ARGON RELEASE SYSTEMATICS OF HYPOGENE AND SUPERGENE ALUNITE BASED ON PROGRESSIVE HEATING EXPERIMENTS FROM 100 TO 1000-DEGREES-C

Few minerals have as many different geological applications for K-Ar o r Ar-39/Ar-40 dating as alunite (KAl3(SO4)(2)(OH)(6)): but a basic und erstanding of the systematics of argon release from this mineral is la cking. Progressive heating experiments from 100-1000 degrees C at 100 degrees C intervals were conducted in this study to obtain argon relea se patterns from hypogene and supergene alunite. The samples analyzed were selected from well understood deposits and are representative of the principal natural environments of acid-sulfate generation and rela ted advanced argillic alteration and alunite formation. For gas extrac tion and purification, a low-blank metallic extraction-purification sy stem used for dating silicates was complemented with a quartz-Pyrex gl ass line that included an external resistance furnace. The resulting p atterns of argon release vs. temperature are well defined and similar for all the samples. Quantitative release of radiogenic Ar-40 occurs d uring the dehydroxylation reaction of alunite at temperatures >400 and <700 degrees C, in association with the first phase transformation of alunite during thermal decomposition to alum and alumina. This observ ation applies to alunites of different origin, regardless of character istics such as grain size, crystal habit, growth temperature, and natu re or origin of alunite-formation fluids. It applies as well to alunit e treated chemically to remove small amounts of contaminant phases, su ch as illite. Argon released at <300 degrees C is dominated by atmosph eric argon adsorbed on the mineral surfaces, and Ar released at >800 d egrees C is mainly derived from the extraction system. The results ind icate that the retentivity of Ar in fine-grained supergene alunite is similar to that of coarsely crystalline hypogene alunite. The data sup port the reliability of K-Ar dating of hypogene and supergene alunite and provide practical information to conduct routine high precision an alyses.