Rb and Sr concentrations were determined by replicate analyses of pressed powder pellets using X-ray fluorscence. USGS rock standards were used for calibraiton; mass absorption coefficients were obtained from MoK alpha Compton scattering measurements. Sr isotopic composition was measured on unspiked samples prepared using standard ion exchange techniques. The mass spectrometer, Vacuum Generators Isomass 54R, has data acquisition digitized and automated using a Hewlett-Packard HP-85 computer. Experimental data have been normalized to a 86Sr/88Sr ratio of 0.1194 and adjusted so that the National Bureau of Standards (NBS) standard SrCO3 (SRM987) gives a 87Sr/86Sr ratio of 0.71020 + or - 0.00002 and the Eimer and Amend Sr a ratio of 0.70800 [...]