High spatial resolution investigation of nucleation, growth and early diagenesis in speleothems as exemplar for sedimentary carbonates

Investigation by high resolution Transmission Electron Microscopy (HR-TEM) and Synchrotron-Radiation based micro X-ray fluorescence (SR-μXRF) of diagenesis in carbonates imposes uncertainties on the boundary between stages of crystal growth and post-depositional processes. Speleothems and dolomite are exemplars of the entire range of processes that result in solid materials consisting of crystals. HR-TEM investigation of speleothems suggests that there are many possible pathways of crystallization comprising classical ion-mediated, particle-mediated and formation of metastable phases. Diverse pathways influence the potential of a primary carbonate to undergo post-depositional transformation, with consequences on the accuracy of the preservation of original chemical and physical properties. The capability to date speleothems with U-series techniques is unique amongst other archives of Earth's history. It has been observed that U mobilization, which results in age inversions and uncertainties, is dependent on both crystallization and diagenetic pathways. Here, it is also proposed that the presence of organic colloids, mostly consisting of humic substances (HS), influences the extent to which U may be mobilized, as well as the capacity of original speleothem fabric to undergo dissolution and re-precipitation. Our hypothesis that colloidal HS protect the initial products of crystallization from subsequent diagenesis could explain the existence of primary dolomite preserved in Triassic sabkha facies influenced by fluvial input of siliciclastics. Primary dolomite nanocrystal aggregates coexist with single crystals, highlighting that multiple formation mechanisms were possible, which explains the variety of micro and nanostructures observed by conventional TEM investigations in dolomites. Similarly, we observed calcite nanocrystal aggregates and single crystals preserved in Triassic shallow marine facies, which are famous for their preservation of original aragonite, influenced by continental siliciclastic input. Our new data on the Triassic carbonates suggest that when HS colloids are abundant, nanocrystal aggregates are commonly preserved, possibly because of a protective coating by organic substances. These hinder both the transformation of the aggregates into larger crystals and multiple-steps diagenetic transformation that obliterate primary environmental signals.As opposed to textbook assumptions, it is here proposed that the existence of multiple crystallization pathways for carbonates within the same depositional environment, such as a cave setting or a sabkha, have repercussion on the early diagenetic processes and the extent to which diagenesis significantly resets the original chemical and physical signals. By using speleothems as paradigm for carbonates formed in natural environments where organic compounds are ubiquitous, the present review corroborates the notion that most mechanisms of carbonate crystallization, when the carbonate is not a biomineral, follow inorganic pathways. However, the role of organic substances, as inhibitors of growth and diagenesis, has profound influences on preservation of initial states of crystal formation.