A PARTIAL AGE SPECTRUM FROM K AND AR RELEASED CHEMICALLY FROM BIOTITE

Stepwise heating of neutron-irradiated biotite gives ⁴⁰Ar/³⁹Ar age spectra that are problematic for interpreting the distribution of radiogenic Ar relative to K within the mica. An alternative way to obtain information on the spatial variation of radiogenic Ar relative to K within biotite is to use an ion-exchange process that displaces K and Ar from interlayer sites, working progressively inward from the periphery of each mica flake. In aqueous solution at room temperature, large alkyammonium ions displace interlayer K from biotite at appropriate rates, and presumably do so with no chemical change to the mineral except in the interlayer. For a first attempt to obtain an age spectrum by stepwise chemical release of K and Ar from biotite, a sample from the Auburn Gneiss having a conventional K-Ar age of 310 ± 9 Ma was chosen. Work by others has indicated some excess radiogenic Ar in biotite from this terrane. Two different kinds of aqueous quaternary ammonium ions were used for ion exchange with 200 mg of the biotite in an evacuated apparatus. Tetramethylammonium (TMA) chloride (0.02 M) was used in a first step, and subsequent steps employed 0.01 M hexadecyltrimethylammonium (HDTMA) bromide. In each step the biotite was immersed in solution within a boat for one day or longer, during which time water vapor pressure in the apparatus was manipulated from time to time to move the solution across the biotite. At the end of each step, the mica was thoroughly rinsed by condensed water draining down through the sample, now lifted above the boat. Argon was removed at the end of each step for isotopic measurement. The K within each boat was measured later. A one-day reaction with TMA displaced 0.05% of the sample’s K but no radiogenic Ar. A two-day reaction with HDTMA released 0.6% of the sample’s K along with radiogenic Ar. The rate of ion exchange by HDTMA decreased greatly during two subsequent steps, and only 1.4% of the K from the sample had been released when the experiment ended. Apparent age decreased from 337 Ma in the first HDTMA step to 314 Ma and 310 Ma in the two subsequent steps, a pattern that suggests concentration of excess radiogenic Ar in peripheral portions of the mica flakes.