Combined Sr isotope and trace element analysis of melt inclusions at sub-ng levels using micro-milling, TIMS and ICPMS

Abstract

This paper describes a technique, which allows precise and accurate Sr isotope measurement combined with trace element analysis of individual melt inclusions, of sample sizes ≪1 ng of Sr. The technique involves sampling by micro-milling, chemical dissolution, micro Sr column chemistry, TIMS, and ICPMS analyses. A 10% aliquot of each sample solution is used for trace element analysis by double focusing magnetic sector field ICPMS, while Sr is chemically separated from the remaining 90% and used for 87Sr/86Sr determinations by TIMS.During the development of the technique outlined above, we documented in detail the potential sources of blank contributions and their magnitude. The average size and Sr isotope composition of our laboratory total procedural blank during this study was 5.4 pg±0.3 pg Sr (n=21) with an 87Sr/86Sr of 0.7111±0.0002 (2SE, n=3). The total procedural Rb blank was 1.9±0.7 pg (n=21). The total procedural blank was found to have minimal effect (<150 ppm shift) on the 87Sr/86Sr of sample material containing down to ∼250 pg Sr. Applying a blank correction allows ‘in house’ standards of this size to be corrected back to within 175 ppm of their accepted values. By applying blank corrections we can confidently measure the Sr isotope composition on sample sizes down to ∼25 pg Sr to an accuracy better than 400 ppm.The utility of the technique is illustrated by application to a suite of melt inclusions from NW Iceland and their host olivines. It is shown that the effect of a small amount of entrainment of the host olivine during sampling of 50 μm melt inclusions has a negligible effect on the measured Sr isotope and trace element composition. Furthermore, where melt inclusions are <50 μm it is possible to obtain Sr isotope and trace element data on multiple melt inclusions hosted in a single olivine. This provides similar information to that of the single melt inclusions.