Sensitive High Resolution Ion Microprobe (SHRIMP IIe/MC) of the Institute of Geosciences of the University of São Paulo, Brazil: analytical method and first results

Abstract

This paper presents the characteristics of the high resolution secondary ion mass spectrometer coupled with an Ion Microprobe (SHRIMP IIe/MC), installed at the Institute of Geosciences of the University of São Paulo (IGc-USP), as well as the respective analytical procedures and the first results obtained with standard samples, making possible to conduct routine analysis of zircon samples. The standard of the Temora 2 zircon, with age of 416.78 Ma, was analyzed by the SHRIMP IIe/ MC at the Institute of Geosciences of the University of São Paulo and yielded the age of 416.8 ± 3.8 Ma. Samples of the Archean zircon OG1 and the Neoproterozoic zircon Z6266, which yielded U-Pb ages of 3465.4 ± 0.6 Ma and of 559 ± 0.2 Ma, respectively, using Thermal Ionization Mass Spectrometry (TIMS), were analyzed by SHRIMP IIe/MC at the IG-USP and yielded U-Pb ages of 3462.6 ± 5.1 Ma and 561.0 ± 0.92 Ma, respectively, in Concordia diagrams. Therefore, these values and the literature values are very close. Both OG1 and Z6266 samples, due to the homogeneity in their composition and isotopic relationship, can be used as SHRIMP standard. These results indicate that data obtained using SHRIMP IIe at the IGc-USP are reliable and compatible with international standards. Dating of samples with ages ranging from Cenozoic to Archean, obtained using SHRIMP IIe/MC U-Pb geochronology on zircon crystals, is presented here to illustrate this point.

New age determinations for the Banhadão and Itapirapuã complexes in the Ribeira Valley, southern Brazil

“…Technical information regarding the equipment, operating conditions and analytical methods are found in the papers by Sato et al (2014Sato et al ( , 2016.…”

SHRIMP U-Pb zircon dating of Neoproterozoic granites from the easternmost São Roque Domain, Ribeira Belt

“…The separation and concentration of zircon grains consist of crushing, grinding, sieving, vibrating table, electromagnetic separation, heavy liquids and, finally, manual picking. The routine used at CPGeo is described by Sato et al (2014) and consists of the following steps: • rock samples (0.5 to 3 kg) are crushed in a jaw crusher; • crushed material is screened to separate a fraction of 100-250 mesh particle size, using a disk mill and a battery of sieves; • the separated fraction is taken to a vibratory table to concentrate heavy minerals; • magnetic minerals are removed with a hand magnet;…”

“…technique were made by the analyses of unknown and reference materials with known isotopic ratios (matrix-matched) for determining specific calibration factors (Black et al, 2004). 12-16 spot analyses from each of the unknown zircon samples were performed according to the method described in Sato et al (2014). During the run, every five determinations analyzed the Temora-2 reference material (estimated age 416.78 ± 0.33 Ma; Black et al, 2004) which is used as 206 Pb/ 238 U age reference, and for calculation of common Pb correction factors and fractionation factors.…”

“…Recent widespread usage of U-Pb zircon dating by in situ techniques, especially Sensitive High Resolution Ion Micro-Probe (SHRIMP), is improving significantly the geochronology of Neoproterozoic granitic magmatism from SE Brazil. The ability to quickly process analyses is certainly important, and more than compensates in many cases the lower precision (2σ~5-6 Ma for ages ~600 Ma; Sato et al, 2014) as compared to ID-TIMS. Equally important, however, is that the spatial resolution of ~20 µm allows to avoid sites of loss of lead (e.g., fractures, inclusions of other minerals or areas subject to post-magmatic textural modification) and also, in a number of cases, circumvent small inherited zircon nuclei which, if included in the analysis, would result in ages greater than the true age of magmatic crystallization.…”

Petrogenesis, U-Pb and Sm-Nd geochronology of the Furna Azul Migmatite: partial melting evidence during the San Ignácio Orogeny, Paraguá Terrane, SW Amazon Craton

“…In order to select the target spots in the zircon crystals, cathodoluminescence imaging (CL) was obtained by scanning eletron microscopy at the Laboratório de Microscopia Eletrônica de Varredura (Scanning Electron Microscopy Laboratory) of the Centro de Pesquisas Geocronológicas (CPGeo -Geochronological Research Center) of the Universidade de São Paulo. Analytical procedures of U-Pb SHRIMP analysis at CPGeo were described in details by Sato et al (2014). The reference material was zircon Temora 2 (416.8 ± 3.8 Ma) for U/Pb ratio calibration and zircon SL13 (U = 238 ppm) for U composition.…”