ABSTRACT

The fossil-bone mineral in dinosaurs from the Morrison Formation is well-crystallized, stoichiometric francolite. The closely packed, subparallel francolite crystals have equidimensional cross sections mostly 10-40 nm wide. In contrast, crystallites in unaltered mammal bone tissue are poorly crystallized, nonstoichiometric carbonate hydroxyapatite, a few nanometers thick, and plate-, rod- or needle-shaped. The diagenetic francolite crystals are elongated in the c axis direction, as are bone-tissue crystallites. Along permeable cracks, francolite crystals grew to 250 nm wide. Compared to unaltered crystallites in mammal bones, the francolite has higher concentrations of Ba, Ce, Cr, F, La, Mn, Ni, Pb, Rb, Sr, Th, U, V, and Y, and perhaps Zr. Na and Mg are lower in concentration. Diagene ic francolite grew from groundwater enriched in ions from silicic ashes in the Brushy Basin Member and from dissolution of bone-tissue crystallites.

After burial, francolite grew on crystallite seeds, filling the space formerly occupied by collagen, perhaps half the tissue volume, and dissolved crystallites. The micron-scale structures such as Haversian canals and laminae are preserved because the diagenetic francolite retains the orientations of the seeds. The laminae, 1-10 µm thick, commonly preserve an orthogonal plywood structure.

The diagenetic growth of francolite during this type of bone fossilization implies that oxygen isotopic ratios measured in dinosaur bones, even those that appear fresh and unaltered, may dominantly reflect groundwater temperature rather than dinosaur body temperature. Trace-element concentrations in the francolite may reflect groundwater composition rather than dinosaur diet.

Pore-filling cements precipitated in the sands and bones at shallow burial depths, as evidenced by the 35-52% minus-cement porosities of the sandstones and numerous uncompacted oversize pores filled by cements. Gypsum and calcite were precipitated from shallow alkaline groundwater. As the groundwater became increasingly dominated by ions from silicic ash, argentite, chalcopyrite, native gold, native silver, tiemannite, and uraninite replaced areas of bone a few microns to tens of microns in size along cracks. A thin chlorite crust commonly mantled francolite crystals along bone cracks and lined openings in bones and sandstones. Pervasive precipitation of a chalcedony crust followed, with pseudomorphs of chalcedony after gypsum. In larger cancellous openings, the remaining space was fi led by the various forms of silica. The bones are brown due to goethite-filled cracks that cut pore-filling minerals.