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Featured article from geoscience journal

Wiley-Blackwell
Geology Today, 2009, Vol 25, Issue 1, p. 29-33
Phreatic cave calcites - archives of two realms
Abstract:

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.