Nb-Ta分异的控制因素及其地质意义
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西北大学

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科技部重点研发计划项目(2023YFF0804200)


Influencing factors of Nb-Ta fractionation and its geological implications
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Northwest University

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    摘要:

    Nb和Ta的地球化学性质相似,但地球各圈层和不同地质体的Nb/Ta比值都表现为低于球粒陨石的特点,并且不同地幔端元、洋中脊玄武岩、大陆地壳等也具有不同的Nb/Ta比值。本文归纳总结了大量模拟实验和天然样品研究得到的矿物/熔体的Nb-Ta分配系数,发现金红石、角闪石和黑云母与硅酸盐熔体之间具有相对较高的Nb-Ta分配系数,Nb/Ta比值变化较大且受控于岩浆体系的成分和物理化学条件。温度和含水量升高会导致金红石与熔体Nb-Ta分配系数的降低。角闪石、黑云母与熔体之间的Nb-Ta分配系数主要受到熔体聚合度(NBO/T)、矿物成分(例如Mg#)和水含量的共同影响,DNb、DTa和DNb/DTa都表现为随着Mg#和水含量的降低而增加。金红石由于具有非常高的Nb-Ta分配系数被认为是重要的Nb-Ta储库,并且其在太古宙TTG岩浆形成过程中是必要的残余相,因此金红石在高压下的分离结晶作用常被用来解释TTG的Nb-Ta亏损,但是由于扩散作用影响的不确定性,能否导致共存熔体中Nb/Ta比值的降低还存在争议。对弧岩浆和长英质熔体的演化模拟实验表明,角闪石和黑云母的分离结晶都会导致熔体Nb/Ta比值呈现降低的趋势,而在岩浆演化晚期阶段黑云母和白云母对熔体Nb/Ta值的降低具有更重要的影响,并且F等挥发分含量的增加会促进熔体中Nb-Ta的富集。俯冲作用过程中富Ti矿物的转变和流体控制着Nb-Ta的储存、迁移和分异,高压下形成的含角闪石和金红石的榴辉岩是潜在的Nb-Ta储库。

    Abstract:

    The geochemical properties of niobium and tantalum are similar, but the Nb/Ta ratios of various spheres of the Earth and different geological units are lower than that of chondrites. Different mantle endmembers, continental crust, and mid-ocean ridge basalts also have different Nb/Ta ratios. This review summarizes the Nb-Ta partition coefficients of mineral-melts obtained from a large number of simulation experiments and natural samples. It is found that rutile, amphibole and biotite have relatively high Nb-Ta partition coefficients with silicate melts. The Nb / Ta ratio varies greatly and is controlled by the composition and physical and chemical conditions of the magmatic system. The increase of temperature and water content will lead to the decrease of Nb-Ta partition coefficient between rutile and melt. The Nb-Ta partition coefficient between amphibole, biotite and melt is mainly affected by the degree of polymerization (NBO/T), mineral composition (such as Mg #), and water content. DNb, DTa and DNb/DTa all increase with the decrease of Mg # and water content. Since rutile is considered as an important reservoir mineral of Nb-Ta due to its high Nb-Ta partition coefficients, as well as a necessary residual phase in the formation of Archean TTG magma, its fractional crystallization is often used to explain the Nb-Ta depletion of TTG. However, due to the uncertainty of its distribution coefficient ratio and diffusion effect, it is still controversial whether it can lead to the decrease of Nb/Ta ratio in coexisting melts. Based on experimental simulations of magma evolution in arc magmas and felsic melts, it is demonstrated that the fractional crystallization of amphibole and biotite may lead to the decrease of Nb/Ta ratio in the melt. In the late stage of magma evolution, biotite and muscovite have an enhanced effect on the decrease of Nb/Ta ratio in the melt, and the increase of volatile content such as F will promote the enrichment of Nb-Ta in the melt. The transformation of Ti-rich minerals and fluids control the storage, migration and differentiation of Nb-Ta during the process of subduction, the resultant amphibole and rutile-bearing eclogite formed under high pressure may be an important reservoir of Nb-Ta.

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  • 收稿日期:2024-10-19
  • 最后修改日期:2024-12-14
  • 录用日期:2024-12-23
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