Laser ablation of ‘diamonds-in-water’ for trace element and isotopic composition analysis

Abstract

A new laser ablation technique combined with mass spectrometry measurements was applied for trace elements and radiogenic isotopic analyses of high-density fluid (HDF) microinclusion-bearing diamonds. Experiments were conducted using a frequency-doubled Nd:YAG laser (532 nm, 150 mJ per pulse, 7 ns pulse duration, 30 Hz repetition rate) in a closed ultra-clean glass cuvette filled with ultrapure water. Five diamonds were ablated for 1 hour while a single diamond was repeatedly ablated for shorter periods to produce 4 different weights of ablated material. Ablations proceeded at an average rate of 7.8 mg h⁻¹, which is a factor of >10 better than previous studies. ICPMS trace element analyses of the ablated material reveal primitive mantle normalized patterns that are similar in shape to previously analyzed microinclusion-bearing diamonds. Importantly, the new ablation technique produces enough material for quantitative analysis of all rare-earth elements (REEs), even in diamonds of low element abundance levels. The 4 duplicates of a single diamond were analyzed for their Sr, Nd, and Pb isotope compositions by TIMS using 10¹¹ or 10¹³ Ω resistors. The results reveal a relationship between decreasing amounts of analyte and increasing Sr and Pb isotope ratios attributed to blank contribution. No blank influence is detected on Nd isotope ratios. Ablations of a few mg provide sufficient amount of analyte to yield comparable Sr–Nd–Pb isotope values that reflect the composition of the ablated diamond. This result also suggests that HDF microinclusions within individual diamonds are rather homogeneous in their isotopic composition.