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The accretion and differentiation of Earth under oxidizing conditions
-
R. Bastian Georg
und
Anat Shahar
Veröffentlicht/Copyright:
24. November 2015
Abstract
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 ΔIW ~ -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 ~3.0 to 2.2 wt%, depending on the initial fO₂ 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‰ 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.
Received: 2014-8-7
Accepted: 2015-5-27
Published Online: 2015-11-24
Published in Print: 2015-11-1
© 2015 by Walter de Gruyter Berlin/Boston
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Schlagwörter für diesen Artikel
Terrestrial accretion;
core formation;
Si isotopes;
terrestrial building blocks
Artikel in diesem Heft
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- Trace element partitioning into sulfide: How lithophile elements become chalcophile and vice versa
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- Quasicrystals at extreme conditions: The role of pressure in stabilizing icosahedral Al63Cu24Fe13 at high temperature
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- Ankerite grains with dolomite cores: A diffusion chronometer for low- to medium-grade regionally metamorphosed clastic sediments
- Crystal structure, high-pressure, and high-temperature behavior of carbonates in the K2Mg(CO3)2–Na2Mg(CO3)2 join
- Temperature micro-mapping in oscillatory-zoned chlorite: Application to study of a green-schist facies fault zone in the Pyrenean Axial Zone (Spain)
- Micro- and nano-characterization of Zn-clays in nonsulfide supergene ores of southern Peru
- Mineralogical characterization of individual growth structures of Mn-nodules with different Ni+Cu content from the central Pacific Ocean
- The magma plumbing system of the Emeishan large igneous province and its role in basaltic magma differentiation in a continental setting
- Weathering of the Ethiopian volcanic province: A new weathering index to characterize and compare soils
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- Tetrahedral plot diagram: A geometrical solution for quaternary systems
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- Dislocation microstructures in simple-shear-deformed wadsleyite at transition-zone conditions: Weak-beam dark-field TEM characterization of dislocations on the (010) plane
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