Molecular Geochemistry of Rubidium: Diverse Earth and Planetary Sciences Generated by Physicochemical Universality at the Molecular Level

2022 Fiscal Year Final Research Report

• Project/Area Number: 19K21893
• Research Category: Grant-in-Aid for Challenging Research (Exploratory)
• Allocation Type: Multi-year Fund
• Review Section: Medium-sized Section 17:Earth and planetary science and related fields
• Research Institution: The University of Tokyo
• Principal Investigator: Takahashi Yoshio 東京大学, 大学院理学系研究科(理学部), 教授 (10304396)
• Co-Investigator(Kenkyū-buntansha): 山本 順司 北海道大学, 総合博物館, 准教授 (60378536) ; 板井 啓明 東京大学, 大学院理学系研究科(理学部), 准教授 (60554467)
• Project Period (FY): 2019-06-28 – 2023-03-31
• Keywords: ルビジウム / EXAS / 同位体 / 吸着構造 / 分子地球化学

Outline of Final Research Achievements

Systematic adsorption experiments showed that the Rb concentration in seawater is dominated by the partitioning between seawater and phyllosilicates; EXAFS analyses showed that Rb in natural sediments is also predominantly inner-sphere complexes with phyllosilicates. From these results, the experimentally determined Rb isotopic fractionation coefficients between seawater and phyllosilicates can be applied to the analysis of natural samples. The solid-liquid partition coefficient of Rb was almost constant in the region where the Rb concentration in solution was sufficiently low, and the Rb concentration in seawater fell within this region. The solid-liquid partition coefficient of Rb, which is the dominant factor for δ87Rb under these conditions, is governed by the water-rock ratio (liquid-solid ratio) in the system and could be applied to estimate past variations in the Earth's seawater volume and the water-rock ratio of planets.

Free Research Field

地球化学、環境化学

Academic Significance and Societal Importance of the Research Achievements

Rb同位体比の分別は、層状ケイ酸塩と特異的に反応して同位体分別を引き起こすため、それを利用することで、様々な宇宙地球化学的情報を引き出すことが可能と考えられ、例えば地球の海水量変動史や地殻物質の上部マントルへのリサイクルに関する研究、惑星の水/岩石比の推定などに応用できると期待される。こうした微量元素を用いた同位体及び化学種の地球化学的指標の開発は、宇宙普遍の物理化学法則に基づく分子地球化学の概念によるため、さまざまな条件下での多様なシステムに適用可能であり、地球表層、地球外物質などの地球惑星科学におけるさまざまな地球化学情報を抽出するために活用できる。