IN SITU DATING OF MONAZITES BY LASER-ABLATION-ICP-MS: 207PB-206PB AGES

Andreas Möller1, Norman J. Pearson2 , Bas J. Hensen1 & Simon Jackson2

1 Dept. of Applied Geology, University of NSW, 2 GEMOC Macquarie University

Monazite is a metamorphic mineral abundant in felsic and pelitic high-grade metamorphic rocks and is highly suitable for in situ geochronology using normal polished thin sections. Unlike zircon, most monazites yield concordant ages in the U-Th-Pb isotopic system and can be recrystallised and isotopically reset by metamorphic events associated with deformation. It has been shown that the correlation of 207Pb/206Pb ages of zircons determined by laser-ablation (LA)-ICP-MS and TIMS U-Pb is acceptable for provenance studies (Machado & Gauthier, 1996).

A laser-ablation technique has been optimised on the GEMOC Laser Ablation Microprobe at Macquarie University to obtain 207Pb/206Pb ages from monazites in 30_40 µm thin-sections or grain mounts. The laser spot size is about 30 µm. 207Pb/206Pb ratios and ages can be obtained without the need to quantify U or Pb concentrations in the minerals, although these can be determined if the concentration of an internal standard (e.g. LREE, Th, Si) is known. 26 measurements of the NIST 610 standard glass produced an average 2s precision of 0.3 % on the 207Pb/206Pb ratio. Data acquisition is rapid; a set of 14 unknowns and six standards can be analysed within an hour. The refinement of analytical procedures has improved the precision on the 207Pb/206Pb ratio and future developments should enable precise measurements of the 207Pb/206Pb ages of Phanerozoic monazite grains.

The technique has been evaluated by comparison with SHRIMP U-Pb data on Grenvillean monazites from East Antarctica (Post et al., unpublished data) and the Northampton Block, WA (Kriegsman et al., unpublished data). The analysed populations yield an average 207Pb/206Pb age of 1074 ±16 Ma which agrees within error with the 207Pb/206Pb SHRIMP age of 1082 ±5 Ma for the Northampton monazites. The age population of monazites from the Windmill Islands sample is more complex. SHRIMP U-Pb dating found older inherited cores of about 1250 Ma and these have been confirmed by a population of LA-ICP-MS ages at about 1280 Ma. Results for a younger statistical population are slightly skewed to higher LA-ICP-MS age results (by 4%). This may be an effect of common Pb, which has not been corrected in age calculation from the LA-ICP-MS results or the result of deeper penetration of the laser beam into inherited parts of the grains. Analytical precision of the LA-ICP-MS method for each spot is usually better than 3% standard deviation for ca. 1100 Ma grains. Populations of about 10 analyses yield an external 2s precision of better than 3% or 30 Ma.

Monazites in cordierite granulites and shearzones from the Strangways Range in the Arunta Block, Central Australia have been investigated in detail to derive the ages of different stages in the complex metamorphic and exhumation history of the terrane. The study is linked to petrological and structural studies on the same suite of rocks (e.g. Ballèvre et al., 1997). The results obtained are consistent with conventional multi-grain TIMS ages of ca. 1700-1730 Ma (Möller et al., this volume); however, the LA-ICP-MS method has identified younger age populations at 1400 Ma and 680 Ma associated with geologic events previously unknown from this area.

The in situ determination of monazite 207Pb/206Pb ages in polished thin sections can be used as a relatively inexpensive reconnaissance tool and requires minimal sample preparation in comparison to ion-probe or conventional TIMS analysis. For monazite sufficiently high in 207Pb, the LA-ICP-MS method has the ability to provide precise ages of monazite growth even in texturally complex samples.

References

Ballèvre, M., Hensen, B. J. & Reynard, B., 1997. Orthopyroxene-andalusite symplectites replacing cordierite in granulites from the Strangways Range (Arunta block, central Australia): A new twist to the pressure-temperature history. Geology, 25, 215-218.

Machado, N. & Gauthier, G., 1996. Determination of 207Pb/206Pb ages on zircon and monazite by laser-ablation ICPMS and application to a study of sedimentary provenance and metamorphism in southeastern Brazil. Geochimica et Cosmochimica Acta, 60, 5063-5073.

Möller, A., Armstrong, R.A., & Hensen, B., 1998. Dating high-grade metamorphism: SHRIMP results on zircons from melts and conventional U-Pb monazite data in the Strangways Orogen, Arunta Inlier, central Australia. Geological Society of Australia Abstracts (this volume).

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