海洋化学と火山岩の古代のつながり(Ancient Connections between Ocean Chemistry and Volcanic Rocks)

2024-10-18 カリフォルニア工科大学(Caltech)

This illustration shows how one oceanic plate subducts underneath another, producing conditions that give rise to magma upwelling that builds islands. Credit: Caltech

＜関連情報＞

• https://www.caltech.edu/about/news/ancient-connections-between-ocean-chemistry-and-volcanic-rocks
• https://www.pnas.org/doi/10.1073/pnas.2401832121

過去20億年にわたる島弧のSr同位体と海水化学の時間的共変動 Temporal covariation of island arc Sr isotopes and seawater chemistry over the past 2 billion years

Amanda L. Bednarick, Claire E. Bucholz, Donald J. DePaolo, and Daniel A. Stolper
Proceedings of the National Academy of Sciences  Published:October 15, 2024
DOI:https://doi.org/10.1073/pnas.2401832121

Significance

Modern island arc basalts have elevated ⁸⁷Sr/⁸⁶Sr ratios as compared to mid-ocean ridge basalts, a feature explained by the incorporation of subducted seawater-derived radiogenic Sr from hydrothermally altered oceanic crust. Whether this explanation is consistent with Sr isotope signatures of ancient island arc magmatic rocks has not been explored. To address this, we compiled island arc magmatic rock ⁸⁷Sr/⁸⁶Sr data spanning from present to Paleoproterozoic. The record shows persistent enrichment in radiogenic Sr in island arcs and can be successfully modeled as resulting from secular variations in seawater Sr concentrations and seawater ⁸⁷Sr/⁸⁶Sr. We conclude that island arc ⁸⁷Sr/⁸⁶Sr_i has responded to changes in seawater chemistry since the Paleoproterozoic.

Abstract

The chemical compositions of island arc basalts (IAB) reflect contributions from the mantle as well as fluids and melts from the subducting slab. Addition of radiogenic seawater Sr to oceanic crust through hydrothermal alteration and subsequent subduction is often invoked to explain elevated ⁸⁷Sr/⁸⁶Sr signatures in modern IAB. However, changes in the ⁸⁷Sr/⁸⁶Sr of island arc magmatic rocks through time has not been investigated, limiting our understanding of the factors influencing the Sr budgets of arcs throughout Earth’s history. To address this, we compiled ⁸⁷Sr/⁸⁶Sr values from island arc magmatic rocks ranging in age from modern to Paleoproterozoic, only including data from island arc localities that best preserve initial magmatic ⁸⁷Sr/⁸⁶Sr. Median initial ⁸⁷Sr/⁸⁶Sr values are consistently elevated compared to depleted mantle ⁸⁷Sr/⁸⁶Sr over this period, indicating persistent enrichment in radiogenic Sr in island arcs. Moreover, the elevation in island arc ⁸⁷Sr/⁸⁶Sr relative to the depleted mantle is variable. A notable rise in island arc ⁸⁷Sr/⁸⁶Sr during the late Neoproterozoic coincides with a steep increase in seawater ⁸⁷Sr/⁸⁶Sr and Sr concentration. To investigate this potential connectivity, we modeled the ⁸⁷Sr/⁸⁶Sr of island arc magmas between 0 and 830 Ma with inputs of depleted mantle ⁸⁷Sr/⁸⁶Sr, seawater ⁸⁷Sr/⁸⁶Sr, and seawater Sr concentration. The model reproduces the overall trajectory of the compiled data. We interpret the observed temporal variation in island arc ⁸⁷Sr/⁸⁶Sr values and its close association with fluctuations in seawater chemistry as evidence that changes in marine geochemistry have strongly influenced the Sr isotopic record of island arc magmas over time.