This thesis is focused on two aspects: one is to obtain high precision measurement of rhenium isotope composition of iron meteorites; the other is to discuss what processes cause systematic isotopic differences in Re between IVB irons and other irons and chondritic metal. As Re has only two isotopes (187Re and 185Re), it is hard to precisely correct the instrumental mass bias. W and Re have similar masses and first ionization potentials, so we used W as an internal standard to correct mass bias on the Multicollector-Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS). The Re isotope ratio was measured with a precision of ~0.02 permil(2SE) at concentration of 100 ppb. Re and W hydrides were identified which interfered with the accurate determination of Re isotopic composition. The Re/W ratio was maintained at a constant value (185Re/184W=0.75) in all samples and standards to minimize this effect. Also, tests have been performed to study if Re fractionates during High-Pressure Asher digestion and to assess the effect of mass dependent fractionation induced in Re during column chromatographic separation. We report precise new d187Re values for six iron meteorites, and combine these with previous data to find that IVB irons exhibit ~0.14 permil Re isotope excess compared to other iron meteorites and chondritic metal. Cosmogenic effects, nucleosynthetic anomalies and mass fractionation are evaluated as possible mechanisms to explain this observation. 185Re has high neutron capture cross section and will be burned out by galactic cosmic rays (GCR) irradiation which increases d187Re. Variations in stable Pt and Os isotope ratios were employed as in situ neutron dosimeters to correct this effect in IVB irons. The pre-GCR irradiation d187Re is lowered, but not enough to explain the deficiency. Nucleosynthetic anomalies reflecting an s-process deficit have been observed in IVB irons on many elements (Mo, Ru, Pd, etc.). Molybdenum nucleosynthetic anomalies were used to model how the s-deficit may have impacted Re isotopes assuming that the s-process deficit was the same in the two elements. 185Re has 75 % r-process nucleosynthetic origin, and 187Re has ~100 % r-process origin so that the modeled s-deficit for Re is only ~0.03 permil, essentially unresolvable. The lack of mass-dependent fractionation on Cu and Mo in IVB irons preclude the possibility that Re fractionates in this way. Nuclear volume-dependent fractionation is controlled by electronic structure and proportional to 1/T, obvious in heavy elements (Hg, Tl, U etc.). Re isotopic variation in terrestrial samples is reported to be ~1 permil. Using this value as a proxy, the modeled nuclear volume-dependent fractionation for Re is 0.17 permil at 1800 K, which is comparable with the observed effect. However, Re as a refractory element is likely to be quantitatively condensed so fractionation between gaseous Re species and the condensed phases is not likely to occur; also Re is a highly siderophile element nearly quantitatively partitioned into the metallic liquid it is difficult to find a second phase with which to exchange isotopes. If metallic solids were isotopically distinct from metallic liquids then Re isotope fractionation is expected to occur during fractional crystallization, which is not observed in the IVBs, the most extensively studied group of irons. Thus, the Re isotopic difference between IVBs and other irons or chondritic metal remains unexplained. / A Thesis submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the Master of Science. / Spring Semester, 2015. / April 3, 2015. / cosmogenic effects, iron meteorites, mass fractionation, nucleosynthetic anomalies, rhenium isotope / Includes bibliographical references. / Munir Humayun, Professor Directing Thesis; Leroy Odom, Committee Member; Vincent J. Salters, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_252992 |
| Contributors | Liu, Rui, 1988- (authoraut), Humayun, Munir (professor directing thesis), Odom, A. L. (A. Leroy) (committee member), Salters, Vincent J. M. (committee member), Florida State University (degree granting institution), College of Arts and Sciences (degree granting college), Department of Earth, Ocean, and Atmospheric Science (degree granting department) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource (52 pages), computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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