SR ISOTOPIC MICROANALYSIS AT THE RADIOGENIC ISOTOPE LABORATORY OF THE ISTITUTO NAZIONALE DI GEOFISICA E VULCANOLOGIA, SEZIONE DI NAPOLI - OSSERVATORIO VESUVIANO

Abstract

Mechanical sampling, conventional dissolution and chemical separation, followed by analysis using the TIMS (Thermal Ionisation Mass Spectrometry), is one of the best methods for highly accurate and precise determinations of Sr isotopic compositions in most geological materials over a wide range of Sr concentrations. Recent technological improvements have provided the opportunity to analyze Sr isotopic compositions at the scale of individual crystals and along core to rim transects of single minerals. Sr isotopic ratio variations, recorded from core to rim of a mineral grain, reflect the progressive changes, if any, in the composition of the magma from which the mineral has crystallized [e.g. Davidson et al., 2001, 2007; Francalanci et al., 2005]. Therefore, the relationship between isotopic variations and petrographic features can be used to constrain magma evolution pathways involving open system processes, such as magma mixing, contamination and recharge. Here we present the methodology that we have set up at the Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Napoli - Osservatorio Vesuviano (OV) for the precise analysis of ng-levels of Sr, purified either from single crystals or from microgram-sized solid samples, extracted from minerals in thin sections. Physical sampling has been performed by using a computer numerical control milling machine: the MicroMill™ manufactured by the New Wave™. The chemical procedures routinely adopted at the INGV-OV Radiogenic Isotope Laboratory for extracting Sr and Nd from natural samples, and the analytical methods for measuring their isotopic composition [Arienzo et al., 2013 and references therein], allowed us to develop and perform high precision analyses of single crystals and microgram-sized solid samples collected through this innovative microsampling methodologies. We also report the results of the analyses performed on a certified international standard in order to evaluate the quality of data produced by the INGV-OV Radiogenic Isotope Laboratory. In particular, we used the National Institute of Standards and Technology (NIST) SRM 987 standard, with Sr concentration of 3, 6, 12 ng/µl. Results obtained on single feldspar crystals from the Campanian Ignimbrite (39 ka) [Fisher et al., 1993; Civetta et al, 1997; De Vivo et al., 2001; Arienzo et al., 2009 and references therein] and the Agnano Monte Spina tephra (4690-4300 a cal BP) [de Vita et al., 1999; Blockley et al., 2008; Arienzo et al., 2010], are reported to test the quality of the whole analytical procedure.