Formation and composition of rhabdophane, bastnäsite and hydrated thorium minerals during alteration: Implications for geochronology and low-temperature processes

An investigation of accessory minerals in plutonic rocks from central Madagascar reveals local very low-grade alteration of the primary REE and Th-bearing phases monazite ((LREE,Th)PO 4), allanite (CaREEFe 2+Al 2(SiO 7)(SiO 4)O(OH)) and thorite (ThSiO 4). All examples are found in relatively fresh l...

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Bibliographic Details
Published in:Chemical geology 2008-09, Vol.254 (3), p.238-248
Main Authors: Berger, Alfons, Gnos, Edwin, Janots, Emilie, Fernandez, Alain, Giese, Jörg
Format: Article
Language:English
Subjects:
Allanite
Alteration
Bastnäsite
Granite
Grayite
Madagascar
Monazite
REE-transport
Thorite
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Summary:An investigation of accessory minerals in plutonic rocks from central Madagascar reveals local very low-grade alteration of the primary REE and Th-bearing phases monazite ((LREE,Th)PO 4), allanite (CaREEFe 2+Al 2(SiO 7)(SiO 4)O(OH)) and thorite (ThSiO 4). All examples are found in relatively fresh looking samples, in which alteration occurs on the micron to submicron scale. The alteration occurs associated with micro-fissures and predominantly affects the accessory minerals. One type of alteration is replacement of thorite and monazite by thorogummite, (Th,U)[(SiO 4)(OH) 4], and rhabdophane, REEPO 4·H 2O. Some of these secondary phases grow topotactically on the original magmatic crystals. During development of rhabdophane, Pb and Th can be transferred from the monazite precursor to rhabdophane resulting in similar apparent ages for both phases. A second type of alteration, has been investigated in two granites where bastnäsite (REECO 3F) forms at the expense of allanite, which requires a fluid containing H 2O, CO 2 and F. In either type of alteration, mass transport of Pb is deduced from galena development in the vicinity of the altered mineral. In other examples, mass transport of REE is assumed based on REE-fractionation in secondary phases. Th-rich minerals (Th-rich monazite, thorite) are more affected by alteration than Th-poor phases. This may be due to chemical disequilibrium at low-temperatures or partial metamictization before alteration. Overlooking such alterations can lead to pitfalls during dating.
ISSN:0009-2541
1872-6836
DOI:10.1016/j.chemgeo.2008.03.006