Low-temperature geochemistry of B in a hypersaline basin of Central Andes: Insights from mineralogy and isotopic analysis (δ11B and 87Sr/86Sr)

Abstract

The dynamics of B isotopes in the hypersaline Olaroz basin located in the Central Andes, was characterized by means of chemical and isotopic analysis (δ11B and 87Sr/86Sr) of brines, warm springs, rivers, evaporites, and regional rocks. The mineralogical composition of evaporites was determined by XRPD and SEM/EDS, while the fractionation of B isotopes was simulated using a batch-type model. The results revealed that the alteration of ignimbrite rocks in the arid climate conditions of the study area is triggered by the circulation of thermal waters in the main tributary's catchments, and generates solutions that preserve the isotopic signature of the basin rocks. The B-rich leachates are transported downflow by runoff and once in the salt flat, the B concentration increases due to evaporation. Modeling results suggest that fractionation occurs during infiltration of water in the salt flat nucleus, by ulexite precipitation. However, shifts of δ11B values between brines and evaporites are low because the trigonal soluble B species, which concentrates the heavier B isotope, predominates at the neutral pH of brines, and is incorporated into the ulexite lattice in a lower proportion than the tetrahedral species (ratio B(OH)3/B(OH)4 − = 2/3). In arid and cold environments such as those of the Altiplano-Puna region, B isotopes are weak proxies of weathering because denudation regimes of the weathering-limited kind result in nearly negligible fractionation of B isotopes by adsorption or co-precipitation onto the solid products of alteration.