Authigenic minerals in fossil bones from the Mesozoic of England: poor correlation with depositional environments

Petrographic microscopy, X-ray diffractometry, and scanning electron microscopy–energy dispersion spectrometry studies of cavity infills of bone samples from English Mesozoic vertebrate deposits (Isle of Wight, Swanage, Lyme Regis, Aust Cliff, Westbury Garden Cliff, Tytherington) allow a better understanding of the distribution of authigenic minerals in bone voids and shed light on the importance of diagenetic processes on bone preservation. Surprisingly few minerals were identified as void fillers in all depositional environments, carbonates, sulphides, oxides, and sulphates being most abundant. Calcite is present in almost all samples. Pyrite is very common in voids as well as incorporated into bone francolite. In many samples several generations of infills are found, often consisting of layered cements. The co-occurrence of the common minerals indicates anaerobic, slightly alkaline, and often sulphate-reducing environments in the bone voids. Later oxidation is common in iron minerals. Calcite, pyrite, and barite lack unambiguous environmental control, whereas sphalerite is possibly an indicator for marine deposits. Freshwater deposits show no particular mineral combination which would help in separating them from other environments. In some instances, sediment infills and pyrite show geopetal structures.