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by admin | Sep 10, 2020 | mainpost, vol15 | 1 comment

N. Pujol-Solà, A. Garcia-Casco, J.A. Proenza, J.M. González-Jiménez, A. del Campo, V. Colás, À. Canals, A. Sánchez-Navas, J. Roqué-Rosell

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Diamond forms during low pressure serpentinisation of oceanic lithosphere

N. Pujol-Solà1,

1Department of Mineralogy, Petrology and Applied Geology, University of Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain

A. Garcia-Casco2,3,

2Department of Mineralogy and Petrology, University of Granada, Av. Fuentenueva s/n, 18071 Granada, Spain
3Andalusian Earth Science Institute (IACT), Spanish Research Council (CSIC)–University of Granada, Av. de las Palmeras 4, 18100 Armilla, Spain

J.A. Proenza1,4,

1Department of Mineralogy, Petrology and Applied Geology, University of Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain
4Institut de Nanociència i Nanotecnologia, IN2UB Facultat de Química, University of Barcelona, Diagonal 645, 08028 Barcelona, Spain

J.M. González-Jiménez2,

2Department of Mineralogy and Petrology, University of Granada, Av. Fuentenueva s/n, 18071 Granada, Spain

A. del Campo5,

5Instituto de Cerámica y Vidrio – CSIC. C. Kelsen, 5, Campus de Cantoblanco, 28049 Madrid, Spain

V. Colás6,

6Institute of Geology, National Autonomous University of Mexico, Ciudad Universitaria, 04510 Ciudad de México, Mexico

À. Canals1,

1Department of Mineralogy, Petrology and Applied Geology, University of Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain

A. Sánchez-Navas2,

2Department of Mineralogy and Petrology, University of Granada, Av. Fuentenueva s/n, 18071 Granada, Spain

J. Roqué-Rosell1,4

1Department of Mineralogy, Petrology and Applied Geology, University of Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain
4Institut de Nanociència i Nanotecnologia, IN2UB Facultat de Química, University of Barcelona, Diagonal 645, 08028 Barcelona, Spain

Affiliations | Corresponding Author | Cite as | Funding information

N. Pujol-Solà
Email: npujolsola@ub.edu

1Department of Mineralogy, Petrology and Applied Geology, University of Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain
2Department of Mineralogy and Petrology, University of Granada, Av. Fuentenueva s/n, 18071 Granada, Spain
3Andalusian Earth Science Institute (IACT), Spanish Research Council (CSIC)–University of Granada, Av. de las Palmeras 4, 18100 Armilla, Spain
4Institut de Nanociència i Nanotecnologia, IN2UB Facultat de Química, University of Barcelona, Diagonal 645, 08028 Barcelona, Spain
5Instituto de Cerámica y Vidrio – CSIC. C. Kelsen, 5, Campus de Cantoblanco, 28049 Madrid, Spain
6Institute of Geology, National Autonomous University of Mexico, Ciudad Universitaria, 04510 Ciudad de México, Mexico

Pujol-Solà, N., Garcia-Casco, A., Proenza, J.A., González-Jiménez, J.M., del Campo, A., Colás, V., Canals, À., Sánchez-Navas, A., Roqué-Rosell, J. (2020) Diamond forms during low pressure serpentinisation of oceanic lithosphere. Geochem. Persp. Let. 15, 19–24.

Research funded by: Fondo Europeo de Desarrollo Regional (FEDER) Funds, Spanish Projects CGL2015-65824, RTI2018-099157-A-I00, PID2019-105625RB-C21, and A.RNM.186.UGR18 Additional funding was provided by the Ramón y Cajal Fellowship RYC-2015-17596 to JMGJ, a FPU-PhD grant to NPS, the Mexican research programs CONACYT-Ciencia Básica (A1-S-14574) and UNAM-PAPIIT grant IA-101419, and received support for analyses at CIC from the University of Granada.

Geochemical Perspectives Letters v15 | doi: 10.7185/geochemlet.2029
Received 6 June 2020 | Accepted 31 July 2020 | Published 10 September 2020

Copyright © 2020 The Authors

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

Keywords: ophiolite, gabbro, exotic minerals, chromitite, olivine, Cuba

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Abstract

Abstract | Introduction | Results | Natural Origin of the Studied Diamond | Processes and Conditions for Diamond Formation | Acknowledgements | References | Supplementary Information

Diamond is commonly regarded as an indicator of ultra-high pressure conditions in Earth System Science. This canonical view is challenged by recent data and interpretations that suggest metastable growth of diamond in low pressure environments. One such environment is serpentinisation of oceanic lithosphere, which produces highly reduced CH4-bearing fluids after olivine alteration by reaction with infiltrating fluids. Here we report the first ever observed in situ diamond within olivine-hosted, CH4-rich fluid inclusions from low pressure oceanic gabbro and chromitite samples from the Moa-Baracoa ophiolitic massif, eastern Cuba. Diamond is encapsulated in voids below the polished mineral surface forming a typical serpentinisation array, with methane, serpentine and magnetite, providing definitive evidence for its metastable growth upon low temperature and low pressure alteration of oceanic lithosphere and super-reduction of infiltrated fluids. Thermodynamic modelling of the observed solid and fluid assemblage at a reference P-T point appropriate for serpentinisation (350 °C and 100 MPa) is consistent with extreme reduction of the fluid to logfO2 (MPa) = −45.3 (ΔlogfO2[Iron-Magnetite] = −6.5). These findings imply that the formation of metastable diamond at low pressure in serpentinised olivine is a widespread process in modern and ancient oceanic lithosphere, questioning a generalised ultra-high pressure origin for ophiolitic diamond.

Figures

Figure 1 (a) Transmitted light photomicrograph (TLP) of olivine hosting a trail of secondary fluid inclusions. The red area defines the zoom in (b-d). (b, c) TLP of fluid inclusions below the surface of olivine with focus at Z (depth) = −1 and 0 μm respectively. (d) Reflected light photomicrograph of (c), showing that the central inclusion is completely below the surface. White rectangles mark the area of (e-g). (e) Fluid inclusion confocal Raman map at Z = −1 μm; different colours represent different phases. (f) Confocal Raman map for the same inclusion at Z = −2 μm. (g) Z-stack of (e, f) showing the inclusion profile. (h) Raman spectra of the identified phases (colour coded). Mapping conditions: 6 × 6 μm, 30 × 30 spectra, Tint = 2 s, 2 mW, 100× objective.

Figure 2 (a) Field emission scanning electron microscope image of olivine-hosted inclusion thinned by focused ion beam. (b,c) TEM image of the diamond and the surrounding serpentine and magnetite; the red square shows the selected area electron diffraction (SAED). (d) SAED pattern confirming the diamond structure of the crystal (the nearly horizontal rows of reflections have indices 111 with d spacing of 2 Å). (e) Electron energy loss near-edge structure of the C K-edge for the diamond showing a major peak due to its sp3 bonding. Abbreviations: Dia-Diamond, Mag-magnetite, Ol-olivine, Srp-serpentine, Pt*-platinum deposited during sample preparation.

Figure 3 Phase relations in the C-O-H system (see methods in SI for details) with indication of fO2 isopleths (black solid lines, log-units) for the diamond-(Dia) saturated portion (log(aC) = 0) of the system at 100 MPa, 350 °C.

