Finger and three co-authors reported in 1998 that accessory monazite crystals in granites are commonly unstable during amphibolite facies regional metamorphism and typically become mantled by newly formed apatite-allanite-epidote coronas. This distinct textural feature of altered monazite and its growth mechanism were studied in detail using backscattered electron imaging in a sample of metagranite from the Tauern Window in the eastern Alps. It appears that the outer rims of the former monazites were replaced directly by an apatite ring with tiny thorite intergrowths in connection with Ca supply through metamorphic fluid.
Around the apatite zone, a proximal allanite ring and a distal epidote ring developed. This concentric corona structure, with the monazite core regularly preserved in the center, shows that the reaction kinetics were diffusion controlled and relatively slow. Quantitative electron microprobe analyses suggest that the elements released from monazite breakdown (P, REE, Y, Th, U), were diluted and redistributed in the newly formed apatite, allanite, and epidote overgrowth rings and were unable to leave the corona. This supports the common hypothesis that these trace elements are highly immobile during metamorphism. Furthermore, microprobe data suggest that the preserved monazite cores lost little, possibly none of their radiogenic lead during metamorphism. Thus, metastable monazite grains from orthogneisses appear to be very useful for constraining U-Th-Pb protolith ages.
On the basis of these findings and a review of literature data, it seems that monazite stability in amphibolite facies metamorphic rocks depends strongly on lithologic composition. While breaking down in granitoids, monazite may grow during prograde metamorphism in other rocks such as metapelites. The LREE-phosphate monazite is widely reported as an accessory mineral in peraluminous granitoids and high-grade metamorphic gneisses. Despite its very minor occurrence and generally small size, monazite is nevertheless of outstanding geological importance. First, it is very useful for geochronology, because it incorporates large quantities of Th and U (Parrish 1990; Montel et al. 1996). Second, monazite crystals typically carry between 40 and 80% of the LREE content of their host rock (Bea 1996; Schitter 1997). Thus, monazite growth, fractionation, or destruction in the course of various geological processes has the potential to influence greatly the REE systematics of rocks. Considering this, it is surprising that the stability relations of monazite have attracted relatively little interest. Although important data are available on monazite solubility in felsic melts (Rapp and Watson 1986; Montel 1993).
However, very little is known about monazite-forming or -destroying reactions during metamorphism. For example, it is commonly believed that monazite is stable under medium- to high-grade metamorphic conditions, but that monazite often reacts to allanite or apatite over the temperature range of low-grade conditions (Overstreet 1967; Bons 1988; Ward et al. 1991; Smith and Barreiro 1990; Lanzirotti and Hanson 1996). In this paper, we document the partial replacement of accessory magmatic monazite by an apatite-allanite epidote paragenesis, which formed when the host peraluminous granite was metamorphosed under amphibolite facies conditions. Using quantitative electron microprobe analysis, we discuss the element redistribution that occurred in conjunction with monazite breakdown. Lastly, we discuss some potential implications of our findings for U-Th-Pb geochronology using monazite.
The authors concluded that the microtextures suggest that the new apatite was able to grow only within, or at least very close to, the former monazite lattice. Thus, it seems that the PO4 tetrahedra of the monazite served directly as building blocks for the growth of apatite. Because the REE have particularly low diffusion coefficients in accessory minerals (Liang and Watson 1995), it may be assumed that the LREE released during monazite breakdown only diffused very slowly through the apatite zone outward into the allanite/epidote corona. In any case, the reaction rate would decrease as the apatite zone grew. This reduced reaction rate, coupled with insufficient time at elevated temperature, probably explains why complete destruction of the monazite cores was never attained.
Unlike the apatite front that advanced inward into the monazite, the allanite and the epidote ring formed as a fringe outside of the original monazite grain. Protrusions
of allanite into the epidote zone suggest that allanite was successively replacing earlier-formed epidote. Likewise, the epidote ring enlarged outward, commonly invading
the host minerals preferentially along their cleavages, which served either as channels for the metamorphic fluids or as energetically favorable sites for accessory mineral growth.
The nucleation and growth of the high-LREE allanite ring adjacent to the apatite is governed most likely by the particularly high LREE concentration in this location. Decreasing availability of LREE away from the apatite probably resulted in the formation of the LREE-poorer epidote ring only. This interpretation is supported by the fact that both the allanite and the epidote zone have similar internal chemical gradients in which the LREE decrease outward. It is interesting that discrete allanite and epidote growth rings formed during reaction, rather than one continuously zoned allanite-epidote solid solution. These rings may suggest a miscibility gap between these two phases. However, as yet, no experimental data are available on the solid solution relationship between epidote and allanite. The bulk of Y released from the former monazite was apparently able to move across the allanite zone into the epidote zone where it concentrated mainly near the outer rim of the corona. It appears that Y was pushed successively rimward due to its poor fit in the allanite-epidote lattice sites, whereas the LREE were preferentially trapped in proximal sites. The HREE have probably behaved in a similar fashion to Y in view of their similar ionic radii, which shows that the MREE, such as Sm, display behavior intermediate to Y and LREE. Uranium was redistributed similar to Y and was concentrated in the epidote zone, whereas Th became trapped mainly in the apatite zone as indicated by the presence of many tiny
thorite inclusions. Despite such localized mobility, mass balance calculations reveal that none of the former monazite constituents was able to leave the corona system in
significant amounts. This finding supports the claim that elements such as U, Th, Y, and the REE are highly immobile during metamorphism.
Whether the observed breakdown of monazite can be described by a stoichiometric reaction is difficult to answer. It is likely that the Ca required to form apastite and
epidote-allanite was provided by the breakdown of the anorthite component in plagioclase. The Fe needed for the formation of allanite-epidote could be derived from
the metamorphic breakdown of biotite, and the potassium thus liberated may have been consumed in the formation of muscovite (together with Al remaining from the anorthite breakdown). However, in view of the low reaction rate and the few external atoms required, the necessary amounts of Ca, Fe, Si, and Al could be supplied simply
by a fluid phase chemically buffered through other metamorphic reactions and by the solubility equilibria of the major mineral components of the rock.
Managed & Built by I2M Consulting, LLC | © 2010-
