Methane occurrence within the Vourinos (a) and Othrys (b) ophiolite rocks. Methane occurs only within chromitite-rich layers (massive and disseminated chromitites, with Platinum Group Elements - PGE) within the ultramafic body. Sample acronyms are described in Supplementary Table 1 (see also Fig. 2). Peridotites and other rocks overlying or underlying the chromitites, i.e. gabbros and basalts (above the chromitites in the Vourinos ophiolite; below the chromitite in the inverted Othrys ophiolite) are devoid of methane. Also all flood basalts that separate the Othrys ophiolite from the basement (Pelagonian Unit) are methane-free.