Contrasting oxygen isotopes in garnet from diamondiferous and barren eclogitic parageneses
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Keywords: oxygen isotopes, subduction, mantle, eclogites, diamonds
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Abstract
Figures
 Figure 1 δ18O histograms for eclogitic garnet in xenoliths and DIs. (a) Comparison of our data with Kaapvaal non-diamondiferous eclogites. (b) Comparison of global data for garnet/majorite DIs with garnets from diamondiferous eclogites worldwide (references are listed in the Supplementary Information). Lines are kernel-smoothed distribution curves. |  Figure 2 Comparison of δ18O in eclogitic minerals for barren (n = 183) and diamondiferous (n = 52) parageneses worldwide (references are given in the Supplementary Information) with a superimposed histogram for δ18O in the Cullinan DIs (this study). |  Figure 3 Comparison of δ18O of eclogitic garnets/majorites and δ13C of the host diamond worldwide (ESM1) with δ18O of Cullinan diamond inclusions. Inclusions with δ13C for studied Cullinan diamonds (Korolev et al., 2018a) are plotted as symbols, δ18O of eclogitic garnets with no information on the host diamond δ13C are shown as the green histogram. The blue hexagon marks the initial magnesite reactant. A blue arrow connects δ18O of the magnesite reactant with the Grt product for modelled combined metasomatic reactions (Reactions 1 and 8 in Table S-4); it is placed at an average mantle value of −6 ‰ for δ13C. The blue field corresponds to δ13C in sedimentary carbonates, the yellow field represents mantle carbon, and the pink field is for organic carbon. |  Figure 4 The observed δ18O contrast between barren and diamondiferous eclogites with superimposed modelled changes of δ18O composition of eclogitic garnets produced in metasomatic reactions (orange field; Table S-4). The initial δ18O in eclogitic garnet (vertical line at +6 ‰) is chosen arbitrarily (see explanations in the text). A detailed description of the geochemical modelling is provided in the Supplementary Information. |
| Figure 1 | Figure 2 | Figure 3 | Figure 4 |
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Introduction
Eclogite, a high grade garnet (Grt)-clinopyroxene (Cpx) rock metamorphosed from the mafic crust, is the most diamondiferous mantle lithology. Diamond concentrations in mantle eclogite can be orders of magnitude higher than the concentration of diamonds in kimberlite – the rock from which they are mined. Over the past 40 years, the oxygen isotope composition (δ18O) of eclogite has become one of the most powerful indicators of its crustal origin in the cratonic mantle (Schulze et al., 2003
Schulze, D.J., Harte, B., Valley, J.W., Brenan, J.M., Channer, D.M.D.R. (2003) Extreme crustal oxygen isotope signatures preserved in coesite in diamond. Nature 423, 68–70. https://doi.org/10.1038/nature01615
), together with stable isotopes of C, N and S, and radiogenic isotopes (Pearson et al., 2003Pearson, D.G., Canil, D., Shirey, S.B. (2003) 2.05 - Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. First Edition, Elsevier, Amsterdam, 171–275. https://doi.org/10.1016/B0-08-043751-6/02005-3
; Jacob, 2004Jacob, D.E. (2004) Nature and origin of eclogite xenoliths from kimberlites. Lithos 77, 295–316. https://doi.org/10.1016/j.lithos.2004.03.038
). Diamond growth, however, is envisioned as a process overprinting the recycled shallow eclogite protolith. Crustal protoliths for the eclogite do not necessarily imply crustal sources for its diamonds, which could inherit shallow C and O, or could be introduced to the eclogite from mantle fluids. A knowledge of diamond formation in eclogites is critically important to unravel the carbon cycle and deep mantle processes. Diamond formation is considered to be partly metasomatic, as suggested by diamond distribution in eclogites (Taylor and Anand, 2004Taylor, L.A., Anand, M. (2004) Diamonds: time capsules from the Siberian Mantle. Geochemistry 64, 1–74. https://doi.org/10.1016/j.chemer.2003.11.006
), δ13C core-to-rim patterns (Smart et al., 2011Smart, K.A., Chacko, T., Stachel, T., Muehlenbachs, K., Stern, R.A., Heaman, L.M. (2011) Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δ13C–N study of eclogite-hosted diamonds from the Jericho kimberlite. Geochimica et Cosmochimica Acta 75, 6027–6047. https://doi.org/10.1016/j.gca.2011.07.028
) and correlations of O isotopes with trace element indicators of metasomatism (Gréau et al., 2011Gréau, Y., Huang, J.-X., Griffin, W.L., Renac, C., Alard, O., O’Reilly, S.Y. (2011) Type I eclogites from Roberts Victor kimberlites: Products of extensive mantle metasomatism. Geochimica et Cosmochimica Acta 75, 6927–6954. https://doi.org/10.1016/j.gca.2011.08.035
; Huang et al., 2012Huang, J.-X., Gréau, Y., Griffin, W.L., O’Reilly, S.Y., Pearson, N.J. (2012) Multi-stage origin of Roberts Victor eclogites: Progressive metasomatism and its isotopic effects. Lithos 142–143, 161–181. https://doi.org/10.1016/j.lithos.2012.03.002
). Diamond precipitates from mantle C-bearing fluids percolating upward and experiencing Raleigh fractionation (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
; Riches et al., 2016Riches, A.J.V., Ickert, R.B., Pearson, D.G., Stern, R.A., Jackson, S.E., Ishikawa, A., Kjarsgaard, B.A., Gurney, J.J. (2016) In situ oxygen-isotope, major-, and trace-element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from Roberts Victor, Kaapvaal Craton. Geochimica et Cosmochimica Acta 174, 345–359. https://doi.org/10.1016/j.gca.2015.11.028
). Possible effects of metasomatic diamond formation on δ18O of eclogitic minerals may be especially notable for diamondiferous parageneses. Our goal is to quantify these δ18O to separate out the signatures of shallower crustal alteration from the changes introduced from deeper-seated diamondiferous fluids.It has been noticed that garnet and clinopyroxene in diamondiferous eclogites are higher in δ18O than their respective phases in barren eclogites (Pearson et al., 2003
Pearson, D.G., Canil, D., Shirey, S.B. (2003) 2.05 - Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. First Edition, Elsevier, Amsterdam, 171–275. https://doi.org/10.1016/B0-08-043751-6/02005-3
