TECHNIQUES FOR CHEMICAL REMOVAL OF BARITE FROM DETRITAL ZIRCON

Barite is a common accessory mineral in sandstone that hinders isolation of detrital zircon. Barite and zircon have similar densities and magnetic susceptibilities, so the minerals remain together after separation from less dense or more magnetic minerals. Dissolution of barite is rarely undertaken because it is little affected by strong acids except hot concentrated sulfuric acid, which is difficult to handle safely. Accordingly, most laboratories make use of barite’s lesser tenacity to remove it from zircon. In this procedure, the barite and detrital zircon grains are milled in a ball mill, preferentially breaking the less-tenacious barite into smaller pieces that are then removed by sieving. However, we show that such milling also breaks zircon crystals. Breaking the detrital zircon grains risks biasing subsequent geochemical analyses in multiple ways: by removing zircon tips that could have geochemical compositions different than grain interiors, due to loss of entire grains that were broken into pieces small enough to pass through the sieve, and by geologists incorrectly considering analyses of multiple fragments as separate grains even though the fragments were all part of one grain prior to milling. In an effort to develop a technique for barite removal that does not adversely affect coexisting zircon, we describe optimization of two methods of chemical removal of barite. The first involves conversion to barium carbonate by boiling in an aqueous solution of sodium carbonate, followed by dissolution of the barium carbonate in boiling nitric acid. Although this method removes barite, it takes five hours to complete and may affect the uranium-lead isotopic system in zircon. The second technique involves boiling in an aqueous solution of diethylenetriaminepentaacetic acid (DTPA) and potassium carbonate. This method requires only one hour to dissolve barite and does not visibly attack zircon, but we have not yet tested whether it affects isotopic systems in zircon. DTPA has been reported to dissolve pyrite, raising the possibility of dissolving both barite and pyrite in one step. Pending further experiments, we suggest that dissolution in an aqueous solution of DTPA and potassium carbonate is a promising option for removal of barite and possibly pyrite without adversely affecting coexisting zircon.