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

溶解于地幔水流体中的硅酸盐组分在1~2 GPa时由安山岩转变为3 GPa时的橄榄岩。我们用多砧型高温、高磷同步辐射X射线照相技术，在Spring-8上直接观测了水流体与高镁安山岩和水流体与海洋沉积物之间的解混和混合。我们观察到水中流体和高镁安山岩(HMA)熔体在低于2.7 Gpa的压力下共存。当高于2.8 GPa时，我们只观察到一个流体相，这表明HMA和水相流体完全混合并成为超临界流体。除了HMA，我们还确定了海洋沉积物和水中流体之间的一个关键终点，该终点位于2.6 Gpa左右。我们在3.8/3.0 GPa时，在含水流体和橄榄岩/a-MORB熔体之间发现了第二个临界终点。根据这些数据，我们认为，在火山弧(3-6 Gpa)下方的下行板，板岩流体应处于超临界条件下。这意味着在地幔楔形底部，从水的流体到含水的熔体不断变化。板条衍生的流体是否具有部分熔融或含水流体的化学特性取决于温度；相对温暖地区的板条衍生超临界流体比相对寒冷地区的板条衍生超临界流体可以溶解更多的硅酸盐组分。在暖地幔楔体底部形成的类熔融态超临界流体将分解成熔融相和流体相，可能导致双重岩浆作用，如埃达克岩和普通岛弧岩或HMA和玄武岩共存。

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.