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A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13C-depleted diamonds

S. Mikhail1,

1The School of Earth and Environmental Sciences, University of St. Andrews, UK

J.C. Crosby1,

1The School of Earth and Environmental Sciences, University of St. Andrews, UK

F.M. Stuart2,

2Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, UK

L. DiNicola2,

2Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, UK

F.A.J. Abernethy3

3Department of Physical Sciences, The Open University, UK

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Mikhail, S., Crosby, J.C., Stuart, F.M., DiNicola, L., Abernathy, F.A.J. (2019) A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13C-depleted diamonds. Geochem. Persp. Let. 11, 39–43.

SM acknowledges support from the National Environmental Research Council (grant no. NE/PO12167/1).

Geochemical Perspectives Letters v11  |  doi: 10.7185/geochemlet.1923
Received 23 April 2019  |  Accepted 28 August 2019  |  Published 10 October 2019
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0


Keywords: diamond, volatiles, noble gases, stable isotopes, deep carbon cycle



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Abstract


Fluid inclusions trapped in fast-growing diamonds provide a unique opportunity to examine the origin of diamonds, and the conditions under which they formed. Eclogitic to websteritic diamondites from southern Africa show 13C-depletion and 15N-enrichment relative to mantle values (δ13C = -4.3 to -22.2 ‰ and δ15N = -4.9 to +23.2 ‰). In contrast the 3He/4He of the trapped fluids have a strong mantle signature, one sample has the highest value so far recorded for African diamonds (8.5 ± 0.4 Ra). We find no evidence for deep mantle He in these diamondites, or indeed in any diamonds from southern Africa. A correlation between 3He/4He ratios and 3He concentration suggests that the low 3He/4He are largely the result of ingrowth of radiogenic 4He in the trapped fluids since diamond formation. The He-C-N isotope systematics can be best described by mixing between fluid released from subducted altered oceanic crust and mantle volatiles. The high 3He/4He of low δ13C diamondites reflects the high 3He concentration in the mantle fluids relative to the slab-derived fluids. The presence of post-crystallisation 4He in the fluids means that all 3He/4He are minima, which in turn implies that the slab-derived carbon has a sedimentary organic origin. In short, although carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamond-formation, helium isotopes reveal an unambiguous mantle component hidden within a strongly 13C-depleted system.

Figures and Tables

Table 1 Data for southern African diamondites. A full data table including all comparative data is located in the Supplementary Information (Table S-1).

Figure 1 New C and N isotope data for southern African diamondites (red circles) and literature data (grey circles: Gautheron et al., 2005; Burgess et al., 1998) Fields are shown for other diamondites (Mikhail et al., 2013), eclogitic monocrystalline diamonds from Jwaneng (Cartigny et al., 1998) and Orapa (Cartigny et al., 1999), the mean mantle and altered oceanic crustal material (Cartigny et al., 2014).

Figure 2 (a) Helium isotope systematics of fluids released by in vacuo crushing of diamondites. The 3He/4He of modern the convecting upper mantle (CUM) and the sub-continental lithospheric mantle (SCLM) are shown for reference. (b) Carbon-helium isotope systematics of southern African diamondites and fibrous diamonds from across southern Africa. The mixing lines are plotted between mantle and crustal fluid sources, the crosses refer to percent of mantle fluid component. The mixing lines plotted in Figure 2b are hyperbolic as [4He]mantle/[4He]crust is assumed to be 10. Comparative data are from Burgess et al. (1998), Gautheron et al. (2005) and Timmermann et al. (2018, 2019a).

Figure 3 Cartoon illustrating the model discussed in the text. Not to scale.

Table 1 Figure 1 Figure 2 Figure 3

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Introduction


Placing diamond-formation into context of large scale tectonothermal processes, such as subduction and plume-lithosphere interaction, is fundamental to understanding the deep terrestrial carbon cycle (Shirey et al., 2013

Shirey, S.B., Cartigny, P., Frost, D.J., Keshav, S., Nestola, F., Nimis, P., Pearson, D.G., Sobolev, N.V., Walter, M.J. (2013) Diamonds and the geology of mantle carbon. Reviews in Mineralogy and Geochemistry 75, 355–421.

).  Diamond is a chemically simple mineral comprised largely of C and trace N (~0.025 %) incorporated as single nitrogen atoms substituting for carbon (Kaiser and Bond, 1959

Kaiser, W., Bond, W.L. (1959) Nitrogen, a major impurity in common type I diamond. Physical Review Letters 115, 857–863.

). The origin of diamond-forming fluids is commonly addressed using the stable isotope values of carbon and nitrogen, where coupled δ13C-δ15N values can indicate diamond growth from mantle-derived fluids (Cartigny et al., 2014

Cartigny, P., Palot, M., Thomassot, E., Harris, J.W. (2014) Diamond formation: A stable isotope perspective. Annual Reviews of Earth and Planetary Sciences 42, 699–732.

). However, many datasets require the contribution of crustal sources for the C and/or N, such as eclogitic diamonds from Dachine (Smith et al., 2016

Smith, C.B., Walter, M.J., Bulanova, G.P., Mikhail, S., Burnham, A.D., Gobbo, L., Kohn, S.C. (2016) Diamonds from Dachine, French Guiana: a unique record of Early Proterozoic subduction. Lithos 265, 82–95.

). Sometimes, C-N isotope systematics do not fully resolve the origin of the diamond-forming fluids. For instance, eclogitic diamonds from Jwaneng (Cartigny et al., 1998

Cartigny, P., Harris, J.W., Javoy, M. (1998) Eclogitic diamond formation at Jwaneng: no room for a recycled component. Science 280, 1421–1424.

) and Orapa (Cartigny et al., 1999

Cartigny, P., Harris J.W., Javoy, M. (1999) Eclogitic, peridotitic and metamorphic diamonds and the problems of carbon recycling—the case of Orapa (Botswana). 7th International Kimberlite Conference Extended Abstracts, 117–124.

) show crust-like low δ13C values yet have mantle-like negative δ15N values (Fig. 1), and  C and N isotope systems can be decoupled during diamond-formation (Mikhail et al., 2014

Mikhail, S., Howell, D., Hutchison, M., Verchovsky, A.B., Warburton, P., Southworth, R., Thompson, A.R., Jones, A.P., Mileage, H.J. (2014) Constraining the internal variability of carbon and nitrogen isotopes in diamonds. Chemical Geology 366, 14–23.

; Hogberg et al., 2016

Hogberg, K., Stachel, T., Stern, R.A. (2016) Carbon and nitrogen isotope systematics in diamond: Different sensitivities to isotopic fractionation or a decoupled origin? Lithos 265, 16–30.

).

Diamonds can trap fluid during their growth, either along the diamond fibres, between interlocking polycrystalline grains, and surrounding diamond-hosted mineral inclusions (Navon et al., 1988

Navon, O., Hutcheon, I.D., Rossman, G.R., Wasserburg, G.J. (1988) Mantle-derived fluids in diamond micro-inclusions. Nature 335, 784–789.

