Mafic phyllosilicates in low-grade metabasites; characterization using deconvolution analysis

The <2 gm clay fractions from low-grade metabasalts of eastern North Greenland are mixtures of mafic phyllosilicates and celadonite that show complex X-ray diffraction (XRD) patterns. Interpretation of such patterns is difficult and subjective using only visual examination of peak positions and shapes. Deconvolution analysis offers a less subjective means of identifying peak positions for overlapping peaks and is applied to several composite peaks in a pattern; the peaks identified are then rationalized in terms of specific mixed-layer phases. Comparison against NEWMOD patterns of the identified clay mixture provides an additional constraint on the phases identified. Three mafic phyllosilicates, discrete chlorite and two chlorite-smectites with 60:40 and 80:20 proportions are demonstrated in the same XRD pattern. Modelling using NEWMOD of a mechanical mixture between these mineral shows an excellent match to the observed XRD pattern. The recognition in low-grade metabasalts of chlorite and random and different varieties of regular mixed-layer chlorite-smectite offers support for the classical model of a tri-smectite to chlorite transition. This is in contrast to an alternative model proposing smectite and chlorite end-members with corrensite as a discrete phase crystallizing directly without intervening mixed-layer chlorite-smectite. Mafic phyllosilicates are ubiquitous in low-grade metabasites of hydrothermal/geo thermal systems and regional metamorphic terranes and comprise the tri-smectite to chlorite series. The classical interpretation of this series is of end-members of smectite and chlorite with intermediate mixed- layer structures, rationalized into random order- ing with high smectite contents and ordered types (R = 1) with low smectite contents (Reynolds, 1980). This interpretation has much in common with the model proposed for the smectite to illite transition. An ordered mixed-layer chlorite-