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，对于理解地球如何运作，以及我们如何最好地管理这些岛屿上的资源和灾害，都是至关重要的。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柱产生的。这些熔岩的一部分以熔岩的形式喷发在地球表面。由于这些来自热点的熔岩是羽状物质的熔体，最终来自地幔最深处，它们的同位素组成提供了有关地球深处成分的关键信息，否则无法访问。对海洋熔岩成分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，必须将几公斤的岩石粉碎并均质。然后对粉末进行同位素组成分析。获取这一信息的另一种方法是使用被困在熔岩中的被称为熔体包裹体的微小熔滴。然而，这些包裹体已被发现表现出与大量熔岩显著不同的高度不均匀的同位素组成。熔体包裹体中这种极端的变异性的起源还不清楚。这项研究的目标是来自萨摩亚熔岩的单个橄榄石软管熔体包裹体。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综合分析，并将进行锶和钕同位素分析。这些地球化学信息全部收集在同一包体上，将有助于阐明萨摩亚熔岩的起源，并有助于解开熔体包裹体携带的复杂地球化学特征，从而能够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Virje大学的荷兰科学家开展国际合作，对科学界代表性较低的群体和少数族裔的本科生进行培训和指导，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巴巴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，被困在岩浆斑晶中的熔体气泡，称为熔体包裹体，使这一图景变得复杂，因为来自单一熔岩的熔体包裹体可以表现出高度不均匀的87Sr/86Sr比值，这一比值跨越了全球大洋熔岩中观察到的大部分变异性。以前的工作发现，在萨摩亚、夏威夷和冰岛热点采集的熔岩中，以橄榄石为宿主的熔体包裹体具有极高的87Sr/86Sr变异性。然而，这种差异的来源仍然是一个争论的来源。从它是来自同位素不同地幔来源的原始岩浆混合的结果，到地壳同化的结果，假说范围很广。本研究采集了萨摩亚岛上的熔体包裹体，并对其主要元素、微量元素和挥发分(S、氯和氟)进行了地球化学分析。还将分析单个熔体包裹体的锶和钕的同位素，以深入了解熔体的来源及其意义。新数据检验了橄榄石熔体包裹体中87Sr/86Sr变异性的三种来源假说：变异性是(1)原始地幔熔体混合的结果，(2)浅层蚀变洋壳的同化，(3)深辉长岩洋壳的同化。这项工作的意义对于解释熔融包裹体以及它们能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.