Sr-Na-bearing åkermanite and nepheline in nephelinite from Nagahama, Hamada, Shimane Prefecture, Southwest Japan

Abstract

Akermanite and nepheline found within melilite-olivine nephelinite in Nagahama Plateau, Hamada city, Shimane Prefecture, Southwest Japan, were investigated to characterize their occurrence and chemical compositions. Samples of massive and dense blue-gray melilite-olivine nephelinite were collected from a Naka-Uchida tunnel construction site. Their bulk composition is characterized by an extremely low SiO2 content of 35.75 wt% and high Sr and Ba contents, reaching 2091 ppm and 958 ppm, respectively. Forsterite, akermanite, augite, ulvospinel and apatite occur as microphenocrysts. Nepheline occurs as an interstitial mineral between these microphenocrysts. Euhedral nepheline, zeolite-group minerals, diopside and calcite occur in druses. Akermanite occurs as prismatic to anhedral microphenocrysts. The rims of the akermanite microphenocrysts are dissolved and overgrown by augite. Akermanite has an average composition of (K0.011Na0.390Ba0.004Sr0.031Ca1.566)Σ2.002(Mn0.007Fe2+0.088Mg0.457Ni0.001Fe3+0.134Cr3+0.001Ti0.003Al0.295)Σ0.986(Al0.074Si1.926)Σ2.000O7, and contains up to 4.62 wt% Na2O, 1.68 wt% SrO, and 0.64 wt% BaO. Calculated Fe2O3 and FeO based on charge balance are 5.36 wt% and 3.11 wt%, respectively. Fe2+/(Fe2+ + Fe3+) ratios estimated from the FeLβ/FeLα intensity ratios range from 0.47 to 0.79, indicating that a considerable amount of Fe3+ is contained within akermanite. The chemical composition of nepheline in the groundmass is (Na0.655K0.304)Σ0.959(Fe3+0.025Al0.978)Σ1.003(Al0.012Si0.988)Σ1.000O4 on average. No excess SiO2 nor anorthite components were detected. The bulk composition of the Hamada melilite-olivine nephelinite and the compositions of akermanite and nepheline are similar to those of equivalents from the carbonatite-nephelinite-phonolite Oldoinyo Lengai volcano in Tanzania. This similarity may support the proposed genetic model that the Hamada nephelinite magma was derived from a source material formed by carbonate metasomatism of a lherzolite upper mantle.