[S-MP30] 地殻-マントル・コネクション

[SMP30-P03] Origin of variable CaO/Al₂O₃ in olivine-hosted melt inclusions from Kibblewhite Volcano, Kermadec Arc

*平井 康裕¹、田村 芳彦²、Kaj Hoernle³、Christian Timm⁴、Folkmar Hauff³、Reinhard Werner³、羽生 毅²、Bogdan Vaglarov²、常 青²、宮崎 隆²、木村 純一² (1.金沢大学大学院 自然科学研究科、2.海洋研究開発機構、3.ゲオマール海洋研究所、4.GNSサイエンス)

キーワード：Subduction zone、Kermadec Arc、Kibblewhite Volcano、SO-255、Olivine-hosted melt inclusion

During R/V SONNE expedition SO-255 mafic basaltic to magnesian andesitic lavas containing melt inclusion hosting forsteritic olivines (Fo_90.3–92.4) were recovered from Kibblewhite Volcano, southern Kermadec Arc. The re-homogenized melt inclusions have compositions similar to primitive basalt (49.6–52.0 wt.% SiO₂; 12.7–15.9 wt.% MgO). Their CaO/Al₂O₃ values, however, range from 0.9 to 1.6, which cannot be explained by melting of lherzolite. Moreover, CaO/Al₂O₃ values in the melt inclusions correlate negatively with Ni contents of their host olivines suggesting that the high CaO/Al₂O₃ values result from interaction between mafic melts and subarc lithospheric mantle. Prior to the interaction the mafic magma must have had high Ni contents and low CaO/Al₂O₃ values, thus they crystallized high Ni olivines trapping low CaO/Al₂O₃ melt inclusions. During the interaction the olivines crystallized from the mafic magmas and its latent heat assimilated pyroxenes from the surrounding lithospheric mantle. This assimilation of pyroxenes allowed the Ni content of the melt to decrease and CaO/Al₂O₃ to increase while buffering the the Mg-number of the mafic magmas during the ascent. Thus, magmas during and after the interaction could crystallize low Ni olivines bearing high CaO/Al₂O₃ melt inclusions.