U-Th-Ra Isotope Systematics of the Historical Lavas of Kilauea Volcano, Hawaii

9628288 Rubin地球化学对比表明，基拉韦厄火山峰顶熔岩来自非均质来源。相比之下，在裂谷带持续喷发期间喷发的熔岩，其不相容的微量元素比值（如Ce/Yb）呈现出快速和系统的时间下降，而Pb、Sr和Nd同位素比值几乎没有变化。这些短期的地球化学关系可能是由一个局部的、相对均匀的地幔源逐渐枯竭造成的。我们对这些观测结果的解释是：(1)基拉韦厄火山熔岩的整体成分变异性是由非均质地幔源熔融造成的。(2)在裂谷带持续喷发过程中，由于岩浆绕过山顶储层，避免了其缓冲和（或）污染作用，导致了微量元素的快速变化；(3)熔融和岩浆搬运的时间尺度较短（1 ~ 2 ka）。为了具体检验上述假设，计划在三个不同的基拉韦厄样品组中确定U、Th和Ra的主要同位素的活性。高精度质谱法将用于测定这些熔岩中的238U、234U、232U、230U、228Ra、226Ra和Ba浓度。将这些测量结果与其他可用的地球化学数据相结合，将使我们能够评估：(1)部分熔融过程的变化与地幔源区的非均质性对于创造基拉韦厄熔岩成分多样性的相对重要性；(2)基拉韦厄浅层岩浆管道系统的结构（几何形状和大小）。

9628288 Rubin Geochemical correlations indicate Kilauea's summit lavas were derived from a heterogeneous source. In contrast, lavas erupted during sustained rift zone eruptions show rapid and systematic temporal decreases in their incompatible trace element ratios (e.g., Ce/Yb) but little or no change in Pb, Sr, and Nd isotope ratios. These short-term geochemical relationships may be caused by progressive depletion of a localized, relatively homogeneous mantle source. Out hypotheses to explain these observations are : (1) the overall compositional variability of Kilauea lavas results from melting a heterogeneous mantle source. (2) The rapid trace element changes during sustained rift-zone eruptions are observed because the magmas for these eruptions bypassed the summit reservoir and avoided it buffering and/or contaminating effects; and (3) The timescale for melting and magma transport are short (1-2 ka). To specifically examine the above hypotheses it is planned to determine the activities of the major isotopes of U, Th, and Ra in three different Kilauea sample suites. High-precision mass spectrometry methods will be used to determine 238U, 234U, 232U, 230U, 228Ra , 226Ra, and Ba concentrations in these lavas. Integrating the results of these measurements with other available geochemical data will allow us to evaluate: (1) the relative importance of variations in the partial melting process versus heterogeneity in the mantle source region for creating the compositional diversity of Kilauea lava, and (2) the structure (geometry and size) of Kilauea's shallow magma plumbing system.