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The accretion and differentiation of Earth under oxidizing conditions

R. Bastian Georg and Anat Shahar
The accretion and differentiation of Earth under oxidizing conditions
American Mineralogist (December 2015) 100 (11-12): 2739-2748


We present a new approach to model planetary accretion and continuous core formation, and discuss the implications if Earth accreted under conditions initially more oxidized than the modern day mantle. The modified model uses the same partitioning data that were previously used to model accretion under reducing conditions, however, changing the partitioning between accreting metal and silicate mantle means that reducing conditions fail to meet expected core/mantle values. Instead, the model requires conditions more oxidized than the modern day mantle to converge and to yield expected elemental core/mantle distribution values for moderately siderophile elements. The initial oxygen fugacity required to provide the crucial level of oxidation is approximately Delta IW approximately -1.2 to -1.7 and thus is in the range of carbonaceous and ordinary chondrites. The range of peak pressures for metal silicate partitioning is 60-6 GPa and oxygen fugacity must decrease to meet modern FeO mantle contents as accretion continues. Core formation under oxidizing conditions bears some interesting consequences for the terrestrial Si budget. Although the presented partitioning model can produce a Si content in the core of 5.2 wt%, oxidizing accretion may limit this to a maximum of approximately 3.0 to 2.2 wt%, depending on the initial f (sub O2) in BSE, which places bulk earth Mg/Si ratio between 0.98-1.0. In addition, under oxidizing conditions, Si starts partitioning late during accretion, e.g., when model earth reached >60% of total mass. As a consequence, the high P-T regime reduces the accompanied isotope fractionation considerably, to 0.07 ppm for 5.2 wt% Si in the core. The isotope fractionation is considerably less, when a maximum of 3.0 wt% in the core is applied. Under oxidizing conditions it becomes difficult to ascertain that the Si isotope composition of BSE is due to core-formation only. Bulk Earth's Si isotope composition is then not chondritic and may have been inherited from Earth's precursor material.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 100
Serial Issue: 11-12
Title: The accretion and differentiation of Earth under oxidizing conditions
Affiliation: Trent University, Water Quality Centre, Peterborough, ON, Canada
Pages: 2739-2748
Published: 201512
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 52
Accession Number: 2016-007461
Categories: Solid-earth geophysicsIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
Secondary Affiliation: Carnegie Institution of Washington, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 201604
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