TRACING THE MATURATION OF BIOGENIC OPAL TO OPAL-CT AND MICROQUARTZ CHERT IN MARINE SEDIMENTS WITH SEM AND SEM/EDS

Biogenic amorphous opal (biogenic opal-A) that accumulates in marine sediments, after significant burial at depths of 100`s of meters, is transformed to more thermodynamically stable mineral forms of opal-CT and microquartz chert. Biogenic opal-A refers to skeletons of silica microfossils that include diatoms, radiolaria, and sponge spicules. The silica which the organisms secrete to make their shell is disorganised and amorphous in nature, hence the name amorphous opal-A. The diagenetic pathway of phase transformations is proposed to be either dissolution of the original biogenic opal-A and repricipitation as a more stable form, opal-CT and microquartz chert, or a direct reorientation of the original biogenic opal-A to opal-CT and microquartz chert without dissolution. The local lithologic environment (e.g., ratio of calcium carbonate to clay) is shown to have influence on which diagenetic pathway will take place. We have retrieved samples from marine sediments from a number of deep sea sediment cores that capture these transitions in both environments. We follow these transitions with d18O measurements of the phases and also XRD, SEM, and SEM/EDS. Our results show, that in lithologic environments with a lot of clay, biogenic opal-A dissolves and reprecipitates as opal-CT and microquartz chert. In lithological environments with a lot of carbonate, opal-CT directly repriciptates from the original biogenic opal-A. This has implications for understanding formation of silica and changes in its mineral structure in different local environments and additionally, paleoenvironmental interpretations of the d18O of opal-CT and microquartz cherts in the geologic record.