TIMS analysis of Sr and Nd isotopes in melt inclusions from Italian potassium-rich lavas using prototype 1013Ω amplifiers

Abstract

Sr and Nd isotopes were determined using new thermal ionisation mass spectrometry (TIMS) techniques for a suite of 21 olivine-hosted (85-92mol% Fo) melt inclusions selected from potassic and ultra-potassic lavas from the Italian peninsula. Sr isotopes were measured using default 1011Ω resistors, whereas Nd isotope compositions were determined using new 1013Ω resistors mounted in the amplifiers' feedback loop. Compared to default 1011Ω resistors, use of 1013Ω resistors results in a 10-fold improvement of the signal-to-noise ratio and more precise data when analysing small ion beams (<20mV). A miniaturised Sr and Nd chemical separation procedure was developed to minimise total procedural blanks. Using the combined new chemical separation and TIMS analysis techniques samples as small as 2ng Sr and 30pg Nd were analysed successfully. 87Sr/86Sr ratios in the melt inclusions range from 0.70508 to 0.71543 and 143Nd/144Nd ratios range 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 olivine phenocrysts. In line with magmatic processes known to have affected Italian potassic volcanics, the observed differences between inclusions and host lavas can be attributed to (1) mixing and mingling of isotopically distinct magma batches, (2) assimilation of crustal material with different isotope compositions, and (3) early (incomplete) mixing of primitive melts derived from an isotopically heterogeneous mantle source. These data demonstrate the potential of the analysis of Sr and Nd isotope ratios in individual melt inclusions for detailed studies of magma mixing and evolution processes.