RADIOGENIC CARBON IN GOETHITE FROM THE UPPER ORDOVICIAN NEDA FORMATION: IMPLICATIONS FOR POST-BURIAL RE-CRYSTALLIZATION

摘要

The Fe(CO_3)OH component of natural goethite from the Upper Ordovician Neda Formation type locality was analyzed for the presence of radiogenic carbon isotopes to test the hypothesis that the goethite has been re-crystallized in the recent past (< 50 ky). Atmospheric Pco_2 of the Ordovician has been estimated using the mole fraction of CO_2 in goethite from this formation (Yapp, 1996) assuming that the goethite acts as a closed system with respect to CO_2. Evidence of the re-crystallization of the hematite (Kean, 1981) in the formation implies that the goethite was similarly affected. The presence of radiogenic carbon indicates that the goethite in these rocks is not a closed system after burial and lithification and therefore, inferences made about the environment of their formation, including atmospheric Pco_2 must take this alteration into account. The absence of radiogenic carbon isotopes indicates that re-crystallization has not occurred in the recent past and supports the theory that the goethite is a closed system.