MELTING OF MANTLE PERIDOTITE AT PRESSURES APPROACHING THE SPINEL TO GARNET TRANSITION - APPLICATION TO MIDOCEAN RIDGE BASALT PETROGENESIS

This paper reports experiments carried out between 1.5 and 2.3 GPa usi ng synthetic analogs of the molten peridotite system to investigate th e effects pf pressure (P), temperature (T), and variable bulk chemistr y on the composition of melts multiply saturated with the minerals pre sent in the upper oceanic mantle: olivine, orthopyroxene, augite, and spinel ( garnet). The multidimensional surface defined by melts coexis ting with the Iherzolite mineral assemblage produced in this study as well as from the literature is fit as a function of the variables: P, melt Mg number, and melt mole percent NaO0.5 KO0.5, Cr2O3, and TiO2, u sing multiple regression techniques. Forward models of polybaric, near -fractional melting using the parameterization presented here demonstr ate that small extent, pooled magmas produced at greater initial press ures of melting differ in composition from small extent, pooled magmas produced at lower initial pressures of melting. Furthermore, the smal l extent, pooled magmas generated at shallower initial pressures of me lting fractionate at low P to magmas that are similar to the high-Na2O , low-FeO mid-ocean ridge basalts of the global array of Klein and Lan gmuir [1987], while the small extent, pooled magmas generated at great er initial pressures of melting fractionate at low P to magmas that do not resemble any mid-ocean ridge basalts present in the global data s et. Preliminary data relevant for melting garnet-lherzolite indicate t hat the systematics of the key major element indicators of depth of me lting (FeO and SiO2, in the melt) observed in melts generated in the s pinel stability field persist in melts generated in the lower P portio n of the garnet stability field.