Figure 1 Figure 2 Figure 3

View all figures and tables





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Introduction

Abstract | Introduction | Results | Natural Origin of the Studied Diamond | Processes and Conditions for Diamond Formation | Acknowledgements | References | Supplementary Information

The discovery of nano- to micrometre scale grains of diamond separated from ophiolitic rocks has recently attracted the attention of geoscientists due to its potential evidence for lithosphere recycling down to, or below, the mantle Transition Zone (e.g., Yang et al., 2007

Yang, J.S., Dobrzhinetskaya, L., Bai, W.J., Fang, Q.S., Robinson, P.T., Zhang, J., Green, H.W. (2007) Diamond- and coesite-bearing chromitites from the Luobusa ophiolite, Tibet. Geology 35, 875–878.

, 2015

Yang, J., Meng, F., Xu, X., Robinson, P.T., Dilek, Y., Makeyev, A.B., Wirth, R., Wiedenbeck, M., Cliff, J. (2015) Diamonds, native elements and metal alloys from chromitites of the Ray-Iz ophiolite of the Polar Urals. Gondwana Research 27, 459–485.

; Griffin et al., 2016

Griffin, W.L., Afonso, J.C., Belousova, E.A., Gain, S.E., Gong, X.H., González-Jiménez, J.M., Howell, D., Huang, J.X., McGowan, N., Pearson, N.J., Satsuawa, T., Shi, R., Williams, P., Xiong, Q., Yang, J.S., Zhang, M., O’Reilly, S.Y. (2016) Mantle Recycling: Transition Zone Metamorphism of Tibetan Ophiolitic Peridotites and its Tectonic Implications. Journal of Petrology 57, 655–684.

). The earlier reports of diamond in nominally low pressure ophiolitic rocks date back to the early 1990s, when diamond was found in heavy mineral concentrates obtained from Tibetan ophiolites (Bai et al., 1993

Bai, W.J., Zhou, M.F., Robinson, P.T. (1993) Possibly diamond-bearing mantle peridotites and podiform chromitites in the Luobusa and Donqiao ophiolites, Tibet. Canadian Journal of Earth Sciences 30, 1650–1659.

). These were initially considered to be due to contamination during sample preparation. The debate, however, rejuvenated after findings of other UHP minerals such as coesite together with super-reduced phases in many chromitites and associated peridotites of ophiolites worldwide (e.g., Griffin et al., 2016

Griffin, W.L., Afonso, J.C., Belousova, E.A., Gain, S.E., Gong, X.H., González-Jiménez, J.M., Howell, D., Huang, J.X., McGowan, N., Pearson, N.J., Satsuawa, T., Shi, R., Williams, P., Xiong, Q., Yang, J.S., Zhang, M., O’Reilly, S.Y. (2016) Mantle Recycling: Transition Zone Metamorphism of Tibetan Ophiolitic Peridotites and its Tectonic Implications. Journal of Petrology 57, 655–684.

). Far-reaching geodynamic models have been proposed based on the assumption that diamond growth took place at UHP conditions in these rocks (e.g., Barron et al., 1996

Barron, L.M., Lishmund, S.R., Oakes, G.M., Barron, B.J., Sutherland, F.L. (1996) Subduction model for the origin of some diamonds in the Phanerozoic of eastern New South Wales. Australian Journal of Earth Sciences 43, 257–267.

; Xiong et al., 2019

Xiong, F., Liu, Z., Kapsiotis, A., Yang, J., Lenaz, D., Robinson, P.T. (2019) Petrogenesis of lherzolites from the Purang ophiolite, Yarlung-Zangbo suture zone, Tibet: origin and significance of ultra-high pressure and other ‘unusual’ minerals in the Neo-Tethyan lithospheric mantle. International Geology Review 61, 2184–2210.

and references therein). Recently, the finding of in situ diamond in chromite-hosted fluid inclusions from ophiolitic chromitites by Farré-de-Pablo et al., (2019a)

Farré-de-Pablo, J., Proenza, J.A., González-Jiménez, J.M., Garcia-Casco, A., Colás, V., Roqué-Rosell, J., Camprubí, A., Sánchez-Navas, A. (2019a) A shallow origin for diamonds in ophiolitic chromitites. Geology 47, 75–78.

provided the first evidence for empirical (Simakov et al., 2015

Simakov, S.K., Kouchi, A., Mel’nik, N.N., Scribano, V., Kimura, Y., Hama, T., Suzuki, N., Saito, H., Yoshizawa, T. (2015) Nanodiamond finding in the hyblean shallow mantle xenoliths. Scientific Reports 5, 10765, doi: 10.1038/srep10765.

, 2020

Simakov, S.K., Scribano, V., Mel’Nik, N.N., Barone, G. (2020) Sicilian serpentinite xenoliths containing abiotic organics with nanodiamond clusters as key model for prebiotic processes. Geoscience Frontiers (in press), doi: 10.1016/j.gsf.2020.04.008.

), experimental (Simakov et al., 2008

Simakov, S.K., Dubinchuk, V.T., Novikov, M.P., Drozdova, I.A. (2008) Formation of diamond and diamond-type phases from the carbon-bearing fluid at PT parameters corresponding to processes in the Earth’s crust. Doklady Earth Sciences 421, 835–837.

) and theoretical (Manuella, 2013

Manuella, F.C. (2013) Can nanodiamonds grow in serpentinite-hosted hydrothermal systems? A theoretical modelling study. Mineralogical Magazine 77, 3163–3174.

; Simakov, 2018

Simakov, S.K. (2018) Nano- and micron-sized diamond genesis in nature: An overview. Geoscience Frontiers 9, 1849–1858.

) work on low pressure growth of diamond. However, the debate on the natural origin of diamond continued (e.g., Farré-de-Pablo et al., 2019b

Farré-de-Pablo, J., Proenza, J.A., González-Jiménez, J.M., Garcia-Casco, A., Colás, V., Roqué-Rosell, J., Camprubí, A., Sánchez-Navas, A. (2019b) A shallow origin for diamonds in ophiolitic chromitites: REPLY. Geology 47, e477–e478, doi: 10.1130/G46602Y.1.

; Massonne, 2019

Massonne, H.J. (2019) Comment: A shallow origin for diamonds in ophiolitic chromitites. Geology 47, e476, doi: 10.1130/G46459C.1.

; Yang et al., 2019

Yang, J., Lian, D., Robinson, P.T., Qiu, T., Xiong, F., Wu, W. (2019) Comment to: A shallow origin for diamonds in ophiolitic chromitites. Geology 47, e475, doi: 10.1130/G46446C.1.

). In this regard, Litasov et al. (2019a

Litasov, K.D., Kagi, H., Voropaev, S.A., Hirata, T., Ohfuji, H., Ishibashi, H., Makino, Y., Bekker, T.B., Sevastyanov, V.S., Afanasiev, V.P., Pokhilenko, N.P. (2019a) Comparison of enigmatic diamonds from the Tolbachik arc volcano (Kamchatka)and Tibetan ophiolites: Assessing the role of contamination by synthetic materials. Gondwana Research 75, 16–27.

,b

Litasov, K.D., Kagi, H., Bekker, T.B., Hirata, T., Makino, Y. (2019b) Cuboctahedral type Ib diamonds in ophiolitic chromitites and peridotites: the evidence for anthropogenic contamination. High Pressure Research 39, 480–488.

