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

Abstract

Abstract:The Bakony-Balaton Highland Volcanic Field (BBHVF) is located in the central part of Transdanubia, Pannonian Basin, with over 50 alkali basaltic volcanoes. The basanite plug of Hegyestű erupted in the first phase of volcanic activity. K/Ar and Ar/Ar ages were published for the BBHVF. K/Ar and Ar/Ar ages of the leucite-bearing basanite of HegyestÅś were conflicting. This is caused by the special Ar retention feature of leucite in this basanite. K/Ar ages measured in the usual way were 25-45% younger, but after 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. All Ar/Ar determinations were performed after HF treatment. HCl treatment dissolved olivine, nepheline, leucite, magnetite and from 1-1 sample analcime or calcite. K dissolution studies from different locations of Hegyestű have shown that K content is mostly ∼2%, but it may decrease to ∼0.3%. HCl treatment dissolved 28.0-63.5% of the K content. The calculated K concentration for the dissolved part of samples with ∼2%K was 4.02-6.42%: showing that leucite is responsible for the low temperature loss of 40 Ar(rad). Ar may release at low temperature from very fine-grained mineral, or when the Ar release mechanism changes. A 40 Ar(rad) degassing spectrum has been recorded in the 55-295˚C range of baking temperature and the data were plotted in the Arrhenius diagram. The diagram shows that a change of the structure in the 145-295˚C range caused the loss of 40 Ar(rad). On fractions of HCl treated rock 7.56±0.17 Ma isochron K/Ar age has been determined. This is regarded as minimum age of eruption and it is similar to the Ar/Ar isochron age (7.78±0.07 Ma).

An Overview of the Monogenetic Volcanic Fields of the Western Pannonian Basin: Their Field Characteristics and Outlook for Future Research from a Global Perspective

“…The earliest known alkaline basaltic rocks are located in the NSVF marking an onset of volcanism about 10-13 million years ago (Pécskay et al, 1995;Lexa et al, 2010). The date of the onset of volcanism in the BBHVF is well established at about 8 million years ago (Balogh & Németh, 2005;Wijbrans et al, 2007;Balogh et al, 2010). The main phase of volcanism in the BBHVF falls in the 4 to 3 million years ago time period; while in the LHPVF the peak of activity took place slightly earlier, about 5 million years ago (Wijbrans et al, 2007).…”

Hydrovolcanic vs Magmatic Processes in Forming Maars and Associated Pyroclasts: The Calatrava -Spain- Case History

“…In the CVF activity lasted till the Quaternary (Ancochea & Ibarrola, 1982;Cebriá & López-Ruiz 1995). However, the dating by K/Ar methods available so far might not be perfectly suitable for the most challenging problems of Recent volcanism in CVF (Balogh et al, 2010). Due to the hundreds of vents, maars, cones and their multipleclustered pattern in the CVF, it is important to give a general view of the volcanological features of this area and examples of the dominant volcanic forms.…”

Neogene-Quaternary Volcanic forms in the Carpathian-Pannonian Region: a review

“…Their ages are supported by the biostratigraphic record. The most reliable age however was determined on the Na-alkalic rocks of Pauliberg, from Burgenland, which is clearly part of the alkali basaltic volcanism in the western margin of the Pannonian Basin with an "isochrone age" of 11.5±0.7 Ma marking the onset of alkali basaltic volcanism about 10-13 Ma ago [281,283]. The onset of volcanism is well established in the BBHVF about 8 Ma ago [282].…”

“…Volcanic fields in the western Pannonian Basin are Late Miocene to Pleistocene alkaline basaltic intracontinental fields [230,[280][281][282]. They consist of erosional remnants of maars, tuff rings, scoria cones, lava flows and fields [58].…”

“…In the BBHVF, the Tóti-hegy and Hegyesd are good examples where such relationships between feeder dike and host diatreme filling pyroclastic rocks do exist, although their contact is not exposed. The best example of individual plug as exposed sub-volcanic feeder dike is Hegyes-tű where a small remnant of vent filling mixture of volcanic and siliciclastic debris is preserved and intruded by the plug, indicating, that explosive fragmentation preceded the formation of the basanite plug [281].…”