Linear correlation between pairs of RbSr isochron ages from coexisting metamorphic micas

Abstract

This paper analyzes the significance of the difference between the Rb/Sr isochron ages of coexistent micas (biotite and muscovite) completely reset during a metamorphic event. Two possible resetting models are considered: (1) “ideal” resetting, when isotopic homogenization is reached between the sample forming minerals; and (2) “negative” resetting, when minerals close, with respect to the radiogenic 87Sr, with a 87 Sr 86 Sr ratio which is lower than that of the whole rock (thus starting with a negative age). In the first case, the ages of the micas are correlated by a simple linear expression: pairs of metamorphic micas which closed at different moments in geologic time and with the same age difference, should align in the biotite vs. muscovite age diagram on a straight line with slope 1 and y-intercept representing the ratio ( T m– T b)/ v ( T m, T b=blocking temperatures of the two micas; v=cooling rate of whole rock system between these temperatures). In the second case, some assumptions must be made in order to have the same simple correlation. Eighteen pairs of micas from different metamorphic units of the Eastern Alps were used to test the equation. All of them align well to form a straight line with slope 0.999 ± 0.045 and y-intercept 9.96 ± 0.99 Ma. Assuming the blocking temperatures of biotite and muscovite to be 300° and 500°C, respectively, and combining the estimated metamorphic thermal gradient with the obtained y-intercept value, it is possible to calculate (a) a value for the average cooling rate of the whole rock system between these temperatures ( ∼ 20°C Ma −1); and (b) a value for the uplifting rate between these temperatures of the actually outcropping rock levels after the Alpine metamorphic climaxes ( ∼ 0.7 mm a −1).