Fossilization of bone studied in-operando by fluid-cell Raman spectroscopy

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Fossil bones represent an important geochemical archive for the reconstruction of the life history of vertebrates on Earth. However, the taphonomy of the bones can change this information or even destroy it completely. Experimental studies of bone taphonomy under controlled boundary conditions are necessary, in particular to understand the processes associated with early fossilization, as these appear to be critical to the preservation of bones. New insights into the mesoscopic partial reactions and transport processes of bone taphonomy can be gained with the help of confocal hyperspectral Raman spectroscopy (RS). This technique enables real-time imaging of moving reaction interfaces during fluid-driven reactions on the micrometre scale and at elevated temperatures using special fluid cells (FC) without the need to interrupt the reaction, i.e., in operando. FC-RS can provide a wealth of information, such as the phase composition, crystallinity, crystallite size, interfacial strain, and at the same time the concentrations of aqueous species in the solution, from which even the local pH value can be estimated. In addition, due to the isotope shift, the FC-RS enables the exchange of oxygen and hydrogen isotopes between solids and aqueous molecular species to be tracked in operando when isotopically enriched precursor materials are used. Intriguing evidence for the feasibility of such in-operando FC-RS studies emerges from the results of initial experiments on dentin and silicate glasses. The results of these experiments have in both cases revealed unforeseen, short-term changes in the mechanism and the overall reaction kinetics. They also show that dynamic phenomena such as re-equilibration reactions and potentially the diffusion of molecular water through bone can be studied and quantified when 18-O and 2-H are used as tracers. First measurements indicate that it will even be possible to follow the diffusion of traces of Nd3+ in bones in-operando with the help of laser-induced photoluminescence/Raman spectroscopy. The FC-RS therefore opens up completely new possibilities to get deeper insights into bone-water reactions. The main goals of the proposed project are to gain a deeper mechanistic understanding of (i) the recrystallization of bone apatite, (ii) bone mineralization, (iii) collagen degradation, and ultimately (iv) reliable, quantitative, kinetic information about the identified transport and reaction processes as a function of the pH value, the composition of the solution (containing carbonate, phosphate, and F) and the temperature. However, the main focus will initially be on room temperature experiments that simulate near-surface conditions.