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

利用 226Ra-230Th-238U 不平衡检验夏威夷地盾火山橄榄岩-辉石岩熔体混合的假说

• 批准号: 0738286
• 负责人: Aaron Pietruszka
• 金额: $ 15.14万
• 依托单位: San Diego State University Foundation
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-01-15 至 2010-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0738286&HistoricalAwards=false
• 关键词: Using; 226Ra; 230Th; 238U; Disequilibria

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.

智力上的优点。人们早已认识到，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）纳入地幔柱的深部源区是产生在洋岛玄武岩中观察到的化学和同位素异质性的重要过程。这种“再循环假说”的一个重要组成部分是，除了通常被认为构成大部分地幔柱的石榴石或尖晶石橄榄岩之外，许多地幔柱还含有大量的辉石岩或榴辉岩（相当于再循环的海洋地壳的变质物）。地幔的。夏威夷火山熔岩的地球化学和同位素分析在这场争论中占据了重要地位。一个重要的观察结果是，夏威夷盾状火山的一些拉斑玄武岩具有异常高的 SiO2 含量（在给定的 MgO 值下），这不可能是由于浅层晶体分异所致。有人提出，这些高 SiO2 玄武岩可能是由少量（按重量计 5%）的古老、回收的含石英榴辉岩部分熔融形成的。最近，研究发现了夏威夷熔岩中辉石岩或榴辉岩成分的其他主要元素和微量元素特征，例如镍丰度和氧化钙含量（分别）过高和过低，无法与橄榄岩源保持平衡。这些研究之所以引人注目有两个原因。首先，它们要求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，它们需要一个不寻常的两阶段熔化过程，其中榴辉岩的深层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。次生辉石岩和剩余的、未反应的橄榄岩都上升到较浅的深度并熔化，形成混合的“橄榄岩-辉石岩”岩浆。如果正确的话，这个模型将代表我们对来源异质性本质和夏威夷盾火山下熔体生成机制的思考发生根本性变化，甚至可能具有全球地球动力学影响。我们建议检验夏威夷地幔柱包含两种不同的岩性（橄榄岩与辉石岩）的假设，这些岩性有助于熔化（以不同的比例混合），形成大量的拉斑玄武岩，构成夏威夷盾状火山的大部分。该分析方法涉及对来自夏威夷两座最活跃的盾状火山莫纳罗亚火山和基拉韦厄火山的特征明确的熔岩的 226Ra-230Th-238U 不平衡进行详细、高精度的研究。 U 系列同位素非常适合研究这个问题，因为从单斜辉石和石榴石为主的辉石岩源与以橄榄石和斜方辉石为主的橄榄岩源中提取的熔体的 226Ra-230Th-238U 不平衡预计会有很大差异。该方法代表了对橄榄岩-辉石岩熔体混合模型的独立测试，因为它不基于主要或相容的微量元素。具有更广泛的影响。圣地亚哥州立大学 (SDSU) 位于与墨西哥和环太平洋地区接壤的一个多元化的大都市区，是该地区科学和技术资源的重要贡献者。这项研究将为我们多元化的学生群体提供教育机会，其中许多人是第一代大学生。至少一名才华横溢的本科生将通过高级论文项目参与样品制备、分析工作和解释。此外，地球化学和同位素分析以及随后的解释将构成圣地亚哥州立大学研究生的硕士学位论文。拟议的研究将充分利用 SDSU 同位素实验室，确保该多用户设施继续成为对地球科学学生进行先进技术指导的宝贵场所。为了扩大向公众传播我们的研究成果，Pietruszka 自愿在圣地亚哥的鲁本·H·弗利特科学中心向学校团体和老年人定期进行科学演示，其中融入了他活跃的研究项目的各个方面。