講演情報
[T1-O-8]アルマンディンざくろ石の局所U–Pb年代で読み解く海洋地殻の冷却時間：中米グアテマラ産ローソン石エクロジャイトの例★「日本地質学会学生優秀発表賞」受賞★
*福島 諒1、辻森 樹1、ベラノアグイレ アラッツ2、アルバート リチャード2、ゲルデス アクセル2、マーシャル ホルスト2 (1. 東北大学、2. フランクフルト大学)
【ハイライト講演】変成作用の年代を決定することは，地球のテクトニックな歴史や物質循環の解読のために非常に重要である．この講演では，世界最先端の分析技術を活用し，従来計測対象外であった鉱物（鉄に富む柘榴石）から変成作用の年代を読み解くことに成功している．この結果を元に，地球のダイナミックな歴史がこれまで以上の解像度で議論されることが期待される．（ハイライト講演とは．．．）
キーワード：
アルマンディンざくろ石、局所U–Pb年代測定、ローソン石エクロジャイト、スラブ年代、ジルコン
Lawsonite eclogite has received attention due to its high H2O retainability, thereby contributing to volatile input into the deep Earth via oceanic plate subduction. Because the pressure–temperature (P–T) stability field of lawsonite is limited to high-P and low-T conditions along a cold thermal gradient (~5–6°C/km), continuous subduction of lawsonite eclogite would require either significant cooling of the oceanic lithosphere before subduction or extensive subduction maturation [1]. However, scarce geochronological data of natural lawsonite eclogites that include both the protolith and metamorphic ages inhibit the quantitative validation of this idea.
Here, we report metamorphic and protolith ages of lawsonite eclogite from the South Motagua Mélange, Guatemala. These samples ubiquitously include well-preserved lawsonite grains both as inclusions in garnet and in the rock matrix, demonstrating that garnet-forming reactions occurred within the lawsonite stability field. For the determination of the eclogite-facies metamorphic age, we performed LA-MC-ICPMS U–Pb dating of almandine-rich garnet grains [2,3] in four thin sections representing four different rock samples. Forty-four spot analyses of the inclusion-free rims in one of the samples plot on a well-defined regression line with a lower intercept of 137 ± 4 Ma. The garnet U–Pb ages from the other samples overlap this value within uncertainty. As for the protolith age, we performed LA-HR-ICPMS U–Pb dating of magmatic zircon in one of the samples, and obtained a concordia age of 187 ± 2 Ma. Hence, the protolith–metamorphic age difference (Δt) is found to be ~50 Myr.
Previously reported Δt values for lawsonite–epidote eclogites from other localities (North Qilian and New Caledonia) are ~20 Myr, which is smaller than our result by ~30 Myr. Those samples formed along warmer thermal gradients than the Guatemalan lawsonite eclogites, as their metamorphic peak conditions were outside the lawsonite stability field. Assuming the subduction duration of eclogite-forming material is generally <10 Myr, we propose that 40–50 Myr of oceanic crust cooling before subduction is required to keep the thermal gradient cold enough for lawsonite stability. This study applied the novel in-situ U–Pb almandine garnet dating method, demonstrating its viability for extracting robust temporal information on convergent plate margin dynamics.
References
[1] Hernández-Uribe, D., & Tsujimori, T. (2023). Geology, 51, 678–682.
[2] Millonig, L. J. et al. (2020). Earth and Planetary Science Letters, 552, 116589.
[3] Shu, Q. et al. (2024). Contributions to Mineralogy and Petrology, 179, 49.
Here, we report metamorphic and protolith ages of lawsonite eclogite from the South Motagua Mélange, Guatemala. These samples ubiquitously include well-preserved lawsonite grains both as inclusions in garnet and in the rock matrix, demonstrating that garnet-forming reactions occurred within the lawsonite stability field. For the determination of the eclogite-facies metamorphic age, we performed LA-MC-ICPMS U–Pb dating of almandine-rich garnet grains [2,3] in four thin sections representing four different rock samples. Forty-four spot analyses of the inclusion-free rims in one of the samples plot on a well-defined regression line with a lower intercept of 137 ± 4 Ma. The garnet U–Pb ages from the other samples overlap this value within uncertainty. As for the protolith age, we performed LA-HR-ICPMS U–Pb dating of magmatic zircon in one of the samples, and obtained a concordia age of 187 ± 2 Ma. Hence, the protolith–metamorphic age difference (Δt) is found to be ~50 Myr.
Previously reported Δt values for lawsonite–epidote eclogites from other localities (North Qilian and New Caledonia) are ~20 Myr, which is smaller than our result by ~30 Myr. Those samples formed along warmer thermal gradients than the Guatemalan lawsonite eclogites, as their metamorphic peak conditions were outside the lawsonite stability field. Assuming the subduction duration of eclogite-forming material is generally <10 Myr, we propose that 40–50 Myr of oceanic crust cooling before subduction is required to keep the thermal gradient cold enough for lawsonite stability. This study applied the novel in-situ U–Pb almandine garnet dating method, demonstrating its viability for extracting robust temporal information on convergent plate margin dynamics.
References
[1] Hernández-Uribe, D., & Tsujimori, T. (2023). Geology, 51, 678–682.
[2] Millonig, L. J. et al. (2020). Earth and Planetary Science Letters, 552, 116589.
[3] Shu, Q. et al. (2024). Contributions to Mineralogy and Petrology, 179, 49.
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