|Silicon and oxygen isotopes unravel quartz formation processes in the Icelandic crust
B.I. Kleine, A. Stefánsson, S.A. Halldórsson, M.J. Whitehouse, K. Jónasson
HTML | PDF | PDF + SI
Geochem. Persp. Let. (2018) 7, 5–11 | doi: 10.7185/geochemlet.1811 | Published 3 April 2018
|Comment on "Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes"
Ballhaus et al. (2017) 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.
W.L. Griffin, D. Howell, J.M. Gonzalez-Jimenez Q. Xiong S.Y. O’Reilly
HTML | PDF
Geochem. Persp. Let. (2018) 7, 1-2 | doi: 10.7185/geochemlet.1809 | Published 9 March 2018Reply to Comment on "Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes"
Griffin et al. (2018) discard our lightning experiments because we did not identify ultra-high pressure (UHP) phases. 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. 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).
C. Ballhaus, R.O.C. Fonseca, A. Bragagni
HTML | PDF
Geochem. Persp. Let. (2018) 7, 3-4 | doi: 10.7185/geochemlet.1810 | Published 9 March 2018