SUPERCRITICAL CO2 HYPOGENE SPELEOGENESIS: A NEW MODEL

The Guadalupe Mountains in southeast New Mexico are known for the enormous caves and spectacular formations of the sulfuric acid speleogenesis model, but little is known about the smaller spar caves that are truncated by this late stage speleogenesis. Attempts have been made to describe a model for these drusy voids, but none to date have been able to adequately explain both the dissolution of the limestone and the deposition of the cave spar. Spar caves are ubiquitous throughout the Guadalupe Mountains and play an important role in the geologic history of the region. These caves (vugs lined with large, euhedral calcite crystals) contain a wealth of information about tectonics, landform evolution and speleogenesis. A new model, invoking the pressure and temperature regime of supercritical CO₂ (scCO₂) and its transition to subcritical CO₂ (subCO₂), is used to describe the speleogenesis of these enigmatic caves. These spar caves formed 500 ± 300 meters below a regional water table during relatively brief periods of increased hydrothermal convection driven by magmatic activity. The ages of the spar crystals correlate precisely with the ignimbrite flare-up, not only supporting the hydrothermal origin, but also linking regional processes to continental scale tectonics. Magma releases CO₂ as it rises and degasses. The CO₂ is in a supercritical phase at the temperatures and pressures at which it is released. ScCO₂ has a very low viscosity (~ 6% that of water) allowing it to move rapidly through porous subsurface materials. As it does so, it displaces brines in the overlying aquifers, but this brine still wets the pore walls and absorbs some of the scCO₂ acidifying the brine and dissolving the rock as it rises and cools. There is a critical depth (~200 to 800 m) at which a small change in temperature can change the scCO₂ to subCO_2­ allowing degassing to occur which in turn raises the pH of the system lowering the solubility of CaCO₃ precipitating spar in the voids left by the earlier dissolution. This small change in temperature is interpreted to be coeval with cessation of the magmatic activity, which also stops supplying CO₂ to the system further decreasing the amount of CaCO₃ that can be held in solution. This model accounts for the dissolution of the voids, the precipitation of the spar, and the simultaneity of the events that created them.