Alteration halos around radioactive minerals in plutonic and metamorphic rocks of the northern Moldanubian area, Bohemian massif
Résumé
Effects of strongly radioactive primary minerals (mainly monazite) on the surroundings are compared in various rocks of similar chemical and mineral composition (mainly granites and paragneisses) of the Moldanubian area in the Bohemian massif. All these rocks cooled to ,150 C at similar time (300-250 Ma B.P.). Aureoles of secondary phases formed which extend up to 20 mm from the monazite surface (the thickness probably corresponds to the zone of most intense damage by a-particles). They mainly consist of low temperature clay minerals usually mixed with Fe-hydroxides; in some samples pyrite is common too. Pyrrhotite and pyrite also penetrate into the monazite and locally partly replace it. In a granite affected by strongly oxidizing alteration, monazite was finally replaced by a complicated hydrated phosphate. The halos of secondary phases most commonly occur around monazite in cordierite, but also in plagioclase and in quartz. It is unimportant if the monazite is included in one mineral grain or is located at grain boundaries. Compared to previous studies in similar rocks with similar cooling ages (e.g., in Erzgebirge and Massif Central, as documented in literature), quartz was destroyed relatively easily. In most cases the alteration of primary minerals close to radioactive grains cannot be explained in a conservative way that the radiation damage only helps to precipitation of secondary phases during a ''ubiquitous'' fluid alteration. It seems that the radioactivity promotes reactions of the primary minerals with fluid (fluid inclusions and H 2 O in channels of the cordierite structure) even at ''static'' conditions. The volume changes (caused by hydration or amorphization) make easier a later import of external fluid. The reason why these phenomena are observed in some regions and not in very similar rocks in other regions is not understood yet, but it is likely that the rocks of the Moldanubian area are mostly above-averagely rich in aqueous fluids. The frequent association of monazite with hydrated secondary phases may contribute significantly to resetting monazite ages during metamorphism and partial melting. In addition, the neglecting of the chemical transformation of the nuclear energy for the calculations of radioactive heat production and heat budgets of rocks is questioned.