JpGU-AGU Joint Meeting 2017

講演情報

[EE] 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-IT22] [EE] 核-マントルの相互作用と共進化

2017年5月20日(土) 13:45 〜 15:15 A05 (東京ベイ幕張ホール)

コンビーナ:土屋 卓久(愛媛大学地球深部ダイナミクス研究センター)、寺崎 英紀(大阪大学大学院理学研究科)、Satish-Kumar Madhusoodhan(Department of Geology, Faculty of Science, Niigata University)、入舩 徹男(愛媛大学地球深部ダイナミクス研究センター)、Hernlund John(Earth-Life Science Institute, Tokyo Institute of Technology)、大谷 栄治(東北大学大学院理学研究科地学専攻)、座長:寺崎 英紀(大阪大学大学院理学研究科)

14:15 〜 14:30

[SIT22-15] Reconciling Magma-Ocean Crystallization Models with the present-day Structure of the Earth’s mantle

★招待講演

*Maxim Ballmer1,2Diogo Lourenco1Kei Hirose2,3Razvan Caracas4Ryuichi Nomura2 (1.Institut für Geophysik, ETH Zürich, Sonneggstrasse 5, 8092 Zurich, Switzerland、2.Earth-Life Science Institute, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan、3.Laboratory of Ocean-Earth Life Evolution Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa 237-0061, Japan、4.Laboratoire de Géologie de Lyon, Université Claude Bernard Lyon 1, Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, 69342 Lyon Cedex 07, France,)

キーワード:Magma Ocean, Large-Low Shear Wave Velocity Province, Lower Mantle

Terrestrial planets are thought to experience episode(s) of large-scale melting early in their history. Studying the crystallization and fractionation of terrestrial magma oceans (MO) can provide constraints for the initial condition and thermochemical evolution of solid-state mantle convection. MO fractionation leads to unstable stratification within the cumulate layer due to progressive iron enrichment upwards, but the effects of incremental cumulate overturn that may occur during MO crystallization remain to be quantitatively explored. Here, we use geodynamic models with a moving-boundary approach to study convection and mixing within the growing cumulate layer, and thereafter within the resulting, fully-crystallized mantle. For fractional crystallization, pronounced stratification leads to incremental cumulate overturns during MO freezing and hence efficient cumulate mixing, except for the most iron-enriched final-stage cumulates, which remain unmixed and persist for billions of years near the base of the mantle. Less extreme crystallization scenarios can lead to somewhat more subtle stratification and more pervasive mixing. However, MO cooling models indicate that fractional crystallization should have been dominant at least during the slow final stages of MO freezing. The long-term preservation of strongly iron-enriched cumulates at the base of the Earth’s mantle as predicted by MO fractional-crystallization models is inconsistent with seismic constraints. Based on scaling relationships, however, we infer that final-stage Fe-rich MO cumulates should thermally equilibrate during overturn and sinking, and hence undergo melting and reaction with the host rock. The resulting moderately iron-enriched hybrid rock assemblages should be preserved in the deep mantle through the present day. In contrast to the original strongly-enriched final-stage cumulates, moderately iron-enriched hybrid rock assemblages can much better reconcile the physical properties of the large low shear-wave velocity provinces in the present-day lower mantle. Thus, we reveal Hadean melting and rock-reaction processes by integrating simplified MO crystallization models with the present-day seismic snapshot.