Abstract
Over the course of Earth’s history, marine sulfate concentrations have been increasing in response to long-term atmospheric oxygenation. In contrast to modern oceans, where abundant sulfate precipitates in hot oceanic crust as anhydrite, Precambrian oceans contained much less (~0–10 mM) sulfate, so that submarine hydrothermal systems were comparatively poor in anhydrite. As a step towards exploring the role of chemical evolution of seawater solutes, we investigate the reaction between basalt and seawater that took place at the ca. 2.43–2.41 Ga Vetreny Belt (Karelia craton, NW Russia) using fluid inclusion and multi-isotope measurements complemented by reactive transport and static aqueous-mineral equilibrium calculations. Using fluid inclusion measurements by LA-ICP-MS, we constrain the Sr concentration in the least modified seawater-derived fluids and address the effect of phase separation. Then, we complement the previous δ18O – Δ′17O datasets with new 87Sr/86Sr measurements performed on 2.41 Ga epidote from the Vetreny Belt, and recent (0–6 Ma) oceanic epidote from Reykjanes, Iceland and the drilling site 504B in the eastern Pacific Ocean. The 2.41 Ga epidote with 87Sr/86Srinitial of 0.7029–0.7042 and Δ′17O of –0.06 to 0.00‰ is best explained by a relatively high fraction (~90%) of marine Sr that was delivered from contemporaneous seawater with 87Sr/86Sr ≈ 0.7045, and without significant removal by early anhydrite. Using Monte-Carlo simulation of a dual-porosity model, we constrain the range of possible exchange trajectories based on the variability of physical parameters (porosity, fluid flow velocity, fracture spacing, recrystallization rates). Further, we use a series of static equilibrium seawater-basalt reaction calculations with emphasis on the possible range of marine Ca/SO4 values at 2.41 Ga. Our calculations demonstrate that co-existing quartz and epidote in absence of feldspars represent equilibrium with less-evolved hydrothermal fluids. Consequently, equilibrium assemblage of quartz and epidote provide an insightful archive for marine Sr. Based on our modeling and assumptions about marine 87Sr/86Sr and Sr/Ca ratios, the 2.41 Ga epidotes document a seawater-basalt reaction where the initial fluid contained between 30 and 40 mM of Ca and 0–10 mM SO4, representing a high marine input and the possible effect of phase separation. Based on our data, we suggest that high Ca/SO4 ratio of seawater (≫1) and low concentration of anhydrite in submarine systems of the contemporaneous oceans promote a higher fraction of seawater Sr to be permanently stored in silicates of altered oceanic crust. In contrast, modern altered oceanic crust is depleted in radiogenic Sr due to partitioning into anhydrite, which partly returns into the ocean upon cooling.
Highlights
Over time scales exceeding 10 Ma, the composition of the Earth’s oceans is modulated by delivery and removal of major elements
As a step towards exploring the role of chemical evolution of seawater solutes, we investigate the reaction between basalt and seawater that took place at the ca. 2.43–2.41 Ga Vetreny Belt (Karelia craton, NW Russia) using fluid inclusion and multi-isotope measurements complemented by reactive transport and static aqueous-mineral equilibrium calculations
The data is reported here (Tables 3 and 4) in two ways: (1) as assemblage ratios to Na (Table 3), which should be preferred for inter pretation given that salinity was not directly determined; and (2) as concentrations calculated using an average salinity of 40 wt% NaCl equivalent and a correction accounting for the presence of other cations as major chloride compounds (i.e. K, Ca as well as Fe and Mn for sample VB8A), determined based on the raw intensity ratios (Table 4)
Summary
Over time scales exceeding 10 Ma, the composition of the Earth’s oceans is modulated by delivery and removal of major elements. Driven by exchange reactions with oceanic crust, hydrothermal circulation of seawater serves both as a sink and a source in budgets of many major and trace elements. The composition of exchanged fluids released at submarine vents (i.e. black smokers) is considered to be rock-buffered with limited influence of seawater dissolved load (e.g. Berndt et al, 1989). The long-term changes of seawater concentrations driven by other factors such as continental weathering can modify the pathways that marine elements undergo during reactive transport within oceanic crust (Kump and Seyfried, 2005; Antonelli et al, 2017). On billion-year timescale, one of the most prominent changes that affected seawater composition is the rise of sulfate. The overall rise of atmospheric pO2 levels over geological time promoted an increase in oxidized marine species, sulfate, which is intimately tied to the increasing oxidative capacity of terrestrial sulfide weathering (Holland, 1984; Canfield, 1998; Farquhar and Wing, 2003; Kah et al, 2004)
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