U-Th-Ra disequilibria in basalts from the Mid-Atlantic Ridge 6-11°S: Constraints on melting, mixing and time-scales of magmatic processes

…我们建议研究6 ~ 11°S之间缓慢扩展的大西洋中脊零年龄MORB样品的U、Th和Ra同位素组成。这部分扩展轴轴深、地壳厚度、样品熔体变化较大，主微量元素地球化学特征及Sr、Nd、Pb同位素组成变化显著。现有资料表明，在熔融上升过程中，地幔源、二元混合和岩浆分异程度的变化是复杂的相互作用。第一年，我们建议测定沿地轴和近地轴年轻海山（20万年）新鲜样品的U、Th和Ra同位素组成，以评估熔融深度和程度、地幔组成、孔隙度和上升流速率以及沿传播脊段的混合过程。第二年，我们拟在U系列同位素资料的基础上，结合工作区内选定火山构造的地层填图和采样，研究该扩张轴上岩浆演化和喷发过程的时间。

... We propose to study the U, Th, and Ra isotopic compositions of zero-age MORB samples from the slow-spreading Mid-Atlantic Ridge between 6 and 11°S. This part of the spreading axis shows large variations in axial depth, crustal thickness and samples melts with significant variations in major and trace element geochemistry as well as Sr, Nd, and Pb isotopic composition. The existing data indicates a complex interplay of variations in mantle sources, binary mixing and degree of magma differentiation during melt ascent. In the first year wie suggest to determine U, Th and Ra isotope compositions of fresh young samples along the axis and from young ( 200.000 years) near- axis seamounts to evalute the depth and extent of melting, the mantle composition, porosity and upwelling rate and the mixing processes along a propagating ridge segment. In the second year wie propose to investigate the timing of magma evolution and eruption processes on this spreading axis on the basis of U series isotope data and stratigraphic mapping and sampling of selected volcanic structures of the working area.