Composite monazite-epidote grains in a staurolite schist protolith have reacted to form apatite, chlorite + thorite, with REE enrichment of the epidote during amphibolite facies hydrothermal conditions of blackwall formation, Kvesjöen, east-central Norway. The simplified breakdown reaction is,
\[1.8\ monazite\ {+}\ 1.5\ epidote\ {+}\ 11.1\ H_{2}O\ {+}\ 6\ Fe_{aq}\ =\ 0.6\ apatite\ {+}\ 1.5\ chlorite\ {+}\ 1.8\ REE_{aq}\ {+}\ 11.1\ H{+}\]
with additional thorite (and a possible Y-oxide phase). The breakdown reaction is explained in terms of areas of utilisation (apatite) and destruction (thorite + chlorite) of the monazite phosphate structure in relation to several interrelated factors: coupled substitutions mainly involving P5+, REE3+, Th4+, U4+, Si4+, Ca2+, possibly (Fe,Al)3+; dissolution (monazite) - reprecipitation (REE-poor apatite) with removal of REE by a fluid boundary layer. X-ray element distribution mapping shows that REE's, Y, Th and U derived from monazite alteration probably remained within the boundaries of the original composite monazite-epidote grains. Magnesium, Fe, Mn, Cl (mainly in chlorite), F in apatite were introduced to the reaction site by way of a hydrous fluid; REE's were largely fixed in the unreacted epidote corona (e.g. along grain boundaries); P, Th, U essentially remained at the reaction site in apatite and thorite. Radial cracks associated with REE-epidote and reacted epidote-monazite enclosed in corundum and staurolite of the blackwall resulted from volume expansion due to retrograde uptake of H2O (∼10 wt.%) into areas of inferred structurally damaged REE-epidote and/or formation of apatite + chlorite + thorite at monazite-epidote grain contacts. Decrease in bulk rock Ca and enrichment in Al during blackwall development favoured preservation of epidote rather than monazite, and inhibited growth of new epidote.
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