幔源岩浆氧化还原状态及对岩浆矿床成矿的制约

岩浆的氧化还原状态是控制许多基本地质过程的关键热动力学参数之一。估算玄武质岩浆和源区岩石氧化还原状态的常用方法主要包括多价元素的价态、多价元素的分配系数、共存矿物对的化学平衡和全岩化学比值。岩石学实验的深入和分析技术手段的快速发展使精确估算岩浆氧逸度成为可能。这有力地促进了对地幔源区成分、岩浆的部分熔融程度和熔融方式与分异演化历史,以及岩浆矿床的成因机制及成矿过程的研究。幔源岩浆的氧化还原状态复杂多变,不仅与构造背景有关,还与地幔深度(压力)、交代作用和部分熔融有着密切联系。而在岩浆到达浅部地壳后,结晶分异、岩浆去气和同化混染等过程也能不同程度地改变岩浆的氧逸度。因此,即使来自同一构造背景的幔源岩浆也呈现出明显的氧逸度不均一性。氧逸度的高低对源区部分熔融过程中金属元素的地球化学行为、岩浆的分异演化趋势、Fe-Ti-V氧化物饱和时间的早晚和S在岩浆中的溶解度具有明显的控制作用。因此,岩浆的氧逸度对钒钛磁铁矿矿床和汇聚板块边缘的岩浆硫化物矿床的成矿过程具有显著的影响。

Redox state of mantle-derived magma and constraints on the genesis of magmatic deposits

The redox state of mantle-derived basaltic magma is one of the key thermodynamic parameters controlling many basic geological processes. Methods commonly used to estimate redox state of basaltic magmas and its source rocks include valence state of multivalent elements, partition coefficient of multivalent elements, chemical equilibrium of coexisting mineral pairs and whole-rock chemical ratios. With the rapid development of petrological experiments and the advances in analytical techniques, it is possible to accurately estimate the oxygen fugacity of mantle-derived basaltic magma and source rocks. This has greatly promoted the studies on composition of mantle source, evolutional history of partial melting, differentiation of mantle-derived basaltic magma, genesis of magmatic deposits, and metallogenic processes. The variable redox state of the mantle-derived magma is not only related to the tectonic settings, but also closely related to the mantle depth (pressure), metasomatism, and partial melting. Fractional crystallization, degassing and contamination of the magma can also change the oxygen fugacity of the magma in variable degrees at the shallow crust. Therefore, the mantle-derived magma exhibits variable oxygen fugacity even from the same geodynamic background. The oxygen fugacity of mantle-derived magma exerts critical controls on geochemical behavior of metals (e.g., PGE, Au, Cu) during partial melting of the source region, magma differentiation trends, relative saturation time of Fe-Ti-V oxide and its V content, and sulfur solubility in the magma. Thus, the ore-forming processes of Fe-Ti-V oxide deposits and Ni-Cu-(PGE) sulfide deposits are significantly influenced by the oxygen fugacity of magma.