87Sr/86Sr ratios as tracer for gypsum sources and redistribution processes

Gypsum group minerals in the Atacama Desert occur in pedogenic profiles or as primary lacustrine sediments and are subject to complex redistribution processes. The 87Sr/86Sr composition of gypsum is a useful tool to trace the source of the gypsum deposits. A sequential digestion method was developed to analyze gypsum and its inherent detrital silicate components separately. Samples were taken from the Central Depression and the Western Cordillera, including  Miocene - Pleistocene lacustrine deposits and some pedogenic gypsum.

The 87Sr/86Sr compositions measured in gypsum range from 0.7065 to 0.7074, and are in most cases similar to that of the detrital components, indicating that the gypsum composition is influenced by the underlying bedrock through parental water-rock interaction. Interestingly, most samples yield lower 87Sr/86Sr ratios than present day waters from the Atacama Desert. The 87Sr/86Sr  ratio varies strongly between the different samples, reflecting the highly variable bedrock composition in the Atacama Desert that was often neglected in previous studies. There are some outliers with strongly enriched 87Sr/86Sr values up to 0.7128 in the detrital components. This mismatch can be explained by various sources of gypsum and detritus, which can be achieved through processes like aeolian redeposition of gypsum into a basin with a bedrock carrying a distinct Sr signal or by dissolution and reprecipitation of the gypsum by waters with different Sr signals. Also, in some cases, the parental brine may reflect the bedrock composition of the water catchment rather than the bedrock composition of the basin, where lacustrine gypsum precipitates.