Journal of Petrology, Volume 40, Issue 4: April 1999.

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

Y. TATSUMI¹, R. ARAI² AND K. ISHIZAKA²

¹INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, BEPPU 874-0903, JAPAN
²GRADUATE SCHOOL OF HUMAN AND ENVIRONMENTAL STUDIES, KYOTO UNIVERSITY, KYOTO 606-8501, JAPAN

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.

Keywords: carbonate metasomatism;hydrothermal alteration; nephelinite; SW Japan; wehrlite source

Pages 497-509