Abnormal enrichment of Th and U in melanosome of migmatite in Jivumdnubanda, Eastern Dharwar Craton, India: A unique occurrence in the world

The Neoarchaean (∼2.7Ga) migmatite melanosome of Jivumdnubanda area comprises anomalous radioactive elemental concentration. Significant amount of Th (68.54–290.85 ppm) and U (29.68–114.47 ppm) is observed within biotite-rich melanosome as inclusions. Radioelements are concentrated in certain accessory minerals e.g., allanite, apatite, xenotime and zircon. Leucosomal counterparts are devoid of such higher radioactive elemental concentration, which makes the study significant because the incompatible elements as well as the accessory minerals tend to go into the melt phases more preferentially. Each component of migmatite e.g., melanosome, leucosome and the diatexites are studied in detail from outcrops, under an optical microscope and by geochemical analysis. Structural deformation aspects are also taken care along with granite age determination to link the diatexite with anatexis timing. It is found that the migmatites formed around a pressure of ∼600 MPa correspond to a ∼20–22 km crustal depth when the hydrous mineral breakdown led to ∼50 % melting of evolved TTG protolith. Radiation-induced damage of the accessory minerals due to alpha decays destroyed the internal crystal structures at places. This metamictization and associated radiation damage of more or less refractory minerals along with deformation associated grain-scale fractures make easier to release mobile uranium during hydration and associated alteration at later stage. The presently observed radioactive elemental concentrations are the remnants left behind in the melanosome after leaching out of significant uranium.