EXPERIMENTAL CONSTRAINTS ON THE GENERATION AND EVOLUTION OF MID-OCEAN RIDGE BASALTS by Rosamond

Experimental petrology and major element chemistry are used to infer the melting processes that operate in the upper oceanic mantle to yield basaltic magma, and the subsequent cooling processes by which these magmas chill to form the oceanic crust. THE PRIMARY MAGMAS OF MID-OCEAN RIDGE BASALTS This paper reports experiments carried out between 9 and 16 kbar (0.9 1.6 GPa) using natural, primitive mid-ocean ridge basalt (MORB) compositions and synthetic analogs of MORB to investigate the effects of pressure, temperature and variable bulk chemistry on the composition of liquids multiply saturated with the minerals present in the upper oceanic mantle: olivine, orthopyroxene, augite and plagioclase or spinel. For this low-variance, 5-phase assemblage, expressions involving pressure, liquid NaK# (Na2 0+K 2 0)/(Na2 0+K 2 0+CaO), Mg# (Mg/[Mg+Fe 2 ]; total iron as Fe2+) and wt. % TiO 2 predict temperature and major element compositions of magmas produced by melting spinel and plagioclase lherzolites over the pressure range of 0.001 16 kbar. The expressions are derived using data from this experimental study and published experimental studies that report compositions of glasses coexisting with olivine, orthopyroxene, augite and plagioclase and/or spinel. A two-part quantitative framework for evaluating the systematics of MORB chemistry is presented: 1) for estimating melts from a depleted MORB-mantle source as a function of pressure and changing mantle composition, and 2) for evaluating the effects of fractional crystallization on these