A Mixed Up Mantle Beneath Ocean Islands?

玄武岩火山岛出现在所有主要的海洋盆地中，它与板块构造理论不太吻合。这些岛屿上的熔岩与在洋中脊喷发的玄武岩的成分截然不同，这表明它们的一些地幔源中存在俯冲的海洋地壳。目前关于洋岛玄武岩形成的模型，要么支持地幔源的两种不同岩石类型——辉石岩和橄榄岩的部分熔融，要么支持地幔源的“肥沃”橄榄岩可以很容易地部分熔融。这些模型是由观测到的再循环海洋地壳的直接部分熔融不能产生玄武岩成分驱动的。因此，这两种来源类型都被认为是由俯冲地壳与地幔中的橄榄岩熔化和融合而形成的。这些来源如何形成和融化，细节上有很大的不同，辉石岩表明大规模的地幔非均质性。一种公认的检测辉石岩的方法是精确测量玄武岩中的橄榄石成分。像锇这样的贵金属也可以用来检测橄榄岩和辉石岩主导的熔融之间的差异，而检测再循环地壳贡献的最灵敏的方法是来自长寿命的放射性同位素，如锶。这项工作将结合这些方法，通过研究两个已知存在重要再循环地壳成分的地点，来测试辉石岩是否存在于海洋岛屿源；库克群岛的曼加亚岛和加那利群岛。新的成果将部分由从事研究的本科生和斯克里普斯本科生研究奖学金（SURF）项目的本科生产生，他们也将帮助公开交流这项工作，这些成果将加强对玄武岩熔体产生的基本过程的理解，并加强对海洋岛屿如何适应板块构造理论的理解。本项目将通过检查两个关键的洋岛玄武岩地点，检查辉石岩-橄榄岩或肥沃橄榄岩熔融在洋岛玄武岩（OIB）形成中的作用。第一个地点是Mangaia（库克群岛），在那里，长寿命的放射性成因同位素（Sr-Nd-Os-Pb）变化将与橄榄石组成相结合，以检查一个没有强烈辉石岩熔融特征，但具有强烈的再循环地壳特征（HIMU）的地点。这项任务的目标是量化OIB岩石成因中极端再循环端元组成的地幔源组成。第二个任务是评估辉石岩-橄榄岩对具有HIMU特征的加那利群岛的贡献。研究认为辉石岩的贡献，但具有不同的端元组成。本文将收集海岛熔岩橄榄石颗粒的高精度电子探针数据，结合Sr-Nd-Os-Pb同位素变化和高亲铁元素丰度。有了这些数据，就有可能区分耗尽或富集的辉石岩来源，并调查是否高压-高温熔融肥沃的橄榄岩可能提供另一种选择。研究将包括整合本科生和未被充分代表的少数民族，他们也将与PI和研究生一起参与将在圣地亚哥地区和加那利群岛举行的外展活动，并通过参与有组织的STEM活动。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Basaltic volcanic islands, which occur in all major ocean basins, do not fit particularly well in plate tectonic theory. The lavas from these islands have distinct compositions from basaltic rocks erupted at mid-ocean ridges, indicating the presence of subducted oceanic crust in some of their mantle sources. Current models for the generation of ocean island basalts favor either partial melting of mantle sources with two different rock-types, pyroxenite and peridotite, or sources with only "fertile" peridotite that can readily partially melt. The models are driven by the observation that direct partial melting of recycled oceanic crust cannot generate basaltic compositions. Both source types are therefore thought to form by melting and melding of subducted crust with peridotite in Earth's mantle. How these sources form and melt, in detail, are quite different, with pyroxenite indicating large-scale mantle heterogeneity. An accepted means for detecting pyroxenite is through precise measurement of olivine compositions in basalts. Precious metals such as osmium can also be used to examine between peridotite versus pyroxenite-dominated melting, and the most sensitive method for detecting recycled crustal contributions is from long-lived radiogenic isotopes, like strontium. This work will combine these methods to test whether pyroxenite exists in oceanic island sources through study of two locations where significant recycled crust components are known to exist; Mangaia in the Cook Islands, and the Canary Islands archipelago. New results, which will be partially generated by undergraduate students engaged in research and in the Scripps Undergraduate Research Fellowships (SURF) program, and who will also help publicly communicate the work, will enhance understanding of the fundamental process of basaltic melt generation, and enhance understanding of how ocean islands fit in plate tectonic theory.This project will examine the role of pyroxenite-peridotite or fertile peridotite melting in the generation of ocean island basalts (OIB) through examination of two key OIB localities. The first locality is Mangaia (Cook Islands), where long-lived radiogenic isotope (Sr-Nd-Os-Pb) variations will be coupled with olivine compositions to examine a location without a strong pyroxenite melting signature but that does have a strong recycled crustal signature (HIMU). The goal of this task is to quantify mantle source composition for an extreme recycled endmember composition in OIB petrogenesis. The second task is to evaluate pyroxenite-peridotite contributions to Canary Islands that also have a HIMU signature. Studies have argued for pyroxenite contributions, but with different endmember compositions. Here, new high-precision electron microprobe data will be collected for olivine grains in lavas from the islands to combine with Sr-Nd-Os-Pb isotope variations and highly siderophile element abundances. With these data, it will be possible to discriminate between depleted or enriched pyroxenite sources and to investigate if high pressure-temperature melting of fertile peridotite might offer an alternative. Research will include integration of undergraduates and underrepresented minorities who will also be engaged, along with the PI and graduate student, in outreach events that will take place in the San Diego region and in the Canary Islands, and through participation in organized STEM events.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.