactions. Nature Communications 6, 8702. https://doi.org/10.1038/ncomms9702
Show in context We use predictive reaction path models of irreversible reactions between rocks and diamond forming aqueous fluids (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023) with thermodynamic data from the DEW model (Huang and Sverjensky, 2019; Sverjensky, 2019) as a process based framework to explore the possibility that mixed paragenesis inclusions can form during a single episode of diamond growth via fluid-rock metasomatism.
View in article
The thermodynamic data set is calibrated using experimentally determined solubilities at upper mantle pressures and temperatures (e.g., Kessel et al., 2015) and has previously been applied to model diamond forming fluids and metasomatic reactions in the lithospheric mantle (Sverjensky and Huang, 2015; Huang and Sverjensky, 2020; Mikhail et al., 2021; Rinaldi et al., 2023).
View in article
Taylor, L.A., Snyder, G.A., Crozaz, G., Sobolev, V.N., Yefimova, E.S., Sobolev, N.V. (1996) Eclogitic inclusions in diamonds: Evidence of complex mantle processes over time. Earth and Planetary Science Letters 142, 535–551. https://doi.org/10.1016/0012-821X(96)00106-9
Show in context The dissolution and precipitation of phases explains why the modal abundance of silicate and oxide inclusions in diamond do not reflect the mineralogy of the mantle (Stachel and Harris, 2008; Stachel and Luth, 2015), and why the major element geochemistry of many garnet and clinopyroxene diamond inclusions do not match the major element geochemistry of the garnets or clinopyroxenes of their host rocks (Ireland et al., 1994; Taylor et al., 1996; Anand et al., 2004; Misra et al., 2004; Stepanov et al., 2008; Liu et al., 2009; Smart et al., 2012; Aulbach and Stachel, 2022).
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Thomson, A.R., Kohn, S.C., Bulanova, G.P., Smith, C.B., Araujo, D., Walter, M.J. (2014) Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): Constraints from carbon isotopes and inclusion compositions. Contributions to Mineralogy and Petrology 168, 1081. https://doi.org/10.1007/s00410-014-1081-8
Show in context For example, fluid-rock interaction can partially dissolve existing mantle minerals and precipitate new phases during diamond growth (e.g., Walter et al., 2011; Aulbach et al., 2013; Thomson et al., 2014; Huang and Sverjensky, 2020; Mikhail et al., 2021; Pasqualetto et al., 2022; Rinaldi et al., 2023; Bruno et al., 2024; Howarth et al., 2025).
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Walter, M.J., Kohn, S.C., Araujo, D., Bulanova, G.P., Smith, C.B., Gaillou, E., Wang, J., Steele, A., Shirey, S.B. (2011) Deep mantle cycling of oceanic crust: Evidence from diamonds and their mineral inclusions. Science 334, 54–57. https://doi.org/10.1126/science.1209300
Show in context For example, fluid-rock interaction can partially dissolve existing mantle minerals and precipitate new phases during diamond growth (e.g., Walter et al., 2011; Aulbach et al., 2013; Thomson et al., 2014; Huang and Sverjensky, 2020; Mikhail et al., 2021; Pasqualetto et al., 2022; Rinaldi et al., 2023; Bruno et al., 2024; Howarth et al., 2025).
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Wang, W. (1998) Formation of diamond with mineral inclusions of mixed eclogite and peridotite paragenesis. Earth and Planetary Science Letters 160, 831–843. https://doi.org/10.1016/S0012-821X(98)00131-9
Show in context Mixed paragenesis diamonds have been suggested to record multiple episodes of diamond growth in different host rocks which requires physical movement of the diamond between different mantle environments (e.g., Wang, 1998; Lai et al., 2022).
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Weiss, Y., Czas, J., Navon, O. (2022) Fluid inclusions in fibrous diamonds. Reviews in Mineralogy and Geochemistry 88, 475–532. https://doi.org/10.2138/rmg.2022.88.09
Show in context Taken together, these trajectories follow the same general compositional gradients defined by natural HDF arrays (Weiss et al., 2022), thus indicating that the diversity of diamond forming fluids can arise from progressive metasomatic reactions akin to those predicted by DEW simulations (Figs. 1–3).
