Variations in radiogenic strontium found in volcanic rocks

Analyses of the Sr isotopic composition of volcanic rocks reveal variations of nearly 2% in the amounts of radiogenic Sr87 that is found in these rocks at their times of eruption. Oceanic basalts exhibit a small but real range in radiogenic Sr which probably reflects regional inhomogeneities in the upper mantle. Many continental basalts are isotopically similar to oceanic basalts; however, most continental felsic volcanics are significantly enriched in radiogenic Sr. The data suggest that continental felsic volcanic rocks have been derived within the crust or have been appreciably contaminated by crustal material.