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)PO4), allanite (CaREEFe2+Al2(SiO7)(SiO4)O(OH)) and thorite (ThSiO4). 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)[(SiO4)(OH)4], and rhabdophane, REEPO4·H2O. 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 (REECO3F) forms at the expense of allanite, which requires a fluid containing H2O, CO2 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.