The study of crystal structure in the evolution of interstratified 2:1 clay minerals, Reindeer D-27 well, Mackenzie Delta-Beaufort Sea region, Arctic Canada.

摘要

Owing to the conflicting results obtained from the application of different analytical and sample preparation techniques, and to the complex nature of interstratified 2:1 clay minerals, the compositional and structural changes that occur with increasing diagenetic grade in illite-smectite (IS) mixed layers had not been adequately deciphered previously. Some of the uncertainties result from different modes of sample preparation, namely ion-beam milled original rock samples and dispersed separates. I have used a combined approach of n-alkylammonium cation exchange of 2:1 clay minerals, XRD, HRTEM, CTEM of Pt-C replicas of adsorbed and freeze-fractured specimens after cryofixation, and K-Ar dating is used to investigate nucleation and growth characteristics of 2:1 clay minerals from argillaceous rocks of the Reindeer D-27 well, Mackenzie Delta-Beaufort Sea (MDBS) region, Arctic Canada. In addition, three reference 2:1 clay minerals (Zempleni illite, rectorite, and Mancos Shale) were studied.; Alkylamrnonium cation exchange of 2:1 clay minerals for investigation by XRD and TEM showed that the diagenetic evolution of smectite to illite in argillaceous rocks of the Reindeer D-27 well cannot be considered as a single, continuous and progressive reaction series as is conventionally proposed. It is rather a prograde sequence of discrete, multiple phases that coexist in overlapping depth intervals. K-Ar dating of illitic phases after treatment with octadecylammonium cations was undertaken to clarify the origin of octadecylammonium intercalated illite, account for its contribution to K-Ar ages, and assess the presence of detrital illitic phases in fine clay-size separates. Results suggested that K and Ar are removed to different degrees from both detrital and diagenetic components with treatment.; By correlating the internal structure of the 2:1 clay minerals with the surface morphology, it was possible to show that the original stacking order of interstratified 2:1 clay minerals can be deciphered using dispersed separates, and that suitable techniques of sample preparation and dispersion can be employed to avoid disrupting the primary texture. In diagenetic samples from Reindeer D-27, the neoformed crystals have unique euhedral morphology, suggesting primary growth as free particles, rather than being disarticulated fragments of larger crystals, as proposed by the investigation of ion-milled specimens of original rock. Investigation of the reference samples of Zempleni illite, rectorite, and Mancos Shale showed that the original stacking order of the 2:1 clay minerals is not destroyed by sample preparation and dispersion. The results suggest that the concept of fundamental particles can be applied to illitic material formed in the early stages of diagenesis, but not necessarily to illitic material formed in other geologic environments such as hydrothermal and contact metamorphic environments. In addition, it was shown that the application of the technique of n-alkylammonium cation exchange for TEM is useful for the study of the evolution of S → I and isotopic dating of illitic clay minerals.