广西新元古代BIF的铁同位素特征及其地质意义

Fe Isotopic Characteristics of the Neoproterozoic BIF in Guangxi Province and its Implications

通过分析广西三江地区新元古代条带状含铁建造的Fe同位素和主量元素组成,对海水的氧化还原状态提供了制约,为富禄期的地球处于间冰期提供了证据。相对于标准物质IRMM-014,新元古代含铁建造不同条带全岩样品的δ57Fe值变化范围1.60‰～2.20‰,平均值为1.85‰,表明BIF样品富集铁的重同位素。条带状含铁建造主要由Fe2O3和SiO2组成,但却具有较高的Al2O3含量。这表明条带状含铁建造样品不是纯净的化学沉积物,而是具有一定的碎屑物质输入。碎屑输入量的不同引起深色和浅色条带之间铁同位素组成存在着0.4‰的差别。剔除碎屑的影响,新元古代BIF从海水中沉淀的赤铁矿δ57Fe的平均值在2‰左右,略高于太古代条带状铁建造的Fe同位素组成,这表明当时海水的氧逸度可能比太古代还低。这说明在富禄期绝大部分海洋仍旧被冰盖覆盖,只在局部出现融化。因此,富禄期的地球可能出于冰期的相对温暖阶段,而不是间冰期。

The increase of atmospheric oxygen fugacity has a significant impact on the geochemical cycling of Fe isotopes,which may be recorded in the Precambrian Banded Iron Formation.This paper studied Fe isotope compositions and major elements of the Neoproterozoic BIF from the Sanjiang area,Guangxi Province.The Fe isotopic compositions of the Neoproterozoic BIF were analyzed using MS-ICP-MC.Relative to the reference material IRMM-014,the iron isotope composition of bulk samples range from 1.60 to 2.20,with an average of 1.85.The results show BIF samples are enriched in Fe heavy isotopes.The δ57Fe value between the dark and light bands has the difference of 0.4‰,which was caused by terrestrial debris input.Without the impact of debris,the δ57Fe of hematite from the seawater precipitate is 2‰,indicating that surface water was suboxic,even lower than the Archean.This shows that the ice still covers the most of ocean in Fulu period,only partial melting.This suggests that the ice covered most of ocean in Fulu period,only partial melting.Therefore,the Fulu period is the relatively warm phase of the glacial age,rather than the interglacial stage.