; Jacob et al., 2014

Jacob, D.E., Dobrzhinetskaya, L., Wirth, R. (2014) New insight into polycrystalline diamond genesis from modern nanoanalytical techniques. Earth-Science Reviews 136, 21–35.

). As well as providing the only direct samples of metasomatic fluids from the mantle (Weiss et al., 2015

Weiss, Y., McNeill, J., Pearson, D.G., Nowell, G.M., Ottley, C.J. (2015) Highly saline fluids from a subducting slab as the source for fluid-rich diamonds. Nature 524, 339–342.

), the trapped fluids enable the application of noble gas isotope tracers to resolve the origin of diamond-forming fluids (Burgess et al., 1998

Burgess, R., Johnson, L., Mattey, D., Harris, J., Turner, G. (1998) He, Ar and C isotopes in coated and polycrystalline diamonds. Chemical Geology 146, 205–217.

; Gautheron et al., 2005

Gautheron, C., Cartigny, P., Moreira, M., Harris, J.W., Allégre, C.J. (2005) Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana). Earth and Planetary Science Letters 240, 559–572.

; Broadley et al., 2018

Broadley, M.W., Kagi, H., Burgess, R., Zedgenizov, D., Mikhail, S., Almayrac, M., Ragozin, A., Pomazansky, B., Sumino, B. (2018) Plume-lithosphere interaction, and the formation of fibrous diamonds. Geochemical Perspectives Letters 8, 26–30.

; Timmermann et al., 2018

Timmermann, S., Honda, M., Phillips, D., Jaques, A.L., Harris, J.W. (2018) Noble gas geochemistry of fluid inclusions in South African diamonds: implications for the origin of diamond-forming fluids. Mineralogy and Petrology 112, 181–195.

, 2019a

Timmermann, S., Yeowa, H., Honda, M., Howell, D., Jaquesa, A.L., Krebs, M.Y., Woodland, S., Pearson, D.G., Ávila, Y.A., Ireland, T.R. (2019a) U-Th/He systematics of fluid-rich ‘fibrous’ diamonds – Evidence for pre- and syn-kimberlite eruption ages. Chemical Geology 515, 22–36.

,b

Timmerman, S., Honda, M., Burnham, A.D., Amelin, Y., Woodland, S., Pearson, D.G., Jaques, A.L., Le Losq, C., Bennett, V.C., Bulanova, G.P., Smith, C.B., Harris, J.W., Tohver. E. (2019b) Primordial and recycled helium isotope signatures in the mantle transition zone. Science 16, 692–694.

). Here we combine C-N isotope and nitrogen abundance data from southern African diamondites with He isotope data from micro-inclusions in the same sample to investigate the origin of carbonaceous fluids in the mantle. These new data shed light on the number of discrete sources required for the generation of carbonaceous high-density fluids responsible for diamond-formation in the SCLM.

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Samples and Methods


Three main diamond types are recognised; monocrystalline, fibrous and polycrystalline. The latter, also known as diamondites, are a mixture of diamond intergrown with silicates and oxides (Kurat and Dobosi, 2000

Kurat, G., Dobosi, G. (2000) Garnet and diopside-bearing diamondites (framesites). Mineralogy and Petrology 69, 143-159.

). Seventeen diamondites in this study are from two collections, the Orapa and the southern African diamondites (n = 10 and 7 respectively; Table 1). These samples are described in detail elsewhere (Kurat and Dobosi, 2000

Kurat, G., Dobosi, G. (2000) Garnet and diopside-bearing diamondites (framesites). Mineralogy and Petrology 69, 143-159.

; Mikhail et al., 2019

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.

; Supplementary Information). The δ13C, δ15N values and N concentrations were obtained using the automated and custom-made Finesse system at the Open University following the method outlined in Mikhail et al. (2014)

Mikhail, S., Howell, D., Hutchison, M., Verchovsky, A.B., Warburton, P., Southworth, R., Thompson, A.R., Jones, A.P., Mileage, H.J. (2014) Constraining the internal variability of carbon and nitrogen isotopes in diamonds. Chemical Geology 366, 14–23.

. The helium isotope composition of trapped fluids released by in vacuo crushing were determined using a ThermoFisher Scientific Helix-SFT mass spectrometer at the Scottish Universities Environmental Research Centre (Carracedo et al., 2019

Carracedo, A. Rodés, Á., Smellie, J., Stuart, F.M. (2019) Episodic erosion in West Antarctica inferred from cosmogenic 3He and 10Be in olivine from Mount Hampton. Geomorphology 327, 438–445.

). These data are provided in Table 1.

Table 1 Data for southern African diamondites. A full data table including all comparative data is located in the Supplementary Information (Table S-1).
SampleLocationParaMineralsR/Ra±4He ccSTP/g3He ccSTP/gδ13C (‰)±N at.ppmδ15N (‰)±
DIA030SAWGnt8.50.41.33 E-071.51 E-12-16.60.28+6.43.9
DIA053SAWGnt2.80.73.20 E-081.18 E-13-20.80.556+2.00.7
DIA057B#1SA0.10.06.79 E-075.81 E-14-21.40.11389
DIA057B#2SA0.10.16.90 E-071.25 E-13-21.40.11389
DIA058BSAEGnt3.90.27.42 E-083.84 E-13-19.10.1
DIA059SAWGnt0.50.21.42 E-071.01 E-13-22.20.313+5.36.1
DIA073BSAWGnt1.90.48.71 E-082.21 E-13-17.40.12812+6.90.5
DIA077SA7.40.51.44 E-071.43 E-12
ORF9OrapaWGnt2.44 E-08-5.30.1
ORF12Orapa6.72 E-08
ORF19OrapaEGnt4.22.79.02 E-085.11 E-13-8.00.1255+5.80.1
ORF26#1OrapaP + ECpx + Gnt4.22 E-09-14.60.238-4.92.2
ORF26#2OrapaP + ECpx + Gnt-17.80.219+23.26.6
ORF28#1Orapa5.37 E-08-4.30.1775+2.90.2
ORF28#2Orapa-4.90.252+19.70.6
ORF41Orapa7.77 E-08-14.80.31146+14.70.2
ORF57OrapaEGnt7.50.72.24 E-072.26 E-12-16.60.217+18.43.0
ORF60OrapaChromite-6.50.11054+12.10.2
ORF91Orapa-20.30.3401+4.40.3
ORF143OrapaEGnt1.61.11.03 E-072.17 E-13-19.90.2647+10.00.2

Abbreviations: W = websteritic, P = peridotitic, E = eclogitic, Gnt = garnet, Cpx = clinopyroxene, SA = southern Africa.

