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

9628288个鲁宾地球化学对比表明，基拉韦厄峰顶熔岩来自一种不均匀的来源。相反，在持续裂谷带喷发期间喷发的熔岩，其不相容的微量元素比值(如Ce/Yb)在时间上迅速而系统地下降，而铅、锶和钕的同位素比值变化很小或没有变化。这些短期的地球化学关系可能是由局部的、相对均一的地幔来源的逐渐枯竭造成的。解释这些观测结果的假设是：(1)基拉韦厄熔岩的总体成分变化是由一个不均匀的地幔来源熔融的结果。(2)裂谷带持续喷发期间，由于岩浆绕过峰顶储集层，避开了顶层的缓冲和/或污染作用，观察到了微量元素的快速变化；(3)熔融和岩浆运移的时间尺度较短(1~2ka)。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测定U、Th和Ra的主要同位素的活度。高精度质谱仪将用于测定熔岩中238U、234U、232U、230U、228Ra、226Ra和Ba的浓度。将这些测量结果与其他现有的地球化学数据结合起来，将使我们能够评估：(1)部分熔融过程中的变化与地幔源区的不均质性对于创造基拉韦厄熔岩成分多样性的相对重要性，以及(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.