Reply to Comment on “Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes” by Griffin et al., 2018: No evidence for transition zone metamorphism in the Luobusa ophiolite
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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., 2017Ballhaus, 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., 2015McGowan, 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., 2016aGriffin, 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., 2015Xiong, 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., 2016bGriffin, 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., 1997Koeberl, 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, 1997Sung, 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., 2016aGriffin, 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, 1986Barnes, S.J. (1986) The distribution of chromium among orthopyroxene, spinel and silicate liquid at atmospheric pressure. Geochimica et Cosmochimica Acta 50, 1889–1909.
; Kinzler, 1997Kinzler 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., 2017Chen, 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., 2017Chen, 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, 1986Essene, E.J., Fisher, D.C. (1986) Lightning strike fusion: Extreme reduction and metal-silicate liquid immiscibility. Science 234, 189–193.
; Plyashkevich et al., 2016Plyashkevich, 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
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.
Show in context
Our experiments (Ballhaus et al., 2017) 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.
View in article
Barnes, S.J. (1986) The distribution of chromium among orthopyroxene, spinel and silicate liquid at atmospheric pressure. Geochimica et Cosmochimica Acta 50, 1889–1909.
Show in context
Are the authors aware that liquidus chromite also incorporates SiO2 and CaO to the tune of 0.3 wt. % each (Barnes, 1986; Kinzler, 1997)?
View in article
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.
Show in context
As for coesite, isochoric heating following lightning strikes may reach pressures well inside coesite stability (Chen et al., 2017).
View in article
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).
View in article
Essene, E.J., Fisher, D.C. (1986) Lightning strike fusion: Extreme reduction and metal-silicate liquid immiscibility. Science 234, 189–193.
Show in context
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; Plyashkevich et al., 2016).
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., 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.
Show in context
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; Griffin et al., 2016a); that chromite crystallised as high pressure polymorph in the calcium ferrite (CF) structure (Xiong et al., 2015); or that the upper mantle is super-reduced (Griffin et al., 2016b).
View in article
Alternative origins are (1) isochoric shock heating following lightning bolts: the Popigai astrobleme (Koeberl et al., 1997) 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) found concentrated in metal inclusions in Luobusa diamonds are used in industry to flux the graphite-diamond transition (Sung and Tai, 1997).
View in article
How could mantle diamonds have coexisted with Fe-free Ni70Mn20Co5 metal melts (Griffin et al., 2016a) when the lithologies that supposedly carried those diamonds (chromitite, harzburgite) are ferrous and ferric iron bearing?
View in article
Griffin et al. (2016a) document oxide and silicate spherules and relate them to an unspecified high temperature event.
View in article
Griffin et al. (2016a), Yamamoto et al. (2009), and Xiong et al. (2015) assert that podiform chromite crystallises (or recrystallises) in the CF structure at >12 GPa because it carries clinopyroxene exsolutions.
View in article
In the Griffin et al. (2016a) scenario, chromite is first enriched to ore grades at low pressure, then subducted to 600 km, then exhumed back to the surface.
View in article
As for the inverse ringwoodite octahedra brought up by Griffin et al. (2016a) 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.
View in article
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.
Show in context
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; Griffin et al., 2016a); that chromite crystallised as high pressure polymorph in the calcium ferrite (CF) structure (Xiong et al., 2015); or that the upper mantle is super-reduced (Griffin et al., 2016b).
View in article
The glass composition Griffin et al. (2016b) report from Mount Carmel has 4.8 wt. % MgO and zero FeO.
View in article
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.
Show in context
Griffin et al. (2018) discard our lightning experiments because we did not identify ultra-high pressure (UHP) phases.
View in article
Griffin et al. (2018) assert that there is no confirmed textural connection of ultra-reduced phases with UHP minerals.
View in article
Griffin et al. (2018) doubt that the diamonds of Luobusa are vapour deposited (CVD) diamonds.
View in article
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.
Show in context
Based on nitrogen aggregation states, Howell et al. (2015) calculate for the Luobusa diamonds residence times of ~100 years.
View in article
Diamonds are reported in ophiolites along that suture for 1300 km (!) along strike (Howell et al., 2015).
View in article
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.
Show in context
Are the authors aware that liquidus chromite also incorporates SiO2 and CaO to the tune of 0.3 wt. % each (Barnes, 1986; Kinzler, 1997)?
View in article
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.
Show in context
Alternative origins are (1) isochoric shock heating following lightning bolts: the Popigai astrobleme (Koeberl et al., 1997) 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) found concentrated in metal inclusions in Luobusa diamonds are used in industry to flux the graphite-diamond transition (Sung and Tai, 1997).
View in article
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.
Show in context
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; Griffin et al., 2016a); that chromite crystallised as high pressure polymorph in the calcium ferrite (CF) structure (Xiong et al., 2015); or that the upper mantle is super-reduced (Griffin et al., 2016b).
View in article
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.
Show in context
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; Plyashkevich et al., 2016).
View in article
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.
Show in context
At the elevation of the Tibetan ophiolites cloud-to-ground lightning bolts are common (Qie et al., 2003).
View in article
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.
Show in context
Alternative origins are (1) isochoric shock heating following lightning bolts: the Popigai astrobleme (Koeberl et al., 1997) 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) found concentrated in metal inclusions in Luobusa diamonds are used in industry to flux the graphite-diamond transition (Sung and Tai, 1997).
View in article
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.
Show in context
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; Griffin et al., 2016a); that chromite crystallised as high pressure polymorph in the calcium ferrite (CF) structure (Xiong et al., 2015); or that the upper mantle is super-reduced (Griffin et al., 2016b).
View in article
Griffin et al. (2016a), Yamamoto et al. (2009), and Xiong et al. (2015) assert that podiform chromite crystallises (or recrystallises) in the CF structure at >12 GPa because it carries clinopyroxene exsolutions.
View in article
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).
Show in context
In the Luobusa ophiolite, the classic ophiolite lithostratigraphy appears to be preserved (Xuchang et al., 1983) even though it may have been modified during obduction.
View in article
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.
Show in context
Griffin et al. (2016a), Yamamoto et al. (2009), and Xiong et al. (2015) assert that podiform chromite crystallises (or recrystallises) in the CF structure at >12 GPa because it carries clinopyroxene exsolutions.
View in article
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.
Show in context
Yang et al. (2007) document symplectites from Luobusa rocks in which Fe-Ti-Si alloys are intergrown with pseudomorphs of coesite after stishovite.
View in article
Zuxiang, Y. (1984) Two new minerals gupeiite and xifengite in cosmic dusts from Yanshan. Acta Petrologica Mineralogica et Analytica 3 (abstract).
Show in context
Zuxiang (1984) suggested the globules are extraterrestrial in origin because he identified Fe-Si alloys in their cores.
View in article