Chemical and isotopic exchange between seawater and oceanic crust is a fundamental mechanism by which Earth’s surface and interior interact. This exchange is fundamental to understanding Earth’s chemical and volatile cycling, climate regulation, continental growth, and habitability. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) are a powerful tracer of this process because the ocean and mantle have distinct isotopic signatures that are reasonably well-constrained through Earth history. The research chapters of the thesis investigate how ocean-mantle interactions have operated on long timescales, centering on hydrothermal alteration at spreading centers as the locus of seawater-seafloor Sr isotopic exchange. Ophiolites and island arcs serve as archives of and windows into ancient, seawater-altered oceanic crust. In Chapter II, we demonstrate that island arc ⁸⁷Sr/⁸⁶Sr has covaried with seawater Sr isotope composition and concentration over the past two billion years. The observation is consistent with modern understanding of geochemical cycling at subduction zones – that seawater chemistry exerts a first-order control on the composition of the subducting oceanic crust, which island arc magmatic rocks inherit – but it is a novel result to show that ⁸⁷Sr/⁸⁶Sr enrichment is a persistent feature of island arcs. The mechanism by which seawater chemistry affects oceanic crust is via large-scale fluid circulation of seawater-derived fluids through nascent seafloor at hydrothermal systems. The result of the interactions at the interface of seawater and seafloor is an isotopic shift in oceanic crust towards more radiogenic, seawater-like ⁸⁷Sr/⁸⁶Sr. Chapter III tests this framework by probing the record of seawater-altered oceanic crust directly, using ophiolites. A compilation of ophiolitic dike samples aged 0-750 Ma suggests that the magnitude of seawater-seafloor Sr isotopic exchange has varied systematically through the Phanerozoic and Neoproterozoic in concert with seawater Sr isotopic composition and concentration, supporting the hypothesis that seawater chemistry has been a first-order control on hydrothermal alteration intensity throughout this interval. Given the purported connection between seawater chemistry and Sr isotopic patterns in ophiolites, we hypothesized that ancient ophiolites may lend insights into seawater chemistry and hydrothermal alteration dynamics during periods of Earth history during which we have few constraints on geochemical cycling and climate feedbacks. To test this, we present ⁸⁷Sr/⁸⁶Sr data through two sections of Precambrian altered oceanic crust – the 1.7 Ga Payson ophiolite and the 2.0 Ga Purtuniq ophiolite – and find that the structure of the ancient ophiolite ⁸⁷Sr/⁸⁶Sr data is largely similar to that from modern and Cretaceous oceanic crust and ophiolites. We interpret this to mean modern-style hydrothermal alteration was ongoing by the Paleoproterozoic. Chapters IV and V expand on this premise, employing complementary geochemical systems to probe seawater-seafloor interactions from different angles. Oxygen isotopes through the 1.7 Ga Payson ophiolite independently constrain the section as representative of seawater-altered oceanic crust. This further supports the idea that modern-style hydrothermal alteration was ongoing by the Paleoproterozoic, lending insight into weathering regimes at the time. Fe³⁺/ΣFe data from both ophiolites tentatively suggest that hydrothermal alteration was driving modest oxidation in the extrusive units of the 1.7 Ga Payson ophiolite relative to the least-altered intrusive units, but not in the 2.0 Ga Purtuniq ophiolite. Chapter VI uses unaltered ophiolitic gabbros and ultramafic rocks to construct an empirical record of depleted mantle ⁸⁷Sr/⁸⁶Sr for the Phanerozoic. The results suggest that the isotopically heterogeneity in the depleted mantle that we recognize today has been long-lived, persisting at least as far back as 500 million years. Together, these findings indicate that the coupling between surface ocean chemistry and the geochemistry of oceanic crust has been operating since at least the Paleoproterozoic. These findings are important for informing how the geochemistry and Earth history communities conceptualize ancient ocean and mantle conditions, geochemical cycling, weathering feedbacks, and surface-mantle connectivity.
Amanda Lee Bednarick (Thu,) studied this question.