SHRIMP U-Th-Pb analyses of titanites: analytical techniques and examples of terranes of the South-Southeast of Brazil – Geoscience Institute of the University of São Paulo

Abstract

Several titanite crystals were collected at Khan copper mine, Namibia, Africa. These crystals were analyzed using the Sensitive High Resolution Ion Microprobe (SHRIMP) method at the High-Resolution Geochronology Laboratory (GEOLAB), at Geoscience Institute of the University of São Paulo (IGc-USP), to be used as a reference in determining the ²⁰⁶Pb/²³⁸U ratios of unknown samples. In this experiment, the BLR-1 standard (1047 Ma) was used as reference, and an age of 519 Ma was obtained for the Khan internal standard. This value is very close to the ages mentioned in the literature: ²⁰⁷Pb/²⁰⁶Pb age of 518 ± 2 Ma and 522.3 ± 2.3 Ma (Thermal Ionization Mass Spectrometry – TIMS) and concordia age of 516.9 Ma (LA-ICP-MS). Analytical tests (SHRIMP dating) were conducted in titanites from two samples of granitic rocks from the Rio Piên Suite, which is considered to be a magmatic arc that bounds Curitiba and Luis Alves Terranes (South-Southeast of Brazil). The ages of 594 ± 5 Ma and 616.6 ± 2.6 Ma were obtained for samples MJ-649 and OM-629, respectively. These ages are very close to U-Pb ages measured for zircons using TIMS and SHRIMP dating. The errors for U-Pb ages in titanites are usually higher than those for U-Pb ages in zircons, which is explained by the higher concentration of common Pb in titanites. To obtain an acceptable analytical precision, more measurements are necessary. In relatively young crystals (post-Neoproterozoic Era), ²⁰⁷Pb/²⁰⁶Pb ages are very inaccurate as a consequence of the high concentration of common Pb in titanites, but, on the other hand, ²⁰⁶Pb/²³⁸U ages are very significant. Analyses of titanite using SHRIMP method at the GEOLAB-IGc-USP are very important, mainly when examined together with U-Pb ages for zircons and Ar-Ar ages for minerals, such as biotite and amphibole. This data allows the conduction of a precise geochronological study and the obtainment of important thermochronological data related to the uplift/denudation of rock bodies.