HOW MAY UPWELLING GEOTHERMAL FLOW AND RETROGRADE SOLUBILITY LEAD TO HYPOGENE SPELEOGENESIS IN CARBONATE AQUIFERS?

Recent studies indicate that extensive karstification and speleogenesis in carbonates are induced by the rise of hydrothermal fluids from depth (>1 km). However, the contribution of different geochemical and hydrogeological mechanisms remains obscure, and a variety of reactivity sources, as well as hydrogeological mechanisms, were suggested. The proposed mechanisms include renewed reactivity by mixing of different solutions or condensation corrosion above the groundwater table. However, the role of cooling and retrograde solubility of carbonates as a major hypogene speleogenesis mechanism was often considered negligible (e.g., [1] & [2] or attributed to the development diffuse karst [3]).

Here, using mathematical modeling, we study speleogenesis induced by deep-origin thermal flow, enriched by deep CO₂ fluxes, that upon cooling leads to large retrograde solubility and extensive dissolution. The conceptual model we suggest, consistent with our case study of hypogene caves [4], considers upwelling of focused channelized thermal flow through faults. Upon approaching an impermeable caprock this flow is diverted sideways and flows radially along permeable bedding planes and fractures in limestone strata (so-called “inception horizon”). The radially dispersed hot flow then cools rapidly via heat transfer to the surrounding rock, leading to focused dissolution and over time-scales of 10⁴-10⁵ yr, to speleogenesis near the inlet. Because the caves are isolated and breakthrough to the surface is not achieved during speleogenesis, the overall permeability and fluid flux do not appreciably change, so that dissolution remains localized, forming a cave. The model also predicts that maximal fluid cooling and dissolution are attained slightly downstream from the inlet, for which corresponding field observations are presented. The findings here, in contrast to previous thinking, show that geothermal heat loss by upwelling of thermal fluids, in conjunction with deep CO₂ fluxes, may shape and extensively karstify carbonate aquifers in the upper crust, with the manifestation of sizable speleological structures.

[1] Palmer, A.N., Geol. Soc. Am. Bull., 103(1), 1-21, 1991

[2] Klimchouk, A.B., In: White, W.B., Culver, D.C. (Eds.), 2nd ed. Academic Press, New York, 748–765, 2012

[3] Andre, B.J. and Rajaram, H., Water Resour. Res., 41(1), 2005

[4] Frumkin, A. et al., Geol. Soc. Am. Bull., 129(11-12), 1636-1659, 2017