Origin of highly heterogeneous Strontium Isotopic Ratio in melt inclusions from oceanic hotspot lavas

了解与地幔物质羽流（称为热点）相关的火山过程，就像那些形成夏威夷和萨摩亚群岛或冰岛的火山一样，对于了解地球如何运作以及我们如何最好地管理这些岛屿上的资源和危害至关重要。热点衍生的岛链是由从下地幔上升并在上地幔部分融化的热岩石的浮力羽流产生的。其中一部分以熔岩的形式在地球表面喷发。因为这些热点衍生的熔岩是羽流物质的熔体，最终来自地幔的最深处，它们的同位素组成提供了关于地球深处成分组成的关键信息，否则这些信息是无法获得的。对海洋熔岩组成的研究传统上集中在对整个岩石的分析上，在进行任何分析之前，必须对几公斤岩石进行粉末化和均质化。然后分析粉末的同位素组成。另一种获得这些信息的方法是利用被困在熔岩中的微小的熔体气泡，称为熔体内含物。然而，这些包裹体显示出高度不均匀的同位素组成，与大块熔岩明显不同。熔体包裹体中这种极端可变性的起源尚不清楚。本研究的目标是萨摩亚熔岩中个别的橄榄石软管熔融包裹体。这些包裹体将使用新的、最先进的同位素和地球化学分析技术进行全面分析，还将分析锶和钕的同位素。这些地球化学信息都是在同一包裹体上收集的，将揭示萨摩亚熔岩的起源，并有助于解开熔体包裹体所携带的复杂地球化学特征，从而更好地解释它们所携带的信息。这项工作的更广泛影响包括与阿姆斯特丹维尔耶大学的荷兰科学家进行国际合作，培训和指导来自科学领域代表性不足的群体和少数民族的本科生，研究生支持和培训，以及将新技术和地球化学方法转让给美国加州大学圣巴巴拉分校新建立的地球化学设施。海洋岛屿玄武岩的整个岩石地球化学分析一直被认为是了解地幔化学成分的窗口。然而，岩浆斑晶中被困的熔体的气泡，称为熔体包裹体，使这一图景复杂化，因为来自单个熔岩的熔体包裹体可以表现出高度不均匀的87Sr/86Sr比率，这跨越了全球海洋熔岩中观察到的大部分可变性。先前的研究发现，在萨摩亚、夏威夷和冰岛的热点地区收集的熔岩中，橄榄石熔融包裹体中87Sr/86Sr的极端变化。然而，这种差异的来源仍然是一个争论的来源。假说的范围从同位素不同的地幔源的原始岩浆混合到地壳同化的结果不等。本研究从萨摩亚岛提取熔体包裹体，并对其主要元素、微量元素和挥发性（S、Cl和F）浓度进行地球化学分析。还将分析单个熔体包裹体的锶和钕的同位素，以深入了解熔体的起源及其意义。新数据检验了橄榄石含矿熔体87Sr/86Sr变异来源的三种假设：变异是(1)原始幔源熔体混合的结果，(2)浅蚀变洋壳同化的结果，以及(3)深辉长岩洋壳同化的结果。这项工作的意义对于解释熔体包裹体以及它们能和不能告诉我们的关于它们的来源、岩浆过程和地幔的信息具有重要的价值。

Understanding volcanic processes associated with plumes of mantle material, called hotspots, like those that form the Hawaiian and Samoan Islands or Iceland, is critical for understanding both how the Earth works and how we can best manage resources and hazards on these islands. Hotspot-derived island chains are generated by buoyant plumes of hot rock that rise from the lower mantle and partially melt in the upper mantle. A portion of this melt is erupted as lava on Earth's surface. Because these hotspot-derived lavas are melts of plume material, ultimately derived from the deepest portion of the mantle, their isotopic compositions provide critical information about the compositional makeup of the deep Earth, which is otherwise inaccessible. Studies examining the composition of oceanic lavas have traditionally focused on analyses of entire rocks, for which several kilograms have to be powdered and homogenized before any analyses can be done. The powder is then analyzed for its isotopic composition. Another way to get this information is to use tiny blebs of melt, called melt inclusions, that are trapped in the lavas. These inclusions, however, have been found to exhibit highly heterogeneous isotopic compositions that are significantly different from the bulk lava. The origin of this extreme variability in melt inclusions is not well understood. This research targets individual olivine-hosed melt inclusions from Samoan lavas. These inclusions will be comprehensively analyzed using novel, state-of-the-art isotopic and geochemical analytical techniques and will also be analyzed for strontium and neodymium isotopes. This geochemical information, all collected on the same inclusion, will shed light on both the origin of Samoan lavas as well as help to unravel the complex geochemical signature carried by melt inclusions, thus, enabling better interpretation of the message they carry. Broader impacts of the work include international collaboration with Dutch scientists at the Virje University-Amsterdam, training and mentoring of undergraduates from groups and minorities underrepresented in the sciences, graduate student support and training, and the transfer of new technology and geochemical methodology to a newly established geochemical facility in the USA at the University of California, Santa Barbara. Whole rock geochemical analyses of ocean island basalts have long been considered windows into the chemical composition of the mantle. However, blebs of trapped melt in magmatic phenocrysts, called melt inclusions, complicate this picture because melt inclusions from a single lava can exhibit highly heterogeneous 87Sr/86Sr ratios that span much of the variability observed in oceanic lavas globally. Previous work identified extreme 87Sr/86Sr variability in olivine-hosted melt inclusions from lavas collected at the Samoan, Hawaiian, and Icelandic hotspots. However, the origin of this variability remains a source of debate. Hypotheses range from it being the result of mixing pristine magmas from isotopically diverse mantle sources to the result of crustal assimilation. This research takes melt inclusions from the Island of Samoa and geochemically analyzes their major, trace element, and volatile (S, Cl and F) concentrations. The isotopes of strontium and neodymium of individual melt inclusions will also be analyzed to provide insight into the origin of the melts and their meaning. The new data examines three hypotheses for the origin of 87Sr/86Sr variability in olivine-hosted melt inclusions: variability is the result of (1) the mixing of pristine mantle-derived melts, (2) the assimilation of shallow altered oceanic crust, and (3) the assimilation of deep gabbroic oceanic crust. Implications of the work have significant value to interpretation of melt inclusions and what they can and cannot tell us about their source, magmatic processes, and the mantle.