Using 226Ra-230Th-238U Disequilibria to Test the Hypothesis of Peridotite-Pyroxenite Melt Mixing at Hawaiian Shield Volcanoes

智力上的优点。长期以来，人们认识到，将古老的、再循环的岩石圈物质(例如，俯冲的洋壳和/或沉积物)并入地幔热柱的深源区，是造成洋岛玄武岩中观察到的化学和同位素不均一性的一个重要过程。这一“循环假说”的一个重要组成部分是，许多地幔岩柱除了通常被认为构成地球地幔大部分的石榴石或尖晶石橄榄岩外，还含有大量的辉石岩或榴辉岩(相当于再循环洋壳的变质作用)。对夏威夷火山熔岩的地球化学和同位素分析在这场辩论中占据了突出地位。一个重要的观察结果是，一些来自夏威夷盾火山的拉斑玄武岩具有异常高的二氧化硅含量(在给定的氧化镁值下)，这不可能是由于浅层次的晶体分馏。有人提出，这些高SiO_2玄武岩可能是由一种古老的、含回收石英的榴辉岩部分熔融的(重量占5%)。最近，研究发现了夏威夷熔岩中辉石岩或榴辉岩组分的其他主元素和微量元素特征，如Ni丰度和CaO含量分别太高和太低，无法与橄榄岩来源平衡。这些研究之所以引人注目，有两个原因。首先，它们需要在夏威夷火山源区内存在大量的辉石岩或榴辉岩(高达100%)。其次，它们需要一个不同寻常的两阶段熔融过程，在这个过程中，榴辉岩的深熔体与周围的橄榄岩基质发生反应，形成固体的次生辉石岩。次生辉石岩和残留的未反应的橄榄岩都上升到较浅的深度并熔融，形成了混合的“橄榄岩-辉石岩”岩浆。如果这一模型是正确的，那么这一模型将代表着我们对物质来源不均质性的本质和夏威夷盾火山之下熔体产生机制的看法发生了根本性的改变，甚至可能对全球地球动力学产生影响。我们建议检验这一假设，即夏威夷地幔热柱包含两种不同的岩性(橄榄岩和辉石岩)，这两种岩性促成了熔体(以不同的比例混合)，形成了巨大的拉斑玄武岩，构成了夏威夷屏蔽式火山的主体。分析方法包括详细、高精度地研究来自毛纳罗亚火山和基拉韦厄火山这两座夏威夷最活跃的盾形火山的特征良好的熔岩的226Ra-230Th-238U不平衡。U系列同位素非常适合于研究这一问题，因为从以单斜辉石和石榴石为主的辉石岩来源提取的熔体的226Ra-230Th-238U不平衡被预测为与以橄榄石和斜方辉石为主的橄榄岩来源的熔体的226Ra-230Th-238U不平衡有很大不同。这种方法代表了对橄榄岩-辉石岩熔体混合模型的独立测试，因为它不是基于主要或相容的痕量元素。圣地亚哥州立大学(圣地亚哥州立大学)位于与墨西哥和环太平洋接壤的一个大而多样的大都市地区，是该地区科技资源的重要贡献者。这项研究将为我们多样化的学生群体提供教育机会，其中许多人是第一代大学生。至少有一名有才华的本科生将通过一个高级论文项目参与样本的准备、分析工作和解读。此外，地球化学和同位素分析，以及随后的解释，将形成SDSU研究生的硕士论文。拟议的研究将充分利用SDSU同位素实验室，确保这一多用户设施继续成为对地球科学专业学生进行高级技术指导的宝贵场所。为了向公众更广泛地传播我们的成果，Pietruszka自愿在圣地亚哥的鲁本·H·舰队科学中心向学校团体和老年人进行定期的科学陈述，纳入了他积极研究项目的各个方面。

Intellectual merit. It has long been recognized that incorporation of ancient, recycled lithospheric materials (e.g., subducted oceanic crust and/or sediment) into the deep source regions of mantle plumes is an important process in creating the chemical and isotopic heterogeneity observed in ocean-island basalts. An important component of this "recycling hypothesis" is the idea that many mantle plumes contain a significant amount of pyroxenite or eclogite (the metamorphic equivalent of the recycled oceanic crust) in addition to the garnet or spinel peridotite that is generally thought to make up most of the Earth's mantle. Geochemical and isotopic analyses of lavas from Hawaiian volcanoes have figured prominently in this debate. An important observation is that some tholeiitic basalts from Hawaiian shield volcanoes have unusually high SiO2 contents (at a given MgO value) that cannot be due to shallow-level crystal fractionation. It has been proposed that these high-SiO2 basalts might result from the partial melting of a modest amount (5% by weight) of an ancient, recycled quartz-bearing eclogite. More recently, studies have identified additional major- and trace-element signatures of a pyroxenitic or eclogitic component in Hawaiian lavas, such as Ni abundances and CaO contents that are too high and too low (respectively) to be in equilibrium with a peridotite source. These studies are notable for two reasons. First, they require the presence of an enormous amount of pyroxenite or eclogite within the source regions of Hawaiian volcanoes (up to 100%). Second, they require an unusual two-stage melting process in which deep melts of eclogite react with the surrounding peridotite matrix to create a solid, secondary pyroxenite. Both the secondary pyroxenite and leftover, unreacted peridotite rise to shallower depths and melt to create a mixed "peridotite-pyroxenite" magma. If correct, this model would represent a fundamental change in our thinking about the nature of source heterogeneity and the mechanism of melt generation beneath Hawaiian shield volcanoes, and may even have global geodynamic implications. We propose to test the hypothesis that the Hawaiian mantle plume contains two distinct lithologies (peridotite vs. pyroxenite) that contribute melts (that mix in variable proportions) to create the voluminous tholeiitic basalts making up the bulk of Hawaiian shield volcanoes. The analytical approach involves detailed, high-precision study of the 226Ra-230Th-238U disequilibria of well-characterized lavas from Mauna Loa and Kilauea, the two most active Hawaiian shield volcanoes. U-series isotopes are ideally suited to investigate this issue because the 226Ra-230Th-238U disequilibria of melt extracted from a clinopyroxene- and garnet-dominated pyroxenite source vs. an olivine- and orthopyroxene-dominated peridotite source is predicted to differ greatly. This approach represents an independent test of the peridotite-pyroxenite melt mixing model because it is not based on major or compatible trace elements.Broader impacts. Located in a large and diverse metropolitan area bordering Mexico and the Pacific Rim, San Diego State University (SDSU) is an important contributor to the scientific and technical resources of this region. This study will provide educational opportunities to our diverse student population, many of who are first-generation college students. At least one talented undergraduate student will be involved in the sample preparation, analytical work, and interpretation via a Senior Thesis project. In addition, the geochemical and isotopic analyses, and subsequent interpretation, will form a Masters Thesis for a graduate student at SDSU. The proposed study will fully utilize the SDSU isotope laboratory, ensuring that this multi-user facility continues to be valuable site for advanced technological mentoring of students in the Geosciences. To broaden dissemination of our results to the general public, Pietruszka has volunteered to conduct regular scientific presentations, incorporating aspects of his active research projects, at the Reuben H. Fleet Science Center in San Diego to school groups and senior citizens.