CompuServe Messages

Jon, I looked once for K39/K40 segregation in the Llano uplift batholith. I had core samples from 3100 meters bearing K rich alkali feldspars, and similar samples from intrusive dikes from the San Saba (edge of uplift) area. To within 0.07%, the isotopic composition is identical. Note that I'm not saying that the two were different by that amount – my instrument (a Finigan quadrapole mass spectrometer) was good to this amount, and as far as I could tell, the isotope ratios were dead on. Having said that, I will tell you that one sometimes sees deuterium differentiation of a few ppm in amphibolite rocks. As Doug points out, deuterium is the most likely isotope to show fractionation, and sure enough if you look (hard) you can see some evidence, down at the limits of detectability. Amphiboles that were created by contact metamorphosis seem to have higher deuterium ratios than those created by regional metamorphosis. The difference is like 0.014723 to 0.014729 in the most differentiated samples I've heard of. — Norm