Temporal covariation of island arc Sr isotopes and seawater chemistry over the past 2 billion years.

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.