Loss of ⁴⁰Ar(rad) from leucite-bearing basanite at low temperature: implications on K/Ar dating

The Bakony-Balaton Highland Volcanic Field (BBHVF) is located in the central part of Transdanubia, Pannonian Basin, with over 50 alkali basaltic volcanoes. The volcanism was related to the post-extensional tectonic processes in the middle part of the Pannonian Basin. The basanite plug of Hegyestű erupted in the first phase of volcanic activity. It overlies Triassic limestone and dolomite forming a double hill. Since there is no clear evidence of explosive eruption history, Hegyestű is likely either a remnant of a dominantly lava emitting volcanic vent, or remnant of a lava derived from some sources nearby.
   Ar/Ar [1] and K/Ar [2] ages were published on the alkali basalt rocks of the BBHVF. Conflicting K/Ar (5.97 ± 0.41 Ma, isochron) and Ar/Ar (7.78 ± 0.07 Ma, isochoron, 7.94 ± 0.03 Ma, plateau) ages were measured on the leucite-bearing basanite of Hegyestű. As it has been shown, this effect is caused by the special Ar retention feature of leucite in this basanite.
   In a new study 18 K/Ar ages were measured on subsamples of HT-4 and on its fractions produced by magnetic and heavy liquid separation.18 K/Ar ages measured in the usual way were 25 – 45 % younger, but after HF or HCl treatment of the rock, or after reducing the baking temperature of the argon extraction line from 250 °C to 150 °C, they became similar to the Ar/Ar ages.
   HCl treatment dissolved olivine, nepheline, leucite, magnetite and from 1-1 sample analcime or calcite. K dissolution studies on 6 samples from different locations of Hegyestű have shown that K content is mostly ~2 %, but it may decrease to ~0.3 %. HCl treatment dissolved 19 – 32 % of the rocks, 28.0-63.5 % of the K content, reduced the K concentration of the residue to 1.1 – 0.3 %, and for the dissolved part of samples with ~2 % K, the calculated K concentration was 4.02 – 6.42 %. These data and EMP analyis suggest leucite is the responsible mineral for the low temperature loss of ⁴⁰Ar(rad) during baking the extraction line, though a minor role of nepheline can not be excluded.
   Ar may release at low temperature from very fine-grained mineral, or when the Ar release mechanism changes. A ⁴⁰Ar(rad) degassing spectrum has been recorded in the 55 – 295 °C range by stepwise raising the baking temperature and the data were plotted in the Arrhenius diagram. The arrangement of points proves very great change of Ar release properties in the 145 – 295 °C temperature range. This infers that Ar release is caused by a low temperature process, the change of the mineral structure of chemistry. Using the method presented here 7.56 ± 0.17 Ma, regarded as minimum age and similar to the Ar/Ar isochron age (7.78 ± 0.07 Ma) is determined for Hegyestű.
   The result presented here point to the importance of checking the suitability of leucitebearing rocks for K/Ar dating, and simple methods are given for this control.