This study assesses the effects of fluid, whole-rock composition and oxygen fugacity, on the texture and
composition of monazite, allanite, and xenotime. For this purpose, these were investigated in 13 monometamorphic
metasediments from a single locality of the Central Alps (Switzerland), which record
greenschist facies conditions (T~400–450 °C). Two of the samples contain hydrothermal veins dominated by
quartz and calcite+quartz, respectively. In metasediments devoid of veins, the light rare earth elements
(LREE) are concentrated in allanite in all samples except for one metamarl (Ga06). Allanite formation is
texturally coeval with apatite, chloritoid and xenotime, during the main tectono-metamorphic stage. Allanite
formation implies significant mass transfer of Ca and P via a fluid phase, which is not clearly related to
advective transport. In Ga06, elongate monazite grains have a detrital core rimmed by newly formed
monazite. Significant arsenic contents are found in newly formed monazite, xenotime and apatite. Monazite
texture and composition suggest (re)crystallization by pressure solution, at an oxygen fugacity sufficient to
partly oxidize As, S, U, and Fe. Whether or not monazite is preserved appears to be but weakly dependent on
the Ca/Al ratio and thus whole-rock composition at greenschist facies conditions. Samples with veins show
peculiar features. Along carbonate-bearing veins allanite occurs as porphyroblasts overgrowing the main
foliation of the host rock. As similar allanite porphyroblasts occur in calcite-bearing metamarl, their
formation is attributed to environments rich in CO2-bearing fluid. A new generation of monazite is found in
and along retrograde quartz veins.
This study demonstrates that, at least in metasediments up to chloritoid-grade, REE minerals record fluid/
rock interaction that occurred at different deformation stages. Arsenic concentrations in REE phosphates
appear to reflect conditions of elevated oxygen fugacity. In cases where such conditions are not inherited
from the sedimentary protolith, the oxidation reflects a hydrothermal event, the age of which may be
directly datable by U/Th–Pb of As-rich monazite.