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Carbon and Nitrogen Geochemistry


δ13C values range from -4.3 to -22.2 ‰ and δ15N range from -4.9 to +23.2 ‰ (Fig. 1). Although they overlap, the southern Africa diamondites have lower mean δ13C (-19.8 vs. -12.1 ‰) and higher average δ15N values (+10.6 vs. +5.2 ‰) than the Orapa diamondites.  There is no systematic relationship between C or N isotope values and the silicate paragenesis, consistent with previous observations (Mikhail et al., 2019

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.

). Although mantle-like C and N isotope values are recorded, no sample plots in the mantle field (Fig. 1).  Diamondites show 13C-depletion and 15N-enrichment relative to mantle values. The Orapa diamondites have a more pronounced 13C-depletion and 15N-enrichment when compared to the eclogitic and peridotitic monocrystalline diamonds from the same kimberlite (Fig. 1). These data indicate crustal fluids sourced from subducted oceanic sediments or altered oceanic crust (Thomazo et al., 2009

Thomazo, C., Pinti, D.L., Busigny, V., Ader, M., Hashizume, K., Philippot, P. (2009) Biological activity and the Earth's surface evolution: Insights from carbon, sulfur, nitrogen and iron stable isotopes in the rock record. Comptes Rendus Palevol 8, 665–678.

). Nitrogen concentrations range from 8–1389 ppm and do not correlate with δ13C or δ15N, consistent with previous observations (Mikhail et al., 2013

Mikhail, S., Kurat, G., Dubosi, G., Verchovsky, A.B., Jones, A.P., Mileage, H.J. (2013) Peridotitic and websteritic diamondites provide new information regarding mantle melting and metasomatism induced through the subduction of crustal volatiles. Geochimica Cosmochimica et Acta 107, 1–11.

, 2014

Mikhail, S., Howell, D., Hutchison, M., Verchovsky, A.B., Warburton, P., Southworth, R., Thompson, A.R., Jones, A.P., Mileage, H.J. (2014) Constraining the internal variability of carbon and nitrogen isotopes in diamonds. Chemical Geology 366, 14–23.

, 2019

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.

). The lack of correlation between nitrogen concentrations with δ13C or δ15N in both datasets may reflect multiple diamond forming events, or isotopic heterogeneity in the source.


Figure 1 New C and N isotope data for southern African diamondites (red circles) and literature data (grey circles: Gautheron et al., 2005

Gautheron, C., Cartigny, P., Moreira, M., Harris, J.W., Allégre, C.J. (2005) Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana). Earth and Planetary Science Letters 240, 559–572.

; Burgess et al., 1998

Burgess, R., Johnson, L., Mattey, D., Harris, J., Turner, G. (1998) He, Ar and C isotopes in coated and polycrystalline diamonds. Chemical Geology 146, 205–217.

) Fields are shown for other diamondites (Mikhail et al., 2013

Mikhail, S., Kurat, G., Dubosi, G., Verchovsky, A.B., Jones, A.P., Mileage, H.J. (2013) Peridotitic and websteritic diamondites provide new information regarding mantle melting and metasomatism induced through the subduction of crustal volatiles. Geochimica Cosmochimica et Acta 107, 1–11.

), eclogitic monocrystalline diamonds from Jwaneng (Cartigny et al., 1998

Cartigny, P., Harris, J.W., Javoy, M. (1998) Eclogitic diamond formation at Jwaneng: no room for a recycled component. Science 280, 1421–1424.

) and Orapa (Cartigny et al., 1999

Cartigny, P., Harris J.W., Javoy, M. (1999) Eclogitic, peridotitic and metamorphic diamonds and the problems of carbon recycling—the case of Orapa (Botswana). 7th International Kimberlite Conference Extended Abstracts, 117–124.

), the mean mantle and altered oceanic crustal material (Cartigny et al., 2014

Cartigny, P., Palot, M., Thomassot, E., Harris, J.W. (2014) Diamond formation: A stable isotope perspective. Annual Reviews of Earth and Planetary Sciences 42, 699–732.

).
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Helium Isotopes


The 3He/4He ratios range from 0.06 to 8.5 Ra, overlapping, but extending, the range previously measured in southern Africa diamondites (Fig. 2a).  The highest 3He/4He in these diamondites (8.5 ± 0.4 Ra) exceeds the highest ratios measured in all other diamondites and fibrous diamonds from southern Africa (Fig. 2a). It is higher than values typical of SCLM (6.1 ± 1 Ra; Gautheron et al., 2005

Gautheron, C., Cartigny, P., Moreira, M., Harris, J.W., Allégre, C.J. (2005) Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana). Earth and Planetary Science Letters 240, 559–572.

) and modern kimberlite from southern Africa (4.2 Ra; Brown et al., 2012

Brown, R.J., Manya, S., Buisman, I., Fontana, G., Field, M., Mac Niocaill, C., Sparks, R.S.J., Stuart, F.M. (2012) Eruption of kimberlite magmas: physical volcanology, geomorphology, and age of the youngest kimberlitic volcanoes known on Earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania). Bulletin of Volcanology 74, 1621–1643, doi: 10.1007/s00445-012-0619-8.

), and overlaps the present day convecting upper mantle (8 ± 1 Ra; Graham et al., 2014

Graham, D.W., Hanan, B.B., Hémond, C., Blichert-Toft, J., Albarède, F. (2014) Helium isotopic textures in Earth’s upper mantle. Geochemistry, Geophysics, Geosystems 15, 2048–2074, doi: 10.1002/2014GC005264.

).

The southern Africa diamondites formed between kimberlite emplacement and craton stabilisation (91 to >3000 Ma; Gurney et al., 2010

Gurney, J.J., Helmstaedt, H.H., Richardson, S.H., Shirey, S.B. (2010) Diamonds through time. Economic Geology 105, 689–712.

). The highly aggregated nitrogen in diamondites is consistent with long mantle residence times (Mikhail et al., 2019

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.

), which implies ages far greater than 91 Ma. This leaves open the likelihood that 4He produced by alpha decay of U and Th in the fluid, or recoil of 4He into the fluids, has decreased the 3He/4He (Timmermann et al., 2019a

Timmermann, S., Yeowa, H., Honda, M., Howell, D., Jaquesa, A.L., Krebs, M.Y., Woodland, S., Pearson, D.G., Ávila, Y.A., Ireland, T.R. (2019a) U-Th/He systematics of fluid-rich ‘fibrous’ diamonds – Evidence for pre- and syn-kimberlite eruption ages. Chemical Geology 515, 22–36.

). A strong relationship between 3He/4He and 3He concentration is apparent from Figure 2a and reflects radiogenic 4He production since the fluids were trapped in the diamond.  In this case, all 3He/4He ratios are minimum values, although it appears that diamonds with >1 x 10-12 ccSTP 3He/g are largely immune to the effect based on Figure 2a.  The diamonds studied here are not alluvial, ruling out cosmogenic 3He implantation during surface exposure (e.g., Yakubovich et al., 2019

Yakubovich, O., Stuart, F.M., Nesterenok, A., Carracedo, A.P. (2019) Cosmogenic 3He in alluvial metal and alloy grains; assessing the potential as a tool for quantifying sediment transport times. Chemical Geology 517, 22–33.

