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Comment

Ballhaus et al. (2017)

Ballhaus, C., Wirth, R., Fonseca, R.O.C., Blanchard, H., Pröll, W., Bragagni, A., Nagel, T., Schreiber, A., Dittrich, S., Thome, V., Hezel, D.C., Below, R., Cieszynski, H. (2017) Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes. Geochemical Perspectives Letters 5, 42-46.

use electric-discharge experiments to argue that lightning strikes could produce ultra-high pressure (UHP) and super-reduced (SuR) phases “identical to those found in ‘high-pressure’ ophiolites” and that thus there is “not sufficient evidence to challenge long-established models of ophiolite genesis”, specifically for the UHP processing of Tibetan ophiolites. However, the authors produced no evidence for UHP phases in their experiments. There are pertinent observations, relevant to the authors’ assertions, in the literature regarding the relationship between the UHP and SuR assemblages in the Tibetan peridotites. Their conclusions are not consistent with this evidence.

(1) There is no clear genetic connection between the UHP phases and the SuR assemblage in the Tibetan ophiolites. The SuR phases, such as moissanite, native metals, carbides, Ti-nitrides and silicides, found in mineral separates or in situ, have no confirmed textural connection with UHP phases (e.g., Robinson et al., 2004

Robinson, P.T., Bai, W.-J., Malpas, J., Yang, J.-S., Zhou, M.-F., Fang, Q.-S., Hu, X.-F., Cameron, S., Staudigel, H. (2004) Ultra-high pressure minerals in the Luobusa ophiolite, Tibet, and their tectonic implications. In: Malpas, J., Fletcher, C.J.N., Ali, J.R., Aitchison, J.C. (Eds.) Aspects of the Tectonic Evolution of China. Geological Society, London, Special Publications, 226, 247-271.

; Xu et al., 2015

Xu, X., Yang, J., Robinson, P.T., Xiong, F., Ba, D., Guo, G. (2015) Origin of ultrahigh pressure and highly reduced minerals in podiform chromitites and associated mantle peridotites of the Luobusa ophiolite, Tibet. Gondwana Research 27, 686-700.

), with the possible exception of a moissanite inclusion in diamond (Moe et al., 2017

Moe, K.S., Yang, J.S., Johnson, P., Xu, X., Wang, W. (2017) Spectroscopic analysis of microdiamonds in ophiolitic chromitite and peridotite. Lithosphere 10, 133-141, doi: 10.1130/L603.1.

), and coesite surrounding an alloy ball (Dobrzhinetskaya et al., 2009

Dobrzhinetskaya, L.F., Wirth, R., Yang, J.S., Hutcheon, I.D., Weber, P.K., Green II, H.W. (2009) High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite. Proceedings of the National Academy of Sciences of the United States of America 106, 19233-19238.

).

(2) The SuR assemblage reported in the Tibetan chromitites is very similar to one documented in Cretaceous mafic pyroclastic rocks in Israel. These SuR phases are interpreted as products of reactions between mantle-derived CH₄-H₂ fluids and basaltic to ultramafic melts (Griffin et al., 2016a

Griffin, W.L., Gain, S.E.M., Adams, D.T., Huang, J.-X., Saunders, M., Toledo, V., Pearson, N.J., O’Reilly, S.Y. (2016a) First terrestrial occurrence of tistarite (Ti₂O₃): Ultra-low oxygen fugacity in the upper mantle beneath Mt Carmel, Israel. Geology 44, 815-818.

; Xiong et al., 2017

Xiong, Q., Griffin, W.L., Huang, J.-X., Gain, S.E.M., Toledo, V., Pearson, N.J., O’Reilly, S.Y. (2017) Super-reduced mineral assemblages in “ophiolitic” chromitites and peridotites: The view from Mt Carmel. European Journal of Mineralogy 29, 557-570.

).

In both the Israeli and the Tibetan examples, many of the SuR phases occur in melt pockets trapped in skeletal corundum crystals, rapidly crystallised from Al₂O₃-supersaturated melts (Xu et al. 2013

Xu, X.Z., Yang, J.S., Guo, G.L., Xiong, F.H. (2013) Mineral inclusions in corundum from chromitites in the Kangjinla chromite deposit, Tibet. Acta Petrologica Sinica 29, 1867-1877 (Chinese, English abstract).

; Griffin et al., 2016a

Griffin, W.L., Gain, S.E.M., Adams, D.T., Huang, J.-X., Saunders, M., Toledo, V., Pearson, N.J., O’Reilly, S.Y. (2016a) First terrestrial occurrence of tistarite (Ti₂O₃): Ultra-low oxygen fugacity in the upper mantle beneath Mt Carmel, Israel. Geology 44, 815-818.

