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

鲁宾地球化学相关性表明，基拉韦厄的山顶熔岩来自一个异质源。 相比之下，在持续的裂谷带喷发期间喷发的熔岩显示出其不相容微量元素比率的快速和系统性的时间减少（例如，Ce/Yb），但Pb、Sr和Nd同位素比值变化不大或没有变化。 这些短期的地球化学关系可能是由局部的、相对均一的地幔源的逐渐耗竭引起的。 解释这些现象的假设是：（1）基拉韦厄火山熔岩的总体成分变化是由于地幔源区的不均匀熔融造成的。 (2)微量元素在裂谷带持续喷发过程中的快速变化是因为这些喷发的岩浆绕过了山顶储层，避免了它的缓冲和/或污染作用;（3）熔融和岩浆运输的时间尺度很短（1-2 ka）。 为了具体检验上述假设，计划确定三个不同的基拉韦厄样品组中U、Th和Ra的主要同位素的活动。 高精度质谱法将用于确定这些熔岩中的238 U，234 U，232 U，230 U，228 Ra，226 Ra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）部分熔融过程中的变化与地幔源区的异质性的相对重要性，以形成基拉韦厄熔岩的成分多样性，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.