).


Figure 2 (a) Helium isotope systematics of fluids released by in vacuo crushing of diamondites. The 3He/4He of modern the convecting upper mantle (CUM) and the sub-continental lithospheric mantle (SCLM) are shown for reference. (b) Carbon-helium isotope systematics of southern African diamondites and fibrous diamonds from across southern Africa. The mixing lines are plotted between mantle and crustal fluid sources, the crosses refer to percent of mantle fluid component. The mixing lines plotted in Figure 2b are hyperbolic as [4He]mantle/[4He]crust is assumed to be 10. Comparative data are from Burgess et al. (1998)

Burgess, R., Johnson, L., Mattey, D., Harris, J., Turner, G. (1998) He, Ar and C isotopes in coated and polycrystalline diamonds. Chemical Geology 146, 205–217.

, Gautheron et al. (2005)

Gautheron, C., Cartigny, P., Moreira, M., Harris, J.W., Allégre, C.J. (2005) Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana). Earth and Planetary Science Letters 240, 559–572.

and Timmermann et al. (2018

Timmermann, S., Honda, M., Phillips, D., Jaques, A.L., Harris, J.W. (2018) Noble gas geochemistry of fluid inclusions in South African diamonds: implications for the origin of diamond-forming fluids. Mineralogy and Petrology 112, 181–195.

, 2019a

Timmermann, S., Yeowa, H., Honda, M., Howell, D., Jaquesa, A.L., Krebs, M.Y., Woodland, S., Pearson, D.G., Ávila, Y.A., Ireland, T.R. (2019a) U-Th/He systematics of fluid-rich ‘fibrous’ diamonds – Evidence for pre- and syn-kimberlite eruption ages. Chemical Geology 515, 22–36.

).
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Tracing the Origin of Diamondite-forming Volatiles


Mixing lines in Figure 2b are drawn between oceanic crust-derived fluids with δ13C = -20 and -30 ‰, and 3He/4He = 0.01 Ra, and mantle-derived fluids with δ13C = -5 ‰ and 3He/4He = 9 Ra. The mantle 3He/4He end member is slightly higher than the present day upper asthenosphere and lithosphere mantle values, reflecting the temporal evolution of 3He/4He in the mantle (Porcelli and Elliot, 2008

Porcelli, D., Elliott, T. (2008) The evolution of He Isotopes in the convecting mantle and the preservation of high 3He/4He ratios. Earth and Planetary Science Letters 269, 175–185.

).

The spread of data can be explained by mixing between He-rich, high 3He/4He mantle source (δ13C = -5 ‰)  and a He-poor, low 3He/4He crustal component (δ13C between -15 and -25 ‰). This is strongly supported by the 15N-enrichment in the 13C-depleted samples which best matches altered oceanic crust (Cartigny et al., 2014

Cartigny, P., Palot, M., Thomassot, E., Harris, J.W. (2014) Diamond formation: A stable isotope perspective. Annual Reviews of Earth and Planetary Sciences 42, 699–732.

) or organic material hosted in sedimentary rocks (Thomazo et al., 2009

Thomazo, C., Pinti, D.L., Busigny, V., Ader, M., Hashizume, K., Philippot, P. (2009) Biological activity and the Earth's surface evolution: Insights from carbon, sulfur, nitrogen and iron stable isotopes in the rock record. Comptes Rendus Palevol 8, 665–678.

). Furthermore, the prevalence of eclogitic to websteritic silicate inclusions/intergrowths in these diamondites further supports a recycled basaltic component (Mikhail et al., 2019

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.

). These observations contrast strongly with the fibrous diamonds which, although they also contain a high density of trapped fluids with evidence of recycled material based on lithophile element geochemistry and Sr-isotopes (Weiss et al., 2015

Weiss, Y., McNeill, J., Pearson, D.G., Nowell, G.M., Ottley, C.J. (2015) Highly saline fluids from a subducting slab as the source for fluid-rich diamonds. Nature 524, 339–342.

), they commonly show C and N isotope values that are indistinguishable from mantle values (see Fig. 2a,b). Despite significantly lower 3He concentrations, they are thus more likely to be affected by radiogenic 4He ingrowth; the highest 3He/4He southern African diamondites are higher than the highest values measured in fibrous diamonds from southern Africa (Fig. 2a). This suggests that there is no genetic link between the fibrous diamonds and diamondites, and they likely formed in different mantle domains or from isotopically distinct sources.

A mantle origin for the helium trapped in the diamondite fluids is incontrovertible (Fig. 2a), but the incorporation mechanism(s) for He into the high density fluids (HDF) is less clear. Possible mechanisms include the assimilation of He from grain boundaries or small volume mantle melts initiated by the expulsion of slab-derived fluids into the mantle wedge or SCLM.  Resolving the origin of the mantle He is hampered by the post-formation ingrowth of radiogenic 4He, and because the 3He/4He ratio of the mantle has reduced over time (e.g., Porcelli and Elliot, 2008

Porcelli, D., Elliott, T. (2008) The evolution of He Isotopes in the convecting mantle and the preservation of high 3He/4He ratios. Earth and Planetary Science Letters 269, 175–185.

). The uncertainty in the diamond age and the He isotope evolution of the mantle makes drawing firm conclusions regarding the source of the mantle He problematic.  One prevailing view posits that the southern African kimberlites originate at the edges of large heterogeneities at the core-mantle boundary (Torsvik et al., 2010

Torsvik, T., Burke, K., Steinberger, B., Webb, S.J., Ashwal, L. (2010) Diamonds sampled by plumes from the core-mantle boundary. Nature 466, 352–355.

). This should be evident from high 3He/4He (e.g., Stuart et al., 2003

Stuart, F.M., Lass-Evans, S., Fitton, J.G., Ellam, R.M. (2003) Extreme 3He/4He in picritic basalts from Baffin Island: role of a mixed reservoir in mantle plumes. Nature 424, 57–59.

) in kimberlitic fluids. In contrast to Brazilian diamonds (Timmermann et al., 2019b

Timmerman, S., Honda, M., Burnham, A.D., Amelin, Y., Woodland, S., Pearson, D.G., Jaques, A.L., Le Losq, C., Bennett, V.C., Bulanova, G.P., Smith, C.B., Harris, J.W., Tohver. E. (2019b) Primordial and recycled helium isotope signatures in the mantle transition zone. Science 16, 692–694.

