Early diagenetic versus hydrothermal signals in pyrite from ancient metamorphic sediment-hosted massive sulfides – implications for the stability of sulfur and iron isotope records in deep time

Stable isotope compositions in pyrite are widely employed for tracing microbial sulfur and iron cycling through geological time. In hydrothermal sulfide systems, however, sulfur and iron pools can be affected by both microbial and abiotic processes, limiting the applicability of the respective stable isotopes as biosignatures. Moreover, the diagenetic and metamorphic stability of sulfur and iron isotope signatures in pyrite under hydrothermal conditions is insufficiently understood. Here, we employed coupled in-situ Secondary Ion Mass Spectrometry (SIMS) triple sulfur (δ34S and Δ33S) and iron (δ56Fe) isotope analysis on morphologically diverse pyrite in ~390 Ma sediment-hosted massive sulfides to better understand biosignature preservation in hydrothermal systems. Petrographic analysis reveals recrystallized or cemented framboid-like pyrite that was locally overgrown by a secondary generation of subhedral pyrite. δ34S and Δ33S signatures of the pyrites (-15.13 to +18.77‰ and -0.21 to +0.26‰, respectively) can be explained by either microbial or thermochemical sulfate reduction. However, the isotopically lightest δ34S value in framboid-like pyrite (-15.13‰) most likely represents a mixed signal of early diagenetic microbial sulfur cycling and later sulfidic hydrothermal fluids driving recrystallization or cementation. The same pyrites show highly variable δ56Fe compositions (-1.30 to +2.19‰), indicating precipitation from hydrothermal Fe(II) at varying rates and/or pyritization of a diagenetically fractionated iron pool. The lower median δ56Fe value in framboid-like versus subhedral pyrite points to a greater expression of kinetic and equilibrium fractionation in the former. This may reflect differences in precipitation rates between early diagenetic (microbial) processes and hydrothermal overprint of the system, consistent with textural evidence for framboid recrystallization or cementation, and overgrowth. Nevertheless, the likely presence of microbially formed pyrite and the incomplete equilibration with hydrothermal fluids highlight that signatures of early diagenetic redox cycling can be preserved in hydrothermal sulfides despite alteration by sulfidic fluids or greenschist metamorphism. Our study stresses the challenges and potentials of coupled textural and in-situ stable isotope analysis for tracing microbial sulfur and iron cycling in hydrothermal sulfide systems through Earth’s history.

黄铁矿（pyrite）的稳定同位素组成被广泛用于示踪地质时间尺度下的微生物硫铁循环。然而，在热液硫化物系统（hydrothermal sulfide systems）中，硫和铁库会同时受到微生物与非生物过程的影响，限制了相应稳定同位素作为生物标志物（biosignatures）的适用性。此外，热液条件下黄铁矿中硫、铁同位素特征的成岩与变质稳定性尚未得到充分认识。本研究针对约390 Ma沉积型块状硫化物（sediment-hosted massive sulfides）中形态多样的黄铁矿，采用耦合的原位二次离子质谱（Secondary Ion Mass Spectrometry，SIMS）三硫同位素（δ34S与Δ33S）及铁（δ56Fe）同位素分析，以更好地理解热液系统中的生物标志物保存机制。岩相分析显示，存在重结晶或胶结形成的莓球状黄铁矿，其局部被次生半自形黄铁矿所增生。黄铁矿的δ34S与Δ33S特征（分别为-15.13‰至+18.77‰、-0.21‰至+0.26‰）可通过微生物硫酸盐还原或热化学硫酸盐还原（thermochemical sulfate reduction）来解释。但莓球状黄铁矿中同位素最轻的δ34S值（-15.13‰）最有可能代表早期成岩微生物硫循环与后期驱动重结晶或胶结作用的硫化物热液流体的混合信号。这类黄铁矿同时显示出变化范围极大的δ56Fe组成（-1.30‰至+2.19‰），表明其沉淀自速率各异的热液亚铁离子，或是源自成岩分馏铁库的黄铁矿化作用。莓球状黄铁矿的δ56Fe中位数低于半自形黄铁矿，说明前者更显著地表现出动力学与平衡分馏效应。这可能反映了早期成岩（微生物）过程与系统的热液叠加作用之间的沉淀速率差异，与莓球状黄铁矿重结晶、胶结及次生生长的结构证据一致。尽管如此，微生物成因黄铁矿的潜在存在以及与热液流体未完全达到平衡，表明尽管经历了硫化物流体改造或绿片岩相变质作用，早期成岩氧化还原循环的特征仍可在热液硫化物中得以保存。本研究强调了耦合结构与原位稳定同位素分析在示踪地球历史中热液硫化物系统内微生物硫铁循环方面所面临的挑战与潜力。