) have recently claimed that most diamonds, if not all, from ophiolitic rocks are not natural but instead have a synthetic origin, and emphasised the need to identify diamond below the polished surface of the host mineral. In this study we report for the first time in situ diamond grains hosted below the polished surface of magmatic olivine from a low pressure gabbro sill of the upper mantle section of the Moa-Baracoa Ophiolitic Massif, eastern Cuba (Supplementary Information; Figs. S-1–S-4, Table S-1), where super-reduced phases formed during serpentinisation have been previously reported (Pujol-Solà et al., 2018

Pujol-Solà, N., Proenza, J., Garcia-Casco, A., González-Jiménez, J., Andreazini, A., Melgarejo, J., Gervilla, F. (2018) An Alternative Scenario on the Origin of Ultra-High Pressure (UHP) and Super-Reduced (SuR) Minerals in Ophiolitic Chromitites: A Case Study from the Mercedita Deposit (Eastern Cuba). Minerals 8, 433, doi: 10.3390/min8100433.

). Diamond grains, which are also present in olivine of associated chromitite, occur in secondary inclusions within olivine. Our observations provide conclusive evidence for the natural formation of metastable diamond at low P (<200 MPa) and low T (<350 °C) during serpentinisation of oceanic mafic and ultramafic rocks, and allow a word of caution on the development of generalised geodynamic models of mantle convection and lithosphere recycling into the deep mantle based on diamond and super-reduced phases alone.

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Results

Abstract | Introduction | Results | Natural Origin of the Studied Diamond | Processes and Conditions for Diamond Formation | Acknowledgements | References | Supplementary Information

We have studied approximately 150 inclusions (96 below the polished surface) in olivine from 5 gabbro thin sections and 16 inclusions (8 below the surface) hosted in olivine in chromitite (representative inclusions in Table S-2). The inclusions align along trails that extend across adjacent mineral grains and delineate healed fractures (Fig. 1a–d). The distribution of these trails is heterogeneous, with some olivine grains showing a high density of trails cross-cutting each other. Inclusions are typically spheroid, with sizes ranging between <1 μm and 14 μm in diameter (Figs. 1a–d, S-2).


Figure 1 (a) Transmitted light photomicrograph (TLP) of olivine hosting a trail of secondary fluid inclusions. The red area defines the zoom in (b-d). (b, c) TLP of fluid inclusions below the surface of olivine with focus at Z (depth) = −1 and 0 μm respectively. (d) Reflected light photomicrograph of (c), showing that the central inclusion is completely below the surface. White rectangles mark the area of (e-g). (e) Fluid inclusion confocal Raman map at Z = −1 μm; different colours represent different phases. (f) Confocal Raman map for the same inclusion at Z = −2 μm. (g) Z-stack of (e, f) showing the inclusion profile. (h) Raman spectra of the identified phases (colour coded). Mapping conditions: 6 × 6 μm, 30 × 30 spectra, Tint = 2 s, 2 mW, 100× objective.
Full size image


Diamond grains were identified with the characteristic Raman peak at 1330 cm−1 with a slight downshift from the typical band to 1326 cm−1 (n = 17; Table S-2) lining the walls of inclusions (Fig. 1e–f). These very small (200–300 nm) diamond grains were better characterised by confocal Raman maps at different depths (Fig. 1e–h). Nanodiamond is usually associated with methane, serpentine (lizardite, polygonal serpentine, chrysotile), and magnetite (Fig. 1h). Daughter minerals include diopside, chlorite, graphite-like amorphous C, and calcite (Table S-2, Fig. S-5). Brucite was not identified in any of the studied inclusions, similar to other locations (e.g., Sachan et al., 2007

Sachan, H.K., Mukherjee, B.K., Bodnar, R.J. (2007) Preservation of methane generated during serpentinization of upper mantle rocks: Evidence from fluid inclusions in the Nidar ophiolite, Indus Suture Zone, Ladakh (India). Earth and Planetary Science Letters 257, 47–59.

). However, brucite is a widespread product in olivine-hosted fluid inclusions in ophiolitic samples (e.g., Klein et al., 2019

Klein, F., Grozeva, N.G., Seewald, J.S. (2019) Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions. Proceedings of the National Academy of Sciences of the United States of America 116, 17666–17672.

; Grozeva et al., 2020

Grozeva, N.G., Klein, F., Seewald, J.S., Sylva, S.P. (2020) Chemical and isotopic analyses of hydrocarbon-bearing fluid inclusions in olivine-rich rocks. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 378, 20180431.

).

TEM observations of a thin foil extracted by focused ion beam (FIB) (Fig. 2a) reveal that nanodiamond is clearly surrounded by polygonal serpentine and associated with magnetite (Fig. 2b,c). The selected area electron diffraction (SAED) pattern of a grain a few hundreds of nanometres in size confirms its diamond structure (with a reciprocal distance of 5 nm−1 corresponding to the d111-spacing of 2 Å; Fig. 2d), while the corresponding electron energy loss spectrum (EELS) indicates that C-type is the diamond allotrope (sp3-hybridised C atoms; Fig. 2e). No polishing debris was observed in the studied inclusions, suggesting no contamination during ion milling of the thin foil except for sublimated Pt used to protect the area. The observed pore space in the inclusions (Figs. 2a, S-6) was likely filled by methane, as indicated by Raman spectroscopy measurements (Table S-2).


Figure 2 (a) Field emission scanning electron microscope image of olivine-hosted inclusion thinned by focused ion beam. (b,c) TEM image of the diamond and the surrounding serpentine and magnetite; the red square shows the selected area electron diffraction (SAED). (d) SAED pattern confirming the diamond structure of the crystal (the nearly horizontal rows of reflections have indices 111 with d spacing of 2 Å). (e) Electron energy loss near-edge structure of the C K-edge for the diamond showing a major peak due to its sp3 bonding. Abbreviations: Dia-Diamond, Mag-magnetite, Ol-olivine, Srp-serpentine, Pt*-platinum deposited during sample preparation.
Full size image


Sub-surface fluid inclusions lack water and are dominated by methane (Table S-2), similarly to olivine-hosted fluid inclusions described by Klein et al. (2019)

Klein, F., Grozeva, N.G., Seewald, J.S. (2019) Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions. Proceedings of the National Academy of Sciences of the United States of America 116, 17666–17672.

. In chromitite, diamond grains have been found in 5 sealed fracture-filling inclusions within interstitial magmatic olivine. One inclusion studied by TEM (Fig. S-7) revealed a <1 μm-sized diamond crystal coexisting with the super-reduced phase native Si. In addition, H2 was identified by Raman spectroscopy in another inclusion (Table S-2). Nevertheless, the phase assemblages are similar to those of the studied inclusions in olivine from the associated gabbro, with serpentine and magnetite (Fig. S-4, Table S-2).

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Natural Origin of the Studied Diamond

Abstract | Introduction | Results | Natural Origin of the Studied Diamond | Processes and Conditions for Diamond Formation | Acknowledgements | References | Supplementary Information

The natural vs. anthropogenic origin of ophiolitic diamond is a hotly debated topic. Whereas in other samples the natural origin of diamond has not been firmly proved (Massonne, 2019

Massonne, H.J. (2019) Comment: A shallow origin for diamonds in ophiolitic chromitites. Geology 47, e476, doi: 10.1130/G46459C.1.

