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

了解与地幔物质羽流相关的火山过程，即所谓的热点，例如形成夏威夷和萨摩亚群岛或冰岛的热点，对于了解地球的运作方式以及我们如何最好地管理这些岛屿上的资源和危险至关重要。热点衍生的岛链是由热岩的浮力羽流产生的，这些热岩从下地幔上升，并在上地幔部分熔化。这些熔岩的一部分在地球表面以熔岩的形式喷发出来。由于这些热点衍生的熔岩是地幔柱物质的熔融物，最终来自地幔的最深处，它们的同位素组成提供了有关地球深部组成的关键信息，否则无法获得。传统上，研究海洋熔岩的成分主要集中在对整个岩石的分析上，在进行任何分析之前，必须将几公斤的岩石磨成粉末并均匀化。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使用被困在熔岩中的微小的熔体气泡，称为熔体包裹体。然而，这些包裹体，已被发现表现出高度不均匀的同位素组成，是显着不同的散装熔岩。熔体包裹体这种极端变异性的起源还没有很好的理解。这项研究的目标是从萨摩亚熔岩的个别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，还将进行锶和钕同位素分析。这些地球化学信息都收集在同一个包裹体上，将揭示萨摩亚熔岩的起源，并有助于解开熔融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开展国际合作，培训和指导在科学领域代表性不足的群体和少数群体的本科生，支持和培训研究生，以及向美国加州大学圣巴巴拉新设立的地球化学设施转让新技术和地球化学方法。洋岛玄武岩的全岩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，气泡被困在岩浆斑晶熔体，称为熔体包裹体，复杂的这一情况，因为熔体包裹体从一个单一的熔岩可以表现出高度不均匀的87 Sr/86 Sr的比例，跨越了大部分的变化，在全球海洋熔岩。以前的工作确定了极端的87 Sr/86 Sr的变化，橄榄石托管熔融包裹体从熔岩收集在萨摩亚，夏威夷和冰岛的热点。 然而，这种差异性的起源仍然是一个争论的来源。各种假设从它是混合原始岩浆同位素不同的地幔来源的结果，地壳同化的结果。本研究采取熔融包裹体从萨摩亚岛和地球化学分析其主要，微量元素和挥发性（S，Cl和F）的浓度。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熔融包裹体中87 Sr/86 Sr变化起源的三种假设：变化是（1）原始地幔熔融混合的结果，（2）浅蚀变洋壳的同化，（3）深部辉长岩洋壳的同化。研究结果对解释熔融包裹体的成因、岩浆作用和地幔等具有重要意义。

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.