; Xiong et al., 2017

Xiong, Q., Griffin, W.L., Huang, J.-X., Gain, S.E.M., Toledo, V., Pearson, N.J., O’Reilly, S.Y. (2017) Super-reduced mineral assemblages in “ophiolitic” chromitites and peridotites: The view from Mt Carmel. European Journal of Mineralogy 29, 557-570.

). These melts were depleted in Fe and Si by the immiscible separation of Fe-Ti-Si-C melts and crystallisation of SiC (crystals >4 mm in the Israeli examples, ≤2 mm in Tibetan ones), leading to very high Al contents. The presence of native vanadium in late-stage Si-depleted melts requires very low oxygen fugacities (ΔIW -11; Griffin et al., 2016a

Griffin, W.L., Gain, S.E.M., Adams, D.T., Huang, J.-X., Saunders, M., Toledo, V., Pearson, N.J., O’Reilly, S.Y. (2016a) First terrestrial occurrence of tistarite (Ti₂O₃): Ultra-low oxygen fugacity in the upper mantle beneath Mt Carmel, Israel. Geology 44, 815-818.

). The abundance of carbides (TiC, SiC) and amorphous carbon indicates high carbon contents in the melts. Textural evidence for the reaction corundum+melt → anorthite in the Israeli examples indicates crystallisation pressures of 9-10 kb, and temperatures of ca. 1450 °C (Goldsmith, 1980

Goldsmith, J.R. (1980) The melting and breakdown reactions of anorthite at high pressures and temperatures. American Mineralogist 65, 272-284.

).

These SuR phases are clearly related to deep magmatic processes in the mantle, rather than lightning strikes.

(3) The diamonds in Tibetan ophiolites (Bai et al., 1993

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

) have been largely ignored by geoscientists, because of their similarity to those grown by high-pressure high-temperature (HPHT) synthesis, and because their lack of nitrogen aggregation (pure Ib) is inconsistent with the originally proposed (Yang et al., 2014

Yang, J.S., Robinson, P.T., Dilek, Y. (2014) Diamonds in ophiolites: a little-known diamond occurrence. Elements 10, 123-126.

) deep origins. Detailed studies of these diamonds provide compelling evidence for their natural origins (Howell et al., 2015

Howell, D., Griffin, W.L., Yang, J.-S., Gain, S., Stern, R.A., Huang, J.-X., Jacob, D.E., Xu, X., Stokes, A.J., O'Reilly, S.Y., Pearson, N.J. (2015) Diamonds in ophiolites: Contamination or a new diamond growth environment? Earth and Planetary Science Letters 430, 284-295.

; Moe et al., 2017

Moe, K.S., Yang, J.S., Johnson, P., Xu, X., Wang, W. (2017) Spectroscopic analysis of microdiamonds in ophiolitic chromitite and peridotite. Lithosphere 10, 133-141, doi: 10.1130/L603.1.

), and describe characteristics that are hard to reconcile with the environment proposed by Ballhaus et al. (2017)

Ballhaus, C., Wirth, R., Fonseca, R.O.C., Blanchard, H., Pröll, W., Bragagni, A., Nagel, T., Schreiber, A., Dittrich, S., Thome, V., Hezel, D.C., Below, R., Cieszynski, H. (2017) Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes. Geochemical Perspectives Letters 5, 42-46.

, which resembles the chemical vapour deposition (CVD) process used to produce some synthetic diamonds. Growth rates of high-quality single crystals in carefully-controlled laboratory CVD synthesis are in the region of 100 µm/hr (e.g., Liang et al., 2009

Liang, Q., Yan, C.-S., Meng, Y., Lai, J., Krasnicki, S., Mao, H.-K., Hemley, R.J. (2009) Recent advances in high-growth rate single-crystal CVD diamond. Diamond and Related Materials 18, 698-703

; Lu et al., 2013

Lu, J., Gu, Y., Grotjohn, T.A., Schuelke, T., Asmussen, J. (2013) Experimentally defining the safe and efficient, high pressure microwave plasma assisted CVD operating regime for single crystal diamond synthesis. Diamond and Related Materials 37, 17-28.

). Thus, the plasma temperatures of the lightning strike would need to be sustained for hours to produce the diamonds (100 - 500 µm) in the Tibetan ophiolites; this would be inconsistent with the lack of nitrogen aggregation (P. Cartigny, pers. comm. 2018). The diamonds clearly formed at high T (metal-alloy inclusions) but the nitrogen-aggregation data are inconsistent with formation in the transition zone. We have suggested (Xiong et al., 2017

Xiong, Q., Griffin, W.L., Huang, J.-X., Gain, S.E.M., Toledo, V., Pearson, N.J., O’Reilly, S.Y. (2017) Super-reduced mineral assemblages in “ophiolitic” chromitites and peridotites: The view from Mt Carmel. European Journal of Mineralogy 29, 557-570.