), the absence of 3He/4He above Phanerozoic upper mantle values in fluid-rich diamonds from southern Africa rules likely reflects the formation of the diamonds prior to the generation of the kimberlite melts, and post-formation isolation from kimberlitic fluids. Furthermore, it is conceivable that high 3He/4He ratios (>10 Ra) in some samples (e.g., Siberian fibrous diamonds; Broadley et al., 2018

Broadley, M.W., Kagi, H., Burgess, R., Zedgenizov, D., Mikhail, S., Almayrac, M., Ragozin, A., Pomazansky, B., Sumino, B. (2018) Plume-lithosphere interaction, and the formation of fibrous diamonds. Geochemical Perspectives Letters 8, 26–30.

) might reflect the higher 3He/4He of the ancient mantle, as opposed to evidence for the role of mantle plumes in the generation of shallow diamonds. While the highest diamondite 3He/4He ratios are above modern SCLM values, they do not allow a distinction to be made between an ancient sub-continental lithospheric or a convecting upper asthenospheric mantle source.  That said, the absence of 3He/4He above Phanerozoic upper mantle values in fluid-rich diamonds from all southern Africa rules out a requirement for deep mantle fluids in the generation of diamonds in the SCLM. Diamondites from southern Africa crystallised from HDFs where the C originates from crustal and mantle sources, the N is mostly slab derived, likely from a subducted sedimentary organic component, but the He is largely derived from the upper mantle.

The carbon and nitrogen isotope data (Fig. 1) are consistent with prevailing models for diamondite-formation, whereby high density fluid from a subducting slab interacts with the SCLM (Mikhail et al., 2013

Mikhail, S., Kurat, G., Dubosi, G., Verchovsky, A.B., Jones, A.P., Mileage, H.J. (2013) Peridotitic and websteritic diamondites provide new information regarding mantle melting and metasomatism induced through the subduction of crustal volatiles. Geochimica Cosmochimica et Acta 107, 1–11.

, 2019

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.

; Jacob et al., 2000

Jacob, D.E., Viljoen, K.S., Grassineau, N., Jagoutz, E. (2000) Remobilization in the cratonic lithosphere recorded in polycrystalline diamond. Science 289, 1182–1185.

, 2014

Jacob, D.E., Dobrzhinetskaya, L., Wirth, R. (2014) New insight into polycrystalline diamond genesis from modern nanoanalytical techniques. Earth-Science Reviews 136, 21–35.

). Mechanically, this journey provides an opportunity for the exchange of material between a subducted fluid acting as a mobilising agent (HDF) with solid residual mantle rocks (illustrated in Fig. 3).  This physical interaction can assimilate mantle volatiles into the HDF resulting in hybridisation (where the HDF is now comprised of subducted + mantle volatiles). If the mantle component in the subduction-derived HDF is small, then the diamond-forming media might not reveal this assimilated mantle component in δ13C-δ15N space (e.g., as shown for most samples in Fig. 1). For example, if the subducted carbon has δ13C = -25 ‰ and assimilated 10 % mantle carbon (δ13C = -5 ‰) with most of the nitrogen provided by the subducted source (δ15N > 0 ‰) then the resulting hybrid would have a δ13C value of -23 ‰ and positive δ15N. Such δ13C-δ15N values do not suggest mixing between mantle and crustal volatiles because they are within the range of crustal organic carbon and distinct from the canonical mean mantle values for both δ13C and δ15N (-5 ± 3 ‰; Cartigny et al., 2014

Cartigny, P., Palot, M., Thomassot, E., Harris, J.W. (2014) Diamond formation: A stable isotope perspective. Annual Reviews of Earth and Planetary Sciences 42, 699–732.

; Mikhail et al., 2014

Mikhail, S., Howell, D., Hutchison, M., Verchovsky, A.B., Warburton, P., Southworth, R., Thompson, A.R., Jones, A.P., Mileage, H.J. (2014) Constraining the internal variability of carbon and nitrogen isotopes in diamonds. Chemical Geology 366, 14–23.

). However, if the 13C-depleted HDF assimilated 50 % mantle carbon and 100 % mantle helium, then the resulting hybrid would have a δ13C value of -15 ‰ and 3He/4He > 1 Ra. This scenario matches the data of samples Dia030 (southern Africa) and ORF57 (Orapa) which show elevated 3He/4He ratios of 8.5 and 7.5 Ra with corresponding δ13C values (-16.6 ‰; Fig. 2b).

We argue that the C-N isotope data commonly identify a mix of multiple sources. In the case of the southern African and Orapa diamondites, the δ13C-δ15N systematics reveal that the subducted component dominates over the mantle component (Fig. 1).  However, the discrete, but important, mantle component is revealed in the fluid-hosted helium isotope data. The higher 3He concentration of the mantle fluids compared to slab-derived fluids means that the helium isotopes trace small contributions of mantle-derived volatiles better than C or N isotopes.  For example, samples DIA058B (southern African) and ORF143 (Orapa) show elevated 3He/4He ratios of 3.9 and 1.6 Ra with correspondingly low δ13C values of -19.1 and -19.9 ‰ (Fig. 2b).  Ergo, helium reveals what carbon and nitrogen cannot. When the carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamond formation (Fig. 1), the helium isotopes reveal an unambiguous mantle component hidden within strongly 13C-depleted diamond (Fig. 2b). This observation speaks to the mechanics of fluid migration through the SCLM. Our data require that subducted material percolates though ambient mantle en route to the SCLM and results in the mechanical re-mobilisation of primary mantle volatiles (Fig. 3). These data further enhance the notion that the volatile-element components within a diamond-forming HDF do not always share a common origin, and indeed, the C-N-He isotopic systems preserved in mantle diamonds record distinct processes and should not be considered coupled isotopic systems, by default.


Figure 3 Cartoon illustrating the model discussed in the text. Not to scale.
Full size image | Download in Powerpoint

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Acknowledgements


SM acknowledges support from the National Environmental Research Council (grant no. NE/PO12167/1). We are grateful to Dr. J.J. Gurney, Dr. Gabor Dobosi, and the late Prof. Gero Kurat for sample provisions, and the editorial handling of Prof. Cin-Ty Lee.

