玄武岩マグマの起源 : 高温高圧実験の結果を踏まえて

抄録

Experimental melting studies on natural peridotite at high pressures are reviewed. Composition of partial melt formed along the dry solidus of the peridotite is quartz tholeiitic up to 0.5 GPa, olivine tholeiite similar to mid-ocean ridge basalt (MORB) at 0.8 to 1.0 GPa, alkali olivine basaltic between 1.5 and 2.5 GPa, picritic between 2.5 and 5 GPa, and komatiitic above 5 GPa. Thus, MgO content of the magma formed along the peridotite dry solidus increases as a function of increasing depth. Using partition coefficients K_D(Fe/Mg), K_D(Ni/Mg) and K_D(Mn/Mg) between olivine and silicate melts, a model is constructed in order to calculate fractional crystallization paths of primitive basaltic magmas. Ni and Mn contents of olivines of 250 spinel lherzolite xenoliths collected from the world were analyzed. Ni and Mn contents of the mantle olivines show narrow and coherent trends as a function of Fo mol%, and their compositional ranges are called as “mantle olivine array for ferro-magnesian elements”. Given the mantle array as a constraint for the fractional crystallization model, composition of primary magmas and Mg^*=100 Mg/(Mg+Fe) of residues are estimated. Estimated primary magma compositions for MORBs, Hawaiian tholeiites, Columbia river basalts and Quaternary basalts from Japanese volcanoes all fall in a range between MgO=9 and 14 wt%. Judging from ubiquitous occurrence of olivine xenocrysts showing deformation texture (kink banding), most picritic basalts (MgO＞15 wt%) erupted on the present day earth are considered to be mixtures of basaltic melts and olivine xenocrysts.