Sr provides unique constraints on subduction magma source models because it is a fluid-mobile element that is
abundant and relatively unradiogenic in arc volcanic rocks. It is common for arc basalts to be 3-4-times more Sr-rich
than similarly evolved MORB (Sr/Nd = 30-50 vs 10-15 in MORB) yet Sr isotopes in arc basalts are usually offset from MORB only slightly ( Sr/ Sr ~0.7034 vs 0.7028 for MORB). This is a puzzle because abundant sources of subducted Sr in sediment (GLOSS II Sr/ Sr = 0.712) and altered oceanic crust ( Sr/ Sr = 0.704-0.705) are more radiogenic than average arc basalts globally ( Sr/ Sr ~ 0.7034)..
