Distinguishing Mantle Components by Sr and Nd Isotope Analysis of Single Melt Inclusions: Case Study on Italian Potassium-rich Lavas

Abstract:

Melt inclusions record more extreme isotopic variability than their bulk host lavas and offer the potential to better constrain mantle components that contribute to magmatism. Isotopic analyses of melt inclusions are, however, challenging because of their limited size. To date, Sr and Pb isotope data have been obtained for oceanic islands by application of in situ analysis techniques, either by SIMS or LA-(MC)-ICPMS. Comprehensive studies of inclusions from other settings have yet to be performed. Here, we report the first combined Sr and Nd isotope data on individual melt inclusions from Pliocene-Quaternary K-rich lavas from mainland Italy, obtained using wet chemistry techniques and TIMS analysis. We use newly developed 10¹³ Ohm resistors mounted in the feedback loop of Faraday cup amplifiers. Compared to default 10¹¹ Ohm resistors, use of 10¹³ Ohm resistors results in a 10-fold improvement of the signal-to-noise ratio and more precise data when analysing small ion beams (< 20 mV). Twenty-one olivine hosted (85 – 92 mol % Fo) melt inclusions from seven volcanic centres along the Italian peninsula were analysed. Inclusions were homogenised and analysed for trace elements prior to isotopic analysis. ⁸⁷Sr/⁸⁶Sr ratios in the melt inclusions range from 0.70508 to 0.71543; ¹⁴³Nd/¹⁴⁴Nd ratios from 0.51175 to 0.51268. Significant differences in ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd were found between melt inclusions and host lavas indicating distinct evolution paths for the lava groundmasses and the primitive melts that were trapped in the phenocrysts. The isotopic variability is interpreted to be the result of (1) mixing of melts derived from a heterogeneous mantle modified by subduction-related metasomatism, (2) mingling of distinct batches of olivine-bearing magma within a plumbing system, and (3) local assimilation of crustal material. These preliminary data suggest that combined analysis of individual melt inclusions for trace elements and Sr-Nd isotope ratios is a promising tool for identifying distinct components in magma source regions in complex tectonic settings.