MINOR AND TRACE ELEMENT ANALYSES ON GYPSUM: AN EXPERIMENTAL STUDY

Abstract

Minor and trace element (Na, Cl, Mg, K, Sr) abundances in gypsum and anhydrite can be used to help elucidate paleosalinity and depositional environments. However, sulfates may contain abundant fluid and solid inclusions which can contribute considerable amounts of elements to bulk-analyses. Previously published element data in gypsum and anhydrite show highly variable values, which may have been significantly influenced by inclusions. This study developed procedures for grinding, heating, and rinsing with alcohol, HNO3, and deionized water to fracture and remove fluid and solid inclusions from gypsum and anhydrite. Samples used for this study are petrographically unaltered Miocene and Holocene marine gypsum crystals ranging from > 10 cm to < 1 mm and were analyzed for Sr, Na, K, Cl and Mg concentrations before and after inclusion-stripping. Results show that Mg and Sr are less sensitive (usually < 30%) to the inclusion-stripping procedures than Na and Cl. Up to 98% of the Cl was removed from bulk gypsum and anhydrite by the grinding, heating and rinsing procedures. Cl contents in treated samples and inclusion-free samples are very low (usually a few ppm). This study, therefore, confirms that Cl does not occur in the lattice of gypsum and anhydrite as proposed by Kushnir (1980). The procedures also removed up to 80% Na from the bulk-samples. Na concentrations in treated gypsum samples are orders of magnitude lower than previously published analyses and the 'marine' Na values calculated by using published KNagypsum. The previously published KNagypsum may need to be revised. Moreover, the methods used in this study can be applied in other evaporative minerals (e.g., halite and dolomite) for minor and trace analyses.