Editor: Cin-Ty Lee

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References



Broadley, M.W., Kagi, H., Burgess, R., Zedgenizov, D., Mikhail, S., Almayrac, M., Ragozin, A., Pomazansky, B., Sumino, B. (2018) Plume-lithosphere interaction, and the formation of fibrous diamonds. Geochemical Perspectives Letters 8, 26–30.
Show in context

As well as providing the only direct samples of metasomatic fluids from the mantle (Weiss et al., 2015), the trapped fluids enable the application of noble gas isotope tracers to resolve the origin of diamond-forming fluids (Burgess et al., 1998; Gautheron et al., 2005; Broadley et al., 2018; Timmermann et al., 2018, 2019a,b).
View in article
Furthermore, it is conceivable that high 3He/4He ratios (>10 Ra) in some samples (e.g., Siberian fibrous diamonds; Broadley et al., 2018) might reflect the higher 3He/4He of the ancient mantle, as opposed to evidence for the role of mantle plumes in the generation of shallow diamonds.
View in article


Brown, R.J., Manya, S., Buisman, I., Fontana, G., Field, M., Mac Niocaill, C., Sparks, R.S.J., Stuart, F.M. (2012) Eruption of kimberlite magmas: physical volcanology, geomorphology, and age of the youngest kimberlitic volcanoes known on Earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania). Bulletin of Volcanology 74, 1621–1643, doi: 10.1007/s00445-012-0619-8.
Show in context

It is higher than values typical of SCLM (6.1 ± 1 Ra; Gautheron et al., 2005) and modern kimberlite from southern Africa (4.2 Ra; Brown et al., 2012), and overlaps the present day convecting upper mantle (8 ± 1 Ra; Graham et al., 2014).
View in article


Burgess, R., Johnson, L., Mattey, D., Harris, J., Turner, G. (1998) He, Ar and C isotopes in coated and polycrystalline diamonds. Chemical Geology 146, 205–217.
Show in context

As well as providing the only direct samples of metasomatic fluids from the mantle (Weiss et al., 2015), the trapped fluids enable the application of noble gas isotope tracers to resolve the origin of diamond-forming fluids (Burgess et al., 1998; Gautheron et al., 2005; Broadley et al., 2018; Timmermann et al., 2018, 2019a,b).
View in article
Figure 1 New C and N isotope data for southern African diamondites (red circles) and literature data (grey circles: Gautheron et al., 2005; Burgess et al., 1998)
View in article
Figure 2 [...] Comparative data are from Burgess et al. (1998), Gautheron et al. (2005) and Timmermann et al. (2018, 2019a).
View in article


Carracedo, A. Rodés, Á., Smellie, J., Stuart, F.M. (2019) Episodic erosion in West Antarctica inferred from cosmogenic 3He and 10Be in olivine from Mount Hampton. Geomorphology 327, 438–445.
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The helium isotope composition of trapped fluids released by in vacuo crushing were determined using a ThermoFisher Scientific Helix-SFT mass spectrometer at the Scottish Universities Environmental Research Centre (Carracedo et al., 2019).
View in article


Cartigny, P., Harris, J.W., Javoy, M. (1998) Eclogitic diamond formation at Jwaneng: no room for a recycled component. Science 280, 1421–1424.
Show in context

For instance, eclogitic diamonds from Jwaneng (Cartigny et al., 1998) and Orapa (Cartigny et al., 1999) show crust-like low δ13C values yet have mantle-like negative δ15N values (Fig. 1), and C and N isotope systems can be decoupled during diamond-formation (Mikhail et al., 2014; Hogberg et al., 2016).
View in article
Figure 1 [...] Fields are shown for other diamondites (Mikhail et al., 2013), eclogitic monocrystalline diamonds from Jwaneng (Cartigny et al., 1998) and Orapa (Cartigny et al., 1999), the mean mantle and altered oceanic crustal material (Cartigny et al., 2014).
View in article


Cartigny, P., Harris J.W., Javoy, M. (1999) Eclogitic, peridotitic and metamorphic diamonds and the problems of carbon recycling—the case of Orapa (Botswana). 7th International Kimberlite Conference Extended Abstracts, 117–124.
Show in context

For instance, eclogitic diamonds from Jwaneng (Cartigny et al., 1998) and Orapa (Cartigny et al., 1999) show crust-like low δ13C values yet have mantle-like negative δ15N values (Fig. 1), and C and N isotope systems can be decoupled during diamond-formation (Mikhail et al., 2014; Hogberg et al., 2016).
View in article
Figure 1 [...] Fields are shown for other diamondites (Mikhail et al., 2013), eclogitic monocrystalline diamonds from Jwaneng (Cartigny et al., 1998) and Orapa (Cartigny et al., 1999), the mean mantle and altered oceanic crustal material (Cartigny et al., 2014).
View in article


Cartigny, P., Palot, M., Thomassot, E., Harris, J.W. (2014) Diamond formation: A stable isotope perspective. Annual Reviews of Earth and Planetary Sciences 42, 699–732.
Show in context

The origin of diamond-forming fluids is commonly addressed using the stable isotope values of carbon and nitrogen, where coupled δ13C-δ15N values can indicate diamond growth from mantle-derived fluids (Cartigny et al., 2014).
View in article
Figure 1 [...] Fields are shown for other diamondites (Mikhail et al., 2013), eclogitic monocrystalline diamonds from Jwaneng (Cartigny et al., 1998) and Orapa (Cartigny et al., 1999), the mean mantle and altered oceanic crustal material (Cartigny et al., 2014).
View in article
This is strongly supported by the 15N-enrichment in the 13C-depleted samples which best matches altered oceanic crust (Cartigny et al., 2014) or organic material hosted in sedimentary rocks (Thomazo et al., 2009).
View in article
Such δ13C-δ15N values do not suggest mixing between mantle and crustal volatiles because they are within the range of crustal organic carbon and distinct from the canonical mean mantle values for both δ13C and δ15N (-5 ± 3 ‰; Cartigny et al., 2014; Mikhail et al., 2014).
View in article


Gautheron, C., Cartigny, P., Moreira, M., Harris, J.W., Allégre, C.J. (2005) Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana). Earth and Planetary Science Letters 240, 559–572.
Show in context

As well as providing the only direct samples of metasomatic fluids from the mantle (Weiss et al., 2015), the trapped fluids enable the application of noble gas isotope tracers to resolve the origin of diamond-forming fluids (Burgess et al., 1998; Gautheron et al., 2005; Broadley et al., 2018; Timmermann et al., 2018, 2019a,b).
View in article
Figure 1 New C and N isotope data for southern African diamondites (red circles) and literature data (grey circles: Gautheron et al., 2005; Burgess et al., 1998)
View in article
It is higher than values typical of SCLM (6.1 ± 1 Ra; Gautheron et al., 2005) and modern kimberlite from southern Africa (4.2 Ra; Brown et al., 2012), and overlaps the present day convecting upper mantle (8 ± 1 Ra; Graham et al., 2014).
View in article
Figure 2 [...] Comparative data are from Burgess et al. (1998), Gautheron et al. (2005) and Timmermann et al. (2018, 2019a).
View in article


Graham, D.W., Hanan, B.B., Hémond, C., Blichert-Toft, J., Albarède, F. (2014) Helium isotopic textures in Earth’s upper mantle. Geochemistry, Geophysics, Geosystems 15, 2048–2074, doi: 10.1002/2014GC005264.
Show in context

It is higher than values typical of SCLM (6.1 ± 1 Ra; Gautheron et al., 2005) and modern kimberlite from southern Africa (4.2 Ra; Brown et al., 2012), and overlaps the present day convecting upper mantle (8 ± 1 Ra; Graham et al., 2014).
View in article


