铁同位素分馏机理以及在镁铁-超镁铁质岩浆作用和成矿作用中的应用

Fractionation mechanism of iron isotope and its application in mafic-ultramafic magmatism and metallogenesis

镁铁-超镁铁质侵入体包含了大量来自地球深部的信息,与其具有成因联系的岩浆矿床具有重要的经济价值。铁作为直接参与成矿的元素,其同位素组成在示踪成矿物质来源与反演成矿过程方面有着天然的优势。近年来,随着MC-ICP-MS的出现与发展,诸多学者对镁铁-超镁铁质侵入体及相关的岩浆矿床开展了一系列铁同位素研究,并取得了重要的研究进展。本文通过总结镁铁-超镁铁质岩浆作用及成矿作用中铁同位素的组成,探讨了镁铁-超镁铁质侵入体中铁同位素的分馏机制。研究表明不同地质储库的铁同位素组成不同,其中球粒陨石代表地球最初始的铁同位素组成,其他岩体均因遭受不同程度的后期地质作用的改造而产生铁同位素分馏。铁同位素的分馏机制包括热力学平衡分馏和动力学分馏,在自然界矿物中,Fe3+形成的化学键一般强于Fe2+,因此富Fe3+的矿物相(如磁铁矿)相对于富Fe2+的矿物相(如磁黄铁矿、镍黄铁矿)更富集重同位素。多种动力学过程包括部分熔融、分离结晶、氧化还原过程和扩散等均会导致铁同位素产生动力学分馏。由于其独特的地球化学性质,铁同位素在与镁铁-超镁铁质侵入体相关的铜镍铂族元素硫化物矿床、钒钛磁铁矿矿床和铬铁矿矿床等领域有着非常广泛的应用及前景。与其他稳定同位素,如锂、硫、镁、镍同位素等相结合,可以有效地用来示踪沉积物质的同化作用,反演硫化物熔体冷凝结晶过程和揭示矿床成因。

The mafic-ultramafic intrusions contain a large amount of information from the deep earth,and the magmatic deposits they contain are of great economic value.Fe is an element directly involved in mineralization,and Fe isotope method has a natural advantage in tracing the source of metallogenic material and metallogenic process.In recent years,with the appearance and development of MC-ICP-MS,many scholars have carried out a series of Fe isotope studies on mafic-ultramafic intrusions and made important progress.In this paper,the fractionation mechanism of Fe isotopes in mafic-ultramafic intrusions is discussed by summarizing the composition characteristics of Fe isotopes in magmatism and metallogenesis.The results show that the composition of Fe isotope in different geological bodies is different.The chondrite represents the initial Fe isotope composition of the earth,and the other geological bodies are subjected to different degrees of later geological processes and produce fractionation of Fe isotope.The framework of Fe isotope fractionation mechanism in mafic-ultramafic intrusions is constructed:Fe isotope fractionation mechanism includes thermodynamic equilibrium fractionation and kinetic fractionation.In natural minerals,Fe3+forms stronger chemical bonds than Fe2+,so mineral rich in Fe3+(such as magnetite)are more enriched in heavy isotopes than mineral rich in Fe2+(such as pyrrhotite and pentlandite).Many kinetic processes such as redox process,fractional crystallization,partial melting and diffusion are the factors leading to kinetic fractionation.Due to its unique geochemical properties,Fe isotopes have a wide application and prospect in the fields of Cu-Ni-PGE sulfide deposits,V-Ti magnetite deposits and chromite deposits related to mafic-ultramafic intrusions.Combined with other stable isotopes,such as Li-S-Mg-Ni isotopes,they can be used to trace the assimilation of sediment,invert the crystallization process of sulfide melt and reveal the genesis of deposit.