). The difference was explained by the origin of garnet in diamondiferous assemblages from the shallow, more altered part of the oceanic crust where δ18O is higher (McCulloch et al., 1981McCulloch, M.T., Gregory, R.T., Wasserburg, G.J., Taylor Jr., H.P. (1981) Sm-Nd, Rb-Sr, and 18O/16O isotopic systematics in an oceanic crustal section: Evidence from the Samail ophiolite. Journal of Geophysical Research: Solid Earth 86, 2721–2735. https://doi.org/10.1029/JB086iB04p02721
; Alt et al., 1986Alt, J.C., Muehlenbachs, K., Honnorez, J. (1986) An oxygen isotopic profile through the upper kilometer of the oceanic crust, DSDP Hole 504B. Earth and Planetary Science Letters 80, 217–229. https://doi.org/10.1016/0012-821X(86)90106-8
; Ickert et al., 2013Ickert, R.B., Stachel, T., Stern, R.A., Harris, J.W. (2013) Diamond from recycled crustal carbon documented by coupled δ18O–δ13C measurements of diamonds and their inclusions. Earth and Planetary Science Letters 364, 85–97. https://doi.org/10.1016/j.epsl.2013.01.008
). This study aims to extend the comparison to eclogitic inclusions in diamonds and make it more statistically robust. In the last 20 years, advances in measurements of O isotopes and new kimberlite discoveries created an abundance of new data. A new summary on δ18O in garnet from diamondiferous and barren parageneses is long overdue. Here we confirm the distinction between δ18O of garnet equilibrated and unequilibrated with diamond and assess how much of this distinction can be assigned to diamond-friendly metasomatism.top
Samples, Methods and Results
We studied diamond inclusions (DIs) from Cullinan Mine (Premier kimberlite) individual raw diamonds. The inclusions are associated with mafic eclogitic and majorite-bearing sublithospheric parageneses. They are derived from a wide interval of temperatures (T) and pressures (P) of 5.5–7.5 GPa from the lithosphere and 10.5–13.5 GPa from the sublithospheric mantle (Korolev et al., 2018a
Korolev, N., Kopylova, M., Gurney, J.J., Moore, A.E., Davidson, J. (2018a) The origin of Type II diamonds as inferred from Cullinan mineral inclusions. Mineralogy and Petrology 112, 275–289. https://doi.org/10.1007/s00710-018-0601-z
). Here we report major element, δ18O, and P-T data for 42 non-touching Grt-Cpx pairs and 8 majorites (Supplementary Information; Table S-5). Analysed δ18O composition of garnet ranges from +5.4 to +10.2 ‰ and covers the oxygen isotope composition of majorites worldwide (+6.0 to +9.4 ‰; Burnham et al., 2015Burnham, A.D., Thomson, A.R., Bulanova, G.P., Kohn, S.C., Smith, C.B., Walter, M.J. (2015) Stable isotope evidence for crustal recycling as recorded by superdeep diamonds. Earth and Planetary Science Letters 432, 374–380. https://doi.org/10.1016/j.epsl.2015.10.023
; Ickert et al., 2015Ickert, R.B., Stachel, T., Stern, R.A., Harris, J.W. (2015) Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters 1, 65–74. https://doi.org/10.7185/geochemlet.1507
).This new dataset of O isotopes in eclogitic garnet DIs enables statistical comparison with global datasets. The most notable pattern is revealed by a comparison of δ18O in garnet/majorite associated with diamond (DIs and diamondiferous eclogites) and garnet in barren eclogites. Statistical t tests determine that the average δ18O and its distribution in garnet from diamondiferous eclogites are distinctly higher than the barren eclogites from the Kaapvaal craton with probability >99.99 % (Supplementary Information). The δ18O compositions of the garnet/majorite inclusions from the Cullinan diamonds are higher than the δ18O of garnet in Kaapvaal barren eclogites (Fig. 1a). While only 24 % of Cullinan DIs demonstrate δ18O < +6 ‰ and values <+5 ‰ are completely absent (Fig. 1a, Table S-5), 67.4 % of garnets from the barren eclogites have δ18O < +6.0 ‰. Diamondiferous eclogites globally show a narrow δ18O distribution with a higher mode than the Kaapvaal barren eclogites (Fig. 1b).
Figure 1 δ18O histograms for eclogitic garnet in xenoliths and DIs. (a) Comparison of our data with Kaapvaal non-diamondiferous eclogites. (b) Comparison of global data for garnet/majorite DIs with garnets from diamondiferous eclogites worldwide (references are listed in the Supplementary Information). Lines are kernel-smoothed distribution curves.
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Discussion
Several explanations may account for the contrasting δ18O in barren and diamondiferous eclogitic parageneses. The latter may have formed deeper (>150 km), at higher pressures and temperatures. A suggested positive covariation of δ18Ogrt with equilibration temperature for Lace eclogites may hint at a wider T-δ18Ogrt correlation in the deep mantle as heavy oxygen may favour garnet with increasing T and P (Aulbach et al., 2017
Aulbach, S., Woodland, A.B., Vasilyev, P., Galvez, M.E., Viljoen, K.S. (2017) Effects of low-pressure igneous processes and subduction on Fe3+/ΣFe and redox state of mantle eclogites from Lace (Kaapvaal craton). Earth and Planetary Science Letters 474, 283–295. https://doi.org/10.1016/j.epsl.2017.06.030
). To test for this, we compiled data for eclogite xenoliths of the Kaapvaal craton (Fig. S-3a) and worldwide occurrences (Fig. S-4). The absence of δ18O correlations of garnet DIs with the P-T of their formation observed in Cullinan (Fig. S-3) is repeated globally. A comparison of the δ18O in Grt-Cpx pairs from eclogite xenoliths worldwide equilibrated at the widest range of temperatures (650–1500 °C) shows that there is no dependence between δ18Ogrt or δ18Ocpx and temperature (Fig. S-4). The difference between δ18Ogrt and δ18Ocpx is constant (±0.87 ‰, 2σ) and does not correlate with temperature (Fig. S-4a). The contrasting δ18O compositions in barren and diamondiferous parageneses do not relate to pressure, which was predicted by Clayton et al. (1975)Clayton, R.N., Goldsmith, J.R., Karel, K.J., Mayeda, T.K., Newton, R.C. (1975) Limits on the effect of pressure on isotopic fractionation. Geochimica et Cosmochimica Acta 39, 1197–1201. https://doi.org/10.1016/0016-7037(75)90062-9
