Experiments on two techniques for the removal of barite from detrital zircon

Abstract

Barite (BaSO 4) is a common mineral in sandstone that must be removed during separation of detrital zircon (ZrSiO 4). One widespread technique for the removal of barite exploits its lesser tenacity by milling the barite and zircon mixture in a ball mill. Here we test the extent to which such milling affects zircon and thus whether the milling could introduce bias into the detrital zircon sample. We then describe a new chemical technique for the removal of barite from detrital zircon. We find that milling a mixture of barite and zircon both breaks and causes loss of zircon grains, potentially introducing bias into a detrital zircon sample. Boiling barite in a 0.94 molar aqueous solution of sodium carbonate for four hours converts most grains to barium carbonate. The barium carbonate grains are opaque white and thus visually distinguishable from zircon, allowing separation by hand under a stereoscopic microscope. Alternatively, the barium carbonate grains can be dissolved by boiling in sixteen weight percent nitric acid for thirty minutes. In our experiments, boiling zircon in sodium carbonate solution and/or concentrated (sixty-five weight percent) nitric acid cleaned the surfaces of and the cracks in the grains but did not visibly change the zircon surfaces in other ways. Boiling only in concentrated nitric acid did not measurably affect the U-Pb and Lu-Hf isotopic systems in zircon interiors and boiling in a sodium carbonate solution followed by nitric acid did not detectably alter the Lu-Hf isotopic system. However, boiling in a concentrated sodium carbonate solution followed by nitric acid did disturb the U-Pb isotopic system in zircon interiors. Our results highlight the importance of proper technique during zircon isolation to minimize the introduction of bias into the sample. This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Removal of barite from zircon using an aqueous solution of diethylenetriaminepentaacetic acid and potassium carbonate

“…Although we did not test whether boiling in DTPA plus potassium carbonate solution affected isotopic systems in zircon, the absence of physical attacks larger than a few nanometers on a zircon crystal surface is an encouraging sign that these chemicals may not greatly affect zircon. In contrast to milling in a ball mill, the DTPA plus potassium carbonate procedure may not introduce bias into the detrital zircon separate, and the new protocol takes about one fifth the time as the sodium carbonate solution method developed by Martin et al (2021). Therefore, pending tests of its effects on isotopic systems in zircon, we suggest that the DTPA plus potassium carbonate solution procedure developed in this paper may be an advantageous replacement for other barite removal methods.…”

“…In all of these applications, it is important to minimize bias introduced into the sample during separation and analysis of the zircon. Milling a detrital zircon and barite mixture to remove the barite can introduce bias by breaking the barite along with the zircon grains (Martin et al, 2021). The alternative offered by Martin et al (2021), boiling in sodium carbonate solution, has two drawbacks: (1) This procedure takes five hours plus preparation and cleanup, and (2) It might affect the U-Pb isotopic system in zircon.…”

“…As the final step in zircon separation, geologists in many laboratories remove barite using a ball mill. A problem with this method is that milling the barite and zircon mixture breaks and causes loss of both minerals, not only the barite (Martin et al, 2021). Loss of zircon can introduce bias into the sample because the lost grains are no longer available for analysis (Slama and Kosler, 2012).…”

“…Breakage of grains into multiple fragments can cause bias even if the pieces are retained because dates from each fragment will be treated as coming from individual detrital grains, which is incorrect because those fragments actually were just one detrital grain in the sedimentary rock. To avoid the breakage and loss caused by mechanical comminution of the grains, Martin et al (2021) developed a chemical method to remove barite from zircon. The heart of this method is conversion of the barite to barium carbonate by boiling in an aqueous solution of sodium carbonate.…”

“…Whereas milling takes about fifteen minutes not including preparation and cleanup, the grains must be boiled in sodium carbonate solution for four hours, followed by removal of the barium carbonate for an hour, plus setup and cleanup. Additionally, boiling in sodium carbonate solution for four hours followed by concentrated nitric acid for one hour slightly decreased the U-Pb isotopic age of standard reference zircon crystal interiors, suggesting that this technique might affect the U-Pb isotopic system in zircon even at the more dilute concentrations of sodium carbonate and nitric acid required to dissolve barite (Martin et al, 2021).…”