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，如形成夏威夷群岛、萨摩亚群岛或冰岛的地幔物质羽流）相关的火山过程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升起并在上地幔部分融化的热岩浮羽形成的。一部分融化物在地球表面以熔岩的形式喷发。由于这些源自热点的熔岩是羽流物质的熔体，最终源自地幔的最深部分，因此它们的同位素成分提供了有关地球深处成分构成的关键信息，而这是其他方式无法获得的。传统上，对海洋熔岩成分的研究主要集中于对整个岩石的分析，在进行任何分析之前，必须将数公斤的岩石磨成粉末并均质化。然后分析粉末的同位素组成。 获取此信息的另一种方法是使用被困在熔岩中的微小熔体气泡，称为熔体包裹体。然而，人们发现这些包裹体表现出高度异质的同位素组成，与大块熔岩显着不同。熔体夹杂物这种极端变化的根源尚不清楚。这项研究的目标是来自萨摩亚熔岩的单个橄榄石熔体包裹体。这些包裹体将使用新颖、最先进的同位素和地球化学分析技术进行全面分析，还将分析锶和钕同位素。这些地球化学信息全部收集在同一包裹体上，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荷兰阿姆斯特丹维尔耶大学科学家的国际合作、对科学领域代表性不足的群体和少数族裔的本科生的培训和指导、研究生支持和培训，以及将新技术和地球化学方法转移到美国加州大学圣塔芭芭拉分校新建立的地球化学设施。长期以来，洋岛玄武岩的全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，岩浆斑晶中被困熔体的气泡（称为熔体包裹体）使这一情况变得复杂，因为来自单个熔岩的熔体包裹体可以表现出高度异质的 87Sr/86Sr 比率，该比率跨越了在全球海洋熔岩中观察到的大部分变化。先前的研究发现，在萨摩亚、夏威夷和冰岛热点地区收集的熔岩中，橄榄石熔体包裹体中的 87Sr/86Sr 存在极大的变异性。 然而，这种变异的起源仍然是一个争论的焦点。假设范围从同位素不同的地幔来源的原始岩浆混合的结果到地壳同化的结果。这项研究采用了来自萨摩亚岛的熔体包裹体，并对其主要元素、微量元素和挥发性元素（S、Cl 和 F）浓度进行了地球化学分析。还将分析各个熔体包裹体的锶和钕同位素，以深入了解熔体的起源及其含义。新数据检验了关于橄榄石熔体包裹体中 87Sr/86Sr 变异性起源的三种假设：变异性是以下因素的结果：(1) 原始地幔衍生熔体的混合，(2) 浅层蚀变洋壳的同化，以及 (3) 深部辉长岩洋壳的同化。这项工作的意义对于解释熔体包裹体以及它们能够和不能告诉我们关于它们的来源、岩浆过程和地幔的信息具有重要价值。

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.