. Only a small proportion of Cullinan Mg-rich DIs demonstrate a local δ18O-T correlation (Supplementary Information; Fig. S-2b). Thus, higher pressures and temperatures of diamondiferous eclogites and DIs cannot account for the heavier oxygen in their garnets.An alternative explanation invokes diffusive buffering of oxygen by the surrounding mantle to explain the δ18O contrast between garnet in barren (xenoliths) and diamondiferous eclogitic parageneses (DIs and xenoliths). DI garnet is shielded from re-equilibration with the ambient mantle oxygen (δ18O = +5.5 ± 0.4 ‰; Mattey et al., 1994
Mattey, D., Lowry, D., Macpherson, C. (1994) Oxygen isotope composition of mantle peridotite. Earth and Planetary Science Letters 128, 231–241. https://doi.org/10.1016/0012-821X(94)90147-3
), while the “exposed” garnet in xenoliths is not. Only silicate inclusions protected by diamonds retained the 18O-enriched compositions (Schulze et al., 2003Schulze, D.J., Harte, B., Valley, J.W., Brenan, J.M., Channer, D.M.D.R. (2003) Extreme crustal oxygen isotope signatures preserved in coesite in diamond. Nature 423, 68–70. https://doi.org/10.1038/nature01615
; Burnham et al., 2015Burnham, A.D., Thomson, A.R., Bulanova, G.P., Kohn, S.C., Smith, C.B., Walter, M.J. (2015) Stable isotope evidence for crustal recycling as recorded by superdeep diamonds. Earth and Planetary Science Letters 432, 374–380. https://doi.org/10.1016/j.epsl.2015.10.023
; Ickert et al., 2015Ickert, R.B., Stachel, T., Stern, R.A., Harris, J.W. (2015) Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters 1, 65–74. https://doi.org/10.7185/geochemlet.1507
) formed via low temperature seawater alteration of the shallowest levels of the former oceanic crust (McCulloch et al., 1981McCulloch, M.T., Gregory, R.T., Wasserburg, G.J., Taylor Jr., H.P. (1981) Sm-Nd, Rb-Sr, and 18O/16O isotopic systematics in an oceanic crustal section: Evidence from the Samail ophiolite. Journal of Geophysical Research: Solid Earth 86, 2721–2735. https://doi.org/10.1029/JB086iB04p02721
; Alt et al., 1986Alt, J.C., Muehlenbachs, K., Honnorez, J. (1986) An oxygen isotopic profile through the upper kilometer of the oceanic crust, DSDP Hole 504B. Earth and Planetary Science Letters 80, 217–229. https://doi.org/10.1016/0012-821X(86)90106-8
). These diamonds and their mineral inclusions originated from carbon and oxygen derived from the sedimentary organic matter or altered oceanic crust (Li et al., 2019Li, K., Li, L., Pearson, D.G., Stachel, T. (2019) Diamond isotope compositions indicate altered igneous oceanic crust dominates deep carbon recycling. Earth and Planetary Science Letters 516, 190–201. https://doi.org/10.1016/j.epsl.2019.03.041
) subducted into the mantle, as evidenced by a correlation of heavy 18O in silicate DIs and light, low 13C/12C carbon (Ickert et al., 2015Ickert, R.B., Stachel, T., Stern, R.A., Harris, J.W. (2015) Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters 1, 65–74. https://doi.org/10.7185/geochemlet.1507
; Li et al., 2019Li, K., Li, L., Pearson, D.G., Stachel, T. (2019) Diamond isotope compositions indicate altered igneous oceanic crust dominates deep carbon recycling. Earth and Planetary Science Letters 516, 190–201. https://doi.org/10.1016/j.epsl.2019.03.041
). The extent of this “diamond shielding” effect can be evaluated by comparing δ18O histograms for garnet in DIs and diamondiferous eclogites. The δ18O mode for the DI garnet is between +7 and +8 ‰, 1 ‰ higher than the mode for the exposed garnet in diamondiferous eclogites (Fig. 1b).One cannot defer to the “diamond shielding” effect to explain the contrast between garnet compositions of diamondiferous and barren xenoliths. The latter show a mode at +5 to +6 ‰, at a lower δ18O than diamondiferous xenoliths, and an extended “tail” of the distribution towards 0 ‰ (Fig. 1a). A clear difference in δ18O was shown for both Cpx and Grt for barren and diamondiferous eclogites worldwide (Fig. 2). Traditionally, this difference would be explained as the contrast in δ18O of the eclogite protoliths is related to their depth position within the slab and the gradual decrease of δ18O with depth in the oceanic crust (McCulloch et al., 1981
McCulloch, M.T., Gregory, R.T., Wasserburg, G.J., Taylor Jr., H.P. (1981) Sm-Nd, Rb-Sr, and 18O/16O isotopic systematics in an oceanic crustal section: Evidence from the Samail ophiolite. Journal of Geophysical Research: Solid Earth 86, 2721–2735. https://doi.org/10.1029/JB086iB04p02721
; Alt et al., 1986Alt, J.C., Muehlenbachs, K., Honnorez, J. (1986) An oxygen isotopic profile through the upper kilometer of the oceanic crust, DSDP Hole 504B. Earth and Planetary Science Letters 80, 217–229. https://doi.org/10.1016/0012-821X(86)90106-8
). In this model, garnet in barren eclogites might have inherited the δ18O from deep gabbro layers of oceanic crust (δ18O = 0 to +5 ‰; Alt et al., 1986Alt, J.C., Muehlenbachs, K., Honnorez, J. (1986) An oxygen isotopic profile through the upper kilometer of the oceanic crust, DSDP Hole 504B. Earth and Planetary Science Letters 80, 217–229. https://doi.org/10.1016/0012-821X(86)90106-8
). Diamondiferous eclogites with higher δ18O, by contrast, may have recorded a higher input from altered oceanic basalts (δ18O = +7 to +15 ‰; McCulloch et al., 1981McCulloch, M.T., Gregory, R.T., Wasserburg, G.J., Taylor Jr., H.P. (1981) Sm-Nd, Rb-Sr, and 18O/16O isotopic systematics in an oceanic crustal section: Evidence from the Samail ophiolite. Journal of Geophysical Research: Solid Earth 86, 2721–2735. https://doi.org/10.1029/JB086iB04p02721
; Alt et al., 1986Alt, J.C., Muehlenbachs, K., Honnorez, J. (1986) An oxygen isotopic profile through the upper kilometer of the oceanic crust, DSDP Hole 504B. Earth and Planetary Science Letters 80, 217–229. https://doi.org/10.1016/0012-821X(86)90106-8
; Eiler, 2001Eiler, J.M. (2001) Oxygen Isotope Variations of Basaltic Lavas and Upper Mantle Rocks. Reviews in Mineralogy and Geochemistry 43, 319–364. https://doi.org/10.2138/gsrmg.43.1.319
; Korolev et al., 2018bKorolev, N.M., Melnik, A.E., Li, X.-H., Skublov, S.G. (2018b) The oxygen isotope composition of mantle eclogites as a proxy of their origin and evolution: A review. Earth-Science Reviews 185, 288–300. https://doi.org/10.1016/j.earscirev.2018.06.007
).Figure 2 Comparison of δ18O in eclogitic minerals for barren (n = 183) and diamondiferous (n = 52) parageneses worldwide (references are given in the Supplementary Information) with a superimposed histogram for δ18O in the Cullinan DIs (this study).
