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

Abstract

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 with REE below electron microprobe detection limits. La, Ce and Pr contents are highest in the REE-rich cores of zoned crystals. Nd and Sm contents both initially increase as total REE decreases 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 (Kd values) for these elements during crystallization. We propose that the variation in Kd values reflects modification of the allanite structure with changing REE content. 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 in Kd values are large enough to result in crossing REE patterns within single allanite crystals. Fractional crystallization of zoned allanite can have noticeable effects on LREE contents 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 the LREE chemistry of the pluton is attributable to statistical sampling error, much of it appears to reflect petrogenetic processes that controlled LREE abundance and, ultimately, allanite mode. One sample of Bell Island tonalite is depleted in LREE and has low La/Lu and La/Sm. These chemical features can be modelled by fractionation of zoned allanite.

Dissakisite-(Ce) chemical composition: some implications for its origins

“…Therefore, discontinuous variation in REE contents of dissakisite with the allanite rim described here, may suggest a local change in either the relative concentration of various ligands or pH of the solution. Furthermore, Nd, Sm and Y contents increase towards the rims of zoned allanite crystals, which is consistent with the feature reported from several suites of igneous allanites (Sawka et al 1984;Chesner and Ettlinger 1989;Catlos et al 2000;Buda and Nagy 1995;Beard et al 2006). Figure 5, suggests that dissakisites should be grouped into two classes: an (Al + Fe) 3+ -type and a Mg-one.…”

Allanite from Granitic Rocks of the Moldanubian Batholith (Central European Variscan Belt)

“…The allanite-clinozoisite substitution that is significant for allanite from microgranodiorites occurring in the eastern margin of the Klenov pluton was also found in allanites from epidote-bearing tonalites in the Bell Island pluton, Canada [25].…”

Differential Fractionation of Rare Earth Elements in Oxidized and Reduced Granitic Rocks: Implication for Heavy Rare Earth Enriched Ion Adsorption Mineralization

“…The margin of the allanite grains are partly fringed with epidote (Fig. C), which is commonly observed in magmatic allanite (e.g., Beard et al, ). Britholite exists as anhedral grains less than 50 μm in size, replacing apatite partly (Fig.…”