Effect of paleoseawater composition on hydrothermal exchange in midocean ridges

Abstract

Variations in the Mg, Ca, Sr, and SO 4 concentrations of paleoseawater can affect the chemical exchange between seawater and oceanic basalt in hydrothermal systems at midocean ridges (MOR). We present a model for evaluating the nature and magnitude of these previously unappreciated effects, using available estimates of paleoseawater composition over Phanerozoic time as inputs and 87 Sr/ 86 Sr of ophiolite epidosites and epidote-quartz veins as constraints. The results suggest that modern hydrothermal fluids are not typical due to low Ca and Sr relative to Mg and SO 4 in modern seawater. At other times during the last 500 million years, particularly during the Cretaceous and Ordovician, hydrothermal fluids had more seawater-derived Sr and Ca, a prediction that is supported by Sr isotope data. The predicted 87 Sr/ 86 Sr of vent fluids varies cyclically in concert with ocean chemistry, with some values much higher than the modern value of ∼0.7037. The seawater chemistry effects can be expressed in terms of the transfer efficiency of basaltic Ca and Sr to seawater in hydrothermal systems, which varies by a factor of ∼1.6 over the Phanerozoic, with minima when seawater Mg and SO 4 are low. This effect provides a modest negative feedback on seawater composition and 87 Sr/ 86 Sr changes. For the mid-Cretaceous, the low 87 Sr/ 86 Sr of seawater requires either exceptionally large amounts of lowtemperature exchange with oceanic crust or that the weathering flux of continentally derived Sr was especially small. The model also has implications for MOR hydrothermal systems in the Precambrian, when low-seawater SO 4 could help explain low seawater 87 Sr/ 86 Sr.paleoseawater | Sr isotopes | hydrothermal systems | midocean ridges M idocean ridge (MOR) hydrothermal circulation, fueled by persistent heat from shallow magma reservoirs, is a key component in the long-term regulation of global climate and ocean chemistry (1). Hydrothermal fluids that emerge from these systems, at temperatures up to ∼400°C, are chemically distinct from seawater due to reactions with newly forming oceanic crust. Relative to seawater, fluids emanating from modern MOR hydrothermal systems are enriched in Ca and transition metals, have lower pH and Eh, and have little to no Mg and SO 4 (2-5).Hydrothermal circulation at MOR is dominated by two important chemical reactions: the removal of seawater SO 4 through mineral precipitation with seawater Ca, and the removal of seawater Mg through precipitation of hydroxy-silicate minerals (3, 6, 7). Seawater SO 4 is primarily lost through the formation of anhydrite (CaSO 4 ) early during hydrothermal circulation, as anhydrite precipitation occurs by simply heating seawater >130°C (8). There can also be minor losses of SO 4 through thermochemical sulfate reduction (9, 10) and bacterial processes.Seawater Mg is lost from hydrothermal fluids at both low and high temperatures, by exchange of seawater Mg for basaltic Ca through the transformation of primary igneous minerals to alteration phases such as montmorillonite ...

Continuously Changing Quartz‐Albite Saturated Melt Compositions to 330 °C With Application to Heat Flow and Geochemistry of the Ocean Crust

“…Sr/ 86 Sr rises as ocean crust production drops off at the end of the Mesozoic. The increased role of mantle (MOR) input explains the very low 87 Sr/ 86 Sr of the Cretaceous, which without it, currently requires an exceptionally low continental riverine flux (Antonelli et al, 2017). My conclusion thus suggests that seawater 87 Sr/ 86 Sr does not adequately track continental weathering.…”

Ocean Alkalinity, Buffering and Biogeochemical Processes

“…where exchanges between the univalent and divalent cations or between magnesium and calcium are considered of less importance for the balance. High-temperature hydrothermal vents result in the removal of Ca 2+ and SO 4 2− via anhydrite precipitation and of Mg 2+ via hydroxy-silicate formation (Antonelli et al, 2017). The latter process generates acidity that enhances release of Ca 2+ from basalt so that charge remains balanced.…”

Experimental partitioning of Ca isotopes and Sr into anhydrite: Consequences for the cycling of Ca and Sr in subseafloor mid-ocean ridge hydrothermal systems

“…In contrast, an ancient ocean with a Ca/SO 4 > 1 (i.e. the Eocene), seawater derived Ca and Sr may persist beyond the zone of anhydrite precipitation near the seafloor and the partially modified isotopic signa-ture of seawater may be incorporated into secondary Ca silicates during exchange and alteration at high temperature in the reaction zone or possibly retained in composition in the hydrothermal fluid emitting from the subseafloor (Lowenstein et al, 2001;Wortmamnn and Paytan, 2012;Coogan and Dosso, 2015;Antonelli et al, 2017). Although seawater Ca/SO 4 ratios likely play an important role in determining the d 44 Ca value of altered oceanic crust, these specific cause and effect processes are not well understood and require experimental and further field data to constrain the Ca isotope composition of altered and subducted oceanic crust.…”