Gurney, J.J., Helmstaedt, H.H., Richardson, S.H., Shirey, S.B. (2010) Diamonds through time. Economic Geology 105, 689–712.
Show in context

The southern Africa diamondites formed between kimberlite emplacement and craton stabilisation (91 to >3000 Ma; Gurney et al., 2010).
View in article


Hogberg, K., Stachel, T., Stern, R.A. (2016) Carbon and nitrogen isotope systematics in diamond: Different sensitivities to isotopic fractionation or a decoupled origin? Lithos 265, 16–30.
Show in context

For instance, eclogitic diamonds from Jwaneng (Cartigny et al., 1998) and Orapa (Cartigny et al., 1999) show crust-like low δ13C values yet have mantle-like negative δ15N values (Fig. 1), and C and N isotope systems can be decoupled during diamond-formation (Mikhail et al., 2014; Hogberg et al., 2016).
View in article


Jacob, D.E., Viljoen, K.S., Grassineau, N., Jagoutz, E. (2000) Remobilization in the cratonic lithosphere recorded in polycrystalline diamond. Science 289, 1182–1185.
Show in context

The carbon and nitrogen isotope data (Fig. 1) are consistent with prevailing models for diamondite-formation, whereby high density fluid from a subducting slab interacts with the SCLM (Mikhail et al., 2013, 2019; Jacob et al., 2000, 2014).
View in article


Jacob, D.E., Dobrzhinetskaya, L., Wirth, R. (2014) New insight into polycrystalline diamond genesis from modern nanoanalytical techniques. Earth-Science Reviews 136, 21–35.
Show in context

Diamonds can trap fluid during their growth, either along the diamond fibres, between interlocking polycrystalline grains, and surrounding diamond-hosted mineral inclusions (Navon et al., 1988; Jacob et al., 2014).
View in article
The carbon and nitrogen isotope data (Fig. 1) are consistent with prevailing models for diamondite-formation, whereby high density fluid from a subducting slab interacts with the SCLM (Mikhail et al., 2013, 2019; Jacob et al., 2000, 2014).
View in article


Kaiser, W., Bond, W.L. (1959) Nitrogen, a major impurity in common type I diamond. Physical Review Letters 115, 857–863.
Show in context

Diamond is a chemically simple mineral comprised largely of C and trace N (~0.025 %) incorporated as single nitrogen atoms substituting for carbon (Kaiser and Bond, 1959).
View in article


Kurat, G., Dobosi, G. (2000) Garnet and diopside-bearing diamondites (framesites). Mineralogy and Petrology 69, 143-159.
Show in context

The latter, also known as diamondites, are a mixture of diamond intergrown with silicates and oxides (Kurat and Dobosi, 2000).
View in article
These samples are described in detail elsewhere (Kurat and Dobosi., 2000; Mikhail et al., 2019; Supplementary Information).
View in article


Mikhail, S., Kurat, G., Dubosi, G., Verchovsky, A.B., Jones, A.P., Mileage, H.J. (2013) Peridotitic and websteritic diamondites provide new information regarding mantle melting and metasomatism induced through the subduction of crustal volatiles. Geochimica Cosmochimica et Acta 107, 1–11.
Show in context

Nitrogen concentrations range from 8–1389 ppm and do not correlate with δ13C or δ15N, consistent with previous observations (Mikhail et al., 2013, 2014, 2019).
View in article
Figure 1 [...] Fields are shown for other diamondites (Mikhail et al., 2013), eclogitic monocrystalline diamonds from Jwaneng (Cartigny et al., 1998) and Orapa (Cartigny et al., 1999), the mean mantle and altered oceanic crustal material (Cartigny et al., 2014).
View in article
The carbon and nitrogen isotope data (Fig. 1) are consistent with prevailing models for diamondite-formation, whereby high density fluid from a subducting slab interacts with the SCLM (Mikhail et al., 2013, 2019; Jacob et al., 2000, 2014).
View in article


Mikhail, S., Howell, D., Hutchison, M., Verchovsky, A.B., Warburton, P., Southworth, R., Thompson, A.R., Jones, A.P., Mileage, H.J. (2014) Constraining the internal variability of carbon and nitrogen isotopes in diamonds. Chemical Geology 366, 14–23.
Show in context

For instance, eclogitic diamonds from Jwaneng (Cartigny et al., 1998) and Orapa (Cartigny et al., 1999) show crust-like low δ13C values yet have mantle-like negative δ15N values (Fig. 1), and C and N isotope systems can be decoupled during diamond-formation (Mikhail et al., 2014; Hogberg et al., 2016).
View in article
The δ13C, δ15N values and N concentrations were obtained using the automated and custom-made Finesse system at the Open University following the method outlined in Mikhail et al. (2014).
View in article
Nitrogen concentrations range from 8–1389 ppm and do not correlate with δ13C or δ15N, consistent with previous observations (Mikhail et al., 2013, 2014, 2019).
View in article
Such δ13C-δ15N values do not suggest mixing between mantle and crustal volatiles because they are within the range of crustal organic carbon and distinct from the canonical mean mantle values for both δ13C and δ15N (-5 ± 3 ‰; Cartigny et al., 2014; Mikhail et al., 2014).
View in article


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.
Show in context

These samples are described in detail elsewhere (Kurat and Dobosi., 2000; Mikhail et al., 2019; Supplementary Information).
View in article
There is no systematic relationship between C or N isotope values and the silicate paragenesis, consistent with previous observations (Mikhail et al., 2019).
View in article
Nitrogen concentrations range from 8–1389 ppm and do not correlate with δ13C or δ15N, consistent with previous observations (Mikhail et al., 2013, 2014, 2019).
View in article
The highly aggregated nitrogen in diamondites is consistent with long mantle residence times (Mikhail et al., 2019), which implies ages far greater than 91 Ma.
View in article
Furthermore, the prevalence of eclogitic to websteritic silicate inclusions/intergrowths in these diamondites further supports a recycled basaltic component (Mikhail et al., 2019).
View in article
The carbon and nitrogen isotope data (Fig. 1) are consistent with prevailing models for diamondite-formation, whereby high density fluid from a subducting slab interacts with the SCLM (Mikhail et al., 2013, 2019; Jacob et al., 2000, 2014).
View in article


Navon, O., Hutcheon, I.D., Rossman, G.R., Wasserburg, G.J. (1988) Mantle-derived fluids in diamond micro-inclusions. Nature 335, 784–789.
Show in context


Porcelli, D., Elliott, T. (2008) The evolution of He Isotopes in the convecting mantle and the preservation of high 3He/4He ratios. Earth and Planetary Science Letters 269, 175–185.
Show in context

The mantle 3He/4He end member is slightly higher than the present day upper asthenosphere and lithosphere mantle values, reflecting the temporal evolution of 3He/4He in the mantle (Porcelli and Elliot, 2008).
View in article
Resolving the origin of the mantle He is hampered by the post-formation ingrowth of radiogenic 4He, and because the 3He/4He ratio of the mantle has reduced over time (e.g., Porcelli and Elliot, 2008).
View in article


