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

...本文拟对南纬6 ° ~ 11°之间缓慢扩张的大西洋中脊的零龄MORB样品的U、Th和Ra同位素组成进行研究。这部分的扩展轴显示出很大的变化，在轴向深度，地壳厚度和样品熔体的主要和微量元素地球化学以及Sr，Nd，Pb同位素组成的显着变化。现有的数据表明，在熔体上升过程中的地幔源，二元混合和岩浆分异程度的变化复杂的相互作用。建议在第一年测定轴向沿着新鲜年轻样品和近轴年轻（20万年）海山样品的U、Th和Ra同位素组成，以评价熔融的深度和程度、地幔成分、孔隙度和上涌速率以及沿着扩张脊段的混合过程。第二年拟根据铀系同位素资料和工作区选定火山构造的地层填图和取样，研究这条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.