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Volume 7

Silicon and oxygen isotopes unravel quartz formation processes in the Icelandic crust

Quartz formation processes in the Icelandic crust were assessed using coupled δ18O and δ30Si systematics of silica deposits formed over a wide temperature range (<150 to >550 °C). Magmatic quartz reveals δ18O (-5.6 to +6.6 ‰) and δ30Si (-0.4 ± 0.2 ‰) values representative of mantle- and crustally-derived melts in Iceland. Hydrothermal quartz and silica polymorphs display a larger range of δ18O (-9.3 to +30.1 ‰) and δ30Si (-4.6 to +0.7 ‰) values. Isotope modelling reveals that such large variations are consistent with variable water sources and equilibrium isotope fractionation between fluids and quartz associated with secondary processes occurring in the crust, including fluid-rock interaction, boiling and cooling. In context of published δ18O and δ30Si data on hydrothermal silica deposits, we demonstrate that large ranges in δ30Si values coupled to insignificant δ18O variations may result from silica precipitation in a hydrothermal fluid conduit associated with near-surface cooling. While equilibrium isotope fractionation between fluids and quartz seems to prevail at high temperatures, kinetic fractionation likely influences isotope systematics at low temperatures.

B.I. Kleine, A. Stefánsson, S.A. Halldórsson, M.J. Whitehouse, K. Jónasson

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

Geochem. Persp. Let. (2018) 7, 1-2 | doi: 10.7185/geochemlet.1809 | Published 9 March 2018

Reply 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

Geochem. Persp. Let. (2018) 7, 3-4 | doi: 10.7185/geochemlet.1810 | Published 9 March 2018