汇聚环境岩浆氧化态来源的进展与展望

汇聚板块边缘的俯冲带是壳幔相互作用最为强烈的区域, 其重要特征是俯冲板片将地表物质带入地球内部并在上覆板块产生强烈的岩浆活动, 同时孕育铜、钼、金和其他具有重要经济价值的金属矿床。现有研究显示, 俯冲带弧岩浆的氧逸度总体高于洋中脊玄武岩, 但对于弧岩浆的高氧逸度的成因存在争议。目前有关弧岩浆高氧逸度的来源存在如下认识: (1)板片俯冲过程中释放的高氧化性流体对地幔楔持续氧化, 该流体可能来源于蚀变基性洋壳、上覆沉积物和/或下伏的蛇纹岩化地幔; (2)地幔部分熔融形成的含水熔体与周围地幔反应过程中, 还原组分(例如H₂)的丢失促使氧化性弧岩浆的形成; (3)上升熔体在深部岩浆房演化过程中, 富Fe²⁺矿物相(石榴子石、角闪石等)的分离结晶和/或去气作用致使残余熔体氧逸度升高。本文系统梳理了弧岩浆高氧逸度成因的最新进展, 从俯冲带不同来源流体(俯冲沉积物、基性洋壳、岩石圈地幔)的氧化性、地幔尺度内熔岩反应、地壳尺度内弧岩浆分离结晶和去气作用三方面进行系统总结, 评述研究现状和争议存在的原因, 并对未来研究进行了展望。

Progress and prospect of the oxidation state of magmas in convergent tectonic settings

The subduction zone at the margin of the convergent plate shows strongest crust-mantle interaction on Earth. The subducted plate brings the surface material into the deep Earth and generates large-scale magmatism, which produces copper, molybdenum, gold and other mineral deposits with important economic value. Current research shows that the oxygen fugacity of arc magma is generally higher than that of mid-ocean ridge basalt, but the origin of high oxygen fugacity of arc magma is still highly debated. Several processes are thought to be the main causes of the high oxygen fugacity of arc magmas: (1) the oxidizing fluid released from altered oceanic crust, overlying sediments, and/or underlying serpentinized mantle during plate subduction continuously oxidizes the overlying mantle wedge; (2) the reaction between the hydrous melt formed by partial melting of the mantle and the surrounding mantle, during which the loss of reduced components (such as H2) promotes the formation of oxidized arc magma; and (3) the fractional crystallization of Fe2+ rich mineral phases (garnet, amphibole, etc.) and/or degassing lead to the increase of oxygen fugacity of the residual melt. This paper reviews the latest progress for the origin of high oxygen fugacity of arc magma from the perspective of the oxidation of fluids from different sources in the subduction zone (sediments, oceanic crust, and lithospheric mantle), the reaction of water-rock in the mantle, the fractional crystallization and degassing of arc magma in the crust level. We also provided further discussions for the causes of these disputes and future research.