弧岩浆演化过程中Nb/Ta分异行为实验研究

Arc magma evolution is a key to understand the formation and evolution of continental crust. Although fractional crystallization is the dominant process during arc magma evolution, the mechanisms that primitive arc basaltic magma transformed to siliceous magma remain unclear. Arc magma evolution is accompanied by decreasing Nb/Ta, which in turn provides insights into how arc magma differentiated. Continental arc cumulates (arclogites) contain rutiles whose Nb/Ta ratios are higher than those in primitive arc basalts, and therefore rutile-bearing arclogites were likely to be responsible for Nb/Ta fractionation and arc magma evolution. However, this model has been challenged by partitioning data, which show that rutile favors Ta over Nb. To investigate the role of rutile on Nb/Ta fractionation during arc magma evolution, this project intend to carry out two experimental studies: (1) determining Nb and Ta partition coefficients between rutile and melts under the conditions of low temperature range (800℃≤T≤1000℃) and low H2O contents (≤10wt%);(2) determining the chemical diffusivities of Nb and Ta in melts to estimate the possible Nb/Ta kinetic fractionation when rutile crystallization proceeds. Base on experimental results, this study will discuss the mechanisms that how arc basaltic melts differentiated into andesitic melts from the perspective of Nb/Ta fractionation.

弧岩浆演化是理解陆壳形成和演化的关键。结晶分异作用主导着弧岩浆的演化，但弧岩浆由玄武质熔体向富硅熔体转变的机制尚有争议。随着弧岩浆演化Nb/Ta不断降低，因此Nb/Ta分异可以为研究弧岩浆演化提供依据。大陆弧堆晶榴辉岩中含有Nb/Ta值高于原始玄武质熔体的金红石，所以含金红石弧榴辉岩的结晶分异可能控制着Nb/Ta分异和弧岩浆演化。然而，现有的金红石/熔体平衡分配系数均表明DNb<DTa，并不能解释高Nb/Ta含金红石榴辉岩的成因。为了探究金红石在弧岩浆演化过程中对Nb/Ta分异的控制作用，本课题拟开展两项实验工作：测定低温（800℃≤T≤1000℃）和低水含量（≤10wt%）条件下Nb和Ta在金红石与熔体间的分配系数；针对金红石结晶过程中Nb/Ta可能产生的动力学分异，测定Nb和Ta在熔体中的扩散速率。根据实验结果，本研究将从Nb/Ta分异角度探讨弧岩浆由玄武质熔体向安山质熔体演变的机制。

弧岩浆结晶分异作用被认为是形成陆壳和富硅岩浆的重要方式之一，然而，幔源玄武质岩浆向长英质转变主要发生在角闪石的稳定域还是石榴石稳定域，还存在很大争议。弧岩浆的演化常伴随Nb/Ta降低，目前有研究认为低Mg#角闪石或金红石的分离结晶会导致岩浆Nb/Ta比值降低。金红石形成于高压环境，常与石榴石伴生。因此，厘清弧岩浆Nb/Ta降低的诱因，可能解决其化学成分由玄武质向长英质转变发生深度的问题。本研究开展了2 GPa, 850-1000℃条件下金红石对Nb/Ta分异效应的高温高压实验研究。通过测定高温高压实验对矿物/熔体分配系数研究，结果显示Ta相对Nb在金红石中更相容，不支持金红石是引起岩浆Nb/Ta降低的推论。在此研究结果的基础上本研究还统计了大洋岛弧、年轻陆弧、成熟陆弧岩浆的Nb/Ta和Dy/Yb数据等，发现Nb/Ta和Dy/Yb呈正相关性，这进一步指示是角闪石分离结晶对弧岩浆Nb/Ta变化起控制作用。本研究认为，幔源岩浆的化学成分由玄武质向长英质转变主要发生在角闪石的稳定域。