Occurrence and genesis of Borjlu travertine located in Nair in Ardabil province based on the geochemical and stable isotopes findings

Travertine refers to all non-marine carbonate sediments consisting of calcite/aragonite formed by calcium and CO2-rich fluids under surface conditions and low pressure. Travertine deposition takes place as a result of the decomposition of calcium bicarbonate and the release of CO2 gas. Based on elemental geochemistry, origin of carbon dioxide gas and composition of stable isotopes, travertines are divided into two groups: thermogenic and meteogenic. Studies of stable isotopes of carbon and oxygen play a fundamental role in determining the origin and type of travertine. Thermogenic travertines, which are often associated with young volcanic areas, were deposited from medium to high temperature fluids and their carbon isotope composition is heavy. While meteogenic travertines are formed from relatively low temperature fluids and their carbon isotope composition is light.

In this study, the mineralogical, geochemical and isotopic characteristics of Borjlu travertine located in the Alborz-Azerbaijan magmatic belt in the northwest of Iran have been investigated based on field and laboratory studies. In this research, petrographic studies were done using microscopic thin sections by polarizing microscope. The samples were analyzed by inductively coupled plasma mass spectroscopy (ICP-MS) to determine Ba and Sr contents. X-ray diffraction (XRD) and scanning electron microscope (SEM) imaging have also been used to study the mineralogy of the samples. The analysis of stable isotopes of carbon and oxygen has been done to determine the isotopic composition of carbonate minerals by mass spectrometer (MS) method.

Three facies including crystalline crust in the form of alternating light and dark layers, raft in the form of discontinuous layers, and shrub consisting of branching structures with rounded and buttons ends can be seen in Barjelo travertine. The result of XRD analysis from the crystalline crust shows that they are formed from pure calcite. SEM image shows the limited presence of microbial accumulations on the surface of calcite crystals. Petrographic studies of the crystalline crust part of travertine show that calcite crystals have grown in three forms of lamellar prisms, fibrous crystals and micrite grains. Some sections also indicate the alternating growth of crystalline crust and shrub facies. The average values of Ba and Sr in the travertine samples are 40.22 and 563.89 ppm, respectively. The values of δ13C(VPDB) and δ18O(VPDB) isotopes of the samples range from +1.44‰ to +2.19‰ and -14.39 to -16.51‰, respectively. The calculated values of δ18O(SMOW) and δ13C(CO2) for the samples show changes from +13.84‰ to +16.02‰ and -7.87 to -8.77‰ respectively.

The comparison of Ba and Sr contents indicates the thermogenic origin of Borjlu travertine. Although the δ13C(VPDB) positive values in Borjlu travertine indicate its thermogenic origin, its values are lower than typical thermogene travertines. Comparison of δ13C(VPDB) and δ18O(SMOW) values showed that Borjlu travertine samples located in the field of thermogene and meteogene travertines, which indicates the mixing of two fluids with heavy and light isotopic composition. The calculated values of δ13C(CO2) indicate the inorganic and internal origin of CO2 in the travertine-forming fluid. Also, in the isotope composition diagram of δ13C(VPDB) versus δ18O(VPDB), carbonate rocks are shown as the source of CO2 gas for the formation of Borjlu travertine. Some field evidences, such as the formation of crystalline crust and shrub facies, the presence of a hot spring with a relatively high temperature in the studied area, and the presence of a geothermal reservoir in the depth are signs of active hydrothermal phenomena in the area. It is thought that the presence of limestone units in the region and their contact with the fault system has provided the possibility of infiltration and circulation of hydrothermal fluids containing CO2 inside them. In this way, the bicarbonate ions necessary for the formation of travertine has been provided by decarbonation and dissolution of these carbonates. It seems that the fault system acted as a conduit for the migration and ascent of fluids containing calcium bicarbonate towards the surface. The mixing of this fluid with meteoric waters near the surface caused lighter isotopic composition of the ascending fluid. Based on isotope composition of travertine samples, the temperature of outflowing fluid is estimated about 70 °C.