Huntite [CaMg3(CO3)4], a Rare Carbonate Phase Formed during Early Diagenesis in Modern Microbialites

Microbialites are sedimentary rocks formed under the influence of microbial communities and environmental factors, hence, particularly suited to look for traces of life and reconstruct palaeoenvironments. However, the mineral phases composing them and the signals they carry may be altered by secondary alteration during, e.g., early diagenesis. While the impact of early diagenesis has already been evidenced on some microbialites, some efforts are still needed for documenting its extent and its dependence on environmental conditions. Here, we analyzed early diagenetic transformations in shallow to deep modern microbialites formed in the seasonally stratified lake Alchichica, Mexico. We combined bulk analyses such as X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), with spatially resolved techniques, including petrographic microscopy, confocal laser scanning microscopy (CLSM), X-ray fluorescence (XRF) mapping, scanning electron microscopy (SEM), focused ion beam (FIB) milling and transmission electron microscopy (TEM). We showed the massive diagenetic formation of huntite (CaMg3(CO3)4), a rare carbonate, in microbialites developing at depths ≥20 m in the Lake Alchichica water column. Huntite replaced aragonite (CaCO3) and hydromagnesite (Mg5(CO3)4(OH)2·4H2O), the main primary phases forming Alchichica microbialites. The bulk isotope compositions of carbon and oxygen in carbonate phases also supported a diagenetic origin of huntite. We propose that specific conditions restricted to the pore water physicochemistry within the deep microbialites control the destabilization of the primary carbonates and the precipitation of huntite. Interestingly, thermodynamics predict that huntite instead of hydromagnesite and aragonite is the most stable phase under Lake Alchichica conditions. Yet the latter phases are dominant in most Alchichica microbialites. This suggests that the formation of huntite, similarly to that of dolomite in many surficial environments, is likely kinetically hindered and that the mineral assemblage composing microbialites does not necessarily follow thermodynamic predictions but is more controlled by kinetics. Last, the loss of luminescence in deep microbialites in comparison with shallow microbialites raises questions about the preservation of some mineralogical and biological signatures of microbialites over time.