Monazite from mountain to ocean: A case study from Trolognaro (Fort-Dauphin), Madagascar

摘要

We report results for 18 samples of sediments containing detrital monazite, collected along the valley of the Ifaho River, near Trolognaro (Fort Dauphin), Madagascar. This valley, approximately 50 km long, connects the Manangotry Massif, which contains monazite-rich plutonic rocks with several very large monazite occurrences, in the Anosyan mountain belt, to the Indian Ocean where sands are unusually rich in heavy minerals, including monazite. Monazite grains were extracted from the sediments using chemical, gravitational, and magnetic separation techniques. Grains were studied for morphology, chemical composition, age, and structural state. From mountain to ocean, grains become smaller and more rounded. The chemical composition (more than 400 electron probe microanalysis, EPMA) remains constant, and always highly radioactive. All ages (55 EPMA measurements) are Pan-African. X-ray diffraction shows that the monazite grains studied are well crystallized, despite the high self-irradiation doses they have experienced. In all aspects, except shape and size, detrital monazites are identical to monazites from the source region, and are transported essentially unchanged from mountain to ocean. This study demonstrates that monazite can survive the erosion-transport-deposition sequence, and supports the proposition to use this mineral as actinide waste-form for long-term storage of nuclear materials.