40Ar/39Ar geochronology of jarosite: The effectiveness of HF in removing silicate contaminants

Supergene and hypogene jarosite have been dated successfully, but accurate dating of weathering-derived jarosite and its application to landscape-evolution studies has been limited because of difficulties in obtaining high-purity mineral separates. Hydrofluoric acid (HF) can remove potassium-bearing silicates from supergene samples, but its effects on jarosite crystal chemistry and age dating are unquantified. Three experiments were conducted to determine whether HF treatment removes silicates without altering the potassium or argon composition of the jarosite prior to dating using the 40Ar/39Ar method. In the first two experiments, pure hypogene jarosite from Peٌa Blanca Mexico (PB; 9.42 ± 0.22 Ma) and a mixture of 85% PB:15% Fish Canyon sanidine (FC-2; 28.02 Ma) were crushed and treated with 25% HF for 0, 30, 240, and 480 min. Jarosite partially dissolves during HF treatment with grains becoming increasingly pitted and rounded with time in acid, but PBʹs potassium concentration and apparent age remained constant regardless of treatment time. FC-2 was absent from all treated samples, suggesting that 30 min in HF is sufficient to remove mechanically mixed sanidine crystals from jarosite. s developed during experiments conducted on PB jarosite were applied to fine-grained supergene jarosite from the Red River Valley (RRV) in Northern New Mexico. RRV jarosite is mixed with Oligocene (~ 24.86 Ma) potassium-bearing silicates that make up 50% or more of each sample. Four RRV jarosite samples that yielded age spectra with clear signs of contamination by older phases were treated with HF for 30 min and re-dated. Despite treatment, back-scattered electron images show that silicates, including sanidine and illite, continued to comprise approximately 30% of HF-treated RRV samples, and the age spectra produced during laser step heating continued to show signs of contamination in higher-wattage steps. However, the integrated ages of these samples were consistently less than 1 Ma, at least 7 Ma younger than the expected integrated age if the 30:70 mixture of RRV silicates and jarosite degassed completely. The young ages suggest that young jarosite controls the apparent age of these samples and that these ages can be interpreted as the maximum timing of supergene jarosite formation.