The second model can explain light C and heavy O isotope compositions of many diamonds and their inclusions, where carbonate in altered mafic-ultramafic oceanic crust with δ18O = +11 to +33 ‰, δ13C = −30 to −5 ‰ (Li et al., 2019
Li, K., Li, L., Pearson, D.G., Stachel, T. (2019) Diamond isotope compositions indicate altered igneous oceanic crust dominates deep carbon recycling. Earth and Planetary Science Letters 516, 190–201. https://doi.org/10.1016/j.epsl.2019.03.041
) and organic C (Fig. 3) contributed to eclogite protoliths. Yet the Cullinan diamonds with eclogitic and sublithospheric majoritic inclusions have the characteristic mantle δ13C of −2.4 to −4.8 ‰ (Fig. 3) indistinguishable from Cullinan peridotitic diamonds (Korolev et al., 2018aKorolev, N., Kopylova, M., Gurney, J.J., Moore, A.E., Davidson, J. (2018a) The origin of Type II diamonds as inferred from Cullinan mineral inclusions. Mineralogy and Petrology 112, 275–289. https://doi.org/10.1007/s00710-018-0601-z
). Thus, the model implying contribution of carbonate in altered mafic-ultramafic oceanic crust cannot be universally applied to all diamonds with inclusions enriched in heavy O, although the model adequately explains compositional patterns in many diamond occurrences.Figure 3 Comparison of δ18O of eclogitic garnets/majorites and δ13C of the host diamond worldwide (ESM1) with δ18O of Cullinan diamond inclusions. Inclusions with δ13C for studied Cullinan diamonds (Korolev et al., 2018a
Korolev, N., Kopylova, M., Gurney, J.J., Moore, A.E., Davidson, J. (2018a) The origin of Type II diamonds as inferred from Cullinan mineral inclusions. Mineralogy and Petrology 112, 275–289. https://doi.org/10.1007/s00710-018-0601-z
) are plotted as symbols, δ18O of eclogitic garnets with no information on the host diamond δ13C are shown as the green histogram. The blue hexagon marks the initial magnesite reactant. A blue arrow connects δ18O of the magnesite reactant with the Grt product for modelled combined metasomatic reactions (Reactions 1 and 8 in Table S-4); it is placed at an average mantle value of −6 ‰ for δ13C. The blue field corresponds to δ13C in sedimentary carbonates, the yellow field represents mantle carbon, and the pink field is for organic carbon.Another factor that may contribute to contrasting δ18O in barren and diamondiferous eclogites are diamond-forming metasomatic reactions. Metasomatism plays a central role in 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
), and its effect on stable isotopes of diamondiferous parageneses ought to be quantitatively assessed. It was proposed that the metasomatism may have modified the eclogitic protolith by diffusional equilibration with a carbonatitic fluid (Lowry et al., 1999Lowry, D., Mattey, D.P., Harris, J.W. (1999) Oxygen isotope composition of syngenetic inclusions in diamond from the Finsch Mine, RSA. Geochimica et Cosmochimica Acta 63, 1825–1836. http://dx.doi.org/10.1016/S0016-7037(99)00120-9
) or with the mantle carbonatitic fluids containing heavy oxygen (δ18O of +5 to +10.5 ‰) (Gréau et al., 2011Gréau, Y., Huang, J.-X., Griffin, W.L., Renac, C., Alard, O., O’Reilly, S.Y. (2011) Type I eclogites from Roberts Victor kimberlites: Products of extensive mantle metasomatism. Geochimica et Cosmochimica Acta 75, 6927–6954. https://doi.org/10.1016/j.gca.2011.08.035
; Huang et al., 2016Huang, J.-X., Xiang, Y., An, Y., Griffin, W.L., Gréau, Y., Xie, L., Pearson, N.J., Yu, H., O’Reilly, S.Y. (2016) Magnesium and oxygen isotopes in Roberts Victor eclogites. Chemical Geology 438, 73–83. https://doi.org/10.1016/j.chemgeo.2016.05.030
). However, any fluid deviating from the mantle O isotopic composition is expected to be short lived, as it would be buffered back to the mantle δ18O values by re-equilibration with ambient peridotite oxygen isotope reservoirs (Riches et al., 2016Riches, A.J.V., Ickert, R.B., Pearson, D.G., Stern, R.A., Jackson, S.E., Ishikawa, A., Kjarsgaard, B.A., Gurney, J.J. (2016) In situ oxygen-isotope, major-, and trace-element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from Roberts Victor, Kaapvaal Craton. Geochimica et Cosmochimica Acta 174, 345–359. https://doi.org/10.1016/j.gca.2015.11.028
).We tested viable diamond-forming reactions that do not involve heavy oxygen-rich fluids for 18O enrichment effects. Diamond can form by oxidation of methane-rich fluids, by reduction of carbonatitic fluids or by isochemical precipitation from cooling or ascending C-H-O fluids (Stachel and Luth, 2015
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
). The isochemical precipitation would not shift δ18O, while oxidation of methane or other reduced fluids equilibrated with H2O would lead to metasomatic silicates with lighter oxygen compositions (Ickert et al., 2013Ickert, R.B., Stachel, T., Stern, R.A., Harris, J.W. (2013) Diamond from recycled crustal carbon documented by coupled δ18O–δ13C measurements of diamonds and their inclusions. Earth and Planetary Science Letters 364, 85–97. https://doi.org/10.1016/j.epsl.2013.01.008
). An origin of diamond from an oxidised medium was suggested on the basis of the core-to-rim increases in δ13C composition of individual diamonds (Smart et al., 2011Smart, K.A., Chacko, T., Stachel, T., Muehlenbachs, K., Stern, R.A., Heaman, L.M. (2011) Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δ13C–N study of eclogite-hosted diamonds from the Jericho kimberlite. Geochimica et Cosmochimica Acta 75, 6027–6047. https://doi.org/10.1016/j.gca.2011.07.028
) and daughter minerals in fluid inclusions in diamonds (Kopylova et al., 2010Kopylova, M., Navon, O., Dubrovinsky, L., Khachatryan, G. (2010) Carbonatitic mineralogy of natural diamond-forming fluids. Earth and Planetary Science Letters 291, 126–137. https://doi.org/10.1016/j.epsl.2009.12.056
). We modelled δ18O effects for metasomatism by oxidising fluids in multiple feasible reactions with the realistic eclogitic mineralogy. The reactions start with the carbonatitic fluid equilibrated with the initial eclogitic garnet (δ18O = +6.0 ‰) and leads to a δ18O value of resulting garnet elevated by as much as 1.5 ‰ (Fig. 4; Supplementary Information). Diamond-forming metasomatising reactions with the strongest δ18O shift upward involve 1) production of O2 or CO2, 2) heavy oxygen supplied by the metasomatic fluid, 3) a sufficiently high fluid/rock ratio (1–3 moles of fluid to 1 mole of garnet), and 4) oxides (rutile or ilmenite) as products rather than reactants. In Reactions 1 and 2 (Table S-4), diamond forms by disproportionation also creating free O2, which is immediately used up to make Fe3+-bearing Grt and Cpx (Reaction 8; Table S-4). Reactions 3–7 (Table S-4) facilitate diamond production indirectly, by adding carbon dioxide to C-O-H mantle fluids that may be parental to diamonds (Stachel et al., 2022Stachel, T., Cartigny, P., Chacko, T., Pearson, D.G. (2022) Carbon and Nitrogen in Mantle-Derived Diamonds. Reviews in Mineralogy and Geochemistry 88, 809–875. https://doi.org/10.2138/rmg.2022.88.15
). The CO2 concentrations in the mantle, however, are expected to be low, buffered by silicate carbonation (Kopylova et al., 2021Kopylova, M.G., Ma, F., Tso, E. (2021) Constraining carbonation freezing and petrography of the carbonated cratonic mantle with natural samples. Lithos 388–389, 106045. https://doi.org/10.1016/j.lithos.2021.106045
). In CO2 producing reactions the δ18O of product garnet is elevated by 0.5 to 0.6 ‰ (Table S-4), and the strongest δ18O upward shift of 1.5 ‰ is observed as a net effect of Reaction 1:
Figure 4 The observed δ18O contrast between barren and diamondiferous eclogites with superimposed modelled changes of δ18O composition of eclogitic garnets produced in metasomatic reactions (orange field; Table S-4). The initial δ18O in eclogitic garnet (vertical line at +6 ‰) is chosen arbitrarily (see explanations in the text). A detailed description of the geochemical modelling is provided in the Supplementary Information.
