Two main types of karst formation are commonly known: the surficial meteoric one and the subsurface (hypogenic) karst, which can be related to both carbonic (H2CO3) and sulfuric (H2S) acids. This paper documents evidence for a third, CO2-regime related, type of karst that is less commonly described. Petrographic and geochemical properties of exhumed Pleistocene phreatic cave deposits from the diapiric Jabal Madar dome in northern Oman are documented and discussed in a process-oriented context. These calcites form at the interface between two fundamentally different diagenetic and hydrogeological domains: the deep-seated, hydrothermal and the near-surficial, meteoric-vadose one.

Four calcite phases are recognized: (i) acicular, (ii) blocky to stubby elongated, (iii) proto-palisade, and (iv) macro-columnar calcites. The macro-columnar calcites, forming the last stage of precipitation, are conspicuous due to their cyclical red zonation, and they form the main (geochemical) focus of this study. Fluid inclusion data point to fluid temperatures of between 30 to 50°C (monophase liquid inclusions) and elevated salinities (1.6 to 7.3 wt.% NaCl equivalent). Low carbon-isotope data (−8 to −9‰) are in agreement with the influx of soil-zone CO2 whereas decreasing δ18O (−15‰) values might point to mixing of saline hydrothermal and 18O depleted, meteoric freshwater, i.e., two fluid sources. Trace-element and stable-isotope data shift between the different cement phases and vary cyclically across the red zoning in macro-columnar calcites. With respect to the intra-crystal variability, these patterns are perhaps best explained in the context of redox potential. Two interpretations are presented; the one favored here suggests that the cyclical red zoning in macro-columnar calcites is controlled by Pleistocene monsoonal climate patterns.

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