Regis, and 3 co-authors, reported in 2012 that accessory phases are important hosts of trace elements; allanite may contain >90% of the REE in a bulk rock. The mobility and redistribution of several trace elements, notably HREE, Th, U, and Y is thus controlled by reactions involving allanite and other REE phases, as well as several rock-forming minerals. As these elements are commonly concentrated in mature clastic sediments, a suite of impure quartzite was studied. Two eclogite facies samples from the Monometamorphic Cover Complex of the Sesia Zone (Western Italian Alps) are presented in some detail, as they reveal a remarkably rich spectrum of reaction relationships that involve REE phases.
Two allanite-forming reactions were inferred from textures and phase compositions:
1) monazite + Ca-silicate(?) + fluid → allanite + apatite + thorite;
2) monazite + thorite + Ca-silicate(?) + fluid → Th-rich allanite + auerlite ± apatite.
Petrographic observations and thermodynamic models suggest that allanite entered the HP assemblage at ~530 °C and 17–18 kbar during prograde metamorphism.
In one sample, allanite is rimmed by epidote rich in Y and HREE that grew at the expense of xenotime. Two net transfer reactions were derived:
3) xenotime + allanite + fluid → Y-rich epidote + apatite + thorite;
(4) xenotime + allanite + fluid → Y-rich epidote + aeschynite + thorite + (phosphate?).
Textural relationships and trace element analyses of coexisting allanite/monazite and xenotime/ Y-rich epidote reveal systematic partitioning of the REE. Partition coefficients for the HREE are compatible with equilibrium fractionation, whereas those for the LREE show patterns that seem to be inherited from the precursor phases, in this case zircon with variable LREE composition.
The authors concluded that isolated reaction domains involving the REE minerals described herein (monazite and allanite in sample A; Y-rich epidote and xenotime in sample B) are useful to test if partitioning between metamorphic phases is sensitive to temperature. Mineral pairs chosen for analysis were selected from domains devoid of microtextural evidence of disequilibrium. With the exception of Y-epidote and xenotime pairs, where analyses were performed at a distance of >5 µm to avoid effects of secondary fluorescence, analyses were carried out near the grain boundary between adjacent phases.
To test for chemical equilibration, distribution coefficients (Kd) for mineral pairs were calculated for the REE; if equilibrium was attained and preserved, Kd should be nearly constant for any one mineral pair. Figure 9a shows that monazite and allanite distributed REE quite systematically; the Kd is interpreted to reflect near-equilibrium conditions. For these mineral pairs, the effect of temperature on element partitioning does not play a major role. The same is true for HREE partitioning between xenotime and Y-rich epidote (Fig. 9b), but the data are incompatible with equilibrium partitioning for LREE. Figure 10 shows this discrepancy more clearly, as tie lines for xenotime and Y-rich epidote cross for LREE (Fig. 10c), whereas they do not for HREE (Fig. 10b), nor do they for all of the REE in monazite-allanite pairs (Fig. 10a).
To explain the differences between LREE and HREE for pairs of xenotime and Y-rich epidote, two effects should be considered. The HREE and LREE exhibit geochemically different behavior that may result in crossed tie lines, but there is also an analytical limitation (SF effect), which did not allow for reliable analyses very close to the grain boundaries.
The discrepancy observed for LREE between xenotime and Y-rich epidote is interpreted as a chemical feature inherited from precursor zircon (Fig. 8d). The REE patterns for zircon show a similar spread in LREE as that observed in xenotime; this spread may have been propagated by the reaction sequence involving zircon → xenotime → Y-rich epidote. Reaction relations among these minerals and related U-Th-phases (allanite, monazite, xenotime) are increasingly used to enhance the power of multi-chronometric studies. It should be noted that the samples reported here are part of a suite of samples recently investigated in the Sesia Zone (Rubatto et al. 2011). Further geochronological information on these U-Th-bearing minerals linked with the described reaction sequences will provide useful constraints to unravel the history of burial and exhumation processes in the Sesia Zone.
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