The Petrology of a Melilite-Olivine Nephelinite from Hamada, SW Japan

Abstract

A melilite–olivine nephelinite lava flow occurring in Hamada, SW Japan, is the most silica-undersaturated lava known to have erupted in Japan (SiO2 content ∼37 wt %), and may be a least fractionated melt from mantle depths. Heterogeneity of elements such as Si, Al, Ca, and alkaline elements is one of the characteristic features of the Hamada nephelinites and is attributed to hydrothermal alteration, because these elements’ concentrations correlate with H2O contents. Melting phase relations at high pressures on the least altered nephelinite sample in the presence both of H2O and CO2 demonstrated that the nephelinite melt is multiply saturated with olivine and clinopyroxene but not with orthopyroxene, suggesting that the nephelinite magma was not derived from a lherzolite source, but possibly from a wehrlite source. Such an unusual mantle material may have been formed by carbonate metasomatism of a lherzolitic upper mantle, which is a process consistent with the geochemical characteristics of the Hamada nephelinite lava.