) that they formed in systems like the Israeli one, but at greater depths.

(4) The Tibetan peridotites and chromitites are typical of those formed at shallow depths, in the mantle wedges of subduction zones (Griffin et al., 2016b

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., Satsukawa, T., Shi, R., Williams, P., Xiong, Q., Yang, J-S., Zhang, M., O’Reilly, S.Y. (2016b) Mantle Recycling: Transition-Zone metamorphism of Tibetan ophiolitic peridotites and its tectonic implications. Journal of Petrology 57, 655-684.

). The evidence for their subsequent subduction to the deep upper mantle or Mantle Transition Zone is difficult to attribute to lightning strikes.

• (a) Exsolution of pyroxenes (+rare coesite) as oriented lamellae in chromite. Yamamoto et al. (2009)

  Yamamoto, S., Komiya, T., Hirose, K., Maruyama, S. (2009) Coesite and clinopyroxene exsolution lamellae in chromites: In-situ ultrahigh-pressure evidence from podiform chromitites in the Luobusa ophiolite, southern Tibet. Lithos 109, 314-322.

  suggested that a UHP precursor with a calcium ferrite structure originally formed at >12.5 GPa, and then decomposed to low-P chromite containing silicate exsolution lamellae. The stability range of this polymorph (14- ≥18 GPa at 1400 °C), and its ability to incorporate percent levels of Ca and Si, have been demonstrated experimentally (Zhang et al., 2017

  Zhang, Y., Jin, Z., Griffin, W.L., Wang, C., Wu, Y. (2017) High-pressure experiments provide insights into the Mantle Transition Zone history of chromitite in Tibetan ophiolites. Earth and Planetary Science Letters 463, 151-158.

  ).
  
  
• (b) Microstructures suggesting that the chromitites recrystallised under static conditions from fine-grained, highly-deformed mixtures of wadsleyite and an orthorhombic polymorph of chromite (Satsukawa et al., 2015

  Satsukawa, T., Griffin, W.L., Piazolo, S., O’Reilly, S.Y. (2015) Messengers from the deep: Fossil wadsleyite-chromite microstructures from the mantle Transition Zone. Scientific Reports 5, 16484.

  ).
  
  
• (c) Harzburgites with coarsely vermicular symplectites of orthopyroxene + Cr–Al spinel ± clinopyroxene. Reconstructions suggest that these are the breakdown products of high-Cr (6-8 wt. % Cr₂O₃; peridotitic) majoritic garnets, with estimated minimum pressures up to ca. 13 GPa (Gong et al., 2016

  Gong, X.-H., Shi, R-D., Griffin, W.L., Huang, Q.-S., Xiong, Q., Chen, S.-S., Zhang, M., O’Reilly, S.Y. (2016) Recycling of ancient subduction-modified mantle domains in the Purang ophiolite (southwestern Tibet). Lithos 262, 11-26.

  ; Griffin et al., 2016b

  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., Satsukawa, T., Shi, R., Williams, P., Xiong, Q., Yang, J-S., Zhang, M., O’Reilly, S.Y. (2016b) Mantle Recycling: Transition-Zone metamorphism of Tibetan ophiolitic peridotites and its tectonic implications. Journal of Petrology 57, 655-684.

  ).
  
  
• (d) The presence in the Luobusa chromitites of an inverse-ringwoodite phase with minor levels of Mg and Al (Griffin et al., 2016b

  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., Satsukawa, T., Shi, R., Williams, P., Xiong, Q., Yang, J-S., Zhang, M., O’Reilly, S.Y. (2016b) Mantle Recycling: Transition-Zone metamorphism of Tibetan ophiolitic peridotites and its tectonic implications. Journal of Petrology 57, 655-684.

  ). This phase has been produced in the magnesiochromite-forsterite system at 20 GPa and 1600 °C (L. Bindi, pers. comm. 2017), further confirming the Transition-Zone metamorphism of the Tibetan chromitites and their host peridotites.
  

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Conclusions, Acknowledgements

The experiments described by Ballhaus et al. (2017)

Ballhaus, C., Wirth, R., Fonseca, R.O.C., Blanchard, H., Pröll, W., Bragagni, A., Nagel, T., Schreiber, A., Dittrich, S., Thome, V., Hezel, D.C., Below, R., Cieszynski, H. (2017) Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes. Geochemical Perspectives Letters 5, 42-46.

did not produce any UHP phases. The large body of evidence for the UHP metamorphism of some collision-zone ophiolites cannot be dismissed on the basis of a speculation that other experiments might do so. However, we thank the authors for the opportunity to provide this clarification for the scientific community.

This is contribution 1106 from CCFS and 1212 from GEMOC. Thanks to Pierre Cartigny and an anonymous referee for useful comments.

Editor: Graham Pearson

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References