Fe-Mo同位素与古海洋化学演化

doi:10.11867/j.issn.1001-8166.2013.09.1049

Fe元素在自然界储量丰富，而Mo元素则是海水中储量最丰富的过渡金属元素。由于Fe，Mo对其所在环境的氧化还原条件非常敏感，近年来随着分析技术的进步，Fe与Mo同位素组成和变化被广泛用于鉴别古代海洋氧化还原状态及其演化。系统总结了古海洋化学研究中Fe与Mo同位素分馏机理和自然分布，并对当前获得的Fe-Mo同位素地史记录及其所指示的古海洋氧化还原状态进行了归纳与部分解释。造成Fe同位素分馏最明显的过程是氧化还原反应，氧化态的Fe通常具有更重的Fe同位素组成；此外，微生物作用及非生物作用下的黄铁矿生成过程也会产生明显的Fe同位素分馏。在海洋环境中Mo同位素的分馏主要与沉积物中铁锰（氢）氧化物吸附过程有关，铁锰（氢）氧化物吸附Mo的过程中，铁锰（氢）氧化物中倾向富集同位素较轻的Mo，造成海水中Mo同位素偏重，而硫化环境下的Mo沉积几乎不造成Mo同位素的分馏。Fe-Mo同位素的地史记录很好地说明了地质历史各时期海洋的氧化还原状态，在2.3 Ga以前海洋主要为铁化的状态，其中在2.6~2.5 Ga时氧含量略有增加；2.3~1.8 Ga之间地球表面初步氧化，硫化物沉积增加；1.8~0.8 Ga时海洋中的硫化环境得到了扩张；0.8 Ga以后地球表层逐步氧化，硫化水体消退。最后，对古海洋化学研究中Fe-Mo同位素研究未来的工作重点给予了展望。

关键词: Fe同位素, Mo同位素, 氧化还原状态, 古海洋化学演化

Iron(Fe) is abundant in nature while molybdenum(Mo) is the most abundant transition metal in seawater. Due to their high sensitivity to the redox state of the environment, the isotopic compositions of Fe and Mo as well as variations have been widely used to probe the redox conditions and the evolution of ancient ocean chemistry in favor of improved analytical techniques. Here, we summarized isotopic fractionation mechanisms and natural distribution of both iron and molybdenum isotopes, and further we summarized and partially reinterpreted the redox evolution of ancient oceans through time based on available Fe-Mo data compiled in this study. The process that causes the largest iron isotope fractionation is redox reaction and the iron in oxidation state is generally enriched in ⁵⁶Fe. Biotic and abiotic pyrite formations also produce a large Fe isotope fractionations. Isotopic fractionation of molybdenum in seawater is mainly caused by the adsorption process of dissolved Mo onto ferromanganese oxides or hydroxides in sediments. Fe-Mn (hydro)oxides tend to adsorb isotopically light molybdenum resulting in the isotopic composition of Mo in seawater heavier. However, the Mo sinks in euxinic settings cause almost no molybdenum isotope fractionation. The FeMo isotope isotopic records through geological timegenerally suggest similar ocean redox evolution: Oceans older than 2.3 Ga was mainly dominated by ferruginous condition, and there was a slight increase in oxygen content between 2.6 and 2.5 Ga. Earth’s surface was initially oxidized during 2.3 to 1.8 Ga, during which euxinic deposition of sulfide was elevated. Euxinic waters may have expanded greatly between 1.8 and 0.8 Ga, and after that, Earth’s surface had being gradually oxidized and the euxinic waters shrank substantially.Finally, suggestions are proposed for further work on the Fe-Mo isotope research in the context of ancient ocean chemistry.

Key words: Iron isotope, Molybdenum isotope, Redox state, Ancient ocean chemistry

中图分类号:

P736.4

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摘要

Iron-Molybdenum Isotopes and the Chemical Evolution of Ancient-Oceans