伊豆-小笠原-马里亚纳岛弧岩浆中硫的地球化学行为研究：对弧岩浆高氧逸度成因的制约

The oxygen fugacity of subduction zone system is an important parameter to study the material recycling of Earth’s surface and interior, as well as the origin and evolution of the hydrosphere, atmosphere and biosphere, and also the key factor for the formation of abundant large-scale porphyry copper deposits in subduction zones. Therefore, it has long been the research frontier of Earth sciences. Arc magmas are known to be more oxidized than mid-ocean ridge basalts. However, it is still highly debated as to whether the high oxygen fugacity is inherited from an oxidized mantle source or arc magma gets more oxidized during evolution. Sulfur is an important multivalent element in the subduction system, which is sulfide at reduced conditions and sulfate at oxidized conditions, and thus, the geochemical behavior of sulfur and sulfur-loving (chalcophile) elements could act as a sensitive tracer for the variations in oxygen fugacity of the arc magmas. Therefore, this study plans to analyze the Cu, platinum-group elements (PGE) and other chalcophile elements as well as Re-Os isotopes in a series of across-arc volcanic rocks from the Izu-Bonin-Mariana arc to investigate the across-arc variations in sulfur behaviors in the arc magmas. Combining analyses on the valence status of multivalent elements S and Fe in the melt inclusions as well as in situ major and trace elemental and isotopic analyses on silicate and sulfide minerals in the samples, the study aims to systematically constrain the variations of oxygen fugacity of the arc magmas. Furthermore, the study would analyze the major and trace element, and B stable isotope compositions on the volcanic rocks to quantify the contribution of volatiles released from the subducted plate to the mantle source of the rocks. Eventually, the study would synthesize all the data to reveal how systematic differences in the volatiles released from the subducted plate at different depths would affect the oxygen fugacity of the arc magmas and to constrain the origin of the high oxygen fugacity of arc magmas globally.

俯冲带体系的氧逸度是研究地表与地球深处物质循环及地表各圈层起源与演化极为重要的参数，也是俯冲带富集大型斑岩铜矿的关键因素，一直是地球科学研究的前沿。弧岩浆普遍比洋脊玄武岩更氧化，但岩浆高氧逸度的成因是“幔源”还是“壳源”存在很大争议。硫是俯冲带体系重要的变价元素，其地球化学行为受控于氧逸度的变化。因此，岩浆中硫及亲硫元素的系统变化可灵敏示踪弧岩浆形成和演化过程中氧逸度的变化。本项目拟对西太平洋伊豆-小笠原-马里亚纳岛弧的跨岛弧的火山岩样品进行系统的Cu和铂族元素等亲硫元素、Re-Os同位素、相关的微区矿物学及熔体包裹体中变价元素S、Fe的价态测定等研究，反演岩浆中硫的地球化学行为，制约不同位置岛弧火山岩的氧化还原状态；结合主微量及B等同位素研究限定俯冲板片的挥发分对火山岩源区的贡献。本项目旨在以不同深度下俯冲板片物质输入对跨岛弧火山岩的氧逸度的影响为突破口，制约弧岩浆高氧逸度的成因。

俯冲带体系的氧逸度是研究地表与地球深处物质循环及地表各圈层起源与演化极为重要的参数，也是俯冲带富集大型斑岩铜矿的关键因素，一直是地球科学研究的前沿。弧岩浆普遍比洋脊玄武岩更氧化，但岩浆高氧逸度的成因是“幔源”还是“壳源”存在很大争议。硫是俯冲带体系重要的变价元素，其地球化学行为受控于氧逸度的变化。因此，岩浆中硫及亲硫元素的系统变化可灵敏示踪弧岩浆形成和演化过程中氧逸度的变化。本项目通过系统分析西太平洋伊豆-小笠原-马里亚纳岛弧不同位置火山岩的亲硫元素地球化学组成，结合B等稳定同位素研究，综合示踪俯冲板片在不同深度下脱水过程对于岛弧地幔氧逸度的影响；汇总全球弧火山岩亲硫元素数据，制约全球弧岩浆高氧逸度的成因是“幔源”；结合全球洋中脊玄武岩汇总研究，制约俯冲板片释放的富水组成在全球软流圈地幔中的分布及其成因。相关成果发表在Science Advances, Nature Communications及G-cubed上。