GEOCHEMICAL DIAGENETIC MODIFICATION OF PINK CRINOID OSSICLES FROM THE LOWER DEVONIAN BECRAFT FORMATION OF NEW YORK

Diagenetic geochemical modification of uniquely whitish-pink crinoid skeletal fragments from the Lower Devonian Becraft Formation of New York can be used to understand the effects of fluid diagenesis on high-magnesium calcite. Petrographic analyses on thin-sections from grainstones from the Lower Devonian Becraft Formation show crinoid ossicles perforated with small pores filled with diagenetic micrite material. SEM secondary imaging on etched and gold coated mounts reveals preservation of patches of the original stereom ultrastructure with distinct differences between crinoid stereom and background calcite cement. EDS elemental analyses on polished and carbon coated thin sections show a modification of the original high-magnesium calcite to a diagenetically-stabilized calcite with concentrations of non-biogenic trace elements aluminum, silicon, and iron. Anhedral microdolomite occurring in angular pore spaces within the surrounding calcite and non-biogenic trace elements within the stereom suggests a low-temperature dissolution-reprecipitation diagenesis of this formation. We interpret that dissolution and transportation of magnesium from the original stereom allowed for microdolomite formation within the surrounding calcite cement and the fluid-controlled environment provided a medium for the transportation non-biogenic elements into the system. This suggests that the non-biogenic elements filled the void sites produced by removing magnesium and calcium cations from the original stereom and altered the stereom mineralogy. Ferrous iron brought into the stereom ultrastructure may have been slightly oxidized due to the redox conditions of the hydrothermal fluid, producing the observed whitish-pink color of the crinoid ossicles. Collectively, these data show the effects of diagenesis on crinoid ossicles and provide further evidence for low-temperature diagenesis of the Becraft Formation.