), here we provide several lines of evidence for a natural origin. Our most significant evidence is that diamond is hosted within olivine well below the mineral’s polished surface (Fig. 1). Such an observation meets the basic requirements for in situ mineral grains, as suggested by Massonne (2019)

Massonne, H.J. (2019) Comment: A shallow origin for diamonds in ophiolitic chromitites. Geology 47, e476, doi: 10.1130/G46459C.1.

. Additional evidence includes: (1) that diamond was found within CH4-bearing fluid inclusions forming linear arrays (healed fractures) in olivine and surrounded by serpentine (Figs. 1–2), and (2) lack of polishing debris and/or resin artificially incorporated (e.g., Dobrzhinetskaya et al., 2014

Dobrzhinetskaya, L., Wirth, R., Green, H. (2014) Diamonds in Earth’s oldest zircons from Jack Hills conglomerate, Australia, are contamination. Earth and Planetary Science Letters 387, 212–218.

).

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Processes and Conditions for Diamond Formation

Abstract | Introduction | Results | Natural Origin of the Studied Diamond | Processes and Conditions for Diamond Formation | Acknowledgements | References | Supplementary Information

The olivine-hosted inclusion trails represent fluids trapped in healed fractures. Fluid infiltration in oceanic lithosphere is commonly associated with sporadic deformation events that trigger an increase of porosity by (micro-)fracturing during cooling below the brittle-ductile transition of olivine (Klein et al., 2019

Klein, F., Grozeva, N.G., Seewald, J.S. (2019) Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions. Proceedings of the National Academy of Sciences of the United States of America 116, 17666–17672.

). In this scenario, fluid-rock interaction changes from a general open system during initial infiltration to closed system micro-reactors once the inclusions are sealed. At the initial trapping pressure and temperature and during subsequent cooling, the trapped aqueous fluids react with the olivine walls of the inclusion, triggering a number of reactions that ultimately result in growth of hydrated minerals and changes in fluid composition. Comprehensive thermodynamic models of these processes in ultramafic and oceanic rocks (McCollom and Bach, 2009

McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875.

; Klein et al., 2019

Klein, F., Grozeva, N.G., Seewald, J.S. (2019) Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions. Proceedings of the National Academy of Sciences of the United States of America 116, 17666–17672.

) show that below ∼350 °C (at <200 MPa) in the stability field of serpentine + brucite “serpentinisation” of the walls of olivine hosting fluid inclusions consumes H2O in the fluid and generates H2 through precipitation of Fe3+-rich phases, particularly magnetite, so that consumption of inorganic carbon (CO2) and formation of abiotic CH4 takes place. In the 6 component system MgO-FeO-SiO2-C-O2-H2, the formation of phase assemblages made of serpentine, brucite, magnetite, diamond and CH4-fluid from an initial assemblage made of olivine and H2O-CO2 fluid can be described by a number of linearly independent reactions. Assuming 11 phase components (Fo, Fa, Mag, Mg-Srp, Mg-Brc, H2O, CO2, CH4, H2, SiO2(aq), C abbreviations after Whitney and Evans, 2010

Whitney, D.L., Evans, B.W. (2010) Abbreviations for names of rock-forming minerals. American Mineralogist 95, 185–187.

) appropriate for the low temperature stage of reaction progress, and excluding magnesite for simplicity, the dimension of the reaction space is 5. Among many, the following set of five linearly independent reactions obtained with the software CSpace (Torres-Roldán et al., 2000

Torres-Roldán, R.L., Garcia-Casco, A., García-Sánchez, P.A. (2000) CSpace: An integrated workplace for the graphical and algebraic analysis of phase assemblages on 32-bit Wintel platforms. Computers and Geosciences 26, 779–793.

) describes the process (commonly used as coupled reactions during serpentinisation, e.g., Lamadrid et al., 2017

Lamadrid, H.M., Rimstidt, J.D., Schwarzenbach, E.M., Klein, F., Ulrich, S., Dolocan, A., Bodnar, R.J. (2017) Effect of water activity on rates of serpentinization of olivine. Nature Communications 8, 1–9.

; Klein et al., 2019

Klein, F., Grozeva, N.G., Seewald, J.S. (2019) Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions. Proceedings of the National Academy of Sciences of the United States of America 116, 17666–17672.

):





As a whole, this set of reactions (and any other independent set, provided that it contains all phase components considered) indicates that the hydration process of olivine triggers the formation of magnetite by oxidation of Fe2+ (from component fayalite), liberating H2 that ultimately favours the consumption of CO2 and the formation of CH4 and C. A bulk mass balance can be obtained after combination of these reactions correspondingly multiplied by a given stoichiometric coefficient. The number of combinations is hence infinite, and combinations that yield zero brucite are possible. A defined mass balance can be calculated only if additional constraints are imposed, such as the bulk composition of the system as long as the relative contribution of the above reactions to the integrated mass balance is a function of the original compositions of olivine and fluid and the initial fluid-olivine ratio. For initial H2O-rich fluid and olivine with the composition of the studied gabbro (Table S-1), the observed stable brucite-lacking assemblage serpentine + magnetite + diamond + CH4-rich fluid can be reached for specific olivine-fluid ratios, such as in the following example obtained using the software CSpace:
This reaction completely consumes both reactants. Under natural conditions in the fluid inclusion, olivine is in excess once H2O and CO2 are completely exhausted. Hence, thermodynamic calculations for a closed system fluid inclusion must consider an olivine-fluid ratio higher than 0.27∶0.3. A bulk composition with an initial olivine:fluid ratio of 0.7∶0.3 produces an assemblage of 51.3 vol. % Ol (Fo = 0.76), 40.2 vol. % antigorite, 6.8 vol. % magnetite, 1.5 vol. % CH4 and 0.12 vol. % diamond at 100 MPa and 350 °C using the Perple_X software (Connolly, 2009

Connolly, J.A.D. (2009) The geodynamic equation of state: What and how. Geochemistry, Geophysics, Geosystems 10, doi: 10.1029/2009GC002540.

). According to the low P conditions at which serpentinisation occurs, the stable C allotrope should be graphite. However, it has been demonstrated that nanodiamond can form at super-reducing conditions (e.g., Manuella, 2013

Manuella, F.C. (2013) Can nanodiamonds grow in serpentinite-hosted hydrothermal systems? A theoretical modelling study. Mineralogical Magazine 77, 3163–3174.

; Simakov, 2018

Simakov, S.K. (2018) Nano- and micron-sized diamond genesis in nature: An overview. Geoscience Frontiers 9, 1849–1858.

). Hence, in the above thermodynamic calculations, diamond has been used instead of graphite in order to simulate its metastable formation. The corresponding calculated log(fO2; MPa) is as low as −45.3, (ΔlogfO2[Iron-Magnetite] = −6.5; Frost, 1991

Frost, B.R. (1991) Introduction to oxygen fugacity and its petrologic importance. Reviews in Mineralogy and Geochemistry 25, 1–9.

), consistent with thermodynamic calculations in the diamond-COH fluid system at 350 °C and 100 MPa (Fig. 3; cf. Schmidt et al., 2014

Schmidt, M.W., Gao, C., Golubkova, A., Rohrbach, A., Connolly, J.A. (2014) Natural moissanite (SiC) – a low temperature mineral formed from highly fractionated ultra-reducing COH-fluids. Progress in Earth and Planetary Science 1, 27.

