Glass inclusions in magnesian olivine phenocrysts from Tonga: evidence for highly refractory parental magmas in the Tongan arc

During the 1984 “Natsushima” cruise to the North Tonga arc, fresh volcanic rocks were recovered from the Tongan forearc. Olivine phenocrysts in some of the dredged rocks are highly magnesian (Fo₉₄) and contain large (up to 0.2 mm) glass inclusions. The glass inclusion chemistry, as analysed by electron microprobe, is characterized by high CaO (> 13 wt.%) and low Na₂O (< 0.8 wt.%) contents, resulting in very high CaO/Na₂O ratios (18–29). Such high SCV002SCV0 CaO/Na₂O ratios in glasses provide a ready explanation for the occurrence of extremely calcic plagioclase (up to SCV002SCV0 An₁₀₀) in the Tongan arc lavas. The prevalence of magma mixing and the existence of a spectrum of liquids, ranging to the extremely refractory glasses analysed, is a characteristic of this intra-oceanic island arc environment. High P_H2O is not important in precipitation of extremely calcic plagioclase from common low CaO/Na₂O liquids, although water SCV002SCV0 plays a significant role in controlling melting in refractory source compositions.