U-Series Isotope Systematics of Continental Rift Basalts: The Rio Grande Rift, USA

阿斯默隆裂谷是一种独特的构造环境，岩浆来源于深部地幔和古老的岩石圈大陆地幔。对裂谷中喷发的大陆玄武岩进行化学和同位素研究，可以为我们提供有关大陆下地幔组成的信息。与长寿命同位素系统相比，短寿命核素，如u系列同位素，使我们能够追踪当今地壳-地幔系统的动力学和组成方面。研究区里约热内卢大裂谷及其克拉通边缘是美国最具特征的大陆裂谷系之一。它含有同时期的年轻碱玄武岩和拉斑玄武岩，推测它们在空间上接近软流圈和岩石圈地幔。u系列同位素数据与这些玄武岩的Pb、Sr和Nd同位素组成将有助于我们理解：1)控制大陆玄武岩中Th富集变化的因素，如熔融程度和速率的差异以及地幔熔融深度（来源）的差异。地幔源随时间变化的信息为大陆裂谷模型提供了约束。2)大陆下地幔Th-U成分的变异，特别是大陆岩石圈地幔232Th/238U比值（k值）的变异。关于k值的信息对于全球质量平衡和作为地幔中再循环和交代等过程的指标是重要的。3)大陆地壳对裂谷岩浆化学性质的影响。

9706373 Asmerom Rifts are unique tectonic settings where magmas derived from both the deep mantle and the old lithospheric continental mantle occur. Chemical and isotopic studies of continental basalts erupted in rifts can provide us with information on the mantle composition under the continents. In contrast to long-lived isotope systems, short-lived nuclides, such as the U-series isotopes, allow us to trace dynamical and compositional aspects of the present-day crust-mantle system. The study area, the Rio Grande Rift and its cratonic margins, is one of the best characterized continental rift systems in the US. It contains contemporaneous young alkali and tholeiitic basalts of presumed asthenospheric and lithospheric mantle origin in close spatial proximity. The U-series isotopic data in conjunction with Pb, Sr and Nd isotopic compositions of these basalts will contribute to our understanding of: 1) controls on the variation of Th enrichment in continental basalts such as differences in degree and rate of melting, and depth (source) of mantle melting. Information on the variation of mantle sources with time provides constrains for models of continental rifting. 2) Th-U compositional variability of the mantle under continents, and in particular the variability in the 232Th/238U ratio (k-value) of the continental lithospheric mantle. Information on the k-value is important with respect to global mass balance and as indicator of processes such as recycling and metasomatism in the mantle. 3) the role of continental crust in modifying the chemistry of rift magmas.