All phases in the proposed reactions are found in cratonic eclogites (e.g., Jacob, 2004
Jacob, D.E. (2004) Nature and origin of eclogite xenoliths from kimberlites. Lithos 77, 295–316. https://doi.org/10.1016/j.lithos.2004.03.038
), and the latter reaction is based on the observed concentrations of Fe3+ in eclogitic minerals (Aulbach et al., 2022Aulbach, S., Woodland, A.B., Stagno, V., Korsakov, A.V., Mikhailenko, D., Golovin, A. (2022) Fe3+ Distribution and Fe3+/ΣFe-Oxygen Fugacity Variations in Kimberlite-Borne Eclogite Xenoliths, with Comments on Clinopyroxene-Garnet Oxy-Thermobarometry. Journal of Petrology 63, egac076. https://doi.org/10.1093/petrology/egac076
). A replacement of eclogitic garnet with a more magnesian garnet has been described in multiple occurrences as part of diamond-friendly metasomatism (e.g., De Stefano et al., 2009De Stefano, A., Kopylova, M.G., Cartigny, P., Afanasiev, V. (2009) Diamonds and eclogites of the Jericho kimberlite (Northern Canada). Contributions to Mineralogy and Petrology 158, 295–315. https://doi.org/10.1007/s00410-009-0384-7
; Korolev et al., 2021Korolev, N., Nikitina, L.P., Goncharov, A., Dubinina, E.O., Melnik, A., Müller, D., Chen, Y.-X., Zinchenko, V.N. (2021) Three Types of Mantle Eclogite from Two Layers of Oceanic Crust: A Key Case of Metasomatically-Aided Transformation of Low-to-High-Magnesian Eclogite. Journal of Petrology 62, egab070. https://doi.org/10.1093/petrology/egab070
). An increase of MgO was found to be the most significant chemical change accompanying δ18O enrichment in garnet from Orapa eclogite xenoliths (Deines et al., 1991Deines, P., Harris, J.W., Robinson, D.N., Gurney, J.J., Shee, S.R. (1991) Carbon and oxygen isotope variations in diamond and graphite eclogites from Orapa, Botswana, and the nitrogen content of their diamonds. Geochimica et Cosmochimica Acta 55, 515–524. https://doi.org/10.1016/0016-7037(91)90009-T
). It is well known that metasomatism oxidises the adjacent metasomatised mantle (Creighton et al., 2009Creighton, S., Stachel, T., Matveev, S., Höfer, H., McCammon, C., Luth, R.W. (2009) Oxidation of the Kaapvaal lithospheric mantle driven by metasomatism. Contributions to Mineralogy and Petrology 157, 491–504. https://doi.org/10.1007/s00410-008-0348-3
). The reactions are equally applicable to majorites in the sublithospheric mantle (Supplementary Information).We conclude that some metasomatic reactions of diamond formation in eclogites may contribute to the observed δ18O contrast between barren and diamondiferous eclogitic assemblages worldwide, yet the strongest upward δ18O shift of all feasible metasomatic reactions (up to 1.5 ‰) achieved in decarbonation followed by metasomatic oxidation is not sufficient to explain the 2.5 ‰ difference in δ18O (Fig. 4). Inheritance of the O isotopic composition from the crustal eclogitic protoliths is the only model that currently offers a satisfactory explanation for the contrast. This implies preferential diamond formation in eclogites with shallow basaltic protoliths with or without contribution of carbonate in altered mafic-ultramafic oceanic crust that experienced stronger low temperature alteration on the seafloor.
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Acknowledgements
We are grateful to J. Davidson (Petra Diamonds), J. Gurney and A. Moore for donation of diamonds for the research. It was funded by NSERC Discovery grant 2019-03988 to MK and by RNF grant № 22-17-00052 to ED. We express our gratitude to Horst R. Marschall, Steven Shirey and an anonymous reviewer for their constructive comments on the manuscript.
Editor: Horst R. Marschall
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References
Alt, J.C., Muehlenbachs, K., Honnorez, J. (1986) An oxygen isotopic profile through the upper kilometer of the oceanic crust, DSDP Hole 504B. Earth and Planetary Science Letters 80, 217–229. https://doi.org/10.1016/0012-821X(86)90106-8
Show in context The difference was explained by the origin of garnet in diamondiferous assemblages from the shallow, more altered part of the oceanic crust where δ18O is higher (McCulloch et al., 1981; Alt et al., 1986; Ickert et al., 2013).
View in article
DI garnet is shielded from re-equilibration with the ambient mantle oxygen (δ18O = +5.5 ± 0.4 ‰; Mattey et al., 1994), while the “exposed” garnet in xenoliths is not. Only silicate inclusions protected by diamonds retained the 18O-enriched compositions (Schulze et al., 2003; Burnham et al., 2015; Ickert et al., 2015) formed via low temperature seawater alteration of the shallowest levels of the former oceanic crust (McCulloch et al., 1981; Alt et al., 1986).
View in article
Traditionally, this difference would be explained as the contrast in δ18O of the eclogite protoliths is related to their depth position within the slab and the gradual decrease of δ18O with depth in the oceanic crust (McCulloch et al., 1981; Alt et al., 1986).
View in article
In this model, garnet in barren eclogites might have inherited the δ18O from deep gabbro layers of oceanic crust (δ18O = 0 to +5 ‰; Alt et al., 1986).
View in article
Diamondiferous eclogites with higher δ18O, by contrast, may have recorded a higher input from altered oceanic basalts (δ18O = +7 to +15 ‰; McCulloch et al., 1981; Alt et al., 1986; Eiler, 2001; Korolev et al., 2018b).
View in article
Aulbach, S., Woodland, A.B., Vasilyev, P., Galvez, M.E., Viljoen, K.S. (2017) Effects of low-pressure igneous processes and subduction on Fe3+/ΣFe and redox state of mantle eclogites from Lace (Kaapvaal craton). Earth and Planetary Science Letters 474, 283–295. https://doi.org/10.1016/j.epsl.2017.06.030
Show in context A suggested positive covariation of δ18Ogrt with equilibration temperature for Lace eclogites may hint at a wider T-δ18Ogrt correlation in the deep mantle as heavy oxygen may favour garnet with increasing T and P (Aulbach et al., 2017).
View in article
Aulbach, S., Woodland, A.B., Stagno, V., Korsakov, A.V., Mikhailenko, D., Golovin, A. (2022) Fe3+ Distribution and Fe3+/ΣFe-Oxygen Fugacity Variations in Kimberlite-Borne Eclogite Xenoliths, with Comments on Clinopyroxene-Garnet Oxy-Thermobarometry. Journal of Petrology 63, egac076. https://doi.org/10.1093/petrology/egac076
Show in context All phases in the proposed reactions are found in cratonic eclogites (e.g., Jacob, 2004), and the latter reaction is based on the observed concentrations of Fe3+ in eclogitic minerals (Aulbach et al., 2022).
View in article
Burnham, A.D., Thomson, A.R., Bulanova, G.P., Kohn, S.C., Smith, C.B., Walter, M.J. (2015) Stable isotope evidence for crustal recycling as recorded by superdeep diamonds. Earth and Planetary Science Letters 432, 374–380. https://doi.org/10.1016/j.epsl.2015.10.023
Show in context Analysed δ18O composition of garnet ranges from +5.4 to +10.2 ‰ and covers the oxygen isotope composition of majorites worldwide (+6.0 to +9.4 ‰; Burnham et al., 2015; Ickert et al., 2015).
View in article
DI garnet is shielded from re-equilibration with the ambient mantle oxygen (δ18O = +5.5 ± 0.4 ‰; Mattey et al., 1994), while the “exposed” garnet in xenoliths is not. Only silicate inclusions protected by diamonds retained the 18O-enriched compositions (Schulze et al., 2003; Burnham et al., 2015; Ickert et al., 2015) formed via low temperature seawater alteration of the shallowest levels of the former oceanic crust (McCulloch et al., 1981; Alt et al., 1986).
