Analysis of Rare Earth Element concentrations in barite (BaSO4).
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Temperature and redox conditions during barite precipitation potentially provide information about levels of oxygen in the Archean (3.8-2.5 Ga) environment and the presence of early microbial life. Rare Earth Element (REE) abundances, especially the preservation of Ce and Eu anomalies, can provide this information, but little is known due to analytical difficulties in measuring trace elements in barite. In situ determination of REE in barite by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is a fast method to obtain these element abundances, but requires calibration to test for accuracy and precision. Here, we develop an Ion Exchange Chromatography technique to measure REE in solutions of dissolved barite while avoiding spectral interferences, such as 137Ba16O on 153Eu. Barite was fused in a 1:10 ratio with Na2CO3 and subsequently dissolved in 6N HCl. More than 99% Of the Ba-ions were removed from this solution after Ion Exchange Chromatography using 2(ethylhexyl)-orthophosporic acid as binding fluid. ICP-MS analyses of solutions from Archean and Paleozoic barites from high- and low-temperature environments showed that REE concentrations are very low. Both solution and in situ techniques suffered from a number of problems including: 1) PrO interferences on Gd, 2) Ce contamination in Na2CO3 flux, 3) residula Ba in both methods resulting in Eu interferences, 4) absence of reliable internal and external standards, 5) presence of interfering accessory phases such as monazite and 6) poor detection limits. However, comparison of REE abundances with the geological settings of the analyzed deposits suggests that measured Ce and Eu anomalies are consistent with the temperature and redox conditions during barite formation. Data corresponds with earlier data on REE fractionation in barite by Guichard (1979) and Barrett et al (1990) with respect to REE abundances but differs with this earlier data with respect to Eu and Ce anomalies. Interpretation of the results in terms of possible microbial versus non-microbial processes and the oxidation state of the Archean environment is difficult and requires the improvement of the methods to overcome remaining questions on the factors controlling REE partitioning in barite.