日本地球惑星科学連合2022年大会

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[E] 口頭発表

セッション記号 S (固体地球科学) » S-IT 地球内部科学・地球惑星テクトニクス

[S-IT21] 惑星中心核:内部構造・形成・進化

2022年5月22日(日) 09:00 〜 10:30 103 (幕張メッセ国際会議場)

コンビーナ:寺崎 英紀(岡山大学理学部)、コンビーナ:大谷 栄治(東北大学大学院理学研究科地学専攻)、McDonough William F(Department of Earth Science and Research Center for Neutrino Science, Tohoku University, Sendai, Miyagi 980-8578, Japan)、コンビーナ:飯塚 理子(東京大学大学院理学系研究科地殻化学実験施設)、座長:寺崎 英紀(岡山大学理学部)、飯塚 理子(東京大学大学院理学系研究科地殻化学実験施設)


10:00 〜 10:15

[SIT21-05] Sulfur content of the Earth and its core

*William F McDonough1,2Takashi Yoshizaki1 (1.Department of Earth Science and Research Center for Neutrino Science, Tohoku University, Sendai, Miyagi 980-8578, Japan、2.Department of Geology, University of Maryland, College Park, Maryland 20742, USA)

キーワード:Earth, core, Pb concentration

Compositional models of the Earth are built on physical and chemical data for the planet and compositional systematics in chondritic meteorites. Chondrites, the building blocks of the terrestrial planets, do not, however, match the Earth's bulk composition. To determine the core's composition we must combine cosmochemical observations with geophysical and geochemical data for the Earth. By defining the composition of the Earth and its layers, we are better equipped to understand its origin and evolution.

At half a radius and a third of it's mass, the Earth's metallic liquid core creates a protective magnetosphere through dynamo action. The core's density does not matching that of an Fe,Ni alloy at core conditions and thus has a density deficit, meaning it contains lighter element(s) to account for its ~10% lower density.

Debate continues regarding the composition and proportion of light element(s) in the Earth's core. It is commonly asserted that the core contains only ≦2 wt% S, plus other elements. This prediction for the S abundance in the core was based upon the Earth's depletion in moderately volatile, lithophile elements, those not partitioned into the core.

Here, we characterized systematic differences between lithophile and non-lithophile (siderophile and chalcophile) elements in chondrites and predict the bulk Earth and its core contains 2.1±0.3 and 6.5±0.8 wt% sulfur, respectively. We propose a 3.3 times increase in the core's S content relative to traditional estimates, which consequently lowers the core's contents of Si (~1 wt%) and/or O (~1 wt%) to compensate for its density deficit. The mean atomic weight for this core composition (24.2) is consistent with that recommended by Birch (1968). These same systematics of siderophile and chalcophile elements in chondrites can be used to constrain the sulfur contents of other terrestrial planets.

Changes to the abundance estimates of other moderately volatile and volatile siderophile elements include the core's Pb abundance and its implications for the lead paradox of the BSE. From this we estimate that the bulk Earth and core contains 0.46±0.13 and 1.1±0.36 microg/g Pb, respectively, assuming 0.15±0.01 microg/g in the BSE.