Bimineralic calcite-aragonite rafts in the hypogeous lakes of Cova dets Ases (Mallorca): controls on precipitation and polymorphism.

Abstract

[eng] Calcite-aragonite bimineralic rafts and related waters from hypogenic lakes in the coastal cave of “Cova dets Ases”, Mallorca, have been analyzed for mineralogy and geochemistry (including stable isotopes). The main objective has been the evaluation of the factors that control the chemical and mineralogical composition of the precipitates as they are another natural example of the classical problem of aragonite vs calcite precipitation. The hydrochemistry of the lakes in these coastal caves is mainly conditioned by the mixture of fresh and marine waters. The percentage of mixing, together with the CO2 degasification from the lakes water, seems to play a fundamental role in the precipitation of the carbonate rafts. However, their presence in the studied lakes does not show a homogeneous distribution in space and time and no clear relation has been found between that distribution and the evolution of any of the monitored environmental variables. With respect to their mineralogy, the main factor that seems to control the dominant bimineralic composition of the rafts studied here is the Mg/Ca ratio in the waters, which means the percentage of marine component in those waters. The variations of this ratio, in combination with changes in the degasification rate and in the oversaturation with respect to calcite and aragonite will condition the presence of one or the other polymorph. Even small microenvironmental variations seem to affect the order of precipitation of these bimineralic rafts. Another important issue in this work is that the data obtained in this natural system have been used to check the most suitable δ18O isotope fractionation equations for the paleotemperature calculations from rafts precip- itated in the past under similar conditions.