DIAGENETIC RESPONSE OF ARAGONITE ARCHIVES TO EXPERIMENTAL ALTERATION (Invited Presentation)

Variations in the diagenetic response of carbonate minerals create difficulties in the reconstruction of paleoenvironmental conditions using proxy data. Due to the inherent variability in the initial diagenetic susceptibility and diagenetic conditions, this work aims to comparatively assess alteration features of different aragonitic materials by experimentally induced diagenesis by hydrothermal alteration. Many factors lead to different initial reactivity and responses of a carbonate mineral, and can provide insight to the interpretation of altered material in the rock record. Chosen sample types used in this study include relatively organic-rich biogenic samples, such as coral skeletons and bivalve shells, and organic-lean abiotic samples, such as speleothems and aragonite single crystals. Elements and organic distribution, carbon and oxygen isotope ratios, and crystallographic restructuring were used to identify alteration features of the experimentally altered material. Differences between observed diagenetic responses include: mineralogy of secondary phases, rate and extent of mineral transformation, foreign ion distribution within the crystal lattice, and the number of diagenetic processes and products along the alteration pathways. Alteration was shown to be primarily controlled by the initial diagenetic susceptibility of the sample (including porosimetry and structural characteristics, organic content, and initial amounts of trace elements within the mineral phase). Primary structural characteristics lead to internally “fluid-” or “rock-buffered” conditions, while differences in the amount of organic content and internal fluids affect transformation rates, secondary mineralogy, and isotope equilibria. The effect of internal fluids and organics was found to generally have a greater influence in samples with low porosity and permeability (rock-buffered conditions). Minerals with relatively high porosity and high Mg/Ca ratios, as well as high mineral transformation rates, may result in preferentially formation of secondary aragonite. This study suggests that aragonite diagenesis is strongly controlled by the internal architecture governing the availability and transfer of internal fluids.