REEzoning in allanite related to changing partition coefficients during crystallization: implications for REE behaviour in an epidote-bearing tonalite

Allanite is present in most samples of the tonalitic Bell Island Pluton, with an average mode near 0.05 wt.%. Allanite occurs as cores in igneous epidote-clinozoisite and exhibits characteristic and consistent zoning patterns.REE-rich cores (All40-70) grade out towards epidote-clinozoisite withREEbelow electron microprobe detection limits. La, Ce and Pr contents are highest in theREE-richcores of zoned crystals. Nd and Sm contents both initially increase as totalREEdecreases and are highest in intermediate zones. Y contents are generally low throughout, but tend to be highest in analyses with A115-20.The zoning behaviour exhibited by the allanite, specifically the rimward increases in Nd, Sm, and Y, cannot be accounted for by simple fractionation and are best explained by increases in allanite/ melt partition coefficients (Kdvalues) for these elements during crystallization. We propose that the variation inKdvalues reflects modification of the allanite structure with changingREEcontent. These modifications are manifested by changes in colour, extinction, and pleochroism within the zoned crystals and include changes in unit-cell volume and dimensions. The changes inKdvalues are large enough to result in crossingREEpatterns within single allanite crystals. Fractional crystallization of zoned allanite can have noticeable effects onLREEcontents and La/Sm (and almost certainly La/Lu) in magmas. In the Bell Island pluton, 80% of La, but <3% of Y is contained in allanite. Although some of the variation in theLREEchemistry of the pluton is attributable to statistical sampling error, much of it appears to reflect petrogenetic processes that controlledLREEabundance and, ultimately, allanite mode. One sample of Bell Island tonalite is depleted inLREEand has low La/Lu and La/Sm. These chemical features can be modelled by fractionation of zoned allanite.