硫对地球早期演化和重大地质事件的制约

Sulfur constraints on Earth early evolution and great geological events

硫是地球上的关键元素,是重要的氧化剂、示踪剂和挥发分元素,在约束地球演化和诸多地质过程方面起着至关重要的作用。硫是地核中最主要的轻元素之一,在核幔分异过程中,大量硫进入地核;整体硅酸盐地球与球粒陨石的δ^(34)S差异暗示核幔分异过程中可能存在硫同位素分馏或地球早期存在显著的H_(2)S逃逸。硫同位素非质量分馏(S-MIF)是限定大氧化事件发生、起始时间以及增氧过程最重要的手段,陆壳出露海面造成火山喷气硫种型的变化可能是大氧化事件形成机制之一;海相硫酸盐与沉积硫化物的δ^(34)S差异是反演地史上海水硫酸盐含量进而限定大气氧含量的有效工具。此外,古老克拉通地体金刚石中S-MIF信号揭示了板块构造的启动时间,特殊地层中S-MIF异常指示了超级火山喷发与生物大灭绝的成因联系,海水δ^(34)S突变可能记录了大火成岩省及其诱发的环境剧变事件。本文对硫在地球早期演化和重大地质事件方面所扮演的角色进行了较为系统的综述和总结。

Among the elements on Earth,sulfur acts as an important oxidant,geochemical tracer and volatile element,playing a critical role in determining Earth’s evolution and many geological processes.As a major light element,sulfur was incorporated into the core during core formation.The light sulfurδ^(34)S composition of the bulk silicate Earth relative to chondrites indicates possible sulfur isotope fractionation during core-mantle differentiation or significant H_(2)S evaporation during the early Earth.Sulfur isotope mass-independent fractionation(S-MIF)is the most important mean to constrain the great oxidation event,its onset time and oxidation process.The emergence of continents leads to a decrease in the average pressure of volcanic degassing,which changes the oxidation state of sulfur in volcanic gases and therefore triggers atmospheric oxygenation.The isotopic offsets between sulfate and sulfide constrain the amount of ancient seawater sulfate and,therefore,reveal the O2 level in the atmosphere.S-MIF is absent in old Slave craton diamonds but is present in young African cratons,suggesting difference in continent construction related to start of plate tectonics.S-MIF anomalies found in some hallmark sedimentary stratigraphy can link stratospheric volcanism to the mass extinction events.In addition,abrupt sulfateδ^(34)S variations in seawater have been ascribed to large igneous provinces and associated climatic perturbations related to the mass extinction.The important roles of sulfur on constraining the evolution of the early Earth and the great geological events are briefly reviewed and summarized in this paper.