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Griffin et al. (2018)

Griffin, W.L., Howell, D., Gonzalez-Jimenez, J.M., Xiong, Q., O'Reilly, S.Y. (2018) Comment on “Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes”. Geochemical Perspectives Letters 7, 1–2.

discard our lightning experiments because we did not identify ultra-high pressure (UHP) phases. Our experiments (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.

) provide the first rational explanation of many unusual findings in the so-called UHP ophiolites and hence undermine the foundations on which the resulting speculative geotectonic scenarios are based. Little room seems left to postulate that ultramafic rocks along the Jarlung-Zangbo suture zone have seen Transition Zone (TZ) pressures (McGowan et al., 2015

McGowan, N.M., Griffin, W.L., Gonzalez-Jiménez, J.M., Belousova, E.A., Afonso, J., Shi, R., McCammon, C.A., Pearson, N.J. & O’Reilly, S.Y. (2015) Tibetan chromitites: Excavating the slab graveyard. Geology 43, 179–182.

; Griffin et al., 2016a

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. (2016a) Mantle recycling: Transition zone metamorphism of Tibetan ophiolitic peridotites and its tectonic implications. Journal of Petrology 57, 655–668.

); that chromite crystallised as high pressure polymorph in the calcium ferrite (CF) structure (Xiong et al., 2015

Xiong, F.H., Yang, J., Robinson, P.T., Xu, X., Liu, Z., Li, Y., Li, J., Chen, S. (2015) Origin of podiform chromitite, a new model based on the Luobusa ophiolite, Tibet. Gondwana Research 27, 525-542.

); or that the upper mantle is super-reduced (Griffin et al., 2016b

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

).

(1) Griffin et al. (2018)

Griffin, W.L., Howell, D., Gonzalez-Jimenez, J.M., Xiong, Q., O'Reilly, S.Y. (2018) Comment on “Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes”. Geochemical Perspectives Letters 7, 1–2.

assert that there is no confirmed textural connection of ultra-reduced phases with UHP minerals. That is incorrect. Yang et al. (2007)

Yang, J., Dobrzhinetskaya, L.F., Bai, W.J., Junfeng Zhang, J., Green II, H.W. (2007) Diamond and coesite-bearing chromitites from the Luobusa ophiolite, Tibet. Geology 35, 875–878.

document symplectites from Luobusa rocks in which Fe-Ti-Si alloys are intergrown with pseudomorphs of coesite after stishovite.

(2) The glass composition Griffin et al. (2016b)

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

report from Mount Carmel has 4.8 wt. % MgO and zero FeO. That melt is not in equilibrium with an upper mantle mineralogy. So how could one speculate that ultra-reduced phases like Ti₂O₃, Fe-Si alloys, Ti nitrides and borides inside that glass are diagnostic of mantle redox states?

(3) Griffin et al. (2018)

Griffin, W.L., Howell, D., Gonzalez-Jimenez, J.M., Xiong, Q., O'Reilly, S.Y. (2018) Comment on “Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes”. Geochemical Perspectives Letters 7, 1–2.

doubt that the diamonds of Luobusa are vapour deposited (CVD) diamonds. We brought up the CVD option because we synthesised shell fullerenes, known to be potential precursors to diamond. Alternative origins are (1) isochoric shock heating following lightning bolts: the Popigai astrobleme (Koeberl et al., 1997

Koeberl, C., Masaitis V.L., Shafranovsky, G.I., Gilmour, I., Langenhorst, F., Schrauder, M. (1997) Diamonds from the Popigai impact structure, Russia. Geology 25, 967–970.

