The high temperature geochemistry of vanadium: From isotope ratios in ocean island basalt to thermal diffusion processes at the core-mantle boundary

Lin, Xiaobao

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https://hdl.handle.net/2142/113295 Copy

Description

Title

The high temperature geochemistry of vanadium: From isotope ratios in ocean island basalt to thermal diffusion processes at the core-mantle boundary

Author(s)

Lin, Xiaobao

Issue Date

2021-07-15

Director of Research (if dissertation) or Advisor (if thesis)

Lundstrom, Craig C.

Doctoral Committee Chair(s)

Lundstrom, Craig C.

Committee Member(s)

• Johnson, Thomas M.
• Liu, Lijun
• Stewart, Michael

Department of Study

Geology

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Vanadium Isotope
• Ocean Island Basalt
• XANES
• Thermal diffusion
• Basal Magma Ocean

Abstract

Vanadium (V) is a redox-active transition metal used widely to understand terrestrial magmatic processes. Measurements of the V isotopic composition of peridotites, the dominant rock of upper mantle, and primitive meteorites show significant offset with the peridotites heavier by ~0.2 ‰, the largest offsets among all metal stable isotope systems. If primitive meteorites represent the primary building blocks of the Earth, Earth’s interior must have a complementary “hidden reservoir” having light V isotope ratio. The two mostly likely reservoirs are the Earth’s deep mantle and its core. To assess the first possibility, I measured the V isotopic composition of ocean island basalts (OIB) which potentially sample materials from the lower mantle rather than upper mantle as represented by mid-ocean ridge basalt (MORB). While there is some variability due to magmatic differentiation processes, the average δ51V of OIB is -0.85 ± 0.13‰ (Two standard deviation (2SD)), identical to that of MORB (0.84 ± 0.10‰, 2SD, Wu et al., 2018). Based on the similarity of MORB and OIB, the mantle appears largely homogeneous and thus the δ51V offset between the upper mantle and chondrites implies the core may be the hidden light reservoir within the Earth. To understand V isotopic fractionation at high temperature, I performed temperature gradient piston cylinder experiments using basaltic melt to probe the melt structure and oxygen fugacity variations during thermal diffusion. Thermal diffusion is a mass diffusion process driven by a temperature gradient which can generate substantial isotopic fractionation in silicate melts, causing heavy isotopes to be enriched at a gradient’s cold end. This could play a role in Earth’s compositional and isotopic evolution given that the core-mantle boundary is the biggest thermal boundary layer (TBL) on earth. However, the valence state of multi-valent elements and thus oxygen fugacity during the thermal diffusion process is largely unconstrained. Trace element and X-ray absorption near-edge spectroscope (XANES) results show: 1). The relationship between ionic size and Soret coefficient of trace element groups with different charge indicates the importance of valence state in thermal diffusion experiments; 2). XANES spectra of basaltic glass from graphite capsule experiments indicate that Fe and V are predominately +2 and +3, respectively, and that compositional effects on Fe XANES spectra are stronger than previously thought as temperature dependent variation of its centroid energy is seen in silicic melt diffusion experiments; 3) the V thermal diffusion isotopic sensitivity is 0.0088 ± 0.0003‰ per degree per AMU fractionation at steady state showing that heavy V isotopes are enriched at the cold end of thermal gradients. Based on this finding, I developed an “ISOTOPE PUMP” model to explain the observed isotopic offsets between BSE and chondrites for siderophile (iron-loving) elements like Fe and V, Model results for these elements were compared to lithophile (rock-loving) elements like Mg and Ca. This model predicts the thermal diffusion isotopic fractionation at the thermal boundary layer between the solid mantle (cold end) and liquid core/basal magma ocean (hot end). Thermal diffusion within the TBL would cause heavy isotopes to be incorporated into the solid mantle and light isotopes to partition into the core. This results in the silicate Earth gradually becoming isotopically heavier as light isotopes are stored in the core for siderophile elements. Since the core is not a reservoir for lithophile elements, their isotopic composition remains similar to the bulk Earth and thus similar to chondrites, broadly agreeing with the observations.

Graduation Semester

2021-08

Type of Resource

Thesis

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http://hdl.handle.net/2142/113295

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Copyright 2021 Xiaobao Lin

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