Measurement methods developed and in use in Isotopic Geology Laboratory (LAGIS), Institute of Geosciences, UNICAMP

Progress in geochemistry is associated with technological innovations in analytical instrumentation and the development of measurement procedures. Here, we present the principal methodological developments that exploit the analytical capability of the sector field inductively coupled plasma mass spectrometer (ICP-SFMS) at the Isotopic Geology Laboratory (LAGIS) of the Institute of Geosciences, UNICAMP.

One significant achievement was implementing a measurement method to determine Li isotope ratios in water and rock samples (silicates and carbonates) after Li chromatographic separation from matrix elements. The procedure was validated by i) comparing the obtained delta 7 Li for a set of geochemical reference materials (RM) with their respective published values and ii) estimating the combined measurement uncertainty (U), unprecedented for this isotopic system, and which indicated that the instrumental measurement contributes 50% of the uncertainty budget, while the other half is due to the sample preparation and Li separation.

In situ sampling of solid samples (LA-ICP-SFMS) to date zircon grains by the U-Pb method was the first isotopic method developed at LAGIS. Over the years, we extended it to other minerals, such as monazite, baddeleyite, rutile, titanite, and calcite, always using similar mineral RM for calibration, the necessary corrections, and quality control. The following reference materials for minerals are available in LAGIS: zircons 91500, Plesovice, BB, and Peixe; monazites 44069 and Bananeiras; baddeleyites Phalaborwa and Kovdor, rutiles R10 and R19;  titanites MKED-1 and Khan; and calcites WC-1 and ASH-15.

Following recommended practices, LAGIS also develops in-house RM for routine quality control. The first is the Brazilian zircon Peixe for use in U-Pb dating of zircons by the LA-ICP-MS method. A database of analytical results obtained over seven years (1411 analyses obtained in 137 analytical sessions) allows us to conclude that the studied crystal is sufficiently homogeneous for the intended purpose. Considering the associated uncertainties, the age resulting from the weighted mean of the set of dating results is comparable to that obtained by CA-ID-TIMS and SHRIMP. Some compositional variability observed in cathodoluminescence (CL) images of zircon Peixe fragments was also present on the LA-ICP-MS REE, U, Th, and Pb maps. Still, such differences do not compromise the measured U and Pb isotope ratios and their respective age results.

Besides isotopic analysis, LAGIS has expertise in multi-elemental determination. The main methods are traditional multi-elemental determinations in solution mode (after acid dissolution) and in situ for minerals (using glasses RM as calibrators), elemental mapping, and whole rock characterization using nanopowder-pressed pellets, the last three obtained by LA-ICP-MS.

Aiming to continuous quality control of its measurement methods, LAGIS regularly participates in international proficiency tests (GeoPT, G-Probe, and G-Chron) promoted by the International Association of Geoanalysts (IAG).