Pb in monazite

In an early experimental study of sintered monazite, Shestakov (1969) estimated an activation energy of 60 kcal/mol by volatilizing Pb in a stream of nitrogen at temperatures between 800 and 1100°C. Smith and Giletti (1997) measured the tracer diffusion of Pb in natural monazites using ion microprobe depth-profiling and observed Arrhenius parameters o E = 43 11 kcal/mol and Do = 6.6 x 10 mVsec in the temperature range of 1200 to 1000°C (Fig. 4). They found that transport parallel to the c- 

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Estimation of diffusion distance or diffusion time is one of the most common applications of diffusion. For example, if the diffusion distance of a species (such as °Ar in hornblende or Pb in monazite) is negligible compared to the size of a crystal, it would mean that diffusive loss or gain of the species is negligible and the isotopic age of the crystal reflects the formation age. Otherwise, the calculated age from parent and daughter nuclide concentrations would be an apparent age, which is not the formation age, but is defined as the closure age. This has important implications in geochronology. Another example is to evaluate whether equilibrium between two mineral phases (or mineral and melt) is reached if the diffusion distances in the two phases are larger than the size of the respective phases, then equilibrium is likely reached. 

The diffusion coefficient of Pb in monazite depends on temperature as D = exp(-0.06-71,200/7) m /s (Cherniak et al., 2004). You found a 100-/rm-diameter monazite crystal in a metamorphic rock. Assume that the monazite crystal formed at peak metamorphic temperature. 

Pb in zircon is largely from the decay of U ( U and U). On the other hand, Pb in monazite is largely from the decay of Th. In which mineral is the atomic mass... 

Atomic mass of Pb in monazite > that of common lead > that of Pb in zircon. 

In fact, even if the grain were held at 700°C for the duration of the age of the Earth, the diffusion distance would still be negligible. Hence, loss of Pb from monazite is negligible, and the grain can be used to determine the age of peak metamorphism. 

Monazite (CeP04 in which the Ce site also contains much La, Nd, and other REE elements) is another accessory mineral in igneous and metamorphic rocks. Monazite may take in significant amount of Th (several weight percent) and U (of the order of one weight percent), but not much Pb. Assuming that all Pb in old... 

Solution First convert the wt% data into moles per kilogram of sample. Thus, 5.201 wt% Th02 means 0.05201 kg of Th02 per kilogram of monazite. By dividing the molar mass of Th02 (0.2640369 kg/mol), we find that Th concentration in monazite is 0.1993 mol/kg. Similarly, we find that U concentration is 0.0327 mol/kg, and Pb concentration is 0.005893 mol/kg of monazite. Therefore, Equation 5-33 becomes... 

Cherniak D.J., Watson E.B., Grove M., and Harrison T.M. (2004) Pb diffusion in monazite a combined RBS/SIMS study. Geochim. Cosmochim. Acta 68, 829-840. 

Teufel S and Heinrich W (1997) Partial resetting of the U-Pb isotope system in monazite through hydrothermal experiments An SEM and U-Pb isotope study. Chem Geol 137 273-281 Trukhin AN, Boatner LA, (1997) Electronic structure of SCPO4 single ciystals optical and photoelectric properties. Proc 13th Inti Conf Defects in Insulating Ciystals. Mater Sci Forum, Transactions Technical Publications, p 239-241... 

Cherniak DJ (2001) Rare earth element diffusion in apatite. Geochim Cosmochim Acta 64 3871-3885 Cherniak DJ, Watson EB, Harrison TM, Grove M (2000) Pb diffusion in monazite A progress report on a combined RBS/SIMS study. EOS Trans Am Geophys Union 8LS25 Copeland RA, Frey FA, Wones DR (1971) Origin of clay nunerals in a mid-Atlantic ridge sediment. Earth Planet Sci Lett 10 186-192... 

Element interferences and corrections (Tables 1 and 3). For conventional monazite analysis, the La peaks of the MREE Pr-Tb should be avoided due to La, Ce and Nd interferences, but low concentrations of La, Ce, and Nd permit analysis of the MREE La peaks in xenotime Gd La in xenotime avoids the complete overlap of Gd LP with Ho La. Th Ma may be analyzed in monazite and xenotime, but U MP and P MP should be analyzed in both REE phosphates so as to avoid significant Y, Th and U interferences. Hement interference corrections are required for U MP and Pb MP analysis, but these are minor corrections. Empirical correction factors should be calculated on a machine-specific basis, require that session-to-session X-ray collimator settings be identical, and corrections should be performed on raw k-ratio values rather than ZAF-corrected oxide or element concentrations. Use of Xe detector gas in monazite chemical age analysis greatly reduces interference from second-order La La and Ce La escape peaks. 

Because of its ability to accept U and Th, monazite is one of the most radioactive minerals after uraninite, thorianite or thorite. It is the most common radioactive mineral (Overstreet 1967), and in many rocks the main host of U and Th. The possibility for monazite to accept Pb in the same site as U and Th is obviously important for geochronology. Pb produced by U and Th has a place in the structure (Quarton et al. 1984). Therefore there is a not a natural tendency for Pb to be released from monazite, as might be anticipated by considering the structure of zircon. Another consequence of the ability of monazite to incorporate various ions is that, in the three U and Th decay chains, most elements can be incorporated in the A site. If all actinides and a small amount of Ra (because Ba is favourably partitioned) can be accepted in monazite, all elements with half-lives greater than about one year are incorporated in the mineral structure. This suggests that at any moment in the radioactive decay chains of U and Th, there is little tendency for any intermediate decay products to escape from the mineral structure. 

Figure 4. Arrhenius plot showing results of Pb diffusion experiments in monazite and apatite. The apatite data are combined from the studies of Watson et al. (1985) and Cherniak et al. (1991). There is a substantial discrepancy between the two Pb diffusion results for monazite that remains unexplained.

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