Change of structure and physical features of aqueous fluids in subduction zones

地幔含水流体中硅酸盐组分的化学性质由1 ~ 2 GPa的安山岩变为3 GPa的橄榄岩。本文利用多顶柱型高t高p仪器的同步加速器x射线摄影技术，直接观察了水流体与高镁安山岩之间以及水流体与海洋沉积物之间的分离和混合。在低于2.7 GPa的压力下，我们观察到含水流体和高mg安山岩（HMA）熔体共存。当温度高于2.8 GPa时，我们只观察到一个流体相，这表明HMA与水溶液完全混合，成为超临界流体。除了HMA之外，我们还确定了海洋沉积物和含水流体之间的一个临界端点，位于2.6 GPa左右。我们发现在水相流体和橄榄岩/ a MORB熔体之间的第二个临界端点分别为3.8/ 3.0 GPa。根据这些资料，我们认为在火山弧下的下行板块（3-6 GPa），板块衍生流体应处于超临界条件下。这意味着在地幔楔基底处从含水流体到含水熔体的连续变化。板状衍生流体是否具有部分熔体或含水流体的化学特征取决于温度；相对温暖地区的板状超临界流体比相对寒冷地区的板状超临界流体能溶解更多的硅酸盐组分。在暖地幔楔基底形成的熔融式超临界流体将分离为熔融期和流体期，可能导致埃达克岩与普通岛弧岩或HMA与玄武岩共存等双重岩浆作用。

Silicate components dissolved in aqueous fluids in mantle change their chemistry from andesitic at 1-2 GPa to peridotitic at 3 GPa. We show direct observations of unmixing and mixing between aqueous fluid and a high-magnesian andesite and between aqueous fluid and an oceanic sediment by use of synchrotron X-ray radiography with multi-anvil type high-T and high-P apparatus at SPring-8. We observed aqueous fluid and a high-Mg andesitic (HMA) melt coexisting in pressures lower than 2.7 GPa. While above 2.8 GPa, we observed only one fluid phase, suggesting that the HMA and aqueous fluids are completely mixing and become supercritical. In addition to the HMA, we determined a critical endpoint between an oceanic sediment and aqueous fluid, which is located at around 2.6 GPa. We found a second critical endpoint between aqueous fluids and a peridotitic/ a MORB melt at 3.8/ 3.0 GPa, respectively. Based on these available data, we suggest that slab-derived fluids should be under supercritical conditions at the downgoing slabs beneath the volcanic arcs (3-6 GPa). This means a continuous change from aqueous fluids to hydrous melts at the base of mantle wedge. Whether the slab-derived fluids have chemical characteristics like a partial melt or an aqueous fluid depends on the temperature; slab derived-supercritical fluids in relatively warm regions can dissolve more silicate components than slab derived-supercritical fluids in relatively cold regions. The melt-like supercritical fluid formed at the base of the warm mantle wedge will separate into a melt phase and a fluid phase, which may result in double-magmatism, for example, coexisting of adakite rocks and common island-arc rocks or HMA and basalt.