Francolite as a diagenetic mineral in dinosaur and other Upper Cretaceous reptile bones (Lano, Iberian Peninsula): microstructural, petrological and geochemical features

Dinosaur and other Upper Cretaceous reptile (crocodile and turtle) bone sam ples from the Lano quarry, in the northern Iberian Peninsula, are composed of well-crystallized francolite (carbonate fluorapatite). The replacement o f biogenic hydroxyapatite by francolite is related to diagenetic changes. M oreover, the crystallinity index (CI) of the Lano bone fossils is lower (0. 2 to 0.3) than that of recent bones (0.39-0.48) and, compared to fresh bone s (0.41-0.54), the francolite has higher concentrations of trace elements a nd rare earth elements (Sigma REE 500 to 900 ppm). The geochemical composit ion and trends of the fossil reptile remains suggest that the diagenetic pr ocesses were relatively uniform at the Lano site. The main fossil accumulat ions of Lano are associated with ferruginous crust surfaces. The reptile bo nes are usually covered with nodule-like iron oxides which are mainly compo sed of goethite and small detrital quartz grains. The diagenetic processes that led to the development of the iron oxides could have been a consequenc e of a hydromorphic process owing to seasonal variations of the phreatic wa ter level. This suggests a climate with dry and wet seasonal periods. (C) 1 999 Academic Press.