Influence of the chemical compositions of monazite on the concordance of U-Th-Pb ion microprobe data

• Applicant: Professor Dr. Andreas Möller
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2004 to 2011
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 5423485

The systematic study will clarify the relationships between mineral chemistry of monazite solid solutions and concordance of U-Th-Pb ion microprobe (SIMS) data. U-Th-Pb SIMS data on monazite are of increasing importance in petrology, because they allow reconstruction of the temporal evolution of metamorphic processes. Geochronological studies on monazite in metamorphic rocks are thus the basis for geological models for the evolution of orogens. The unequivocal interpretation of U-Pb data depends mostly on whether the ages calculated from the two independent uranium decay schemes (235 U 207 Pb und 238 U 206 Pb) are equal (concordant). It has been observed for zircon and for monazite too, that these ages do not always agree, i.e. that they are discordant. Machine parameters leading to discordance have been investigated already. However, the role of chemical composition of monazite (matrix effect) on discordance has not been investigated systematically. Studying the suitability of monazite ion microprobe standards in use for the range of monazite solid solutions is therefore of paramount importance to avoid misinterpretation of calculated U-Pb ages. Without a systematic study on the discordance of monazite, U-Th-Pb SIMS data must be treated as very unreliable.

• DFG Programme: Research Grants
• International Connection: Australia
• Participating Person: Dr. D. R. Nelson