Earth and Planetary Sciences ETDs

Publication Date

7-10-2013

Abstract

The interior structure of Earth, particularly the metal core, is responsible for the core dynamo and thus our magnetic field and supplying the mantle with heat to encourage solid state convection and thus decompression melting and hot spot volcanism. Therefore determining how Earth's core formed and if that process was unique to our planet in the solar system is of great scientific interest. Core formation can be studied experimentally by examining the metal-silicate partitioning behavior of siderophile elements and using the results to explain their observed upper mantle depletions relative to bulk Earth abundances. This study investigated the partitioning behavior of the moderately siderophile elements molybdenum and tungsten by conducting experiments using multi-anvil presses and obtaining compositional analyses of the run products using electron probe microanalysis. Molybdenum was found to dissolve as Mo4+ in silicate melts, whereas tungsten dissolved as W6+. The partition coefficients [Di = ci(metal)/ci(silicate)] for both molybdenum and tungsten decrease with increasing pressure; however, DW increases slightly with increasing temperature whereas DMo decreases. Both elements become less siderophile as silicate melt polymerization decreases. The addition of carbon to the metal phase causes DMo and DW to increase, while addition of sulfur causes DMo and DW to decrease. Parameterization of the data from this study and literature data allowed for core formation modeling to determine what conditions could explain Earth's mantle abundances of molybdenum, tungsten, and nickel (the most extensively studied element). The modeling suggests that the abundances of these elements were set by a global magma ocean near the end of accretion with conditions of 35-37 GPa (~1100 km depth) and 2950-3000 K with XC = 0.07, XS = 0.05, and XSi = 0.06. Thus implying that large impacts in the late stages of Earth's accretion were energetic enough to re-equilibrate the already differentiated metal and silicate, leaving the distinct chemical signature of a single equilibration event in the mantle. Applying the parameterizations to other differentiated bodies for which we have compositional data indicates that magma oceans were common occurrences in the early solar system, but each body underwent a unique differentiation history.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Brearley, Adrian

Second Committee Member

Shearer, Charles

Third Committee Member

McCubbin, Francis

Fourth Committee Member

Li, Jie

Language

English

Document Type

Dissertation

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