View in article
Clayton, R.N., Goldsmith, J.R., Karel, K.J., Mayeda, T.K., Newton, R.C. (1975) Limits on the effect of pressure on isotopic fractionation. Geochimica et Cosmochimica Acta 39, 1197–1201. https://doi.org/10.1016/0016-7037(75)90062-9
Show in context The contrasting δ18O compositions in barren and diamondiferous parageneses do not relate to pressure, which was predicted by Clayton et al. (1975).
View in article
Creighton, S., Stachel, T., Matveev, S., Höfer, H., McCammon, C., Luth, R.W. (2009) Oxidation of the Kaapvaal lithospheric mantle driven by metasomatism. Contributions to Mineralogy and Petrology 157, 491–504. https://doi.org/10.1007/s00410-008-0348-3
Show in context It is well known that metasomatism oxidises the adjacent metasomatised mantle (Creighton et al., 2009).
View in article
Deines, P., Harris, J.W., Robinson, D.N., Gurney, J.J., Shee, S.R. (1991) Carbon and oxygen isotope variations in diamond and graphite eclogites from Orapa, Botswana, and the nitrogen content of their diamonds. Geochimica et Cosmochimica Acta 55, 515–524. https://doi.org/10.1016/0016-7037(91)90009-T
Show in context An increase of MgO was found to be the most significant chemical change accompanying δ18O enrichment in garnet from Orapa eclogite xenoliths (Deines et al., 1991).
View in article
De Stefano, A., Kopylova, M.G., Cartigny, P., Afanasiev, V. (2009) Diamonds and eclogites of the Jericho kimberlite (Northern Canada). Contributions to Mineralogy and Petrology 158, 295–315. https://doi.org/10.1007/s00410-009-0384-7
Show in context A replacement of eclogitic garnet with a more magnesian garnet has been described in multiple occurrences as part of diamond-friendly metasomatism (e.g., De Stefano et al., 2009; Korolev et al., 2021).
View in article
Eiler, J.M. (2001) Oxygen Isotope Variations of Basaltic Lavas and Upper Mantle Rocks. Reviews in Mineralogy and Geochemistry 43, 319–364. https://doi.org/10.2138/gsrmg.43.1.319
Show in context Diamondiferous eclogites with higher δ18O, by contrast, may have recorded a higher input from altered oceanic basalts (δ18O = +7 to +15 ‰; McCulloch et al., 1981; Alt et al., 1986; Eiler, 2001; Korolev et al., 2018b).
View in article
Gréau, Y., Huang, J.-X., Griffin, W.L., Renac, C., Alard, O., O’Reilly, S.Y. (2011) Type I eclogites from Roberts Victor kimberlites: Products of extensive mantle metasomatism. Geochimica et Cosmochimica Acta 75, 6927–6954. https://doi.org/10.1016/j.gca.2011.08.035
Show in context Diamond formation is considered to be partly metasomatic, as suggested by diamond distribution in eclogites (Taylor and Anand, 2004), δ13C core-to-rim patterns (Smart et al., 2011) and correlations of O isotopes with trace element indicators of metasomatism (Gréau et al., 2011; Huang et al., 2012).
View in article
It was proposed that the metasomatism may have modified the eclogitic protolith by diffusional equilibration with a carbonatitic fluid (Lowry et al., 1999) or with the mantle carbonatitic fluids containing heavy oxygen (δ18O of +5 to +10.5 ‰) (Gréau et al., 2011; Huang et al., 2016).
View in article
Huang, J.-X., Gréau, Y., Griffin, W.L., O’Reilly, S.Y., Pearson, N.J. (2012) Multi-stage origin of Roberts Victor eclogites: Progressive metasomatism and its isotopic effects. Lithos 142–143, 161–181. https://doi.org/10.1016/j.lithos.2012.03.002
Show in context Diamond formation is considered to be partly metasomatic, as suggested by diamond distribution in eclogites (Taylor and Anand, 2004), δ13C core-to-rim patterns (Smart et al., 2011) and correlations of O isotopes with trace element indicators of metasomatism (Gréau et al., 2011; Huang et al., 2012).
View in article
Huang, J.-X., Xiang, Y., An, Y., Griffin, W.L., Gréau, Y., Xie, L., Pearson, N.J., Yu, H., O’Reilly, S.Y. (2016) Magnesium and oxygen isotopes in Roberts Victor eclogites. Chemical Geology 438, 73–83. https://doi.org/10.1016/j.chemgeo.2016.05.030
Show in context It was proposed that the metasomatism may have modified the eclogitic protolith by diffusional equilibration with a carbonatitic fluid (Lowry et al., 1999) or with the mantle carbonatitic fluids containing heavy oxygen (δ18O of +5 to +10.5 ‰) (Gréau et al., 2011; Huang et al., 2016).
View in article
Ickert, R.B., Stachel, T., Stern, R.A., Harris, J.W. (2013) Diamond from recycled crustal carbon documented by coupled δ18O–δ13C measurements of diamonds and their inclusions. Earth and Planetary Science Letters 364, 85–97. https://doi.org/10.1016/j.epsl.2013.01.008
Show in context The difference was explained by the origin of garnet in diamondiferous assemblages from the shallow, more altered part of the oceanic crust where δ18O is higher (McCulloch et al., 1981; Alt et al., 1986; Ickert et al., 2013).
View in article
The isochemical precipitation would not shift δ18O, while oxidation of methane or other reduced fluids equilibrated with H2O would lead to metasomatic silicates with lighter oxygen compositions (Ickert et al., 2013).
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Ickert, R.B., Stachel, T., Stern, R.A., Harris, J.W. (2015) Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters 1, 65–74. https://doi.org/10.7185/geochemlet.1507
Show in context Analysed δ18O composition of garnet ranges from +5.4 to +10.2 ‰ and covers the oxygen isotope composition of majorites worldwide (+6.0 to +9.4 ‰; Burnham et al., 2015; Ickert et al., 2015).
View in article
DI garnet is shielded from re-equilibration with the ambient mantle oxygen (δ18O = +5.5 ± 0.4 ‰; Mattey et al., 1994), while the “exposed” garnet in xenoliths is not. Only silicate inclusions protected by diamonds retained the 18O-enriched compositions (Schulze et al., 2003; Burnham et al., 2015; Ickert et al., 2015) formed via low temperature seawater alteration of the shallowest levels of the former oceanic crust (McCulloch et al., 1981; Alt et al., 1986).
View in article
These diamonds and their mineral inclusions originated from carbon and oxygen derived from the sedimentary organic matter or altered oceanic crust (Li et al., 2019) subducted into the mantle, as evidenced by a correlation of heavy 18O in silicate DIs and light, low 13C/12C carbon (Ickert et al., 2015; Li et al., 2019).
View in article
Jacob, D.E. (2004) Nature and origin of eclogite xenoliths from kimberlites. Lithos 77, 295–316. https://doi.org/10.1016/j.lithos.2004.03.038
Show in context Diamond concentrations in mantle eclogite can be orders of magnitude higher than the concentration of diamonds in kimberlite – the rock from which they are mined. Over the past 40 years, the oxygen isotope composition (δ18O) of eclogite has become one of the most powerful indicators of its crustal origin in the cratonic mantle (Schulze et al., 2003), together with stable isotopes of C, N and S, and radiogenic isotopes (Pearson et al., 2003; Jacob, 2004).
