The Ab Ask mineral springs are located 85 km northeast of Tehran, in the southern range of the Damavand volcano. Deposits of these calcareous springs are mainly precipitated as travertine. Petrographical, mineralogical, and stable isotope studies were conducted on different types of travertine to determine their genesis and factors that govern carbonate precipitation. Based on sedimentation consequence and lithofacies these travertines are categorized as first type (fresh travertine), second type (fissure-ridge, dam, and cascade), and third type (laminated) travertines, illustrating a specific condition of formation, deposition and diagenesis. Combined XRD and microscopic investigations show that the Ab Ask travertines are is composed of about 95 % calcite and a minor amount of quartz along with iron oxide impurities. The origin and transport of springs water from which travertine was precipitated are elucidated by 13 C and 18 O isotopic studies of the travertines.δ 18 O and δ 13 C values of travertines increase (from -13.0 to -6.3‰ VPDB and from 6 to 9.8‰ VPDB, respectively) with increasing distance from the spring orifice. This significant increase is attributed to temperature decrease, rapid degassing of CO 2 , and biological activities. It seems that CO 2 content of these fluids may have originated from limestone decarbonation. Based on the integrated petrographic and stable isotope study, the Ab Ask travertines can could be thermogenic in origin.

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