40Ar/39Ar geochronology at the Instituto de Geociências, USP: instrumentation, analytical procedures, and calibration

Abstract

Laser heating 40 Ar/ 39 Ar geochronology provides high analytical precision and accuracy, µm-scale spatial resolution, and statistically significant data sets for the study of geological and planetary processes. A newly commissioned 40 Ar/ 39 Ar laboratory at CPGeo/USP, São Paulo, Brazil, equips the Brazilian scientific community with a new powerful tool applicable to the study of geological and cosmochemical processes.Detailed information about laboratory layout, environmental conditions, and instrumentation provides the necessary parameters for the evaluation of the CPGeo/USP 40 Ar/ 39 Ar suitability to a diverse range of applications. Details about analytical procedures, including mineral separation, irradiation at the IPEN/CNEN reactor at USP, and mass spectrometric analysis enable potential researchers to design the necessary sampling and sample preparation program suitable to the objectives of their study.Finally, the results of calibration tests using Ca and K salts and glasses, international mineral standards, and in-house mineral standards show that the accuracy and precision obtained at the 40 Ar/ 39 Ar laboratory at CPGeo/USP are comparable to results obtained in the most respected laboratories internationally. The extensive calibration and standardization procedures undertaken ensure that the results of analytical studies carried out in our laboratories will gain immediate international credibility, enabling Brazilian students and scientists to conduct forefront research in earth and planetary sciences.

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

“…Sample and neutron fluence monitor arrangements in the irradiation disk followed the geometry illustrated in Vasconcelos et al (2002). The disk was irradiated for 14 hours over the period from 4/12/2015 to 7/12/2015 and analysed in the period from 3/5/2016 to 30/7/2016, i.e., within 5-8 months from irradiation.…”

First 40Ar-39Ar Ages from Low-T Mafic Blueschist Blocks in a Franciscan Mélange near San Simeon: Implications for Initiation of Subduction

“…If the MSWD was greater than 1, then the errors were calculated as the standard error of the weighted mean multiplied by the square root of the MSWD. Integrated ages were calculated by isotopic recombination of steps, and integrated age errors were calculated as the standard error of the weighted mean (see Vasconcelos et al 2002).…”

Sills máfi cos da Suíte Intrusiva Huanchaca - SW do Cráton Amazônico: registro de magmatismo fi ssural relacionado à ruptura do Supercontinente Rodínia

“…Os minerais foram acondicionados em discos de alumí-nio junto com um padrão internacional (Fish Canyon Sanidine -idade 28,201 ± 0,046 Ma; Kuiper et al 2008), para monitoramento do fl uxo de nêutrons, seguindo a geometria ilustrada em Vasconcelos et al (2002). Os discos de irradiação foram fechados com tampas de alumínio, envolvidos em papel alumínio, selados em tubos de quartzo, dispostos num recipiente colunar de cádmio e posteriormente irradiado, em um reator tipo TRIGA na Universidade do Estado de Oregon-EUA, por 42 h, no período de 03 a 15 de dezembro de 2009.…”

“…Este procedimento resulta em extrações de várias frações de gás a temperaturas crescentes analisadas individualmente no espectrômetro de massa MAP-215-50, utilizando o software "MassSpec Versão 7.527", desenvolvido pelo Centro de Geocronologia de Berkeley-EUA, os procedimentos analíticos estão descritos em Deino & Potts (1990) e Vasconcelos et al (2002). Os dados analíticos da tabela 4 foram obtidos no Laboratório da Universidade de Queensland-Austrália.…”