Seven Bonneterre and Davis Formation glauconite samples from the Magmont mine area, Viburnum Trend, southeast Missouri, yield a 359 + or - 22-m.y. Rb-Sr isochron with an initial 87 Sr/ 86 Sr ratio of 0.7234 + or - 0.0273 (2 sigma). Gangue calcite from the ore zone has an 87 Sr/ 86 Sr ratio of approximately 0.7112. The isochron age for the Magmont glauconite samples is about 30 percent younger than the age of their Cambrian host rocks.The Rb/Sr ratios for Magmont glauconite samples are typical of many early Paleozoic glauconites but distinctly lower than ratios for glauconites from unmineralized localities in basinal Bonneterre facies, some distance from the Vibrunum Trend.Because the Rb-Sr data from Magmont glauconite samples define an isochron, it appears that the glauconites behaved as a cogenetic suite which was isotopically homogenized 359 m.y. ago. It is very unlikely that episodic or incomplete loss or exchange of radiogenic strontium, or the addition of rubidium, would result in the formation of an isochron. Addition of common strontium (mixing) will lower and produce a greater spread in Rb/Sr ratios but will not significantly change the calculated Rb-Sr isochron age for the glauconites. Because the total thickness of stratigraphic cover in the southeast Missouri region has never exceeded a kilometer, it is unlikely that the anomalously young glauconite isochron age resulted from heating associated with deep burial. Mississippi Valley-type ore fluids offer a hot, chemically reactive medium which could promote isotopic homogenization in glauconites. Therefore, the 359-m.y. age probably reflects a real geologic event, most likely the time of ore formation in southeast Missouri.A review of other attempts at dating early Paleozoic glauconites from the midcontinental United States indicates that all Rb-Sr and K-Ar glauconite ages are 10 to greater than 30 percent younger than their known stratigraphic ages. These glauconites are from Paleozoic formations on the stable craton, and again, resetting due to burial metamorphism is unlikely. Because there was no immediately obvious explanation for the discordant young ages, early Paleozoic glauconites were simply considered to be unsuitable geochronometers. It is suggested that the anomalously young radiometric ages for early Paleozoic glauconites record disturbances in the Rb-Sr and K-Ar isotopic systems, the result of Mississippi Valley-type fluid migration in the midcontinent region.

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