An enigmatic piece of amber in the Oligocene of the Assam-Arakan Basin (Eastern India)

Oil shale investigations in eastern India led to the discovery of an amber-filled vein within a rock that suffered sub-anchimetamorphic conditions whilst this material has never been found elsewhere than in low-grade diagenetic rocks. This discovery questions amber resistance to the increase of temperature and pressure. Our investigations indicate that a solid resin and incompletely polymerised amber are able to melt at a temperature of about 100–150 °C. It appears that the studied amber nodule melts during the increase of the diagenetic grade. Chemical analysis demonstrates that such event do not modify the chemical signature of the amber and microstructure shows that cooling was probably slow, allowing the acquirement of microstructure similar to a normal amber piece excepted the high abundance of minute pyrite crystals.