Fig. 1: Location and characteristics of the magmatic rock samples.

a High resolution bathymetric map of the Atlantis Massif hosting the Lost City hydrothermal field. The massif is mainly composed of mantle and mantle-derived magmatic rocks exhumed along the Mid-Atlantic Ridge (MAR) parallel to the Atlantis transform fault (“m.b.s.l.” stands for meters below sea level). The inset shows its location at the MAR scale. b, c Thin section scans in natural and cross-polarized light, respectively, of a characteristic troctolite sample used in this study and recovered at 1100 meters below sea floor by drilling the Atlantis massif during the IODP Expedition 304–305 Leg 1309D (sample 1309D–228R2). d Optical view in transmitted cross-polarized light of olivine (Ol) kernels hosted in the same troctolite. Red arrows show planes of secondary fluid inclusions within olivine crystals. e Optical photomicrographs in transmitted plane-polarized light of olivine-hosted multiphasic fluid inclusions.