EXCESS ARGON IN MICAS FROM THE CLASSIC BARROVIAN METAMORPHIC SEQUENCE IN DUTCHESS COUNTY, NEW YORK

New ⁴⁰Ar/³⁹Ar age data from step-heating of biotite-white mica (WM) pairs across the classic Barrovian sequence in Dutchess County, N.Y. show that micas in these rocks incorporated excess argon. Twelve biotite-WM pairs were dated from rocks in the biotite, garnet, staurolite, and kyanite zones. WM separates from these samples consistently produce U-shaped age spectra with minimum ages of ~380 Ma. Isotope correlation analysis from several of these samples yields ages identical (within uncertainty) to the minimum ages, and a pair of closely spaced (< 50 m) samples also records consistent age minima, indicating the WM age minima are reproducible and likely geologically meaningful. The correlations document initial ⁴⁰Ar/³⁶Ar ratios up to 900 ± 400 and are consistent with the U-shapes of the spectra being the result of excess argon. In contrast, biotite separates from all of these samples produce plateau or near-plateau age spectra that vary from 380 to 435 Ma over the study area, and biotite ages from the two closely spaced samples disagree by > 20 Ma. Furthermore, in 11 of 12 samples the biotite total gas age exceeds the WM total gas age, and in most samples the biotite plateau age exceeds the age of every step in the WM age spectrum despite the higher nominal closure temperature of WM. Thus, both biotite and white mica argon isotopic systems incorporate excess ⁴⁰Ar, but their release spectra show very different behavior during step-heating experiments. Step-heating of WM resolves argon isotopic gradients as U-shaped age spectra, such that excess ⁴⁰Ar can be identified. However, step-heating of biotite produces “plateaus,” suggesting that each step samples a similar set of sites (likely due to isotopic homogenization during natural exhumation or during the experiment),thus making the step-heating method incapable of detecting excess argon. We conclude that [1] either ⁴⁰Ar is more soluble in biotite than it is in muscovite (a relationship expected from crystal chemical arguments) or that grain boundaries were richer in ⁴⁰Ar during cooling through biotite closure than WM closure, [2] these metamorphic rocks had a low permeability during exhumation, such that radiogenic ⁴⁰Ar was not lost to migrating metamorphic fluids, and [3] caution is needed in interpretation of metamorphic biotite ages, even when a simple plateau is produced.