) was also short-lived but did produce diamonds millimetres in size, so the size-time argument may not be valid; and (2) contamination: all transition elements Griffin et al. (2016a)

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. (2016a) Mantle recycling: Transition zone metamorphism of Tibetan ophiolitic peridotites and its tectonic implications. Journal of Petrology 57, 655–668.

found concentrated in metal inclusions in Luobusa diamonds are used in industry to flux the graphite-diamond transition (Sung and Tai, 1997

Sung, C.-M., Tai, M.-F. (1997) Reactivities of transition metals with carbon: Implications to the mechanism of diamond synthesis under high pressure. International Journal of Refractory Metals and Hard Materials 15, 237–256.

). We consider a mantle origin unlikely. How could mantle diamonds have coexisted with Fe-free Ni₇₀Mn₂₀Co₅ metal melts (Griffin et al., 2016a

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. (2016a) Mantle recycling: Transition zone metamorphism of Tibetan ophiolitic peridotites and its tectonic implications. Journal of Petrology 57, 655–668.

) when the lithologies that supposedly carried those diamonds (chromitite, harzburgite) are ferrous and ferric iron bearing? Based on nitrogen aggregation states, Howell et al. (2015)

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

calculate for the Luobusa diamonds residence times of ~100 years. Why are the implications of this important finding being ignored?

(4) Griffin et al. (2016a)

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. (2016a) Mantle recycling: Transition zone metamorphism of Tibetan ophiolitic peridotites and its tectonic implications. Journal of Petrology 57, 655–668.

document oxide and silicate spherules and relate them to an unspecified high temperature event. Are the authors aware of magmatic activity that produces near-perfectly spherical wüstite globules? Zuxiang (1984)

Zuxiang, Y. (1984) Two new minerals gupeiite and xifengite in cosmic dusts from Yanshan. Acta Petrologica Mineralogica et Analytica 3 (abstract).

suggested the globules are extraterrestrial in origin because he identified Fe-Si alloys in their cores. We reproduce those spherules with electric discharges in all detail, and we offer a sensible explanation: they are ejecta of plasma fountains released from lightning flash tubes, quenched and oxidised extremely rapidly in air.

(5) Griffin et al. (2016a)

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. (2016a) Mantle recycling: Transition zone metamorphism of Tibetan ophiolitic peridotites and its tectonic implications. Journal of Petrology 57, 655–668.

, Yamamoto et al. (2009)

Yamamoto, S., Komiya, T., Hirose, K., Shigenori 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, 314–332.

, and Xiong et al. (2015)

Xiong, F.H., Yang, J., Robinson, P.T., Xu, X., Liu, Z., Li, Y., Li, J., Chen, S. (2015) Origin of podiform chromitite, a new model based on the Luobusa ophiolite, Tibet. Gondwana Research 27, 525-542.

assert that podiform chromite crystallises (or recrystallises) in the CF structure at >12 GPa because it carries clinopyroxene exsolutions. In the Griffin et al. (2016a)

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. (2016a) Mantle recycling: Transition zone metamorphism of Tibetan ophiolitic peridotites and its tectonic implications. Journal of Petrology 57, 655–668.

scenario, chromite is first enriched to ore grades at low pressure, then subducted to 600 km, then exhumed back to the surface. Along that path, magmatic chromite would recrystallise twice: first at high pressure in the CF structure to incorporate the silicate component, then back to spinel to exsolve silicate in the form of clinopyroxene needles. Are the authors aware that liquidus chromite also incorporates SiO₂ and CaO to the tune of 0.3 wt. % each (Barnes, 1986

Barnes, S.J. (1986) The distribution of chromium among orthopyroxene, spinel and silicate liquid at atmospheric pressure. Geochimica et Cosmochimica Acta 50, 1889–1909.

; Kinzler, 1997

Kinzler R.J. (1997) Melting of mantle peridotite at pressures approaching the spinel to garnet transition: Application to mid-ocean ridge basalt petrogenesis. Journal of Geophysical Research 102, 853–874.

