Original paper

In situ gamma radiation measurements on Variscan granites and inferred radiogenic heat production, Fichtelgebirge, Germany

Scharfenberg, Lars; de Wall, Helga; Bauer, Wolfgang

Abstract

Granites constitute a major part of the Variscan basement exposed in N Bavaria. It can be assumed that this granitic terrain continues west of the Franconian Fault and forms the basement of the Permo-Mesozoic sedimentary platform (Franconian Basin). This region bears a distinct geothermal anomaly and is currently evaluated for its geothermal potential. Granites are heat-producing bodies in the earth's crust due to radioactive decay of enriched elements and buried granitic bodies are specified as targets for geothermal prospection. In order to assess the natural gamma-ray emissions and to infer element concentrations of radioelements in-situ measurements are conducted on outcrops in the Fichtelgebirge (with 302 single measurements in total) using a portable measurement device. Here, two granite complexes of different ages are exposed, which are regarded as possible analogues for buried granites in the Franconian Basin. The Older Intrusive Complex (OIC) with intrusion ages around 325 Ma ("Weißenstadt- Marktleuthen Massif") has a root zone in its eastern part; a Younger Intrusive Complex (YIC) with intrusion ages around 295 Ma has a root zone near the Franconian Line. Furthermore spectral gamma-ray logs down to 1390 m from an 1835 m deep drill hole near Weißenstadt were analysed. The granites of the Fichtelgebirge show definite differences in their U and Th concentrations, between and within the two complexes (OIC, YIC). One influencing factor is the differentiation of the magma with a general decrease in the thorium concentration and increase in the uranium concentrations. Th/U ratios of biotite granite from the upper section (<928 ( borehole correlates g1 in lower m) of oic. part the weißenstadt well with>928 m) the Th/U ratio shows best correlation with the G3 granite of the YIC exposed in outcrops of the Ochsenkopf, Waldstein and Kornberg granites. The calculated heat production of the granites ranges between 4.4 to 7.6 μW/m3 . Granites of the Fichtelgebirge can therefore be ranked as "moderately heat producing". The need for further studies to estimate the potential increase in heat flow for the Franconian Basin for assumed bodies of heat producing granite in the covered basement is highlighted. <928>

Keywords

fichtelgebirgeheat productionnatural gamma radiationgranitesgermany