ABSTRACT

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 the
mechanisms 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.

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