Ba同位素在花岗质岩浆演化及流体活动中的应用与展望

Application and Prospect of Ba Isotopes in the Evolution of Granitic Magmas and Fluid Activities

钡(Ba)作为一种非传统的稳定同位素,逐渐展现出其作为示踪花岗质岩浆演化和流体活动的独特优势。目前,Ba同位素的地球化学储库已经基本建立。地壳的重熔过程可以分馏出Ba同位素,导致生成的熔体富集重Ba同位素。然而,在喜马拉雅康巴淡色花岗岩中观察到的轻δ^(138/134)Ba值(-1.32‰~+0.12‰)很可能与来自大型深层岩浆储库的流体有关。深层岩浆储库释放的流体对康巴浅色花岗岩的岩浆演化过程产生了重要影响,导致了较低的轻Ba同位素组成(<0)。这些流体不仅改变了浅色花岗岩的Ba同位素组成,还可能输运了一些可溶性金属元素,有助于稀有金属矿床的形成。与花岗岩熔体相比,钾长石富含轻Ba同位素。高分异花岗岩中的钾长石分离结晶会导致明显升高的δ^(138/134)Ba值(>0),而具有岩浆-热液特征的高分异花岗岩的δ^(138/134)Ba值明显低于大陆地壳(~0‰)。在俯冲带流体过程中,Ba同位素可以大量分馏,轻Ba同位素优先富集在矿物中,导致变质流体的δ^(138/134)Ba值远高于俯冲物质(如蚀变洋壳)。俯冲带中重Ba同位素(>0)可能与蚀变洋壳的流体贡献有关,而轻Ba同位素(<0)可能反映了俯冲碎屑沉积物的增加。超高压变质岩和高压脉可以记录俯冲带变质作用和流体—岩石相互作用过程中的Ba同位素分馏,而变质再平衡则可以改变地幔楔和弧火山岩的Ba同位素组成。然而,在使用Ba同位素来量化不同来源的贡献时,尤其是涉及变质脱水和流体活动时,需要仔细考虑。综上所述,总结了Ba同位素在花岗岩及流体活动中的应用现状和进展,并针对未来的研究方向和挑战进行了展望。

Barium(Ba),as a non-traditional stable isotope,gradually reveals its unique advantage as a tracer for granite magma evolution and fluid activity.Currently,the geochemical reservoir of Ba isotopes has been established.Crustal remelting can fractionate Ba isotopes,resulting in the enrichment of heavy Ba isotopes in the produced melt.However,light Ba isotopeδ138/134 Ba values(-1.32‰to+0.12‰)observed in the Himalayan Kampa leucogranites are likely related to fluid exsolution from large deep-seated magma reservoirs.The fluid released from deep magma reservoirs has significant effects on the magma evolution of Kampa leucogranites,leading to low light Ba isotope compositions(<0).These fluids not only alter the Ba isotope compositions of leucogranites but also transport some mobile metal elements,potentially contributing to the formation of rare metal deposits.In contrast to granite melts,potassium feldspar is enriched in light Ba isotopes.Fractional crystallization of potassium feldspar in highly fractionated granites result in significantly elevatedδ138/134 Ba values(>0),while highly fractionated granites with characteristics of magmatic-hydrothermal interaction exhibit considerably lowerδ138/134 Ba values compared to continental crust(~0‰).In fluid processes within subduction zones,Ba isotopes can undergo extensive fractionation,with light Ba isotopes preferentially enriching in minerals,leading to much higherδ138/134 Ba values in metamorphic fluids than in subducted materials(e.g.,altered oceanic crust).Heavy Ba isotopes(>0)in subduction zones are likely associated with fluid contributions from altered oceanic crust,while light Ba isotopes(<0)may reflect an increased contribution of subducted volcaniclastic sediments.Ultra-high pressure metamorphic rocks and high-pressure veins can record Ba isotope fractionation during subduction zone metamorphism and fluid-rock interactions,while the metamorphic re-equilibration can alter the Ba isotope compositions of mantle wedge and arc volcanic rocks.However,careful consideration is needed when using Ba isotopes to quantify contributions from different sources,particularly in the context of metamorphic dehydration and fluid activities.In summary,this paper summarizes the application and progress of Ba isotopes in granite and fluid activity,and provides an outlook on future research directions and challenges.