Mineralogical, geochemical and isotopic characteristics of Quaternary calcretes in the Adana region, southern Turkey: Implications on their origin

Calcretes are widespread, and form as nodular, columnar/tubular, fracture-infill, laminated hardpan and conglomeratic crust in the Adana region. X-ray powder diffractometry (XRD), scanning electron microscopy and energy-dispersive analyses (SEM–EDX) and differential thermal analysis−thermal gravimetry (DTA–TG) reveal that calcretes and their host-rock mudstones are composed predominantly of calcite and smectite, respectively. The other minerals determined in the samples are palygorskite, quartz, and feldspar. Palygorskite is a minor component of the calcretes. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) analyses reveal that the calcrete samples are characterized by high CaO and LOI values whereas the host-rock mudstones contain relatively high values of SiO2, Al2O3, Fe2O3 and MgO. The cross-plots of SiO2–Al2O3, CaO–SiO2, CaO–MgO and CaO–Fe2O3 show positive and negative correlations. Their gradual change is due to the degree of the progressive calcretization causing mudstone replacement. The δ18O and δ13C values range from − 3.8 to − 5.7 (mean − 4.4) and from − 7.7 to − 10.0 (mean − 8.6) ‰ PDB, respectively which are consistent with values of pedogenic calcretes reported in the literature from worldwide sites. The δ18O values indicate formation from meteoric water either by replacement or precipitation at estimated temperatures from ~ 21 to 23 °C. The δ13C values are typical for pedogenic calcretes, reflecting development under the C3-dominated vegetation cover and semiarid or seasonally arid climatic conditions.