Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes

doi: 10.7185/geochemlet.1744 Since ultra-high pressure (UHP) minerals have been discovered in ophiolites from Tibet and the Polar Urals, it is speculated that the mantle sections of ophiolites may originate deep within the mantle. The UHP minerals are frequently found together with ultra-reduced silicides, carbides, and nitrides. Consequently, it is argued that the deep mantle, or at least domains within it, must be highly reduced, so reduced that practically all transition elements at depth are present in the metallic state. We find it problematic to rewrite the history of ophiolite complexes based on these observations and suggest we should search for alternative and more realistic modes of origin. Electric discharge experiments at >6000 K reported here show that the UHP and highly reduced phase assemblages may precipitate from plasmas. We argue that the mineral assemblages may originate by lightning strikes. As such, they may not record the origin and emplacement history of the mantle lithologies within which they occur. Received 6 July 2017 | Accepted 31 October 2017 | Published 20 November 2017 1. Steinmann Institute, University of Bonn, Germany * Corresponding author (email: ballhaus@uni-bonn.de) 2. Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany 3. Wehrtechnische Dienststelle, WTD 81, Greding, Germany 4. Institute of Geology and Mineralogy, University of Cologne, Germany 5. Department of Geoscience, University of Århus, Denmark 6. Fraunhofer Institute for Building Physics Holzkirchen, Germany Introduction Until recently it was accepted that ophiolites form at low pressure along intra-oceanic spreading ridges (Lago et al., 1982; Zhou et al., 1996; Matveev and Ballhaus, 2002). The identification of ultra-high pressure (UHP) phases in chromitites and harzburgites of ophiolite complexes (Robinson et al., 2004; Yang et al., 2014, 2015) appears to be changing that view. It is now being argued that mantle sections of ophiolites either originate in, or were processed within, the Earth’s mantle at depths as great as 600 km (Xiong et al., 2015; Xu et al., 2015; Griffin et al., 2016). Furthermore, these UHP phases are often found associated with ultra-reduced minerals: native metals, silicides, carbides, and nitrides (Dobrzhinetskaya et al., 2009; Xu et al., 2015). Consequently, it is speculated that the mantle source regions of ophiolites must be ultra-reduced. We question that the UHP and ultra-reduced phases are sufficient evidence to challenge long-established models of ophiolite genesis. Rather, alternative ways should be explored as to how these unusual mineral assemblages may have formed. We suggest that they may be the products of lightning strikes (Essene and Fisher, 1986; Daly et al., 1993). Novel ultra-high temperature experiments at >6000 K reported here show that ultra-reduced phases can precipitate directly from plasmas. We do not identify diamond because our quench rates are too high, however, we do synthesise concentric shell fullerenes that may serve as nanoscopic pressure cells to vapour-deposited (CVD) diamonds (Banhart and Ayajan, 1996; Sorkin et al., 2011). The Luobusa ophiolite in Tibet where the unusual mineral assemblages are documented in most detail is situated at an altitude in excess of 4000 m, in a region where lightning strikes are frequent (Kumar and Kamra, 2012). The lithologies in which UHP and ultra-reduced minerals are found – serpentinised harzburgite and podiform chromitite – are electrically highly conductive lithologies and prone to attract lightning strikes. Experimental Strategy Basaltic samples are exposed to electric discharges that correspond in terms of current (30 kA) to natural cloud-to-ground lightning strikes (Uman, 1986). The samples are cut in cuboids, set in epoxy resin, and encased by steel sleeves. Tungsten