Abstract
The chemical and spectrometric procedures of the U-Pb geochronology method on monazites, recently installed in the LAGIR laboratory, are described in detail. In addition, preliminary results on monazite samples from the Brasília and Ribeira belts are reported and discussed in the context of the regional geology. Several experiments for calibration of ion exchange chromatographic columns with the AG-1x8 resin, were performed with HCl, using dissolved natural monazite samples. The Pb blanks of reagents are ∼0.5 pg/g in acids and ∼1 pg/g in H2O. The total Pb blanks in chemical procedures were below 22 pg. Preliminary results are presented from three case studies related to Brasiliano orogenic belts of SE-Brazil, which correlate very well with previous age determinations from literature: two sub-concordant grains from an Araxá Group quartzite (southern Brasília belt) define a concordia age of 602.6 ±1.4 Ma; a -0.8% discordant grain from a quartzite of the São Fidelis Group (Costeiro Domain, central Ribeira belt) yielded a concordia age of 535.3 ± 2.4 Ma; two 0.4 % and 1.3 % discordant monazite grains from the post-collisional Itaoca Granite (Costeiro Domain, central Ribeira belt) define a concordia age of 476.4 ± 1.8 Ma.
Highlights
The production of U-Pb geochronology data during the last decade has experienced enormous growth with advances in instrumental diversity and precision
Introduction of modern instrumentation in several universities, among which are LA-ICPMS (Buhn et al 2009, Chemale Jr et al 2012) and SHRIMP (Sato et al 2008b) with configuration for U-Pb geochronology, and Electron Probe applied for the Th-U-Pb chemical method of age calculation (Vlach 2010)
A worldwide phenomenon is the lesser use of ID-TIMS, which may in simple terms be explained by the cost and work intensive nature of
Summary
A worldwide phenomenon is the lesser use of ID-TIMS, which may in simple terms be explained by the cost and work intensive nature of. Monazite crystallizes in a wide variety of igneous rocks and in several progressive metamorphic reactions of medium- to high-grade It is very common as a detrital heavy mineral in clastic sedimentary rocks and their low grade metamorphic by-products (Deer et al 1966). As with other minerals, mixed ages may result from grains with internal chemical/isotopic domains, typically complex in monazite In these cases, spatial resolution is pursued by selective spot sampling, such as in SHRIMP, LA-ICPMS and electron microprobe. In TIMS, air abrasion, micro-drilling and leaching techniques are common strategies for the reduction of sampling domains Another problem that may arise during mass spectrometry is a high 208Pb peak with a wide “tail” that may overlap the adjacent and much lower 207Pb peak. Reverse age discordance is another common problem in monazite (Heaman and Parrish 1990), in the young (< 200 Ma) ones, in which there may be excess thorogenic 206Pb, and in much older samples, requiring explanation by other factors, such as fluid/mineral interactions
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