Magnesium Isotopic Investigations of Oceanic Basalts and Olivines

智力上的功绩。地球的镁同位素组成可能会对早期太阳系的形成和演化造成强烈的制约，但镁同位素系统学仍然鲜为人知，对现有数据的解释也存在争议。对地幔岩浆(如大洋玄武岩)的研究不仅可以揭示地球的组成，还可以揭示地壳循环产生的地幔不均一性。在这里，PI建议在全球范围内研究大洋中脊玄武岩(MORBs)和洋岛玄武岩(OIB)中稳定的镁同位素变化。以前的研究已经证明，镁同位素在玄武岩分异过程中不会分馏，因此玄武岩可以用来限制其来源的同位素组成。到目前为止，镁同位素数据仍然非常稀少，仅有约20种玄武岩分析：其中大部分来自夏威夷基拉韦厄Iki熔岩湖，其余为玄武岩标准。在这项研究中，将分析46个MORBs和63个OIB的镁同位素。所有样品都已由PI和同事进行了铁同位素分析。将对一等分的原液进行镁同位素分析。这些研究将有助于更好地约束全球大洋玄武岩中的镁同位素变化(如果有的话)，澄清地壳物质在造成地幔不均匀中的作用，并提供对块体地球镁同位素组成的估计。玄武岩和橄榄石中镁同位素变化的研究也有助于揭示高温下稳定同位素的分馏机制。最近的实验研究发现，在化学和热扩散过程中，镁和铁的同位素分馏显著。在夏威夷的天然玄武岩和橄榄石中也观察到了显著的Fe同位素变化；这要么被解释为由于层析效应而增强的平衡同位素分馏，要么被解释为与热扩散和化学扩散有关的动力学同位素分馏。对同一橄榄石颗粒的镁同位素分析将有助于阐明这一过程；即，橄榄石颗粒中缺乏镁同位素变化表明存在平衡的同位素分馏。因此，这里将测量与铁同位素研究中使用的等电点相同的橄榄石颗粒(N=77)的镁同位素组成。此外，还将对几个大的橄榄石颗粒进行微钻孔，并对其进行镁、铁同位素和铁/镁比的分析。这些现场分析将进一步研究在岩浆分异过程中，哪些过程分馏了铁(镁)同位素。这项拟议的工作是PI在芝加哥和马里兰州博士后和论文工作的后续工作。这项提议将使国际和平研究所能够首次在大学建立最先进的地球化学设施。阿肯色州。阿肯色州周围的学生和社区成员将有机会参观实验室，了解分析方法的背景，并分析样品和标准。这项建议将支持一名全日制博士生和几名本科生研究助理，其中一些人预计将从事镁同位素研究的高级论文，最终，这项建议将帮助PI在阿肯色州发展自己的研究小组。

Intellectual Merit. Magnesium isotopic composition of the Earth can potentially place strong constraints on the formation and evolution of the early solar system but Mg isotope systematics is still poorly known and interpretation of existing data under debate. Studies of mantle-derived magmas (e.g., oceanic basalts) can shed light not only on the composition of the Earth but also on mantle heterogeneity produced by crustal recycling. Here, the PI proposes to investigate stable Mg isotopic variations globally, in well-characterized midoceanic ridge basalts (MORBs) and oceanic island basalts (OIBs). Previous studies have documented that Mg isotopes do not fractionate during basalt differentiation, hence basalts can be used to constrain the isotopic composition of their sources. To date, Mg isotopic data are still very sparse with only ~20 analyses available for basalts: most of them are from Kilauea Iki lava lake, Hawaii while the others are basalt standards. In this study, Mg isotopes will be analyzed for 46 MORBs and 63 OIBs. All samples have been analyzed for Fe isotopes by the PI and coworkers. An aliquot of the stock solution will be analyzed for Mg isotopes. These studies will help to better constrain Mg isotopic variations (if any) in global oceanic basalts, clarify the role of crustal material in creating mantle heterogeneity and provide estimates on Mg isotopic composition of the bulk Earth.Studies of Mg isotopic variations in basalts and olivines can also help to unravel themechanisms that fractionate stable isotopes at high temperatures. Recent experimental studies have found significant Mg and Fe isotope fractionation during chemical and thermal diffusion. Significant Fe isotopic variations have also been observed in natural basalts and olivines from Hawaii; this has been interpreted as either equilibrium isotope fractionation enhanced by chromatographic effects or kinetic isotope fractionation associated with thermal and chemical diffusion. Analyses of Mg isotopes for the same olivine grains will help to elucidate the process; i.e., a lack of Mg isotopic variation in olivine grains indicates an equilibrium isotope fractionation. Therefore, Mg isotopic compositions for the same olivine grains (N = 77) that the PI used in Fe isotopic studies will be measured here. In addition, a few large olivine grains will also be micro-drilled and analyzed for Mg and Fe isotopes and Fe/Mg ratios. These in-situ analyses will further examine which process fractionated Fe (Mg) isotopes during magmatic differentiation.Broader Impact. The proposed work is a follow-up of the PI's postdoc and dissertation work at Chicago and Maryland. This proposal will enable the PI to establish for the first time a state of the art geochemical facility at Univ. of Arkansas. Students and members of the community around Arkansas will have a chance to visit the laboratory to gain background in analytical methods and to analyze samples and standards. This proposal will support a full-time PhD student and several undergraduate research assistants, some of whom are expected to undertake their senior theses on Mg isotopic studies, and ultimately, this proposal will help the PI to develop his own research group at Arkansas.