表生环境中镁同位素的地球化学循环

近些年表生环境中镁同位素分馏取得了一系列重要研究进展,这些新认识为深入理解表生环境中镁同位素地球化学循环奠定了基础。表生环境中镁同位素的地球化学循环主要涉及风化、河流搬运、碳酸盐沉淀、水岩反应等重要地质过程。风化过程中镁同位素发生显著分馏,硅酸盐风化产物中富集重的镁同位素,轻的镁同位素易进入水体。河流搬运过程中,镁同位素不发生分馏,但外源输入可能影响水体的镁同位素组成。河水汇入海洋后,碳酸盐沉淀过程可导致轻的镁同位素以碳酸盐的形式从海水中移出。在海底高温水岩反应过程中,海水中绝大多数的镁(80%~87%)都进入岩石,循环后的热液可能富集轻的镁同位素。海底低温水岩反应过程中海水的镁可以进入岩石并形成次生矿物,此过程的镁同位素分馏主要与次生矿物的形成有关。此外,海水中的镁易与黏土矿物发生交换反应,此过程黏土矿物倾向于吸附轻的镁同位素。总之,在表生环境中上地壳的镁(δ26Mg约为-0.22‰)经历风化作用、河流搬运、海洋贮存,最终以碳酸盐岩(δ26Mg一般小于-1‰)或与玄武岩发生反应的形式重新回到岩石圈。

Mg Isotope Geochemical Cycle in Supergene Environment

Mg isotope fractionation in the supergene environment have been obtained many important advances in recent years, and these new knowledge supply the clues for further understanding Mg isotope geochemical cycle in the supergene environment. Mg isotope geochemical cycle in the supergene environment involves some important geological processes, such as weathering, river transportation, carbonate sedimentation and water-rock reaction. Mg isotope fractionates dramatically in weathering process. Silicate weathering products enrich in ²⁶Mg (secondary clay minerals prefer to combine with ²⁶Mg) and ²⁴Mg prefer to be into the aqueous phase. Although there is not significant Mg isotope fractionation during river transportation, Mg isotope of river water could still be affected by the supplement of external sources. Most magnesium from river water are transported into the ocean, and then the carbonate precipitation prefers to remove ²⁴Mg from ocean as carbonate minerals. Submarine low temperature water rock reaction could transport less magnesium from ocean into the rocks during secondary mineral formation, which is associated with Mg isotope fractionation. However, most of magnesium (80%~87%) in seawater prefer to combine with the rock in high temperature water rock reaction and recirculated hydrothermal fluid may be enriched in ²⁴Mg. In brief, magnesium from upper crust (δ²⁶Mg about -0.22‰) experiences weathering and transports by river water to ocean in the supergene environment. The magnesium in ocean could recycle into the rocks by the process of carbonate precipitation (δ²⁶Mg is less than -1‰) or reacting with submarine basalt to form secondary mineral.