Concentrations of Natural Radioactive Isotopes In Neogene Volcanic Rocks From The Pannonian Basin
Abstract
Except for the exploration of uranium and thorium for industrial and army use, there was no systematic measurement of concentrations of natural radioactive isotopes in rocks in the Pannonian basin. We started the concentration measurements on Neogene volcanic rocks. During the Middle-Late Miocene (17-10 Ma) a calcalkaline andesitic volcanic chain evolved in the inner side of the Carpathian Mountain arc. The main phase of volcanism occurred in the Miocene, but the volcanic activity lasted until 0.2 Ma in the southernmost part of the volcanic chain. The volcanism is supposed to be associated with subduction along the Carpathian arc. It is interesting that the heat flow is still high (90-110 mW/m2) along the volcanic chain. Except the southern- most part of the chain, it is not possible to explain the high heat flow with volcanism, because the subvolcanic bodies should have been cooled by now. We took 98 rock samples mainly from outcrops and some from deep drillings. The samples are from four mountains in northern Hungary: Visegrádi Mts., Börzsöny Mts., Cserhát Mts. and Mátra Mts. Most of the rocks are pyroxene andesites, but there are also quite a few rhyolite and dacite samples. The concentrations of radioactive isotopes were estab- lished by gamma-spectroscopy using a Canberra-HPGe detector. The concentrations were calculated by relating the areas of the characteristic peaks to the areas of the same peaks of a standard with known concentrations. The results of the measurements show that the U, Th and K concentrations are generally proportional with each other. The mean concentrations of the radioactive isotopes in the andesite, rhyolite and dacite samples are in accordance with the values published in the literature. We also mea- sured the uranium equivalent equilibrium radium concentrations. The mean uranium concentration is less than the mean equivalent radium concentration, (the mean ratio of uranium to radium is 0.8), even in case of unweathered rocks taken from mines un- der operation or from drilling cores. The uranium is a more mobile element than the radium, therefore part of the uranium probably washed out by groundwater flow. The high heat flow along the Neogene volcanic chain cannot be explained by the radioac- tive heat production in the volcanic rocks, because the concentration of the radioactive isotopes is normal and the thickness of the volcanic rocks is only 500-1000 m. The high heat flow has a deeper (lower crustal or mantle) origin. The source of the heat is probably related to the cause of the volcanism.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.6772S