Abstract

On geological timescales, the volume of CO2 in the atmosphere is influenced by processes such as silicate weathering. In particular, the weathering of basaltic rocks increase the availability of divalent cations such as Mg2+ and Ca2+ in natural waters enabling carbonate formation under atmospheric conditions. Therefore, the existence of carbonate deposits in basaltic provinces provides an excellent opportunity to contribute for the regional environmental record. The Pali Aike Volcanic Field consist of basaltic rocks that covers an area of ca. 4500 km2 in southernmost South America, which were sourced from a deep Mantle source since ca. 4 Ma until the Holocene. The Laguna Timone is a maar filled by a brine developed after explosive volcanic eruptions and constitutes an endorheic hydrological system where carbonate precipitation (calcite and magnesium calcite) is controlled by enrichment of Ca2+ and Mg2+ ions released during the weathering of alkaline basalts. X- ray fluorescence analyses in basaltic rocks reveal high concentration of elements such as CaO (9.73 - 10.57 wt.%) and MgO (9.49-12.76 wt.%). X-ray diffraction results verify that basalts contain pyroxene (Na, Ca) (Mg, Fe, Al) (Al, Si)2O6, olivine (Fe, Mg)2SiO4 and plagioclase NaAlSi3O8-CaAl2Si2O8 which are suitable phases for the mineral carbonation process. The δ13C DIC values of lake range between -12‰ and -16‰ while for the water of the river values of -7‰. These isotopic ratios are associated to three possible sources/processes: weathering of silicates by carbonic acid, atmospheric CO2 and degradation of organic matter. Furthermore, Sr isotope ratios of carbonates (tufa fragments and thin crust in pebbles) define a range between 0.70408 and 0.70475 which is discussed on basis of the data of basaltic rocks (0.70316 to 0.70351) and top soils (0.705382) in the PAVF. Although the Sr isotope ratios of carbonates are indicative of their derivation from the weathering of basalts an exogenous input from relatively enriched 87Sr material is required, this could be associated to 87Sr leached from the top soils and transported by strong wind. Contrastingly a carbonate vein preserved in the tuff ring deposit of the maar has Sr isotope ratios ranging from 0.70265 to 0.70314, similar to the compositional range of the Mantle xenoliths (0.70264 to 0.70431) and basalts. These data indicate that primary sources of carbonates in the lake are related to in-situ weathering of mafic and ultramafic rocks.