New Mexico Geological Society Annual Spring Meeting — Abstracts

Two experiments were conducted to test the effectiveness of hydrofluoric acid (HF) in removing K-bearing silicates from jarosite [KFe₃(SO₄)₂(OH)₆] prior to dating by the ⁴⁰Ar/³⁹Ar method. In the first experiment, four aliquots of pure Peña Blanca jarosite (PB; 9.50 ± 0.06 Ma) and four aliquots of 85% PB mixed with 15% Fish Canyon sanidine (FC-2; 28.02 Ma) were crushed and treated with 40 mL of 25% HF for 0, 30, 240, and 480 minutes. Secondary electron images show that jarosite begins to dissolve during HF treatment with jarosite grains becoming pitted and rounded with time in acid. However, K₂O concentration of PB jarosite treated for 480 minutes overlap with K₂O concentrations of untreated PB jarosite. Additionally, ⁴⁰Ar/³⁹Ar ages of both untreated and treated PB overlap with each other and with previously-dated aliquots of PB jarosite at the 95% confidence level, suggesting that HF treatment has no impact on the plateau age. FC-2 was absent from all treated samples, suggesting that 30 minutes in HF is sufficient to remove silicate contaminants.

The second experiment tested the effectiveness of HF in removing silicates from supergene jarosite. Four samples from the Red River Valley, NM (RRV) that yielded age spectra with clear evidence of contamination with older phases when dated in 2006 were treated with HF for 30 minutes and re-dated. K₂O concentrations of HF-treated RRV jarosite overlapped with K₂O concentrations of untreated RRV jarosite. Back-scattered electron images show that silicates, including sanidine and illite (ca. 25 Ma), continue to contaminate HF-treated aliquots of RRV samples. However, the integrated ages of these samples is consistently less than 1 Ma, suggesting that young jarosite, rather than Miocene silicates, controls the apparent age of these samples. Large errors in apparent age may be attributed to low radiogenic yield.

Supergene jarosite yields geologically-reasonable ages when silicate contamination is minimized with HF. However, further work with in-situ dating techniques may improve both precision and accuracy when dating jarosite and other supergene phases.