NEW INSIGHTS INTO THERMAL HISTORY FROM SINGLE GRAIN AR-40/AR-39 ANALYSIS OF BIOTITE

Single grains of biotite extracted from two hand samples of two granitoid rocks from the Red Lake greenstone belt of northwestern Ontario were examined using a high-sensitivity Ar-40/Ar-39 laser dating system for variations in age. For small grains a striking correlation is seen between the ages and the grain sizes, as measured by the cylindrical radii, with smaller samples yielding younger ages. This effect saturates at a radius of about 225-mu-m, above which the integrated age is independent of grain size. By combining this grain size effect with biotite diffusion parameters taken from the literature, continuous cooling curves were derived for the region. These curves suggest the region cooled at a rate of about 0.3-degrees-C/Myr between 2635 and 2570 Ma. With its ability to define a grain size effect, the laser-probe mass spectrometer system provides new insight into thermal history.