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Ternary diagram comparing predicted model fluid compositions (curves and arrows) with high density fluid (HDF) compositions in fibrous diamonds (coloured fields from Weiss et al., 2022).
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Supplementary Information
The Supplementary Information includes:
- Table S-1
- Supplementary Information References
Download the Supplementary Information (PDF)
Download Table S-1 (xlsx)
Figures
Figure 1 Predicted mineral precipitation during reaction progress in two DEW fluid-rock interaction models. Log mineral abundance (mol) is plotted against log reaction progress (ξ), where ξ represents the extent of fluid-rock interaction simulated by irreversible mass transfer in EQ6. (a) Model I predicts progressive garnet growth (Py 0.46–0.83), followed by omphacite (Jd ∼0.67), diamond precipitation, and late olivine (Mg# ∼0.95), with minor orthopyroxene and kyanite at advanced reaction progress. (b) Model II shows a simpler sequence dominated by garnet and omphacite formation followed by diamond precipitation. Values in parentheses indicate representative mineral compositions predicted by the models. The simulations illustrate mineral assemblages produced during mantle fluid-rock interaction capable of forming diamond bearing eclogitic-peridotitic assemblages.
Figure 2 Conceptual illustrations of metasomatic fluid-rock interaction represented by Models I and II (top and bottom). Fluids infiltrate mantle rocks along veins and react with the surrounding lithologies, precipitating minerals and diamond along the reaction path. Although the DEW models have no spatial dimension, the cartoons illustrate processes observed in diamondiferous mantle xenoliths (Howarth et al., 2025
Howarth, G.H., Shaw Kahle, B., Janney, P.E., Gurney, J.J. (2025) Diamond–silicate–sulphide–oxide textural relationships in diamondiferous eclogites from the Kalahari craton revealed by X-ray computed tomography. Journal of Petrology 66, egaf062. https://doi.org/10.1093/petrology/egaf062
). Progressive diffusive equilibration may remove the original metasomatic veins, leaving inclusion-bearing and inclusion-free diamonds preserved within the mantle matrix.
Figure 3 Ternary diagram comparing predicted model fluid compositions (curves and arrows) with high density fluid (HDF) compositions in fibrous diamonds (coloured fields from Weiss et al., 2022
Weiss, Y., Czas, J., Navon, O. (2022) Fluid inclusions in fibrous diamonds. Reviews in Mineralogy and Geochemistry 88, 475–532. https://doi.org/10.2138/rmg.2022.88.09
). Solid curves and arrows show fluid evolution paths predicted by DEW models from this study (Models I and II). Dashed curves and arrows show paths from Rinaldi et al. (2023)Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
, and Huang and Sverjensky (2020)Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
. Models I and II trajectories reproduce the transition from silicic to carbonatitic compositions and overlap natural HDF arrays. The simulations of Rinaldi et al. (2023)Rinaldi, M., Mikhail, S., Sverjensky, D.A., Kalita, J. (2023) The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta 356, 105–115. https://doi.org/10.1016/j.gca.2023.06.025
represent reactions between fluids of different initial compositions (eclogitic, silicic, transitional silicic-carbonatitic, and carbonatitic) and mantle lithologies including carbonated peridotite (Models 151 and 152) and eclogite or websterite (Models 154 and 158) and overlap numerous natural fluid inclusion trends. The model of Huang and Sverjensky (2020)Huang, F., Sverjensky, D.A. (2020) Mixing of carbonatitic into saline fluid during Panda diamond formation. Geochimica et Cosmochimica Acta 284, 1–20. https://doi.org/10.1016/j.gca.2020.06.011
describes the evolution of saline fluids as they mix and react with highly Mg-rich carbonatitic compositions.