).


Figure 3 Phase relations in the C-O-H system (see methods in SI for details) with indication of fO2 isopleths (black solid lines, log-units) for the diamond-(Dia) saturated portion (log(aC) = 0) of the system at 100 MPa, 350 °C.
Full size image


Admittedly, calculations in a more complex system, with additional components and minerals (e.g., CaO, clinopyroxene, talc, carbonates etc.) and constraints (e.g., dissolved silica in the fluid), and at other P-T conditions over which serpentinisation takes place, would yield a more intricate picture of the basic process outlined above and different absolute values of fO2 (e.g., Klein et al., 2019

Klein, F., Grozeva, N.G., Seewald, J.S. (2019) Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions. Proceedings of the National Academy of Sciences of the United States of America 116, 17666–17672.

). However, a highly reduced environment (particularly at lower T, see e.g., Klein et al., 2019

Klein, F., Grozeva, N.G., Seewald, J.S. (2019) Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions. Proceedings of the National Academy of Sciences of the United States of America 116, 17666–17672.

) should develop if CH4 is to be the main fluid species in the inclusions, making possible the metastable formation of nanodiamond in low pressure, olivine-bearing oceanic rocks during low T infiltration of H2O-CO2 fluid mixtures.

The undoubted natural origin of diamond hosted with serpentine, magnetite and CH4 in sealed fluid inclusions within magmatic olivine from a gabbro sill and associated chromitite should be related to the generalised release of CH4 during hydrothermal alteration (partial hydration) of shallow oceanic lithosphere (e.g., Klein et al., 2019

Klein, F., Grozeva, N.G., Seewald, J.S. (2019) Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions. Proceedings of the National Academy of Sciences of the United States of America 116, 17666–17672.

). This finding implies that the formation of nanodiamond in altered olivine-bearing rocks can be a widespread process. The presence of diamond (Farré-de-Pablo et al., 2019a

Farré-de-Pablo, J., Proenza, J.A., González-Jiménez, J.M., Garcia-Casco, A., Colás, V., Roqué-Rosell, J., Camprubí, A., Sánchez-Navas, A. (2019a) A shallow origin for diamonds in ophiolitic chromitites. Geology 47, 75–78.

) and other highly reduced phases (e.g., metallic Si, moissanite; Pujol-Solà et al., 2018

Pujol-Solà, N., Proenza, J., Garcia-Casco, A., González-Jiménez, J., Andreazini, A., Melgarejo, J., Gervilla, F. (2018) An Alternative Scenario on the Origin of Ultra-High Pressure (UHP) and Super-Reduced (SuR) Minerals in Ophiolitic Chromitites: A Case Study from the Mercedita Deposit (Eastern Cuba). Minerals 8, 433, doi: 10.3390/min8100433.

) in these rocks, in particular chromitites, cannot hence be taken as a general indication of ultra-high pressure conditions (e.g., Yang et al., 2007

Yang, J.S., Dobrzhinetskaya, L., Bai, W.J., Fang, Q.S., Robinson, P.T., Zhang, J., Green, H.W. (2007) Diamond- and coesite-bearing chromitites from the Luobusa ophiolite, Tibet. Geology 35, 875–878.

, 2015

Yang, J., Meng, F., Xu, X., Robinson, P.T., Dilek, Y., Makeyev, A.B., Wirth, R., Wiedenbeck, M., Cliff, J. (2015) Diamonds, native elements and metal alloys from chromitites of the Ray-Iz ophiolite of the Polar Urals. Gondwana Research 27, 459–485.

; Griffin et al., 2016

Griffin, W.L., Afonso, J.C., Belousova, E.A., Gain, S.E., Gong, X.H., González-Jiménez, J.M., Howell, D., Huang, J.X., McGowan, N., Pearson, N.J., Satsuawa, T., Shi, R., Williams, P., Xiong, Q., Yang, J.S., Zhang, M., O’Reilly, S.Y. (2016) Mantle Recycling: Transition Zone Metamorphism of Tibetan Ophiolitic Peridotites and its Tectonic Implications. Journal of Petrology 57, 655–684.

). The small size of diamond (<1 μm) and its scarcity in the fluid inclusions are important handicaps in searching for “the needle in the haystack”, thus explaining its apparent absence in other case studies of altered oceanic rocks.

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Acknowledgements

Abstract | Introduction | Results | Natural Origin of the Studied Diamond | Processes and Conditions for Diamond Formation | Acknowledgements | References | Supplementary Information

We thank the editor H.R. Marschall and the reviewers F. Klein, I. Graham and C. Ballhaus for their constructive comments. This research was funded by Fondo Europeo de Desarrollo Regional (FEDER) Funds, Spanish Projects CGL2015-65824, RTI2018-099157-A-I00, PID2019-105625RB-C21, and A.RNM.186.UGR18. Additional funding was provided by the Ramón y Cajal Fellowship RYC-2015-17596 to JMGJ, a FPU-PhD grant to NPS, the Mexican research programs CONACYT-Ciencia Básica (A1-S-14574) and UNAM-PAPIIT grant IA-101419, and received support for analyses at CIC from the University of Granada.

Editor: Horst R. Marschall

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References

Abstract | Introduction | Results | Natural Origin of the Studied Diamond | Processes and Conditions for Diamond Formation | Acknowledgements | References | Supplementary Information

Bai, W.J., Zhou, M.F., Robinson, P.T. (1993) Possibly diamond-bearing mantle peridotites and podiform chromitites in the Luobusa and Donqiao ophiolites, Tibet. Canadian Journal of Earth Sciences 30, 1650–1659.
Show in context

The earlier reports of diamond in nominally low pressure ophiolitic rocks date back to the early 1990s, when diamond was found in heavy mineral concentrates obtained from Tibetan ophiolites (Bai et al., 1993).
View in article


Barron, L.M., Lishmund, S.R., Oakes, G.M., Barron, B.J., Sutherland, F.L. (1996) Subduction model for the origin of some diamonds in the Phanerozoic of eastern New South Wales. Australian Journal of Earth Sciences 43, 257–267.
Show in context

Far-reaching geodynamic models have been proposed based on the assumption that diamond growth took place at UHP conditions in these rocks (e.g., Barron et al., 1996; Xiong et al., 2019 and references therein).
View in article


Connolly, J.A.D. (2009) The geodynamic equation of state: What and how. Geochemistry, Geophysics, Geosystems 10, doi: 10.1029/2009GC002540.
Show in context

A bulk composition with an initial olivine:fluid ratio of 0.7∶0.3 produces an assemblage of 51.3 vol. % Ol (Fo = 0.76), 40.2 vol. % antigorite, 6.8 vol. % magnetite, 1.5 vol. % CH4 and 0.12 vol. % diamond at 100 MPa and 350 °C using the Perple_X software (Connolly, 2009).
View in article


Dobrzhinetskaya, L., Wirth, R., Green, H. (2014) Diamonds in Earth’s oldest zircons from Jack Hills conglomerate, Australia, are contamination. Earth and Planetary Science Letters 387, 212–218.
Show in context

