超深層のダイヤモンドに記録された石層から下層のマントルへの炭素循環
M E Regier1, D G Pearson2, T Stachel2
1Canadian Centre for Isotopic Microanalysis, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada. margoregier@gmail.com.
Nature
|September 10, 2020
まとめ
沈殿物ではなく 炭酸化海殻が 深い地層の炭素の 主な源です ダイヤモンドインクルージョンは,この炭素を移行領域に吸収し,ダイヤモンドと下層マントルの炭素バリアを形成します.
科学分野:
- 地化学
- ミネラロジー
- 地質学
背景:
- 地球のマントルへの炭素輸送は,気候と酸化還元状態に影響を及ぼし,炭素循環にとって極めて重要です.
- 炭素貯蔵庫 (沈殿物,海殻) の沈殿深さや,マントルの深層部における炭素収縮への貢献については,不確実性がある.
- サブリトスフィアのダイヤモンド (深さ >250 km) は,地球深層のプロセスと炭素輸送の洞察を提供します.
研究 の 目的:
- 地球のマントルの炭素貯蔵庫の起源と潜水経路を調査する
- 沈殿物や海殻が潜水中の主要な炭素媒介者かどうかを決定する.
- 炭素沈殿の深さを制限し,深層マントルの炭素収縮におけるその役割を制限する.
主な方法:
- 超深層のダイヤモンド (リトスフィアからマントル下部) の鉱物含有物の酸素同位体測定.
- ホストダイヤモンドの炭素と窒素同位体の分析
- 亜岩層と岩層のダイヤモンドインクルージョンの同位体シグネチャーの比較
主要な成果:
- 沈殿物ではなく,炭化岩石の海殻は,移行地帯の深さ (<660 km) に沈殿されたスラブの支配的な炭素貯蔵庫です.
- サブリトスフィアのインクルージョンは18Oで濃縮され,炭酸塩に富んだ海殻の融解から結晶化を示しています.
- 下層マントルのインクルージョン (> 660 km) は,変化しないマントルの典型的な同位体値を示し,同位体干渉なしに炭素の動員とダイヤモンドの形成を示唆している.
結論:
- 沈殿した海殻は 移行地帯に閉じ込められた炭素の 主な源です
- 炭酸板の溶解 (移行区) から板の脱水 (下層) への移行は,深層の炭素沈殿に障壁を生じます.
- 炭素は動員され,マントルの下部に濃縮され,周囲のマントルの同位体シグネチャを変化させずにダイヤモンドを形成します.
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