Phreatic cave calcites – archives of two realms

Two main types of karst formation are commonly known: the surficial meteoric one and the subsurface (hypogenic) karst, related to both carbonic (H₂CO₃) and sulphuric (H₂S) acids. Nevertheless, a third, less well studied type exists that is referred to here as CO₂-regime related karst. This article describes Pleistocene phreatic cave spar from the diapiric Jabal Madar dome in Northern Oman. These caverns and their precipitates probably represent examples of CO₂-related karsting and subsequent calcite precipitation. Phreatic calcites form at the interface between two fundamentally different diagenetic and hydrogeological domains: the deep-seated, hydrothermal and the near-surficial, meteoric–vadose one. Changes in the ratio between hydrothermal upwelling and meteoric influx are recorded in the geochemistry and mineralogy of these calcites making them uncommon archives of two contrasting realms.