MIXED DIOCTAHEDRAL-TRIOCTAHEDRAL SMECTITES AS INDICATORS OF REACTION PROGRESS AND METAMORPHIC GRADE IN VERY LOW-GRADE METABASITES

The composition, proportions, and structure of clay minerals are widely employed as indicators of reaction progress and metamorphic grade in metasedimentary and metavolcanic rocks. For instance, the transition from dioctahedral smectite to mixed-layer chlorite-smectite to chlorite as a function of both extent of reaction and temperature is-well established in upper zeolite facies and prehnite-pumpellyite (actinolite) facies metabasalts. Several recent studies have demonstrated the existence of intimate mixtures of dioctahedral and trioctahedral smectites in lower zeolite facies samples. The structural nature of these mixtures remains unclear, and previous workers have variously suggested both mechanical mixtures and true interlayering of dioctahedral and trioctahedral smectites to explain these materials. Electron probe microanalysis of mixed dioctahedral-trioctahedral smectites from regionally metamorphosed basalts of the North Atlantic Tertiary Igneous Province (specifically in eastern Iceland, East Greenland, and West Greenland) indicate that they are ubiquitous at grades less than or equal to mesolite-scolecite zone alteration. They are present both as replacements of mesostasis and phenocryst phases and as rims (coatings) on vesicles. In thin section they are typically brown in color and have no discernable structure aside from banding parallel to vesicle walls. Systematic compositional observations have been made as a function of both depth (paleotemperature) and relative crystallization age (based on systematic analyses of vesicle coatings as a function of distance from the vesicle wall). These data suggest that the proportion of trioctahedral to dioctahedral smectites in these materials increases with both metamorphic grade (eventually giving way to pure trioctahedral smectites) and crystallization time. This is similar to observations of mixed layer chlorite-smectite at slightly higher grades. It thus appears that the compositions of these materials may be used with caution to infer relative changes in alteration temperature within the uppermost portions of basaltic crust.