The Boda Claystone Formation (BCF) is an extended sedimentary sequence formed in a shallow-water salt lake under semi-arid to arid climatic conditions during the middle Permian period. The rock was formed predominantly from denuded and altered products of three primary felsic sources, the Mórágy Metagranite Complex, the Baksa Metamorphic Complex and the rhyolitic Gyűrűfű Formation, resulting in the recent dominant sheet silicate components, illite and chlorite. BCF has been considered a potential host rock for high-level nuclear waste, too. Thus, it has been characterized by several powerful methods so far (X-ray diffraction (XRD), transmission electron microscopy (TEM), etc.). ⁵⁷Fe Mössbauer spectroscopy may provide a unique additional tool to study iron-bearing minerals. Iron is dominantly present in a ferrous form in minerals of the fresh parent rocks (in the biotite group and amphibole), and in a ferric oxide, hematite, in altered Gyűrűfű Formation. During transformations of biotite group minerals and amphibole, the partial release of ferrous iron or its conversion to ferric form takes place with the stabilization of recent illite and chlorite, while the original layered structure is still preserved. Mössbauer spectroscopy revealed the dominant presence of ferrous iron located in cis-M2 octahedral sites both in parent biotite group minerals and in the final illite, as well as chlorite in both stages. The proportion of ferrous iron in biotite group minerals was halved during the stages of evolution by conversion to ferric iron still in sheet silicate illite or by segregation into separate hematite inclusions. The transformation process of biotite group minerals and amphibole of the source rocks is connected only to the iron-bearing smaller fraction of sheet silicates in the BCF clay mineral assemblage. Determination of Fe²⁺/Fe³⁺ ratios in sheet silicates was also pertinent in two sections of BCF. Namely, in samples from the Gorica region, Fe³⁺ was dominant, siting in illite, whereas Fe²⁺ was also present in significant portions in chlorite in samples from the Western Mecsek Anticline. The interpretation is deduced in correspondence with results of extended XRD, and high-resolution TEM studies.

Continental rifting of the Tisza microplate started during the Late Jurassic and resulted in phreatic eruptions, peperite, and the construction of a volcanic edifice in the Early Cretaceous in the Mecsek Mountains (South Hungary). In the SE direction from the volcanic edifice at Zengővárkony, a shallow marine (depth 100–200 m) carbonate sediment hosted a vent environment, and iron ore deposition occurred at the end of the Valanginian to early Hauterivian, hosting a diverse, endemic fauna of approximately 60 species. The detailed mineralogical analysis of the transport conduits included Fe oxides (ferrihydrite, goethite, hematite, and magnetite), quartz, mixed carbonate, pyrite, feldspar, Fe-bearing clay minerals, apatite, sulfates (barite, gypsum, and jarosite), and native sulfur. Filamentous, microbially mediated microtextures with inner sequented, necklace-like spheric forms (diameter of 1 μm) and bacterial laminae are also observed inside decapod crustacean coprolites (Palaxius tetraochetarius) and in the rock matrix. This complex ecological and mineralogical analysis provided direct evidence for the presence of bacteria in fossil sediment-hosted vent (SHV) environments on the one hand and for the intimate connection between bacteria and decapod crustaceans in hydrothermal environments 135 Ma before. This observation completes the fossil food chain of chemosynthesis-based ecosystems, from primary producers to the top carnivores reported for the first time from this locality.

Clumped isotope measurements of carbonates and stable isotope analyses of water trapped in fluid inclusions are both promising techniques to determine carbonate formation temperatures. Cave-hosted carbonate deposits (speleothems) would be excellent targets for such studies, but kinetic fractionations and diagenetic influences frequently deteriorate the temperature data obtained with these methods. However, subaqueous carbonate deposits may provide reliable data as kinetic fractionations are less significant in underwater environment. In the present study subaqueous carbonate formations were investigated, whose formation temperatures were directly measured in the water. Additionally, temperatures calculated from oxygen isotope fractionations between calcite and fluid inclusion hosted water were compared with clumped isotope temperatures obtained for subaqueous carbonate formations in cave-hosted lakes. The clumped isotope temperatures fit the measured and calculated temperatures within the analytical precisions. Our study shows that subaqueous carbonate speleothems are useful targets for clumped isotope and inclusion water analyses, and therefore they are valuable paleotemperature archives.

Continental rifting of the Tisza microplate has started during the Late Jurassic that resulted phreatic eruptions, peperites, and built up a volcanic edifice in the Early Cretaceous of the Mecsek Mountains (South Hungary). In SE direction from the volcanic edifice at Zengővárkony a shallow marine (depth 100 – 200 m) carbonate sediment hosted vent environment and iron-ore deposition occurred at end Valanginian too early Hauterivian hosting a diverse, endemic fauna of around 60 species. Detailed mineralogical analysis of the transport channels include Fe oxides (ferrihydrite, goethite, hematite, magnetite), quartz, mixed carbonate, pyrite, feldspar, Fe-bearing clay minerals, apatite, sulphates (barite, gypsum, jarosite) and native sulphur. Filamentous, microbially mediated microtexture with inner sequented, necklace-like spheric forms (diameter 1 μm) and bacterial laminae are observed inside the decapod crustacean coprolites (Palaxius tetraochetarius) and in the rock matrix, too. This complex, ecological and mineralogical analysis provided direct evidence for the presence of bacteria at fossil SHV environments on one hand, and the intimate connection between bacteria and decapod crustaceans at hydrothermal environments 135 Ma before. This observation completes this chemosymbiotic fossil food chain from the primary producers to the top carnivores reported for the first time from this locality.