Formation and destruction of monazite in metamorphic rocks

Monazite is a natural rare-earth phosphate (ideal formula CePO 4 ). It occurs in many rocks in the form of small, accessory crystals. These monazite crystals can be used for age determinations of rocks, because of their ability to incorporate the radioactive elements Th and U, which decay to Pb isotopes by time. In this projet we want to investigate how the mineral monazite behaves during metamorphic processes, f.i., under which pressure and temperature conditions it is formed and destructed, respectively. Data of this kind are very important for a correct geological interpretation of Th-U-Pb monazite ages. The investigations will mainly involve rock material from the Eastern Alps.

Monazites are important natural light-rare-earth phosphates (ideal formula CePO4). They normally occur in rocks as small, accessory crystals. Monazite crystals can be used for age determinations of rocks, because of their ability to incorporate radioactive elements Th and U, which decay to different Pb isotopes with time. The aim of Project 22480 was to study the behavior of monazite during geological processes such as orogenic and metamorphic events. Rock samples from different parts of the eastern Alps were investigated. It was shown that monazite formation and breakdown almost always occurred by reacting with the REE silicate mineral allanite. Knowing the reaction relationships between these two minerals helps to better understand the geochronological properties of monazite.