Additional evidence includes: (1) that diamond was found within CH4-bearing fluid inclusions forming linear arrays (healed fractures) in olivine and surrounded by serpentine (Figs. 1–2), and (2) lack of polishing debris and/or resin artificially incorporated (e.g., Dobrzhinetskaya et al., 2014).
View in article


Farré-de-Pablo, J., Proenza, J.A., González-Jiménez, J.M., Garcia-Casco, A., Colás, V., Roqué-Rosell, J., Camprubí, A., Sánchez-Navas, A. (2019a) A shallow origin for diamonds in ophiolitic chromitites. Geology 47, 75–78.
Show in context

Recently, the finding of in situ diamond in chromite-hosted fluid inclusions from ophiolitic chromitites by Farré-de-Pablo et al., (2019a) provided the first evidence for empirical (Simakov et al., 2015, 2020), experimental (Simakov et al., 2008) and theoretical (Manuella, 2013; Simakov, 2018) work on low pressure growth of diamond.
View in article
The presence of diamond (Farré-de-Pablo et al., 2019a) and other highly reduced phases (e.g., metallic Si, moissanite; Pujol-Solà et al., 2018) in these rocks, in particular chromitites, cannot hence be taken as a general indication of ultra-high pressure conditions (e.g., Yang et al., 2007, 2015; Griffin et al., 2016).
View in article


Farré-de-Pablo, J., Proenza, J.A., González-Jiménez, J.M., Garcia-Casco, A., Colás, V., Roqué-Rosell, J., Camprubí, A., Sánchez-Navas, A. (2019b) A shallow origin for diamonds in ophiolitic chromitites: REPLY. Geology 47, e477–e478, doi: 10.1130/G46602Y.1.
Show in context

However, the debate on the natural origin of diamond continued (e.g., Farré-de-Pablo et al., 2019b; Massonne, 2019; Yang et al., 2019).
View in article


Frost, B.R. (1991) Introduction to oxygen fugacity and its petrologic importance. Reviews in Mineralogy and Geochemistry 25, 1–9.
Show in context

The corresponding calculated log(fO2; MPa) is as low as −45.3, (ΔlogfO2[Iron-Magnetite] = −6.5; Frost, 1991), consistent with thermodynamic calculations in the diamond-COH fluid system at 350 °C and 100 MPa (Fig. 3; cf. Schmidt et al., 2014).
View in article


Griffin, W.L., Afonso, J.C., Belousova, E.A., Gain, S.E., Gong, X.H., González-Jiménez, J.M., Howell, D., Huang, J.X., McGowan, N., Pearson, N.J., Satsuawa, T., Shi, R., Williams, P., Xiong, Q., Yang, J.S., Zhang, M., O’Reilly, S.Y. (2016) Mantle Recycling: Transition Zone Metamorphism of Tibetan Ophiolitic Peridotites and its Tectonic Implications. Journal of Petrology 57, 655–684.
Show in context

The discovery of nano- to micrometre scale grains of diamond separated from ophiolitic rocks has recently attracted the attention of geoscientists due to its potential evidence for lithosphere recycling down to, or below, the mantle Transition Zone (e.g., Yang et al., 2007, 2015; Griffin et al., 2016).
View in article
The debate, however, rejuvenated after findings of other UHP minerals such as coesite together with super-reduced phases in many chromitites and associated peridotites of ophiolites worldwide (e.g., Griffin et al., 2016).
View in article
The presence of diamond (Farré-de-Pablo et al., 2019a) and other highly reduced phases (e.g., metallic Si, moissanite; Pujol-Solà et al., 2018) in these rocks, in particular chromitites, cannot hence be taken as a general indication of ultra-high pressure conditions (e.g., Yang et al., 2007, 2015; Griffin et al., 2016).
View in article


Grozeva, N.G., Klein, F., Seewald, J.S., Sylva, S.P. (2020) Chemical and isotopic analyses of hydrocarbon-bearing fluid inclusions in olivine-rich rocks. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 378, 20180431.
Show in context

However, brucite is a widespread product in olivine-hosted fluid inclusions in ophiolitic samples (e.g., Klein et al., 2019; Grozeva et al., 2020).
View in article


Klein, F., Grozeva, N.G., Seewald, J.S. (2019) Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions. Proceedings of the National Academy of Sciences of the United States of America 116, 17666–17672.
Show in context

However, brucite is a widespread product in olivine-hosted fluid inclusions in ophiolitic samples (e.g., Klein et al., 2019; Grozeva et al., 2020).
View in article
Sub-surface fluid inclusions lack water and are dominated by methane (Table S-2), similarly to olivine-hosted fluid inclusions described by Klein et al. (2019).
View in article
Fluid infiltration in oceanic lithosphere is commonly associated with sporadic deformation events that trigger an increase of porosity by (micro-)fracturing during cooling below the brittle-ductile transition of olivine (Klein et al., 2019).
View in article
Comprehensive thermodynamic models of these processes in ultramafic and oceanic rocks (McCollom and Bach, 2009; Klein et al., 2019) show that below ∼350 °C (at <200 MPa) in the stability field of serpentine + brucite “serpentinisation” of the walls of olivine hosting fluid inclusions consumes H2O in the fluid and generates H2 through precipitation of Fe3+-rich phases, particularly magnetite, so that consumption of inorganic carbon (CO2) and formation of abiotic CH4 takes place. In the 6 component system MgO-FeO-SiO2-C-O2-H2, the formation of phase assemblages made of serpentine, brucite, magnetite, diamond and CH4-fluid from an initial assemblage made of olivine and H2O-CO2 fluid can be described by a number of linearly independent reactions.
View in article
Among many, the following set of five linearly independent reactions obtained with the software CSpace (Torres-Roldán et al., 2000) describes the process (commonly used as coupled reactions during serpentinisation, e.g., Lamadrid et al., 2017; Klein et al., 2019).
View in article
Admittedly, calculations in a more complex system, with additional components and minerals (e.g., CaO, clinopyroxene, talc, carbonates etc.) and constraints (e.g., dissolved silica in the fluid), and at other P-T conditions over which serpentinisation takes place, would yield a more intricate picture of the basic process outlined above and different absolute values of fO2 (e.g., Klein et al., 2019).
View in article
However, brucite is a widespread product in olivine-hosted fluid inclusions in ophiolitic samples (e.g., Klein et al., 2019; Grozeva et al., 2020).
View in article
However, a highly reduced environment (particularly at lower T, see e.g., Klein et al., 2019) should develop if CH4 is to be the main fluid species in the inclusions, making possible the metastable formation of nanodiamond in low pressure, olivine-bearing oceanic rocks during low T infiltration of H2O-CO2 fluid mixtures.
View in article
The undoubted natural origin of diamond hosted with serpentine, magnetite and CH4 in sealed fluid inclusions within magmatic olivine from a gabbro sill and associated chromitite should be related to the generalised release of CH4 during hydrothermal alteration (partial hydration) of shallow oceanic lithosphere (e.g., Klein et al., 2019).
View in article


Lamadrid, H.M., Rimstidt, J.D., Schwarzenbach, E.M., Klein, F., Ulrich, S., Dolocan, A., Bodnar, R.J. (2017) Effect of water activity on rates of serpentinization of olivine. Nature Communications 8, 1–9.
Show in context

Among many, the following set of five linearly independent reactions obtained with the software CSpace (Torres-Roldán et al., 2000) describes the process (commonly used as coupled reactions during serpentinisation, e.g., Lamadrid et al., 2017; Klein et al., 2019).
View in article


