Anhydrous partial melting of an iron-rich mantle and implications for basalt pertrogenesis

Anhydrous partial melting experiments, at 10 to 30 kbar from solidus to near liquidus temperature, have been performed on an iron-rich Martian mantle composition, DW. At 15 kbar both primary melt and coexisting mineral compositions were determined at increasing degrees of partial melting. The composition of primary melts near the solidus was determined with basalt-peridotite sandwich experiments. In order to evaluate the approach of the liquids to equilibrium with a DW assemblage, experiments were also performed to establish the liquidus mineralogy of the primary melts.; The DW solidus assemblage from {dollar}leq{dollar}5 kbar to at least 24 kbar is a spinel lherzolite. At 25 kbar garnet is stable at the solidus along with spinel. The clinopyroxene stable on the DW solidus at and above 10 kbar is a pigeonitic clinopyroxene. Pigeonitic clinopyroxene is the first phase to melt out of the spinel lherzolite assemblage at less than 20{dollar}spcirc{dollar}C above the solidus. Spinel melts out of the assemblage about 50{dollar}spcirc{dollar}C above the solidus. Primary melt compositions produced from an iron-rich mantle are more picritic than those produced from an iron-poor mantle. The melts produced within 20{dollar}spcirc{dollar}C of the DW solidus range from picritic alkali basalts to picritic tholeiites. Those at 60{dollar}spcirc{dollar}C above the solidus are periodotitic komatiites. Decreasing the mg#, (mg# = atomic (Mg/(Mg + Fe){dollar}sp*{dollar}100)), of a model mantle composition results in the production of picritic and komatiitic magmas at lower percentages of melting and temperatures closer to the solidus. Terrestrial iron-rich primitive volcanics may be the partial melting products of iron-rich, mg# {dollar}geq{dollar} 80, source regions.; Pigeonitic clinopyroxene is stable on the solidus at lower pressure in an iron-rich mantle (mg# = 75, {dollar}geq{dollar}10 kbar), than in an iron-poor mantle (mg# = 90, {dollar}>{dollar}20 kbar). If pegionitic clinopyroxene is stable at a lherzolite solidus then orthopyroxene is in a reaction relation with the melt. Partial melts in equilibrium with a pigeonitic-bearing spinel lherzolite assemblage have a higher CaO/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} ratio than melts in equilibrium with an augite-bearing spinel lherzolite assemblage. In an iron-rich Martian mantle, the dominant crystallization product of partial melts in a 10 to 25 kbar pressure region may be a spinel-clinopyroxene assemblage. The fractionation of clinopyroxene and spinel at high pressure from DW near solidus primary melts can produce basaltic komatiite compositions similar to those proposed for the parent magmas of the SNC (shergottites, nakhlites, chassignites) meteorites.