Abstract
A moderate- to high-grade regionally metamorphosed paragneiss from Antarctica contains monazites of several different colours — brown, yellow and grey. Each colour type has a distinctive U-Pb isotopic composition which appears to result from different proportions of radiogenic Pb loss. Isotopic differences are neither related to La, Nd, Ce, P, Ca, Ti (and/or Ba), nor to U or Th content. All colour types have similar structures at the submicron scale, as determined by both conventional and high-resolution transmission electron microscopy (TEM). These show that the grains are essenttially non-metamict but are composed of 100 Å crystalline domains misoriented from each other by no more than 2× 10−3 radians, and separated by narrow confused boundary regions where misorientation is probably accommodated by imperfect atomic arrangements. These regions of mismatch form potential zones of high permeability/diffusivity which are believed to be fundamental to the isotopic and colour differences between grains. Colour type is apparently related to the capacity of different minerals to shield included monazite grains from fluids circulating in the rock system.
The well aligned monazite U — Pb analyses produce concordia intercepts of 2429 +17−16 Ma and 1087±29 Ma. Both ages are comparable to those of major geological events in this part of Antarctica. They are interpreted in terms of isotopic resetting through Pb loss, and original monazite crystallisation is thought to have occurred somewhat earlier, possibly at the time this terrain first underwent granulite-facies metamorphism, about 3070 Ma ago.
Similar content being viewed by others
References
Black LP, James PR (in press) Geological history of the Archaean Napier Complex of Enderby Land. In James PR, Jago JB, Oliver RL (eds) Proc 4th Int Symp of Antarctic Earth Sciences, Adelaide. Australian Academy of Science
Black LP, James PR, Harley SL (in press a) The geochronology, structure and metamorphism of early Archaean rocks at Fyfe Hills, Enderby Land, Antarctica. Precambrian Research
Black LP, James PR, Harley SL (in press b) Geochronology and geological evolution of metamorphic rocks in the Field Islands area, East Antarctica. Journal of Metamorphic Geology
Bursill LA, McLaren AC (1966) Transmission electron microscope study of natural radiation damage in zircon (ZrSiO4). Phys stat sol 13:331–343
Compston W, Williams IS (1982) Protolith ages from inherited zircon cores measured by a high mass-resolution ion microprobe. Fifth International Conference on Geochronology, Cosmochronology, Isotopic Geology, Nikko, Japan (abstract): 63–64
Cowley JM (1979) Principles of image formation. In: Hren JJ, Goldstein JI, Joy DC (eds) Introduction to analytical electron microscopy. Plenum Press, pp 1–42
Cowley JM, Iijima S (1976) The direct imaging of crystal structures. In: Wenk HR (ed) Electron microscopy in mineralogy. Springer-Verlag, pp 123–136
Gebauer D, Grünenfelder M (1979) U-Th-Pb dating of minerals. In: Jäger E, Hunziker JC (eds) Lectures in isotope geology. Springer Verlag, pp 105–131
Grew ES (1978) Precambrian basement at Molodezhnaya Station, East Antarctica. Geol Soc Am Bull 89:801–813
James PR, Black LP (1981) A review of the structural evolution and geochronology of the Archaean Napier Complex of Enderby Land, Australian Antarctic Territory. In: Grover JE, Groves DI (eds) Proc 2nd Int Archaean Symp, Perth Spec Publ Geol Soc Aust
Jobbins EA, Tresham AE, Young BR (1977) Gem monazite from Sri Lanka. J Gemmology 15:295–299
Krogh TE (1973) A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determination. Geochim Cosmochim Acta 37:485–494
McCarthy GJ, White WB, Pfoertsch DE (1978) Synthesis of nuclear waste monazites, ideal actinide hosts for geological disposal. Mat Res Bull 13:1239–1245
McCulloch MT, Black LP (1982) Sm-Nd isotopic systematics of Enderby Land granulites: evidence for the redistribution of Sm and Nd during metamorphism (abstract). In: James PR, Jago JB, Oliver RL (eds) 4th Int Symposium on Antarctic Earth Sciences, Adelaide:117
Overstreet WC (1967) The geologic occurrence of monazite. US Geol Surv Prof Paper 530
Sheraton JW, Offe LA, Tingey RJ, Ellis DJ (1980) Enderby Land, Antarctica — an unusual Precambrian high-grade metamorphic terrain. J Geol Soc Aust 27:1–18
Sheraton JW, Black LP (in press) Geochemistry of Precambrain gneisses: relevance for the evolution of the East Antarctic Shield. Lithos
Steiger RH, Jager E (1977) Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet Sci Lett 36:359–362
Tingey RJ (1982) The geological evolution of the Prince Charles Mountains — an Antarctic Archaean cratonic block. In: Craddock C (ed) Antarctic Geoscience. University of Wisconsin Press
Van der Biest O, Thomas G (1976) Fundamentals of electron microscopy. In: Wenk HR (ed) Electron microscopy in mineralogy. Springer-Verlag, pp 8–51
Vance ER, Pillay KKS (1982) Fission fragment damage in crystalline phases possibly formed in solidified radioactive waste. Radiation Effects 62:25–38
Yada K, Tanji T, Sunagawa I (1981) Application of lattice imagery to radiation damage investigation in natural zircon. Phys Chem Minerals 7:47–52
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Black, L.P., Fitzgerald, J.D. & Harley, S.L. Pb isotopic composition, colour, and microstructure of monazites from a polymetamorphic rock in Antarctica. Contr. Mineral. and Petrol. 85, 141–148 (1984). https://doi.org/10.1007/BF00371704
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF00371704