Litasov, K.D., Kagi, H., Voropaev, S.A., Hirata, T., Ohfuji, H., Ishibashi, H., Makino, Y., Bekker, T.B., Sevastyanov, V.S., Afanasiev, V.P., Pokhilenko, N.P. (2019a) Comparison of enigmatic diamonds from the Tolbachik arc volcano (Kamchatka)and Tibetan ophiolites: Assessing the role of contamination by synthetic materials. Gondwana Research 75, 16–27.
Show in context

In this regard, Litasov et al. (2019a,b) have recently claimed that most diamonds, if not all, from ophiolitic rocks are not natural but instead have a synthetic origin, and emphasised the need to identify diamond below the polished surface of the host mineral.
View in article


Litasov, K.D., Kagi, H., Bekker, T.B., Hirata, T., Makino, Y. (2019b) Cuboctahedral type Ib diamonds in ophiolitic chromitites and peridotites: the evidence for anthropogenic contamination. High Pressure Research 39, 480–488.
Show in context

In this regard, Litasov et al. (2019a,b) have recently claimed that most diamonds, if not all, from ophiolitic rocks are not natural but instead have a synthetic origin, and emphasised the need to identify diamond below the polished surface of the host mineral.
View in article


Manuella, F.C. (2013) Can nanodiamonds grow in serpentinite-hosted hydrothermal systems? A theoretical modelling study. Mineralogical Magazine 77, 3163–3174.
Show in context

Recently, the finding of in situ diamond in chromite-hosted fluid inclusions from ophiolitic chromitites by Farré-de-Pablo et al., (2019a) provided the first evidence for empirical (Simakov et al., 2015, 2020), experimental (Simakov et al., 2008) and theoretical (Manuella, 2013; Simakov, 2018) work on low pressure growth of diamond.
View in article
However, it has been demonstrated that nanodiamond can form at super-reducing conditions (e.g., Manuella, 2013; Simakov, 2018).
View in article


Massonne, H.J. (2019) Comment: A shallow origin for diamonds in ophiolitic chromitites. Geology 47, e476, doi: 10.1130/G46459C.1.
Show in context

However, the debate on the natural origin of diamond continued (e.g., Farré-de-Pablo et al., 2019b; Massonne, 2019; Yang et al., 2019).
View in article
Whereas in other samples the natural origin of diamond has not been firmly proved (Massonne, 2019), here we provide several lines of evidence for a natural origin.
View in article
Such an observation meets the basic requirements for in situ mineral grains, as suggested by Massonne (2019).
View in article


McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875.
Show in context

Comprehensive thermodynamic models of these processes in ultramafic and oceanic rocks (McCollom and Bach, 2009; Klein et al., 2019) show that below ∼350 °C (at <200 MPa) in the stability field of serpentine + brucite “serpentinisation” of the walls of olivine hosting fluid inclusions consumes H2O in the fluid and generates H2 through precipitation of Fe3+-rich phases, particularly magnetite, so that consumption of inorganic carbon (CO2) and formation of abiotic CH4 takes place.
View in article


Pujol-Solà, N., Proenza, J., Garcia-Casco, A., González-Jiménez, J., Andreazini, A., Melgarejo, J., Gervilla, F. (2018) An Alternative Scenario on the Origin of Ultra-High Pressure (UHP) and Super-Reduced (SuR) Minerals in Ophiolitic Chromitites: A Case Study from the Mercedita Deposit (Eastern Cuba). Minerals 8, 433, doi: 10.3390/min8100433.
Show in context

In this study we report for the first time in situ diamond grains hosted below the polished surface of magmatic olivine from a low pressure gabbro sill of the upper mantle section of the Moa-Baracoa Ophiolitic Massif, eastern Cuba (Supplementary Information; Figs. S-1–S-4, Table S-1), where super-reduced phases formed during serpentinisation have been previously reported (Pujol-Solà et al., 2018).
View in article
The presence of diamond (Farré-de-Pablo et al., 2019a) and other highly reduced phases (e.g., metallic Si, moissanite; Pujol-Solà et al., 2018) in these rocks, in particular chromitites, cannot hence be taken as a general indication of ultra-high pressure conditions (e.g., Yang et al., 2007, 2015; Griffin et al., 2016).
View in article


Sachan, H.K., Mukherjee, B.K., Bodnar, R.J. (2007) Preservation of methane generated during serpentinization of upper mantle rocks: Evidence from fluid inclusions in the Nidar ophiolite, Indus Suture Zone, Ladakh (India). Earth and Planetary Science Letters 257, 47–59.
Show in context

Brucite was not identified in any of the studied inclusions, similar to other locations (e.g., Sachan et al., 2007).
View in article


Schmidt, M.W., Gao, C., Golubkova, A., Rohrbach, A., Connolly, J.A. (2014) Natural moissanite (SiC) – a low temperature mineral formed from highly fractionated ultra-reducing COH-fluids. Progress in Earth and Planetary Science 1, 27.
Show in context

The corresponding calculated log(fO2; MPa) is as low as −45.3, (ΔlogfO2[Iron-Magnetite] = −6.5; Frost, 1991), consistent with thermodynamic calculations in the diamond-COH fluid system at 350 °C and 100 MPa (Fig. 3; cf. Schmidt et al., 2014).
View in article


Simakov, S.K. (2018) Nano- and micron-sized diamond genesis in nature: An overview. Geoscience Frontiers 9, 1849–1858.
Show in context

Recently, the finding of in situ diamond in chromite-hosted fluid inclusions from ophiolitic chromitites by Farré-de-Pablo et al., (2019a) provided the first evidence for empirical (Simakov et al., 2015, 2020), experimental (Simakov et al., 2008) and theoretical (Manuella, 2013; Simakov, 2018) work on low pressure growth of diamond. However, the debate on the natural origin of diamond continued (e.g., Farré-de-Pablo et al., 2019b; Massonne, 2019; Yang et al., 2019).
View in article
However, it has been demonstrated that nanodiamond can form at super-reducing conditions (e.g., Manuella, 2013; Simakov, 2018).
View in article


Simakov, S.K., Dubinchuk, V.T., Novikov, M.P., Drozdova, I.A. (2008) Formation of diamond and diamond-type phases from the carbon-bearing fluid at PT parameters corresponding to processes in the Earth’s crust. Doklady Earth Sciences 421, 835–837.
Show in context

Recently, the finding of in situ diamond in chromite-hosted fluid inclusions from ophiolitic chromitites by Farré-de-Pablo et al., (2019a) provided the first evidence for empirical (Simakov et al., 2015, 2020), experimental (Simakov et al., 2008) and theoretical (Manuella, 2013; Simakov, 2018) work on low pressure growth of diamond. However, the debate on the natural origin of diamond continued (e.g., Farré-de-Pablo et al., 2019b; Massonne, 2019; Yang et al., 2019).
View in article


Simakov, S.K., Kouchi, A., Mel’nik, N.N., Scribano, V., Kimura, Y., Hama, T., Suzuki, N., Saito, H., Yoshizawa, T. (2015) Nanodiamond finding in the hyblean shallow mantle xenoliths. Scientific Reports 5, 10765, doi: 10.1038/srep10765.
Show in context

