ISOTOPIC EQUILIBRATION DURING PARTIAL MELTING - AN EXPERIMENTAL TEST OF THE BEHAVIOR OF SR

Experiments using isotopically enriched, Sr doped minerals designed to test for isotopic equilibrium between source and melt during partial melting reveal that Sr-87/Sr-86 ratios of the liquids are primarily de termined by the proportions of minerals consumed and vary with the adv ancement of the melting reaction. The experiments were performed at I atm on model crustal assemblages composed of pairs of natural plagiocl ase (An(68); Sr-87/Sr-86=0.701) and synthetic fluorphlogopite doped wi th 90 ppm Sr having Sr-87/Sr-86=4.2. SIMS traverses showed that during the initial stages of the reaction, liquids are isotopically zoned. A ll the analyzed melts have Sr-87/Sr-86 markedly higher than that of th e bulk starting assemblage (i.e. the source), because of the faster me lting rate of fluorphlogopite. At 1200 degrees C and 1250 degrees C me lting occurs above a critical temperature, wherein the dissolution rat es of the crystals are controlled by diffusion of species in the melt, and reactants and reaction products are out of isotopic equilibrium. This is due to faster melt-crystal boundary migration when compared to Sr diffusion in the crystals. Equilibration is possible only if melti ng stops. Calculations show that total equilibration between melt and residue by Sr tracer diffusion in the crystals takes 10(4)-10(6) yr(fo r temperature and grain size ranging, respectively, from 800 degrees C to 1000 degrees C, and 0.1 to 1 cm). When compared to the proposed re sidence time of crustal magmas at their sources, this result strongly suggests that magmas that do not reflect the bulk isotopic characteris tics of their source regions can be produced.