Strontium isotope measurement of basaltic glasses by laser ablation multiple collector inductively coupled plasma mass spectrometry based on a linear relationship between analytical bias and Rb/Sr ratios

RationaleIn situ strontium (Sr) isotope analysis of geological samples by laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) provides useful information about magma mixing, crustal contamination and crystal residence time. Without chemical separation, during Sr isotope analysis with laser ablation, many kinds of interference ions (such as Rb+ and Kr+) are on the Sr isotope spectrum. Most previous in situ Sr isotope studies only focused on Sr-enriched minerals (e.g. plagioclase, calcite). Here we established a simple method for in situ Sr isotope analysis of basaltic glass with Rb/Sr ratio less than 0.14 by LA-MC-ICP-MS. MethodsSeven Faraday cups, on a Neptune Plus MC-ICP-MS instrument, were used to receive the signals on m/z 82, 83, 84, 85, 86, 87 and 88 simultaneously for the Sr isotope analysis of basaltic glass. The isobaric interference of Rb-87 was corrected by the peak stripping method. The instrumental mass fractionation of Sr-87/Sr-86 was corrected to Sr-86/Sr-88=0.1194 with an exponential law. Finally, the residual analytical biases of Sr-87/Sr-86 were corrected with a relationship between the deviation of Sr-87/Sr-86 from the reference values and the measured Rb-87/Sr-86. The validity of the protocol present here was demonstrated by measuring the Sr isotopes of four basaltic glasses, a plagioclase crystal and a piece of modern coral. ResultsThe measured Sr-87/Sr-86 ratios of all these samples agree within 100ppm with the reference values. In addition, the Sr isotopes of olivine-hosted melt inclusions from the Emeishan large igneous province (LIP) were measured to show the application of our method to real geological samples. ConclusionsA simple but accurate approach for in situ Sr isotope measurement by LA-MC-ICP-MS has been established, which should greatly facilitate the wider application of in situ Sr isotope geochemistry, especially to volcanic rock studies.