A Mixed Up Mantle Beneath Ocean Islands?

玄武岩火山岛存在于所有主要洋盆中，并不特别适合板块构造理论。来自这些岛屿的熔岩与在大洋中脊喷发的玄武岩具有不同的成分，表明它们的一些地幔来源中存在俯冲洋壳。目前的洋岛玄武岩形成模式要么支持两种不同岩石类型的地幔来源--辉石岩和橄榄岩--部分熔融，要么是来源中只有“富饶的”橄榄岩容易部分熔融。这些模型是由这样一个观察驱动的，即再循环洋壳的直接部分熔融不能产生玄武岩成分。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融形成的。这些源岩的形成和熔融细节是完全不同的，辉石岩表明了大规模的地幔不均一性。一种公认的探测辉石岩的方法是通过精确测量玄武岩中的橄榄石成分。像Os这样的贵金属也可以用来研究橄榄岩和辉石岩为主的熔融之间的关系，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结壳成分的地点--库克群岛的芒加亚和加那利群岛--来测试大洋岛屿来源中是否存在辉石岩。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)项目产生，他们也将帮助公开交流这项工作，这将增强对玄武岩熔体产生的基本过程的理解，并增强对洋岛如何适应板块构造理论的理解。该项目将通过对两个关键的洋岛玄武岩(OIB)地点的研究，检验辉石岩-橄榄岩或富饶的橄榄岩熔融在洋岛玄武岩(OIB)产生中的作用。第一个地点是Mangaia(库克群岛)，在那里，长寿命的放射性同位素(Sr-ND-Os-Pb)变化将与橄榄石成分相结合，以检查没有强烈辉石岩熔融特征但确实具有强烈再循环地壳特征(HIMU)的位置。这项任务的目标是量化OIB成岩作用中极端再循环端元组成的地幔来源成分。第二项任务是评估辉石岩-橄榄岩对加那利群岛的贡献，这些贡献也具有HIMU特征。研究认为辉石岩有贡献，但端元组成不同。在这里，将收集新的高精度电子探针数据，从岛屿熔岩中的橄榄石颗粒，结合锶-钕-Os-铅同位素变化和高度亲铁元素丰度。有了这些数据，就有可能区分贫化或浓缩的辉石岩来源，并调查可育橄榄岩的高压-高温熔融是否可能提供替代方案。研究将包括本科生和代表不足的少数群体，他们也将参与在圣地亚哥地区和加那利群岛举行的外联活动，并通过参与有组织的STEM活动来参与。该奖项反映了NSF的法定使命，并通过使用基金会的智力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.