Purifying barite for oxygen isotope measurement by dissolution and reprecipitation in a chelating solution.
In the laboratory, barite precipitated from a solution with a high nitrate/sulfate ratio can have a significant amount (up to 28% by weight) of nitrate occluded in barite crystals that cannot be simply washed away. The impurity poses a serious problem for an accurate measurement of the oxygen isotope compositions for atmospheric sulfate, since atmospheric nitrate bears extremely positive Delta17O and delta18O values. Currently available methods for removing the occluded nitrate are either ineffective or not tested for oxygen isotope exchange. Here, I report a DTPA (a chelating solution) dissolution and reprecipitation (DDARP) method that is simple and effective in removing nitrate and other contaminants in barite. A series of barite dissolution and reprecipitation experiments that utilize 17O-anomalous solutions or barite crystals is conducted to examine the effect on oxygen isotopes during various treatments. It is established that no oxygen isotope exchange occurs between sulfate and water during DDARP treatment at two experimental temperatures (21 and 70 degrees C). Occlusion of DTPA itself in barite is negligible. Upon acidification, barite reprecipitation from a DTPA solution is quantitative (approximately 100%). Partially extracted barite may have slightly lower delta18O or delta34S values than the originals but no effect on Delta17O values. It is also demonstrated that heavily nitrate-contaminated barite samples are free of nitrate occlusion after two dissolution-reprecipitation cycles.