Abstract
Water-rock interactions in mid-ocean ridge hydrothermal systems are a critical part of Earth system evolution. Extensive insights have been developed from vent fluid chemistry and laboratory experiments, but these leave unanswered many questions about the temporal evolution and spatial structure of the hydrothermal systems that can only be addressed with reactive transport simulations. Other issues are the effects of changing spreading rates and seawater chemistry through Earth history. We are addressing this problem using the Toughreact code, starting with 2D static (no seafloor spreading) simulations of the near-axis region where most of the interaction occurs. The simulations use a dual-permeability grid to represent fractured rocks, and also have a formulation for Sr isotope exchange. Vent fluid Ca, Mg, SO4, and Na concentrations and Sr isotopes can be used as a guide to fluid chemical evolution. Initial simulations reproduce modern vent fluid chemistry even with maximum temperature only at 380°C, and suggest that fluids need not be in equilibrium with the rocks at any point in the system. Model fluids continue to evolve chemically even in the upflow zone prior to venting. The effects of different seawater chemical composition, as proposed for the Cretaceous, for example, can be captured with charge-balance models.
Highlights
1.1 Scientific backgroundMid-ocean ridge (MOR) hydrothermal systems are a major component of Earth’s geochemical cycles
The chemical exchange that occurs as seawater circulates through basalt and gabbro of varying temperature affects the balance of ions in seawater, and influences the carbon cycle through the coupling of Ca and Mg and carbonate sedimentation
Understanding the geologic history of the oceans, and the relationships between seawater chemistry and climate, requires that the chemical exchange between seawater and oceanic crust be understood in its present form, but as it might have been at different stages of Earth history extending back to earliest Precambrian times
Summary
Mid-ocean ridge (MOR) hydrothermal systems are a major component of Earth’s geochemical cycles. The chemical exchange that occurs as seawater circulates through basalt and gabbro of varying temperature affects the balance of ions in seawater, and influences the carbon cycle through the coupling of Ca and Mg and carbonate sedimentation. Understanding the geologic history of the oceans, and the relationships between seawater chemistry and climate, requires that the chemical exchange between seawater and oceanic crust be understood in its present form, but as it might have been at different stages of Earth history extending back to earliest Precambrian times. There is a need to formulate better models for the effects of changing seafloor spreading rates and the relative magnitudes and effects of high-temperature and low-temperature seawater-basalt exchange
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