地幔氧逸度的时空变化

Spatio-temporal variation of mantle oxygen fugacity

地幔氧逸度是反映地幔氧化还原程度的参量,由温度、压力、岩石化学成分、矿物结构等共同作用控制。目前对上地幔氧逸度的研究主要针对镁橄榄石-磁铁矿-石英体系、含角闪石的橄榄岩体系和玄武岩(熔体)体系,通过实验岩石学方法进行。地幔氧逸度在垂直深度上随深度增加而减小受到普遍认可。天然样品和理论研究认为,岩石圈地幔底部的软流圈氧逸度高于上部岩石圈地幔,垂直方向上地幔氧逸度存在逆梯度,地幔过渡带相对上地幔可能也存在类似情况;水平方向上,不同构造环境、相似构造环境的不同地区氧逸度之间的差异都指示地幔氧逸度存在水平不均一性。不同圈层之间物质交换和氧逸度变化的关系是氧逸度未来研究的重点之一。俯冲带是研究圈层之间物质交换和氧逸度变化关系的天然实验室,同时氧逸度反过来也影响着各种挥发性元素(如C、S等)在不同圈层之间的赋存形式和循环通量。

Oxygen fugacity is a parameter that reflects the oxidation state of the mantle,and it is controlled by temperature,pressure,chemical composition of rock,mineral structure and some other factors.The studies of the oxygen fugacity of the upper mantle are mainly focused on the aspects of the magnesium-olivine-magnetite-quartz system,amphibole-bearing peridotite,basalt(melt)and experimental petrology method.It is widely accepted that mantle oxygen fugacity decreases with the increase of the depth.Nevertheless,natural samples and theoretical studies suggest that the oxygen fugacity of asthenosphere is higher than that of the overlying lithosphere mantle,and even that such an inverse gradient may exist for mantle transition zone compared with the upper mantle.The differences between different tectonic environments and different regions with similar tectonic environments indicate the heterogeneity of oxygen fugacity in horizontal direction.The relationship between substances flux of different layers and the variance of oxygen fugacity is one of the focus in future research on oxygen fugacity.The subduction zone is a great natural laboratory for exploration.Meanwhile,oxygen fugacity in turn influences species of volatiles and their circulating fluxes in different reservoirs,such as carbon and sulfur.