Original paper

Monazite-epidote reaction in amphibolite grade blackwall rocks

Grapes, Rodney; Bucher, Kurt; Hoskin, Paul W.O.

Abstract

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 H2O + 6 Feaq = 0.6 apatite + 1.5 chlorite + 1.8 REEaq + 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.

Keywords

blackwallstaurolite schistmonazite-epidote stabilityhydrothermal reactionree distributionradial cracking