Mineralogical analysis in bones of Cretaceous reptiles: Geochemical attack during fossil diagenesis or depositational nucleus of Fe and Mn oxides-hydroxides?

CLAUDIA DI LELLO; SILVIA AMETRANO; SEBASTIÁN APESTEGUÍA; LUCIANA JUNCAL; ROSANA M. ROMANO; CARLOS O. DELLA VÉDOVA

Mendoza

Congreso; 18th Internacional Sedimentological Congress ISC; 2010

Presence of Fe and Mn oxide-hydroxides on bones of fossil vertebrates is frequent in sedimentary deposits such as concretions lying in mottled levels and directly associated to the fossil-diagenetic process of the remains. These concretions are a consequence of precipitation of minerals present in gels formed within aqueous solutions. Bone pieces may be unaltered or modified – at least superficially – and are usually found within the same level. Such a taphonomic attribute elicits questions on the driving process that concentrates these minerals on the bone nuclei. To understand this pattern, the chromatic variation of the bones of Kaikaifilusaurus avelasi (Apesteguía and Novas, 2003) [MPCA-365] was analyzed. The specimens were collected from the Candeleros Formation (Cenomanian–Turonian) exposed at “Huene” quarry (S39º36.831´/W68º40.254´), Rio Negro Province. Analyses included X-ray diffraction [XRD], infra-red spectroscopy [IR-FT] and scanning electron microscope [SEM].  Following the Cailleux soil color chart, they were classified into 5 categories from A to E: very pale chestnut-brown brun trés pale (10 YR 8/3); weak red rouge faible (10 R 5/3); reddish chestnut-brown brun rouge (5 YR 4/3); dark reddish chestnut-brown brun rouge foncé (5 YR 3/2) and very dark gray gris tres foncé (2.5 Y 3/0). Finally, following the “Bone Weathering Stages” proposed by Behrensmeyer (1978), they were grouped into stages 1 (A-B) and 2 (C-D-E). From the XRD analysis it was determined that all the pieces contained varieties of Apatite [Francolite, Carbonatehydroxyapatite and Carbonatefluoroapatite] in different proportions. In addition, A presents traces of Analcime; C contains a higher Hematite concentration than B; D is richer in this oxide than in phosphate, with subordinate Pyrolusite. Finally, E has subordinate Hematite to Pyrolusite and Manganite, with scarce Apatite. The IR spectrum indicates that A is composed of 65% Apatite and 35% Carbonatehydroxyapatite; and that in D and E there is a relative decrease of the phosphate bands. Macroscopically, A and B show fractures parallel to the bone fibers and SEM of A shows unaltered periosteal bone. On the other hand, damaged and mosaic-patterned cracking and flaking bone surfaces are clear in C, D and E. This is more clearly noticeable in C because of the thinner coating, while in D and E it is obscured by the almost complete absence of bone material. This study suggests a differential preservation of fossil remains from the same strata. Such a difference appears early in the biostratinomic process with the weathering of the carcass altering the surfaces of the bones. The extended exposure of the remains would cause the pre-burial damage observed in the periosteal bone. This renders easier the formation of oxide and hydroxide coatings over the material.  During fossil-diagenesis the bones act as crystallization nuclei, and the formation of concretions over them is enhanced by circulation of fluids loaded with oxides and hydroxides within the fossil-bearing layer. This differential precipitation produces anomalous concentrations (mottles) of Hematite, Pyrolusite and Manganite over the most heavily weathered bones. A remarkable direct correlation exists between the weathering stage and the increasing (from C to E) clouding of boundaries between bone and oxide-hydroxides resulting from phosphate/Fe - Mn reactions (Staley et al., 1992; Cornell & Schwertmann, 2003).