Shirey, S.B., Cartigny, P., Frost, D.J., Keshav, S., Nestola, F., Nimis, P., Pearson, D.G., Sobolev, N.V., Walter, M.J. (2013) Diamonds and the geology of mantle carbon. Reviews in Mineralogy and Geochemistry 75, 355–421.
Show in context

Placing diamond-formation into context of large scale tectonothermal processes, such as subduction and plume-lithosphere interaction, is fundamental to understanding the deep terrestrial carbon cycle (Shirey et al., 2013).
View in article


Smith, C.B., Walter, M.J., Bulanova, G.P., Mikhail, S., Burnham, A.D., Gobbo, L., Kohn, S.C. (2016) Diamonds from Dachine, French Guiana: a unique record of Early Proterozoic subduction. Lithos 265, 82–95.
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However, many datasets require the contribution of crustal sources for the C and/or N, such as eclogitic diamonds from Dachine (Smith et al., 2016).
View in article


Stuart, F.M., Lass-Evans, S., Fitton, J.G., Ellam, R.M. (2003) Extreme 3He/4He in picritic basalts from Baffin Island: role of a mixed reservoir in mantle plumes. Nature 424, 57–59.
Show in context

This should be evident from high 3He/4He (e.g., Stuart et al., 2003) in kimberlitic fluids.
View in article


Thomazo, C., Pinti, D.L., Busigny, V., Ader, M., Hashizume, K., Philippot, P. (2009) Biological activity and the Earth's surface evolution: Insights from carbon, sulfur, nitrogen and iron stable isotopes in the rock record. Comptes Rendus Palevol 8, 665–678.
Show in context

These data indicate crustal fluids sourced from subducted oceanic sediments or altered oceanic crust (Thomazo et al., 2009).
View in article
This is strongly supported by the 15N-enrichment in the 13C-depleted samples which best matches altered oceanic crust (Cartigny et al., 2014) or organic material hosted in sedimentary rocks (Thomazo et al., 2009).
View in article


Timmermann, S., Honda, M., Phillips, D., Jaques, A.L., Harris, J.W. (2018) Noble gas geochemistry of fluid inclusions in South African diamonds: implications for the origin of diamond-forming fluids. Mineralogy and Petrology 112, 181–195.
Show in context

As well as providing the only direct samples of metasomatic fluids from the mantle (Weiss et al., 2015), the trapped fluids enable the application of noble gas isotope tracers to resolve the origin of diamond-forming fluids (Burgess et al., 1998; Gautheron et al., 2005; Broadley et al., 2018; Timmermann et al., 2018, 2019a,b).
View in article
Figure 2 [...] Comparative data are from Burgess et al. (1998), Gautheron et al. (2005) and Timmermann et al. (2018, 2019a).
View in article


Timmermann, S., Yeowa, H., Honda, M., Howell, D., Jaquesa, A.L., Krebs, M.Y., Woodland, S., Pearson, D.G., Ávila, Y.A., Ireland, T.R. (2019a) U-Th/He systematics of fluid-rich ‘fibrous’ diamonds – Evidence for pre- and syn-kimberlite eruption ages. Chemical Geology 515, 22–36.
Show in context

As well as providing the only direct samples of metasomatic fluids from the mantle (Weiss et al., 2015), the trapped fluids enable the application of noble gas isotope tracers to resolve the origin of diamond-forming fluids (Burgess et al., 1998; Gautheron et al., 2005; Broadley et al., 2018; Timmermann et al., 2018, 2019a,b).
View in article
This leaves open the likelihood that 4He produced by alpha decay of U and Th in the fluid, or recoil of 4He into the fluids, has decreased the 3He/4He (Timmermann et al., 2019a).
View in article
Figure 2 [...] Comparative data are from Burgess et al. (1998), Gautheron et al. (2005) and Timmermann et al. (2018, 2019a).
View in article


Timmermann, S., Honda, M., Burnham, A.D., Amelin, Y., Woodland, S., Pearson, D.G., Jaques, A.L., Le Losq, C., Bennett, V.C., Bulanova, G.P., Smith, C.B., Harris, J.W., Tohver. E. (2019b) Primordial and recycled helium isotope signatures in the mantle transition zone. Science 16, 692–694.
Show in context

As well as providing the only direct samples of metasomatic fluids from the mantle (Weiss et al., 2015), the trapped fluids enable the application of noble gas isotope tracers to resolve the origin of diamond-forming fluids (Burgess et al., 1998; Gautheron et al., 2005; Broadley et al., 2018; Timmermann et al., 2018, 2019a,b).
View in article
In contrast to Brazilian diamonds (Timmermann et al., 2019b), the absence of 3He/4He above Phanerozoic upper mantle values in fluid-rich diamonds from southern Africa rules likely reflects the formation of the diamonds prior to the generation of the kimberlite melts, and post-formation isolation from kimberlitic fluids.
View in article


Torsvik, T., Burke, K., Steinberger, B., Webb, S.J., Ashwal, L. (2010) Diamonds sampled by plumes from the core-mantle boundary. Nature 466, 352–355.
Show in context

One prevailing view posits that the southern African kimberlites originate at the edges of large heterogeneities at the core-mantle boundary (Torsvik et al., 2010).
View in article


Weiss, Y., McNeill, J., Pearson, D.G., Nowell, G.M., Ottley, C.J. (2015) Highly saline fluids from a subducting slab as the source for fluid-rich diamonds. Nature 524, 339–342.
Show in context

As well as providing the only direct samples of metasomatic fluids from the mantle (Weiss et al., 2015), the trapped fluids enable the application of noble gas isotope tracers to resolve the origin of diamond-forming fluids (Burgess et al., 1998; Gautheron et al., 2005; Broadley et al., 2018; Timmermann et al., 2018, 2019a,b).
View in article
These observations contrast strongly with the fibrous diamonds which, although they also contain a high density of trapped fluids with evidence of recycled material based on lithophile element geochemistry and Sr-isotopes (Weiss et al., 2015), they commonly show C and N isotope values that are indistinguishable from mantle values (see Fig. 2a,b).
View in article


Yakubovich, O., Stuart, F.M., Nesterenok, A., Carracedo, A.P. (2019) Cosmogenic 3He in alluvial metal and alloy grains; assessing the potential as a tool for quantifying sediment transport times. Chemical Geology 517, 22–33.
Show in context

The diamonds studied here are not alluvial, ruling out cosmogenic 3He implantation during surface exposure (e.g., Yakubovich et al., 2019).
View in article



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Supplementary Information


The Supplementary Information includes:
  • Sample Descriptions
  • Carbon and Nitrogen Isotope Analysis
  • Helium Isotope Analysis
  • Table S-1
  • Supplementary Information References

Download the Supplementary Information (PDF).

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