Recently, the finding of in situ diamond in chromite-hosted fluid inclusions from ophiolitic chromitites by Farré-de-Pablo et al., (2019a) provided the first evidence for empirical (Simakov et al., 2015, 2020), experimental (Simakov et al., 2008) and theoretical (Manuella, 2013; Simakov, 2018) work on low pressure growth of diamond. However, the debate on the natural origin of diamond continued (e.g., Farré-de-Pablo et al., 2019b; Massonne, 2019; Yang et al., 2019).
View in article


Simakov, S.K., Scribano, V., Mel’Nik, N.N., Barone, G. (2020) Sicilian serpentinite xenoliths containing abiotic organics with nanodiamond clusters as key model for prebiotic processes. Geoscience Frontiers (in press), doi: 10.1016/j.gsf.2020.04.008.
Show in context

Recently, the finding of in situ diamond in chromite-hosted fluid inclusions from ophiolitic chromitites by Farré-de-Pablo et al., (2019a) provided the first evidence for empirical (Simakov et al., 2015, 2020), experimental (Simakov et al., 2008) and theoretical (Manuella, 2013; Simakov, 2018) work on low pressure growth of diamond. However, the debate on the natural origin of diamond continued (e.g., Farré-de-Pablo et al., 2019b; Massonne, 2019; Yang et al., 2019).
View in article


Torres-Roldán, R.L., Garcia-Casco, A., García-Sánchez, P.A. (2000) CSpace: An integrated workplace for the graphical and algebraic analysis of phase assemblages on 32-bit Wintel platforms. Computers and Geosciences 26, 779–793.
Show in context

Among many, the following set of five linearly independent reactions obtained with the software CSpace (Torres-Roldán et al., 2000) describes the process (commonly used as coupled reactions during serpentinisation, e.g., Lamadrid et al., 2017; Klein et al., 2019).
View in article


Whitney, D.L., Evans, B.W. (2010) Abbreviations for names of rock-forming minerals. American Mineralogist 95, 185–187.
Show in context

Assuming 11 phase components (Fo, Fa, Mag, Mg-Srp, Mg-Brc, H2O, CO2, CH4, H2, SiO2(aq), C abbreviations after Whitney and Evans, 2010) appropriate for the low temperature stage of reaction progress, and excluding magnesite for simplicity, the dimension of the reaction space is 5.
View in article


Xiong, F., Liu, Z., Kapsiotis, A., Yang, J., Lenaz, D., Robinson, P.T. (2019) Petrogenesis of lherzolites from the Purang ophiolite, Yarlung-Zangbo suture zone, Tibet: origin and significance of ultra-high pressure and other ‘unusual’ minerals in the Neo-Tethyan lithospheric mantle. International Geology Review 61, 2184–2210.
Show in context

Far-reaching geodynamic models have been proposed based on the assumption that diamond growth took place at UHP conditions in these rocks (e.g., Barron et al., 1996; Xiong et al., 2019 and references therein).
View in article


Yang, J.S., Dobrzhinetskaya, L., Bai, W.J., Fang, Q.S., Robinson, P.T., Zhang, J., Green, H.W. (2007) Diamond- and coesite-bearing chromitites from the Luobusa ophiolite, Tibet. Geology 35, 875–878.
Show in context

The discovery of nano- to micrometre scale grains of diamond separated from ophiolitic rocks has recently attracted the attention of geoscientists due to its potential evidence for lithosphere recycling down to, or below, the mantle Transition Zone (e.g., Yang et al., 2007, 2015; Griffin et al., 2016).
View in article
The presence of diamond (Farré-de-Pablo et al., 2019a) and other highly reduced phases (e.g., metallic Si, moissanite; Pujol-Solà et al., 2018) in these rocks, in particular chromitites, cannot hence be taken as a general indication of ultra-high pressure conditions (e.g., Yang et al., 2007, 2015; Griffin et al., 2016).
View in article


Yang, J., Meng, F., Xu, X., Robinson, P.T., Dilek, Y., Makeyev, A.B., Wirth, R., Wiedenbeck, M., Cliff, J. (2015) Diamonds, native elements and metal alloys from chromitites of the Ray-Iz ophiolite of the Polar Urals. Gondwana Research 27, 459–485.
Show in context

The discovery of nano- to micrometre scale grains of diamond separated from ophiolitic rocks has recently attracted the attention of geoscientists due to its potential evidence for lithosphere recycling down to, or below, the mantle Transition Zone (e.g., Yang et al., 2007, 2015; Griffin et al., 2016).
View in article
The presence of diamond (Farré-de-Pablo et al., 2019a) and other highly reduced phases (e.g., metallic Si, moissanite; Pujol-Solà et al., 2018) in these rocks, in particular chromitites, cannot hence be taken as a general indication of ultra-high pressure conditions (e.g., Yang et al., 2007, 2015; Griffin et al., 2016).
View in article


Yang, J., Lian, D., Robinson, P.T., Qiu, T., Xiong, F., Wu, W. (2019) Comment to: A shallow origin for diamonds in ophiolitic chromitites. Geology 47, e475, doi: 10.1130/G46446C.1.
Show in context

However, the debate on the natural origin of diamond continued (e.g., Farré-de-Pablo et al., 2019b; Massonne, 2019; Yang et al., 2019).
View in article



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Supplementary Information

Abstract | Introduction | Results | Natural Origin of the Studied Diamond | Processes and Conditions for Diamond Formation | Acknowledgements | References | Supplementary Information


The Supplementary Information includes:
  • Geological Setting and Petrography
  • Methods
  • Tables S-1 and S-2
  • Figures S-1 to S-7
  • Supplementary Information References


Download the Supplementary Information (PDF).
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Figures



Figure 1 (a) Transmitted light photomicrograph (TLP) of olivine hosting a trail of secondary fluid inclusions. The red area defines the zoom in (b-d). (b, c) TLP of fluid inclusions below the surface of olivine with focus at Z (depth) = −1 and 0 μm respectively. (d) Reflected light photomicrograph of (c), showing that the central inclusion is completely below the surface. White rectangles mark the area of (e-g). (e) Fluid inclusion confocal Raman map at Z = −1 μm; different colours represent different phases. (f) Confocal Raman map for the same inclusion at Z = −2 μm. (g) Z-stack of (e, f) showing the inclusion profile. (h) Raman spectra of the identified phases (colour coded). Mapping conditions: 6 × 6 μm, 30 × 30 spectra, Tint = 2 s, 2 mW, 100× objective.
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Figure 2 (a) Field emission scanning electron microscope image of olivine-hosted inclusion thinned by focused ion beam. (b,c) TEM image of the diamond and the surrounding serpentine and magnetite; the red square shows the selected area electron diffraction (SAED). (d) SAED pattern confirming the diamond structure of the crystal (the nearly horizontal rows of reflections have indices 111 with d spacing of 2 Å). (e) Electron energy loss near-edge structure of the C K-edge for the diamond showing a major peak due to its sp3 bonding. Abbreviations: Dia-Diamond, Mag-magnetite, Ol-olivine, Srp-serpentine, Pt*-platinum deposited during sample preparation.
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Figure 3 Phase relations in the C-O-H system (see methods in SI for details) with indication of fO2 isopleths (black solid lines, log-units) for the diamond-(Dia) saturated portion (log(aC) = 0) of the system at 100 MPa, 350 °C.
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