Other
Scientific paper
Nov 1963
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1963gecoa..27.1097r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 27, Issue 11, pp.1097-1112
Other
9
Scientific paper
Coexisting illites and potassium feldspars have been studied from the clay size fractions of Proterozoic carbonate rocks from the Northern Rocky Mountains. The illites are predominantly of the 1Md type, suggesting (1) an authigenic origin, and (2) an essentially unmetamorphosed condition for most specimens of the Belt and Jasper series. A series is present between 1Md-rich and 2M-rich illites. The series correlates well with petrographic evidence of metamorphism, suggesting that the ratio 2M/1Md constitutes a useful measure of metamorphic intensity. Microcline of high triclinicity is the dominant feldspar. In one specimen, the triclinicity values show a good inverse correlation with particle size. If this relation is valid for other rocks, then highly triclinic microlines may be common in the clay size fractions of carbonate rocks; highly triclinic microlines may therefore be more common than hitherto reported by other workers, who have studied only coarser fractions. Microclines and illites each have a characteristic K / Rb ratio. The K / Rb ratio in microcline is high, indicating an authigenic origin. The K / Rb ratios in microclines show a 2.3-fold increase over K / Rb ratios in coexisting 1Md-rich illites. It is suggested that this is an equilibrium value for the fractionation of K / Rb between illitic micas and potassium feldspars. The K / Rb ratios of the 1Md-rich illites are similar to K / Rb ratios in post-Precambrian illites and glauconites; these data indicate that little change has occurred in the K / Rb ratio in sea water since lower Beltian time.
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