Origin of CaCl2 brines by basalt-seawater interaction: Insights provided by some simple mass balance calculations

Modern rift zone hydrothermal brines are typically CaCl₂-bearing brines, an unusual chemical signature they share with certain oil field brines, fluid inclusions in ore minerals and a few uncommon saline lakes. Many origins have been suggested for such CaCl₂ brines but in the Reykjanes, Iceland, geothermal system a strong empirical case can be made for a basalt-seawater interaction origin. To examine this mechanism of CaCl₂ brine evolution some simple mass balance calculations were carried out. Average Reykjanes olivine tholeiite was “reacted” with average North Atlantic seawater to make an albite-chlorite-epidotesphene rock using Al₂O₃ as the conservative rock component and Cl as the conservative fluid component. The excess components released by the basalt to the fluid were “precipitated” at 275° C as quartz, calcite, anhydrite, magnetite and pyrite to complete the conversion to greenstone. The resulting fluid was a CaCl₂ brine of seawater chlorinity with a composition remarkably similar to the actual Reykjanes brine at 1750 m depth. Thus, the calculations strongly support the idea that the Reykjanes CaCl₂ brines result from “closed system” oceanic basalt-seawater interaction (albitization — chloritization mechanism) at greenschist facies temperatures. The calculation gives a seawater: basalt mass ratio of 3∶1 to 4∶1 (vol. ratio of 9∶1 to 12∶1), in keeping with experimental results, submarine vent data and with ocean crust cooling calculations. The brine becomes anoxic because there is insufficient dissolved or combined oxygen to balance all the Fe released from the basalt during alteration. Large excesses of Ca are released to the fluid and precipitate out in the form of anhydrite which essentially sweeps the brine free of sulfate leaving an elevated Ca concentration. The calculated rock-water interaction basically involves Na + Mg + SO₄ ? Ca + K, simulating chemical differences observed between oceanic basalts and greenstones from many mid-ocean ridges.