View in article
All phases in the proposed reactions are found in cratonic eclogites (e.g., Jacob, 2004), and the latter reaction is based on the observed concentrations of Fe3+ in eclogitic minerals (Aulbach et al., 2022).
View in article
Kopylova, M., Navon, O., Dubrovinsky, L., Khachatryan, G. (2010) Carbonatitic mineralogy of natural diamond-forming fluids. Earth and Planetary Science Letters 291, 126–137. https://doi.org/10.1016/j.epsl.2009.12.056
Show in context An origin of diamond from an oxidised medium was suggested on the basis of the core-to-rim increases in δ13C composition of individual diamonds (Smart et al., 2011) and daughter minerals in fluid inclusions in diamonds (Kopylova et al., 2010).
View in article
Kopylova, M.G., Ma, F., Tso, E. (2021) Constraining carbonation freezing and petrography of the carbonated cratonic mantle with natural samples. Lithos 388–389, 106045. https://doi.org/10.1016/j.lithos.2021.106045
Show in context The CO2 concentrations in the mantle, however, are expected to be low, buffered by silicate carbonation (Kopylova et al., 2021).
View in article
Korolev, N., Kopylova, M., Gurney, J.J., Moore, A.E., Davidson, J. (2018a) The origin of Type II diamonds as inferred from Cullinan mineral inclusions. Mineralogy and Petrology 112, 275–289. https://doi.org/10.1007/s00710-018-0601-z
Show in context They are derived from a wide interval of temperatures (T) and pressures (P) of 5.5–7.5 GPa from the lithosphere and 10.5–13.5 GPa from the sublithospheric mantle (Korolev et al., 2018a).
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Yet the Cullinan diamonds with eclogitic and sublithospheric majoritic inclusions have the characteristic mantle δ13C of −2.4 to −4.8 ‰ (Fig. 3) indistinguishable from Cullinan peridotitic diamonds (Korolev et al., 2018a).
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Inclusions with δ13C for studied Cullinan diamonds (Korolev et al., 2018a) are plotted as symbols, δ18O of eclogitic garnets with no information on the host diamond δ13C are shown as the green histogram.
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Korolev, N.M., Melnik, A.E., Li, X.-H., Skublov, S.G. (2018b) The oxygen isotope composition of mantle eclogites as a proxy of their origin and evolution: A review. Earth-Science Reviews 185, 288–300. https://doi.org/10.1016/j.earscirev.2018.06.007
Show in context Diamondiferous eclogites with higher δ18O, by contrast, may have recorded a higher input from altered oceanic basalts (δ18O = +7 to +15 ‰; McCulloch et al., 1981; Alt et al., 1986; Eiler, 2001; Korolev et al., 2018b).
View in article
Korolev, N., Nikitina, L.P., Goncharov, A., Dubinina, E.O., Melnik, A., Müller, D., Chen, Y.-X., Zinchenko, V.N. (2021) Three Types of Mantle Eclogite from Two Layers of Oceanic Crust: A Key Case of Metasomatically-Aided Transformation of Low-to-High-Magnesian Eclogite. Journal of Petrology 62, egab070. https://doi.org/10.1093/petrology/egab070
Show in context A replacement of eclogitic garnet with a more magnesian garnet has been described in multiple occurrences as part of diamond-friendly metasomatism (e.g., De Stefano et al., 2009; Korolev et al., 2021).
View in article
Li, K., Li, L., Pearson, D.G., Stachel, T. (2019) Diamond isotope compositions indicate altered igneous oceanic crust dominates deep carbon recycling. Earth and Planetary Science Letters 516, 190–201. https://doi.org/10.1016/j.epsl.2019.03.041
Show in context These diamonds and their mineral inclusions originated from carbon and oxygen derived from the sedimentary organic matter or altered oceanic crust (Li et al., 2019) subducted into the mantle, as evidenced by a correlation of heavy 18O in silicate DIs and light, low 13C/12C carbon (Ickert et al., 2015; Li et al., 2019).
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The second model can explain light C and heavy O isotope compositions of many diamonds and their inclusions, where carbonate in altered mafic-ultramafic oceanic crust with δ18O = +11 to +33 ‰, δ13C = −30 to −5 ‰ (Li et al., 2019) and organic C (Fig. 3) contributed to eclogite protoliths.
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Lowry, D., Mattey, D.P., Harris, J.W. (1999) Oxygen isotope composition of syngenetic inclusions in diamond from the Finsch Mine, RSA. Geochimica et Cosmochimica Acta 63, 1825–1836. https://dx.doi.org/10.1016/S0016-7037(99)00120-9
Show in context It was proposed that the metasomatism may have modified the eclogitic protolith by diffusional equilibration with a carbonatitic fluid (Lowry et al., 1999) or with the mantle carbonatitic fluids containing heavy oxygen (δ18O of +5 to +10.5 ‰) (Gréau et al., 2011; Huang et al., 2016).
View in article
Mattey, D., Lowry, D., Macpherson, C. (1994) Oxygen isotope composition of mantle peridotite. Earth and Planetary Science Letters 128, 231–241. https://doi.org/10.1016/0012-821X(94)90147-3
Show in context DI garnet is shielded from re-equilibration with the ambient mantle oxygen (δ18O = +5.5 ± 0.4 ‰; Mattey et al., 1994), while the “exposed” garnet in xenoliths is not. Only silicate inclusions protected by diamonds retained the 18O-enriched compositions (Schulze et al., 2003; Burnham et al., 2015; Ickert et al., 2015) formed via low temperature seawater alteration of the shallowest levels of the former oceanic crust (McCulloch et al., 1981; Alt et al., 1986).
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McCulloch, M.T., Gregory, R.T., Wasserburg, G.J., Taylor Jr., H.P. (1981) Sm-Nd, Rb-Sr, and 18O/16O isotopic systematics in an oceanic crustal section: Evidence from the Samail ophiolite. Journal of Geophysical Research: Solid Earth 86, 2721–2735. https://doi.org/10.1029/JB086iB04p02721
Show in context The difference was explained by the origin of garnet in diamondiferous assemblages from the shallow, more altered part of the oceanic crust where δ18O is higher (McCulloch et al., 1981; Alt et al., 1986; Ickert et al., 2013).
View in article
DI garnet is shielded from re-equilibration with the ambient mantle oxygen (δ18O = +5.5 ± 0.4 ‰; Mattey et al., 1994), while the “exposed” garnet in xenoliths is not. Only silicate inclusions protected by diamonds retained the 18O-enriched compositions (Schulze et al., 2003; Burnham et al., 2015; Ickert et al., 2015) formed via low temperature seawater alteration of the shallowest levels of the former oceanic crust (McCulloch et al., 1981; Alt et al., 1986).
View in article
Traditionally, this difference would be explained as the contrast in δ18O of the eclogite protoliths is related to their depth position within the slab and the gradual decrease of δ18O with depth in the oceanic crust (McCulloch et al., 1981; Alt et al., 1986).
View in article
Diamondiferous eclogites with higher δ18O, by contrast, may have recorded a higher input from altered oceanic basalts (δ18O = +7 to +15 ‰; McCulloch et al., 1981; Alt et al., 1986; Eiler, 2001; Korolev et al., 2018b).
