Selenite

The isotopic composition of gypsum hydration water (d18O and dD) in several selenite speleothems from the caves of the Naica Mine (Chihuahua, Mexico) has been investigated. The gypsum samples were collected at different depths from “Sword Cave” (at -120 m Level), “Crystales Cave” and “Ojo de la Reina Cave” (both at -290 m Level). d18O ranged between -4.66 and -3.26‰, whilst D varied between -81.78 and -71.43‰, relative to V-SMOW. The isotopic composition of the Naica aquifer was calculated using isotopic fractionation factors for dD and d18O during subaqueous gypsum precipitation from hydrothermal solutions (αDgyp – H2O = 0.980 and α18Ogyp – H2O = 1.004), which are independent of temperature (at low values),  revealing that 18O of the Naica aquifer water ranged between -8.62 and -7.23‰, whilst D was between -63.04 and -52.48‰. The data are described by a line (dD= 7.97 d18O +5.81) that is close to the current meteoric water line at the setting of Naica. Furthermore, at presesent deep water of Naica mineshows isotopic values that also fit with the inferred palaeogroundwater values. The differences observed between gypsum at the levels -120 m and -290 m could be explained by their crystallization under different climatic conditions in a span of time around 150 ka, as revealed by previous geochronological studies. Changes in the main moisture source of precipitation (Pacific Ocean/Gulf of Mexico) could have affected the isotopic composition of the infiltrating meteoric water in this area during the Quaternary. Alternatively, 18O and D of gypsum precipitated during the Holocene at -120 m Level show that evaporation of the shallower aquifer affected the isotopic composition of the groundwater during that period. In conclusion, the huge gypsum speleothems of the Naica caves precipitated from water of meteoric origin thatfed the hydrothermal aquifer of Naica, with indication of evaporation during the Holocene. These preliminary results suggest that phreatic gypsum speleothems constitute a potentially promesing archive for palaeogroundwater and palaeoclimate reconstruction.