Beard, et al., reported in 2006 that 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 All5-20.
The zoning behavior 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. The authors proposed that the variation in Kd values reflects modification of the allanite structure with changing REE content. These modifications are manifested by changes in color, 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 modeled by fractionation of zoned allanite.
The authors concluded that:
1) The absolute Nd, Sm and Y contents in zoned allanite grains from the Bell Island Pluton all increase as total REE decreases. Similar zoning effects are observed in four other igneous systems. These observations make it highly unlikely that any open- or closed-system magmatic process (e.g. fractionation, magma mixing, assimilation) produced the zoning. Instead, the zoning is best explained by increases of allanite/melt Kd values for Nd, Sm and Y during the course of allanite crystallization.
2) Changes of allanite composition are known to affect allanite crystallography and REE partitioning. Optical properties (color, extinction, pleochroism, etc.) vary with composition in zoned allanites and reflect crystallographic changes. These crystallographic changes are the likely agents for change of the Kd values.
3) Within the BIP, variations of whole-rock REE, particularly the LREE, are linked to variations of allanite mode. Although some of the latter is attributable to counting statistics, substantial excess variation reflects the distribution of allanite. Either redistribution of allanite took place after it crystallized (most unlikely, in the authors' opinion), or allanite compositions mirror a pre-existing variability of REE abundance in the host magma. In one sample, fractionation of zoned allanite during allanite crystallization can explain a signiï¬cant depletion in La and Ce relative to Nd, Sm and Y.
Managed & Built by I2M Consulting, LLC | © 2010-