View in article
Pearson, D.G., Canil, D., Shirey, S.B. (2003) 2.05 - Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. First Edition, Elsevier, Amsterdam, 171–275. https://doi.org/10.1016/B0-08-043751-6/02005-3
Show in context Diamond concentrations in mantle eclogite can be orders of magnitude higher than the concentration of diamonds in kimberlite – the rock from which they are mined. Over the past 40 years, the oxygen isotope composition (δ18O) of eclogite has become one of the most powerful indicators of its crustal origin in the cratonic mantle (Schulze et al., 2003), together with stable isotopes of C, N and S, and radiogenic isotopes (Pearson et al., 2003; Jacob, 2004).
View in article
It has been noticed that garnet and clinopyroxene in diamondiferous eclogites are higher in δ18O than their respective phases in barren eclogites (Pearson et al., 2003).
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Riches, A.J.V., Ickert, R.B., Pearson, D.G., Stern, R.A., Jackson, S.E., Ishikawa, A., Kjarsgaard, B.A., Gurney, J.J. (2016) In situ oxygen-isotope, major-, and trace-element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from Roberts Victor, Kaapvaal Craton. Geochimica et Cosmochimica Acta 174, 345–359. https://doi.org/10.1016/j.gca.2015.11.028
Show in context Diamond precipitates from mantle C-bearing fluids percolating upward and experiencing Raleigh fractionation (Stachel and Luth, 2015; Riches et al., 2016).
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However, any fluid deviating from the mantle O isotopic composition is expected to be short lived, as it would be buffered back to the mantle δ18O values by re-equilibration with ambient peridotite oxygen isotope reservoirs (Riches et al., 2016).
View in article
Schulze, D.J., Harte, B., Valley, J.W., Brenan, J.M., Channer, D.M.D.R. (2003) Extreme crustal oxygen isotope signatures preserved in coesite in diamond. Nature 423, 68–70. https://doi.org/10.1038/nature01615
Show in context Diamond concentrations in mantle eclogite can be orders of magnitude higher than the concentration of diamonds in kimberlite – the rock from which they are mined. Over the past 40 years, the oxygen isotope composition (δ18O) of eclogite has become one of the most powerful indicators of its crustal origin in the cratonic mantle (Schulze et al., 2003), together with stable isotopes of C, N and S, and radiogenic isotopes (Pearson et al., 2003; Jacob, 2004).
View in article
DI garnet is shielded from re-equilibration with the ambient mantle oxygen (δ18O = +5.5 ± 0.4 ‰; Mattey et al., 1994), while the “exposed” garnet in xenoliths is not. Only silicate inclusions protected by diamonds retained the 18O-enriched compositions (Schulze et al., 2003; Burnham et al., 2015; Ickert et al., 2015) formed via low temperature seawater alteration of the shallowest levels of the former oceanic crust (McCulloch et al., 1981; Alt et al., 1986).
View in article
Smart, K.A., Chacko, T., Stachel, T., Muehlenbachs, K., Stern, R.A., Heaman, L.M. (2011) Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δ13C–N study of eclogite-hosted diamonds from the Jericho kimberlite. Geochimica et Cosmochimica Acta 75, 6027–6047. https://doi.org/10.1016/j.gca.2011.07.028
Show in context Diamond formation is considered to be partly metasomatic, as suggested by diamond distribution in eclogites (Taylor and Anand, 2004), δ13C core-to-rim patterns (Smart et al., 2011) and correlations of O isotopes with trace element indicators of metasomatism (Gréau et al., 2011; Huang et al., 2012).
View in article
An origin of diamond from an oxidised medium was suggested on the basis of the core-to-rim increases in δ13C composition of individual diamonds (Smart et al., 2011) and daughter minerals in fluid inclusions in diamonds (Kopylova et al., 2010).
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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 Metasomatism plays a central role in diamond formation (Stachel and Harris, 2008), and its effect on stable isotopes of diamondiferous parageneses ought to be quantitatively assessed.
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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 Diamond precipitates from mantle C-bearing fluids percolating upward and experiencing Raleigh fractionation (Stachel and Luth, 2015; Riches et al., 2016).
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Diamond can form by oxidation of methane-rich fluids, by reduction of carbonatitic fluids or by isochemical precipitation from cooling or ascending C-H-O fluids (Stachel and Luth, 2015).
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Stachel, T., Cartigny, P., Chacko, T., Pearson, D.G. (2022) Carbon and Nitrogen in Mantle-Derived Diamonds. Reviews in Mineralogy and Geochemistry 88, 809–875. https://doi.org/10.2138/rmg.2022.88.15
Show in context Reactions 3–7 (Table S-4) facilitate diamond production indirectly, by adding carbon dioxide to C-O-H mantle fluids that may be parental to diamonds (Stachel et al., 2022).
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Taylor, L.A., Anand, M. (2004) Diamonds: time capsules from the Siberian Mantle. Geochemistry 64, 1–74. https://doi.org/10.1016/j.chemer.2003.11.006
Show in context Diamond formation is considered to be partly metasomatic, as suggested by diamond distribution in eclogites (Taylor and Anand, 2004), δ13C core-to-rim patterns (Smart et al., 2011) and correlations of O isotopes with trace element indicators of metasomatism (Gréau et al., 2011; Huang et al., 2012).
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Supplementary Information
The Supplementary Information includes:
Download the Supplementary Information (PDF)
Download Table S-5 (xlsx)
Figures
Figure 1 δ18O histograms for eclogitic garnet in xenoliths and DIs. (a) Comparison of our data with Kaapvaal non-diamondiferous eclogites. (b) Comparison of global data for garnet/majorite DIs with garnets from diamondiferous eclogites worldwide (references are listed in the Supplementary Information). Lines are kernel-smoothed distribution curves.
Figure 2 Comparison of δ18O in eclogitic minerals for barren (n = 183) and diamondiferous (n = 52) parageneses worldwide (references are given in the Supplementary Information) with a superimposed histogram for δ18O in the Cullinan DIs (this study).
Figure 3 Comparison of δ18O of eclogitic garnets/majorites and δ13C of the host diamond worldwide (ESM1) with δ18O of Cullinan diamond inclusions. Inclusions with δ13C for studied Cullinan diamonds (Korolev et al., 2018a
Korolev, N., Kopylova, M., Gurney, J.J., Moore, A.E., Davidson, J. (2018a) The origin of Type II diamonds as inferred from Cullinan mineral inclusions. Mineralogy and Petrology 112, 275–289. https://doi.org/10.1007/s00710-018-0601-z
) are plotted as symbols, δ18O of eclogitic garnets with no information on the host diamond δ13C are shown as the green histogram. The blue hexagon marks the initial magnesite reactant. A blue arrow connects δ18O of the magnesite reactant with the Grt product for modelled combined metasomatic reactions (Reactions 1 and 8 in Table S-4); it is placed at an average mantle value of −6 ‰ for δ13C. The blue field corresponds to δ13C in sedimentary carbonates, the yellow field represents mantle carbon, and the pink field is for organic carbon.
Figure 4 The observed δ18O contrast between barren and diamondiferous eclogites with superimposed modelled changes of δ18O composition of eclogitic garnets produced in metasomatic reactions (orange field; Table S-4). The initial δ18O in eclogitic garnet (vertical line at +6 ‰) is chosen arbitrarily (see explanations in the text). A detailed description of the geochemical modelling is provided in the Supplementary Information.