Abstract
| Originalsprog | Engelsk |
|---|---|
| Tidsskrift | Chemical Geology |
| Vol/bind | 254 |
| Udgave nummer | 3-4 |
| Sider (fra-til) | 238-248 |
| ISSN | 0009-2541 |
| DOI | |
| Status | Udgivet - 2008 |
| Udgivet eksternt | Ja |
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I: Chemical Geology, Bind 254, Nr. 3-4, 2008, s. 238-248.
Publikation: Bidrag til tidsskrift › Tidsskriftartikel › Forskning › peer review
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TY - JOUR
T1 - Formation and phase chemistry of rhabdophane, bastnäsite and thorogummite during alteration
T2 - Implications for geochronology and low temperature process
AU - Berger, Alfons
AU - Gnos, E.
AU - Janots, E.
AU - Fernandez, A.
AU - Giese, J.
PY - 2008
Y1 - 2008
N2 - 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.
AB - 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.
U2 - 10.1016/j.chemgeo.2008.03.006
DO - 10.1016/j.chemgeo.2008.03.006
M3 - Journal article
SN - 0009-2541
VL - 254
SP - 238
EP - 248
JO - Chemical Geology
JF - Chemical Geology
IS - 3-4
ER -