)? That is more than enough to exsolve clinopyroxene needles during annealing. As for the inverse ringwoodite octahedra brought up by Griffin et al. (2016a)

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. (2016a) Mantle recycling: Transition zone metamorphism of Tibetan ophiolitic peridotites and its tectonic implications. Journal of Petrology 57, 655–668.

in support of their UHP model, we wait for the site occupancies of the cations based on structural refinement data and/or High Resolution Transmission Electron Microscopy (HRTEM) images. As for coesite, isochoric heating following lightning strikes may reach pressures well inside coesite stability (Chen et al., 2017

Chen, J., Elmy C., Goldsby, D., Gieré, R. (2017) Generation of shock lamellae and melting in rocks by lightning-induced shock waves and electrical heating. Geophysical Research Letters 44, 8757–8768.

).

(6) If the mantle sections of the Tibetan ophiolites were subducted to 600 km then exhumed, why after emplacement are ultramafic lithologies juxtaposed to gabbros? Gabbro cumulates do not tolerate pressures above 1 GPa, neither texturally nor mineralogically. In the Luobusa ophiolite, the classic ophiolite lithostratigraphy appears to be preserved (Xuchang et al., 1983

Xuchang, X., Ziyi, W., Guangcen, L., Yougong, C., Xiang, Z. (1983) On the tectonic evolution of the Yarlung Zangbo (Tsangpo) suture zone and its adjacent areas. Acta Geological Sinica 2, 9 (abstract).

) even though it may have been modified during obduction. Should we assume then that at Luobusa, the juxtaposition of harzburgite and dunite to gabbros is coincidental? Or did the gabbros wait patiently in place while the ultramafic sections of the ophiolite were being cycled down and up through the TZ for 1200 km?

(7) We are not convinced that lithologies with zero pressure densities around 3300 kg m^-3 could have been exhumed so easily from 600 km depths over 2000 km along the Jarlung-Zangbo suture zone. Diamonds are reported in ophiolites along that suture for 1300 km (!) along strike (Howell et al., 2015

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

). No numerical model covers exhumations from TZ pressures on such grand scales.

Thirty years of research failed to acknowledge similarities between phases in the so-called UHP ophiolites and in fulgurites. This is surprising. Chromitites and serpentinised (magnetite bearing) harzburgites are electrically quite conductive. At the elevation of the Tibetan ophiolites cloud-to-ground lightning bolts are common (Qie et al., 2003

Qie, X., Toumi, R., Yuan, T. (2003) Lightning activities on the Tibetan Plateau as observed by the lightning imaging sensor. Journal of Geophysical Research 108, 4551, doi:10.1029/2002JD003304, D17.

). When lightning hits solid rock, a thermal pulse is generated that may impose extreme shock pressures in excess of 10 GPa (Chen et al., 2017

Chen, J., Elmy C., Goldsby, D., Gieré, R. (2017) Generation of shock lamellae and melting in rocks by lightning-induced shock waves and electrical heating. Geophysical Research Letters 44, 8757–8768.

). The fulgurite glasses resulting may carry a wide range of super-reduced minerals including metallic Fe, Si, Fe-S-Ti alloys, and moissanite (Essene and Fisher, 1986

Essene, E.J., Fisher, D.C. (1986) Lightning strike fusion: Extreme reduction and metal-silicate liquid immiscibility. Science 234, 189–193.

; Plyashkevich et al., 2016

Plyashkevich, A.A., Minyuk, P.S., Subbotnikova, T.V., Alshevsky, A.V. (2016) Newly formed minerals of the Fe-P-S system in Kolyma fulgurite. Doklady Earth Sciences 467, 380–383.

). If these exotic phases are recovered from oxidised FeO-Cr₂O₃ bearing lithologies, should we not search for other terrestrial occurrences in rocks from tectonic settings that cannot have seen high pressure?

Griffin and coworkers should analyse Luobusa diamonds for radiogenic carbon.

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Acknowledgements

We thank Griffin et al. (2018) for giving us the opportunity to communicate more details.

Editor: Graham Pearson

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References