宾川苦橄岩的水含量:对峨眉山大火成岩省成因的启示

Water Content of the Binchuan Picrites: Implications for the Genesis of Emeishan Large Igneous Province

近几年来对西伯利亚、哥伦比亚河、塔里木等大火成岩省原始岩浆水含量的分析表明,其源区的显著水化是造成地幔熔融异常的重要原因之一。峨眉山大火成岩省是全球二叠纪大火成岩省的重要组成之一,前期对其位于宾川剖面底部的大理苦橄岩水含量的分析表明地幔的水化在峨眉山大火成岩省形成中起到了重要作用。然而大理苦橄岩只能代表峨眉山大火成岩省初始阶段的熔融条件,而在其后的喷发历史中水是否也起到了关键作用还不清楚。定量分析后阶段喷发的苦橄岩水含量有助于更完整评估水在大火成岩省形成中的作用。本文报道了位于峨眉山大火成岩省西部岩带宾川剖面中上部的宾川苦橄岩的水含量,结果表明原始岩浆水含量可高达2.8 wt%,比大理苦橄岩的水含量略低,但仍显著高于MORB和OIB,达到了岛弧玄武岩的水含量范围。这表明水对整个大火成岩省的形成中都扮演了重要角色。

Large igneous provinces (LIPs) represent melting anomalies of the Earth. Recent investigations of water content of the primary magma in the global LIPs show that source hydration had played an important role in their genesis. The Emeishan LIP is one of Permian LIPs in the world. Previous work on the Dali picrites, within the initial stage of eruption, indicates that water largely promoted partial melting of mantle. However, whether water played a significant role in other stages of eruption is not clear, which is critical to understand the genesis model of LIPs. Here, we report water content of the picrites from Binchuan, within the upper part of eruption sequences of ELIP. The result shows that the primary magma could have water content as high as 2.8 wt.%, slightly lower than that of Dali picrites, but still considerably higher than that of MORB and OIB. Combined with the previous work, this study shows that mantle hydration would be essential for the whole eruption of ELIP.